IONIC RECEPTORS TO REGULATE THE POLYSULFIDE SHUTTLE IN LITHIUM-SULFUR BATTERIES
This disclosure describes a composition including polymers and high dipole moment anion and cation receptors for use in high energy density Li—S batteries and Li-ion sulfur batteries. Methods of making such batteries are also disclosed.
The present disclosure relates to power storage and battery devices. More particularly, the present disclosure relates to materials capable of utilizing lithium/sulfur chemistries for use in such power storage and battery devices.
Li-ion batteries (LIBs) are used as power sources in portable electronics and now are being considered to meet other applications, from electric vehicles, to space shuttles, to mitigating global warming issues. However, the capability of energy storage is still a concern due to limited room for developments in conventional electrode materials.
Beyond the limitations of Li-ion batteries towards efficient energy storage, lithium-sulfur (Li—S) systems are promising in the areas of high-energy density, safety, low cost, and long cycle life for electric-based transportation. This is mainly attributed to the high theoretical capacity, wide range of temperature operation, and the low cost and eco-friendliness of sulfur-based cathodes. However, practical applications of Li—S batteries are still hindered due to short cycle life, poor coulombic efficiency, poisoning of the Li-anode, and self-discharge. These performance limitations generally originate from the insulating nature of sulfur, the shuttling effect of dissolved lithium polysulfide (LiPS) species, and their parasitic reactions with the highly reactive negative electrode. Dissolved LiPS diffuses into the separator as electrostatic attraction proceeds between charged polysulfide species and metallic lithium. Further loss of electroactive species continues while cycling, owing to chemical potential difference, and a concentration gradient can lead to mass transport in the electrolyte.
Though work has been done on cathode frameworks by constructing mesoporous carbon-sulfur composites towards realizing specific capacity, cycle life in the long run still falls short in practical applications due to dissolution of LiPS from cathode. Poor adsorption (only or mostly van der Waals) of polysulfides and the sulfiphobic nature of carbon surfaces drives this phenomenon.
Progressively, polysulfide (Sn2−) species pass through the separator, causing undesired mass transport of electroactive species and spontaneous reduction of Sn2− at the anode surface, which eventually results in poisoning of the lithium electrode. In this regard, various materials and additives having polarity and capable of interacting strongly with dissolved polysulfides effectively have been studied, including polymers, metal oxides, and metal sulfides.
It has been a challenge to construct electrode structures with improved pore volume and polarity to enhance battery performance, and to increase coulombic efficiency, rate capability, and progressive cycling limits for practical applications.
BRIEF SUMMARY OF THE INVENTIONIn one aspect, the present disclosure provides a separator for a battery. The separator may include, or be coated with, or be decorated by inorganic MxNy, in which M may be selected from among boron, silicon, phosphorous, aluminum, carbon, and tin, among others, and N represents nitrogen. In some embodiments, MxNy may be boron nitride. The boron nitride may be provided as nanosheets. Similarly, organic moieties such as amides and their derivatives of formula R′—CONR; thioamides and their derivatives of formula R—CS—NR—NR′R″; and imidazolium and its derivatives of formula C3—X5—N2 may be employed as anion receptors, wherein in the preceding formulas, R, R′, R″ may include hydrogen and linear or branched alkyl groups of 1 to about 30 carbons, or any number in between, and the R/R′/R″ groups of a molecule may be the same as, or different from, one another. The battery may be a lithium-sulfur, or a lithium ion-sulfur battery.
In another aspect, the present disclosure provides a battery including ionic receptors to regulate polysulfide shuttling, inorganic MxNy compounds and organic amides as a part of cathode (electrode), as interlayer between cathode and separator, and ionic receptors contained layer on anode (protective layer). The battery may be a lithium-sulfur, or a lithium ion-sulfur battery.
As used herein with regard to a range, the term “between” is inclusive of the endpoints of said range, unless it is clear that the endpoints are excluded. For example, when “an integer between 1 and 3” is recited, the integer may have a value of 1, or of 2, or of 3.
As used herein, the terms “substantially” and “about” mean “approximately but not necessarily equal to,” and when used in the context of a numerical value or range set forth means a variation of ±20%, or less, of the numerical value. For example, a value differing by ±20%, ±15%, ±10%, or ±5%, or any value in the range between −20% and +20%, would satisfy the definition of “substantially” or “about.”
Conventional polymer based separators and electrodes are not ideal for preventing LiPS migration towards negative electrode which in turn shorten the battery performance and cycle life. Further, in a rechargeable Li-battery, lithium dendrite formation, mossy metal deposits and ipenetration through the separator (causing an internal short circuit) during the charge/discharge process may be risks.
Towards this direction, a modified separator or interlayer between the sulfur cathode and separator to localize dissolved polysulfides at cathode surface is described herein. Several materials have been utilized, from simple carbon paper, to modified graphene structures with tuning of their conductivity, porosity and functional groups. Other candidate materials include mesoporous carbon-sulfur, graphene-sulfur, and carbon nanotubes-sulfur composites. However, these structures use involved process in fabrication as free standing films, and lithium deposition in the separator pores pose challenges. Moreover, side reactions of highly active polysulfides and their radical anions with functional groups of conductive carbons and polymers may lead to performance degradation of the cell.
Therefore, separator or electrode modification with nonconductive materials (including certain polymers) that have strong dipoles (high dipole moment anion/cation receptors) to inhibit polysulfides to travel towards anode by adsorbing strongly may be more suited to such an application. In other aspects, a coating layer which may include polysulfide receptors on a metallic lithium or a silicon anode, may result in a way to regulate polysulfide shuttling, thereby improving electrochemical performance.
Herein, ionic receptor-based battery components including electrodes, separators, interlayers, and protective layers are disclosed. These components effectively bind ionic species involved in the system (including polysulfides) via Lewis acid-base interactions may be employed for long cycling and high energy density Li—S batteries.
A separator according to one embodiment of the present disclosure may include, or consist of, one or more chemical compounds having a formula of MxNy. M may be selected from among boron, silicon, phosphorous, aluminum, carbon, and tin, among others. N represents nitrogen. x may have a value of between 1 and 3. y may have a value of between 1 and 4.
The chemical compound of the separator may be an inorganic anion or cation receptor. It may be provided to the separator either in bulk, or as micron-sized particles, or as nanoparticles, or two-dimensional nanosheets, or nanorods, or in a structure having a porous form, or any combination thereof. The chemical compound may instead be an organic compound or organic moiety, such as an amide, (R′—CONK), a thioamide (R—CS—NR—NR′R″), an imidazolium (C3-R5-N2) anion receptor, wherein each of R, R′, and R″ includes linear or branched alkyl groups of 1 to about 30 carbons, or any number in between. The R groups are independently selected from one another, such that R, R′, and R″ may be the same or different alkyl structures for one another. The separator may also or instead include any of calixarene, crown ethers, and cryptand-based cation receptors.
Another separator configuration may include, for example, the above-mentioned anion or cation receptors in a part of an electrode, or an interlayer, or a protective layer on the anode. Such a configuration with, ionic receptor-based membranes and electrodes, may demonstrate a three-fold improvement in higher energy density, with stability over hundreds of cycles, and improved safety features in terms of avoiding internal short circuits.
As an example of an ionic receptor, boron nitride (BN) nanosheets may be used in a separator. Such nanosheets exhibit favorable properties, acting in the separator like an insulator, chemically and electrochemically stable. Such a material may be composed of mixed sp2 hybridized Lewis acid (boron atoms) and Lewis base (nitrogen atoms), leading to facile bonding with polysulfide species. Boron atoms in the BN have a high tendency to accept anion molecules (Sn2−) and the lone pair of electrons on nitrogen is capable of attracting the positive end of polysulfide species (for example, lithium). Together, this leads to effective complex formation under electric field to minimize the diffusion of soluble polysulfides across the separator membrane.
Presently, Li—S batteries are being researched, as these are limited for meeting present and future demands, in part due to polysulfides shuttling which may reduce cycle life. Herein, we propose a new approach of using ionic receptors with dipoles in the separator or interlayer to inhibit polysulfides from reaching metallic lithium and mass transport from the cathode. A comprehensive analysis of the ionic receptors on the physical, chemical, and electrochemical properties in Li—S chemistry has been performed. Two-compartment H-cells, coupled with spectroscopic studies such as UV absorption and FTIR are used to demonstrate the effectiveness of the ionic receptors' interactions with polysulfide species. Further, excellent specific capacity (1210 mAh/g) and good cycle life (320 cycles), including a self-discharge study, provide evidence for the feasibility of the modified separator with ionic receptors for commercial applications.
Example 1: Fabrication of BN Nanosheets-Modified SeparatorCommercial boron nitride (Sigma Aldrich) having 2-micron particle size was subjected to liquid phase exfoliation by an ultra-sonication method in aqueous medium. Here, sodium cholate hydrate was used as a surfactant material in a ratio of 1 part boron nitride to 5 parts sodium cholate, and sonicated for about 24 hours (hr) with 10 min. rest time after every 2 hr of sonication in order to avoid an excessive increase in temperature of the solution. Then, the homogenous dispersion obtained was centrifuged at 2000 rpm for about 90 minutes (repeated 2 times) in a separation step to segregate the exfoliated BN nanosheets from the bulk samples. Finally, supernatant liquid was drawn off from the residue in the centrifuged tube in a decanting step and used for further experiments. Afterwards, exfoliated BN nanosheets contained in solution may be filtered through a conventional polymer separator (such as celgard) and dried for 12 hr at about 120° C. under vacuum. In one embodiment, the separator may be a trilayer polypropylene-polyethylene-polypropylene membrane, but other membranes as are known in the art may be substituted in this protocol. The uniformly deposited BN nanosheets may be calculated per area of polymer film by adjusting the concentration of BN nanosheets in solution. Such modified films with different amounts of BN nanosheets were used as separators in the example of the present disclosure in comparison with bare polymer film.
Example 2: Preparation of Lithium Polysulfides (Li2S4 and Li2S8)In the present study, an active material of sulfur is used in the form of catholyte solutions with general lithium polysulfides formula of Li2S8 and Li2S4 in tetra ethylene glycol dimethyl ether (TEGDME). This solution may be prepared, for example, from stoichiometric amounts of Li2S and S to form desired LiPS in TEGDME while stirring at about 90° C. for about 12 hr. The catholyte solution (Li2S8) used for the electrochemical studies desired herein is 10 μl of 600 mM per cell which corresponds to 1.61 mg of sulfur per cm2 of electrode.
Example 3: Cell FabricationStandard 2032 coin cells were used to examine the electrochemical performance of ionic receptor modified separator in conventional Li-polysulfides battery configuration. Gas diffusion layer (GDL) paper has been used as positive electrode cut into circular discs of 12.7 mm diameter. An active material containing catholyte (Li2S8) with the molar concentration of 600 mM was used along with an electrolyte including, or consisting of, 1 M of lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) and 0.5 M of lithium nitrate (LiNO3) in TEGDME.
Pertaining to the polymer (separator) membrane, it may include or consist of polyethylene oxide (PEO) and polypropylene. The polymer may also be selected from at least one of polyethylene, polypropylene, polyvinyledene fluoride, and polyacrylic acid. The preceding polymers may be combined with a compound of formula MxNy as described herein. The battery may include an organic, liquid-based electrolyte. The Li—S battery may have a carbon-sulfur or carbon-liquid polysulfide as cathode, and metallic lithium as anode. These constructions provide improved results over electrode materials with limited specific capacities, such as graphite and LiMO2, and will assist in increasing the current limitations of presently available lithium ion batteries (typically 150-200 Wh/kg) due to its high theoretical energy density (of about 2567 Wh/kg). The energy density of a battery in accordance with this invention may be at least 250 Wh/kg, or at least about 300 Wh/kg, or at least about 400 Wh/kg, or at least about 500 Wh/kg, or at least about 750 Wh/kg, or at least about 1000 Wh/kg, or at least about 1500 Wh/kg, or at least about 2000 Wh/kg.
Results and Discussion
Boron nitride nanosheets have been obtained by liquid phase exfoliation using sonication to produce large-scale nanosheets of 2D materials. In a controlled experiment, energies from sound waves, such as ultrasound energies of about 50-60 hertz, are effective to overcome the weak forces of attraction between the interlayer of hexagonal crystalline boron nitride (h-BN) to produce nanosheets. Surfactant has been used to disintegrate and stabilize exfoliated nanosheets in liquid medium, and the ratio of BN and surfactant are selected as it may define the quality of nanosheets. Exfoliated BN nanosheets may be collected through repeated centrifugation, which separate out the nanosheets of h-BN from the un-exfoliated counterpart. Such nanosheets-containing solution may be washed thoroughly to remove surfactant and vacuum dried overnight, and subsequently subjected to microscopy studies as shown in
Further, a modified separator may be prepared using the celgard membrane decorated or coated with BN nanosheets via a filtration process wherein celgard acts as filter membrane for the solution BN-containing nanosheets. Prior to use as separator for the construction of Li—S cell, a modified separator may be dried under vacuum at 100° C. and tested for its robustness as displayed in
Electrochemical compatibility of a BN nanosheets-modified separator, and its unmodified counterpart, has been evaluated by assembling standard 2032 coin cells. The cells include a gas diffusion layer on carbon paper as working electrodes vs. metallic lithium foil as a counter/reference electrode and celgard membrane or modified celgard with BN nanosheets as a separator. The active species may be in the form of LiPS (0.6M of Li2S8) in TEGDME solution, containing 1M of lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) and 0.1M lithium nitrate (LiNO3). Electrochemical properties with and without BN nanosheets were measured using 10 μl of Li2S8 (1.52 mg/cm2) at a constant current rate of 0.2 C in a potential window between 3.0-1.5 V. The quantity of BN per cm2 of polymer membrane yields effective adsorption of LiPS by conducting systematic electrochemical experiments in Li-polysulfides cell configuration.
Further, the cycling behavior of Li-polysulfide cells with various concentrations of BN nanosheets on a polymer membrane as a separator has been displayed in
The adsorption of LiPS on BN nanosheets may be evaluated using visual inspection and ultra-violet absorption studies. A transparent two-compartment H-cell has been used to visualize the effect of a BN-modified separator to stop polysulfides from reaching the anode compartment as shown in
Stability in polarization, even after several cycles, confirms the suitability of BN nanosheets-modified separators for use in longer cycling for Li-polysulfide batteries. Furthermore, owing to their large surface areas and polarity, exfoliated BN nanosheets are effective to control polysulfide shuttling to enhance cycle life as demonstrated in
Self-discharge deteriorates a battery with age and cycling. Li—S chemistry is likely to increase self-discharge, as negatively charged intermediate species tend to escape from the cathode due to electrostatic attraction forces from metallic lithium. To understand the effect of the modified separator, especially with cycling, cells have been subjected to self-discharge studies in between cycles at the 2nd, 20th and 40th cycles, as shown in
The present disclosure provides an improvement in electrochemical performance using a BN nanosheets-modified separator. The high surface area and polar nature allows for strong adsorption of polysulfides without crossover to reach metallic lithium. Visual observation with a two-compartment H-cell and spectroscopic studies reveal the interactions between BN nanosheets and polysulfide species under electrochemical cycling. From the detailed electrochemical studies, high specific capacity of 1210 mAh g−1 at 0.1 C rate with excellent capacity retention over the illustrated number of cycles has been realized (see
Numerous possible uses for such a battery separator, or battery, exist, including in portable electronics, battery-operated mobile tools, and in hybrid electric vehicles. The separator and battery technology has a high theoretical energy density, safety, wide temperature range in which it may operate, low cost, and good environmental friendliness. The limitations of conventional Li—S batteries (short cycle life, lower coulombic efficiency, self-discharge, and poisoning of the Li-anode) are overcome by the aspects of the present disclosure because of avoiding of the insulating nature of sulfur and minimizing the shuttling effect of dissolved lithium polysulfide species and their reactions with the reactive negative electrode, which in turn shortens battery performance and cycle life.
Claims
1. A battery comprising:
- at least one component selected from the group consisting of a separator, an interlayer, a protective layer, and an electrode, the at least one component comprising a material of formula MxNy, wherein M is selected from the group consisting of B, Si, P, Al, C, and Sn, N represents nitrogen, and wherein x=1 to 3, and y=1 to 4.
2. The battery of claim 1, wherein the material of formula MxNy is the form of one of bulk, micron-sized, nanoparticles, 2D sheets, nanotubes, nanorods, and a porous structure.
3. The battery of claim 1, wherein the material of formula MxNy is combined with at least one polymer selected from the group consisting of polyethylene, polypropylene, polyvinyledene fluoride, and polyacrylic acid.
4. The battery of claim 1, wherein the material of formula MxNy is part of one of a mesoporous carbon-sulfur composite, a graphene-sulfur composite, and a carbon nanotubes-sulfur composite.
5. The battery of claim 1, wherein the material of formula MxNy is incorporated into at least one of a separator and an electrode.
6. The battery of claim 1, wherein the battery is one of a lithium-sulfur, a lithium-polysulfide, a Li-ion polysulfide, and a lithium ion-sulfur battery.
7. The battery of claim 1, wherein M is boron, and x=1, and y=1.
8. The battery of claim 7, wherein the boron nitride is provided as nanosheets.
9. The battery of claim 8, wherein the nanosheets have a thickness between about 100 nanometers and about 30 microns.
10. The battery of claim 7, wherein the separator is decorated with the nanosheets.
11. The battery of claim 1, having a capacity of between about 800 mAh/g and about 1200 mAh/g.
12. The battery of claim 1, wherein at least one of the separator, the electrode, the interlayer, and the protective layer comprises at least one organic ionic receptor selected from an amide (—CONR), a thioamide (R—CS—NR—NR′R″), and an imidazolium anion receptor, wherein R, R′, and R″ are each independently linear or branched alkyl groups of 1 to 30 carbons.
13. A battery comprising:
- a separator decorated with boron nitride, the battery being one of a lithium-sulfur, a lithium-polysulfide, a Li-ion polysulfide, and a lithium ion-sulfur battery and having a capacity of between about 800 mAh/g and about 1200 mAh/g
14. The battery of claim 13, wherein the boron nitride is present as a plurality of nanosheets.
15. A battery component selected from a separator, an electrode, an interlayer, and a protective layer, the battery component comprising a material of formula MxNy, wherein M is selected from the group consisting of B, Si, P, Al, C, and Sn, N represents nitrogen, and wherein x=1 to 3, and y=1 to 4.
16. The battery component of claim 15, wherein the at least one organic ionic receptor is part of a polymer selected from at least one of polyethylene, polypropylene, polyvinyledene fluoride, and polyacrylic acid.
17. The battery component of claim 15, wherein the at least one organic receptor is functionalized on at least one of mesoporous carbon, graphene, carbon nanotubes for sulfur composites, and wherein the battery component comprises an electrode.
18. The battery component of claim 15, wherein the battery is one of a lithium-sulfur, lithium-polysulfide, Li-ion polysulfide and a lithium ion-sulfur battery.
19. The battery component of claim 15, wherein M is boron, and x=1, and y=1.
20. The battery component of claim 19, wherein the boron nitride is provided as nanosheets.
Type: Application
Filed: Jan 25, 2018
Publication Date: Nov 7, 2019
Inventors: Leela Mohana Reddy Arava (Farmington Hills, MI), Ganguli Babu (Detroit, MI), Abdulrazzag Sawas (Dearborn, MI), Nareshkumar Thangavel (Detroit, MI)
Application Number: 15/879,822