Patent Application
20220231340
The present invention relates to novel non-aqueous electrolyte compositions, a sodium-ion cell comprising said novel non-aqueous electrolyte compositions and energy storage devices such as batteries, rechargeable batteries, electrochemical devices, and electrochromic devices which include said non-aqueous electrolyte compositions.
Sodium-ion batteries are analogous in many ways to the lithium-ion batteries that are in common use today; they are both reusable secondary batteries that comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material, both are capable of storing energy, and they both charge and discharge via a similar reaction mechanism. When a sodium-ion (or lithium-ion) battery is charging, Na+ (or Li+) ions de-intercalate from the cathode and insert into the anode. Meanwhile charge balancing electrons pass from the cathode through the external circuit containing the charger and into the anode of the battery. During discharge the same process occurs but in the opposite direction.
Lithium-ion battery technology has enjoyed a lot of attention in recent years and provides the preferred portable battery for most electronic devices in use today. However, lithium is not a cheap metal to source and is considered too expensive for use in large scale applications. By contrast sodium-ion battery technology is still in its relative infancy but is seen as advantageous; sodium is much more abundant than lithium and some researchers predict this will provide a cheaper and more durable way to store energy into the future, particularly for large scale applications such as storing energy on the electrical grid. Nevertheless, a lot of work has yet to be done before sodium-ion batteries are a commercial reality.
One area that needs more attention is the development of suitable electrolyte compositions, particularly for sodium-ion batteries.
Although the design of suitable electrolyte compositions is given less attention than active materials (electrodes), their importance should not be overlooked as they are in large part the key to battery life and for determining the practical performance achievable by a cell, for example in terms of capacity, rate capability, safety etc. However, to be a suitable electrolyte composition, it must fulfil a long list of attributes, these include:
In lithium-ion cells, the most common electrolyte compositions contain either LiPF6 or LiBF4 dissolved in organic carbonate-based solvents; electrolyte compositions comprising 1M LiPF6 in either a mixture of EC (ethylene carbonate)/DMC (dimethyl carbonate) or a mixture of EC (ethylene carbonate)/EMC (ethyl methyl carbonate) are regarded by most workers as the “standard” Li-ion cell electrolytes.
In the case of sodium-ion cells, the sodium analogue, NaPF6, may be used in place of the LiPF6, but a much more cost effective alternative is NaBF4; the latter also has the benefit of improved thermal stability compared with NaPF6. Unfortunately, however, NaBF4 has very low solubility in organic carbonate-based electrolyte solvents, and this results in the ionic conductivity of the resulting electrolyte compositions being generally too low for practical application. Thus, poor solubility of NaBF4-containing electrolyte compositions in traditional organic carbonate-based solvents produce inferior electrochemical performance when compared against an equivalent cell using NaPF6.
Furthermore, it should also be stated that although some materials used in lithium systems are able to be carried over to their sodium counterpart, it is by no means safe to assume that this will always be the case, due, in part, to the larger atomic radius of sodium compared to that of lithium. Thus, solvent systems that work for lithium-ion batteries might not work for sodium-ion batteries and vice-versa. A prominent example of this difference is the incompatibility of propylene carbonate (PC) solvent in lithium-ion batteries with graphite anode (the most commonly used anode in commercial lithium-ion systems currently). Due to this incompatibility, commercial lithium-ion batteries typically use EC/DMC or EC/EMC solvent system, as mentioned previously. By contrast, sodium-ion systems will happily tolerate an electrolyte solvent that includes PC.
However, it is desirable to find electrolyte solvent systems which are optimal for promoting high sodium-ion battery performance.
The aim of the present invention therefore is to provide improved sodium ion conducting electrolyte compositions (that is, they are electrolyte compositions which are designed for use in sodium-ion secondary cells) which use sodium-containing salts dissolved in a suitable solvent system. The electrolyte compositions of the present invention will be cost effective, will be non-flammable, will demonstrate enhanced separator wettability, particularly with polyolefin separators, and will demonstrate excellent electrochemical performance in sodium-ion cells. The electrolyte compositions of the present invention will be especially useful in sodium-ion cells which employ an anode electrode which comprises a non-graphitic carbon-containing material such as a hard carbon-containing material, or an anode which comprises a sodium insertion material or a conversion-alloying anode material.
The present invention achieves these aims by providing a novel non-aqueous electrolyte composition comprising:
The one or more sodium-containing salts preferably comprise one or more weakly coordinating anions. The term “weakly coordinating anion” or “WCA” is well known to those skilled in the art and is used to refer to anions which comprise several (more than one) elements, which may contain halogen atoms and/or oxygen atoms, and which share a single negative charge. This means that the negative charge is spread out over the anion and makes the coordinating ability of the anion comparatively weak, for example relative to the coordinating ability of an anion which has a concentrated negative charge or one with multiple negative charges. Preferably, the sodium-containing salt is selected from one or more compounds of the formula NaMmXx; one or more compounds of the formula NaXO4; one or more fluoro sulfonyl-containing compounds; one or more fluoro sulfonate-containing compounds; one or more oxalato borate compounds; and compounds that contain a tetrahedral anion such as tetrakis[3,5-bis(trifluoromethyl)phenylborate anion (B[3,5-(CF3)2C6H3]−4), tris(pentafluorophenyl)borate anion (B(C6F5)−4), and tetrakis carboxy (trifluoromethyl)aluminate anion (Al[OC(CF3)3]−4).
In the preferred one or more sodium-containing salts of the formula NaMmXx, M is one or more metals and/or non-metals and X is a group that comprises or consists of one or more halogens, preferably selected from fluorine, chlorine, bromine and iodine, further preferably fluorine. The amount x of halogen X is preferably x=1 to 16, further preferably x=4 or 6. The amount, m, of the one or more metals and/or non-metals, M, is preferably m=1 to 3, further preferably m=1 to 2, and particularly preferably m=1. Ideally, the one or more metals and/or non-metals, M, is preferably selected from aluminium, boron, gallium, indium, iridium, platinum, scandium, Yttrium, lanthanum, antimony, arsenic and phosphorus. Particularly preferably, M is selected from aluminium, boron, gallium, phosphorus and arsenic. The most preferred sodium-containing salts of the general formula NaMnXx are one of more selected from sodium tetrafluoroborate (NaBF4), sodium hexafluorophosphate (NaPF6).
In the preferred one or more sodium compounds of the formula NaXO4, the element X is preferably one or more halogens selected from fluorine, chlorine, bromine and iodine. Chlorine is particularly preferred, and the most preferred sodium-containing salt of the formula NaXO4 is NaClO4.
Preferred sodium salts of one or more fluoro sulfonyl- and/or sodium salts of one or more fluoro sulfonate-containing compounds include:
Preferred sodium salts of one or more oxalate borate compounds include:
It is convenient to express the amount of the one or more sodium-containing salts in terms of “the molarity of the components in a) in the solvent system b)”; that is, the total number of moles of the one or more sodium-containing salts, per litre of the solvent system b). Preferably, the molarity of each of the sodium-containing salt individually in a) is in the range 0.1M to 5M, and further preferably in the range 0.1M to ≤2M. Highly preferably, particularly when the component in a) is NaBF4 or NaPF6, the molarity is in the range 0.1M to ≤2M. The total molarity of all of the sodium-containing salts in a) combined, is preferably in the range 0.1 M to 10 M, and more preferably in the range 0.1 M to 6 M, and most preferably in the range 0.1 M to ≤4 M.
The one or more organo carbonate-based solvents present in the first solvent component i), may be cyclic or non-cyclic compounds that are characterised by the fact that they contain a carbonate ester group, i.e. a carbonyl group that is flanked by one or two alkoxygroups: R1O(C═O)OR2. The R1 and R2 groups are preferably independently selected (i.e. they may be the same or different from each other) from either hydrogen; or a C1 to C20-cyclic or non-cyclic, branched or unbranched, substituted or unsubstituted alkyl group; or a C1 to C20-cyclic or non-cyclic, branched or unbranched, substituted or unsubstituted alkenyl group; or a C1 to C20-branched or unbranched, substituted or unsubstituted cycloalkyl-, phenyl- or heterocycle-containing group. Highly suitable electrolyte solvents include C3-C10 cycloalkyl organo carbonates, such as propylene carbonate (C4H6O3) and ethylene carbonate (C3H4O3). Propylene carbonate (C4H6O3) shows particularly favourable compatibility with electrode materials and the high solubility, wide liquidus range and a high boiling point of this material also makes this solvent advantageous for use in sodium-ion batteries. Other highly suitable organo carbonate-based compounds include diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylene carbonate (EC), ethyl methyl carbonate (EMC), fluoroethylene carbonate (FEC) and vinylene carbonate (VC).
Highly preferred electrolyte compositions of the present invention comprise a first solvent component which consists of propylene carbonate either alone or alternatively in combination with one or more further organo carbonate-based compounds. When a mixture of organo carbonate-based compounds is used, it is convenient to express the amount of each of the one or more organo carbonate-based solvents in terms of a weight ratio. For example, a preferred organo carbonate-based solvent might contain a mixture of ethylene carbonate, diethyl carbonate and propylene carbonate, and this would ideally be in the weight ratio range 1 to 4:1 to 10:1 to 10 wt/wt and further ideally in the weight ratio 1 to 3:1 to 5:1 to 5 wt/wt and most ideally 1:2:1 wt/wt.
Extremely advantageous electrolyte compositions of the present invention include propylene carbonate in an amount of at least 40%, preferably at least 50%, further preferably at least 60%, by weight of the organo carbonate-based solvent. Highly preferably propylene carbonate is the sole organo carbonate-based solvent i.e it is present in an amount of 100% by weight of the organo carbonate-based solvent.
An alternative preferred electrolyte composition of the present invention comprises a mixture of ethylene carbonate and diethyl carbonate in the absence of propylene carbonate. These organo carbonate-based components are preferably present in the weight ratio 1 to 20:1 to 20 wt/wt, further preferably in the weight ratio 1 to 10:1 to 10 wt/wt, also preferably 1 to 5:1 to 5 wt/wt, and most preferably in the weight ratio 1:1 wt/wt.
The one or more surfactants used in the second solvent component ii) of the present invention are preferably selected to enhance the ability of the electrolyte composition to wet the separator (particularly a polyolefin separator) of the battery which, in turn, advantageously promotes a longer battery cycle life. Such surfactants are ideally one or more selected from:
Most preferably, the second solvent component ii) of the electrolyte composition of the present invention contains one or more non-ionic block copolymer-containing surfactants.
The total amount of the one or more surfactants used in an electrolyte composition of the present invention, is >0.5 to ≤10% by weight of the solvent system, preferably ≥0.6 to ≤6% by weight of the solvent system, further preferably ≥0.6 to ≤4 wt %, very preferably ≥0.6 to ≤3 wt % and ideally 0.6 wt % to ≤2.5 wt %, based on the total weight of the electrolyte solvent system used in the electrolyte composition.
In a preferred embodiment, the non-aqueous electrolyte composition according to the present invention further comprises at least one additional compounds which may or may not be a solvent, and is preferably selected from a sulfur-containing compound, a flame retardant compound (such as a polyalkly phosphate-containing compound, preferably a non-fluorinatedpolyalkyl phosphate-containing compound), a diluent (such as a hydrofluoroether-containing compound, preferably a hydrofluoroalkyl ether-containing compound), a glycol diether (also known as a “glyme”), a glycol ether acetate, an ionic liquid, a nitrile-containing compound (such as adiponitrile, glutaronitrile), and any solvent which is capable of promoting the reduction of viscosity of the electrolyte by acting as an inert diluent (such as a hydrofluoroalkyl ether, preferably 1,1,2,2-Tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (HFE or TTE)). For the avoidance of doubt, the at least one additional compound does not include any of organo carbonate-based solvents listed in the first solvent component, or a surfactant as listed in the second solvent component.
Suitable sulfur-containing compounds include cyclic and/or non-cyclic sulfur-containing compounds and are preferably selected to enable the formation of a stable cathode-electrolyte interphase (CEI) on the cathode and/or a stable solid-electrolyte interphase (SEI) on the anode, which in turn leads to advantages such as enhanced 1st cycle coulombic efficiencies and cycle life enhancement. Preferably, the one or more sulfur-based compound is a sulfone, i.e. it has a sulfonyl functional group which is attached to two carbon atoms. The central hexavalent sulfur atom is doubly-bonded to each of two oxygen atoms and has a single bond to each of two carbon atoms. The general formula of such compounds is R—SO2—R′, in which R and R′ may be independently selected (i.e. they may be the same or different from each other) from any straight or branched, substituted or unsubstituted C1 to C6-alkyl group; a straight or branched, substituted or unsubstituted C1 to C6-alkenyl group; a straight or branched, substituted or unsubstituted C1 to C6-alkoxy group; or a substituted or unsubstituted C3-C6-cycloalkyl-, phenyl- or heterocycle-containing group. Further preferably, the sulfur-based compound is selected from a cyclic sulfone, non-cyclic sulfone and a cyclic sultone. Suitable examples include sulfolane ((CH2)4SO2), 3-methyl sulfolane ((CH3)CH(CH2)3SO2),) 1,3-propanediolcyclic sulfate (PCS) also known as 1,3,2-Dioxathiane 2,2-dioxide (DTD or (CH2)3SO4)), trimethyl sulfone, 1-propene 1,3-sultone ((CH)2CH2SO3), 1,3-propane sultone (CH2)3SO3, trimethylanesulfone ((CH2)3SO2) and methyl phenyl sulfone ((CH3)(C6H5)SO2). One or more cyclic sulfones and/or cyclic sultones and/or 1,3-propanediolcyclic sulfate are particularly preferred. Highly preferably the sulfur-containing compound used in the solvent system does not comprise a sulfoxide-containing compound, a sulfite-containing compound or a sulfide-containing compound, and ideally, the electrolyte composition does not comprise a sulfoxide-containing compound, a sulfite-containing compound or a sulfide-containing compound.
When used in an electrolyte composition of the present invention, the total amount of the one or more sulfur-based compounds is preferably ≤80 wt %, further preferably ≤60 wt %, ideally 0 wt % to ≤60 wt % and most ideally 1 wt % to ≤55 wt %, based on the total weight of the solvent system used in the electrolyte composition.
Suitable optional flame retardant materials include one or more polyalkylphosphate-containing compounds which, when present in the electrolyte composition of the present invention, advantageously not only produce batteries with reduced flammability, but also their use has been discovered to significantly enhance battery cycle life, as well as promoting favourable separator wetting, particularly in the case of polyolefin separator wetting, as compared against conventional organo carbonate electrolyte which lacks a polyalkyl phosphate-containing compound.
Suitable polyalkyl phosphates include trialkyl phosphates and particularly preferably trimethyl phosphate and/or triethyl phosphate.
When used in an electrolyte composition of the present invention, the total amount of the one or more polyalkyl phosphate-containing compounds is preferably at least 1 wt % to 90 wt %, further preferably at least 2 wt % to 70 wt %, more preferably at least 5 wt % to 70 wt % and ideally greater than 10 wt % to 70 wt %, of the total weight of the solvent system used in the electrolyte composition. In some embodiments, an amount of <5 wt % of the one or more polyalkyl phosphate-containing compounds is found to be effective to reduce flammability and/or to enhance cycle life and/or to increase wettability of the separator.
Suitable glymes may be selected from: ethylene glycol dimethyl ether (monoglyme CH3-O—CH2 CH2-O—CH3); diethylene glycol dimethyl ether (diglyme CH3-O—(CH2-O)2-CH3); triethylene glycol dimethyl ether (triglyme, CH3-O—(CH2-O)3-CH3); tetraethylene glycol dimethyl ether (tetraglyme, CH3-O—(CH2-O)4-CH3); ethylene glycol diethyl ether (ethyl glyme, CH3CH2-O—CH2 CH2-O—CH2 CH3); diethylene glycol diethyl ether (ethyl diglyme, CH3CH2-O—(CH2-O)2-CH2 CH3); diethylene glycol dibutyl ether (butyl diglyme, CH3CH2 CH2-O—(CH2-O)2-CH2 CH3); poly(ethylene glycol) dimethyl ether (polyglyme, CH3-O—(CH2-O)n-CH3); and dipropylene glycol dimethyl ether (proglyme, CH3-O—(CH2 CHCH3-O)2-CH3).
Diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethyl ether (triglyme) and tetraethylene glycol dimethyl ether (tetraglyme) are especially preferred.
Suitable one or more glycol ether acetates may be selected from: propylene glycol methyl ether acetate; dipropylene glycol methyl ether acetate; ethylene glycol monobutyl ether acetate; ethylene glycol monomethyl ether acetate; ethylene glycol monoethyl ether acetate; diethylene glycol monobutyl ether acetate; diethylene glycol monoethyl ether acetate; and diethylene glycol monoethyl ether acetate.
Ethylene glycol monomethyl ether acetate and ethylene glycol monoethyl ether acetate are particularly preferred.
Suitable ionic liquids may combine any of the following: cations based on imidazolium, pyrrolidinium, and ammonium families, and anions based on tetrafluoroborate (BF4−), bis(fluorosulfonyl)imide (FSI), tri(fluoromethanesulfonyl)imide (TFSI), perchlorate (ClO4−), hexafluorophaphate (PF6−), and ferric perchlorate (FeCl4−).
The amount of the one or more additional compounds used in the electrolyte compositions of the present invention is preferably in the range 0.1 wt % to 80 wt %, preferably 0.1 wt % to 70 wt %, further preferably 0.2 wt % to 55 wt % and most preferably 0.5 wt % to 50 wt % of the total weight of the solvent system used in the electrolyte composition.
The electrolyte composition according to the present invention may also optionally include one or more performance additives, typically an amount of <15 wt %, preferably <10 wt % and further preferably 0.1 wt % to <5 wt %, of the total weight of the solvent system used in the electrolyte composition, and preferably act at the electrolyte-electrode interface rather than in the bulk of the electrolyte. Such performance additives are useful, particularly in liquid electrolyte systems where insoluble degradation products form a solid passivation layer on the surface of the negative electrode (SEI), to optimise the ion and electron conductivity of the SEI and also to maximise its strength and adhesion to the surface of the negative electrode. Performance additives are also useful to increase the wetting of the surface of the separator, to protect the cell against overcharging events and to act as fluidity enhancers, viscosity reducers, and/or impurity or radical scavengers. Furthermore, such performance additives may render additional benefits to the cell performance such as providing a fail-safe in case the cell overcharges or impart further non-flammability characteristics to the electrolyte.
Suitable performance additives may be selected from: polymerizable additives for promoting overcharge protection such as biphenyl, diphenylamine, dimethoxydiphenylsilane (DDS), 3-chloroanisole (3CA), N-phenylmaleimide, xylene (methyl-substituted benzene) and cyclohexylbenzene; additives for promoting overcharge protection based on redox-shuttle mechanism such as 2,5-di-tert-butyl-1,4 dimethoxybenzene (DDB), 4-tert-butyl-1,2-dimethoxybenzene (TDB), 1,4 bis(trimethylsilyl)-2,5-dimethoxybenzene (BTMSDB) and 1,4-bis(2-methoxyethoxy)-2,5 di-tert-butylbenzene; additives for imparting further flame retardancy attributes to the electrolyte such as dimethyl methylphosphonate (DMMP), ethoxy-pentafluoro-cyclotriphosphazene (N3P3F5OCH2CH3, EFPN), tri(2,2,2-trifluoroethyl) phosphite and/or tri(2,2,2-trifluoroethyl) phosphate (TFEP), methyl nonafluorobuyl ether (MFE) and silane-Al2O3 nanoparticles; and additives for promoting better high temperature cycling such as succinic anhydride.
The electrolyte compositions of the present invention are preferably employed in a sodium-ion cell. Consequently, in a further aspect, the present invention provides a sodium-ion cell comprising a negative electrode, a positive electrode, and an electrolyte composition according to the present invention described above. Such sodium-ion cells may be used in an energy storage device, for example a battery, a rechargeable battery, an electrochemical device and an electrochromic device.
Ideally, the electrolyte compositions of the present invention are used in sodium-ion cells which employ a carbon-containing material as the active negative (anode) electrode material. Preferably the carbon-containing material is a non-graphitisable carbon-containing material. Such carbon materials include, but are not limited to modified graphitic carbon, non-graphitic carbon, isotropic carbon, partially graphitic carbon, exfoliated graphite, expanded graphite, amorphous carbon, non-crystalline carbon, soft carbon and hard carbon materials. For the avoidance of doubt, “non-graphitisable carbon material”, as used herein, does not include any carbon with a natural graphite structure in a long-range order (prevalent in the bulk of the structure) or a synthesised graphite material with a structure identical to that of natural graphite material. Typical methods of producing non-graphitisable carbon employ heating starting materials, for example, sucrose, corn starch, glucose, organic polymers (e.g. polyacrylonitrile or resorcinol-formaldehyde gel), cellulose, petroleum coke or pitch coke, to about 1200° C. Commercially available non-graphitisable carbon materials are termed as “hard carbon materials” and include those sold under the name “Carbotron™ hard carbon material” from Kureha Corporation, and “Bio-Carbotron™ hard carbon material” from Kuraray Chemical Company and Kureha Corporation.
Ideally, the sodium-ion cells of the present invention uses anode active materials that contain non-graphitisable carbon (e.g. hard carbon) alone, however it is also advantageous to employ materials that combine the non-graphitisable carbon with one or more other materials (as a mixture or as a composite) such as a Na-storable metal or alloy, or a metal or a non-metal in its elemental or compound form. Particularly preferred anode active materials include hard carbon/X materials, where X may be one or more elements such as antimony, tin, phosphorus, sulfur, boron, aluminium, gallium, indium, germanium, lead, arsenic, bismuth, titanium, molybdenum, selenium, tellurium, silicon or carbon. Hard carbon/Sb, hard carbon/Sn, hard carbon/SbxSny and hard carbon/P are all suitable hard carbon-containing materials.
The non-graphitisable carbon materials termed as “expanded” or “exfoliated” graphite, have a structure which is recognizable as being similar to the structure of natural graphite but not identical to it because it is modified so that its carbons exhibit an interlayer spacing in the (001) direction which is greater than 3.35 Angstrom.
It is most preferable that the non-aqueous electrolyte compositions of the present invention are used with a non-graphitisable carbon-containing anode, preferably a hard carbon anode.
In a highly preferred embodiment therefore, the present invention provides a non-aqueous electrolyte composition described above which is suitable for use in a sodium-ion cell which contains an anode comprising non-graphitisable carbon material, preferably a hard carbon anode.
In an alternative example, the electrolyte compositions of the present invention may be used in sodium-ion cells which employ an anode which comprises a sodium insertion material such as a titanate compound (either undoped or doped with small quantities of various other metals and/or Ti substituted with other transition metal and/or O substituted with other halogens), for example Na2Ti3O7; NaHTi3O7; Na2Ti6O13; Na2Li2Ti5O12, K2Ti3O7, K2Ti6O13, TiO2, NaTiO2 and H2Ti3O7. Other sodium insertion materials such as undoped or doped (with small quantities of various other metals and/or halogens) or partially substituted (with any metal, non-metal, metalloid and/or halogen) NaTi2(PO4)3, and TiS2, either in conjunction with, or as an alternative to, a non-graphitisable carbon material.
In a further alternative example, the electrolyte compositions of the present invention may be used in sodium-ion cells which employ an anode which comprises a conversion and/or alloying material and a material that displays conversion and alloying type reaction mechanism to store sodium, for example tin, antimony, molybdenum, phosphorus, sulfur, indium, lead, iron, manganese and germanium, either in elemental form or in compound form, preferably with one or more selected from oxygen, carbon, nitrogen, phosphorus, sulfur, silicon, fluorine, chlorine, bromine and iodine (such as SnO, SnO2, SnF2, SiP2, Fe2O3, Fe3O4, FeS, FeS2, MoS2, MoO3, Sb, Sb2O3, SnSb and SbO) either in conjunction with, or as an alternative to, a non-graphitisable carbon material.
Secondary carbon-containing materials may also be used in combination with the above mentioned anode active materials, interalia, to improve the conductivity of the anode, for example: activated carbon materials, particulate carbon black materials, graphene, carbon nano-tubes and graphite. Example particulate carbon black materials include: “C65™ carbon (also known as Super P™ carbon black) (BET nitrogen surface area 62 m2/g) (available from Timcal Limited) although other carbon blacks are also available with a BET nitrogen surface area of <900 m2/g, for example “Ensaco 350g™’ which is a carbon black with a BET nitrogen surface area of 770 m2/g (available from Imerys Graphite and Carbon Limited as specialty carbons for rubber compositions). Carbon nano-tubes have a BET nitrogen surface area of 100-1000 m2/g, graphene around 2630 m2/g and activated carbon materials have a BET nitrogen surface area of >3000 m2/g.
The present invention also provides a sodium-ion cell comprising a negative electrode, a positive electrode and a non-aqueous electrolyte composition according to the present invention as described above, wherein the negative electrode comprises a non-graphitisable carbon material, preferably a hard carbon material. Such sodium-ion cells may be used in an energy storage device, for example a battery, a rechargeable battery, an electrochemical device, and an electrochromic device. The electrolyte compositions according to the present invention find particular utility when employed in a sodium-ion cell which comprises a polyolefin separator.
The sodium-ion secondary cells according to the present invention may comprise any cathode electrode active material, however preferred cathode electrode active materials are of the general formula:
A1±δM1VM2WM3XM4YM5ZO2−c
wherein
A is one or more alkali metals selected from sodium, potassium and lithium;
M1 comprises one or more redox active metals in oxidation state +2, preferably selected from the group consisting of nickel, copper, cobalt and manganese;
M2 comprises a metal in oxidation state greater than 0 to less than or equal to +4;
M3 comprises a metal in oxidation state +2;
M4 comprises a metal in oxidation state greater than 0 to less than or equal to +4;
M5 comprises a metal in oxidation state +3;
0≤δ≤1;
V is >0;
W is ≥0;
X is ≥0;
Y is ≥0;
at least one of W and Y is >0
Z is ≥0;
C is in the range 0≤c<2
wherein V, W, X, Y, Z and C are chosen to maintain electrochemical neutrality.
Ideally, metal M2 comprises one or more transition metals, and is preferably selected from manganese, titanium and zirconium; M3 is preferably one or more selected from magnesium, calcium, copper, tin, zinc and cobalt; M4 comprises one or more transition metals, preferably selected from manganese, titanium and zirconium; and M5 is preferably one or more selected from aluminium, iron, cobalt, tin, molybdenum, chromium, vanadium, scandium and yttrium.
A particularly preferred cathode electrode active material will be a nickelate-based material.
An cathode electrode active material with any crystalline structure may be used, however, preferably the structure will be O3 or P2 or a derivative thereof, but, specifically, it is also possible that the cathode electrode active material will comprise a mixture of phases, i.e. it will have a non-uniform structure composed of several different crystalline forms. For example, the cathode active material will comprise a compound with the general formula detailed above in a mixture of O3 and P2 phases. The ratio of O3:P2 phases is preferably 1 to 99:99 to 1.
As demonstrated in the specific examples presented below, the electrolyte compositions of the present invention provide surprising and significant advantages, particularly regarding enhanced cycle life, and enhanced capacity and non-flammability, when compared against electrolyte compositions which fall outside the composition of the present invention, i.e compared against compounds that do not include one or more organo carbonate-based solvents in combination with one or more surfactants as described above.
In an alternative aspect, there is provided a non-aqueous electrolyte composition comprising: a) one or more sodium-containing salts; and b) a solvent system which comprises i) a first solvent component which comprises one or more organo carbonate-based solvents, and a ii) second solvent component which comprises at least one compound selected from, a glyme, a glycol ether acetate, an ionic liquid, a nitrile-containing compound, a diluent (such as a hydrofluoroalkyl ether diluent), a sulfur-containing compound, and a flame retardant (such as a polyalkly phosphate-containing compound); optionally in combination with one or more surfactants. Preferably, suitable glymes, glycol ether acetates, ionic liquids, nitrile-containing compounds, diluents, hydrofluoroalkyl ether diluents, sulfur-containing compounds, flame retardants, polyalkly phosphate-containing compounds and surfactants, are as described above.
In further alternative aspects, there is provided a sodium-ion cell and an energy storage device which comprise a non-aqueous electrolyte composition according to the alternative aspect described above.
The present invention will now be described with reference to the following figures in which:
The electrolyte compositions under investigation were prepared using the following general procedure: appropriate amounts of the solvents for the desired solvent system were weighed out in an argon-filled glove box and added to a brown glass bottle. To dry the formed solvent thoroughly, 4 Å molecular sieves (Sigma-Aldrich) were added and allowed to dry the solvent mixture for at least 24 h. Afterwards, this solvent mixture was transferred to another brown glass bottle which contained a magnetic stir bar while still in the glove box. The correct amount of one or more sodium-containing compounds was then weighed out in the argon-filled glovebox and slowly added to the solvent mixture under continued stirring. The electrolyte mixture was stirred until the salt was visually determined to have dissolved on a magnetic stirrer plate. The electrolyte mixture was then removed from the stirrer plate and stored and used in the argon-filled glovebox.
The precise composition of each of the electrolyte compositions investigated, is detailed in Table 1 below:
EC=Ethylene carbonate, DEC=Diethyl carbonate, PC=Propylene carbonate, TMP=Trimethyl phosphate, PCS=1,3-propanediolcyclic sulfate, P123=Poloxamer (Pluronic) P123, F127=Poloxamer (Pluronic) F127, TEP=Triethyl phosphate, HFE=1,1,2,2-Tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, TMSB=Tris(trimethylsilyl) borate, D2=1,1,2,2-Tetrafluoroethyl 2,2,2-trifluoroethyl ether, Tetraglyme=Tetraethylene glycol dimethyl ether.
Please note that in the above Table 1, the wt % of the various solvents and/or additives are mentioned with respect to the total solvent weight (they do not account for the weight(s) of the one or more sodium containing salts).
Cell Construction
Generic Procedure to Make a Hard Carbon Na-Ion Cell
Sodium ion pouch cells were built using active material electrodes, separator and electrolyte; aluminium tabs were connected to each of the electrodes and the cell was encased in a polymer-coated aluminium pouch.
The positive (cathode) electrode is prepared by solvent-casting a slurry of the active material, conductive carbon, binder and solvent. The conductive carbon used is C65 (Imerys). PVdF co-polymer (e.g. W #7500 from Kureha Chemicals) is used as the binder, and NMP is employed as the solvent. The slurry is then cast onto carbon-coated aluminium foil and dried at about 80° C. The electrode film contains the following components, expressed in percent by weight: 89% active material, 5% C65 carbon, and 6% W #7500 binder.
The hard carbon negative (anode) electrode is prepared by solvent-casting a slurry of the hard carbon active material (Kuranode Type 1, supplied by Kureha), conductive carbon, binder and solvent. The conductive carbon used is C65 (Imerys). PVdF co-polymer (e.g. W #9300, Kureha Chemicals) is used as the binder, and NMP is employed as the solvent. The slurry is then cast onto carbon-coated aluminium foil and dried at about 80° C. The electrode film contains the following components, expressed in percent by weight: 88% active material, 3% C65 carbon, and 9% W #9300 binder.
Cell Testing
The cells are tested as follows using Constant Current (Galvanostatic) Cycling techniques.
The cell is cycled at a given current density between pre-set voltage limits (at top of charge, a constant voltage step is administered). A commercial battery cycler from Maccor Inc. (Tulsa, Okla., USA) is used). On charge, alkali ions are inserted into the anode material. During discharge, alkali ions are extracted from the anode and re-inserted into the cathode active material.
Two block copolymer non-ionic surfactant materials were investigated, Poloxamer (Pluronic) P123 and Poloxamer (Pluronic) F127. Electrolyte compositions TEL 3, TEL 3a, TEL 3b, TEL 4, TEL 4a, TEL 4b, TEL 9, TEL 10, as indicated in Table 1 above, were prepared, and the up-take of each of these samples by a separator (Asahi ND525) and by a hard carbon electrode (Kuraray's Type 1 hard carbon) was measured and the results compared against the uptake of a non-surfactant-containing control electrolyte composition, sample TEL 0.
The electrolyte uptake values were calculated using:
Where, M0 was the weight of the bare separator (or hard carbon electrode) and M was the weight of the separator (or hard carbon electrode) after it was allowed to soak the electrolyte. For this purpose, all separators/electrodes were submerged in a pool of electrolyte for 1 h. The standard deviation is calculated from 3 different measurements:
As the results in
These are statistically significant values and Example 2 below was used to test whether or not improved up-take translates into better electrochemical performance.
Using the generic method described above, each of the sample electrolyte compositions TEL 3, TEL 3a, TEL 4, TEL 4a, and the control electrolyte composition TEL 0, were employed to prepare 10 mAh test cells. The electrochemical performance of each test cell was then investigated by cycling between 4.2 and 1.0 V. From the results shown in
From
Taken in combination, the results shown in
1,3-Propanediolcyclic sulfate (PCS) was added to the control electrolyte composition TEL 0 to produce electrolyte composition samples, namely: TEL 19 (1 wt % PCS), TEL 19a (2 wt % PCS) and TEL 19b (3 wt % PCS), not according to the present invention
As shown in
It was also observed that PCS-containing samples gave a 1st cycle coulombic efficiency which increased from 77% (for the cell which used the TEL 0 electrolyte composition) to 80% (for cells which used the TEL 19, 19a and 19b electrolyte compositions). This increase in performance could explain the higher capacities observed for PCS-based electrolytes as lesser amount of Na+ from the cathode was irreversibly consumed.
The best cycling performance was observed for the cell which used an electrolyte composition which contained 1 wt % PCS additive (sample TEL 19).
The long-term cycling performance was investigated for 10 mAh cells which contained either the electrolyte composition of TEL 0 (control), or the composition of TEL 0 with 1% PCS (sample TEL 19), or the composition of TEL 0 and 1 wt % PCS+1 wt % P123 (sample TEL 24), or the composition of TEL 0 and 0.5 wt % PCS+0.5 wt % P123 (sample TEL 24a).
From
The long-term cycling performance was further investigated for larger-scale 0.6 Ah cells which contained either the electrolyte composition of TEL 0 (control), or the compositions of the present invention: TEL 0 with 1% PCS (sample TEL 19), or the composition of TEL 0 and 1 wt % PCS+1 wt % P123 (sample TEL 24).
As shown in
Advantageously, it was also found that the delivered capacity could be further enhanced by increasing the C/A mass balance from 2.27 to 2.61 and 2.95.
The long-term cycling performance was investigated, between 4.2 to 1 V for 10 mAh, for cells which used an electrolyte composition that contained either pure propylene carbonate (control sample TEL 36), or a mixture of propylene carbonate and 1 wt % PCS and 1 wt % P123, (sample TEL 37), or the control electrolyte composition (control sample TEL 0).
As the results in
The long-term cycling performance between 4.2 to 1 V was investigated for 10 mAh cells which used an electrolyte composition with a first solvent component containing either ethylene carbonate and diethyl carbonate (i.e. no propylene carbonate) (control sample TEL 38), or ethylene carbonate and diethyl carbonate (i.e. no propylene carbonate) with 1 wt % PCS and 1 wt % P123, (sample TEL 38a), or the control electrolyte composition (sample TEL 0).
As shown in
The long-term cycling performance between 3.75-2.5 V was also investigated for two electrode (2E) 10 mAh cells which used an electrolyte composition with a first solvent component containing either ethylene carbonate and diethyl carbonate (i.e. no propylene carbonate) and TMP (1:1 wt/wt) (sample TEL 39), not according to the present invention, or ethylene carbonate and diethyl carbonate (i.e. no propylene carbonate) and TMP (1:1 wt/wt) (sample TEL 39) with 1 wt % PCS and 1 wt % P123, (sample TEL 39a according to the present invention), or the control electrolyte composition (sample TEL 0).
As the results in
The long-term cycling performance was investigated between 4.2 to 1 V for 10 mAh cells which used an electrolyte composition which contained either a 1:1 wt/wt ratio of organo carbonate:sulfolane with 1 wt % of the surfactant P123 (sample TEL 17), or a mixture of 5 wt % sulfolane and 1 wt % surfactant P123 (sample TEL 17b), or 1 wt % 1,3-propanediolcyclic sulfate (sample TEL 19, not according to the present invention) or 1 wt % 1-propene 1,3-sultone (sample 34), or 1 wt % 1,3 propanesultone (sample TEL 35, not according to the present invention), or the control electrolyte composition (control sample TEL 0).
As the results in
Four different electrolyte compositions were prepared, one contained the control electrolyte composition TEL 0, another contained the control electrolyte composition TEL 0+, another contained a 1:1 wt/wt mixture of (EC:DEC:PC=1:2:1 wt/wt):trimethyl phosphate (sample TEL 2), and the fourth contained a 1:1 wt/wt mixture of (EC:DEC:PC=1:3:1 wt/wt):trimethyl phosphate (sample TEL2a). The same amount of each electrolyte composition was then directly lit on fire with a lighter and the time which elapsed between the moment the electrolyte composition caught fire and the moment it completely self-extinguished was recorded and used to calculate the “Self-Extinguishing Time” (SET). Each of the four electrolyte compositions were tested three times and the results used to produce an average SET for each composition. A SET on <6 s/g correlates to a non-flammable electrolyte composition. The SETs for each of the electrolyte compositions tested are detailed in Table 2 below:
It is readily apparent from the results shown in Table 2 that the trimethyl phosphate (TMP)-containing electrolyte compositions samples TEL 2 and TEL 2a, are non-flammable and are therefore highly advantageous over non-TMP-containing compositions.
Samples of electrolyte compositions, TEL 0, TEL 1a, TEL 2, TEL 2a, TEL 9 and TEL 10 (the compositions as detailed in Table 1 above) were prepared and their up-take by an Ashai ND525 separator was measured.
As shown in
The long-term cycling performance (between 4.2-1 V) for 0.5 Ah cells with C/A mass balances 1.52, 1.94 and 2.26 and the electrolyte composition TEL 2 (a combination of electrolyte composition TEL 0 with trimethyl phosphate (TMP) in a solvent ratio of 1:1 wt/wt).
As shown in
As seen from
The 1st cycle coulombic efficiency was investigated between 4.2 to 1 V for three-electrode (3E) full cells which used an electrolyte compositions TEL 2 and TEL 23 and the results were compared against those for a similar cell which used the control TEL 0 electrolyte composition.
As
2E 10 mAh cells were prepared using electrolyte compositions TEL 2, TEL 14b, TEL 14c, TEL 17 and TEL 23, and results for the long-term cycling performance (between 3.75-2.5 V) were obtained and compared against cycling results of a similar cell using the control electrolyte composition TEL 0.
As mentioned above, a 3.75-2.5 V window simulates the voltage profile for Starter-Ignition-Lighting (SLI) batteries currently used in automobiles, therefore is highly advantageous that the results shown in
Thus, it is confirmed that NaBF4 may be used in place of NaPF6 (sample TEL 14b), and that NaBF4-based TMP electrolytes are also compatible with 1 wt % PCS and 1 wt % P123 additives (TEL 14c). Significant cost savings and greater flexibility in the choice of electrolyte salts are expected to be provided by the electrolyte compositions of the present invention.
Charge acceptance is commercially significant for any battery technology as it dictates how quickly a battery can be safely charged. Fast charging increases likelihood of alkali metal plating on the anode in alkali-ion cells (especially if low voltage anodes are used) and this can lead to explosions—a fact that is well known to anyone skilled in the art. Whilst it is generally known in the lithium-ion battery prior art that increasing electrolyte conductivity by increasing salt concentration can influence charge acceptance, such demonstrations have not been shown before in sodium-ion systems.
These results demonstrate that embodiments of the present invention can lead to superior charge acceptance of sodium-ion cells even at fast rates and at varying voltage windows. From these experiments, it should be obvious to anyone skilled in the art that by varying the C/A mass balance and voltage window of the cell, even faster charge acceptance can be achievable with such electrolytes. Furthermore, it will be obvious to anyone skilled in the art that alternate modes of charging such as tapered charging (ultra-fast charge rates such as 4C or 10C to a set % SOC such as 60 or 80% SOC before reducing the charge rate to lower values such as 2C or C/5 to 80, 90 or 100% SOC) would also be possible with the results disclosed herein.
In order to enhance electrolyte performance by decreasing total electrolyte viscosity,
Experiment 15a extends Experiment 14 in the domain of sodium-ion electrolytes containing majority PC as the solvent, in line with embodiments of the present invention.
Experiment 15b extends Experiment 15a into the domain of large-scale sodium-ion ion pouch cells of the type that would actually be used in commercial applications, such as 1 Ah pouch cells.
The reason for the non-cycling at all for the PC-based TEL 37a electrolyte at the 1 Ah scale is the well-known relatively higher viscosity of PC solvent—as demonstrated herein, this higher viscosity can still enable small-scale cells (such as 10 mAh pouch cells or Swagelok or coin-cells predominantly used in prior arts) to function effectively but the high viscosity of PC-only electrolytes imposes insurmountable barriers when used in actual commercial-scale cells. In particular, the high viscosity of PC-only electrolytes leads to very poor electrolyte wettability on polyolefin separators that are used in commercial batteries. The only other way to enable such PC-only electrolytes to function in large-scale cells would be to use glass fibre separators that typically hold a large amount of electrolyte per unit weight—this might overcome the wettability issues but would impose a severe weight (and cost) penalty on the resulting battery due to the usage of much more amount of electrolyte in a similar capacity-rated cell vs the case of a battery that uses polyolefin separators. Therefore, the resulting energy density of the battery would be significantly lower—this is obviously not appealing commercially. It should, hence, be appreciated by anyone skilled in the art that prior arts which showed good performance of PC-only electrolytes (with/without small amounts of additives) in small-scale cells are not relevant commercially at all and that the burden of proof of commercial viability of such PC-only electrolytes should lie on the particular prior art to demonstrate using commercially-relevant materials and methodologies at commercial scales (large-scale cells). Keeping this in mind,
Experiment 16 provides another example of a diluent that can be used to enable PC-dominant electrolytes to function in large-scale commercial sodium-ion cells. In this example, cycle lives at a fast ±1C were investigated at 30° C. within the 4.2-1 V window using either HFE-based TEL 43e (FPC200116) or the D2-based TEL 58a (FPC191023) electrolyte, as shown in
Based on these results, it should be understood that other types of diluent solvents can also elicit such great performance form PC-dominant sodium-ion electrolytes. Some examples include, but not limited to the following: other hydrofluoroethers, low viscosity ethers or carbonate-ester or polyalkyl solvents with appropriate permittivity, inertness/stability and coordination properties such that they do not significantly influence solvation of the salt with PC solvent while still influencing the physical properties of the resulting electrolyte.
Experiment 17 provides examples on using other types of electrolyte additives in PC+diluent-based electrolytes in realistic large-scale 0.1 Ah pouch cells.
The move to PC-dominant electrolytes can have another positive effect apart from enhanced cycling stabilities—that of higher oxidative stability of the electrolyte in general which would prevent less amount of gas generation in such cells. It is well known in the prior art, especially for lithium-ion batteries, that EC has excellent reductive stability (at low voltages) but poor oxidative stability (at high voltages): EC is well known to decompose on the surface of the graphite anode used in lithium-ion batteries to form stable a SEI. While it is known that PC has better oxidative stability in lithium-ion systems, PC-only or PC-dominant electrolytes cannot be used in lithium-ion batteries as PC exfoliates the graphite anode resulting in significantly poor cycling performance of such batteries. On the other hand, as mentioned previously and demonstrated here in numerous examples, PC-only or PC-dominant based electrolytes can actually enable more superior cycling of sodium-ion cells than state-of-the-art EC-based sodium-ion electrolytes.
One of the reasons why this occurs will be explained here in Experiment 17 which quantitatively investigates the oxidative stability of four sodium-ion electrolytes through Linear Sweep Voltammetry (LSV) loops. In these LSV experiments, 3E pouch cells were fabricated with the same cathode and anode footprint as used in 10 mAh pouch cells shown at several places in this disclosure, but with one main difference: both the cathode and anode were just pure Al current collector foils. In the LSV experiment, these cells were filled with the concerned electrolyte and subjected to a slow 2 mV/s scan rate to different upper cut-off potentials, before being cycled down to 2.5 V and then beginning the process again. Hence, such an experiment is a cross between a cyclic voltammetry and LSV experiment. The advantage of such a modified LSV experiment is that one can accurately calculate the charge passed due to electrolyte oxidation on Al current collector at different V windows, such as 3.8-4.2 V, 4.2-4.4 V, 4.4-5 V and 5-8 V. Since the electrodes are just pure Al current collector foils with no active material, all charge passed should be correlated to electrolyte oxidation in theory. In such an experiment, the lower the charge consumed, the lesser would the electrolyte be prone to oxidation and hence, the better should the electrolyte's performance be when used in actual sodium-ion cells charged to around those voltages. Even though most cells discussed in this document were cycled to 4 V or 4.2 V, it should be appreciated by anyone skilled in the art that electrolyte oxidation is catalysed by the presence of cathode active material and conductive carbon present in the cathode and that the modified LSV experiment represents an ideal-case scenario which is not attainable in real-world settings. As such, it can be understood by anyone skilled in the art that the charge consumed due to electrolyte oxidation at higher potential bands in this modified LSV experiment (such as 4.4-5 V and 5-8 V) might be more representative to what the electrolyte actually experiences in terms of oxidation in actual sodium-ion cells discussed in this disclosure (which were generally charged to 4 V or 4.2 V) due to the catalytic effects of various components.
With this stated,
In
Table 3 provides quantitative values to the amount of charge passed, Q (in units of mC), in the process of oxidation of the different electrolytes in the various voltage ranges as shown. From Table 3, it becomes obvious that TEL 24, TEL 37e and TEL 36 resulted in significantly lesser amount of charge consumed due to electrolyte oxidation than the values seen for the control TEL 0 electrolyte. For example, within the 4.4-5 V range, TEL 24 consumed 30.4% lesser charge with respect to the charge consumed for TEL 0 indicating the positive effects of the PCS and P123 additives in shielding the base TEL 0 electrolyte from electrolyte oxidation (TEL 24 is simply TEL 0 with PCS and P123 additives). Shifting to PC-dominant electrolytes, TEL 36, being pure PC-based with no additives, resulted in the lowest amount of charge passed within this voltage range (61.4% lower than the TEL 0 sample) while TEL 37e, being based on PCS, P123 and EC additives, consumed 28.6% lesser charge than TEL 0. However, PC-only or PC-dominant electrolytes also benefit from additives as disclosed in this invention and this can be appreciated by considering the total amount of charge passed across 3.8-8 V range: in this full voltage range, TEL 37e actually displayed the lowest electrolyte oxidation and hence, it becomes obvious that additives such as PCS, P123 and EC help in making the PC-based electrolyte more resistant to oxidation. As also seen from
Such oxidatively stable electrolytes would also ensure that resultant sodium-ion cells do not exhibit significant gassing—as is well known to those skilled in the art of high voltage battery systems, the high voltages used for such batteries can lead to significant gas generation due to electrolyte oxidation and that this gassing can then have seriously detrimental knock-on effects such as capacity deterioration, metal plating due to uneven pressure distribution in the sealed cells or in the worst case, serious explosions. Hence, such LSV experiments conclusively demonstrate that the additives mentioned in this disclosure can significantly enhance the oxidative stability of sodium-ion electrolytes, irrespective of the solvent system used.
The results contained in Experiments 15-18 also demonstrate, for the first time, that PC-dominant based electrolytes, with the right additives and diluting co-solvents as disclosed herein, can function effectively with cycle lives much higher than current state-of-the-art electrolytes in large-scale commercial sodium-ion cells at low temperatures—these results were previously thought to be not achievable.
The flammability of commercial lithium-ion batteries has been well documented in recent years—there have been various reports on lithium-ion batteries catching fire and exploding in various applications such as different types of consumer electronics or electric vehicles. As is well known to those skilled in the art, one of the major causes for these fires/explosions in lithium-ion batteries is the high flammability of the liquid electrolytes used in them. Simply put, such commercial lithium-ion batteries use electrolytes with low flash points. The flash point is not an inherent physico-chemical property of a liquid, but varies with the type of measurement technique and instrument used. As is well known in this field and described in various prior arts such as in Hess et al. (Journal of The Electrochemical Society, 162 (2) A3084-A3097 (2015)), standardised closed-cup flash point testing methods such as the Abel method (for liquids demonstrating a flash point between −30 to 70° C.) and Pensky-Martens method (for liquids demonstrating a flash point>40° C.) generally deliver the most accurate results in terms of flash point testing. Hence, herein, these methods are used as well and the results of the flash point measurements shown in Table 4 below.
Referring to Table 4, the first entry is that of a typical commercial lithium-ion electrolyte, LP 30 (control sample). As can be seen, a flash point of 32° C. was obtained for LP 30 indicating that it is a flammable liquid and consistent with prior reports, such as Hess et al. Furthermore, two other control sodium-ion control electrolytes (TEL 0+ and 151) were also found to be flammable. The control TEL 0 electrolyte demonstrated a higher flash point of 38.5° C. indicating lower flammability than amongst other control samples. As TEL 24b is based on the same solvent system as the control TEL 0 just with PCS and P123 additives, it also delivered the same flash point value of 38.5° C. indicating that these PCS and P123 additives do not affect the flash point. TEL 14d, being based on the non-flammable TMP solvent, demonstrated a higher flash point of 44° C. indicating its superior thermal stability. Furthermore, as already demonstrated in Experiment 7, TMP-based electrolytes self-extinguish very rapidly even if it catches fire indicating that such TMP-based electrolytes are essentially non-flammable irrespective of the flash point.
As expected, the PC-dominant (TEL 43c, TEL 43e and TEL 58a) or PC+TMP-based (TEL 49d) electrolytes were found to be extremely thermally stable: in fact, no flash points were measured until 120° C. whereupon the tests were ceased due to significant emission of black smoke. There results indicate that these PC-dominant electrolytes would rather decompose as opposed to catch fire. From a battery safety viewpoint, this is very appealing in case there are any catastrophic events which trigger thermal runaway—an electrolyte simply decomposing as opposed to catching fire would prevent explosions from taking place. As another attractive point, increasing the diluent wt % form 20 wt % (for TEL 43e) to 40 wt % (for TEL 43c) did not affect thermal behaviour of the electrolyte. Higher proportion of the diluent in an electrolyte blend would further decrease its viscosity and enhance electrochemical performance without (as the flash point results show) affecting thermal stability. This is obviously a huge benefit from a commercial viewpoint.
In summary, it has been demonstrated that various embodiments of the present invention lead to significantly more thermally stable electrolytes as well apart from enhanced electrochemical performance.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1907612.4 | May 2019 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2020/051317 | 5/29/2020 | WO | 00 |
