LITHIUM SULFUR CELL WITH DOPANT

Abstract

Among other things, the present disclosure provides a particle comprising a form of sulfur and/or lithium sulfide (Li.sub.2S) that is doped with a group VIA element, such as selenium (e.g. Se34), tellurium (e.g. Te52), or polonium (e.g. Po84). The present disclosure also provides a cell comprising a negative electrode, a separator, and a positive electrode comprising the particles of the present disclosure.

Claims

1. A particle comprising Li.sub.2S.sub.1−x−ySe.sub.xM.sub.y, wherein M is a Group VIA element, and x<0.1; and y<0.05.

2. The particle of claim 1, further comprising a coating around the particle.

3. The particle of claim 1, wherein M is Te or Po.

4. The particle of claim 1, wherein the coating is a mixed electronic and Li.sup.+ conductor.

5. The particle of claim 4, wherein the coating is carbon or Ti.sub.2S.

6. The particle of claim 1, wherein the coating is an insulator.

7. The particle of claim 6, wherein the coating is Al.sub.2O.sub.3.

8. A lithium-sulfur cell comprising: (a) a negative electrode; (b) a separator; and (c) a positive electrode comprising the particle of claim 1.

9. The lithium-sulfur cell of claim 8, wherein the particle is present in an amount of at least about 40 volume % of the positive electrode.

10. The lithium-sulfur cell of claim 8, wherein the particle is present in an amount that provides at least about 2 mAh of reversible capacity per cm.sup.2 of composite electrode.

11. The lithium-sulfur cell of claim 8, wherein the positive electrode is more than about 30 microns thick.

Description

DETAILED DESCRIPTION

[0013] Before any examples of the disclosure are explained in detail, it is to be understood that the disclosure is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The disclosure is capable of other examples and of being practiced or of being carried out in various ways.

[0014] In one aspect, the present disclosure provides a lithium-sulfur cell with improved cycle life and energy density. The cathodes according to the present disclosure may provide an increased cycle life for Li/S cells; improved electrical conductivity of Li.sub.2S by introducing dopants; the ability to use of larger particles, which can result in the use of less “dead mass,” thereby increasing the cell-level volumetric and gravimetric energy density. The improved conductivity may yield effectively higher diffusion coefficients for the material, thereby enabling higher volumetric and gravimetric power density. In one aspect, the present disclosure allows one to increase the sulfur loading and utilization of the cell; to decrease the charging time; and/or to increase the power delivered from the battery.

[0015] In an aspect, the cell comprises a negative electrode, a separator, and a positive electrode. In an example, the positive electrode is connected to an electronically-conductive current collector (e.g., Al metal). In an example, the negative electrode may further comprise have a current collector, such as copper metal. Alternatively, Li metal may be used to conduct electrons to and from the electrode. In some examples, the lithium-sulfur cells of the present disclosure have improved properties, such as conductivity, cycle life, and/or energy density.

[0016] In an example, the cell described above may be double sided (i.e., symmetric about the Al current collector as follows: negative electrode/separator/positive electrode/positive current collector/positive electrode/separator/negative electrode/(negative current collector) and stacked (as a “stack”) or wound (as a “jellyroll”) so as to increase the capacity per unit volume, or it may be stacked in a bipolar design (negative electrode/separator/positive electrode/positive current collector/bipolar plate/negative electrode) so as to increase the cell voltage. This electrochemically active volume may be enclosed in a cell housing, the terminals of which are wired to the negative and positive poles of the stack or jellyroll.

[0017] The negative electrode suitably comprises a Li-insertion material, such as Li metal, that can reversibly insert and extract Li ions electrochemically. In examples, the negative electrode is a copper current collector with a protective solid electrolyte coating (e.g. an ionically conductive ceramic, such as lithium phosphorous oxynitride (LiPON); lithium lanthanum zirconate (LLZO); Li.sub.3N; Li.sub.3P, lithium lanthanum titanate (LLTO); a sulfidic lithium conductor such as Li.sub.3PS.sub.4+n (n=0 to 9), Li.sub.10GeP.sub.2S.sub.12, Li.sub.4P.sub.2S.sub.7, Li.sub.2S—P.sub.2S.sub.5—LiI [e.g., Li.sub.3PS.sub.4—LiI, especially Li.sub.7P.sub.2S.sub.8I], Li.sub.3+xGe.sub.xAs.sub.1−xS.sub.4where x=0 to 0.50; BN; Li.sub.2CO.sub.3, etc., and variants thereof). In examples, there may be some Li metal between the current collector and the protective coating, or the Li may be deposited there in situ during the initial charging of the battery cell. In examples, the negative electrode may be a bare copper (or other, e.g., Ni) current collector, or it may be a Li foil.

[0018] The separator is electronically insulating. In an example, the separator comprises a Li-conducting solid electrolyte and/or a porous material with Li-conducting liquid electrolyte in the pores. The separator may include lithium phosphorous oxynitride (LiPON), Li-conducting garnet, Li-conducting sulfide (e.g., Li.sub.2S—P.sub.2S.sub.5), Li-conducting polymer (e.g., polyethylene oxide), Li-conducting metal organic frameworks, thioLiSiCONs, Li-conducting NaSICONs, Li.sub.10GeP.sub.2S.sub.12, lithium polysulfidophosphates, lithium aluminum titanium silicon phosphate (LATSP); or other solid Li-conducting material. In examples, the separator may be a porous polyolefin layer. At least some of the pores may be filed with the electrolyte used in the positive electrode. In some examples, such as where the protective layer is a solid electrolyte, no additional separator is necessary.

[0019] In an example, the positive electrode comprises a form of sulfur and/or lithium sulfide (Li.sub.2S) that is doped with a group VIA element, such as selenium (e.g. Se34), tellurium (e.g. Te52), or polonium (e.g. Po84). In some examples, the positive electrode further comprises one or more of (1) additional Li-insertion materials, (2) an electronically conducting material (e.g., carbon fragments, graphite, and/or carbon black), and (3) a Li-conducting phase (e.g., liquid electrolyte and/or solid electrolyte), and optionally polymeric binder (e.g., PVDF). In an example, the doped Li.sub.2S particle is coated, suitably with a mixed conducting material such as carbon or TiS.sub.2 that is minimally or not electrochemically active in the voltage window over which the cathode material is cycled (e.g., 1.5 to 2.5 V vs. Li). Because Li.sub.2S and Li.sub.2S.sub.2 are electronically insulating, the practical cyclable domain size is very small (several to tens of nm). However, doping these materials with elements such as Se can dramatically increase the electronic conductivity, thereby increasing both the rate capability and the utilization of the sulfide material.

[0020] Suitably, the doped Li.sub.2S particle has the formula Li.sub.2S.sub.1−x−ySe.sub.xM.sub.y particle, wherein M is a second Group VIA metal, such as Te or Po, and wherein x<0.1; y<0.05. The Li.sub.2S.sub.1−x−ySe.sub.xM.sub.y may be made from commercially available Li.sub.2S that is milled together with small amounts of a group VIA element. In examples, the Li.sub.2S is milled into small grains about 100 nm to about 10 μm in diameter. In examples, the Li.sub.2S may be obtained from synthesized nano-Li.sub.2S (less than about 1 μm in diameter). In an example, the Group VIA element may be a lithiated Group VIA element. Suitably, the Group VIA element is about 100 nm to about 10 μm in diameter. Alternatively, the Group VIA element may be a nanoparticle. The Li.sub.2S.sub.1−x−ySe.sub.xM.sub.y particles may be any shape, but are suitably spherical.

[0021] In an example, the Group VIA element is present in an amount of from about 0 to about 7 atomic weight %. In an example, a second Group VIA element may be present in an amount of up to about 1 atomic weight %. Suitably, the Group VIA element is selenium. Suitably, the second Group VIA element is not selenium.

[0022] In an example, the Li.sub.2S.sub.1−x−ySe.sub.xM.sub.y is coated with a material that prevents it from reacting with the electrolyte or, in the case of a liquid electrolyte, dissolving into the electrolyte, even after Li is extracted electrochemically from the Li.sub.2S.sub.1−x−ySe.sub.xM.sub.y. Suitably, the coating material is preferably a mixed electronic and Li.sup.+ conductor, such as carbon or Ti.sub.2S, but it may also be an insulator such as Al.sub.2O.sub.3. The coating should be very thin such that it comprises no more than about 20% of the mass of the coated Li.sub.2S.

[0023] In various examples, the coating may be deposited via a chemical vapor deposition (CVD), atomic layer deposition (ALD), physical vapor deposition (PVD) or other coating process. When carbon is the coating, it can be deposited on the surface of the Li.sub.2S particles by CVD using gaseous C.sub.2H.sub.2 as a precursor. The CVD may be carried out at approximately 400° C. under slowly-flowing argon. The coating procedure may be carried out several times in order to ensure complete coverage of the particles.

[0024] Nano-Li.sub.2S can be synthesized using a solution-based reaction of elemental sulfur with 1M Li(CH.sub.2CH.sub.3).sub.3BH solution in THF. Other suitable techniques of synthesizing nano-Li.sub.2S are known to one of ordinary skill in the art.

[0025] The dopants may be introduced into commercial Li.sub.2S or synthesized nano-Li.sub.2S by any number of doping techniques employed in the semiconducting industry, including diffusion and ion implantation, and should not be limited to the methods described herein.

[0026] Sometimes the above methods for synthesizing coated nanopowders result in a mixture of completely coated (i.e., pinhole-free) Li.sub.2S.sub.1−x−ySe.sub.xM.sub.y and some uncoated and/or partially coated Li.sub.2S.sub.1−x−ySe.sub.xM.sub.y. The presence of the latter may limit the capacity retention of the Li/S cell over its cycle life, because the imperfectly coated Li.sub.2S.sub.1−x−ySe.sub.xM.sub.y could react with the electrolyte or dissolve into the electrolyte. Subsequently, this dissolved sulfide may form a dissolved lithium polysulfide (Li.sub.2S.sub.x, 1<x<=8) that reacts with the negative electrode, resulting in loss of active sulfur and therefore a reduction in the capacity of the cell. Thus, in some examples, the uncoated and/or partially coated Li.sub.2S.sub.1−x−ySe.sub.xM.sub.y and the completely coated Li.sub.2S.sub.1−x−ySe.sub.xM.sub.y may be separated, and use only the completely coated used in fabricating the positive electrode of the cell.

[0027] In one aspect, the presence of a coating enables the doping of Li.sub.2S ex situ, prior to cell fabrication. Without wishing to be bound by theory, the coating suitably maintains the structure of the doped material thereby avoiding the need to re-dope the material every time the cell is cycled. Moreover, the coating prevents reaction of Li.sub.2S, S, and lithium polysulfides (Li.sub.2S.sub.x, 1<x<=8) with the electrolyte. The coating also may prevent dissolution of Li.sub.2S, S, and lithium polysulfides (Li.sub.2S.sub.x, 1<x<=8) into the electrolyte.

[0028] In examples, the positive electrode is a mixture of the active material described herein, and one or more of: a binder (e.g. PVDF) and additional carbon additives to improve conductivity of the matrix, and with pores that are filled with electrolyte (e.g., LiPF.sub.6 in blend of carbonates or DOL/DME blend with LiPF.sub.6 or LiTFSI salt). In examples, the composite cathode may be in contact with a current collector such as an Al foil.

[0029] Suitably, the positive electrode is at least about 30 microns in thickness.

[0030] In examples, the active material (e.g., coated Li.sub.2S.sub.1−x−ySe.sub.xM.sub.y) is present in at least about 40 volume % of the cathode, suitably at least about 50 volume %. The loading of the sulfide material should be sufficient to achieve at least about 1 mAh/cm.sup.2, suitably at least about 3 mAh/cm.sup.2. Suitably, the active material is present in an amount sufficient to provide at least about 2 mAh of reversible capacity per cm.sup.2 of composite electrode.

[0031] Thus, the cathode material described herein suitably allows for doping of Li.sub.2S prior to its introduction into the battery cell. Without wishing to be bound by theory, the coating may prevent polysulfide dissolution, thereby maintaining the structure of the doped material as it is delithiated during cell charging and lithiated during cell discharging.

[0032] In an example, compared to active electrode materials without dopants, the ones described herein may have enhanced electronic conductivity, and therefore high rate capability may be enabled with larger particles of the material. Larger particles may achieve higher packing densities and require less inactive material additive (including less coating on a wt % basis); hence, it is desirable to use large particles in order to achieve higher energy densities.

[0033] The positive electrode is suitably made using a standard method for fabricating battery electrodes, that is, mixing the solid particles with conductive additives, binder, and optionally solid electrolyte powders (e.g., lithium-conducting garnet or lithium-conducting sulfide glasses or ceramics) in a carrier solvent (e.g., NMP). The mixture forms a slurry that can be coated onto the positive current collector. The electrode is then heated so that the solvent evaporates. The electrode may then be densified using a roll press. Optionally, some pores are left in the electrode such that they can be filled with liquid electrolyte during cell fabrication.

[0034] In one example, the dissolved lithium polysulfide may be electrochemically oxidized to form sulfur, which in turn can be recycled and used again in the Li.sub.2S purification process. A byproduct of this second process could be the plating of Li metal.

[0035] In one aspect, the coated and doped lithium sulfide material according to the present disclosure may also be incorporated into a solid-state cathode using a ceramic and or polymer electrolyte and optionally some electronically conductive additive and/or binder material.

[0036] Additionally, graphene oxide can be mixed with the doped Li.sub.2S material prior to coating in order to further enhance electronic conductivity and connectivity of active material grains.

[0037] The present disclosure also provides a solid state battery cell wherein the electrolyte, in the cathode and separator and optionally the protection layers, is a solid electrolyte, such as a polymer electrolyte or a ceramic electrolyte, e.g., LLZO, LATP, LATSP, lithium sulfide, or any combination of ceramic or polymer electrolyte materials.

[0038] The cathode and lithium-sulfur cell of the present disclosure may be used in any way other cathodes and lithium-sulfur cells are used.

[0039] Various features and advantages of the disclosure are set forth in the following claims.