WO2024127150A1 - Liquid electrolyte - Google Patents

Liquid electrolyte Download PDF

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Publication number
WO2024127150A1
WO2024127150A1 PCT/IB2023/062194 IB2023062194W WO2024127150A1 WO 2024127150 A1 WO2024127150 A1 WO 2024127150A1 IB 2023062194 W IB2023062194 W IB 2023062194W WO 2024127150 A1 WO2024127150 A1 WO 2024127150A1
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Prior art keywords
alkali metal
liquid electrolyte
water
lithium
salt
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PCT/IB2023/062194
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French (fr)
Inventor
Yu Hu
Matthew Roberts
Liyu JIN
Laís DIAS FERREIRA
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Dyson Technology Limited
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Publication of WO2024127150A1 publication Critical patent/WO2024127150A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
    • H01M10/0564—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of organic materials only
    • H01M10/0566—Liquid materials
    • H01M10/0568—Liquid materials characterised by the solutes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/054—Accumulators with insertion or intercalation of metals other than lithium, e.g. with magnesium or aluminium
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
    • H01M10/0564—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of organic materials only
    • H01M10/0566—Liquid materials
    • H01M10/0567—Liquid materials characterised by the additives
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
    • H01M10/0564—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of organic materials only
    • H01M10/0566—Liquid materials
    • H01M10/0569—Liquid materials characterised by the solvents
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M2300/00—Electrolytes
    • H01M2300/0017—Non-aqueous electrolytes
    • H01M2300/002—Inorganic electrolyte
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M2300/00—Electrolytes
    • H01M2300/0017—Non-aqueous electrolytes
    • H01M2300/0025—Organic electrolyte
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M2300/00—Electrolytes
    • H01M2300/0017—Non-aqueous electrolytes
    • H01M2300/0025—Organic electrolyte
    • H01M2300/0028—Organic electrolyte characterised by the solvent
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Definitions

  • the present invention relates to a liquid electrolyte for a lithium-ion secondary cell.
  • the invention also relates to electrochemical cells and energy storage devices comprising such liquid electrolyte, and methods of preparing the liquid electrolyte. Also disclosed are compositions for use as water-insensitive liquid electrolyte.
  • Lithium-ion secondary batteries are the leading battery technology currently used in applications from small personal devices to electric vehicles. Lithium-ion batteries are favoured for their high energy density and long cycle life, among other benefits. They contain a plurality of lithium-ion secondary cells.
  • LiPFe lithium hexafluorophosphate
  • This lithium salt is thermally stable up to around 106.9 °C in dry inert atmosphere, after which it decomposes into solid LiF and gaseous products including PF5.
  • the decomposition temperature is lowered, and a thermal reaction occurs between LiPFs and water vapour to form POF3 and HF as gaseous products.
  • These gaseous products pose a serious health and safety hazard. Therefore, moisture and water vapour are tightly controlled at the ppm level in electrolyte and cell manufacture.
  • current lithium-ion secondary battery manufacture uses low humidity dry room facilities to prevent such decomposition of LiPFe salt. These low humidity dry room facilities, although improving safety, have a high financial cost.
  • manufacture of lithium-ion secondary batteries must balance good cell performance of the electrolyte with safety and cost-effectiveness.
  • liquid electrolytes containing lithium salts other than LiPF 6 can tolerate the presence of high levels of water without compromising performance.
  • the present invention is based on this finding.
  • compositions which is free of LiPF 6 for use as a water-insensitive liquid electrolyte in an alkali metal secondary cell contain water in an amount which is not tolerated by corresponding conventional electrolytes containing LiPFe.
  • the inventors have found that the present electrolytes show promising compatibility with conventional electrodes, good cell electrochemical and mechanical behaviours such as long cycle life, thermal stability, shelf-life storage, as well as cell safety.
  • liquid electrolytes are compatible with processes for forming solid polymer batteries (including, for example, a gel battery having one or more of at least one gel electrode, and/or a gel separator) as well as conventional batteries, without requiring superconcentrated electrolytes or electrolytes based on linear carbonate solvents such as are disclosed in US 2019/0123390 A1 .
  • LiPFe is not usually compatible with processes for forming solid polymer batteries because it is not stable above temperatures around 60°C.
  • a first aspect of the invention concerns a liquid electrolyte for a lithium ion secondary cell, the electrolyte comprising a non-aqueous solvent, a lithium salt other than lithium hexafluorophosphate (LiPFe), and water present in an amount of between 20 and 5000 ppm based on the total amount of electrolyte, said electrolyte being free of lithium hexafluorophosphate.
  • LiPFe lithium hexafluorophosphate
  • ppm is ppm by weight.
  • a second aspect of the invention relates to an electrochemical cell comprising a liquid electrolyte according to the first aspect.
  • a third aspect of the invention relates to an electrochemical energy storage device comprising an electrochemical cell according to the second aspect.
  • a fourth aspect of the invention relates to a method of preparing a liquid electrolyte according to the first aspect, comprising blending a non-aqueous solvent and a lithium salt other than lithium hexafluorophosphate (LiPFe), and introducing water in an amount of between 20 and 5000 ppm based on the total amount of electrolyte.
  • LiPFe lithium hexafluorophosphate
  • a method according to the fourth aspect can be carried out without the need for a low humidity dry room and does not require the use of gloveboxes. In this way, the financial cost is substantially reduced, and there is no compromise with safety because the LiPF 6 is not present to provide such hazard.
  • a fifth aspect of the invention relates to the use of a composition comprising non-aqueous solvent and a lithium salt other than lithium hexafluorophosphate (LiPF 6 ) as a waterinsensitive liquid electrolyte, said composition being free of lithium hexafluorophosphate (LiPF 6 ).
  • an alkali metal salt other than lithium e.g. sodium in a sodium ion secondary cell. Accordingly, further aspects of the invention relate to aspects corresponding to the above first to fifth aspects, except that they relate to other alkali metal salts, such as sodium salts, instead of lithium salts. That is, in each of the following first to fifth further aspects, the alkali metal (M) can be any one of the alkali metals (group I of the periodic table) i.e. lithium, sodium, potassium etc. Preferably, the alkali metal (M) is lithium or sodium, and most preferably lithium as set out above.
  • a first further aspect relates to a liquid electrolyte for an alkali metal (M) ion secondary cell, the electrolyte comprising a non-aqueous solvent, an alkali metal (M) salt other than the alkali metal hexafluorophosphate (MPFe), and water present in an amount of between 20 and 5000 ppm based on the total amount of electrolyte, said electrolyte being free of the alkali metal (M) hexafluorophosphate.
  • M alkali metal
  • M alkali metal
  • the first further aspect thus encompasses a liquid electrolyte for a sodium ion secondary cell, the electrolyte comprising a non-aqueous solvent, a sodium salt other than sodium hexafluorophosphate (NaPFe), and water present in an amount of between 20 and 5000 ppm based on the total amount of electrolyte, said electrolyte being free of sodium hexafluorophosphate.
  • a second further aspect relates to an electrochemical cell comprising a liquid electrolyte of the first further aspect.
  • a third further aspect relates to an electrochemical energy storage device comprising an electrochemical cell according to the second further aspect.
  • a fourth further aspect relates to a method of preparing a liquid electrolyte according to the first further aspect, comprising blending a nonaqueous solvent and an alkali metal (M) salt other than the alkali metal (M) hexafluorophosphate (MPFe), and introducing water in an amount of between 20 and 5000 ppm based on the total amount of electrolyte.
  • the fourth further aspect thus encompasses a method of preparing a liquid electrolyte according to the first further aspect, comprising blending a non-aqueous solvent and a sodium salt other than sodium hexafluorophosphate (NaPF 6 ), and introducing water in an amount of between 20 and 5000 ppm based on the total amount of electrolyte.
  • Figure 1 shows rate vs discharge capacity for electrochemical cells containing liquid electrolyte.
  • Figure 2 shows rate vs discharge capacity for electrochemical cells containing liquid electrolyte.
  • Methods described herein usually employ ambient temperature of a typical laboratory, which is typically between 20 and 30°C and preferably around 25°C, at atmospheric pressure, unless a different condition is defined herein or is more usually employed e.g. for a particular apparatus.
  • liquid electrolyte of the invention can also apply to the compositions of the invention, where appropriate.
  • the present invention generally describes the use of alkali metal (M) hexafluorophosphate- free (MPFe- free), and particularly LiPFe-free, compositions for use as water-insensitive liquid electrolytes.
  • M alkali metal
  • M hexafluorophosphate- free
  • LiPFe-free LiPFe-free compositions for use as water-insensitive liquid electrolytes.
  • M alkali metal
  • M alkali metal
  • the liquid electrolytes are suitable for use in alkali metal (M) secondary batteries. Therefore, they contain alkali metal (M)-containing electrolyte salts, but are free of MPFe.
  • liquid electrolytes are suitable for use in lithium secondary batteries. Therefore, they contain lithium-containing electrolyte salts, but are free of LiPFe which is conventionally used.
  • the alkali metal (M) salt(s) may be any suitable kind. That is, the alkali metal (M) salt(s) may be any kind which is water insensitive.
  • water-insensitive as used herein, means that the electrolyte can function at a practical or useful level in the presence of water in the amounts described herein, such as at least 20 ppm water. In some embodiments, the function of a water-insensitive electrolyte is not compromised - and may be improved - in the presence of water in the amounts described herein. In some embodiments, a water-insensitive electrolyte of the present invention has at least 95% of the function of known electrolytes.
  • this is relative to a corresponding electrolyte which is free or substantially free of water. In some embodiments, this is relative to a corresponding electrolyte which has a conventional amount of water, such as one having less than 20 ppm water.
  • the most preferred lithium salt(s) may be any suitable kind. That is, the lithium salt(s) may be any kind which is water insensitive.
  • water-insensitive as used herein, means that the electrolyte can function at a practical or useful level in the presence of water in the amounts described herein, such as at least 20 ppm water.
  • the function of a water-insensitive electrolyte is not compromised - and may be improved - in the presence of water in the amounts described herein.
  • a water-insensitive electrolyte of the present invention has at least 95% of the function of known electrolytes. In some embodiments, this is relative to a corresponding electrolyte which is free or substantially free of water. In some embodiments, this is relative to a corresponding electrolyte which has a conventional amount of water, such as one having less than 20 ppm water.
  • the function referred to herein includes one or more of, such as all of, the following: energy, cycle life and rate performance.
  • energy storage is usually defined by the first discharge after formation cycling, while cycle life is usually determined by the rate at which the discharge energy fades as a function of cycle number.
  • the liquid electrolyte comprises one or more alkali metal (M) salt(s). In some embodiments, the liquid electrolyte comprises one or two alkali metal (M) salt(s). In some embodiments, the liquid electrolyte comprises two alkali metal (M) salts.
  • the anion of the or each alkali metal (M) salt may be any suitable anion.
  • the or each alkali metal (M) salt is independently selected from: an alkali metal (M) borate salt, an alkali metal (M) imide salt, and an alkali metal (M) imidazolide salt.
  • the liquid electrolyte comprises MDFOB and MFSI.
  • any suitable amount of the or each alkali metal (M) salt may be used in the liquid electrolytes and compositions of the present invention.
  • the or each alkali metal (M) salt may be present in an amount of at least 0.8M, at least 1 ,0M or at least 1 ,2M. In some embodiments, the or each alkali metal (M) salt may be present in an amount of up to 2.5M, up to 2.2M or up to 2.0M.
  • the or each alkali metal (M) salt may be present in an amount of between 0.8M and 2.5M.
  • Other combinations of the above values may be combined to form a suitable range, for example between 0.8M and 2.2M, between 1.0M and 2.2M or between 1.2M and 2.0M.
  • the total amount of alkali metal (M) ion in the electrolyte is between 0.1 and 35 wt%.
  • Other combinations of the preceding values may be combined to form a suitable range, for example between 0.1 and 30 wt%, between 0.5 and 30 wt%, or between 1.0 and 25 wt%.
  • the wt% values here are relative to the total weight of the electrolyte.
  • alkali metal (M) salts such as those described above can tolerate the presence of water in substantially higher quantities than corresponding electrolytes comprising MPFe. Therefore, a liquid electrolyte having comparatively significant amounts of water as described further herein can be used as practical alternatives electrolytes, i.e. water-insensitive electrolytes, to those used in conventional alkali metal (M) ion batteries.
  • M is preferably lithium, sodium or potassium. More preferably, M is lithium or sodium. Most preferably, M is lithium.
  • the liquid electrolyte contains at least one lithium salt, for example a plurality of lithium salts.
  • the liquid electrolyte may further comprise other metal salt(s), including other alkali metal salt(s).
  • they are each a lithium salt i.e. the alkali metal (M) is lithium.
  • the anion of the or each lithium salt may be any suitable anion.
  • the or each lithium salt is independently selected from: a lithium borate salt, a lithium imide salt, and a lithium imidazolide salt.
  • the liquid electrolyte comprises one or more of lithium bis(oxalato)borate, lithium tetrafluoroborate, lithium difluoro(oxalate)borate (LiDFOB), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl) imide (LiFSI) and lithium 2-trifluoromethyl-4,5-dicyanoimidaxolide (LiTDI).
  • the lithium salt is one or more of LiFSI, LiDFOB and LiTDI.
  • the liquid electrolyte comprises LiDFOB and LiFSI.
  • any suitable amount of the or each lithium salt may be used in the liquid electrolytes and compositions of the present invention.
  • the or each lithium salt may be present in an amount of at least 0.8M, at least 1 ,0M or at least 1 ,2M. In some embodiments, the or each lithium salt may be present in an amount of up to 2.5M, up to 2.2M or up to 2.0M.
  • the or each lithium salt may be present in an amount of between 0.8M and 2.5M.
  • Other combinations of the above values may be combined to form a suitable range, for example between 0.8M and 2.2M, between 1.0M and 2.2M or between 1.2M and 2.0M.
  • the total amount of lithium ion in the liquid electrolyte is up to 35 wt%, such as up to 30 wt% or up to 25 wt%.
  • a lower limit of lithium ion in the electrolyte is an amount needed for useful function, but may be at least 0.1 wt%, at least 0.5 wt% or at least 1.0 wt%.
  • the total amount of lithium ion in the electrolyte is between 0.1 and 35 wt%.
  • Other combinations of the preceding values may be combined to form a suitable range, for example between 0.1 and 30 wt%, between 0.5 and 30 wt%, or between 1.0 and 25 wt%.
  • the wt% values here are relative to the total weight of the electrolyte.
  • lithium salts such as those described above can tolerate the presence of water in substantially higher quantities than corresponding electrolytes comprising LiPFe. Therefore, a liquid electrolyte having comparatively significant amounts of water as described further herein can be used as practical alternatives electrolytes, i.e. water-insensitive electrolytes, to those used in conventional lithium ion batteries.
  • compositions of the invention are expected to have some measurable, practical function at the water contents defined herein.
  • the amount of water present in the compositions may be varied according to other desired factors. For example, conductivity and viscosity potential stability may be improved by increasing the water content. In other examples, having a lower water content may improve the ability to form a solid electrolyte interface (SEI).
  • SEI solid electrolyte interface
  • An SEI is a passivating layer that often forms on an anode and to some extent a cathode in lithium ion batteries and advantageously can prevent solvent molecules from further decomposition while allowing lithium ions to continue to move.
  • having a lower water content may also or alternatively improve aluminium corrosion resistance.
  • the water content of the liquid electrolytes of the invention is at least 20 ppm.
  • the water is present in an amount of at least 50 ppm, at least 100 ppm, at least 150 ppm or at least 200 ppm, preferably at least 300 ppm, more preferably at least 400 ppm, and particularly preferably at least 500 ppm.
  • the water content of the liquid electrolytes of the invention is 5000 ppm or less. In some embodiments, the water is present in an amount of up to 3000 ppm, up to 2500 ppm, up to 2000 ppm, preferably up to 1500 ppm and particularly preferably up to 1200 ppm.
  • the water content of the liquid electrolytes of the present invention is between 20 ppm and 5000 ppm.
  • Other combinations of the above values may be combined to form a suitable range, such as between 20 and 2500 ppm, between 50 and 2500 ppm, between 100 and 2000 ppm, or between 200 and 2000 ppm.
  • the water concentration in the liquid electrolyte is at least 0.1 mM, such as at least 0.3mM or at least 0.5mM. In some embodiments, the water concentration in the liquid electrolyte is up to 30mM, such as up to 25mM or up to 20mM.
  • the water concentration in the liquid electrolyte is between 0.1 mM and 30mM.
  • Other combinations of the preceding values may be combined to form a suitable range, such as between 0.1 and 25mM, between 0.3 and 25mM, or between 0.5 and 20mM.
  • the water is deionized water, although other forms are expected to be suitable for achieving the effects of the invention.
  • the water content in the electrolyte may be measured by any suitable method.
  • An example provided herein includes the use of a trace moisture measuring unit.
  • Such a unit may measure moisture by calculation using a measured amount of electricity required for electrolysis (applying the principle of Karl-Fisher reaction to coulometric titration).
  • a series of standard measurements is made and subsequently a sample titrated.
  • An amount of iodine required for titration is generated internally by electrolysis and measured by the unit.
  • An exemplary unit is a CA-310.
  • the liquid electrolyte of the invention contains a non-aqueous solvent in addition to the lithium salt and water.
  • the non-aqueous solvent used in the present invention is a solvent which is typically used in the manufacture of electrodes for alkali metal secondary cells.
  • the non-aqueous solvent should suitably be capable of being blended with the lithium salt and water.
  • Such solvents are typically used in electrolytes for lithium ion batteries and examples will be known to the skilled person.
  • the solvent comprises a carbonate, such as one or more cyclic or linear carbonate compounds. In some embodiments the solvent comprises one or more cyclic carbonate compounds. In some embodiments, the non-aqueous solvent comprises one or more of: an acyclic aprotic or cyclic aprotic carbonate.
  • the non-aqueous solvent comprises one or more of: a C1-C4 acyclic aprotic carbonate or a C4 cyclic aprotic carbonate.
  • the non-aqueous solvent comprises one or more of: ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, fluoropropylene carbonate, or a butyrolactone such as y-butyrolactone.
  • the non-aqueous solvent comprises one or more of: ethyl methyl carbonate, dimethyl carbonate, ethylene carbonate, propylene carbonate, vinylene carbonate, fluoroethylene carbonate, or butyrolactone.
  • the non-aqueous solvent comprises one or more of the above- mentioned non-aqueous solvents, two or more of the above-mentioned non-aqueous solvents, such as three or four of the above-mentioned non-aqueous solvents.
  • the molar ratio may be adjusted as desired.
  • Exemplary considerations include viscosity and ability to dissolve desired salts.
  • a 1:1 molar ratio may be appropriate in some embodiments, or a 1:2 molar ratio or a 1 :3 molar ratio in other embodiments.
  • a 1 :1 :1 molar ratio may be appropriate in some embodiments, or a 1 :2:1 or 2:1 :2 molar ratio in other embodiments.
  • the total amount of non-aqueous solvent in the liquid electrolyte is at least 8.0wt%, such as at least 9.0wt% or at least 10wt%. In some embodiments, the total amount of non-aqueous solvent in the liquid electrolyte is up to 40wt%, such as up to 35wt% or up to 30wt%.
  • the total amount of non-aqueous solvent in the liquid electrolytes is between 8.0 and 40wt%.
  • Other combinations of the preceding values may be combined to form other ranges, such as between 8.0 and 35wt%, between 9.0 and 35wt% and between 10 and 30wt%.
  • wt% is relative to the total amount (100 wt%) of the liquid electrolyte.
  • the non-aqueous solvent concentration in the liquid electrolyte is at least 4.0M, such as at least 4.2M or at least 4.5M. In some embodiments, the non-aqueous solvent concentration in the liquid electrolyte is up to 8.0M, such as up to 7.8M or up to 7.5M.
  • the non-aqueous solvent concentration in the liquid electrolyte is between 4.0 and 8.0M.
  • Other combinations of the preceding values may be combined to form other ranges, such as between 4.0 and 7.8M, between 4.2 and 7.8M or between 4.5 and 7.5M.
  • the molar ratio of non-aqueous solvent: water in the liquid electrolyte is at least 400:3, such as at least 600:3 or at least 800:3. In some embodiments, the molar ratio of non-aqueous solvent: water in the liquid electrolyte is at up to 80,000:1, such as up to 60,000:1 or up to 40,000:1.
  • the molar ratio of non-aqueous solvent: water in the liquid electrolyte is between 400:3 to 80,000:1.
  • Other combinations of the above values may be combined to form other ranges, such as between 400:3 to 60,000:1 , between 400:3 to 40,000:1, or between 800:3 to 40,000:1.
  • the molar ratio of alkali metal (M) salt: water in the liquid electrolyte is at least 80:3, such as at least 200:3 or at least 250:3. In some embodiments, the molar ratio of alkali metal (M) salt: water in the liquid electrolyte is up to 25,000:1, such as up to 15,000:1 or up to 8,000:1.
  • the molar ratio of alkali metal (M) salt: water in the liquid electrolyte is between 80:3 to 25,000:1.
  • Other combinations of the preceding values may be combined to form other ranges, such as between 200:3 and 25,000:1, between 200:3 and 15,000:1, or between 250:3 and 15,000:1.
  • the molar ratio of alkali metal (M) salt: non-aqueous solvent in the liquid electrolyte is at least 5:8, such as at least 1 :2 or at least 5:16. In some embodiments, the molar ratio of alkali metal (M) salt: non aqueous solvent in the liquid electrolyte is up to 1:10, such as up to 1 :8 or up to 1 :5.
  • the molar ratio of alkali metal (M) salt: non-aqueous solvent in the liquid electrolyte is between 5:8 and 1:10.
  • Other combinations of the preceding values may be combined to form other ranges, such as between 5:8 and 1:8, between 5:8 and 1 :5, or between 5:16 and 1:5.
  • the alkali metal (M) may preferably be lithium, sodium or potassium, more preferably lithium or sodium, and most preferably lithium.
  • the molar ratio of lithium salt: water in the liquid electrolyte is at least 80:3, such as at least 200:3 or at least 250:3. In some embodiments, the molar ratio of lithium salt: water in the liquid electrolyte is up to 25,000:1, such as up to 15,000:1 or up to 8,000:1.
  • the molar ratio of lithium salt: water in the liquid electrolyte is between 80:3 to 25,000:1.
  • Other combinations of the preceding values may be combined to form other ranges, such as between 200:3 and 25,000:1, between 200:3 and 15,000:1, or between 250:3 and 15,000:1.
  • the molar ratio of lithium salt: non-aqueous solvent in the liquid electrolyte is at least 5:8, such as at least 1:2 or at least 5:16. In some embodiments, the molar ratio of lithium salt: non aqueous solvent in the liquid electrolyte is up to 1:10, such as up to 1:8 or up to 1:5.
  • the molar ratio of lithium salt: non-aqueous solvent in the liquid electrolyte is between 5:8 and 1:10.
  • Other combinations of the preceding values may be combined to form other ranges, such as between 5:8 and 1:8, between 5:8 and 1 :5, or between 5:16 and 1 :5.
  • the electrolyte of the present invention can optionally include any other additives typically used in conventional liquid electrolytes for alkali metal (M), especially lithium, ion batteries. Examples include salts other than lithium salts.
  • any fire-retardant compound typically, however, and especially in view of the water-insensitive nature of the present liquid electrolytes, they do not contain any fire-retardant compound.
  • Such compounds are known in the art and examples thereof include organic compounds having a phosphorus- containing functional group such as phosphate, phosphite, phosphonate, and the like.
  • the electrolytes of the present invention have low volatility.
  • the non-aqueous solvent(s) may have a boiling point of more than about 180 °C, particularly more than 200 °C.
  • an electrochemical secondary cell comprising such liquid electrolyte.
  • the cell may be an alkali metal ion secondary cell, for example a sodium-ion secondary cell or a lithium-ion secondary cell.
  • the cell is a lithium-ion secondary cell.
  • the electrochemical secondary cell comprises a liquid electrolyte according to the invention, the electrochemical cell having a first electrode which is a cathode, and a second electrode which is an anode, and a liquid electrolyte according to the invention between the cathode and the anode.
  • the electrochemical secondary cell comprises an electrode laminated with a current collector, for example a metallic foil.
  • an electrochemical energy storage device comprising an electrochemical secondary cell of the invention.
  • the electrochemical energy storage device is a battery.
  • the electrochemical energy storage device is a lithium-ion battery.
  • the present invention also provides a composition for use as a water-insensitive electrolyte.
  • the composition comprises a non-aqueous solvent and an alkali metal (M) salt other than MPFe; the composition is free of MPFe.
  • M alkali metal
  • the composition can be used as an electrolyte that is capable of tolerating water while providing a practical function in the presence of water in the amounts described herein, such as at least 20 ppm water.
  • the alkali metal (M) may preferably be lithium, sodium or potassium, more preferably lithium or sodium and most preferably lithium.
  • the present invention also provides in preferred embodiments in which M is lithium a composition for use as a water-insensitive electrolyte, the composition comprising a nonaqueous solvent and a lithium salt other than LiPFe; the composition is free of LiPFe.
  • the composition can be used as an electrolyte that is capable of tolerating water while providing a practical function in the presence of water in the amounts described herein, such as at least 20 ppm water.
  • the use of the composition provides very similar rates of discharge capacity when compared to corresponding compositions which do not contain water (i.e. which are not used as water-insensitive electrolytes).
  • the use of the composition is for improving rate of discharge capacity.
  • the inventors find that compositions of the invention when used as water-insensitive electrolytes (e.g. in the presence of at least 20 ppm water, such as 500 ppm or 1000 ppm water) provide improved rate of discharge capacity compared to corresponding, water-free electrolyte compositions.
  • the capacity may be improved (increased) by at least 10%, such as at least 15% or at least 20% at 5C rates of discharge compared to corresponding, water-free electrolyte compositions.
  • An upper limit of improvement is not particularly limited, but the present water-insensitive electrolytes are expected to show an improvement in capacity of up to 30%, such as up to 25% at 5C rates of discharge compared to corresponding, water- free electrolyte compositions.
  • the “C-rate” is a standard measure of battery discharge rate in which a higher C-rate of discharge means that a battery has a higher “rate capability” or “rate performance”. For example, a discharge rate of 1C means the cell will fully discharge in an hour, so 5C means five times the rate which would discharge the battery in an hour.
  • compositions - in particular the lithium salt(s), non-aqueous solvent, and any additives - correspond with those set out above for the liquid electrolyte of the first aspect.
  • Another aspect of the invention relates to a method of preparing a liquid electrolyte according to the present invention.
  • the non-aqueous solvent and the alkali metal (M) salt where M is preferably lithium, sodium or potassium, more preferably lithium or sodium and most preferably lithium, other than alkali metal (M) hexafluorophosphate (MPFe) are blended.
  • M is preferably lithium, sodium or potassium, more preferably lithium or sodium and most preferably lithium, other than alkali metal (M) hexafluorophosphate (MPFe)
  • Blending may be carried out by any suitable method.
  • water is introduced to the blended non-aqueous solvent and alkali metal (M), most preferably where M is lithium as discussed above, salt in an amount of between 20 and 5000 ppm based on the total amount of electrolyte.
  • M alkali metal
  • the water may be introduced in any suitable manner.
  • the water may be dissolved in a composition already containing the non-aqueous solvent(s) and alkali metal (M) salt(s).
  • the introduction of water can be done prior to filling the electrochemical cell. In such cases cell assembly may take place in an environment with high control over moisture.
  • cell assembly may advantageously take place in an environment having relatively lower control over moisture thereby reducing production costs.
  • the water may be introduced by preparing the composition in an environment comprising water, such as a high humidity environment.
  • the water may be introduced by blending the non-aqueous solvent and alkali metal (M) salt, most preferably where M is lithium as discussed above, in an environment comprising water.
  • the environment comprising water is a high humidity environment.
  • cell assembly is carried out in a dry room having a dewpoint of -40°C to -50°C.
  • cell assembly can be carried out in a dry room having a dewpoint of -20°C or -30°C.
  • the water may be introduced by separate addition e.g. as an aliquot. For example, a calculated amount of water may be provided and added to the liquid electrolyte at any suitable time, such as during the blending.
  • the water may alternatively be provided together with the non-aqueous solvent or alkali metal (M) salt(s), where M is most preferably lithium as discussed above, prior to blending, or the water may be added after blending. If the liquid electrolyte contains one or more additives as set out above, these may be introduced at any suitable stage such as during the blending or before or after the addition of water.
  • the liquid electrolyte is subsequently processed e.g. integrated into a battery by addition to a side of the battery or through an extrusion method.
  • LiDFOB lithium difluoro(oxalato)borate
  • LiTDI lithium 2-trifluoromethyl-4,5-dicyanoimidazolide
  • LiFSI lithium bis(fluorosulfonyl)imide
  • LiPFe lithium hexafluorophorsphate
  • FEC fluoroethylene carbonate
  • a commercially available liquid electrolyte comprising a lithium salt containing LiPFe and a non-aqueous electrolyte which includes a carbonate.
  • a liquid electrolyte free of LiPFe having the following composition was prepared:
  • Lithium salt 0.2M LiDFOB, 0.8M LiFSI
  • EMC:DMC 1:1 ;
  • VC: FEC 66:34;
  • VC:FEC makes up 8wt% of total of solvent
  • the ratios of this composition are weight ratios.
  • a liquid electrolyte free of LiPF 6 having the following composition was prepared:
  • Lithium salt 0.2M LiDFOB, 0.8M LiFSI
  • Figure 1 shows the results for Reference Example 1 as a dashed line.
  • Figure 1 shows the results for Comparative Example 1 as squares and triangles (representing three measurements carried out on the same composition, labelled 1-1, 1-2 and 1-3 respectively).
  • Figure 1 shows the results for Example 1 as circles and crosses (representing two measurements carried out on the same composition, labelled 1-1 and 1-2, respectively).
  • the liquid electrolyte according to the present invention consistently shows a higher discharge capacity at a given rate and is improved or very similar to the discharge capacity of Reference Example 1.
  • the FSI-based electrolytes gained around 20% improvement in capacity at 5C rates of discharge.
  • compositions according to Example 1 are optically improved over compositions according to Comparative Example 1 (the former being transparent, the latter being opaque).
  • a liquid electrolyte corresponding to Example 1 was prepared, except that it contains 1000 ppm water (H 2 O).
  • Figure 2 shows five measurements carried out on the same composition.
  • the liquid electrolyte of the invention provides a practical discharge capacity vs rate.
  • the results are consistent.
  • electrolytes are prepared by a method which is as follows:
  • the water can be added as described elsewhere herein.
  • the electrolyte solution is filtered before use.
  • the dissolution step occurs over several hours, such as from about 1 hour to about 24 hours, or from about 4 hours to about 24 hours, or from about 6 hours to about 12 hours.
  • the dissolution step occurs with heating, such as heating to at least 40°C, to at least 50°C or to at least 60°C.
  • the upper limit of the temperature of dissolution may be guided by the flash point(s) or boiling point(s) of the nonaqueous solvent(s) and the water, depending on the point of addition of water.
  • the dissolution step may occur with agitation.
  • the dissolution step occurs with agitation and heating to at least 40°C and occurs for about 1 to 24 hours.
  • Electrochemical evaluations of the electrolytes were carried out with Swagelok or pouch type cells. All the cells have one layer of cathode with areal coating weight over 150 g/m 2 , which consists of over 90wt% a high nickel NMC active materials and one layer of anode with areal coating weight over 100 g/m 2 , which consists of over 90wt% graphite/SiOx mixed active materials.
  • All the cells were electrochemically formed at 30°C.
  • a cell was initially charged with a current of C/20 (a current with which it takes 20 hours to fully charge or discharge the cell) for the first hour and then increased to C/10 for the rest of charging until the cell voltage reaching the cutoff voltage of 4.2V. Then the cell is discharged at C/10 until the cut-off voltage of 2.5V. The cell cycles two more cycles with the same cut-off voltages at C/10 for both charging and discharging.
  • the first-cycle efficiency was determined by the first cycle charging capacity divided by first cycle discharging capacity and presented as percentage.
  • rate capability was tested at 30°C and 45°C, sequentially.
  • the C-rates were calculated based on cathode nominal capacity (active material weight times its theoretical capacity). In a rate capability test, all the charging was carried out at current of C/5 while the discharging ranging from C/10 to 10C. The rate capacities were thus determined, which can be further normalised by dividing the C/5 capacity from the same test.

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Abstract

This application concerns a liquid electrolyte for an alkali metal (M) ion secondary cell. Electrolytes disclosed herein comprise a non-aqueous solvent, an alkali metal (M) salt other than the alkali metal hexafluorophosphate (MPF6), and water present in an amount of between 20 and 5000 ppm based on the total amount of electrolyte. The electrolyte is free of the alkali metal (M) hexafluorophosphate. Also disclosed are methods of preparing the liquid electrolyte, an electrochemical cell comprising the liquid electrolyte, and an electrochemical energy storage device comprising the electrochemical cell. Also disclosed is use of a composition comprising non-aqueous solvent and an alkali metal (M) salt other than the alkali metal (M) hexafluorophosphate (MPF6) as a water-insensitive liquid electrolyte, said composition being free of the alkali metal (M) hexafluorophosphate MPF6.

Description

LIQUID ELECTROLYTE
Field of the Invention
The present invention relates to a liquid electrolyte for a lithium-ion secondary cell. The invention also relates to electrochemical cells and energy storage devices comprising such liquid electrolyte, and methods of preparing the liquid electrolyte. Also disclosed are compositions for use as water-insensitive liquid electrolyte.
Background of the Invention
Lithium-ion secondary batteries are the leading battery technology currently used in applications from small personal devices to electric vehicles. Lithium-ion batteries are favoured for their high energy density and long cycle life, among other benefits. They contain a plurality of lithium-ion secondary cells.
Conventional lithium-ion secondary batteries with liquid electrolytes comprise lithium hexafluorophosphate (LiPFe). This lithium salt is thermally stable up to around 106.9 °C in dry inert atmosphere, after which it decomposes into solid LiF and gaseous products including PF5. In the presence of water, the decomposition temperature is lowered, and a thermal reaction occurs between LiPFs and water vapour to form POF3 and HF as gaseous products. These gaseous products pose a serious health and safety hazard. Therefore, moisture and water vapour are tightly controlled at the ppm level in electrolyte and cell manufacture. Thus, current lithium-ion secondary battery manufacture uses low humidity dry room facilities to prevent such decomposition of LiPFe salt. These low humidity dry room facilities, although improving safety, have a high financial cost.
Accordingly, manufacture of lithium-ion secondary batteries must balance good cell performance of the electrolyte with safety and cost-effectiveness.
There is a need for liquid electrolyte compositions which satisfy the above criteria.
Electrolyte compositions containing salts other than LiPFe, and LiPFe-free electrolytes, are described in the literature. For example, US 2019/0123390 A1 discloses low flammability and non-flammable localized super-concentrated electrolytes that may be free of LiPFe. These electrolytes contain a flame-retardant compound in a solvent and contain active salt concentrations of more than 3 M making the electrolytes proposed therein highly viscous and expensive. Other examples include WO 2018/200631 A1, US 2016/0149263 A1 , US 2018/0331393 A1, EP 3072178 A1 , US 2019/0319299 A1, ON 109103490 A, ON 110212160 A, CN 108987810 A, CN 103943884 A and CN 108258316 A. Their use as water-insensitive electrolytes or as liquid electrolytes containing a comparatively large amount of water, is not described.
Summary of the Invention
The present inventors have surprisingly discovered that liquid electrolytes containing lithium salts other than LiPF6 can tolerate the presence of high levels of water without compromising performance. The present invention is based on this finding.
A general proposal of the present invention is a composition which is free of LiPF6 for use as a water-insensitive liquid electrolyte in an alkali metal secondary cell. Accordingly, compositions of the invention when used as liquid electrolytes contain water in an amount which is not tolerated by corresponding conventional electrolytes containing LiPFe. The inventors have found that the present electrolytes show promising compatibility with conventional electrodes, good cell electrochemical and mechanical behaviours such as long cycle life, thermal stability, shelf-life storage, as well as cell safety. In addition, the present liquid electrolytes are compatible with processes for forming solid polymer batteries (including, for example, a gel battery having one or more of at least one gel electrode, and/or a gel separator) as well as conventional batteries, without requiring superconcentrated electrolytes or electrolytes based on linear carbonate solvents such as are disclosed in US 2019/0123390 A1 . In contrast, LiPFe is not usually compatible with processes for forming solid polymer batteries because it is not stable above temperatures around 60°C.
A first aspect of the invention concerns a liquid electrolyte for a lithium ion secondary cell, the electrolyte comprising a non-aqueous solvent, a lithium salt other than lithium hexafluorophosphate (LiPFe), and water present in an amount of between 20 and 5000 ppm based on the total amount of electrolyte, said electrolyte being free of lithium hexafluorophosphate.
Unless stated explicitly otherwise herein, ppm is ppm by weight.
A second aspect of the invention relates to an electrochemical cell comprising a liquid electrolyte according to the first aspect.
A third aspect of the invention relates to an electrochemical energy storage device comprising an electrochemical cell according to the second aspect. A fourth aspect of the invention relates to a method of preparing a liquid electrolyte according to the first aspect, comprising blending a non-aqueous solvent and a lithium salt other than lithium hexafluorophosphate (LiPFe), and introducing water in an amount of between 20 and 5000 ppm based on the total amount of electrolyte.
Advantageously, a method according to the fourth aspect can be carried out without the need for a low humidity dry room and does not require the use of gloveboxes. In this way, the financial cost is substantially reduced, and there is no compromise with safety because the LiPF6 is not present to provide such hazard.
A fifth aspect of the invention relates to the use of a composition comprising non-aqueous solvent and a lithium salt other than lithium hexafluorophosphate (LiPF6) as a waterinsensitive liquid electrolyte, said composition being free of lithium hexafluorophosphate (LiPF6).
The inventors believe that corresponding advantages apply when an alkali metal salt other than lithium is used (e.g. sodium in a sodium ion secondary cell). Accordingly, further aspects of the invention relate to aspects corresponding to the above first to fifth aspects, except that they relate to other alkali metal salts, such as sodium salts, instead of lithium salts. That is, in each of the following first to fifth further aspects, the alkali metal (M) can be any one of the alkali metals (group I of the periodic table) i.e. lithium, sodium, potassium etc. Preferably, the alkali metal (M) is lithium or sodium, and most preferably lithium as set out above. Thus, a first further aspect relates to a liquid electrolyte for an alkali metal (M) ion secondary cell, the electrolyte comprising a non-aqueous solvent, an alkali metal (M) salt other than the alkali metal hexafluorophosphate (MPFe), and water present in an amount of between 20 and 5000 ppm based on the total amount of electrolyte, said electrolyte being free of the alkali metal (M) hexafluorophosphate. The first further aspect thus encompasses a liquid electrolyte for a sodium ion secondary cell, the electrolyte comprising a non-aqueous solvent, a sodium salt other than sodium hexafluorophosphate (NaPFe), and water present in an amount of between 20 and 5000 ppm based on the total amount of electrolyte, said electrolyte being free of sodium hexafluorophosphate. A second further aspect relates to an electrochemical cell comprising a liquid electrolyte of the first further aspect. A third further aspect relates to an electrochemical energy storage device comprising an electrochemical cell according to the second further aspect. A fourth further aspect relates to a method of preparing a liquid electrolyte according to the first further aspect, comprising blending a nonaqueous solvent and an alkali metal (M) salt other than the alkali metal (M) hexafluorophosphate (MPFe), and introducing water in an amount of between 20 and 5000 ppm based on the total amount of electrolyte. The fourth further aspect thus encompasses a method of preparing a liquid electrolyte according to the first further aspect, comprising blending a non-aqueous solvent and a sodium salt other than sodium hexafluorophosphate (NaPF6), and introducing water in an amount of between 20 and 5000 ppm based on the total amount of electrolyte. A fifth further aspect relates to the use of a composition comprising non-aqueous solvent and an alkali metal (M) salt other than the alkali metal hexafluorophosphate (MPF6) as a water-insensitive liquid electrolyte, said composition being free of the alkali metal (M) hexafluorophosphate (MPF6). The fifth further aspect thus encompasses the use of a composition comprising non-aqueous solvent and a sodium salt other than sodium hexafluorophosphate (NaPF6) as a water-insensitive liquid electrolyte, said composition being free of sodium hexafluorophosphate (NaPF6).
Brief Description of the Drawings
Figure 1 shows rate vs discharge capacity for electrochemical cells containing liquid electrolyte.
Figure 2 shows rate vs discharge capacity for electrochemical cells containing liquid electrolyte.
Detailed Description and Preferred Embodiments
Preferred and/or optional features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention unless the context demands otherwise. Any of the preferred and/or optional features of any aspect may be combined, either singly or in combination, with any aspect of the invention unless the context demands otherwise.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Methods and materials are described herein for use in the present disclosure; other, suitable methods and materials known in the art can also be used. The materials, methods, and examples are illustrative only and not intended to be limiting. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control.
Methods described herein usually employ ambient temperature of a typical laboratory, which is typically between 20 and 30°C and preferably around 25°C, at atmospheric pressure, unless a different condition is defined herein or is more usually employed e.g. for a particular apparatus.
Where embodiments discussed herein use the term “comprises” or the like, corresponding embodiments using the term “consists of” should be considered explicitly disclosed.
Herein, references to (total) amounts I concentrations of a component of the composition other than water refer to (total) amounts I concentrations of the component excluding water. By way of example, a reference to “0.2M lithium salt” means that a composition without water added, has a lithium salt concentration of 0.2M; and a reference to “8wt% solvent” means that a composition without water added, has 8wt% solvent.
In the following, features defined for the liquid electrolyte of the invention can also apply to the compositions of the invention, where appropriate.
The present invention generally describes the use of alkali metal (M) hexafluorophosphate- free (MPFe- free), and particularly LiPFe-free, compositions for use as water-insensitive liquid electrolytes. In the following, we refer primarily to lithium salts, for use in lithium secondary batteries. Corresponding remarks also apply to other alkali metal (M) salts, for use in other alkali metal (M) secondary batteries, such as sodium metal salts for use in sodium secondary batteries, and such should be considered as explicitly disclosed throughout. That is, where lithium or Li is mentioned, other alkali metal (M) especially sodium or Na should be considered as explicitly disclosed throughout.
The liquid electrolytes are suitable for use in alkali metal (M) secondary batteries. Therefore, they contain alkali metal (M)-containing electrolyte salts, but are free of MPFe.
The most preferred liquid electrolytes are suitable for use in lithium secondary batteries. Therefore, they contain lithium-containing electrolyte salts, but are free of LiPFe which is conventionally used.
The alkali metal (M) salt(s) may be any suitable kind. That is, the alkali metal (M) salt(s) may be any kind which is water insensitive. By ‘water-insensitive’ as used herein, means that the electrolyte can function at a practical or useful level in the presence of water in the amounts described herein, such as at least 20 ppm water. In some embodiments, the function of a water-insensitive electrolyte is not compromised - and may be improved - in the presence of water in the amounts described herein. In some embodiments, a water-insensitive electrolyte of the present invention has at least 95% of the function of known electrolytes. In some embodiments, this is relative to a corresponding electrolyte which is free or substantially free of water. In some embodiments, this is relative to a corresponding electrolyte which has a conventional amount of water, such as one having less than 20 ppm water.
The most preferred lithium salt(s) may be any suitable kind. That is, the lithium salt(s) may be any kind which is water insensitive. By ‘water-insensitive’ as used herein, means that the electrolyte can function at a practical or useful level in the presence of water in the amounts described herein, such as at least 20 ppm water. In some embodiments, the function of a water-insensitive electrolyte is not compromised - and may be improved - in the presence of water in the amounts described herein. In some embodiments, a water-insensitive electrolyte of the present invention has at least 95% of the function of known electrolytes. In some embodiments, this is relative to a corresponding electrolyte which is free or substantially free of water. In some embodiments, this is relative to a corresponding electrolyte which has a conventional amount of water, such as one having less than 20 ppm water.
In some embodiments, the function referred to herein includes one or more of, such as all of, the following: energy, cycle life and rate performance. As a skilled person would understand, energy storage is usually defined by the first discharge after formation cycling, while cycle life is usually determined by the rate at which the discharge energy fades as a function of cycle number.
In accordance with the present invention, the liquid electrolyte contains at least one alkali metal (M) salt, for example a plurality of alkali metal (M) salts. The liquid electrolyte may further comprise other metal salt(s). Typically, but not exclusively, when more than one salt is present, they are each the same alkali metal (M).
In some embodiments, the liquid electrolyte comprises one or more alkali metal (M) salt(s). In some embodiments, the liquid electrolyte comprises one or two alkali metal (M) salt(s). In some embodiments, the liquid electrolyte comprises two alkali metal (M) salts.
The anion of the or each alkali metal (M) salt may be any suitable anion. In some embodiments, the or each alkali metal (M) salt is independently selected from: an alkali metal (M) borate salt, an alkali metal (M) imide salt, and an alkali metal (M) imidazolide salt. In some embodiments, the liquid electrolyte comprises one or more of alkali metal (M) bis(oxalato)borate, alkali metal (M) tetrafluoroborate, alkali metal (M) difluoro(oxalate)borate (MDFOB), alkali metal (M) bis(trifluoromethanesulfonyl)imide (MTFSI), alkali metal (M) bis(fluorosulfonyl) imide (MFSI) and alkali metal (M) 2-trifluoromethyl-4,5-dicyanoimidaxolide (MTDI). In some embodiments, the alkali metal (M) salt is one or more of MFSI, MDFOB and MTDI.
In some embodiments, the liquid electrolyte comprises MDFOB and MFSI.
Any suitable amount of the or each alkali metal (M) salt may be used in the liquid electrolytes and compositions of the present invention.
In some embodiments, the or each alkali metal (M) salt may be present in an amount of at least 0.8M, at least 1 ,0M or at least 1 ,2M. In some embodiments, the or each alkali metal (M) salt may be present in an amount of up to 2.5M, up to 2.2M or up to 2.0M.
Accordingly, in some embodiments, the or each alkali metal (M) salt may be present in an amount of between 0.8M and 2.5M. Other combinations of the above values may be combined to form a suitable range, for example between 0.8M and 2.2M, between 1.0M and 2.2M or between 1.2M and 2.0M.
In some embodiments, the total amount of alkali metal (M) ion in the liquid electrolyte is up to 35 wt%, such as up to 30 wt% or up to 25 wt%. A lower limit of alkali metal (M) ion in the electrolyte is an amount needed for useful function, but may be at least 0.1 wt%, at least 0.5 wt% or at least 1.0 wt%.
Accordingly, in some embodiments, the total amount of alkali metal (M) ion in the electrolyte is between 0.1 and 35 wt%. Other combinations of the preceding values may be combined to form a suitable range, for example between 0.1 and 30 wt%, between 0.5 and 30 wt%, or between 1.0 and 25 wt%. The wt% values here are relative to the total weight of the electrolyte.
The inventors have surprisingly found that in the absence of MPFe, alkali metal (M) salts such as those described above can tolerate the presence of water in substantially higher quantities than corresponding electrolytes comprising MPFe. Therefore, a liquid electrolyte having comparatively significant amounts of water as described further herein can be used as practical alternatives electrolytes, i.e. water-insensitive electrolytes, to those used in conventional alkali metal (M) ion batteries.
In the above, M is preferably lithium, sodium or potassium. More preferably, M is lithium or sodium. Most preferably, M is lithium.
In accordance with the most preferred embodiments of the present invention in which M is lithium, the liquid electrolyte contains at least one lithium salt, for example a plurality of lithium salts. The liquid electrolyte may further comprise other metal salt(s), including other alkali metal salt(s). Typically, but not exclusively, when more than one salt is present, they are each a lithium salt i.e. the alkali metal (M) is lithium.
In some embodiments, the liquid electrolyte comprises one or more lithium salt(s). In some embodiments, the liquid electrolyte comprises one or two lithium salt(s). In some embodiments, the liquid electrolyte comprises two lithium salts.
The anion of the or each lithium salt may be any suitable anion. In some embodiments, the or each lithium salt is independently selected from: a lithium borate salt, a lithium imide salt, and a lithium imidazolide salt.
In some embodiments, the liquid electrolyte comprises one or more of lithium bis(oxalato)borate, lithium tetrafluoroborate, lithium difluoro(oxalate)borate (LiDFOB), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl) imide (LiFSI) and lithium 2-trifluoromethyl-4,5-dicyanoimidaxolide (LiTDI). In some embodiments, the lithium salt is one or more of LiFSI, LiDFOB and LiTDI.
In some embodiments, the liquid electrolyte comprises LiDFOB and LiFSI.
Any suitable amount of the or each lithium salt may be used in the liquid electrolytes and compositions of the present invention.
In some embodiments, the or each lithium salt may be present in an amount of at least 0.8M, at least 1 ,0M or at least 1 ,2M. In some embodiments, the or each lithium salt may be present in an amount of up to 2.5M, up to 2.2M or up to 2.0M.
Accordingly, in some embodiments, the or each lithium salt may be present in an amount of between 0.8M and 2.5M. Other combinations of the above values may be combined to form a suitable range, for example between 0.8M and 2.2M, between 1.0M and 2.2M or between 1.2M and 2.0M.
In some embodiments, the total amount of lithium ion in the liquid electrolyte is up to 35 wt%, such as up to 30 wt% or up to 25 wt%. A lower limit of lithium ion in the electrolyte is an amount needed for useful function, but may be at least 0.1 wt%, at least 0.5 wt% or at least 1.0 wt%.
Accordingly, in some embodiments, the total amount of lithium ion in the electrolyte is between 0.1 and 35 wt%. Other combinations of the preceding values may be combined to form a suitable range, for example between 0.1 and 30 wt%, between 0.5 and 30 wt%, or between 1.0 and 25 wt%. The wt% values here are relative to the total weight of the electrolyte.
The inventors have surprisingly found that in the absence of LiPFe, lithium salts such as those described above can tolerate the presence of water in substantially higher quantities than corresponding electrolytes comprising LiPFe. Therefore, a liquid electrolyte having comparatively significant amounts of water as described further herein can be used as practical alternatives electrolytes, i.e. water-insensitive electrolytes, to those used in conventional lithium ion batteries.
The amount of water present in the compositions will depend to some extent on various factors including for example the choice of lithium salt(s) present. That is, and without wishing to be bound by theory, it is expected that different kinds of lithium salt may function optimally at different water contents. Even so, the compositions of the invention are expected to have some measurable, practical function at the water contents defined herein.
The amount of water present in the compositions may be varied according to other desired factors. For example, conductivity and viscosity potential stability may be improved by increasing the water content. In other examples, having a lower water content may improve the ability to form a solid electrolyte interface (SEI). An SEI is a passivating layer that often forms on an anode and to some extent a cathode in lithium ion batteries and advantageously can prevent solvent molecules from further decomposition while allowing lithium ions to continue to move. In addition, having a lower water content may also or alternatively improve aluminium corrosion resistance. The water content of the liquid electrolytes of the invention is at least 20 ppm. In some embodiments, the water is present in an amount of at least 50 ppm, at least 100 ppm, at least 150 ppm or at least 200 ppm, preferably at least 300 ppm, more preferably at least 400 ppm, and particularly preferably at least 500 ppm.
The water content of the liquid electrolytes of the invention is 5000 ppm or less. In some embodiments, the water is present in an amount of up to 3000 ppm, up to 2500 ppm, up to 2000 ppm, preferably up to 1500 ppm and particularly preferably up to 1200 ppm.
Accordingly, in some embodiments, the water content of the liquid electrolytes of the present invention is between 20 ppm and 5000 ppm. Other combinations of the above values may be combined to form a suitable range, such as between 20 and 2500 ppm, between 50 and 2500 ppm, between 100 and 2000 ppm, or between 200 and 2000 ppm.
In some embodiments, the water concentration in the liquid electrolyte is at least 0.1 mM, such as at least 0.3mM or at least 0.5mM. In some embodiments, the water concentration in the liquid electrolyte is up to 30mM, such as up to 25mM or up to 20mM.
Accordingly, in some embodiments, the water concentration in the liquid electrolyte is between 0.1 mM and 30mM. Other combinations of the preceding values may be combined to form a suitable range, such as between 0.1 and 25mM, between 0.3 and 25mM, or between 0.5 and 20mM.
In general, when we refer to water herein, we refer to H2O. Preferably, the water is deionized water, although other forms are expected to be suitable for achieving the effects of the invention.
The water content in the electrolyte may be measured by any suitable method. An example provided herein includes the use of a trace moisture measuring unit. Such a unit may measure moisture by calculation using a measured amount of electricity required for electrolysis (applying the principle of Karl-Fisher reaction to coulometric titration). In such a unit, a series of standard measurements is made and subsequently a sample titrated. An amount of iodine required for titration is generated internally by electrolysis and measured by the unit. An exemplary unit is a CA-310.
The liquid electrolyte of the invention contains a non-aqueous solvent in addition to the lithium salt and water. The non-aqueous solvent used in the present invention is a solvent which is typically used in the manufacture of electrodes for alkali metal secondary cells. The non-aqueous solvent should suitably be capable of being blended with the lithium salt and water. Such solvents are typically used in electrolytes for lithium ion batteries and examples will be known to the skilled person.
In some embodiments, the solvent comprises a carbonate, such as one or more cyclic or linear carbonate compounds. In some embodiments the solvent comprises one or more cyclic carbonate compounds. In some embodiments, the non-aqueous solvent comprises one or more of: an acyclic aprotic or cyclic aprotic carbonate.
In some embodiments, the non-aqueous solvent comprises one or more of: a C1-C4 acyclic aprotic carbonate or a C4 cyclic aprotic carbonate.
In some embodiments, the non-aqueous solvent comprises one or more of: ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, fluoropropylene carbonate, or a butyrolactone such as y-butyrolactone.
In some embodiments, the non-aqueous solvent comprises one or more of: ethyl methyl carbonate, dimethyl carbonate, ethylene carbonate, propylene carbonate, vinylene carbonate, fluoroethylene carbonate, or butyrolactone.
In some embodiments, the non-aqueous solvent comprises one or more of the above- mentioned non-aqueous solvents, two or more of the above-mentioned non-aqueous solvents, such as three or four of the above-mentioned non-aqueous solvents.
In embodiments where more than one of the above-mentioned non-aqueous solvents is employed, the molar ratio may be adjusted as desired. Exemplary considerations include viscosity and ability to dissolve desired salts. For examples where two non-aqueous solvents are included, a 1:1 molar ratio may be appropriate in some embodiments, or a 1:2 molar ratio or a 1 :3 molar ratio in other embodiments. For examples where three nonaqueous solvents are included, a 1 :1 :1 molar ratio may be appropriate in some embodiments, or a 1 :2:1 or 2:1 :2 molar ratio in other embodiments.
In some embodiments, the total amount of non-aqueous solvent in the liquid electrolyte is at least 8.0wt%, such as at least 9.0wt% or at least 10wt%. In some embodiments, the total amount of non-aqueous solvent in the liquid electrolyte is up to 40wt%, such as up to 35wt% or up to 30wt%.
Accordingly, in some embodiments, the total amount of non-aqueous solvent in the liquid electrolytes is between 8.0 and 40wt%. Other combinations of the preceding values may be combined to form other ranges, such as between 8.0 and 35wt%, between 9.0 and 35wt% and between 10 and 30wt%. Here, wt% is relative to the total amount (100 wt%) of the liquid electrolyte.
In some embodiments, the non-aqueous solvent concentration in the liquid electrolyte is at least 4.0M, such as at least 4.2M or at least 4.5M. In some embodiments, the non-aqueous solvent concentration in the liquid electrolyte is up to 8.0M, such as up to 7.8M or up to 7.5M.
Accordingly, in some embodiments, the non-aqueous solvent concentration in the liquid electrolyte is between 4.0 and 8.0M. Other combinations of the preceding values may be combined to form other ranges, such as between 4.0 and 7.8M, between 4.2 and 7.8M or between 4.5 and 7.5M.
In some embodiments, the molar ratio of non-aqueous solvent: water in the liquid electrolyte is at least 400:3, such as at least 600:3 or at least 800:3. In some embodiments, the molar ratio of non-aqueous solvent: water in the liquid electrolyte is at up to 80,000:1, such as up to 60,000:1 or up to 40,000:1.
Accordingly, in some embodiments, the molar ratio of non-aqueous solvent: water in the liquid electrolyte is between 400:3 to 80,000:1. Other combinations of the above values may be combined to form other ranges, such as between 400:3 to 60,000:1 , between 400:3 to 40,000:1, or between 800:3 to 40,000:1.
In some embodiments, the molar ratio of alkali metal (M) salt: water in the liquid electrolyte is at least 80:3, such as at least 200:3 or at least 250:3. In some embodiments, the molar ratio of alkali metal (M) salt: water in the liquid electrolyte is up to 25,000:1, such as up to 15,000:1 or up to 8,000:1.
Accordingly, in some embodiments, the molar ratio of alkali metal (M) salt: water in the liquid electrolyte is between 80:3 to 25,000:1. Other combinations of the preceding values may be combined to form other ranges, such as between 200:3 and 25,000:1, between 200:3 and 15,000:1, or between 250:3 and 15,000:1.
In some embodiments, the molar ratio of alkali metal (M) salt: non-aqueous solvent in the liquid electrolyte is at least 5:8, such as at least 1 :2 or at least 5:16. In some embodiments, the molar ratio of alkali metal (M) salt: non aqueous solvent in the liquid electrolyte is up to 1:10, such as up to 1 :8 or up to 1 :5.
Accordingly, in some embodiments, the molar ratio of alkali metal (M) salt: non-aqueous solvent in the liquid electrolyte is between 5:8 and 1:10. Other combinations of the preceding values may be combined to form other ranges, such as between 5:8 and 1:8, between 5:8 and 1 :5, or between 5:16 and 1:5.
In any of the above, the alkali metal (M) may preferably be lithium, sodium or potassium, more preferably lithium or sodium, and most preferably lithium.
In some of the most preferred embodiments in which M is lithium, the molar ratio of lithium salt: water in the liquid electrolyte is at least 80:3, such as at least 200:3 or at least 250:3. In some embodiments, the molar ratio of lithium salt: water in the liquid electrolyte is up to 25,000:1, such as up to 15,000:1 or up to 8,000:1.
Accordingly, in some embodiments, the molar ratio of lithium salt: water in the liquid electrolyte is between 80:3 to 25,000:1. Other combinations of the preceding values may be combined to form other ranges, such as between 200:3 and 25,000:1, between 200:3 and 15,000:1, or between 250:3 and 15,000:1.
In some embodiments, the molar ratio of lithium salt: non-aqueous solvent in the liquid electrolyte is at least 5:8, such as at least 1:2 or at least 5:16. In some embodiments, the molar ratio of lithium salt: non aqueous solvent in the liquid electrolyte is up to 1:10, such as up to 1:8 or up to 1:5.
Accordingly, in some embodiments, the molar ratio of lithium salt: non-aqueous solvent in the liquid electrolyte is between 5:8 and 1:10. Other combinations of the preceding values may be combined to form other ranges, such as between 5:8 and 1:8, between 5:8 and 1 :5, or between 5:16 and 1 :5. The electrolyte of the present invention can optionally include any other additives typically used in conventional liquid electrolytes for alkali metal (M), especially lithium, ion batteries. Examples include salts other than lithium salts.
Typically, however, and especially in view of the water-insensitive nature of the present liquid electrolytes, they do not contain any fire-retardant compound. Such compounds are known in the art and examples thereof include organic compounds having a phosphorus- containing functional group such as phosphate, phosphite, phosphonate, and the like.
In some embodiments, the electrolytes of the present invention have low volatility. In particular, the non-aqueous solvent(s) may have a boiling point of more than about 180 °C, particularly more than 200 °C.
Also provided as an aspect of the invention is an electrochemical secondary cell comprising such liquid electrolyte. The cell may be an alkali metal ion secondary cell, for example a sodium-ion secondary cell or a lithium-ion secondary cell. Preferably the cell is a lithium-ion secondary cell. In some embodiments the electrochemical secondary cell comprises a liquid electrolyte according to the invention, the electrochemical cell having a first electrode which is a cathode, and a second electrode which is an anode, and a liquid electrolyte according to the invention between the cathode and the anode.
In some embodiments the electrochemical secondary cell comprises an electrode laminated with a current collector, for example a metallic foil.
Also provided herein as an aspect of the invention is an electrochemical energy storage device comprising an electrochemical secondary cell of the invention. In some embodiments, the electrochemical energy storage device is a battery. In some embodiments, the electrochemical energy storage device is a lithium-ion battery.
The present invention also provides a composition for use as a water-insensitive electrolyte. The composition comprises a non-aqueous solvent and an alkali metal (M) salt other than MPFe; the composition is free of MPFe. As will be understood from the above, the composition can be used as an electrolyte that is capable of tolerating water while providing a practical function in the presence of water in the amounts described herein, such as at least 20 ppm water. The alkali metal (M) may preferably be lithium, sodium or potassium, more preferably lithium or sodium and most preferably lithium. The present invention also provides in preferred embodiments in which M is lithium a composition for use as a water-insensitive electrolyte, the composition comprising a nonaqueous solvent and a lithium salt other than LiPFe; the composition is free of LiPFe. As will be understood from the above, the composition can be used as an electrolyte that is capable of tolerating water while providing a practical function in the presence of water in the amounts described herein, such as at least 20 ppm water.
In some embodiments, the use of the composition provides very similar rates of discharge capacity when compared to corresponding compositions which do not contain water (i.e. which are not used as water-insensitive electrolytes).
In some embodiments, the use of the composition is for improving rate of discharge capacity. In particular, the inventors find that compositions of the invention when used as water-insensitive electrolytes (e.g. in the presence of at least 20 ppm water, such as 500 ppm or 1000 ppm water) provide improved rate of discharge capacity compared to corresponding, water-free electrolyte compositions.
For example, the capacity may be improved (increased) by at least 10%, such as at least 15% or at least 20% at 5C rates of discharge compared to corresponding, water-free electrolyte compositions. An upper limit of improvement is not particularly limited, but the present water-insensitive electrolytes are expected to show an improvement in capacity of up to 30%, such as up to 25% at 5C rates of discharge compared to corresponding, water- free electrolyte compositions. As the skilled person will understand and for completeness, the “C-rate” is a standard measure of battery discharge rate in which a higher C-rate of discharge means that a battery has a higher “rate capability” or “rate performance”. For example, a discharge rate of 1C means the cell will fully discharge in an hour, so 5C means five times the rate which would discharge the battery in an hour.
The options and preferences for the components of the composition - in particular the lithium salt(s), non-aqueous solvent, and any additives - correspond with those set out above for the liquid electrolyte of the first aspect.
Another aspect of the invention relates to a method of preparing a liquid electrolyte according to the present invention.
In the method of the invention, the non-aqueous solvent and the alkali metal (M) salt, where M is preferably lithium, sodium or potassium, more preferably lithium or sodium and most preferably lithium, other than alkali metal (M) hexafluorophosphate (MPFe) are blended.
Blending may be carried out by any suitable method.
In the method of the invention, water is introduced to the blended non-aqueous solvent and alkali metal (M), most preferably where M is lithium as discussed above, salt in an amount of between 20 and 5000 ppm based on the total amount of electrolyte.
The water may be introduced in any suitable manner. For example, the water may be dissolved in a composition already containing the non-aqueous solvent(s) and alkali metal (M) salt(s).
The introduction of water can be done prior to filling the electrochemical cell. In such cases cell assembly may take place in an environment with high control over moisture.
Alternatively, the introduction of water can take place during cell assembly. In such cases cell assembly may advantageously take place in an environment having relatively lower control over moisture thereby reducing production costs.
In some embodiments, the water may be introduced by preparing the composition in an environment comprising water, such as a high humidity environment.
In some embodiments, the water may be introduced by blending the non-aqueous solvent and alkali metal (M) salt, most preferably where M is lithium as discussed above, in an environment comprising water. In some embodiments, the environment comprising water is a high humidity environment.
Typically, cell assembly is carried out in a dry room having a dewpoint of -40°C to -50°C. In the present invention, in some embodiments, cell assembly can be carried out in a dry room having a dewpoint of -20°C or -30°C.
In some embodiments, the water may be introduced by separate addition e.g. as an aliquot. For example, a calculated amount of water may be provided and added to the liquid electrolyte at any suitable time, such as during the blending. The water may alternatively be provided together with the non-aqueous solvent or alkali metal (M) salt(s), where M is most preferably lithium as discussed above, prior to blending, or the water may be added after blending. If the liquid electrolyte contains one or more additives as set out above, these may be introduced at any suitable stage such as during the blending or before or after the addition of water.
The options and preferences for the components of the liquid electrolyte prepared by the method of the invention - in particular the alkali metal (M) salt(s), non-aqueous solvent, water and any additives - correspond with those set out above for the liquid electrolyte of the first aspect.
In some embodiments, the liquid electrolyte is subsequently processed e.g. integrated into a battery by addition to a side of the battery or through an extrusion method.
Examples
The present invention will now be exemplified by the following, which do not limit the scope of the invention as defined in the claims.
The following abbreviations are used in these examples:
LiDFOB: lithium difluoro(oxalato)borate
LiTDI: lithium 2-trifluoromethyl-4,5-dicyanoimidazolide
LiFSI: lithium bis(fluorosulfonyl)imide
LiPFe: lithium hexafluorophorsphate
EMC: ethyl methyl carbonate
DMC: dimethyl carbonate
EC: ethylene carbonate
PC: propylene carbonate
GBL: y-butyrolactone
VC: vinylene carbonate
FEC: fluoroethylene carbonate
Experiment 1
Reference Example 1
A commercially available liquid electrolyte comprising a lithium salt containing LiPFe and a non-aqueous electrolyte which includes a carbonate.
Comparative Example 1
A liquid electrolyte free of LiPFe having the following composition was prepared:
Lithium salt: 0.2M LiDFOB, 0.8M LiFSI Non-aqueous solvent: EMC:DMC = 1:1 ; VC: FEC = 66:34; VC:FEC makes up 8wt% of total of solvent)
Water content: <10 ppm
The ratios of this composition are weight ratios.
Example 1
A liquid electrolyte free of LiPF6 having the following composition was prepared:
Lithium salt: 0.2M LiDFOB, 0.8M LiFSI
Non-aqueous solvent: EMC:DMC = 1:1 ; VC: FEC = 66:34; VC:FEC makes up 8wt% of total of solvent)
Water content: 500 ppm
The discharge capacity (mAh/g) vs rate performance (C) of the liquid electrolytes of Reference Example 1, Comparative Example 1 and Example 1 were measured. The results are shown in Figure 1.
Figure 1 shows the results for Reference Example 1 as a dashed line.
Figure 1 shows the results for Comparative Example 1 as squares and triangles (representing three measurements carried out on the same composition, labelled 1-1, 1-2 and 1-3 respectively).
Figure 1 shows the results for Example 1 as circles and crosses (representing two measurements carried out on the same composition, labelled 1-1 and 1-2, respectively).
As can be seen from Figure 1 , the liquid electrolyte according to the present invention consistently shows a higher discharge capacity at a given rate and is improved or very similar to the discharge capacity of Reference Example 1.
The FSI-based electrolytes gained around 20% improvement in capacity at 5C rates of discharge.
In addition, it was noted that the compositions according to Example 1 are optically improved over compositions according to Comparative Example 1 (the former being transparent, the latter being opaque). Experiment 2
Example 2
A liquid electrolyte corresponding to Example 1 was prepared, except that it contains 1000 ppm water (H2O).
The discharge capacity (mAh/g) vs rate performance (C) of the liquid electrolytes of Example 2 were measured. The results are shown in Figure 2.
Figure 2 shows five measurements carried out on the same composition.
As can be seen from Figure 2, even at comparatively high amounts of water, the liquid electrolyte of the invention provides a practical discharge capacity vs rate. In addition, the results are consistent.
In general, electrolytes are prepared by a method which is as follows:
(i) provide pre-measured amount of non-aqueous solvent(s) and
(ii) dissolve pre-measured amount of solid(s) with the non-aqueous solvent(s) to form an electrolyte solution.
The water can be added as described elsewhere herein.
In some embodiments, the electrolyte solution is filtered before use.
In some embodiments, the dissolution step occurs over several hours, such as from about 1 hour to about 24 hours, or from about 4 hours to about 24 hours, or from about 6 hours to about 12 hours. In some embodiments, the dissolution step occurs with heating, such as heating to at least 40°C, to at least 50°C or to at least 60°C. The upper limit of the temperature of dissolution may be guided by the flash point(s) or boiling point(s) of the nonaqueous solvent(s) and the water, depending on the point of addition of water. In some embodiments, the dissolution step may occur with agitation.
In some embodiments, the dissolution step occurs with agitation and heating to at least 40°C and occurs for about 1 to 24 hours.
Electrochemical evaluations of the electrolytes were carried out with Swagelok or pouch type cells. All the cells have one layer of cathode with areal coating weight over 150 g/m2, which consists of over 90wt% a high nickel NMC active materials and one layer of anode with areal coating weight over 100 g/m2, which consists of over 90wt% graphite/SiOx mixed active materials.
Cell assembly was carried out in a dry-room with Dew point less than -40°C. By design, the nominal capacity was about 3.5 mAh or 40.0 mAh for Swagelok or pouch type cells, respectively. The capacity balance was controlled at about 85-90% utilisation of the anode. For all the cells, glass fibre separators were used and 70 pl or 1 ml of an electrolyte was added for Swagelok or pouch cells, respectively.
All the cells were electrochemically formed at 30°C. A cell was initially charged with a current of C/20 (a current with which it takes 20 hours to fully charge or discharge the cell) for the first hour and then increased to C/10 for the rest of charging until the cell voltage reaching the cutoff voltage of 4.2V. Then the cell is discharged at C/10 until the cut-off voltage of 2.5V. The cell cycles two more cycles with the same cut-off voltages at C/10 for both charging and discharging. The first-cycle efficiency was determined by the first cycle charging capacity divided by first cycle discharging capacity and presented as percentage. Once a cell passed this formation step, rate capability was tested at 30°C and 45°C, sequentially. The C-rates were calculated based on cathode nominal capacity (active material weight times its theoretical capacity). In a rate capability test, all the charging was carried out at current of C/5 while the discharging ranging from C/10 to 10C. The rate capacities were thus determined, which can be further normalised by dividing the C/5 capacity from the same test.

Claims

Claims
1. A liquid electrolyte for a lithium ion secondary cell, the electrolyte comprising a non-aqueous solvent, a lithium salt other than lithium hexafluorophosphate (LiPFe), and water present in an amount of between 20 and 5000 ppm based on the total amount of electrolyte, said electrolyte being free of lithium hexafluorophosphate.
2. A liquid electrolyte according to claim 1 , wherein the lithium salt is present in the electrolyte in an amount of between 0.8M and 2.5M and/or wherein the solvent is present in the electrolyte in an amount of between 4.0M and 8.0M.
3. A liquid electrolyte according to claim 1 or claim 2, wherein the water is present in an amount of at least 50 ppm.
4. A liquid electrolyte according to claim 3, wherein the water is present in an amount of at least 300 ppm.
5. A liquid electrolyte according to claim 4, wherein the water is present in an amount of at least 500 ppm.
6. A liquid electrolyte according to any one of the preceding claims, wherein the water is present in an amount of up to 3000 ppm.
7. A liquid electrolyte according to any one of the preceding claims, wherein the lithium salt comprises one or more of: a lithium borate salt, a lithium imide salt, and a lithium imidazolide salt.
8. A liquid electrolyte according to claim 7, wherein the lithium salt comprises one or more of: lithium bis(oxalato) borate, lithium tetrafluoroborate, lithium difluoro(oxalate)borate (LiDFOB), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl) imide (LiFSI) and lithium 2-trifluoromethyl-4,5- dicyanoimidaxolide (LiTDI).
9. A liquid electrolyte according to any one of the preceding claims, wherein the nonaqueous solvent comprises one or more of: an acyclic aprotic or cyclic aprotic carbonate.
10. A liquid electrolyte according to claim 9, where in the non-aqueous solvent comprises one or more of: a C1-C4 acyclic aprotic carbonate or a C4 cyclic aprotic carbonate.
11. A liquid electrolyte according to claim 10, wherein the non-aqueous solvent comprises one or more of: ethyl methyl carbonate, dimethyl carbonate, ethylene carbonate, propylene carbonate, vinylene carbonate, fluoroethylene carbonate, or butyrolactone.
12. A liquid electrolyte according to any one of the preceding claims, which is free of flame-retardant compound.
13. An electrochemical cell comprising a liquid electrolyte according to any one of the preceding claims.
14. An electrochemical energy storage device comprising an electrochemical cell according to claim 13.
15. Use of a composition comprising non-aqueous solvent and a lithium salt other than lithium hexafluorophosphate (LiPFe) as a water-insensitive liquid electrolyte, said composition being free of LiPFe.
16. Use of a composition according to claim 15 for improving rate of discharge capacity.
17. A method of preparing a liquid electrolyte according to any one of claims 1 to 12, comprising blending a non-aqueous solvent and a lithium salt other than lithium hexafluorophosphate (LiPF6), and introducing water in an amount of between 20 and 5000 ppm based on the total amount of electrolyte.
18. A method according to claim 17, wherein said introducing comprises blending in an environment comprising water.
19. A liquid electrolyte for an alkali metal (M) ion secondary cell, the electrolyte comprising a non-aqueous solvent, an alkali metal (M) salt other than the alkali metal hexafluorophosphate (MPF6), and water present in an amount of between 20 and 5000 ppm based on the total amount of electrolyte, said electrolyte being free of the alkali metal (M) hexafluorophosphate.
20. A liquid electrolyte according to claim 20, wherein the alkali metal (M) salt is present in the electrolyte in an amount of between 0.8M and 2.5M and/or wherein the solvent is present in the electrolyte in an amount of between 4.0M and 8.0M.
21. A liquid electrolyte according to any one of claims 19 to 20, wherein either:
(i) the water is present in an amount of at least 50 ppm; and/or
(ii) the water is present in an amount of at least 300 ppm; and/or
(iii) the water is present in an amount of at least 500 ppm; and/or
(iv) the water is present in an amount of up to 3000 ppm.
22. A liquid electrolyte according to any one of claims 19 to 21 , wherein the alkali metal salt (M) comprises one or more of: an alkali metal (M) borate salt, an alkali metal (M) imide salt, and an alkali metal (M) imidazolide salt; and optionally wherein the alkali metal (M) salt comprises one or more of: alkali metal (M) bis(oxalato)borate, alkali metal (M) tetrafluoroborate, alkali metal (M) difluoro(oxalate)borate (MDFOB), alkali metal (M) bis(trifluoromethanesulfonyl)imide (MTFSI), alkali metal (M) bis(fluorosulfonyl) imide (MFSI) and alkali metal (M) 2-trifluoromethyl-4,5-dicyanoimidaxolide (MTDI).
23. A liquid electrolyte according to any one of claims 19 to 22, wherein the nonaqueous solvent comprises:
(i) one or more of: an acyclic aprotic or cyclic aprotic carbonate; and/or
(ii) one or more of: a C1-C4 acyclic aprotic carbonate or a C4 cyclic aprotic carbonate; and/or
(iii) one or more of: ethyl methyl carbonate, dimethyl carbonate, ethylene carbonate, propylene carbonate, vinylene carbonate, fluoroethylene carbonate, or butyrolactone.
24. A liquid electrolyte according to any one of claims 19 to 23, which is free of flameretardant compound.
25. A liquid electrolyte according to any one of claims 19 to 24, wherein the alkali metal (M) is sodium (Na).
26. An electrochemical cell comprising a liquid electrolyte according to any one of claims 19 to 25.
27. An electrochemical energy storage device comprising an electrochemical cell according to claim 26.
28. Use of a composition comprising non-aqueous solvent and an alkali metal (M) salt other than the alkali metal (M) hexafluorophosphate (MPFe) as a water-insensitive liquid electrolyte, said composition being free of the alkali metal (M) hexafluorophosphate MPFe, preferably wherein the alkali metal (M) is sodium so that the use is the use of a composition comprising non-aqueous solvent and a sodium salt other than sodium hexafluorophosphate (NaPFe) as a water-insensitive liquid electrolyte, said composition being free of sodium hexafluorophosphate NaPFe.
29. Use of a composition according to claim 28 for improving rate of discharge capacity.
30. A method of preparing a liquid electrolyte according to any one of claims 19 to 25, comprising blending a non-aqueous solvent and an alkali metal (M) salt other than the alkali metal (M) hexafluorophosphate (MPFe), and introducing water in an amount of between 20 and 5000 ppm based on the total amount of electrolyte, and optionally wherein said introducing comprises blending in an environment comprising water.
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