BATTERY CONTROL SYSTEMS
Various embodiments herein provide methods and apparatus for conditioning a battery. The conditioning may be undertaken to restore charge capacity or power capability, especially when practiced on a battery having a lithium negative electrode and/or a positive electrode having a conversion material. In one embodiment, the conditioning method includes applying a substantially constant current or power until a conditioning voltage is reached, where the conditioning voltage is higher than the maximum voltage used during normal cycling. The method further includes continuing to charge the battery at the conditioning voltage until a maximum conditioning charge is reached. Next, the method includes discharging the battery to about the maximum charge voltage prior to using the battery in an end use. Also provided is a conditioning apparatus that is configured to perform the conditioning method.
This application claims benefit of U.S. Provisional Patent Application Ser. No. 61/707,405, filed Sep. 28, 2013, and titled “BATTERY CONTROL SYSTEMS FOR VEHICLES,” which is herein incorporated by reference in its entirety and for all purposes.
BACKGROUNDIn the recent years, with shortage of fossil-fuel based energy and adverse environmental effects from the consumption of fossil fuel, both public and private sectors have poured valuable resources into clean technologies. An important aspect of clean technologies is energy storage, or simply battery systems. One field in which battery technology has made inroads is electric and hybrid vehicles. However, the battery technology successfully deployed in such vehicles has been limited to certain systems, notably metal hydride and lithium ion batteries. There is a need for systems that can accommodate higher performance battery systems.
SUMMARYCertain embodiments herein relate to methods and apparatus for conditioning a battery. In one aspect of the embodiments herein, a method is provided for conditioning a positive electrode of a battery, including (a) cycling the battery while deployed for end use, where cycling during end use is limited to a maximum charge voltage, and where the battery includes a positive electrode having an electrochemically active conversion material; (b) applying a substantially constant current or substantially constant power to charge the battery until a conditioning voltage is reached, where the conditioning voltage is substantially higher than the maximum charge voltage; (c) continuing to charge the battery at the conditioning voltage until the battery reaches a maximum conditioning charge; and (d) prior to using the battery for the end use, discharging the battery to about the maximum charge voltage.
In some embodiments, the method is simultaneously performed on a plurality of batteries that are connected in a battery pack. The battery may remain installed in the battery pack during conditioning. The battery pack may be installed in a vehicle, and may remain in the vehicle during conditioning. More generally speaking, the battery may remain installed in an end use apparatus during operations (a) through (d). After the method is performed, a charge capacity of the battery may be greater than before the method is performed. In various implementations, the battery includes a lithium metal negative electrode. The method may increase the uniformity of a distribution of lithium on the negative electrode. The method may also result in at least partial de-agglomeration of metal agglomerates present in the conversion material of the positive electrode. In certain embodiments, the method substantially de-lithiates the positive electrode. The method may include breaking bonds between Li and F in the positive electrode and forming bonds between Fe and F in the positive electrode. In other embodiments, other materials may be used. In that case, the method may include breaking bonds between a reducing cation species and an oxidizing species, and forming bonds between a metal and the oxidizing species.
The method may be initiated in several ways. In certain cases the method is initiated in response to a scheduled event such as performing a threshold number of charge-discharge cycles or passing a threshold number of coulombs or amount of energy to the battery. The scheduled event may also be a threshold time interval, a threshold distance driven by a vehicle in which the battery is installed, or other event. In certain other cases, the conditioning method is initiated based on a trigger event. The trigger event may be, for example, a threshold loss of battery capacity or a threshold increase in overpotential or hysteresis.
Various battery designs may be used. In some embodiments, the conversion material includes fluorine and a metal selected from the group consisting of cobalt, copper, nickel, manganese, and iron. In certain cases, the battery includes a stack of all solid state materials.
In certain embodiments, the conditioning voltage is at least about 4 V vs. lithium, or at least about 4.5 V vs. lithium. In these or other cases, the maximum charge voltage may be about 4 V vs. lithium, or lower, in a particular case, the maximum charge voltage is about 3.8 V vs. lithium, or lower. In carrying out operation (c), a time-varying current or voltage signal may be superimposed on the conditioning voltage. In many cases the method will also include heating the battery to a conditioning temperature. The heating may include delivering energy to a heater while circulating fluid from the heater proximate the battery to thereby heat the battery to the conditioning temperature. The conditioning temperature may be at least about 90° C., or at least about 100° C., or at least about 110° C., or higher. In various embodiments, the maximum conditioning charge charges the battery to a state of charge substantially greater than about 100%. For example, the maximum conditioning charge may correspond to a state of charge of at least about 105%.
In some implementations, operation (d) includes discharging energy from the battery into a second battery. In these or other embodiments, operation (d) includes discharging energy from the battery through a heater and/or heat exchange system. Further, operation (d) may include discharging energy from the battery to an external charger. In one embodiment, operation (d) includes preferentially discharging energy from the battery (i) to a second battery; or (ii) to an external charger, if (i) is not available; or (iii) to a heater and/or heat exchange system, if (i) and (ii) are not available.
In another aspect of the disclosed embodiments, a method of conditioning a positive electrode of a battery is provided, including deploying the battery in an end use, where the battery includes a positive electrode having an electrochemically active conversion material; charging and discharging the battery multiple times under operating conditions designed for the end use, where the operating conditions for the end use include a maximum charge voltage; determining that conditioning is to be performed; and conditioning the battery by a process including (i) applying a substantially constant current or substantially constant power to charge the battery until a conditioning voltage is reached, where the conditioning voltage is substantially higher than the maximum charge voltage; (ii) continuing to charge the battery at the conditioning voltage until the battery reaches a maximum conditioning charge; and (iii) prior to using the battery for the end use, discharging the battery to about the maximum charge voltage. The battery may also include a lithium metal negative electrode.
In a further aspect of the disclosed embodiments, a battery conditioning apparatus is provided, including contacts for electrically connecting to terminals of a battery during conditioning of a battery; and a power supply coupled to said contacts and configured to control the current, voltage, and/or power delivered to the battery during conditioning; where the apparatus is further configured to (i) apply a substantially constant current or substantially constant power to charge the battery until a conditioning voltage is reached, wherein the conditioning voltage is substantially higher than a maximum charge voltage; (ii) continue to Charge the battery at the conditioning voltage until the battery reaches a maximum conditioning charge; and (iii) prior to using the battery for the end use, discharging the battery to about the maximum charge voltage.
In certain implementations, the apparatus is designed to accommodate in situ conditioning. The power supply may be configured to initiate conditioning based on a trigger event such as a threshold increase in overpotential or hysteresis, or a threshold loss of battery capacity. In these or other cases, the power supply may be configured to initiate conditioning based on a schedule. The schedule may be based on the number of charge-discharge cycles, the amount of current or power delivered to the battery, the distance driven by a vehicle in which the battery is installed, etc.
In some embodiments, the maximum charge voltage is about 4 V vs. lithium, or less, for example about 3.8 V vs. lithium, or less. In these or other cases, the conditioning voltage may be about 4 V vs. lithium, or higher, for example about 4.5 V vs. lithium, or higher. The maximum conditioning charge may correspond to a state of charge of at least about 105% in certain cases. The power supply may be further configured to heat the battery until the battery reaches a conditioning temperature of at least about 90° C., for example at least about 100° C. In a particular embodiment, the apparatus is configured to perform (iii) by preferentially discharging energy from the battery (a) to a second battery; or (b) to an external charger, if (a) is not available; or (c) to a heater and/or heat exchange system, if (a) and (b) are not available.
These and other features will be described below with reference to the associated drawings.
The following description is presented to enable one of ordinary skill in the art to make and use the invention and to incorporate it in the context of particular applications. Various modifications, as well as a variety of uses in different applications will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to a wide range of embodiments. Thus, the present invention is not intended to be limited to the embodiments presented, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
In the following detailed description, numerous specific details are set forth in order to provide a more thorough understanding of the present invention. However, it will be apparent to one skilled in the art that the present invention may be practiced without necessarily being limited to these specific details. In other instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention.
All the features disclosed in this specification, (including any accompanying claims, abstract, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.
Furthermore, any element in a claim that does not explicitly state “means for” performing a specified function, or “step for” performing a specific function, is not to be interpreted as a “means” or “step” clause as specified in 35 U.S.C. Section 1.12, Paragraph f. In particular, the use of “step of” or “act of” in the Claims herein is not intended to invoke the provisions of 35 U.S.C. 112, Paragraph f.
Please note, if used, the labels left, right, front, back, top, bottom, forward, reverse, clockwise and counter clockwise have been used for convenience purposes only and are not intended to imply any particular fixed direction. Instead, they are used to reflect relative locations and/or directions between various portions of an object.
The disclosed embodiments concern systems and methods of using batteries having a high capacity positive electrode material that reversibly undergoes electrochemical transition over many cycles of charge and discharge. In some implementations, the material reversibly undergoes a redox reaction at a high rate. There are many examples of suitable materials, which are sometimes referred to herein as “conversion” materials.
Disclosed are systems and algorithms for using batteries in electric vehicles and other products. In various embodiments, the batteries have high operating temperatures; that is, they operate most effectively at temperatures higher than ambient. In one example, the high temperature batteries are solid state lithium-metal fluoride batteries. The batteries are typically provided as packs containing multiple batteries connected in parallel. Some aspects of the disclosure focus on hybrid pack architecture and thermal management. Hybrid pack architectures include at least two battery packs, at least one of which contains the high operating temperature batteries. This pack serves as the primary energy supply for propelling the vehicle most of the time during in normal operation. The other battery pack, referred to herein as a “boost” pack, facilitates vehicle operation when the primary pack is cold. The boost pack comprises batteries that operate effectively at ambient temperatures. The boost pack provides energy for propelling the vehicle after a cold start and provides energy for heating the primary energy pack batteries to a temperature at which they can propel the vehicle.
Certain aspects of the disclosure also concern thermal management systems for controlling the temperature of the batteries in the primary energy pack. Some aspects concern charging systems for charging the primary energy pack and/or the boost pack.
In general, intercalation and/or conversion materials can be used in battery systems described herein. For example, a positive electrode material may be used for intercalation or conversion with lithium. Intercalation materials, which can be prepared at a macro scale or at a nano scale, are commonly used and typically have relatively low energy density (e.g., less than about 800 Wh/kg of active material). Conversion materials, in contrast, can provide much higher energy density (e.g., about 1000-2500 Wh/kg of active material). Battery systems and structures utilizing conversion material are described in U.S. Provisional Patent Application No. 61/778,455, filed Mar. 13, 2013, titled “IRON, FLUORINE, SULFUR COMPOUNDS FOR. BATTERY CELL POSITIVE ELECTRODES”, and U.S. Provisional Patent Application No. 61/674,961, filed Jul. 24, 2012, and titled “NANOSCALE LITHIUM COMPOUND AND METAL ELECTRODES”, and U.S. patent application Ser. No. 13/922,214, filed Jun. 19, 2013, and titled “NANOSTRUCTURED MATERIALS FOR ELECTROCHEMICAL CONVERSION REACTIONS”, each of which is incorporated by reference herein in its entirety.
In certain embodiments, the conversion material includes an oxidizing species, a reducing cation species, and a metal species. These species are sometimes referred to herein as constituents or components. The oxidizing species is typically a strongly electronegative element, compound, or anion. Examples of oxidizing species anions include halides (fluorides, chlorides, bromides, and iodides), oxides, sulfides, and the like. The reducing cation species is typically an electropositive element or cation such as lithium, sodium, potassium, or magnesium and ions thereof. The metal species is typically less electropositive than the reducing cation species. Transition metals are sometimes used as the metal species. Examples include cobalt, copper, nickel, manganese, and iron. The conversion material may contain two or more oxidizing species, two or more reducing cation species, and/or two or more metal species.
As is understood in the art, batteries and their electrodes undergo electrochemical transitions during discharge and—in the case of secondary or rechargeable batteries—charge. The charge and discharge states of certain conversion materials will now be described.
Discharged state—hi the discharged state, the metal species in the positive electrode are generally more reduced than in the charged state. For example, the metal species is an elemental state or a lower oxidation state or positive valence (e.g., +2 rather than +3). Further, during discharge the oxidizing species will unpair from the metal species and pair with the reducing cation species. Still further, during discharge, the reducing cation species tends to move into the positive electrode where it becomes oxidized by pairing with the oxidizing species. Pairing is typically manifest by formation of a chemical bond such as a covalent or ionic bond.
Depending on the implementation, in the discharged state, the conversion material may include an elemental metal material, one or more oxidizing species, and a reducing cation material. As an example, the discharge state may include at least an elemental metal such as iron and a reducing cationhalide such as lithium fluoride. The constituents of the discharged conversion material may be intimately distributed with one other in the discharged material. As described more fully below, these materials may be intermixed or distributed at a scale of about 20 nm or smaller.
It should be understood that a positive electrode of the type described herein may exist in various states of charge. In some cases, a battery is designed or operated so that full discharge is never attained. Thus, if the fully charged conversion material is ferric fluoride for example, the “fully” discharged positive electrode may contain a mixture of elemental iron, lithium fluoride, and some ferric fluoride, and possibly some ferrous fluoride. The use of “discharged” or “discharged state” herein is a relative term, referring only to a state of a conversion material that is more discharged than a charged state of the material. By the same token, the use of “charged” or “charged state” herein refers to a state of the conversion material that is more charged than a corresponding discharge state for the material.
Charged state—In a charged state, the metal species tends to pair with the oxidizing species, often forming a compound. During charging, the oxidizing species tends to unpair from the reducing cation species and pair with the metal species. The reducing cation species tend to move out of the positive electrode and migrate and/or diffuse to the negative electrode where they exist in a more strongly reduced state (e.g., as an element metal such as lithium metal or lithium inserted in a matrix such as carbon or silicon).
As an example, during charge, elemental iron may pair with fluoride anions to form ferric fluoride and/or ferrous fluoride. Concurrently, fluoride anions may Impair from a reducing cation fluoride such as lithium fluoride. The now freed lithium cation migrates and/or diffuses to the positive electrode where it is reduced to elemental lithium or a lithium intercalation material.
“State of charge” (SOC) is used to describe the relative amount of charge that a battery or battery electrode holds at a given time. It is also a measure of the fraction of the available current capacity of the battery or electrode. In other words, SOC is a charge-weighted range between two resting voltages. The units of SOC are percentage points, where 100% indicates that the battery is “fully charged,” and 0% indicates that the battery is “fully discharged.” The amount of charge between 0-100% SOC decreases over the life of the battery, but 100% SOC is always defined/rated to correspond to a particular voltage value. In order to prevent damage to the battery that may occur if charging continues above the fully charged state, or if discharging continues past the fully discharged state, these fully charged and fully discharged states do not typically correspond to the outermost attainable states of the battery. Instead, the fully charged and fully discharged states of the battery are chosen (i.e., rated) to be within the outermost attainable states, at values that are considered safe for operation and proper battery maintenance. One of ordinary skill in the art would understand that a battery may achieve a SOC that is above 100% or below 0%, though this may carry the risk of battery damage depending on the type of battery used and the conditions of the overcharge or discharge.
Because the batteries capacity may change over time and use, one cannot directly measure the SOC of a battery based on charge passed. There are various methods for indirectly measuring the SOC including measuring the battery voltage and other techniques. In the voltage method, a known discharge curve for a battery is used to translate the battery's open circuit voltage readings into SOC values. Higher voltages correspond to higher SOCs. As the battery discharges, the voltage and SOC decrease together. In some cases SOC readings compensate for temperature and/or rate.
A “unit cell” refers to the smallest unit of layers that functions as a battery, and includes a positive current collector, a positive electrode, an electrolyte, a negative electrode, and a negative current collector. A unit cell is often found in the context of a battery stack.
A “stack” or “battery stack” refers to a collection of unit cells stacked on top of one another, and may or may not include terminals electrically connected with the positive and negative current collectors. The unit cells of the battery stack may be connected in parallel. Alternatively, the cells may be connected in series using, for example, a bipolar arrangement.
A “cell” or “battery cell” refers to an electrochemical device having at least one unit cell and positive and negative terminals for connecting to an outside load and/or to other cells in a battery pack. In certain embodiments, a cell contains a plurality of unit cells, sometimes in the range of about 100-200 unit cells. A cell may include a rigid can or other containment structure surrounding the unit cells, though this is not always the case. In some cases, the containment structure is a flexible pouch.
A “pack” or “battery pack” or “energy pack” refers to a collection of battery cells that are electrically connected to one another.
A “battery” may refer to a battery cell or battery pack, and does not typically refer to a unit cell or a battery stack lacking terminals or a containment structure.
While the following disclosure focuses on electric vehicle applications, it also applies to other applications. Therefore, when the disclosure refers to vehicles, other appropriate end uses can be substituted. Examples of such other end uses include uninterruptible power supplies (UPS), consumer electronics (especially portable electronics), power tools, etc. UPSs provide emergency power to a load in the event that a primary source of power goes down. UPSs are commonly used to ensure near instantaneous protection from loss due to power outages for data centers, telecommunications equipment, medical equipment, and other critical electrical equipment.
Typically, a battery installed for a particular end use is charged and discharged to defined levels at defined rates. These parameters are set to provide safe and effective battery performance for the desired end use. The charge and discharge levels are frequently defined in terms of the battery's state of charge, as described above, which may be monitored by measuring the open circuit potential of the battery's terminals. The parameters may be controlled by circuitry installed in a battery pack and/or the end use apparatus.
Elevated Temperature BatteriesSystems and methods presented herein may be tailored to the characteristics of certain types of elevated temperature batteries. In some examples, the batteries of the primary energy pack have one or more of the characteristics described here. One such Characteristic is an all solid state format, in which the batteries have a solid-state electrolyte as well as solid positive and negative electrodes. Manifestations of the all solid phase form of the battery may include one or more of the following: (1) an elevated operating temperature (e.g., over 50° C. or over 80° C.), (2) high temperature stability, (3) comparatively little sensitivity to over and under voltages, (4) rigid and possibly fragile active materials, and (5) high energy density. These characteristics are made in comparison to a conventional liquid electrolyte lithium ion battery.
The energy density of the solid phase battery may be significantly higher than conventional lithium ion batteries. In some cases, the positive electrode materials have an energy density of at least about 2000 Whit, or at least about 2750 Wh/L, or at least about 3250 Wh/L when discharged in the voltage range of 1 to 4.5 V vs. a lithium metal electrode at 120° C. at 400 mA/g. In some implementations, aggressive electrode and cell design (e.g., high mass loading, low carbon and porosity content, etc.) the energy density in the active material is sufficient to enable an energy density of over 500 Wh/kg and over 1200 Wh/L at the cell level. The positive electrode active material may have a specific energy of at least about 600 mWh/g at a voltage of at least about 2.9 V relative to lithium, when charged to a voltage of 3.9 V relative to lithium and discharged at a rate of about 400 mA/g a to temperature of about 120° C. In these or other embodiments, the positive electrode active material may have a specific energy of at least about 1000 mWh/g at a voltage of at least about 1 V relative to lithium, with the other parameters as described above.
The use of a solid state electrolyte allows the battery to be exposed to higher voltages than normally permitted with conventional liquid electrolyte lithium ion batteries. While normal operating voltages may be in the neighborhood of 4 volts, some solid state batteries will tolerate significantly higher voltages, e.g., about 5 volts. Most solid electrolytes are more stable at high voltages than most liquid electrolytes. Nevertheless, it is expected that the cell voltage in the battery pack may need to be monitored to ensure that none of the cells experience a voltage outside an operable range. If the voltage increases or drops outside the acceptable range, an interrupt or other mechanism for stopping the charge or discharge may need to be activated. Various mechanisms for interrupting charge or discharge are described elsewhere herein.
In some cases, the batteries of the primary energy pack employ a positive electrode having multiple morphological states. Each such morphological state may be associated with a different phase and may be associated with a different state of charge regime. Among the relevant characteristics of batteries containing an electrode that exhibits phase changes are (1) the possible need for reformatting or reconditioning during the battery cycle life, (2) a macroscopic volume change over the course of the charge-discharge cycle, (3) the susceptibility to degradation of the battery in certain phases, and (4) a pronounced hysteresis in the charge-discharge curves (voltage versus state of charge). As described below, the batteries of the primary energy pack may be subject to reconditioning at various stages of their lives to ensure that their positive electrodes and possibly other components continue to operate at a desired level of performance. Also, as described below, the batteries and the battery packs may be designed to accommodate volumetric changes during cycling.
Another characteristic of certain batteries that may be used in the primary energy pack is a multi-plateaued discharge curve (voltage versus state of charge) for the positive electrode material. Yet another characteristic of certain batteries used in the primary energy pack is a high fluorine content. For example, the positive electrode may contain significant quantities of fluorine.
Examples of batteries possessing some or all of these features are presented in U.S. Provisional Patent Application No. 61/674,961, filed Jul. 24, 2012, and titled “NANOSTRUCTURED MATERIALS FOR ELECTROCHEMICAL CONVERSION REACTIONS”; and U.S. patent application Ser. No. 13/922,214, filed Jun. 19, 2013, and titled “NANOSTRUCTURED MATERIALS FOR ELECTROCHEMICAL CONVERSION REACTIONS”, which are incorporated herein by reference in their entirety. In general, these batteries are lithium batteries having positive electrodes containing a metal fluoride or other conversion material in their fully charged states. During discharge, lithium ions are inserted into the positive electrode matrix. At some point, the metal fluoride or other conversion material will react to form lithium fluoride and elemental metal. Examples of the metals used to fabricate the positive electrodes include iron, cobalt, copper, manganese, nickel, bismuth, and alloys of any of these materials. In some cases, the discharged positive electrode is comprised of particles having domains of metal, lithium fluoride and possibly other additives. In some cases, the median characteristic dimension (e.g., diameter) of these domains is about 20 nm or less. The particles containing these domains may have a median characteristic dimension in the range of about 100 to 1000 nm. In certain embodiments, the metal is iron or cobalt and the mole ratio of lithium fluoride to metal is between about 2 to 4 in the fully discharged state. In some cases, the conversion materials are provided in a glassy state.
Hybrid System Architecture for Electric VehiclesCentral to this architecture is the use of two separate battery packs. The primary energy pack 10 contains high operating temperature batteries, while the boost pack 12 contains ambient operating temperature batteries. The boost pack 12 generally has less capacity than the primary energy pack 10, but its energy may be delivered at high power. The boost pack 12 may contain a conventional form of battery such as a liquid electrolyte lithium ion battery. Alternatively, the boost pack 12 may contain batteries having a similar chemistry to the batteries of the primary energy pack 10 but with chemistry and architecture optimized to run at lower temperatures.
The primary energy battery pack 10 is used to power the vehicle, i.e., to provide power to the drive train 22. The boost pack 12 performs one or more of the following functions: (1) power the vehicle from a cold start and (2) heat the batteries of the primary energy pack 10 to a temperature at which the batteries operate effectively. Various algorithms may be used to determine which of the two battery packs 10 and 12 is deployed and for Which purpose under the various conditions dictated by environmental parameters such as ambient temperature, driving conditions, states of charge of the primary energy pack 10 and the boost pack 12, etc. These algorithms may comprise stored instructions and a processor for executing the instructions. Alternatively, the algorithms may be implemented as dedicated hard coded logic.
In various embodiments, the algorithms are implemented in a battery management system (BMS) 14. The BMS 14 contains the necessary logic (processor and instructions for example) and electrical interfaces to sensors, switches, etc. In some cases, a single BMS 14 controls both the primary energy pack 10 and the boost pack 12. In other cases, separate BMSs 14 may be provided for the two packs 10 and 12, in which case there may be a separate master controller providing high level instructions to the BMSs 14 and coordinating their actions. In another implementation, one of the two BMSs is a “master” and the other is a “slave”.
Overall, the hybrid system should be operated in a manner that controls heating and electrical performance. The system should ensure that the primary energy pack 10 operates within a defined temperature window chosen to provide safe and efficient operation of the elevated temperature batteries of the primary energy pack 10. The system should additionally ensure that the states of charge of the batteries in the primary energy pack 10 and the boost pack 12 are maintained at appropriate levels to support long battery life, efficient operation, and acceptable cold start performance.
As explained, the primary energy pack 10 may contain solid phase high energy density batteries such as those employing an iron and fluorine containing positive electrode. The boost pack 12 may contain a more conventional type of battery. Examples of conventional batteries that may be used in a boost pack include, but are not limited to, lithium ion batteries employing conventional cathodes (e.g., lithium iron phosphate), nickel metal hydride batteries, etc.
Typically, the primary energy pack 10 is not suitable for cold start applications. This is because the batteries operate with good performance only at a relatively higher temperature, e.g., about 25° C. to 80° C. The boost pack 12 employs batteries Which can operate adequately at a comparatively low temperature (e.g., about −30° C. or warmer) and can be used to propel the vehicle from a cold start, power vehicle accessories, and also deliver energy to heat the primary energy pack 10 so that it can take over for the boost pack 12 in powering the vehicle shortly after the cold start.
The boost pack 12 should operate at ambient temperatures (e.g., about −30 to 60° C.). It should be able to deliver relatively high power (e.g., about 60-200 kW peak and 30-80 kW continuous) and have a modest capacity (e.g., about 1-20 kWh, for example at least about 5 kWh). This implies a discharge (C-rate) of about 5-50 C peak and about 2-20 C continuous. Charge acceptance for regenerative braking is in the range of about 1-10 C. The voltage range of the booster pack 12 may be anywhere from about 150 to 450V and may generally have a more narrow operational voltage range than the primary energy pack 10.
The primary energy pack 10 should have a significantly greater capacity (e.g., about 50-150 kWh, for example 80 kWh) as compared to the boost pack 12. The primary energy pack 10 should be capable of both powering the vehicle and recharging the boost pack 12. Power capability should be about 60 to 300 kW peak and about 30-100 kW continuous. This implies discharge (C-rates) of about 1-6 C peak and about 0.2 to 2 C continuous. Charge acceptance for regenerative braking is in the range of about 0.1 to 1 C. The voltage range of the primary energy pack 10 may be anywhere from about 100 to 450V.
The electrical system of the depicted hybrid architecture includes an HV (high voltage) junction 16 that connects both the boost pack 12 and the primary energy pack 10 to the drive train 22. It switches between these two packs 10 and 12 as appropriate depending upon the state of charge and temperature of the energy pack. It may switch in response to instructions from the battery management system 14. The HV junction 16 may also control delivery of charge from a slow charger 28 to either the boost pack 12 or the primary energy pack 10. The HV junction 16 may also have a low capacity energy storage capability such as a capacitor to ease the transition between sourcing current between the boost pack 12 and primary energy pack 10. The electrical system may also include a fully capable DC-DC power conversion unit 18. In some embodiments, the HV junction 16 and the DC couple 18 described below are the same device.
A DC couple 18 allows the primary energy pack 10 to charge the boost pack 12 under certain circumstances, which are described more fully below. Basically, the boost pack 12 must have sufficient energy to execute a cold start, i.e., a start in which the primary energy pack 10 is too cold to effectively deliver energy to propel the vehicle, including powering vehicle accessories. To ensure that the boost pack 12 has sufficient energy for the next cold start, the primary energy pack 10 may charge it through the DC couple 18. Of course, the primary energy pack 10 should be at a sufficiently high temperature during this charge. The DC couple 18 is capable of accepting voltages in the range of about 100-450 V and outputting power in what may be a more narrow voltage range between about 150 and 450 V. Example ranges may be about 150-350 V input and about 220-300 V output. The power capability of the DC couple 18 should be sufficient to recharge the boost pack 12 in a range of about 20-120 minutes, and/or between about 3-30 kW. The DC couple 18 may be unidirectional and isolated. The DC couple 18 and HV junction 16 can be constructed for custom applications or can be purchased from an auto supply company such as Delphi Corporation, of Troy, Mich.
In certain embodiments, there may also be a third pack (not shown in
Each of the electrical connections (shown in
Relevant aspects of a thermal management system in this architecture include a heater 20, which may be a resistive heater powered by the boost pack 12, and a fluidic heat transfer system containing fluid carrying conduits and associated pumps and valves. A reservoir 24 may be used to store heat transfer fluid. The fluidic heat transfer system communicates with the heater 20 and the primary energy pack 10. In certain embodiments, it delivers a heat transfer fluid to the heater 20, where the fluid is heated before delivery to the primary energy pack 10. More generally, the fluidic heat transfer system may be configured to deliver hot or cold fluid as necessary to control the temperature of the primary energy pack 10. Stated another way, the primary energy pack 10 can receive or donate thermal energy. In addition to reservoir 24, the fluidic heat transfer system includes appropriate plumbing to transport heated fluid throughout the vehicle. The heat transfer system will include, for example, appropriate valving such as three-way butterfly valves. In certain cases, the reservoir 24 and associated plumbing may be referred to as a cooling system.
Another source of thermal energy that may be used to heat the primary energy pack 10 is current from the pack 10 itself flowing through a relatively low impedance load. This heat source is described in more detail below. This form of heating may be employed during a cold start to reduce the requirements of the boost pack 12 to provide energy for elevating the temperature of the primary energy pack 10. The conditions under which this short circuit is executed are chosen and managed via feedback control administered by the BMS 14 so that the voltage of the primary energy pack series elements does not drop to a level at which the cell degrades. For example, the voltage should be maintained above the potential at which electrochemical dissolution of the anode current collector may take place. Additionally, the current flow should be controlled in a manner to avoid heating the energy pack to a temperature that might damage the cells.
During a cold start and during a charge, thermal energy must be delivered to the primary energy pack 10 to heat it to a temperature that is adequate for high-performance operation. In certain embodiments, the operating temperature of the primary energy pack 10 is maintained between approximately 25° C. to 80° C. After the primary energy pack 10 is in operating mode (i.e., above the minimum operating temperature and providing energy to propel the vehicle), it is no longer necessary to heat the primary energy pack 10. In fact, the pack 10 may generate too much heat, in which case it should be cooled. The fluidic heat transfer system may be employed to provide cooling. If the batteries of the primary energy pack 10 operate at temperatures above the operating range (e.g., temperatures above about 100° C.), they may degrade the primary energy pack 10, for example by degrading the performance of structural or insulating materials such as thermoplastics within the primary energy pack 10.
In certain implementations of this system, an external charger 26, sometimes referred to as a fast charger, is available to provide high power electrical energy to rapidly charge the primary energy pack 10. In the depicted embodiment, fast charger 26 provides not only electrical power to charge the primary energy pack 10, but may also provide heat energy via a heat transfer fluid. Charging of the primary energy pack 10 may be conducted much faster if the battery pack 10 is heated. Therefore, the heat transfer fluid provided from the fast charger 26 may deliver thermal energy to the primary energy pack 10 during charging and/or shortly before charging. In other words, if the primary energy pack 10 is too cool for efficient charging, it may be heated using the heat exchanger provided with the fast charger 26. If, on the other hand, the primary energy pack 10 becomes hot during the charge, the heat exchanger of the fast charger 26 may provide low temperature fluid to cool it. The flow rate and pressure drop of fluid during the fast charge may be higher than during normal operation or during a slow charge, as the off-board fast charge fluid pumps may be more capable than the on-board fluid circulation pumps.
As depicted, the fluidic lines for the fast charger 26 are coupled to the lines associated with the vehicle's dedicated heat transfer system. In some embodiments, the external charging station contains heat transfer fluid for heating or cooling the primary energy pack 10. In some embodiments, the heat transfer fluid stored within the vehicle is simply pumped through the primary energy pack 10 during charging. For example, the fast charger 26 may provide instructions to pump the fluid from reservoir 24 to the primary energy pack 10 during charging. Heater 20 may need to be activated during this procedure.
In certain embodiments, the fast charger 26 is a very high power external device that is arranged to mate with the underside of the vehicle. It may have terminals for delivering high power electrical charge (on the order of megawatts) and an appropriate registration system for aligning to terminals of the primary energy pack 10. When connected to the underside of the vehicle, as opposed to the top or side of the vehicle, the fast charger 26 is less likely to represent a danger to vehicle operators or other persons.
A slow charger 28 may be a conventional charger powered by a conventional electrical outlet in a garage or other suitable location. It may be used to charge the boost pack 12, although it may also be used to charge the primary energy pack 10, albeit relatively slowly. In some examples, the slow charger 28 first charges the boost pack 12, which then heats the primary energy pack 10 to an elevated temperature at Which it can be charged by the slow charger.
Systems having hybrid architecture for electric vehicles are further described in U.S. patent application Ser. No. 13/763,636, filed on Feb. 9, 2013, and titled “BATTERY SYSTEM WITH SELECTIVE THERMAL MANAGEMENT,” which is herein incorporated by reference in its entirety.
In one aspect of the disclosed embodiments, a battery power system for an electric vehicle is provided, including a primary energy pack having multiple elevated temperature batteries that operate effectively only at a temperature significantly above ambient temperature; a boost pack having multiple ambient temperature batteries that operate efficiently at ambient temperature; and a battery management system configured to discharge the ambient temperature batteries of the boost pack during a cold start to (i) power a drive train of the vehicle, and (ii) heat the elevated temperature batteries of the primary energy pack to a temperature at which they operate effectively.
In certain embodiments the primary energy pack may have a capacity that is at least about twice the capacity of the boost pack. The elevated temperature batteries may include lithium ion batteries having positive electrodes containing a metal fluoride. The elevated temperature batteries may have a lithium metal anode. In some cases the elevated temperature batteries have a solid state electrolyte, while in other cases they may have a liquid electrolyte. The ambient temperature batteries may be lithium batteries with a liquid electrolyte. The lithium batteries may contain a positive electrode containing metal fluoride.
In some implementations, the battery power system also includes a heater for heating the elevated temperature batteries of the primary energy pack. The heater may be electrically connected to the boost pack, for example to permit the boost pack to power the heater. The system may also include a fluidic heat transfer system arranged to supply heat to and remove heat from the batteries of the primary energy pack. In certain cases the system may also include terminals for electrically connecting the batteries of the primary energy pack to a fast external charger. The terminals may be located on the underside of the vehicle, for example. In a particular embodiment, the battery power system further includes a low impedance load adapted for connection to the primary energy pack, where when connected to the low impedance load, the elevated temperature batteries in the primary energy pack discharge through the low impedance load and thereby generate heat to raise the temperature of the elevated temperature batteries. The battery management system may also be configured to perform conditioning on the elevated temperature batteries.
Charging the Boost Pack in Idle ModeAs mentioned, the boost pack is frequently, if not always, responsible for providing power to the drive train when the vehicle starts cold. A cold start is one in which the temperature of the primary energy pack is below its operating temperature. Frequently, the temperature of primary energy pack at the beginning of a cold start is the ambient temperature, which is well below the operating temperature. For some elevated temperature batteries, the operating temperature is, as mentioned, approximately 80° C.
During a cold start, when the key is turned on, the boost pack starts to heat the charged primary energy pack to permit the primary energy pack to discharge effectively and power the drive train. Before the primary energy pack can take over, the boost pack must power the drive train. Each cold start consumes some charge of the boost pack. As an example, each cold start may consume about one-third of the energy stored in the boost pack. When the state of charge of the boost pack drops below a threshold value (e.g., 45%), the boost pack should be recharged prior to initiating another cold start.
In order to ensure that the boost pack can reliably perform its role in cold starting, the hybrid system may utilize a procedure to ensure that the boost pack has sufficient charge before each cold start. Many mechanisms can be employed for this purpose. For example, an external charger may be employed to charge the boost pack while the vehicle is parked near a charger. A slow charger 28 as depicted in
In some cases, a control protocol provides an idle recharge state in which the vehicle is idle but the primary energy pack is engaged to recharge the boost pack. The idle recharge state may be triggered by various events. In one embodiment, it is triggered by the driver turning off the key of the vehicle. More generally, “key off” means that the vehicle is no longer being powered by the battery packs.
In the case Where the boost pack state of charge is below the desired threshold (i.e., it is too low for the next key on event), and an external charger is not deployed to charge the boost pack, the vehicle enters the idle state where the primary energy pack is used to charge the boost pack. In some cases, energy from the boost pack may be required to heat the primary energy pack before or during charge of the boost pack. Specifically, to permit the primary energy pack to effectively charge the boost pack, the boost pack may need to utilize some of its remaining charge to first heat the primary energy pack. Thus, before and/or during the charge, the boost pack itself may discharge to a degree in order to drive a heater to heat the primary energy pack.
In one aspect of the disclosed embodiments, a battery power system for an electric vehicle is provided, including a primary energy pack having multiple elevated temperature batteries that operate effectively only at a temperature significantly above ambient temperature; a boost pack comprising multiple ambient temperature batteries that operate effectively at ambient temperature; and a battery management system configured to recharge the ambient temperature batteries of the boost pack during an idle period prior to a cold start by (i) discharging the ambient temperature batteries of the boost pack to heat the elevated temperature batteries of the primary energy pack to a temperature at which they operate effectively, and (ii) discharging the elevated temperature batteries of the primary energy pack to charge the ambient temperature batteries of the boost pack. In some embodiments, the battery management system is further configured to determine whether the ambient temperature batteries of the boost pack have a state of charge below a threshold value prior to recharging the ambient temperature batteries.
In Situ ConditioningIn certain embodiments, in situ conditioning of the batteries in the primary energy pack is performed to maintain high level performance throughout the life of the batteries in the pack. By in situ, it is meant that the batteries of the primary energy pack remain installed in the vehicle or other end use product during conditioning. It is assumed that the positive electrode, and possibly the negative electrode and/or the electrolyte undergo chemical and/or morphological changes over repeated cycling that can degrade their performance. A purpose of the conditioning is to undo and reverse these chemical and/or morphological changes to ensure continued high performance operation of the batteries in the primary energy pack. For example, one type of degradation that may occur over repeated cycling is capacity fade. This refers to a decrease in the amount of charge a battery is able to hold. Conditioning may be used to restore some or all of the capacity that was originally achievable before capacity fade occurred. Conditioning may also be used in certain embodiments to restore power capability. Because the hysteresis behavior of the cell may increase over time, conditioning may be used to restore functionality.
Regarding conditioning of the positive electrode, it is the believed that metal-anion containing electrochemically active material (e.g., conversion materials such as iron- and fluoride-containing materials) produces small-scale agglomerates of iron (e.g., multiple attached iron domains within a particle) after repeated charge-discharge cycles. The high surface energy of discrete nano-domains promotes this agglomeration, which corresponds to a lower surface energy state. High rate discharge of metal fluoride electrodes may be dependent on maintaining discrete metal domains of small length-scale, e.g., about 10 nm or less such that a low mobility anion and metal species are in close proximity. Therefore, agglomeration is likely detrimental to performance, as it becomes more difficult to effectively Charge the positive electrode to a fully fluorinated state in which all of the iron atoms are converted to the fully charged metal-anion compound (e.g., a conversion material, MAx, as a particular example FeF3). This agglomeration may result in capacity fade or other battery degrading processes. Other morphological changes may include the redistribution of species within the cathode.
Additionally, the negative electrode may undergo morphology changes that can benefit from conditioning, even if the conditioning is designed to address changes in the positive electrode. For instance, lateral redistribution of lithium on the negative electrode may occur, causing unequal pressure within a unit cell or cell. Where the thermal profile within a cell is non-uniform during cycling, lithium metal may be selectively plated on hotter regions of the negative electrode as opposed to cooler regions of the negative electrode. This selective plating occurs because hotter areas have lower impedance, resulting in an increased current through the hot spot, and more lithium plating in the hot spot. The increased lithium deposition results in an increased pressure at the hot spot. The local increase in pressure further decreases the local impedance, and causes further plating to occur in this area. This positive feedback loop may lead to redistribution of lithium within the negative electrode.
Conditioning can be performed on a schedule, in response to a particular diagnosis, in response to a conditioning request made by a user, or some combination thereof. Examples of schedules include conditioning after a defined number of charge-discharge cycles, conditioning after a defined number of coulombs have passed (per unit of active material), conditioning after a defined amount of energy (Wh) has passed, conditioning at defined time intervals (e.g., every two months), and conditioning after a defined distance has been travelled by the vehicle. The threshold values for initiating conditioning (e.g., the threshold number of cycles) may be set based on the overall history of the battery since fabrication, and/or based on what has occurred since the most recent conditioning event. Conditioning can also be triggered by certain diagnostic events. Such events may indicate performance degradation or signatures associated with certain morphological characteristics. One example is the detected loss of charge capacity after a typical charge. For example, if the battery can only charge to about 80-95% (or less) of its nominal capacity, this may signal a need for conditioning. In one embodiment, a controller is configured to initiate conditioning when the battery can only reach about 80%, 90%, or 95% of its nominal charge capacity. The charge capacity may be measured by integrating the current used to charge the battery over time. Another example is an increase in over-potential or voltage hysteresis during a typical charge. These or other events may be used to trigger conditioning.
Another possibility is that a user may request that conditioning take place (or not take place). Because conditioning takes a certain amount of time, during which the vehicle is not able to be driven, a user may desire that conditioning occur (or not occur) at specific times. For example, in certain embodiments a control panel may prompt the user before conditioning is performed, in order to determine whether the user wants conditioning to occur at that time. In a particular embodiment, when a controller determines that conditioning should occur, it prompts the user as to whether or not they wish that conditioning take place at the next opportunity (e.g., after the vehicle is turned off and plugged in to a charger or dedicated conditioning apparatus). The prompt may also inform the user as to how long the conditioning process is expected to last. In various embodiments the prompt may occur right away (i.e., when the trigger occurs), or at a later time (e.g., after the vehicle is turned off). The user may then input a response such that conditioning does or does not occur at the next opportunity. This user input may be helpful in ensuring that conditioning doesn't occur at a time that is inconvenient to the user, for example while they are making a relatively short stop where a full conditioning process cannot effectively take place.
As mentioned above, in some embodiments, the triggering event does not immediately cause the conditioning process to occur, but instead causes a conditioning process to be scheduled for a future time. For example, where the trigger occurs while the vehicle is being driven, it would be undesirable for the conditioning to happen immediately. Instead, when the trigger condition is met, a conditioning cycle may be scheduled for the next available conditioning opportunity (e.g., when the car is turned off and plugged in to an appropriate device).
Conditioning typically occurs after a battery has been fabricated and deployed for its end use, not during initial fabrication. Where conditioning happens as a result of a triggering event, the triggering event does not occur during initial fabrication. In other words, the trigger thresholds may be set to values that are not typically seen during fabrication. For example, if the trigger event is the number of cycles and a typical battery will go through 10 cycles during fabrication, the threshold number of cycles may be set at 50 cycles. Because conditioning occurs after battery fabrication, it generally occurs outside of a battery fabrication facility. In some cases, conditioning occurs at a user's residence, work, parking garage, parking lot, etc.
Conditioning to maintain performance of conversion type positive electrodes may be accomplished using various conditioning protocols. In certain conditioning processes, the process substantially de-lithiates the positive electrode and restores its base morphological state (e.g., an amorphous state). By removing lithium (which may be present in the cathode as LiF), the transition metal and anion (e.g., Fe3+ and F−) in the positive electrode are encouraged to rejoin one another as FeF3. Recombination of Fe3+ and F− is energetically favored once the lithium is removed. Any iron agglomerates that were present disappear during conditioning as the iron recombines with fluoride ions. During conditioning, the positive electrode goes back to its base amorphous state, in effect resetting the nanostructure of the positive electrode. One way to achieve conditioning of the positive electrode is to apply relatively high voltages over relatively long timeframes.
In certain embodiments the electrode is held at a relatively high potential, often substantially higher than a maximum normal charging voltage usually used to drain a healthy cathode of lithium. For example, normal charging may reach 100% SOC after holding the voltage at about 3.5 or about 3.8 V vs. Li, while the electrochemical stability window of the electrolyte may extend to about 5 V or higher. An example of a conditioning voltage may be about 4 V or 4.5 V (e.g., at least about 4 V). In many cases, the typical operating voltages during a normal charging cycle are about 4 V or lower, for example about 3.8 V or lower. As used in the context of comparing the normal charging voltage and the conditioning voltage, the phrase “substantially higher than” means 0.1 V higher or more. In many cases, the difference between the normal charging voltage and the conditioning voltage is larger, for example at least about 0.3, 0.5, or 0.7 V.
In certain embodiments, the electrode is held at an elevated temperature during conditioning, e.g., between about 80 and 100° C. or for high temperature pack designs up to about 200° C. or higher. In certain embodiments, the electrode is cycled repeatedly over a narrow range of SOC to promote deagglomeration. For example, the cell may be cycled with constant current or power between about 3.5 V and about 3.8 V, or between about 4.0 V and 4.5 V such that the charge and discharge capacity is identical and the SOC before the conditioning is the same as the SOC after conditioning. In some embodiments, there will be zero net charging during the in situ conditioning. Conditioning may be done with time-varying (e.g., AC, pulsed, etc.) current or voltage signals to induce motion over kinetic barriers; this time-varying signal may be superimposed on top of the charging signal or during a high voltage hold step.
The conditioning temperature may correspond to the highest normal operating temperature (e.g., about 100° C. In other embodiments, the conditioning temperature is above the highest normal operating temperature. The conditioning temperature is chosen to be at or near the highest temperature to which the battery pack may be reliably raised without damaging the most heat/temperature sensitive component of the battery pack. In other words, the conditioning temperature is at or near the highest temperature at which the battery pack may be processed without damage. In certain embodiments, the conditioning temperature is at least about 100° C., at least about 110° C., or at least about 120° C. In these or other cases, the conditioning temperature may be between about 100-130° C., for example between about 110-120° C. The heating operation 324 may be different from that of a typical charging cycle. In a normal charging cycle, the heater is only used to bring the primary energy pack up to a normal operating temperature, and then heat exchange is used to prevent the pack from reaching a temperature beyond the range of acceptable operating temperatures. Instead, operation 324 is used to bring the battery pack up to a higher temperature, as described above.
Next, at operation 326, a constant or substantially constant current or power is applied to the battery. As used herein, substantially constant current or power means that the current or power does not vary by more than about 20% (in some cases a variation of no more than about 10%, or no more than about 5%) during the substantially constant current or power process. The applied current or power will be chosen for the battery size and type. The constant current or power is applied until a specified conditioning voltage is reached. The conditioning voltage is typically at or above the voltage that corresponds to 100% SOC before conditioning takes place. In one example, 100% SOC as rated is about 4.2 V, and the conditioning voltage is about 4.5 V. In certain embodiments, the conditioning voltage is between about 0.1 and 1 V (e.g., between about 0.1-0.5 V) above the voltage associated with 100% SOC for the end use. The voltage associated with 100% SOC for an end use may also be referred to as the “maximum charge voltage.” However, the maximum charge voltage may correspond to a SOC below 100% if the battery is not able to reach 100% SOC during a normal charge. In this case, the “maximum charge voltage” corresponds to the highest level of SOC attainable after a normal charge.
One of ordinary skill in the art would understand that the SOC as rated for a particular application falls within the theoretical limits of battery performance, and that the SOC may go above 100% or below 0% as rated by changing the distribution of lithium between the positive and negative electrodes.
After reaching the conditioning voltage at operation 326, the conditioning voltage continues to be applied and the current is monitored and recorded (operation 328). This operation continues until a threshold battery charge (the maximum conditioning charge) has been delivered (typically measured by the current delivered integrated over time). This battery charge delivered will bring the battery to a SOC at or above 100% or 105%, for example between about 100-110%, or between about 100-105%, or between about 105-110%. The appropriate level of charge may be determined by setting a current decay limit at the target voltage. In another case, the appropriate level of charge may be determined by using a recalibrated charge counting amount. For example, if the battery has experienced a. 50% capacity fade, as determined by recent cycling performance, for example, the amount of charge required to bring the battery from 100% to 110% SOC is half of what it would have required originally. A power supply or other controller will typically be able to measure the current device capacity (and hence capacity fade) based on the battery's recent charge/discharge cycles. The relatively high voltage and/or charge delivered during conditioning aggressively removes lithium from the positive electrode to thereby return the electrode to its base state by effectively “overcharging” the battery pack.
Next, the battery is discharged to a safe operating state of charge for its end use, at operation 330. In one example, the battery is discharged to about 100% SOC. This discharge ensures that the battery pack operates within its safe operating parameters. The discharge may be effected in several ways. In one case, the primary energy pack may discharge its extra energy into a boost pack or other ancillary battery after conditioning. In another case, the extra energy is “burned off” through a heater and/or heat exchange system. In another example, the extra energy is returned to an external charger, where possible.
Because some of these possibilities are more attractive than others, the controller may be used to determine how the extra energy is dissipated. For example, the controller may have instructions to preferentially dissipate the energy by (1) recharging the boost pack, if possible, (2) returning the energy to the external charger, if possible, and (3) dissipating the extra energy through a heater/heat exchange as a last resort. In other words, option (2) would only occur if option (1) was unavailable, and option (3) would only occur if both options (1) and (2) were unavailable.
After discharging in operation 330, the battery may be cycled according to normal charge and discharge methods for the particular end use. In certain embodiments, conditioning is performed over a period of about 12 hours or less, or about 6 hours or less, or about 3 hours or less. The method 320 may also be performed on a single battery cell, rather than on a battery pack.
Conditioning is substantially different from battery formation or other treatment associated with conventional battery systems such as lithium ion batteries. With conventional batteries, the positive and negative electrodes can only accept a certain amount of charge in the form of lithium ions. Beyond this point, the crystal lattice of the negative and/or positive electrode begins to destabilize, leading to electrode damage and battery failure. Further, when a conventional lithium-ion battery is overcharged, it tends to release oxygen from decomposition of liquid electrolyte. This gas release, coupled with flammable solvents in the electrolyte, can lead to the battery igniting. Thus, it is extremely unwise to overcharge a conventional liquid electrolyte containing lithium-ion battery. Therefore many commercial lithium ion batteries include mechanisms to prevent such overcharging. However, where the batteries being conditioned include a lithium metal negative electrode (as opposed to a conventional lithium ion negative electrode), a virtually unlimited amount of lithium may be driven into the negative electrode during a charging process. Often, such batteries have a solid electrolyte, further reducing the safety hazards associated with overcharging.
Stated another way, the battery damage that may occur if the SOC strays above 100% is substantially more likely to occur in conventional lithium ion batteries than with the conversion material batteries used in many of the embodiments herein. When a conventional lithium ion battery is charged, lithium ions are incorporated into the anode. However, the anode is only capable of accepting a certain amount of lithium ions. When the anode has reached its lithium ion capacity, additional charging (with the corresponding additional lithium delivery to the anode) may cause significant damage to the anode. In contrast, where a battery includes a positive electrode having a conversion material and a negative electrode that is elemental metal, e.g., a metallic lithium anode, there is no chemistry-dependent upper limit to the amount of lithium that the anode may accept. Thus, a battery of this type may be more safely “overcharged” (i.e., charged beyond 100% SOC) as compared to a conventional lithium ion battery.
Further, high temperature batteries of the types described herein may have structures that are more likely to require conditioning compared to conventional batteries. The positive electrodes of the high temperature batteries are often made of a conversion material, as opposed to an insertion material as used in conventional lithium ion batteries. The conversion material goes through substantial changes as the battery charges and discharges. In particular, covalent bonds are broken and reformed during each charge-discharge cycle. This repeated bond breakage and formation may make occasional battery conditioning more necessary for extending battery life than with conventional lithium ion batteries that do not experience such bonding and debonding events.
Conditioning may also be employed to improve the performance of the negative electrode and/or electrolyte. Methods for conditioning the negative electrode are further described in U.S. Provisional Patent Application No. 61/839,339 filed Jun. 25, 2013, and titled “PULSE PLATING OF LITHIUM MATERIAL IN ELECTROCHEMICAL DEVICES,” which is herein incorporated by reference in its entirety. Over the life of the solid state battery, the distribution of lithium on the anode may become uneven. In certain embodiments, the negative electrode is conditioned in a manner that removes essentially all the lithium and replaces it by discharging the cathode electrode to near or just below zero volts vs. lithium and then slowly recharging it. The recharging may be accomplished using a constant-current charging profile, a pulsed charge profile, or an alternating pulsed charge and discharge profile with a net charge capacity such as two charge pulses and one discharge pulse and/or longer charge pulses than discharge pulses. During the conditioning, the lithium is redistributed within the cathode. This process may be encouraged by increasing the temperature of the cells. This process effectively reforms the entire negative electrode to, in the case of a lithium metal anode, provide a fresh and even distribution of lithium metal.
Conditioning might be performed in a manner that improves the lithium ion mobility in the electrolyte. Solid state electrolytes containing structural lithium species may develop a potential vs. lithium if the structure changes and is no longer stoichiometric. The conditioning algorithm may serve to deplete or replace lithium to restore the original stoichiometry.
In some implementations, the conditioning is performed during external charging of the primary energy pack. In some embodiments, the boost pack assists with the charge and discharge by shuttling energy back and forth, particularly in cases where the external charger cannot accept charge as a load.
In some embodiments, provisions for thermal management of the battery during conditioning are made. These provisions may include, for example, a heat removal system such as the fluidic heat exchange system described above in relation to
While much of the discussion above focuses on conditioning that occurs in situ, the methods may also be practiced on batteries ex situ. In other words, conditioning may also be performed on batteries that are not installed in an end use device. In this case, the conditioning apparatus may be especially adapted to connect with individual batteries or battery packs. In certain embodiments, the conditioning apparatus may be adapted to hold or otherwise support the battery or battery pack during conditioning.
Heating the Energy Pack by Discharge Through a Low Impedance LoadAs mentioned, the batteries of the primary energy pack may be discharged through a low impedance load to raise their temperature. In some embodiments, the temperature is raised to a point where the primary energy pack can power the vehicle. This heating mechanism may be executed during a cold start while the boost pack powers the vehicle. This heating mechanism may also be employed to raise the temperature of the primary energy pack batteries in preparation for their own charging by an external charger. Still further, the mechanism may be employed to heat the batteries in preparation for charging the batteries of the boost pack.
As explained above, it is often desirable and sometimes necessary to heat the primary energy pack before it is discharged in normal operation. In particular, effective use of the primary energy pack for vehicle propulsion may require that the pack be heated to an operating temperature above the ambient.
As mentioned, the primary energy pack batteries can also be heated by a heater powered by the boost pack. See heater 20 of
In some implementations, it may be appropriate to use a short circuit through the low impedance mechanism even when the batteries of the primary energy pack are fully discharged (dead). The cells will still have an open circuit potential and may provide some current that will be effective only in heating the cells. A battery pack using normal ambient batteries could not do this because it could not tolerate the resulting low voltages. However, the open circuit potential of a fully discharged positive electrode in some elevated temperature batteries is sufficiently distant from potentials at which damage such as dissolution of the current collector would occur. For example, fully discharged lithium ion batteries having positive electrode materials such as metal—lithium fluoride may be discharged through a low impedance load without the low voltage problems encountered by conventional ambient temperature batteries.
101.331 The disclosed short circuit discharge mechanism may be conveniently executed using any of various simple mechanisms. Typically, the system will include a switch to shunt current from the batteries to the low impedance load. A connector may be employed to discharge some or all of the batteries in the primary energy pack through the load. The low impedance mechanism may include switching behavior using components such as IGBT (insulated-gate bipolar transistor) solid state switches to modulate the current draw from the energy pack.
Various constraints may be placed on the self short circuit low impedance heating procedure. An algorithm may be defined to control the temperature of the primary energy pack and/or the duration of the self short circuit in order to avoid damage to the pack or other components and to ensure optimal performance. Short-circuit conditions may be controlled by various mechanisms in the load and associated control system. For example, the current level and/or impedance of the load may be independently controlled over the course of the self discharge and heating. Further, the temperature may be monitored to provide feedback to a controller to allow the controller to make decisions on how much current or impedance to permit. The voltage of each series element in the battery pack may be monitored as part of the feedback loop to modulate the self short circuit load. The voltage of each series element should remain balanced, or within a certain allowable range. As the cells warm, their internal impedances may diverge, possibly leading to negative voltages in the remaining high impedance cells if the feedback control loop is not in place.
The current will flow through the low impedance load until it heats the cells to a temperature at which they can charge or discharge effectively. At that point, the current will cease flowing. If the current was permitted to continue to flow, there could be a dangerous positive feedback situation.
Since the internal impedance of the cells, and hence their loaded voltage at a given pack current, may vary sensitively with temperature, a method of thermally balancing the cells involved in the internal self short circuit may be desired. One embodiment employs a circulating fluid in good thermal communication with the cells involved.
The impedance of such load may be variable or constant over the course of the circuit discharge. Therefore, in certain cases, the load is a variable impedance load.
In a hybrid pack configuration, the load may be charging the “boost” pack or assisting with powering the vehicle. If the primary energy pack is cold, the majority of the discharge energy may be deposited within the primary energy pack as heat.
The heating mechanism may be triggered prior to the key on. It may be triggered by a remote signal, for example, a wireless signal sent by the driver to the car, an automatic communication of a given proximity sensed in communication between the car and a GPS near the driver (on a key fob or smartphone or other device).
Typically, the low impedance discharge is limited to prevent a situation where all negative electrode material is stripped from the negative electrode current collector. There may be a specific initial plating formation cycle (for example, pulsed) that would need to be executed if all the lithium is removed. Otherwise the fresh plating may nucleate unevenly.
In some cases, a large negative voltage may result when discharging cold or unresponsive devices. This can result because some cells might warm up more quickly and have higher fully loaded voltages. For example, if the overall loaded pack voltage is 30 V and there are 50 series elements out of 100 with 1 V each, the remaining 50 series elements will total −20 V. In some cases, the impedance is an exponential or other non-linear function of temperature. To address this issue, the primary energy pack may be designed with a thermal redistribution system Which circulates heat around and keeps all cells at approximately the same temperature.
In certain embodiments a fluid heater is employed to dump power from the primary energy pack. For example, with a 10 kW fluid heater, the primary energy pack could discharge through it until it reached a point above 10% efficiency, at which time a peak of 90 kW would be dissipated through cell internal resistance to heat the pack.
Cell Design Accommodating Volumetric Changes During Cell CyclingIn the depicted design, the current collectors 413 and 415 are arranged in an orientation that is substantially orthogonal to that of terminals 403 and 405 in the battery pack. In other words, the longer direction of the terminals 403 and 405 is perpendicular or substantially perpendicular to the lengthwise direction of the current collectors 413 and 415. This perpendicular orientation is depicted in
The positive and negative terminals 403 and 405 are provided in close proximity to a cooling element such as a water filled conduit 411. As depicted in
As noted, the stack terminals 403 and 405 have a longer dimension that is oriented in a direction that is substantially perpendicular to the direction of the current collectors. Further, the shorter dimension of the terminal (i.e., the left-to-right thickness of terminals 403 and 405 shown in
The current collectors of a given polarity may join to form a current bus that runs the entire length of the active portion of the cell inside the cell packaging. This bus may be one and the same as the terminal 403 or 405 or may be attached (for example, welded) to the terminal 403 or 405 after the bus and active portion of the cell are installed in the package.
In the depicted embodiment, the current collectors 413 and 415 provide triple use paths—the thermal, mechanical (strain), and electrical pathways are all the same. The current collectors 413 and 415 may be relatively thin, e.g., 100s of nanometers to 10s of micrometers thick. The thin cells may enable fast charging and discharging (e.g., about 10 C or greater).
In one aspect of the disclosed embodiments, a battery stack is provided, including a plurality of battery unit cells, each having a positive current collector and a negative current collector, where the positive and negative current collectors of the battery stack are substantially parallel in orientation; a positive terminal connecting the positive current collectors in parallel and optionally having a long dimension that is oriented in a direction that is substantially perpendicular to the direction of the current collectors; a negative terminal connecting the negative current collectors in parallel and optionally having a long dimension that is oriented in a direction that is substantially perpendicular to the direction of the current collectors; and a cooling mechanism proximate the positive and negative terminals.
In certain embodiments, the cooling mechanism includes a fluid. In these or other embodiments, the cooling mechanism may not be necessary to conduct heat from portions of the battery stack other than the positive and negative terminals. The shorter dimension of the terminal may be in substantially the same direction as a current collector, and this shorter dimension may be relatively small to minimize thermal and electrical resistance. Certain implementations may utilize battery unit cells that are arranged in a bi-polar electrical configuration in the battery stack. Alternatively or in addition, the battery stack may also include void spaces and/or compressible regions disposed between the individual unit cells of the battery stack.
The battery 601A may be of various dimensions. In certain embodiments, the width of the battery (as measured left to right in
Further expanding on this concept, batteries 601C and 601D (
A parameter that may characterize the batteries shown in
One feature that is not shown in
As mentioned above, there are numerous ways to implement the compressible regions (also referred to as compression regions). In one embodiment, the compressible region is made of a sheet or block of porous or textured resilient material. The material may be provided in an open cell format or a closed cell format. In another embodiment, the compressible region includes one or more rigid, semi-rigid, or flexible plates in combination with one or more mechanical compression members.
A mechanical compression member is one that contracts upon application of a force and expands to its original state (or substantially its original state) after the force is removed. A good quality compression member will be able to expand and contract many times without losing its “springiness.” In more technical terms, this means that the compression member should have a low compression set. Steel is one example of a material that has a low compression set, as it generally requires heating to a very high temperature before it deforms. In certain embodiments, the material used to form the compression region (and any structures therein) should be able to withstand high temperatures (e.g., 60-120° C., for example 60-100° C.) such that it does not break down during cycling, conditioning, or other typical processes. The material may also need to be stable at low temperatures (e.g., as low as about −40° C., or −20° C., or 0° C.). The materials should be stable at the relevant operating pressures, which may be as low as about 1 kPa absolute. In some embodiments, the operating life of the battery is expected to be about 12 years and/or about 1000 full cycles. Thus, the materials should be stable over this timeframe and cycling demand. Where planes are used, the through plane thermal conductivity may be greater than about 0.1 W/mK.
In one example embodiment, the compression region has an area of about 70 mm by 80 mm. The free thickness (Tfree) of the compressible region is about 6 mm or greater, and the solid thickness (Tsolid) is about 2 mm or less. This results in a working thickness of about 4 mm.
One type of mechanical compression member is a spring. Many different styles of springs may be used. Another type of compression member is a punched plate. A third type is a gas-containing sealed package. A fourth type is a simple sheet or block of porous open or closed cell compressible material (either without plates, as described above, or between plates). In an open cell format, adjacent pores in the material are substantially connected, allowing any gas present in the pores to escape when the material is compressed. In a closed cell format, the adjacent pores are not substantially connected to one another, such that when the material is compressed, gas present in the pores does not escape, but rather is compressed with the material. A block of material in an open cell format may also be referred to as a sponge. Other types of compression members may be used, as well.
The properties of an interface between a compressible region and an electrochemical region may affect the performance of a battery. As mentioned, this interface may include a rigid, semi-rigid, or flexible plate in some embodiments. In other embodiments, this plate is absent. The use of more rigid plates may help promote uniform application of pressure to the electrochemical battery material region. In certain embodiments, the compressible structure may have a stiffness of between about 1-10 atm per mm. However, in some implementations it may be preferable for the plates to be more flexible, such that they can accommodate a degree of non-uniform volume expansion within the electrochemical battery material region. In embodiments where no plate is used at the interface, the ability to compensate for non-uniform volume expansion may be especially great. The design of the interface should balance competing concerns regarding the desired force to be applied by the compressible region, as well as the desired uniformity of that force.
Where the compression region is made of an inherently springy material, the material should be able to expand and contract within the battery can. In some embodiments the springy material is a sheet or block of porous material such as foam made from silicone, metal, or a polymer. These foam materials may have a sponge-like structure containing many pores/paths, as shown in the metal foam drawing in
Shapes other than sheets or blocks may be used as well, depending on the particular application. One way to vary the properties of the compression region is to use fiber reinforcement to orient the fibers of the compressible region in a certain direction. This fiber reinforcement may help provide some anisotropic properties to the compressible region, which may be beneficial in certain embodiments.
Another way to implement a compressible region is to include a layer of compressible material directly in a stack of electrochemically active battery materials.
In a similar embodiment, compressible layers are added to a stack of electrochemically active battery materials, but instead of replacing the current collectors, are included as an additional component.
In other embodiments, the compressible layers may be positioned directly between adjacent positive and negative current collectors. In such embodiments, the material of the compressible layer should not be conductive, or the layer should be coated with a non-conductive material, to prevent current from traveling through the compressible layer between a positive and negative current collector. Where the compressible layer is positioned between two current collectors of the same polarity, the conductive properties of the layer are less important. Examples of suitable conductive materials include, but are not limited to, carbon nanofoams, aerogels, and sheets of non-woven carbon fiber.
Another way to accommodate the volumetric change in the electrochemically active materials is to address it at the battery pack level. In these embodiments, a single mechanism may act to accommodate volumetric expansion in many battery cells at once.
In this context, when it is said that the straps may substantially surround the plates and battery cells, this is defined to mean that the straps may surround enough of these elements to keep them together in a compressible manner. One example of a design in which the straps substantially surround the plates and battery cells (but do not literally surround them) is where a first strap attaches to an edge of each plate, and a second strap attaches to the opposite edges of each plate. In that design, one strap will run down each side of the battery pack (at least two sides). In comparison with
Where the compressible region is outside a cell, one can replace the compressible region easily and safely, if necessary. Further, where the compressible region is outside a cell, there is relatively little risk of adverse reactions happening between the compressible material and the electrochemically active battery materials. Where the compressible region is inside a cell, however, care should be taken to ensure that the materials used in the compressible region are either protected from the electrochemically active materials (e.g., the electrolyte, especially where liquid electrolyte is used), or are resistant to the materials and any byproducts that are released from the electrochemically active materials (e.g., any offgas). The use of pouches may help in this regard.
The battery stack/cell/pack designs disclosed herein may be used in a variety of contexts, and are in no way limited to the field of automobiles. The disclosed embodiments may also be used in uninterruptible power supplies, consumer electronics, etc.
Thermally Interruptible Components in CellsVarious embodiments concern thermally interruptible components of the battery that control thermal excursions that might damage the battery or other components of the vehicle. The embodiments described herein concern thermally interruptible cathode current collectors and thermally interruptible solid-state electrolytes.
Safety concerns may arise when the cells of the primary energy pack reach a high temperature—e.g., about 100° C. or higher (or about 200° C. or higher). In some scenarios, a short in a cell or multiple cells in a stack results in a local hot spot in the stack. Such a short can produce a temperature of about 300-400° C. or higher (e.g., over 2,000° C. locally). Such problems may be mitigated by designing cells and/or packs to terminate operation by one or more mechanisms that dramatically decrease their electronic or ionic conductivity.
A distinction may be drawn between a soft short that slowly heats up the cell in a controlled manner and a harder, local short. In the cases of a soft short, interrupting the electrolyte of the entire cell may be desirable as this works by shutting down the whole cell. The cell may be isolated from other cells in parallel through another mechanism such as an over-temperature shutdown device at the terminal. With a harder, local short, the pack may benefit from cutting off the electron supply locally.
One mechanism makes use of the fact that a battery stack contains a parallel architecture such as that depicted in
In some designs, the positive electrode and associated positive current collector interface is thermodynamically unstable, but kinetically stable at normal operating temperatures, e.g., about 100° C. or less. However, the instability is manifest by a chemical reaction at higher temperatures such as the temperatures where damage from thermal excursions is likely. For example, the interface may react at temperatures of about 300° C. or greater (e.g., 800° C. locally), for example, in the range about 300 to 400° C.
The thermodynamic instability is used to advantage by a careful choice of a particular positive electrode current collector material and positive electrode active material. For example, various embodiments employ fluorinated iron or other fluorinated metals in the positive electrode. By choosing a current collector material such as a metal that tends to react with the charged positive electrode material, one has effectively built in a thermally interruptible positive electrode current collector. The reaction of the current collector metal and the positive electrode material degrades the quality of the positive electrode and prevents it from conducting further current, thereby shutting off a runaway reaction to thereby prevent or minimize damage to individual batteries in the primary energy pack or other components in the vehicle. This is especially effective when the cathode material contains metals in a high oxidation state and the current collector contains metals in the neutral state. For example, an iron current collector in proximity with an iron trifluoride (Fe″) cathode is thermodynamically unstable, as iron trifluoride is a strong fluorinator. At high temperatures, the iron trifluoride may fluorinate the iron current collector, thereby degrading the positive electrode to prevent it from conducting further current.
Another type of thermal protection is provided by electrolyte designs in which high temperatures reduce the electrolyte conductivity. Conventionally, this protection is afforded by ion conducting elements that become non-conducting at high temperatures as a result of polymer electrolyte flowing into and blocking pores. However, this protection is limited to polymer based electrolytes. In certain embodiments, a thermally interruptible solid-state electrolyte may be employed to protect against runaway reactions in the batteries of the primary energy stack.
In some examples, the electrolyte undergoes a phase transition from an ion conducting phase to and ion insulating phase at temperatures deemed to present a dangerous condition, e.g., about 300° C. In one example, the lithium ion conductivity in the low temperature stable phase may be about 1e-4 S/cm, and about 1e-8 S/cm in the high temperature phase. In other embodiments, the electrolyte does not change phase at elevated temperatures but does have a lower ion conductivity at higher temperatures. For example, in an amorphous or glassy electrolyte, the structure may change in a manner that decreases the ion conductivity though the phase is still amorphous or glassy.
Operational Characteristics for Multi-Plateau Discharge CurvesIn various embodiments, a discharge and/or charge control algorithm is employed that operates the batteries of the primary energy pack in particular regions of the charge discharge curve for the battery.
In the depicted example of
In one example, a charging algorithm is employed that maintains the batteries of the primary energy pack in the region of the discharge curve defined by intercalation of lithium ions. Such operation may be most appropriate when frequent driving or other cause of frequent cycling is employed for the primary energy pack. It is believed that operating in the intercalation region, particularly during frequent cycling, reduces the amount of degradation of the positive electrode material and thereby prolongs it cycle life and reduces the frequency of required conditioning.
In certain embodiments, the charge algorithm or other battery management control algorithm is implemented to operate the batteries of the primary energy pack in a sulfide discharge region when the battery is to operate in a “reserve” mode. Reserve mode is implemented when the cells of the primary energy pack are in the lower voltage region of a multi-plateau discharge curve such as the one shown in
In another aspect, the batteries of the primary energy pack are divided into various stages or portions, each comprising a group of batteries or stacks. An algorithm for heating and/or charging and/or discharging these various batteries in the individual stages is implemented. In certain embodiments, the algorithm delivers heat to one or more stages while one or more other stages discharge or charge at peak performance. In some implementations, each stage is capable of powering the vehicle by itself. In such designs, a cold start may proceed as follows: first the boost pack powers the vehicle and heats a first stage of the energy pack until the batteries of the first stage reach their operational temperature, then first stage takes over powering the vehicle while it and/or the boost pack heat a second stage of the energy pack until the batteries of that stage reach their operational temperature, the second stage then participates in powering the vehicle, and so on.
Each chain or stage of the pack must have its own set of connections for charging or discharging the vehicle. In some implementations, the individual stages may be connected in series. In some cases, a separate DC bus is provided to each chain.
In another aspect, a fuel is carried onboard the vehicle. The fuel may be burned to heat the battery pack prior-to and/or during cold-starts to warm the energy pack. The energy can be delivered to the entire energy pack immediately or delivered to stages sequentially.
External Fast ChargersVarious charging mechanisms may be employed. As described with reference to
The fast charger may provide significantly higher voltages and/or currents in order to very rapidly charge the primary energy pack. In certain embodiments, the fast charger is physically embodied with an in situ conditioning apparatus as described above. In such embodiments, the apparatus will be configured to operate differently during fast charge for normal end use and for in situ conditioning. Different energy, voltage, and/or current control algorithms are encoded for the two operations. In some cases, to promote safety and ease of use, the fast charge mechanism is provided via a coupling mechanism for engaging with the underside of the vehicle. An auto registration mechanism may be employed for final alignment between the external charger and the charging terminals for the primary energy pack.
The high power electricity delivered by the fast charger requires some safety considerations. This is one reason the charge mechanism is designed to engage with the underside of the vehicle. As an example of the charging process, the vehicle initially drives over the location of the fast charger, then a panel opens and a connection comes out, The connection is registered with appropriate charge terminals on the underside of the vehicle, and then charging occurs. In some implementations, all charging happens underneath the vehicle. In certain embodiments, the fast charger aligns with both the terminals of the primary energy pack and a heat exchange system/reservoir.
In certain embodiments, a fluid coupling is sized to handle up to 100 psi, 200 LPM, heat exchange fluid at low temperatures for cooling (e.g., about 10° C.) and high temperatures for heating (e.g., between about 80-100° C.) to a fast charge temperature. The high voltage electrical connection may be rated from about 400 to 1,000 V with a current capability of about 300-1,000 A.
Claims
1.-47. (canceled)
48. A lithium ion battery comprising:
- a thermally interruptible solid-state electrolyte;
- a positive electrode comprising lithium iron phosphate; and
- a negative electrode comprising lithium metal.
Type: Application
Filed: Sep 6, 2019
Publication Date: Jan 9, 2020
Inventor: Weston Arthur Hermann (Palo Alto, CA)
Application Number: 16/563,487
