Additive for lithium sulfur battery

Abstract

The disclosed technology relates to a rechargeable electrochemical cell comprising electroactive sulfur-containing material and ion conductive salt, as well as thiophosphate redox mediator.

Description

TITLEADDITIVE FOR LITHIUM SULFUR BATTERY BACKGROUND OF THE INVENTION

[0001] The disclosed technology relates to a rechargeable electrochemical cell comprising electroactive sulfur-containing material and ion conductive salt, as well as thiophosphate redox mediator.

[0002] Lithium-sulfur batteries are a grouping of battery technologies that have and will surpass some of the theoretical limits of lithium-ion battery performance attributes. It is anticipated that on full scale commercialization optimized lithiumsulfur batteries will be safer, cheaper and have a lower environmental impact than lithium-ion batteries. In particular, the elemental sulfur used in the cathode construction is in abundance and generated as a waste product of the oil and gas industry. Unlike lithium-ion batteries or next generation metal air batteries, lithium-sulfur batteries do not require compounds that are comprised of, rare or low abundance elements, such as Ni, Mn, Co, Pt, Au and / or Ag, etc (in most iterations). One advantage lithium-sulfur batteries have over lithium-ion batteries is their potentially higher gravimetric energy density (Wh kg-1), this means they will be useful in electrified applications where lowering weight is a priority. Particularly in the aerospace industry including electric planes, satellites, UAVs (drones) and other military applications but also other applications such as electrified heavy-duty trucks and marine applications. To reach the gravimetric energy densities required for lithium-sulfur batteries to be useful in more widespread applications, such as electrified trucks, improvements in the utilization of the sulfur contained in the cell and more specifically the sulfur contained in the cathode material, are required. There are a number of companies and academic consortiums actively developing lithium-sulfur batteries as next generation battery devices for these reasons.

[0003] In the fundamental lithium-sulfur secondary cell, the discharge involves a complex multistep redox cascade that occurs from elemental sulfur (S8) to lithium sulfide (Li2S), according to the following proposed reactions:

[0004] Both sulfur and lithium sulfide are electrically insulating and mostly insoluble in the electrolyte, however the lithium polysulfide intermediates in the redox cascade are highly soluble (in useful liquid electrolytes) and can migrate away from the cathodic region even reaching the anodic region of the cell, causing issues with battery cycling due to the competing chemical reduction that can take place when the sulfur containing intermediates migrate to the anode. This can both reduce the sulfur available for subsequent electrochemical cycling but also lead to infinite recharge cycles where the polysulfide species ‘shuttle’ between the anode and cathode undergoing chemical redox cycles rather than electrochemical redox cycles. Additionally, this shuttling phenomena can corrode the anode and shorten the cell life. Additives, that can mediate the electrochemical redox transformations quickly can restrain this phenomenon and enable the cell to retain a higher capacity for longer.

[0005] Additionally, only a proportion of the insulating elemental sulfur present as a dispersed fine powder in the cathode composite material is in electrical contact with the electrically conductive carbon matrix of the cathode. To increase the utilization of sulfur in the battery cell (during cell discharge), an additive which is mobile and can access non-electrically contacted regions of both S8 (during cell discharge) and deposited Li2S (during cell charge), as well as redox intermediates, can bridge this gap through additional redox processes and catalysis. Such an additive would enable the deposited battery sulfur, charge and discharge products, to be more fully utilized.

[0006] Thus, there is a need for an additive for batteries containing an electroactive sulfur and an ion conductive salt, such as lithium sulfur batteries, to mediate the electrochemical redox transformations and more fully utilize the charge and discharge products thereof.SUMMARY OF THE INVENTION

[0007] The disclosed technology therefore provides a thiophosphate redox mediator additive for batteries containing an electroactive sulfur and an ion conductive salt to mediate the electrochemical redox transformations and more fully utilize the charge and discharge products thereof.

[0008] In particular, the technology includes an electrolyte composition containing a medium (such as a polymer medium or organic solvent), metal salt, and thiophosphate redox mediator. Metal salts can include, for example, alkali metal, alkaline earth metal, transition or post transition metal.

[0009] Also provided is a cathode containing an electroactive sulfur-containing material, and thiophosphate redox mediator.

[0010] More completely, the electrolyte and the cathode can be used together in an electrochemical cell, which would then contain cathode, the cathode itself containing electroactive sulfur-containing material, and thiophosphate redox mediator, anode, and electrolyte, the electrolyte containing polymer medium or solvent, ion conductive salt, and thiophosphate redox mediator, so long as the concentration of the thiophosphate redox mediator in at least one of the cathode and electrolyte is not 0.

[0011] The technology also includes a method of mediating electrochemical redox transformations in an ion conductive salt-electroactive sulfur-containing electrochemical cell, comprising preparing an electrochemical cell comprising a) cathode comprising electroactive sulfur-containing material, b) anode, and c) electrolyte, and preparing said electrolyte comprising i) polymer medium or solvent, ii) ion conductive salt, and iii) thiophosphate redox mediator, and operating said electrochemical cell.

[0012] The technology also provides the use of thiophosphate redox mediator to mediate electrochemical redox transformations in an ion conductive salt-electro- active sulfur-containing electrochemical cell.DETAILED DESCRIPTION OF THE INVENTION

[0013] Various preferred features and embodiments will be described below by way of non-limiting illustration.

[0014] One aspect of the technology provided is a method of mediating electrochemical redox transformations in an ion conductive salt-electroactive sulfur- containing rechargeable electrochemical cell with thiophosphate redox mediator. The thiophosphate redox mediator can be put into an electrolyte or a cathode that is employed in an ion conductive salt-electroactive sulfur-containing rechargeable electrochemical cell. The electrolyte within the electrochemical cell can be either liquid, gel, or solid state..

[0015] In an embodiment, the technology includes an electrolyte containing medium, metal salt, and thiophosphate redox mediator.

[0016] The electrolyte can be in the form of a liquid, quasi-solid or of a solid at room temperature.

[0017] In an embodiment, the electrolyte can be in the form of a liquid at room temperature. In some liquid electrolyte the medium can include aprotic organic solvent (or simply “solvent” for short). Suitable solvents include any of the basic(cation-complexing) aprotic polar solvents known or used for lithium-sulfur batteries generally, such as polyethers, ethers, such as dimethoxyethane, sulfolane, dimethyl sulfoxide, dimethyl acetamide, tetramethyl urea, N-methyl pyrroli- dinone, tetraethyl sulfamide; ethers such as tetrahydrofuran, methyl-THF, 1,3- dioxolane, diglyme, and tetraglyme, and mixtures thereof; carbonates such as ethylene carbonate, propylene carbonate, dimethylcarbonate, diethylcarbonate, ethylmethylcarbonate, methylpropylcarbonate, ethylpropylcarbonate and the like; as well as esters such as methylacetate, ethyl acetate, propylacetate, and gammabutyrolactone. Fluorinated solvents, deep eutectics and solvate (salt in solvent) may also be employed. The electrolyte may comprise a single such solvent or a mixture of such solvents. Any of the polar aprotic polymers known in the battery art could also be employed as part of the medium as a single polymer or as part of a polymer mixture or along with solvent. The polymeric material may take the form of a gel. Suitable polymers for use in the electrolyte may include, for example, polyethylene oxide, polyethersulfone, polyvinylalcohol, and polyimide . Such liquid mediums are known in the art.

[0018] The liquid electrolyte can also be an ionic liquid or sulfone electrolyte. Ionic liquids, which are known in the art, are liquids at room temperature and composed entirely of ions. Sulfone electrolytes likewise are known in the art and are, in particular, acyclic aliphatic sulfones that exhibit high chemical and thermal stability, such as, for example, ethyl methyl sulfone.

[0019] The liquid electrolyte medium may be divided between compartments in an electrochemical cell. One compartment may comprise an electrolyte in contact with a cathode (the electrolyte in such compartment may be referred to as a catholyte). Another compartment may comprise an electrolyte in contact with an anode (the electrolyte in such compartment may be referred to as an anolyte). The anolyte and the catholyte may be the same as, or different from, each other and only one or both may contain the thiophosphate redox mediator.

[0020] The electrolyte can also be in the form of a solid, wherein the medium includes a solid polymer medium. Such solid polymer mediums are known in the art. Examples of solid state polymer mediums can include sulfide based solid polymer mediums, which can include, for example, lithium, phosphorus, and halogen components as well as metals, metalloids and lanthanides.

[0021] The electrolyte can also be in the form of a quasi-solid, containing some amount of liquid electrolyte in a solid electrolyte.

[0022] The electrolyte composition, in addition to having a solid and / or liquid medium will include one or more salts which impart ions and / or ionic conductivity to the electrolyte medium. This can include metal salts. The metal of the metal salt can be alkali metal, alkaline earth metal, transition metal or post transition metal. Examples of metals include lithium, sodium, magnesium, and aluminum and alloys or composites thereof. Conductive salts are well known in the battery art and include, for example, lithium salts of (CF3SO2)2N-, CF3SO3 -, CH3SO3-, C1O4-, PF6-, AsF6-, halogen or the like.

[0023] Often, conductive salts are lithium salts, such as LiPF6, LiBF4, LiCI04, LiAsF6, LiCF3SO3, LiC(CnF2n+l S02)3, lithium imides such as LiN(CnF2n+lS02)2, where n can be an integer in the range from 1 to 20, LiN(SO2F)2, Li2SiF6, LiSbF6, LiAlC14, and salts of the general formula (CnF2n+lSO2)mXLi, where m can be defined as follows: m = 1 when X can be selected from oxygen and sulfur, m = 2 when X can be selected from nitrogen and phosphorus, and m = 3 when X can be selected from carbon and silicon. Preferred conductive salts are selected from LiC(CF3SO2)3, LiN(CF3SO2)2, LiPF6, LiBF4, LiClO4, particular preference being given to LiPF6 and LiN(CF3SO2)2.

[0024] In one embodiment the ion conductive salt can include an alkali metal salt, such as a lithium salt. Sodium and other alkali metal salts and mixtures thereof may also be used.

[0025] The electrolyte composition will further include a thiophosphate redox mediator. The electrolyte can include one thiophosphate redox mediator or a mixture of multiple different thiophosphate redox mediators. Thiophosphates useful as the thiophosphate redox mediator are known in the art and the literature and can include, for example, amide-containing dithiophosphorus acid esters, dithiophosphate bis amide esters, dithiophosphoric acid ester, and mixtures thereof.

[0026] The thiophosphate redox mediator may be an ionic salt or a covalent (nonionic) compound. Typical thiophosphate compounds, which may also be useful as the thiophosphate redox mediator, include dihydrocarbyl-substituted thiophosphates and include mono- and di- thiophosphoric acid esters, amine or ammonium salts of the acid, and any combination thereof. Esters of thiophosphoric acids include hydrocarbyl esters containing hydrocarbyl groups of 4 to 20 carbon atoms, or 6 to 12 carbon atoms, or 6 to 8 carbon atoms. The hydrocarbyl groups may be linear or branched, aliphatic or aromatic groups.

[0027] In one embodiment, the thiophosphate redox mediator can be a dithiophosphate ester of a branched alkyl group containing 6 to 12 carbon atoms. In oneembodiment, the ester group of the thiophosphoric acid ester may contain an acylated hydrocarbyl group. Acylated hydrocarbyl groups include ester and amide linkages and contain 4 to 20 carbon atoms, or 4 to 12 carbon atoms. The thiophosphate redox mediator may be represented by the formula (I):R iQj ..........(I) wherein X is oxygen or sulfur; each R1 is independently a linear or branched aliphatic hydrocarbyl group containing 3 to 18 carbon atoms, 3 to 10 carbon atoms, or 6 to 8 carbon atoms; R2 is a branched or linear hydrocarbyl group containing 4 to 20 carbon atoms, or an acylated hydrocarbyl group (-R3-(C=O)-Y- R4) containing 4 to 20 carbon atoms; and R3 is a hydrocarbyl group containing 1 to 6 carbon atoms, Y is O or N-R5, R4 and R5 are each independently hydrocarbyl groups of 1 to 12 carbon atoms. Suitable alkyl groups (Rland R2) include ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, t-butyl, amyl, methylamyl, n-pentyl, n- hexyl, n-octyl, iso-octyl, 2-ethylhexyl, n-decyl, isodecyl, n-dodecyl, 2,4,4-trime- thylpentyl, 2-octyl, 2-decyl, and 2-dodecyl.

[0028] In one embodiment, the thiophosphate ester may be a thiophosphate ester of an acylated hydrocarbyl group of the formula (II):wherein X is oxygen or sulfur; each R1is independently a linear or branched aliphatic hydrocarbyl group containing 3 to 18 carbon atoms, 3 to 10 carbon atoms, or 6 to 8 carbon atoms; Y is O or NR5; R4and R5are each independently H or hydrocarbyl groups of 1 to 12 carbon atoms; and R6is hydrogen or a hydrocarbyl group of 1 to 4 carbon atoms. Suitable alkyl groups (R4and R5) include methyl, ethyl, propyl, butyl, sec-butyl, pentyl, hexyl, and 2-ethylhexyl. In some embodiments, R4may be a hydrocarbyl group attached to two or more amide nitrogen atoms, resulting in bridged dimeric, oligomeric, or even polymeric compositions.

[0029] Other covalent thiophosphate compounds that may be employed as the thiophosphate redox mediator include disulfide compounds of the formula (III):wherein X is oxygen or sulfur; n is 0, 1 or 2; each R1is independently a linear or branched aliphatic hydrocarbyl group containing 3 to 18 carbon atoms, 3 to 10 carbon atoms, or 6 to 8 carbon atoms. Suitable alkyl groups (Rl) include include ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, t-butyl, amyl, methylamyl, n-pen- tyl, n-hexyl, n-octyl, iso-octyl, 2-ethylhexyl, n-decyl, isodecyl, and n-dodecyl.

[0030] Ionic thiophosphate compounds include metal, amine, and ammonium salts of dihydrocarbyl-thiophosphoric acid. As above these thiophosphoric acid salts may be mono- or di- thiophosphates or mixtures thereof. In one embodiment, the thiophosphate salt may be a salt of dialkylthiophosphoric acid, wherein the alkyl groups are independently linear or branched aliphatic hydrocarbyl group containing 3 to 18 carbon atoms, 3 to 10 carbon atoms, or 6 to 8 carbon atoms.

[0031] Amine salts of thiophosphate may be salts of primary, secondary, or tertiary hydrocarbyl amines, wherein each hydrocarbyl group independently contains 1 to 20 carbon atoms; 4 to 12 carbon atoms; or 6 to 10 carbon atoms. In one embodiment, a secondary or tertiary amine may be selected such that 2 or more of the hydrocarbyl groups taken together form one or more 5 or 6-membered cyclic structures. Suitable amines include ethylamine, diethylamine, triethylamine, propylamine, isopropylamine, dipropylamine, diisopropylamine, butylamine, dibutylamine, sec-butylamine, tert-butyl amine, pentylamine, 2-ethylhexylamine, bis(2-ethylhexylamine), tris(2-ethylhexylamine), decylamine, undecylamine, dodecylamine, piperidine, tetramethyl piperidine, aniline, and alkylated anliline.

[0032] Ammonium dithiophosphates may be salts of quaternary ammonium or mixtures thereof. The cationic form of the ammonium salt may be represented by the formula (IV):wherein, Ra, Rb, Rc, and Rdare each independently H or hydrocarbyl groups of 2 to 24 carbon atoms, 4 to 20 carbon atoms, or 6 to 18 carbon atoms. Examples of suitable hydrocarbyl groups include ethyl, propyl, butyl, amyl, methylamyl, pentyl, n-hexyl, n-octyl, iso-octyl, 2-ethylhexyl, n-decyl, isodecyl, n-dodecyl, isododecyl, isotridecyl, n-tetradecyl, n-hexadecyl, n-octadecyl, oleyl, and includes other common named groups, such as lauryl, coco, oleyl, tallow, stearyl, cetyl, and behenyl.

[0033] In one embodiment, the ionic phosphorus-containing compound may be a metal salt of a dialkyldithiophosphoric acid, which may include a zinc dialkyldithiophosphate. Such salts are often referred to as dialkyldithiophosphates (DDP) or simply dithiophosphates (DP). They are well known and readily available to those skilled in the art of lubricant formulation. Metal dithiophosphates (MDDP or MDP) include salts of zinc, antimony, calcium, magnesium, lithium, copper, cobalt, iron, manganese, molybdenum, tungsten, vanadium, tin, and combinations thereof. In one embodiment, the metal dithiophosphate may be zinc or lithium. Examples of suitable metal dialkyldithiophosphate useful as the thiophosphate redox mediator include metal salts of the formula (V):wherein each R1is independently a linear or branched aliphatic hydrocarbyl group containing 3 to 18 carbon atoms, 3 to 10 carbon atoms, or 6 to 8 carbon atoms; n is an integer of 1, 2, 3 or 4 matching the valence of the metal; and M is a metal having a valence “n” and generally incudes zinc, copper, iron, cobalt, antimony, lithium, manganese, and combinations thereof.

[0034] The concentration of the thiophosphate redox mediator in the electrolyte composition can, broadly speaking, be in the range from 0 or 0.1 mM to 100 mM, and could be in the range of 0.5 to 50 mM, or 1 to 40 mM or even from 1.5 to 30 mM. In many embodiments, the concentration of the redox mediator in theelectrolyte composition can be in the range from 0 or 0.5 mM to 10 mM, in particular in the range from 1 mM to 9 mM or from 1.5 to 8 mM.

[0035] The electrolyte as described above can be employed in an electrochemical cell further including an anode (i.e., the oxidizing electrode during discharge cycles) and a cathode (i.e., the reducing electrode during discharge cycles). Of a pair of electrodes used in a battery, which serves as a device which converts between chemical energy and electrical energy, the electrode on the side having a higher electrochemical potential is referred to as the positive electrode while the electrode on the side having a lower electrochemical potential is referred to as the negative electrode. As used herein, the conventional nomenclature for batteries is employed wherein the terms “cathode” or “positive electrode” and “anode” or “negative electrode” refer to the electrochemical functions of the electrodes during discharge of the cell to convert the stored chemical energy to electrical energy. During the charging portion of the cycle, the actual electrochemical functions of an electrode are reversed versus that which occurs during discharge.

[0036] The cathode can be prepared from an electroactive sulfur-containing material and optionally a thiophosphate redox mediator as described above. The concentration of the redox mediator in the cathode can, broadly speaking, be in the range from 0 or 0.1 mM to 100 mM, and could be in the range from 0.5 mM to 50 mM, or in the range from 1 mM to 40 mM or even from 1.5 to 30 mM. In many embodiments, the concentration of the redox mediator in the electrolyte composition can be in the range of 0 to 10 mM, or in the range from 0.5 to 10, or from 1 mM to 9 mM, or from 1.5 to 8 mM.

[0037] The cathode comprises elemental sulfur, elemental selenium or a mixture of elemental chalcogens. In one embodiment, the cathode is additionally comprised of one or more electroactive sulfur-containing material. The cathode may additionally and / or alternatively be comprised of a polymer medium and / or an electrically conductive additive. Suitable polymers mediums include, for example, polyvinyl alcohol, polyacrylonitrile, polyvinylidene fluoride (PVDF), polyvinyl fluoride, polytetrafluoroethylene (PTFE), copolymers from tetrafluoroeth- ylene and hexafluoropropylene, copolymers from vinylidene fluoride and hex- afluoropropylene, copolymers from vinylidene fluoride and tetrafluoroethylene, ethylene-propylene-diene monomer rubber (EPDM), and polyvinyl chloride (PVC). The electrically conductive additive may be, for example, a carbon in electrically conductive form such as graphite, graphene, carbon fibers, carbon nanotubes, carbon black, or carbon in its pre-conductive form such aspolyacrylonitrile or pitch, or soot (e.g., lamp or furnace soot). The cathode may be present in a battery or electrochemical cell in combination with a current collector, such as any of the current collectors known in the battery or electrochemical cell art. For example, the cathode may be coated on the surface of a metallic current collector.

[0038] Thus, in one simple embodiment, the technology provides an electrochemical cell having a cathode, the cathode itself having an electroactive sulfur-containing material, and optional (e.g., 0 to 100 mM) thiophosphate redox mediator, anode, and electrolyte, the electrolyte containing polymer medium or solvent, ion conductive salt, and optional (e.g., 0 to 100 mM) thiophosphate redox mediator; so long as the concentration of at least one of the cathode thiophosphate redox mediator and electrolyte thiophosphate redox mediator is not 0.

[0039] Electroactive sulfur-containing materials for the cathode are known in the art and can be either covalent compounds like elemental sulfur or polymers comprising polysulfide bridges or ionic compounds like salts of sulfides or polysulfides. Preferred electroactive sulfur-containing materials are electroactive sulfur- containing materials comprising at least one Li-S-group, like Li2S, lithium polysulfides (Li2S2 to Li2S) or lithiated thioles (lithium thiolates), in particular Li2S.

[0040] In one embodiment the electroactive sulfur-containing material of the cathode can be Li2S.

[0041] During the charging process of the electrochemical cell the cathode can include a mixture of different electroactive sulfur-containing materials as more and more S-S bonds are formed.

[0042] In addition to electroactive sulfur-containing material, the cathode may include one or further constituents. For example, cathode may comprise carbon in a conductive polymorph, for example selected from graphite, carbon black, carbon nanotubes, graphene, or carbon in its pre-conductive form such as polyacrylonitrile or pitch, or mixtures of at least two of the aforementioned substances. Suitable carbons in a conductive polymorph are described in WO 2012 / 168851 page 4, line 30 to page 6, line 22, which is incorporated herein.

[0043] In addition, cathode may include one or more polymer mediums, for example one or more organic polymers. Suitable polymer mediums are described in WO 2012 / 168851 page 6, line 40 to page 7, line 30, which is incorporated herein. Particularly suitable polymer mediums for the cathode include polyvinyl alcohol, poly(ethylene oxide), carboxymethyl cellulose (CMC) and halogenated (copolymers, for example polyvinyl chloride or polyvinylidene chloride, especiallyfluorinated (co)polymers such as polyvinyl fluoride and especially polyvinylidene fluoride, lithiated Nafion and polytetrafluoroethylene.

[0044] In one embodiment, cathode can include in the range from 10 to 80% by weight, preferably 30 to 60% by weight, of sulfur, determined by elemental analysis, based on the total mass of the sum of all electroactive sulfur-containing materials, all carbon in a conductive polymorph and all polymer mediums.

[0045] In one embodiment, cathode can contain in the range from 0.1 to 60% by weight of carbon in a conductive polymorph, preferably 1 to 40% by weight based on the total mass of the sum of all electroactive sulfur-containing materials, all carbon in a conductive polymorph and all polymer mediums. This carbon can likewise be determined by elemental analysis, for example, in which case the evaluation of the elemental analysis has to take into account the fact that carbon also arrives in organic polymers representing polymer mediums, and possibly further sources.

[0046] In one embodiment, cathode can contain in the range from 0.1 to 20% by weight of polymer medium, preferably 1 to 15% by weight and more preferably 3 to 10% by weight, based on the total mass of the sum of all electroactive sulfur- containing materials, all carbon in a conductive polymorph and all polymer mediums.

[0047] In addition, cathode may have further constituents customary per se, for example a current collector, which may be configured in the form of a metal wire, metal grid, metal mesh, expanded metal, metal sheet, metal foil or carbon pa- per / cloth. Suitable metal foils are especially aluminum foils.

[0048] In one embodiment, cathode has a thickness in the range from 25 to 200 pm, preferably from 30 to 100 pm, based on the thickness without current collector.

[0049] The anode of the electrochemical cell can be composed, for example, of material that can accept Li-ions into the metallic form, such as alkali metal such as lithium or sodium or another active material or composition. Anode materials can include metallic lithium, alloys of lithium, metallic sodium, alloys of sodium, alkali metals or alloys thereof, metal powders, alloys of lithium and aluminum, bismuth, magnesium, copper, silicon, and / or tin, alkali metal-carbon and alkali metal-graphite intercalates, hard carbon, soft carbon, graphitic carbon, amorphous carbon, graphene, graphite, and especially graphite, intercalated graphite and mixtures of two or more of the aforementioned carbons, compounds capable of reversibly oxidizing and reducing with an alkali metal ion, and mixtures thereof.The metal or metal alloy (e.g., metallic lithium) may be contained as one film within a battery or as several films, optionally separated by a ceramic material. Suitable ceramic materials include, for example, silica, alumina, or lithium-con- taining glassy materials such as lithium phosphates, lithium aluminates, lithium silicates, lithium phosphorus oxynitrides, lithium tantalum oxide, lithium aluminosilicates, lithium titanium oxides, lithium silicosulfides, lithium ger- manosulfides, lithium aluminosulfides, lithium borosulfides, lithium phosphosulfides and mixtures thereof

[0050] The anode may further contain one or more polymer mediums. The polymer medium selected may be one or more of the aforementioned polymer mediums specified in the context of the description of the cathode.

[0051] In addition, anode may have further constituents customary per se, for example a current collector which may be configured in the form of a metal wire, metal grid, metal mesh, expanded metal, or a metal foil or metal sheet. Suitable metal foils are especially copper foils.

[0052] In one embodiment, anode has a thickness in the range from 15 to 1000 pm, preferably from 30 to 750 pm or from 60 to 500 pm, based on the thickness without current collector.

[0053] The rechargeable electrochemical cells as described above may also have constituents customary per se, for example separators, cable connections and housing.

[0054] The rechargeable electrochemical cells described herein give a high voltage and are notable for a high energy density and good stability. More particularly, they are notable for an increase in capacity and improved cycling stability due to the thiophosphate redox mediator.

[0055] The inventive rechargeable electrochemical cells can be assembled to alkali metal ion batteries, preferably rechargeable alkali metal ion batteries, especially to rechargeable lithium ion batteries.

[0056] Inventive rechargeable electrochemical cells can be combined with one another in inventive rechargeable alkali metal ion batteries, especially in rechargeable lithium ion batteries, for example in series connection or in parallel connection. Series connection is preferred.

[0057] Inventive rechargeable electrochemical cells are notable for particularly high capacities, high performances even after repeated charging and greatly retarded cell death. Inventive rechargeable electrochemical cells are very suitable for use in motor vehicles, bicycles operated by electric motor, for examplepedelecs, aircraft, ships or stationary energy stores. Such uses form a further part of the subject matter of the present invention.

[0058] The present invention further provides for the use of inventive rechargeable electrochemical cells as described above in motor vehicles, bicycles operated by electric motor, aircraft, ships or stationary energy stores.

[0059] The use of inventive rechargeable metal ion batteries, especially rechargeable lithium ion batteries, in devices gives the advantage of prolonged run time before recharging and a smaller loss of capacity in the course of prolonged run time. If the intention were to achieve an equal run time with electrochemical cells with lower energy density, a higher weight for electrochemical cells would have to be accepted.

[0060] The present invention therefore also further provides for the use of inventive rechargeable metal-ion batteries, especially rechargeable lithium-ion batteries, in devices, especially in mobile devices. Examples of mobile devices are vehicles, for example motor vehicles (including cars, trucks, buses, and any other motorized vehicle), bicycles, aircraft, or water vehicles such as boats or ships. Other examples of mobile devices are those which are portable, for example computers, especially laptops, telephones or electrical power tools, for example from the construction sector, especially drills, battery-driven screwdrivers or battery- driven trackers. The use provides an increase in the capacity of the cell over baseline (i.e., without the thiophosphate redox mediator) and a sustained increase in capacity of the cell over baseline over battery cycling.

[0061] The present invention further provides a device comprising at least one inventive rechargeable electrochemical cell as described above.

[0062] The technology further provides a method of mediating electrochemical redox transformations in an ion conductive salt-electroactive sulfur-containing electrochemical cell, comprising preparing an electrochemical cell comprising a) cathode comprising electroactive sulfur-containing material, b) anode, and c) electrolyte, and preparing said electrolyte comprising i) polymer medium or solvent, ii) ion conductive salt, and iii) thiophosphate redox mediator, and operating said electrochemical cell.

[0063] Additionally provided is the use of thiophosphate redox mediator to mediate electrochemical redox transformations in an ion conductive salt-electroactive sulfur-containing electrochemical cell.

[0064] As used herein, the term “hydrocarbyl” refers to a group having a carbon atom directly attached to the remainder of the molecule, where the group includesat least carbon and hydrogen atoms. If the hydrocarbyl group comprises more than one carbon atom, then those carbons need not necessarily be linked to each other. For example, at least two of the carbons may be linked via a suitable element or group. In various embodiments, the term “hydrocarbyl” refers to a group having a carbon atom directly attached to the remainder of the molecule, where the group consists of carbon, hydrogen, optionally one or more heteroatoms provided the heteroatoms do not alter the predominantly hydrocarbon nature of the substituent. The heteroatom may link at least two of the carbons in the hydrocarbyl group, and optionally no more than two non-hydrocarb on substituents. Suitable heteroatoms will be apparent to those skilled in the art and include, for instance, sulphur, nitrogen, oxygen, phosphorus and silicon. Where the hydrocarbyl contains heteroatoms, optionally, no more than two heteroatoms will be present for every ten carbon atoms in the hydrocarbyl group. Suitable non-hydro- carbon substituents will also be apparent to those skilled in the art and include, for instance, halo, hydroxy, alkoxy, mercapto, alkylmercapto, nitro, nitroso, and sulphoxy.

[0065] Examples of hydrocarbyls within the context of the present technology therefore include:

[0066] hydrocarbon groups selected from aliphatic (e.g., alkyl or alkenyl), alicyclic (e.g., cycloalkyl, cycloalkenyl, cycloalkadienyl), and aromatic groups;

[0067] substituted hydrocarbon groups, selected from hydrocarbon groups defined in (i) substituted with no more than two non-hydrocarbon substituents and / or one or more hydrocarbon substituents, the non-hydrocarbon substituents being selected from the group consisting of halo, hydroxy, alkoxy, mercapto, alkylmercapto, nitro, nitroso, and sulphoxy;

[0068] hetero-containing hydrocarbon groups, selected from hydrocarbon groups defined in (i) containing one or more heteroatom in the ring or chain, provided that the group has no more than two heteroatoms present for every ten carbon atoms in the group, the heteroatoms being selected from sulphur, nitrogen, oxygen, phosphorus and silicon. The hetero-containing hydrocarbon groups may be substituted with no more than two non-hydrocarbon substituents and / or one or more hydrocarbon substituents.

[0069] In some embodiments, the term “hydrocarbyl” refers to a group having a carbon atoms directly attached to the remainder of the molecule, where the group consists of carbon and hydrogen atoms.

[0070] It is known that some of the materials described above may interact in the final formulation, so that the components of the final formulation may be different from those that are initially added. For instance, metal ions can migrate to other acidic or anionic sites of other molecules. The products formed thereby, including the products formed upon employing the composition of the present invention in its intended use, may not be susceptible of easy description. Nevertheless, all such modifications and reaction products are included within the scope of the present invention; the present invention encompasses the composition prepared by admixing the components described above.

[0071] EXAMPLES Example of a processes for making 3 dithiophosphates, used to prepare examples 1 to 12..

[0072] Dithiophosphate 1: O,O-di(isooctyl) S-hydrogen phosphorodithi- oate

[0073] To a 1 L flange flask in a heating mantle with PTFE stirrer and thermocouple, N2 inlet on top of powder addition funnel with rubber bung, water cooled condenser with exhaust set up to 20 wt% caustic scrubber with bleach Dreschel, iso-octyl alcohol (IOOL) (352 g) was charged and heated to 70 °C. P2S5 (150 g) was added in 20 g portions over 1 hour before the powder additions funnel was replaced for a subsurface N2 sparge and the reaction heated to 110 °C for 2 hours before cooling to RT. Subsequent filtration yielded dithiophosphate 1 as a paleyellow liquid in a quantitative yield.

[0074] Dithiophosphate 2: Mixed Isobutyl and Amyl DithioacidSR°'6RSHR = Isobutyl / Amyl (61 / 39 wt%)

[0075] To a 1 L flange flask in a heating mantle with PTFE stirrer and thermocouple, N2 inlet on top of powder addition funnel with rubber bung, water cooled condenser with exhaust set up to 20 wt% caustic scrubber with bleach Dreschel, 61 / 39 wt. % mix of amyl alcohol and isobutanol (396 g) were charged and heated to 65 °C. Phosphorus(V) pentasulfide (200 g) was added in 20 g portions over 1 hour before replacing the powder addition funnel for a subsurface N2 inlet andheating to 100 °C for 2 hours. The reaction was subsequently reconfigured for vacuum distillation and residual alcohols removed under reduced pressure to give dithiophosphate 2 as a pale-yellow liquid in a quantitative yield.

[0076] Dithiophosphate 3: O,O-bis(4-methylpentan-2-yl) S-hydrogen phos- phorodithioate

[0077] To a 1 L flange flask in a heating mantle with PTFE stirrer and thermocouple, N2 inlet on top of powder addition funnel with rubber bung, water cooled condenser with exhaust set up to 20 wt% caustic scrubber with bleach Dreschel, 4-methyl-2-pentanol (MPL) (312 g) was charged and heated to 70 °C. P2S5 (150 g) was added in 20 g portions over 1 hour before the powder additions funnel was replaced for a subsurface N2 sparge and the reaction heated to 110 °C for 2 hours before cooling to RT. The reaction was subsequently reconfigured for vacuum distillation and residual alcohol removed under reduced pressure to give dithiophosphate 3 as a pale-yellow liquid in a quantitative yield.

[0078] Preparative examples (1-12):

[0079] Example 1 : S,S'-((methylenebis(azanediyl))bis(3-oxopropane-3,l- diyl)) O,O,O',O'-tetrakis(isooctyl) bis(phosphorodithioate)

[0080] To a 1 L flange flask in a heating mantle with N2 inlet, condenser, PTFE thermocouple and overhead stirrer and caustic Dreschel bottle, dithiophosphate 1 (483 g) was heated to 65 °C. N,N’ -methylene bis-acrylamide (84 g) was added portion wise over 45 mins and the reaction held at 65 °C for 4 hours. Upon completion, reaction taken up in heptane (100 ml) and washing with 1 M NaOH (3 x 50 ml) before drying (MgSC ) and concentrating under reduced pressure to yield- Example 1 as a pale-yellow oil (245 g).

[0081] Example 2: Methyl 3-((di-alkoxyphosphorothioyl)thio)propanoate (alkyl = Isobutyl & Amyl)R = Isobutyl / Amyl (61 / 39 wt %)

[0082] To a 250 ml round bottomed flask on a hotplate-stirrer with N2 inlet, condenser, PTFE thermocouple and stirrer flea and caustic Dreschel bottle, dithiophosphate 2 (100 g) was charged and heated to 65 °C. Methyl acrylate (31 g) added portion wise by dropping additions funnel over 30 minutes, keeping the temperature below 70 °C. Upon complete addition, the reaction was held at 60 °C for 4 hours before cooling to RT. The resulting oil was taken up in heptane (100 ml) and washing with 1 M NaOH (3 x 50 ml) before drying (MgSC ) and concentrating under reduced pressure to yield Example 2 as a pale-yellow oil (115 g)-

[0083] Example 3: S,S',S"-((l,3,5-triazinane-l,3,5-triyl)tris(3-oxopropane-3,l- diyl)) O,O,O',O',O",O"-hexa(isooctyl) tris(phosphorodithioate)

[0084] To a 100 ml round bottomed flask on a hotplate-stirrer with N2 inlet, condenser, PTFE thermocouple and stirrer flea and caustic Dreschel bottle, dithiophosphate 1 (50 g) was heated to 65 °C. 1,3, 5 -Tri acryloyl hexahydro-1, 3, 5-tria- zine (10.5 g) was added portion wise over 45 mins and the reaction held at 65 °C for 4 hours. Upon completion, reaction taken up in heptane (100 ml) and washing with 1 M NaOH (3 x 50 ml) before drying (MgSO4) and concentrating under reduced pressure. Further purification is possible by column chromatography, eluting with EtOAc / heptane1 : 1 gradient). This yielded Example 3 as a paleyellow oil (15 g).

[0085] Example 4: S-(3-(diethylamino)-3-oxopropyl) O,O-di(isooctyl) phos- phorodithioate

[0086] To a 100 ml round bottomed flask on a hotplate-stirrer with N2 inlet, condenser, PTFE thermocouple and stirrer flea and caustic Dreschel bottle, dithiophosphate 1 (50 g) was heated to 65 °C. Diethyl acrylamide (16 g) was added portion wise via dropping additions funnel over 45 mins and the reaction held at 65 °C for 4 hours. Upon completion, reaction taken up in heptane (100 ml) and washing with 1 M NaOH (3 x 50 ml) before drying (MgSC ) and concentrating under reduced pressure. Further purification is possible by column chromatography, eluting with EtOAc / heptane (1 :91: 1 gradient). This yielded Example4 as a pale-yellow oil (13 g).

[0087] Example 5: Methyl 3-((bis(isooctyloxy)phosphorothioyl)thio)propanoate

[0088] To a 100 ml round bottomed flask on a hotplate-stirrer with N2 inlet, condenser, PTFE thermocouple and stirrer flea and caustic Dreschel bottle, dithiophosphate 1 (50 g) was heated to 65 °C. Methyl acrylate (10.8 g) was added portion wise via dropping additions funnel over 45 mins and the reaction held at 65 °C for 4 hours. Upon completion, reaction taken up in heptane (100 ml) and washing with 1 M NaOH (3 x 50 ml) before drying (MgSO4) and concentrating under reduced pressure. Further purification is possible by column chromatography, eluting with EtOAc / heptane (1 :9 1: 1 gradient). This yielded the title compound Example 5 as a pale-yellow oil (20 g).

[0089] Example 6: 2-ethylhexyl 3-((di-alkoxyphosphorothioyl)thio)propanoate (alkyl = Isobutyl & Amyl)

[0090] To a 100 ml round bottomed flask on a hotplate-stirrer with N2 inlet, condenser, PTFE thermocouple and stirrer flea and caustic Dreschel bottle,dithiophosphate 2 (25 g) was heated to 65 °C. 2-ethylhexyl acrylate (16.6 g) was added portion wise via dropping additions funnel over 45 mins and the reaction held at 65 °C for 4 hours. Upon completion, reaction taken up in heptane (100 ml) and washing with 1 M NaOH (3 x 50 ml) before drying (MgSCh) and concentrating under reduced pressure. Further purification is possible by column chromatography, eluting with EtOAc / heptane (1 :9 -> 1 : 1 gradient). This yielded Example 6 as a pale-yellow oil (10 g).

[0091] Example 7: S-(3-amino-3-oxopropyl) O,O-di(isooctyl) phosphorodithio- ate

[0092] To a 100 ml round bottomed flask on a hotplate-stirrer with N2 inlet, condenser, PTFE thermocouple and stirrer flea and caustic Dreschel bottle, dithiophosphate 1 (50 g) was heated to 65 °C. Methyl acrylate (9 g) was added portion wise over 45 mins and the reaction held at 65 °C for 4 hours. Upon completion, reaction taken up in heptane (100 ml) and washing with 1 M NaOH (3 x 50 ml) before drying (MgSO4) and concentrating under reduced pressure. Further purification is possible by column chromatography, eluting with EtOAc / heptane (1 :9 -> 1:1 gradient). This yielded Example 7 as a pale-yellow oil (7 g).

[0093] Example 8: S,S'-((methylenebis(azanediyl))bis(3-oxopropane-3,l-diyl)) O,O,O',O'-tetra(isooctyl) bis(phosphorothioate)

[0094] To a 100 ml round bottomed flask on a hotplate-stirrer with N2 inlet, condenser, PTFE thermocouple and stirrer flea and caustic Dreschel bottle, dithiophosphate 1 (50 g) was heated to 65 °C with water (3.5 ml) for 12 hours. Subsequently, N,N’ -methylene bis-acrylamide (8.6 g) was added portion wise over 45 mins and the reaction held at 65 °C for 4 hours. Upon completion, reaction taken up in heptane (100 ml) and washing with 1 M NaOH (3 x 50 ml) before drying (MgSO4) and concentrating under reduced pressure. Further purification is possible by column chromatography, eluting with acetone / SBP-3 (1 :91: 1 gradient). This yielded Example 8 as a pale-yellow oil (8 g).

[0095] Example 9: 2-Ethylhexylammonium O,O-di(isooctyl) phosphorodithioate

[0096] To a 100 ml round bottomed flask on a hotplate-stirrer with N2 inlet, condenser, PTFE thermocouple and stirrer flea and caustic Dreschel bottle, dithiophosphate 1 (50 g) was charged. 2-ethylhexylamine (18.2 g) was added dropwise via dropping addition funnel over 45 mins, maintaining a reaction temperature below 30 °C. Upon complete addition the reaction was heated to 60 °C for 4 hours. This yielded Example 9 as a pale-yellow oil in a quantitative yield.

[0097] Example 10: S,S'-((methylenebis(azanediyl))bis(3-oxopropane-3,l-diyl)) O,O,O',O'-tetrakis(4-methylpentan-2-yl) bis(phosphorodithioate)

[0098] To a 500 ml round bottomed flask on a hotplate-stirrer with N2 inlet, condenser, PTFE thermocouple and stirrer flea and caustic Dreschel bottle, dithiophosphate 3 (207 g) was heated to 65 °C. N,N’ -methylene bis-acrylamide (52 g) was added portion wise over 45 mins and the reaction held at 65 °C for 4 hours. Upon completion, reaction taken up in heptane (100 ml) and washing with 1 M NaOH (3 x 50 ml) before drying (MgSC ) and concentrating under reduced pressure to yield Example 10 as a pale-yellow oil (121 g).

[0099] Example 11 : O,O-bis(4-methylpentan-2-yl) phosphorodithioate Disulphide

[0100] To a 3 L jacketed vessel with N2 inlet, dropping funnel, PTFE overhead stirrer and PTFE thermocouple, condenser and caustic scrubber, dithiophosphate 3 (1476 g) was added and heated to 35 °C. H2O2 (364 g) was pumped in with peristaltic pump (approx. 3.1 g / min) over 2 hours. The reaction was then help at35 °C for 3 hrs before cooling and leaving to stand overnight. The aqueous layer was separated and the organic layer dried (MgSC ) before concentrating under reduced pressure to give Example 11 as a pale-yellow liquid in a quantitative yield.

[0101] Example 12: O,O-bis(4-methylpentan-2-yl) S-(octan-2-yl) phosphorodi- thioate

[0102] To a 500 ml round bottomed flask on a hotplate-stirrer with N2 inlet, condenser, PTFE thermocouple and stirrer flea and caustic Dreschel bottle, dithiophosphate 3 (182 g) and 1-octene (68 g) were heated to 90 °C. Upon completion, reaction taken up in heptane (100 ml) and washing with 1 M NaOH (3 x 50 ml) before drying (MgSCE) and concentrating under reduced pressure to yield Example 12 as a pale-yellow liquid (230 g).

[0103] It has been found that at a low treat and low contributions to the total battery cell sulfur content (e.g., 1% additional sulfur), thiophosphate redox mediators have increased the cyclable discharge capacity of a model lithium -sulfur rechargeable battery in the formfactor of a standard Swagelok cell. The improvement to discharge capacity was by an impressive, > 70% relative to the un-additized cell within the first 20 cycles.

[0104] New materials and electrolytes for lithium sulfur batteries should be tested on the scale and in the form factor they will eventually be used however for screening purposes, the use of coin cells is necessary. Additional testing at 1 amp-hour pouch cell scale can demonstrate performance for a more real world, commercial-like battery configuration.

[0105] The testing relating to the samples above was conducted using three different cell configurations, namely Swagelock cells, coin cells and pouch cells. Below is a description of the configuration and materials required for coin cell fabrication which can be repeated by those skilled in the art.

[0106] Coin Cell construction: All coin cell construction was carried out within a glove box with an argon atmosphere.

[0107] Electrolyte composition: The electrolyte composition consists of a 1 : 1 dioxolane (DOL): dimethoxy ethane (DME) (anhydrous) solvent. Additional to the solvent are the following materials: LiTFSI (bis(trifluoromethane)sulfonimidelithium) at 1 M concentration and LiN03 at up to 0.8 M concentration, except that the inventive examples were employed at 5 mM concentration for the data contained within Table 1. The electrolyte solution is prepared in a glove box under an argon atmosphere. The solvents are additionally dried over molecular sieves prior to use. The LiTFSI is weighed out, followed by the LiNO3 (of sufficient quantity to achieve a final molarity of 0.8M) which are then combined and the pre-mixed DOL:DME solution is then added.

[0108] Cathode: The cathode material used was commercially sourced sulfurcarbon composites (provided by NEI Corporation). The cathode material was cut into 10-12mm discs prior to use in the coin cells, which were then dried under vacuum at room temperature.

[0109] Separator: The separator was Celguard-2400 and was pre-punched into discs of 15 mm and dried for 72 h under vacuum at 40°C.

[0110] Anode Preparation: The anode used were lithium metal discs (kept within a glovebox under argon atmosphere) and were punched into 12-14mm discs. The surface of the lithium needed gentle rubbing with a plastic tool ahead of coin cell construction to remove any layers that built up from contaminants / surface reactions. The lithium was then gently rolled flat to avoid any changes in size during coin cell crimping.[OHl] Coin cell components: The following were dried before use under vacuum at 70°C, then stored in the glovebox under an argon atmosphere;• Coin cell top casing (large)• Coin cell bottom casing (small)• Coin cell gasket (often already attached to the coin cell bottom casing)• 2 X 0.5 mm stainless steel spacers• Coin cell wave spring• Electrolyte Solution• 15 mm Celgard 2400 separator• 12-14 mm lithium (anode) disc, and• 10-12 mm cathode disc.

[0112] The cell was constructed as follows, adding each component to the starting bottom casing, spacer disc, lithium Disc, separator disc, electrolyte via micropipette (electrolyte volume is dependent on sulfur loading, the electrolyte to sulfur ratio is typically kept below 30pL / Mg), cathode, spacer disc, wave spring, top casing of coin cell (finish). After construction, the coin cell was crimped to a pressure of 50 Kg nr2.

[0113] Coin cell testing: After assembly, the coin cells were taken out of the glovebox and into an oven where they are cycled at 40 °C. They were rested for 8 hours. Starting with the discharge process where negative current is applied, the cells were cycled galvanostatically (starting with discharge), using a constant current between the potential limits of 1.8 and 2.6 V. No constant voltage step was used.

[0114] At the formation step, the cell underwent three cycles at a symmetric charge-discharge rate of 1 / 20 C before the desired testing can begin at 1 / 10 C rate symmetric charge-discharge cycles. The cycling rate rate of 1 C corresponds to the current required to be applied for the cell to be fully charged or discharged within one hour. Therefore, a charge or discharge rate of, for example, 1 / 10 C means the charge or discharge current is such that the anticipated capacity should be fully charged or discharged in 10 hours. The exact current depends on the total sulfur mass in the cell (since these cells were built for the sulfur-containing cathode to be rate limiting).

[0115] Coin cell performance was investigated through charge-discharge cycling using commercially available battery cyclers (e.g. Ivium, BioLogic).Table 1

[0116] It is worth noting that the battery cells used above were constructed with a commercially available carbon-sulfur cathode and not a highly optimized, bespoke cathode construction. This makes the improvements more impressive and potentially would mean this finding would be advantageous in improving the performance of commercially manufactured lithium -sulfur battery cells (through likely high throughput manufacturing techniques currently known in the art). It is thought that the additive has brought about this improvement by interacting advantageously with the sulfur to Li2S, multistep redox process and that interaction has increased the utilization of available sulfur in the in thecell moving the battery closer towards its theoretical maximum capacity. Advantageously the additive of this disclosure is also free from scarce metals and easy to produce relative to other redox mediators demonstrated in the literature, making it highly cost effective and contributing to a more sustainable battery with lower environmental im- pact.

[0117] Each of the documents referred to above is incorporated herein by reference. The mention of any document is not an admission that such document qualifies as prior art or constitutes the general knowledge of the skilled person in any jurisdiction. Except in the Examples, or where otherwise explicitly indicated, all numerical quantities in this description specifying amounts of materials, reaction conditions, molecular weights, number of carbon atoms, and the like, are to be understood as modified by the word "about." It is to be understood that the upper and lower amount, range, and ratio limits set forth herein may be independently combined. Similarly, the ranges and amounts for each element of the invention can be used together with ranges or amounts for any of the other elements. As used herein, the expression "consisting essentially of' permits the inclusion of substances that do not materially affect the basic and novel characteristics of the composition under consideration.

Claims

What is claimed is:

1. An electrolyte composition comprising a. medium, b. ion conductive salt, and c. thiophosphate redox mediator.

2. The electrolyte composition of claim 1, wherein the medium comprises polymer medium.

3. The electrolyte composition of claim 1, wherein the medium comprises aprotic organic solvent.

4. The electrolyte composition of any previous claim, wherein the ion conductive salt comprises a metal salt.

5. The electrolyte composition of claim 4, wherein the metal comprises alkali metal.

6. The electrolyte composition of claim 4, wherein the metal comprises alkaline earth metal.

7. The electrolyte composition of claim 4, wherein the metal comprises transition or post transition metal.

8. The electrolyte composition of claim 4, wherein the metal comprises lithium, sodium, magnesium, aluminum and alloys or composites thereof.

9. The electrolyte composition of any previous claim, wherein the thiophosphate compound comprises a compound of formula (I)R ' Q ..........(I) wherein X is oxygen or sulfur; each R1is independently a linear or branched aliphatic hydrocarbyl group containing 3 to 18 carbon atoms; R2is a branched or linear hydrocarbyl group containing 4 to 20 carbon atoms, or an acylated hydrocarbyl group (-R3-(C=O)-Y-R4) containing 4 to 20 carbon atoms where R3is a hydrocarbyl group containing 1 to 6 carbon atoms, Y is O or N-R5, and R4and R5are each independently H or hydrocarbyl groups of 1 to 12 carbon atoms.

10. The electrolyte composition of any previous claim wherein the thiophosphate compound comprises a compound of formula (II)wherein X is oxygen or sulfur; each R1is independently a linear or branched aliphatic hydrocarbyl group containing 3 to 18 carbon atoms; Y is O or NR5; R4and R5are each independently hydrocarbyl groups of 1 to 12 carbon atoms; and R6is hydrogen or a hydrocarbyl group of 1 to 4 carbon atoms.

11. The electrolyte composition of any previous claim wherein the thiophosphate compound comprises a compound of formula (III)wherein X is oxygen or sulfur; each R1is independently a linear or branched aliphatic hydrocarbyl group containing 3 to 18 carbon atoms.

12. The electrolyte composition of any previous claim wherein the thiophosphate compound comprises a dialkylthiophosphoric acid.

13. The electrolyte composition of any previous claim wherein the thiophosphate compound comprises an amine salt of thiophosphate.

14. The electrolyte composition of any previous claim wherein the thiophosphate compound comprises ammonium dithiophosphate.

15. The electrolyte composition of any previous claim wherein the thiophosphate compound comprises a metal salt of dialkyldithiophosphoric acid.

16. A cathode comprising a. electroactive sulfur-containing material, and b. thiophosphate redox mediator.

17. An electrochemical cell comprising a. cathode comprisingi. electroactive sulfur-containing material, and ii. 0 to 100 mM thiophosphate redox mediator, b. anode, and c. electrolyte comprising i. medium, ii. ion conductive salt, and iii. 0 to 100 mM thiophosphate redox mediator, so long as the concentration of at least one of a(ii) and c(iii) is not 0.

18. A method of mediating electrochemical redox transformations in an ion conductive salt-electroactive sulfur-containing electrochemical cell, comprising preparing an electrochemical cell comprising a) cathode comprising electroactive sulfur-containing material, b) anode, and c) electrolyte, and preparing said electrolyte comprising i) polymer medium or solvent, ii) ion conductive salt, and iii) thiophosphate redox mediator, and operating said electrochemical cell.

19. Use of thiophosphate redox mediator to mediate electrochemical redox transformations in an ion conductive salt-electroactive sulfur-containing electrochemical cell.

20. The use of claim 18, wherein the use provides an increase in the capacity of the cell over baseline (i.e., without the thiophosphate redox mediator).

21. The use of claim 18, wherein the use provides a sustained increase in capacity of the cell over baseline over battery cycling.

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