LITHIUM-CONTAINING TRANSITION METAL SULFIDE COMPOUNDS

- ITI SCOTLAND LIMITED

The present invention provides a convenient process for making lithium-containing transition metal sulfides involving heating at least on transition metal sulfide with a lithium-containing compound, wherein the lithium-containing compound is selected from one or more of lithium oxide, lithium sulfate, lithium carbonate, anhydrous lithium hydroxide, lithium hydroxide monohydrate, lithium oxalate, lithium nitrate, and any material that is a precursor for any of these lithium-containing compounds.

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Description
CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a United States national phase application under 35 U.S.C. §371 of International Patent Application No. PCT/GB2009/051311 filed on Oct. 6, 2009, and claims the benefit of Great Britain Patent Application No. 0818758.5 filed on Oct. 14, 2008, both of which are herein incorporated in their entirety by reference. The International Application was published as International Publication No. WO 2010/043883 on Apr. 22, 2010.

FIELD

The present invention relates to lithium-containing transition metal sulfide compounds, methods of manufacturing lithium-containing transition metal sulfide compounds, a method of manufacturing a cell or battery comprising lithium-containing transition metal sulfide compounds, the use of lithium-containing transition metal sulfide compounds in electrode materials for lithium ion cells or batteries and the use of such cells or batteries in commercial products.

BACKGROUND

Lithium ion batteries are secondary batteries that comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material. They operate by the transfer of lithium ions between the anode and the cathode, and they are not to be confused with lithium batteries, which are characterized by containing metallic lithium. Lithium ion batteries are currently the most commonly used type of rechargeable battery and typically the anode comprises an insertion material, for example, carbon in the form of coke or graphite. An electroactive couple is formed using a cathode that comprises a lithium-containing insertion material. Typical lithium-containing insertion materials are lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), and lithium manganese oxide (LiMn2O4). In its initial condition, this type of cell is uncharged; therefore, to deliver electrochemical energy the cell must be charged to transfer lithium to the anode from the lithium-containing cathode. Upon discharge, the lithium ions are transferred from the anode back to the cathode. Subsequent charging and discharging operations transfer the lithium ions back and forth between the cathode and the anode over the life of the battery. A review of the recent developments and likely advantages of lithium rechargeable batteries is provided by Tsutomu Ohzuku and Ralph Brodd in Journal of Power Sources 2007.06.154.

Unfortunately, lithium cobalt oxide is a relatively expensive material and the nickel compounds are difficult to synthesize. Not only that, cathodes made from lithium cobalt oxide and lithium nickel oxide suffer from the disadvantage that the charge capacity of a cell is significantly less than its theoretical capacity. The reason for this is that less than 1 atomic unit of lithium engages in the electrochemical reaction. Moreover, the initial capacity is reduced during the initial charging operation and still further reduced during each charging cycle. Prior art U.S. Pat. No. 4,828,834 attempts to control capacity loss through the use of a cathode mainly composed of LiMn2O4. U.S. Pat. No. 5,910,382 on the other hand, describes another approach using lithium-mixed metal materials such as LiMPO4 where M is at least one first row transition metal. Preferred compounds include LiFePO4, LiMnPO4, LiCoPO4, and LiNiPO4 and mixed transition metal compounds such as Li1-2xFe1-xTixPO4 or Li1-2xFe1-xMnxPO4 where 0<x<1.

The use of lithium ion rechargeable batteries is limited by the prohibitive cost of providing the lithium electrode material, particularly in the case of lithium cobalt oxide. Consequently, current commercialization is restricted to premium applications such as portable computers and mobile telephones. However, it would be highly desirable to gain access to wider markets, for example, the powering of electric vehicles and work has been ongoing in recent years to produce materials that maintain the high performance of lithium ion batteries, but which at the same time, are much cheaper to produce. To achieve this goal, it has been suggested, for example, in JP Kokai No 10208782 and Solid State Ionics 117 (1999) 273-276), that sulfides may be used in place of oxides as cathode materials. Although the use of many sulfides achieves less voltage measured against lithium of the corresponding oxides, the capacity of some sulfide-based cathodes, measured in milliampere hours per gram, can be as much as about 3 times greater. Based on this, some sulfide-based cathodes achieve an overall advantage of about 1.5 times in terms of cathode energy density for batteries measured against a lithium metal anode, as compared against their oxide counterparts. This makes the use of these sulfide a very attractive proposition. For example, in the case of lithium iron sulfide, a theoretical capacity of 400 mAhg−1 may be obtained with an average operating voltage of 2.2V versus a lithium metal anode.

Thus, lithium-containing transition metal sulfides will be a convenient substitute material for the lithium metal oxides described above, with lithium iron sulfide already described in the patent literature, for example in U.S. Pat. No. 7,018,603, to be a useful cathode material in secondary cells. The commercialization of lithium-containing transition metal sulfides will depend largely on their cost of production. Taking lithium iron sulfide as a specific example, the conventional process for making this material is via a solid-state reaction in which lithium sulfide (Li2S) and ferrous sulfide (FeS) are intimately mixed together and heated under an inert atmosphere at a temperature of about 800° C. The starting materials, ferrous sulfide (FeS) and iron disulfide (FeS2), are relatively inexpensive as they are naturally occurring materials and are dug out of the ground. However, a notable disadvantage of the reaction process is that the other starting material, Li2S, is not only expensive but also highly moisture sensitive. The latter problem in particular has obvious implications for the complexity, and therefore the cost, of storing and handling the starting material, especially for large-scale commercial production. In addition, the kinetics of this reaction are reported in U.S. Pat. No. 7,018,603 to be very slow and it can apparently take up to one month to complete the reaction. Thus, this route is believed to be highly unfavourable in terms of energy costs and not commercially viable for the production of electrode materials.

As an alternative route for making lithium-containing transition metal sulfides, U.S. Pat. No. 7,018,603 discloses reacting a transition metal sulfide such as FeS with lithium sulfide in a reaction medium comprising molten salt or a mixture of molten salts at high temperature (temperatures of 450° C. to 700° C. are exemplified). The preferred molten salts are lithium halides. Whilst this reaction proceeds at a good rate there are still several issues that make it less than ideal. Firstly, the fact it uses Li2S as a starting material leads to the handling and storage problems described above. Secondly, it is very difficult to separate the reaction medium (molten lithium halide used in 1.5 molar excess) from the desired reaction product by means other than solvent extraction, which is expensive. Further, even after rigorous purification as much as 8% of the reaction medium salt is still present in the reaction product. This level of impurity is detrimental to the charge capacity per gram of lithium iron sulfide.

Given the problems associated with the above synthetic routes to make lithium transition metal sulfides, it is highly desirable to find further alternative routes which rely on inexpensive and non-moisture sensitive starting materials, and which involve a simple, energy efficient reaction method to produce a clean product.

SUMMARY OF THE INVENTION

Thus, in the first aspect, the above invention provides a method of producing a lithium-containing transition metal sulfide, which comprises the steps of

a) mixing at least one transition metal sulfide with a lithium-containing compound;

b) heating the resultant mixture to effect evolution of sulfur from the transition metal sulfide; and

c) allowing sufficient time for the resulting lithium-containing transition metal sulfide to form,

wherein sulfur is retained within the reaction vessel for reaction with the lithium-containing compound and further wherein the lithium-containing compound is selected from one or more of lithium oxide, lithium carbonate, anhydrous lithium hydroxide, lithium hydroxide monohydrate, lithium oxalate and lithium nitrate and any material that is a precursor for any of these lithium-containing compounds during the heating step.

Preferably the transition metal sulfide made by the above method is of the formula Li2-x-yAyFe1-zMzS2 where x=0 to 1.5, preferably x=0 to 1, further preferably x=0 to 0.5 and particularly preferably x=0 to 0.3; y=0 to 1; z=0 to 1, A is selected from one or more of silver (Ag), sodium (Na), copper (Cu(I)) and potassium (K) and M is a generic representation for one or more transition metals. The preferred lithium-containing compounds comprise lithium oxide and/or one or more materials that is a precursor for lithium oxide. Preferably, the precursor materials for lithium oxide decompose to give lithium oxide during the heating step of the method of the present invention. Also during the heating step, it is advantageous that the transition metal sulfide decomposes to release sulfur which in turn reacts with the lithium- containing compound to form in situ the transition metal, sulfur and lithium-containing compounds required to produce the lithium-containing transition metal sulfide.

To improve the yield and efficiency of reaction it is highly advantageous if the method of the invention is conducted under a non-oxidizing atmosphere and/or reducing conditions. Many of the well-used non-oxidizing atmosphere and/or reducing conditions may be employed, however, preferred examples involve one or more reducing gases, such as carbon monoxide, hydrogen, reforming gas (mixture of hydrogen and nitrogen), hydrogen sulfide, methane and other gaseous alkanes. One or more reducing agents such as carbon may also be used either alone or in combination with a reducing gas and/or non-oxidizing atmosphere. In the present invention, it is highly preferred that the reducing conditions do not reduce the oxidation state of the transition metal ion.

The ideal reaction temperature used in the present invention is that which, on the one hand, is sufficient to cause the transition metal sulfide to decompose to evolve sulfur so that it is available at the correct temperature for reaction with the lithium-containing compound. On the other hand, the reaction temperature should not be so high as to cause the decomposition to occur too quickly and for the sulfur to be lost before the reaction occurs. In this latter situation, when a high temperature is used, the level and number of impurities, caused for example, by the over-reduction of the transition metal ion to the zero oxidation or metallic state, is also found to increase. Therefore, the actual temperature usedwill depend on the chosen starting materials, at least one transition metal sulfide and lithium-containing compound. As a general rule, the reaction temperature is conveniently from 500 to 1500° C., preferably from 550 to 1500° C., further preferably from 550 to 950° C. and particularly preferably from 550 to 750° C. The reaction time varies according to reaction temperature and as one might expect, the higher the temperature, the faster the reaction. By way of example, a suitable reaction temperature/time profile that will produce the desired lithium-containing transition metal sulfide includes heating the reaction mixture for 12 hours at 650° C. Alternatively, one could heat the reaction mixture for 4 hours at 950° C.

The at least one transition metal sulfide used in the method of the present invention may be one or more sulfide compounds comprising one or more transition metals. This includes the use of single and/or mixtures of several transition metals in the sulfide, as well as the use of mono- and/or di-sulfides. Particularly suitable transition metals comprise one or more of manganese, iron, cobalt, nickel copper and zinc. Preferably, the transition metals are selected from manganese, iron, cobalt and nickel. Sulfides that comprise iron are the most preferred transition metal sulfides.

Although the starting materials are not air or moisture sensitive, and these positive attributes aid the storage and handling of these materials, the reaction product is itself reactive towards water. Therefore, it is advantageous to form and handle the lithium-containing transition metal sulfides under a dry and inert atmosphere such as argon or nitrogen.

Suitable reaction vessels comprise glassy carbon or graphite crucibles that generally have a loose fitting lid. However, a sealed pressurized vessel may also be used. For commercial scale production, it is advantageous to use a continuous process, for example, a rotary tube furnace, although a retort batch process may also be used.

The reaction of the present invention is a solid-state reaction and this means that all reactants are in solid form and are without the use of a reaction medium such as a solvent. The reactants are solid materials that are first grounded using a ball mill to produce a fine powder that can either be used directly or pressed into a pellet.

In the case where the method of the present invention involves heating the starting materials in the presence of carbon, the reaction may be termed a “carbon assisted” process. Carbon, as with the other means of providing reducing conditions, is useful to reduce the lithium sulfate (Li2SO4), which forms as a side reaction during the reaction process, to lithium sulfide (Li2S), which in turn reacts with the transition metal sulfide to form the desired lithium-containing transition metal sulfide. It is highly desirable that the reducing conditions in the process do not directly reduce the oxidation state of the transition metal ion. Any amount of carbon may be used but it is convenient not to use too much to prevent it from becoming a significant impurity in the reaction product. Having said this, it has been found to be of significant advantage, particularly to the conductivity of the target material, for at least a small amount of carbon to be present in the reaction product. Moreover, there are further specific advantages to be gained in the carbon being residual from the carbon assisted reaction process of the present invention, as opposed to it merely being added later to a sample of the target lithium-containing transition metal sulfide material; during the carbon assisted process, the carbon is considerably intimately mixed with the lithium-containing transition metal sulfide product. The degree of mixing described as “intimate” refers specifically to the chemical as opposed to physical mixing that is achieved when carbon is used, at least in part, to provide the reducing conditions in the process of the present invention. This “intimate” mixing is quite different from the degree of mixing that would ever be achieved using ball milling or other physical mixing apparatus. In particular, in the carbon-assisted process of the present invention, the carbon is dispersed at the microscopic level on individual particles of the lithium-containing transition metal sulfide.

The ratio of reaction starting materials is typically 1 mole of transition metal sulfide:the equivalent of from 0.5 to 4 moles of lithium ion in the lithium containing material:from 0 to 5 moles of the one or more reducing agent. The preferred ratio of starting materials, that is transition metal sulfide:number of mole equivalents of lithium supplied by the lithium-containing compound:number of moles of one or more reducing agent, is 1:0.5-2:0.25-5, further preferably 1:0.5-1:0.25-0.5. In the case of the carbon assisted process, the most preferred ratio of reactants, that is, transition metal sulfide:number of mole equivalents of lithium supplied by the lithium-containing compound:carbon, is 1:1:0.5.

As a general rule, lower amounts of carbon are required when a reducing gas and/or a reducing agent is used.

When present, the carbon used may be in any suitable form, for example, graphite, charcoal and carbon black. Although, it is preferred to use high surface area carbons that are typically used in electrode formulations, for example, Super P, Denka Black, Ensaco etc.

An alternative source of carbon may be derived in situ from any suitable carbonaceous material, for example, by the thermal decomposition of an organic material such as lithium acetate, dextrin, starch, flour cellulosic substance or sucrose or a polymeric material such as polyethylene, polyethylene glycol, polyethylene oxide, and ethylene propylene rubber. In fact, most carbon containing materials may be used, provided their thermal decomposition does not involve the production of detrimental by-products.

The target lithium-containing transition metal sulfide compound produced by the first aspect of the present invention is generally found to contain less than 2 atoms of lithium per molecule of product. Even though 1.72 atoms of lithium is the typical number, the true number will depend on the reaction temperature, duration of the heating step and the particular lithium-containing transition metal sulfide being prepared.

In order to reduce the quantity of impurities formed and to optimize the reaction conditions, it has been found advantageous to add a flux agent, also known as a mineralizer, to the reaction mixture. Flux agents or mineralizers are commonly used in the ceramics industry to lower the reaction temperature and shorten reaction times. Mineralizers such as sodium chloride, borax, lithium chloride, lithium fluoride, sodium fluoride, lithium borate and sodium carbonate are known. The present applicant has found that using a very small amount of a mineralizer, in particular, an alkali metal halide, will result in a lithium-containing transition metal sulfide product that exhibits enhanced crystallinity with lower levels of impurities. Any alkali metal halide may be used, but lithium chloride and lithium iodide are most preferred. Alternatively, sodium carbonate or sodium chloride may be used. However, in this case it is likely that at least some substitution of the lithium for sodium will occur in the target product. The amount of mineralizer found to be beneficial in the present invention is from 1 to 5% by weight of the starting materials, preferably from 1 to 3% by weight and further preferably 1% by weight of the starting materials.

A typical electrode comprises 94% of a lithium-containing material, 3% of a binder and 3% of a carbon-containing material. In this aspect of the present invention, the lithium-containing material is preferably a lithium-containing transition metal sulfide and further, preferably one that is made by the method of the present invention described above. The binder can be any material known in the art to be suitable for use as a binder, usually a highly inert polymer such as polytetrafluoroethylene (PTFE), polymers of ethylene propylene diamine monomer (EPDM), polyethylene oxide (PEO), polyacrylonitrile and polyvinylidene fluoride. The Applicant's preferred binder is ethylene propylene diene monomer (EPDM). The key feature of the binder is that it needs to be able to form a slurry or paste with the lithium-containing transition metal sulfide, which in turn may be coated onto a current collector. Conveniently, mixing the binder with a solvent facilitates coating. Any solvent may be used, provided it is nonpolar, dry and does not react with either the binder or the lithium-containing transition metal sulfide. Desirably, the solvent is reasonably volatile to facilitate its removal at room temperature. Suitable solvents might include low molecular weight halogenated compounds, particularly halogenated hydrocarbons such as methylene chloride or low molecular weight materials, such as cyclohexane, trimethylbenzene (TMB), toluene, and xylene, or low molecular weight alcohols, such as methanol and mixtures of any of these compounds. Trimethylbenzene is a preferred solvent.

The binder/solvent/lithium-containing transition metal sulfide slurry/paste may also include additives adapted to modify the properties of the binder. The chosen additive must be naturally compatible with the binder, the lithium-containing transition metal sulfide and the electrolyte, and must not affect the performance of the finished cell.

In working to find a reaction process for the preparation of lithium-containing transition metal sulfides, the Applicants have discovered a highly convenient route for the preparation of lithium sulfide. Consequently, in a second aspect of the present invention, there is provided a method of preparing lithium sulfide comprising heating one or more lithium-containing materials in the presence of sulfur. Preferred lithium-containing materials may be selected from lithium carbonate, lithium oxide, anhydrous lithium hydroxide, lithium hydroxide monohydrate lithium oxalate, lithium nitrate and any other material that is a precursor for these lithium-containing materials. Also preferably, the reaction is carried out under non-oxidizing and/or reducing conditions.

The reaction according to the second aspect is conveniently heated to a temperature from 500° C. to the decomposition temperature of the lithium-containing material, preferably from 550° C. to the decomposition temperature of the lithium-containing material. As described above, one or more reducing gas and/or at least one reducing agent, for example carbon, may be used to provide the optional reducing conditions in the method of this aspect of the present invention. In the case where carbon is used, carbon black is preferred. The sulfur may be generated by the thermal decomposition of a suitable sulfur-containing compound such as pyrite (FeS2) or cobalt sulfide (CoS2). Alternatively, flowers of sulfur may be used.

The lithium-containing transition metal sulfide materials produced by the method of the present invention are useful in a wide variety of applications where a low voltage rechargeable battery power source may be used, for example, in mobile phones, vehicles, lap top computers, computer games, cameras, personal CD and DVD players, drills, screw drivers and flash lights and other hand-held tools and appliances.

In order for the lithium-containing transition metal sulfides of the present invention to be used in such applications it is necessary to construct them into an electrochemical cell. Different methods of making such cells are described in the literature, but one particularly convenient example is described in EP 1 295 355 B1. In this case, an electrochemical cell is assembled comprising a plurality of anode plates and a plurality of cathode plates, each comprising respective insertion materials for example graphite in the anode plates and the lithium-containing transition metal sulfides of the present invention in the cathode plates. In particular, the method involves forming a stack of discrete, separate cathode plates and discrete, separate anode plates stacked alternately, each comprising a layer of a respective ion insertion material bonded to a metal current collector, and interleaving a continuous separator/electrolyte layer between successive plates so it forms a zigzag.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be particularly described by way of example with reference to the following drawings in which:

FIG. 1 shows the overall reaction scheme for the process of the present invention when lithium carbonate is used as the lithium-containing compound. In this case, the reaction proceeds by the thermal decomposition of lithium carbonate to lithium oxide followed by the reaction of the latter with sulfur evolved from the transition metal sulfide and carbon to produce lithium sulfide, which in turn reacts with the transition metal sulfide to produce the target lithium-containing transition metal sulfide.

FIG. 2 shows multiple constant current cycling data for the compound with the formula Li2-xFeS2 made according to Example 1 of the present invention.

FIG. 3 shows multiple constant current cycling data for the compound with the formula Li2-xFeS2 made according to Example 2 of the present invention.

FIG. 4 shows a series of overlaid X-ray diffraction spectra to show the effect on the purity and crystal structure of the addition of 0%/wt, 1%/wt and 3%/wt lithium chloride mineraliser in compounds with the formula Li2-x-yAyFe1-zMzS2 made according to Examples 1, 2 and 3 respectively.

FIG. 5 shows a series of overlaid X-ray diffraction spectra to show the effect of carbon on the purity of Li2-xFeS2 compounds made according to the method of the present invention as described in Examples 1 and 4.

 DESCRIPTION OF THE EMBODIMENTS

General Laboratory Scale Method a) For Making Compounds with the Formula Li2-x-yAyFe1-zMzS2 Using a Reducing Agent to Provide the Reducing Conditions.

The lithium containing compound, transition metal sulfide and reducing agent are weighed out into a ball mill pot, this mixture is milled for 1-12 hrs depending on the size of the precursor mix at a rate of 200-350 rpm. The precursor mix is then pelletized, and placed into a glassy carbon crucible with a lid. The carbon crucible is placed into the furnace under a gentle flow of an inert gas, and heated between 500 and 1500° C. at a rate of 1 to 10° C. per minute over a period of 1 to 12 hours. The crucible is allowed to cool under the inert gas flow and transferred directly into a glove box. The resulting product is ground initially using a pestle and mortar and then milled more finely using a ball mill. The lithium-containing transition metal sulfide product may then be analyzed using X-ray diffraction and/or electrochemical techniques. A suitable furnace for carrying out the above process may be a graphite lined rotary furnace, a retort furnace or a static tube furnace.

General Laboratory Scale Method b) For Making Compounds with the Formula Li2-x-yAyFe1-zMzS2 Using a Reducing Gas to Provide the Reducing Conditions.

The lithium-containing compound and transition metal sulfide are weighed out into a ball mill pot. This mixture is milled for 1-12 hrs depending on the size of the precursor mix at a rate of 200-350 rpm. The precursor mix is then pelletized, and placed into a glassy carbon crucible with a lid. The carbon crucible is placed into the furnace under a gentle flow of a reducing gas, and heated between 500 and 800° C. for 1 -20 hours dwell. The crucible is allowed to cool under the inert gas flow and transferred directly into a glove box and processed and analysed as described above.

Compounds with the Formula Li2-x-yAyFe1-zMzS2 (x, y and z are as defined above) were prepared according to Examples 1 to 8 summarised in Table 1 below:

TABLE 1 Reaction Li-containing Transition Reducing conditions Example compound metal sulfide conditions Mineraliser Temp/Time 1 Li2CO3 FeS2 Carbon None 135° C. 2 hr (111 g; (179 g; Denka Black 900° C. 2 hrs 1.5 moles) 1.5 moles) (9 g; 0.75 moles) 3° min−1 2 Li2CO3 FeS2 Carbon LiCl 135° C. 2 hr (111 g; (179 g; Denka Black (2 g; 1 wt %) 750° C. 9 hrs 1.5 moles) 1.5 moles) (9 g; 0.75 moles) 3° min−1 3 Li2CO3 FeS2 Carbon LiCl 135° C. 2 hr (111 g; (179 g; Denka Black (6 g; 3 wt %) 750° C. 9 hrs 1.5 moles) 1.5 moles) (9 g; 0.75 moles) 3° min−1 4 Li2CO3 FeS2 None None 135° C. 2 hr (5.53 g; (8.99 g; 750° C. 9 hrs 0.075 moles) 0.075 moles) 3° min−1 5 Li2CO3 FeS2 Carbon Na2CO3 135° C. 2 hr (111 g; (179 g; Denka Black (0.79 g; 0.01 moles), 750° C. 9 hrs 1.5 moles), 1.5 moles) (9 g; 0.75 moles) LiCl (2 g; 3° min−1 0.05 moles) 6 Li2CO3 CoS2 Carbon None 135° C. 2 hr (111 g; (179 g; Denka Black 800° C. 9 hrs 1.5 moles) 1.5 moles) (9 g; 0.75 moles) 3° min−1 7 Li2CO3 FeS2 H2/N2 mixture None 500-800° C. (73.89 g; (119.98 g; for 1-20 1 mole) 1 mole) hours dwell 8 Li2CO3 FeS2 Carbon None 500-800° C. (73.89 g; (119.98 g; monoxide for 1-20 1 mole) 1 mole) hours dwell

General Procedure to Determine the Capacity of the Li2-x-yA-yFe1-zMzS2 Compounds Made According to the Present Invention.

Materials were initially tested using a small pouch type cell, tags of aluminium one side and nickel on the opposing side were sealed into the sides of a pouch, a stack of Li/Ni separator and the cathode coating was made and inserted into the pouch in between the two tags. Electrolyte was pipetted onto the separator and the end of the pouch was then vacuum-sealed. Constant current tests were performed on a MACCOR between the voltage limits 2.65V and 1.45V using a rate of 10 mAg−1.

Determination of X-Ray Diffraction Data

Powder X-ray diffraction data was obtained using a SIEMANS D5000 using a copper Kα1 and Kα2 source. The sample was placed into an air sensitive holder, which consisted of a Perspex dome, which sealed over the sample, thus preventing degradation of the material during data collection. Phase analysis data were collected over a period of 4 hours 10-80° 2 theta, whilst high quality data were collected 10-90° 2 theta over a period of 13 hours.

Determination of the Capacity of the Li2-x-yAyFe1-zMzS2 compound made using Example 1

The major phase present in the product synthesised in Example 1 was found to be Li2-xFeS2, with low levels of impurities observed by x-ray diffraction. As shown in FIG. 2, constant current data was obtained for the product using the above general testing procedure. In particular, multiple constant current cycling data was obtained using 1.2M LiPF6 in EC:EMC 20:80 as the electrolyte and cycling against a lithium metal anode between the voltage limits 2.65V and 1.45V at room temp at a current of 10 mAg−1. An initial charge capacity of 290 mAhg−1 versus lithium was observed. A reversible capacity of 320 mAhg−1 was observed over the subsequent cycles.

Investigation of the Effect of Adding 1%/wt Mineraliser During the Preparation of Li2-xFeS2

The Li2-xFeS2 compound made according to Example 2 includes 1%/wt LiCl mineralizer in its formulation. The effect of this mineralizer on the capacity of Li2-xFeS2 is indicated by constant current cycling data shown in FIG. 3. The cycling data was obtained using the above general procedure, using 1.2M LiPF6 in EC:EMC 20:80 as the electrolyte and cycling against a lithium metal anode between the voltage limits 2.65V and 1.45V at room temp at a current of 10 mAg−1. An initial charge capacity of 290 mAhg−1 and a discharge capacity of 320 mAhg−1 versus lithium was observed. A reversible capacity of 320 mAhg−1 is observed over the subsequent 6 cycles. Also, a slight reduction in polarization was observed for the sample made using a lithium chloride mineralizer compared to the sample synthesized with none, as shown in FIG. 2.

Comparison of the Effect of Adding 0 wt %, 1 wt % Versus 3 wt % Mineraliser During the Preparation of Li2-xFeS2

The Li2-xFeS2 compounds made according to Examples 1, 2 and 3 involved the use of 0 wt %, 1 wt % and 3 wt % respectively of lithium chloride as a mineralizer.

FIG. 4 shows a pattern at the bottom of the three graphs that depicts the calculated positions for the reflections associated with Li2-xFeS2. As the level of LiCl additive increases the reflection peaks for Li2-xFeS2 appear sharper, which suggests a larger particle size, and less impurity peaks appear in the 20° 2 theta region. A small level of LiCl is observed as impurity in the 3 w % LiCl containing-material, and capacity data for the 3 wt % LiCl-containing material is reduced due to the LiCl being present as an impurity. Such reduction in capacity is not observed for the Li2-xFeS2 compound made with 1 wt % LiCl.

Determination of the Effect of Carbon Levels on the Purity of Li2-xFeS2 Compounds

The Li2-xFeS2 compound of Example 4 was made without carbon in an argon atmosphere, and X-Ray diffraction data was obtained using the general procedure outlined above. Similar X-ray data was obtained for the compound of Example 1 and both sets of results are illustrated in FIG. 5. The peaks at the bottom of the graphs show the calculated position for Li2FeS2. The sample containing iron sulfide, the amount of lithium ion in lithium carbonate, and carbon in the ratio 1:1:0.5 (Example 1), shows low impurity levels compared to a similar compound but with no carbon (Example 4). The sample made with no carbon shows impurities of Li2SO4 and FeS depicted by ⋄ and X respectively above the reflection peaks. These results show that Li2-xFeS2 compounds can be made with no carbon being present, however impurity phases of Li2SO4 and FeS are present. Thus a key feature of the reducing agent (for example the carbon) is to reduce the Li2SO4 to Li2S, such that the remaining FeS can react with lithium sulfide to form the target Li2-xFeS2 compound as detailed in the reaction scheme in FIG. 1.

Claims

1. An in situ method of producing a lithium-containing transition metal sulfide, characterised in that it comprises the steps of

d) mixing at least one transition metal sulfide with a lithium-containing compound;
e) heating the resultant mixture to effect evolution of sulfur from the transition metal sulfide; and
f) allowing sufficient time for the resulting lithium-containing transition metal sulfide to form,
wherein sulfur is retained within the reaction vessel for reaction with the lithium-containing compound and further wherein the lithium-containing compound is selected from one or more of lithium oxide, lithium carbonate, anhydrous lithium hydroxide, lithium hydroxide monohydrate, lithium oxalate and lithium nitrate and any material that is a precursor for any of these lithium-containing compounds during the heating step.

2. A method of producing a lithium-containing transition metal sulfide according to claim 1 wherein the lithium-containing transition metal sulfide is of the formula Li2-x-yAyFe1-zMzS2 where x=0 to 0.5, y=0 to 1; z=0 to 1, A is selected from one or more of silver (Ag), sodium (Na), copper (Cu(I)) and potassium (K) and M is a generic representation for one or more transition metals.

3. A method of producing a lithium-containing transition metal sulfide according to claim 2 wherein the lithium-containing transition metal sulfide has less than 2 atoms of lithium per molecule.

4. A method of producing a lithium-containing transition metal sulfide according to claim 1 wherein the at least one transition metal sulfide and lithium-containing compound are heated to a temperature of 500 to 1500° C.

5. A method of producing a lithium-containing transition metal sulfide according to claims 1 wherein the at least one transition metal sulfide comprises one or more of manganese, iron, cobalt, nickel, copper and zinc.

6. A method of producing a lithium-containing transition metal sulfide according to claim 5 wherein the at least one transition metal sulfide comprises one or more of manganese, iron, cobalt and nickel.

7. A method of producing a lithium-containing transition metal sulfide according to claim 4 wherein the at least one transition metal sulfide comprises one or more of a monosulfide and/or a disulfide.

8. A method of producing a lithium-containing transition metal sulfide according to claim 1, wherein the reaction is carried out under a non-oxidising atmosphere and/or reducing conditions.

9. A method of producing a lithium-containing transition metal sulfide according to claim 8, wherein the reducing conditions are provided by one or more reducing gases and/or one or more reducing agents.

10. A method of producing a lithium-containing transition metal sulfide according to claim 9, wherein the one or more reducing gases is selected from carbon monoxide, hydrogen, reforming gas (mixture of hydrogen and nitrogen), hydrogen sulfide, methane and other gaseous alkanes and the one or more reducing agent is selected from carbon and any carbonaceous material suitable to provide a source of carbon during the heating step.

11. A method of producing a lithium-containing transition metal sulfide according to claim 1, wherein the at least one transition metal sulfide and the lithium-containing compound are in powder form.

12. A method of producing a lithium-containing transition metal sulfide according to claim 9 wherein the starting materials are in the ratio 1 mole of transition metal sulfide:the equivalent of from 0.5 to 4 moles of lithium ion in the lithium containing material:from 0 to 5 moles of the one or more reducing agent.

13. A method of producing a lithium-containing transition metal sulfide according to claim 12, wherein the starting materials are in the ratio 1 mole of transition metal sulfide:the equivalent of 1 mole of lithium ion in the lithium containing material:0.5 moles of carbon.

14. A method according to claim 1, further comprising the addition of one or more mineralisers in a total amount of from 1 to 5% by weight of the starting materials.

15. A method of claim 14 wherein the mineraliser comprises an alkali metal halide.

16. A method of producing a lithium-containing iron sulfide, characterised in that it comprises the steps of

a) mixing iron sulfide (FeS2) with lithium carbonate (Li2CO3) and carbon;
b) heating the resultant mixture to effect evolution of sulfur from the transition metal sulfide; and
c) allowing sufficient time for the resulting lithium-containing transition metal sulfide to form,
wherein sulfur is retained within the reaction vessel for reaction with the lithium carbonate and further wherein the lithium carbonate is used directly or is formed using any material that form lithium carbonate during the heating step.

17. Use of one or more lithium transition metal sulfides having less than 2 atoms of lithium per molecule, together with a binder and a solvent comprising a non-polar low molecular weight hydrocarbon, in the preparation of an electrode.

18. An electrode made according to claim 17 wherein the binder comprises ethylene propylene diene monomer.

19. An electrode made according to claim 17 wherein the solvent comprises trimethylbenzene.

20. An electrode made according to claim 17 used, in conjunction with a counter electrode and an electrolyte, in a low voltage rechargeable battery power source.

21. A lithium ion battery comprising a cathode comprising an electrode made according to claim 17.

22. A lithium ion battery comprising a cathode comprising an electrode made according to claim 19.

Patent History
Publication number: 20110198532
Type: Application
Filed: Oct 6, 2009
Publication Date: Aug 18, 2011
Applicant: ITI SCOTLAND LIMITED (GLASGOW)
Inventors: Jeremy Barker (Oxfordshire), Emma Kendrick (Hampshire)
Application Number: 13/124,031
Classifications
Current U.S. Class: Having Utility As A Reactive Material In An Electrochemical Cell; E.g., Battery, Etc. (252/182.1); Sulfur Or Compound Thereof (423/511)
International Classification: H01M 4/58 (20100101); C01B 17/00 (20060101);