Electrolyte composition

Abstract

To provide an electrolyte composition for anode-free metal battery cells, which significantly reduces a risk of precipitation of a salt.SOLUTION: The present invention discloses an electrolyte composition for an anode-free metal battery cell. The metal is an alkali metal, an alkaline earth metal, or a metal of Group IIIa of a periodic table. The electrolyte composition includes a first solvent, the salt of an alkali metal, an alkaline earth metal or a metal of Group IIIa of the periodic table, and an additive, the salt being soluble in the first solvent. Concentration of the salt in the electrolyte composition is 2M to 3M. The additive is a perfluorinated organic compound including at least one halogen atom. The halogen is chlorine, bromine or iodine. The present invention also discloses an anode (5)-free metal battery cell (1) including ab electrolyte (3).SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to an electrolyte composition for an anode-free metal battery cell.The present invention further relates to an anode-free metal battery cell comprising an electrolyte according to the present invention. [Background technology]

[0002] Metal batteries based on alkali metals, alkaline earth metals or metals from group IIIa of the periodic table, such as lithium metal batteries, magnesium metal batteries, aluminum metal batteries and sodium metal batteries, have been around for some time. For example, the lithium metal secondary battery. The optimization of this battery technology, from the viewpoints of energy density, cost and safety, has led to the so-called "anode-free" configuration. In this configuration, the anode consists essentially of a bare current collector, such as a copper current collector. In particular, the graphite coating typically used on the copper current collector is not used.

[0003] The charge / discharge mechanism in an anode-free battery is different from that of a traditional rechargeable battery because there is no anode following the classical concept / composition of an anode. An anode-free battery cell does not contain excess amounts of either alkali metals, alkaline earth metals, or metals from group IIIa of the periodic table, either in the electrolyte or in the electrolyte. However, despite the absence of an anode, an alkali metal such as lithium or sodium, an alkaline earth metal such as magnesium, or a metal from group IIIa of the periodic table such as aluminum is required because metal ions are deposited on the anode current collector during charging. This forms a lithium deposit along with an electrochemically stable solid electrolyte interphase (SEI). This lithium deposit is the only lithium available for discharging the battery cell, i.e., there is no active lithium source on the anode side. This significantly reduces the safety risks that can occur in lithium metal batteries.

[0004] As a result, the consumption of electrolyte and the formation of unavailable alkali or alkaline earth metals leads to capacity loss and poor performance of the battery cell. Therefore, for the system to function efficiently, all cell components must be perfectly coordinated with each other, which is currently a challenge.

[0005] The electrolyte is a critical component in any battery cell. It acts as a carrier for the alkali and alkaline earth metal ions and is the primary contact medium between the interfaces.

[0006] International patent application WO2021213743 discloses an anodeless lithium-ion battery comprising a cathode including a current collector, a current collector as an anode, a liquid electrolyte between the anode and the cathode current collector, and a separator. The liquid electrolyte comprises at least one lithium salt and a solvent mixture of at least 70% by volume relative to the total volume of the electrolyte composition. The solvent mixture comprises at least one fluorinated ether compound and at least one non-fluorinated ether compound. However, it is known that fluorinated ether compounds are poorly or not soluble in lithium salts, while the addition of fluorinated ether compounds makes it possible to reduce the overall concentration of lithium salts. Thus, the local concentration of lithium salts in the non-fluorinated ether compounds remains high, which leads to undesirable precipitation of lithium salts in the solvent mixture despite the low overall concentration of lithium salts.

[0007] Electrolytes known to be used in anode-free metal battery cells are so-called high-concentration electrolytes (HCE) or ultra-high-concentration electrolytes. Typically, such electrolytes contain metal salts, i.e. alkali metal salts or alkaline earth metal salts, in concentrations greater than 3M, often 4M or more, even 5.5M. A known example of such an electrolyte contains 5.5M lithium bis(fluorosulfonyl)imide (LiFSI) in dimethyl carbonate, which is stable, has a high cathode voltage, and prevents corrosion of aluminum current collectors. For example, the article 'High rate and stable cycling of lithium metal anode', J. Qian, WA Henderson, et al., Nature Communications, 6, 6362 (2015) discloses that 4M LiFSI in DMC can increase the coulombic efficiency of lithium metal anodes up to 99.2%.

[0008] The salts used often contain fluorine, and due to the high anion concentration of the salts, a fluorine-rich SEI is formed at both the anode and cathode, which reduces or inhibits the formation of dendrites on the metal surface and increases the plating / stripping (charge / discharge) coulombic efficiency of the battery cell.

[0009] Another electrolyte concept is the locally high concentration electrolyte (LHCE) or locally ultra-high concentration electrolyte, which requires the use of a diluent to reduce the overall salt concentration in the final electrolyte composition while maintaining the concentration of the salt-solvent clusters formed between the electrolyte solvent and the salt when the salt dissolves in the solvent, but the concentration remains higher than 3M. The diluent is inert to the high concentration electrolyte, in which the salt is little or not soluble. It is also easily miscible with the solvating solvent, preventing phase separation. The diluent is also required to maintain the high concentration of salt-solvent clusters in the formed electrolyte. In other words, the diluent is required to maintain the local coordination environment of the HCE, i.e., the coordination of Li with the solvating solvent. + Li so that the ionic coordination is maintained.+ The diluent has low solvating ability for the ions. It surrounds the anion of the Li salt and the solvating solvent of HCE.

[0010] International patent application WO2020106762 discloses an embodiment of a localized very-high concentration electrolyte (LSE), also called a localized high concentration electrolyte (LHCE), and a device including such an LSE. The LSE is used in rechargeable alkali metal and alkali ion batteries, including anode-free battery cells. The electrolyte includes an active salt, a solvent in which the active salt is soluble, and a diluent in which the active salt is insoluble or poorly soluble. The diluent is a fluorinated orthoformate. The salt is present in the solvent at a concentration of greater than 3M, such as greater than 3M and up to 10M. The low solubility of the salt in the diluent leads to a localized high concentration of the salt, which causes undesirable precipitation of the salt in the solvent mixture. This phenomenon is particularly evident during storage.

[0011] The disadvantage of such high and ultra-high concentration electrolytes is that the salt must be highly soluble in the solvent. Another disadvantage is that high localized salt concentrations can cause undesirable precipitation of the salt in the electrolyte. Also, the dissociation constant of the salt must be high in HCE and LHCE. Therefore, some commonly used salts cannot be used. Another disadvantage is that the amount of diluent added to the electrolyte is significantly increased. The diluent is often very expensive, which makes the electrolyte expensive. Summary of the Invention [Problem to be solved by the invention]

[0012] The present invention aims to overcome one or more of the above problems. One of the objects of the present invention is to provide an electrolyte composition for anode-free metal battery cells, which significantly reduces the risk of salt precipitation. It is a further object of the present invention to provide an electrolyte composition that can use a wide range of commercially available salts. It is a further object of the present invention to provide an electrolyte that is less expensive than known electrolytes.

[0013] The present invention further aims to provide an anode-free metal battery cell that allows for the formation of a stable SEI and has a high power density and good stability against repeated charge and discharge.The present invention further aims to provide an anode-free metal battery cell that has an increased lifetime, i.e., can be charged and discharged more than known anode-free metal battery cells.The present invention further aims to provide an inexpensive anode-free metal battery cell.

[0014] In the context of this invention, the terms "battery" and "battery cell" have the same meaning.

[0015] A first aspect of the present invention relates to an electrolyte composition for an anode-free metal battery cell, comprising a first solvent, a metal salt and an additive, and optionally a second solvent.

[0016] The metal salt comprises or consists essentially of a salt of an alkali metal, an alkaline earth metal, or a metal from Group IIIa of the periodic table. Preferably, the metal is lithium, magnesium, sodium, or aluminium, preferably lithium or magnesium. The concentration of the salt in the electrolyte composition is 2M to 3M; The additive is a perfluorinated organic compound containing at least one halogen atom; The halogen is chlorine, bromine or iodine.

[0017] The metal salt is soluble in the first solvent. Preferably, when the electrolyte composition contains a second solvent, the metal salt is also soluble in the second solvent. The concentration of the salt in the electrolyte is 0.5M to 3.25M, preferably 1M to 3.1M, and more preferably 2M to 3M.

[0018] Preferably, the first solvent is selected from the group consisting of carbonates, ethers, nitriles, sulfones, sulfoxides, esters, and alkyl esters of ionic liquids. For example, the first solvent can comprise or consist essentially of 1,2-dimethoxyethane (DME).

[0019] Ionic liquids belong to the class of non-molecular compounds that consist only of ions, i.e., anions and cations. In the context of the present invention, ionic liquids suitable as the first solvent in the electrolyte composition preferably comprise a cation that comprises or essentially consists of pyrrolidinium, piperidinium, cyclic saturated ammonium, or a combination thereof. Preferably, the anion comprises or essentially consists of bis(fluorosulfonyl)imide (FSI), bis(trifluoromethanesulfonyl)imide (TFSI), or a combination thereof.

[0020] The additive is a perfluorinated organic compound further comprising at least one halogen atom. The halogen is chlorine (Cl), bromine (Br) or iodine (I). Preferably, the additive is soluble in the first solvent. Preferably, the additive is soluble in the second solvent.

[0021] The additive has the formula (I): XCF 2 -(CF 2 ) x -CF 3-y Y y (I) where X is Cl, Br or I, Y is Cl, Br or I, x is 0 to 10, preferably 1 to 8, more preferably 2 to 6, and y is 0 to 3, preferably 0 or 1.

[0022] Preferably, at least X is I. Preferably, X and Y are I. That is, the additive of the perfluoroalkyl halide is a perfluoroalkyl iodide. Examples of perfluoroalkyl iodides include 1-iodoperfluorobutane (i.e., X is I, x is 2, y is 0, and since y is 0, Y can be either I, Cl or Br), 1,4-diiodoperfluorobutane (i.e., X is I, Y is I, x is 2, y is 1), 1-iodoperfluorohexane (i.e., X is I, x is 4, y is 0, and since y is 0, Y can be either I, Cl or Br), 1,6-diiodoperfluorohexane (i.e., X is I, Y is I, x is 4, y is 1), and 1,8-diiodoperfluorooctane (i.e., X is I, Y is I, x is 6, y is 1). However, the present invention is not limited thereto.

[0023] When the first solvent is an ionic liquid, additives according to formula (I), in particular perfluoroalkyl iodides, are particularly preferred. Such additives are highly soluble in ionic liquids, i.e. in solutions with high ionic strength.

[0024] The additive has the formula (II): C 6 F 6-q Z q (II) where Z is Cl, Br, I or C a F 2a+1-b Q b wherein Q is Cl, Br or I, q is 1 to 5, preferably 1 to 4, more preferably 1 to 3, a is 1 to 10, preferably 1 to 6, more preferably 1 to 4, and b is a value such that 2a+1-b is 1 or greater.

[0025] Preferably, Z is I. Examples of suitable additives according to formula (II) include, but are not limited to, 1,4-diiodo-2,3,5,6-tetrafluorobenzene (i.e., Z is I and q is 2), 1-iodo-2,3,4,5,6-pentafluorobenzene (i.e., Z is I and q is 1).

[0026] Preferably, the electrolyte composition contains the additive in an amount of 0.01 to 20% by weight, preferably 0.05 to 15% by weight, more preferably 0.1 to 10% by weight, based on the total weight of the electrolyte composition.

[0027] Preferably, the electrolyte composition further comprises a second solvent, The second solvent acts as a diluent. The second solvent can comprise or consist essentially of a hydrofluoroether, a partially fluorinated alkyl ether, or a combination thereof.

[0028] A second aspect of the present invention relates to an anode-free metal battery cell comprising an electrolyte composition according to the present invention. Preferably, the anode-free metal battery cell is a secondary battery cell.

[0029] Preferably, the metal of the anode-free battery cell is an alkali metal, an alkaline earth metal, or a metal from group IIIa of the periodic table. Preferably, the metal is an alkali metal or an alkaline earth metal. Preferred examples of alkali metals are lithium (Li) (in which case the anode-free metal battery cell is an anode-free lithium metal battery cell), sodium (Na), and potassium (K). Preferred examples of alkaline earth metals are magnesium (Mg) (in which case the anode-free metal battery cell is an anode-free magnesium metal battery cell), and calcium (Ca). A preferred example of a group IIIa metal is aluminum.

[0030] Preferably, the metal of the metal salt and the metal of the anode-free battery cell are the same. Preferably, the metal salt is a lithium salt or a magnesium salt and the battery cell is a anode-free lithium metal battery cell or anode-free magnesium metal battery cell, respectively.

[0031] Preferably, the anode of the anode-free battery cell comprises a bare current collector, which in the context of the present invention means a current collector without a layer, such as a graphite layer, deposited thereon.

[0032] The electrolyte composition according to the present invention makes it possible to obtain a metal battery cell with a free anode, which allows stable formation of SEI. The electrolyte according to the present invention also makes it possible to obtain a higher specific discharge capacity during repeated charging and discharging. That is, the electrolyte according to the present invention makes it possible to improve the stability of the battery cell during repeated charging and discharging.

[0033] The electrolyte composition according to the invention also has the advantage that a wide range of commercially available metal salts can be used, thereby obtaining battery cells with the properties required for the intended application. The electrolyte according to the invention also avoids metal loss, the so-called "dead metal generation", thereby increasing the life and power density of the battery cells. This favorable effect is further strengthened by the reduced mobility of anions and the increased mobility of metal ions in the electrolyte according to the invention.

[0034] The electrolyte has the further advantage that the presence of a second solvent such as a diluent is not essential and, if present, only a small amount is sufficient. Diluents are known to be expensive and the electrolyte of the present invention can be cheaper than electrolytes known in the art.

[0035] Certain aspects of the present invention will now be described in detail with reference to the accompanying drawings, in which like reference numerals represent like elements, and in which: [Brief description of the drawings]

[0036] [Figure 1] Schematic diagram of a two-electrode metal cell with a free anode. [Diagram 2] FIG. 1 shows the specific capacity of the two-electrode cells, in which the electrolyte does not contain an additive (reference battery cell) and does not contain a perfluoroalkyl iodide additive (battery cell according to the invention), respectively. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] The electrolyte composition according to the present invention comprises a first solvent, an additive, and a metal salt.

[0038] The metal salt comprises or consists essentially of a salt of an alkali metal, an alkaline earth metal, or a metal from Group IIIa of the periodic table. Preferred examples of alkali metals are lithium (Li), sodium (Na), and potassium (K). Preferred examples of alkaline earth metals are magnesium (Mg) and calcium (Ca). A preferred example of a metal from Group IIIa is aluminum (Al). Preferably, the metal consists essentially of lithium, magnesium, aluminum, or sodium, preferably lithium or magnesium.

[0039] The electrolyte composition can include a combination of two or more metal salts. When two or more metal salts are present, the metal salts can include the same metal, in particular all of the salts can be lithium salts, or the metal salts can include different metals, in particular lithium and magnesium salts.

[0040] Examples of lithium salts that can be used in the electrolyte composition of the present invention include lithium hexafluorophosphate (LiPF 6 ), lithium perchlorate (LiClO 4 ), lithium hexafluoroarsenate (LiAsF 6 ), lithium tetrafluoroborate (LiBF4 ), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), lithium trifluoromethanesulfonate (LiTf), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), and derivatives thereof. However, they are not limited thereto.

[0041] Derivatives of LiFSI and LiTFSI preferably include lithium bis(R-sulfonyl)imide and lithium (R 1 ,R 2 -sulfonyl)imide salts, but are not limited thereto. Here, R, R 1 , R 2 is an at least partially fluorinated alkyl group. For example, R 1 and R 2 can be, independently of each other, fluoro, trifluoromethyl, pentafluoroethyl, nonafluorobutyl. Examples of such derivatives include lithium (fluorosulfonyl)(trifluoromethanesulfonyl)imide and lithium (pentafluoroethanesulfonyl)(trifluoromethanesulfonyl)imide.

[0042] An example of a magnesium salt is magnesium perchlorate (Mg(ClO 4 ) 2 ), magnesium bis(trifluoromethanesulfonimide) (Mg(TFSI) 2 ), and the formula: Mg(((R 3 R 4 R 5 R 6 )B) 2 Examples of the magnesium (fluoroalkoxy)borate salts include, but are not limited to, magnesium (fluoroalkoxy)borate salts of the formula: Here, R 3 , R 4 , R 5 and R 6 is an at least partially fluorinated alkoxy group.3 ~R 6 Two or more of the following may be the same: 3 ~R 6 can be different from each other. 3 ~R 6 One or more of R may be fully fluorinated. 3 ~R 6 can be monodentate or bidentate. Examples of such magnesium salts are:

[0043] JPEG0007671801000001.jpg31161

[0044] An example of a sodium salt is sodium perchlorate (NaClO 4 ), sodium hexafluorophosphate (NaPF 6 ), sodium hexafluoroarsenate (NaAsF 6 ), sodium tetrafluoroborate (NaBF 4 ), sodium trifluoromethanesulfonate (NaTf), sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), and sodium bis(fluorosulfonyl)imide (NaFSI), but are not limited to these.

[0045] Preferred derivatives of NaFSI and NaTFSI include sodium bis(R-sulfonyl)imide and sodium (R 1 ,R 2 -sulfonyl)imide salts, but are not limited thereto. Here, R, R 1 , R 2 is an at least partially fluorinated alkyl group. For example, R 1 and R 2can be, independently of each other, fluoro, trifluoromethyl, pentafluoroethyl, nonafluorobutyl. Examples of such derivatives include sodium (fluorosulfonyl)(trifluoromethanesulfonyl)imide and sodium (pentafluoroethanesulfonyl)(trifluoromethanesulfonyl)imide.

[0046] The salt of the metal can be dissolved in the first solvent. The concentration of the salt in the electrolyte is 0.5M to 3.25M, for example, 1M to 3.1M, preferably 1.5M to 3.05M, and more preferably 2M to 3M.

[0047] The inventors have found that at these moderate salt concentrations, it is not necessary for the metal salt to have a high solubility and dissociation constant in the first solvent, and therefore a wide range of commercially available metal salts can be used in the electrolyte compositions of the present invention.

[0048] The inventors have further surprisingly found that at such moderate salt concentrations, it is possible to obtain the formation of a stable SEI layer with a moderately high concentration of fluorine ions without compromising the solubility of the metal salt in the solvent. As is known in the art, the presence of fluorine ions in the SEI tends to suppress the overall metal loss as the amount of non-reactive metal (i.e., separated and electrically isolated metal) formed is significantly reduced. This prevents electrolyte decomposition. As a result, efficiency losses are reduced or even prevented, and coulombic efficiency may be improved. Thus, the presence of fluorine ions in the SEI layer contributes to better battery cell performance.

[0049] The electrolyte composition according to the present invention further contains a first solvent. Preferably, the electrolyte contains the first solvent in an amount of 2 to 40% by weight, preferably 5 to 30% by weight, more preferably 7 to 20% by weight, for example 10 to 15% by weight, based on the total weight of the electrolyte composition.

[0050] The first solvent can comprise or consist essentially of an organic solvent such as alkyl esters of carbonates, ethers, nitriles, sulfones, sulfoxides, esters, or a combination of any two or more thereof.

[0051] Examples of organic solvents include, but are not limited to, 1,2-dimethoxyethane (DME), dimethyl carbonate, ethylene carbonate, ethyl methyl carbonate, dimethyl ether, dimethoxyethane, tetrahydrofuran, dioxane, 1,3-dioxolane, tetraethylene glycol dimethyl ether, acetonitrile, succinonitrile, adiponitrile, sulfolane, dimethyl sulfoxide, methyl acetate, and methyl propionate.

[0052] Alternatively and / or additionally, the first solvent may comprise or consist essentially of an ionic liquid. Ionic liquids belong to a class of non-molecular compounds that consist only of ions, i.e. anions and cations. They are known to have negligible vapor pressure and high thermal stability at room temperature, making them suitable for use instead of or in addition to traditional (organic) solvents.

[0053] In the context of the present invention, ionic liquids suitable as the first solvent in the electrolyte composition preferably contain a cation that comprises or consists essentially of pyrrolidinium, piperidinium, imidazolium, ammonium, such as cyclic saturated ammonium, or combinations thereof. The cyclic saturated ammonium may conform to the following formula (III): where X is CH 2 Or O, and n is 1 to 3.

[0054] JPEG0007671801000002.jpg21158

[0055] In the context of the present invention, ionic liquids suitable as the first solvent in the electrolyte composition preferably comprise an anion that comprises or essentially consists of bis(fluorosulfonyl)imide (FSI), bis(trifluoromethanesulfonyl)imide (TFSI), hexafluorophosphate, dicyanamide, tetrachloroaluminate, or combinations thereof. Preferred examples of anions are bis(fluorosulfonyl)imide (FSI) and bis(trifluoromethanesulfonyl)imide (TFSI).

[0056] The additive of the electrolyte composition according to the invention is a perfluorinated organic compound containing at least one further halogen atom. Preferably, said halogen is chlorine (Cl), bromine (Br) or iodine (I). Preferably, said additive is soluble in the first solvent.

[0057] The inventors have found that by using a perfluorinated organic compound in the electrolyte composition, the halogen atoms of this compound bond with the anions of the metal salt. It has been found that the halogen bond interaction between the anions of the salt and the halogen atoms of the additive is strong enough to destroy the interaction between the anions and cations of the metal salt. Thus, such halogen bonds allow the metal ions of the metal salt to move freely. Also, the halogen bond between the halide and the anion reduces the mobility of the anion. As a result, the number of movements of the ions of the alkali metal or alkaline earth metal in the electrolyte can be increased.

[0058] The transfer number is defined as the ratio of the current derived from the cations, here the cations of alkali or alkaline earth metals, to the total current. It is known that a high transfer number allows to reduce the electrolyte concentration deviation during the charge and discharge steps and therefore to increase the power density.

[0059] Thus, the use of perfluorinated organic compounds as additives in the electrolyte composition of the present invention results in battery cells with higher power density than battery cells without the additive. The inventors have also found that the use of the additive of the present invention reduces any parasitic reactions with the battery, which reduces the life of the battery cell.

[0060] The halogen bond is strongest when the halogen is iodine (I), followed by bromine (Br) and chlorine (Cl). Thus, the halogen is preferably iodine (I).

[0061] The additive is represented by the formula (I): XCF 2 -(CF 2 ) x -CF 3-y Y y (I) Here, X is Cl, Br or I, Y is Cl, Br or I, x is 0 to 10, preferably 1 to 8, more preferably 2 to 6, and y is 0 to 3, preferably 0 or 1.

[0062] The perfluoroalkyl halides according to formula (I) can be linear or branched.

[0063] Preferably, at least X is I. Preferably, X and Y are I. That is, the additive of the perfluoroalkyl halide is preferably a perfluoroalkyl iodide. Examples of suitable perfluoroalkyl iodides include 1-iodoperfluorobutane (i.e., X is I, x is 2, y is 0, and since y is 0, Y can be either I, Cl or Br), 1,4-diiodoperfluorobutane (i.e., X is I, Y is I, x is 2, y is 1), 1-iodoperfluorohexane (i.e., X is I, x is 4, y is 0, and since y is 0, Y can be either I, Cl or Br), 1,6-diiodoperfluorohexane (i.e., X is I, Y is I, x is 4, y is 1), and 1,8-diiodoperfluorooctane (i.e., X is I, Y is I, x is 6, y is 1). However, this is not limited thereto.

[0064] Alternatively, X and Y can be Cl. That is, the additive is preferably a perfluoroalkyl chloride. Examples of suitable perfluoroalkyl chlorides include 1-chloroperfluorobutane (i.e., X is Cl, x is 2, y is 0, and since y is 0, Y can be either I, Cl or Br), 1,4-dichloroperfluorobutane (i.e., X is Cl, Y is I, x is 2, y is 1), and 1,6-dichloroperfluorohexane (i.e., X is Cl, Y is I, x is 4, y is 1). However, this is not limited thereto.

[0065] Alternatively, X and Y can be Br. That is, the additive is preferably a perfluoroalkyl bromide. Examples of suitable perfluoroalkyl bromides include 1-bromoperfluorobutane (i.e., X is Br, x is 2, y is 0, and since y is 0, Y can be either I, Cl or Br), 1,4-dibromoperfluorobutane (i.e., X is Br, Y is I, x is 2, y is 1), and 1,6-dibromoperfluorohexane (i.e., X is Br, Y is I, x is 4, y is 1). However, this is not limited thereto.

[0066] Additives according to formula (I), in particular perfluoroalkyl iodides as described above, are particularly preferred when the first solvent is an ionic liquid, as such molecules are known to be highly soluble in ionic liquids, i.e. in solutions with high ionic strength.

[0067] The additive may be an aromatic compound according to formula (II). C 6 F 6-q Z q (II) where Z is Cl, Br, I, or C a F 2a+1-b Q b wherein Q is Cl, Br or I, q is 1 to 5, preferably 1 to 4, more preferably 1 to 3, a is 1 to 10, preferably 1 to 8, for example 1 to 6, more preferably 1 to 4, and b is selected such that 2a+1-b is 1 or greater.

[0068] Preferably, Z is I. Examples of suitable additives according to formula (II) include, but are not limited to, 1,4-diiodo-2,3,5,6-tetrafluorobenzene (i.e., Z is I and q is 2), 1-iodo-2,3,4,5,6-pentafluorobenzene (i.e., Z is I and q is 1).

[0069] Instead, Z is C a F 2a+1-b Q b Z is C a F 2a+1-b Q b In the case of , it can be linear or branched (branched is only possible if C is 3 or more). For example, the additive is 1-[difluoro(iodo)methyl]-2,3,4,5,6-pentafluorobenzene (i.e., Q is I, q is 1, a is 1, b is 1, therefore Z is CF 2 I).

[0070] Preferably, the electrolyte composition contains 0.01 to 20% by weight, preferably 0.05 to 15% by weight, more preferably 0.1 to 10% by weight of the additive, based on the total weight of the electrolyte.

[0071] Optionally, the electrolyte composition further comprises a second solvent. Preferably, the second solvent acts as a diluent. Preferably, the additive is soluble in the second solvent. Preferably, the metal salt is soluble in the second solvent.

[0072] Preferably, when the electrolyte contains a second solvent, the electrolyte contains the second solvent in an amount of 2 to 40 wt %, preferably 5 to 30 wt %, more preferably 7 to 20 wt %, for example 10 to 15 wt %, based on the total weight of the electrolyte composition.

[0073] The second solvent can comprise or consist essentially of a hydrofluoroether, a partially fluorinated alkyl ether, or a combination thereof. For example, the solvent can comprise one or more hydrofluoroethers and / or one or more partially fluorinated alkyl ethers. Examples of second solvents include, but are not limited to, bis(2,2,2-trifluoroethyl)ether (BTFE), 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE).

[0074] The inventors have surprisingly found that when the electrolyte composition includes the additive of the present invention, the amount of the second solvent, when included, can be lower than that of electrolytes in the art, thus making the electrolyte of the present invention, and the battery cells including such electrolytes, less expensive, since these solvents are the most expensive compounds in the electrolyte.

[0075] In a second aspect of the present invention, there is provided an anode-free metal battery cell comprising an electrolyte according to the present invention. Preferably, the anode-free battery cell is a secondary battery cell.

[0076] 1 shows an exemplary embodiment of an anode-free metal battery cell 1. The cell 1 can have a stacked configuration. The battery cell 1 includes an anode 5 and a cathode 2. The battery cell 1 further includes an electrolyte composition 3 according to the present invention.

[0077] Preferably, the cathode 2 comprises an alkali metal, an alkaline earth metal, or a metal from group IIIa of the periodic table. Preferred examples of alkali metals are lithium (Li) (in which case the metal battery cell is a lithium metal battery cell), sodium (Na), and potassium (K). Preferred examples of alkaline earth metals are magnesium (Mg) (in which case the battery cell is a magnesium metal battery cell) and calcium (Ca). A preferred example of a metal from group IIIa is aluminum. Preferably, the metal is an alkali metal or an alkaline earth metal. Preferably, the metal is lithium, magnesium, aluminum, or sodium. Examples of suitable cathode compositions include lithium iron phosphate (LiFePO 4 , often abbreviated as LFP), lithium nickel manganese cobalt oxide (abbreviated as NMC, e.g., NMC types NMC622 and NMC811), high voltage spinel lithium manganese nickel oxide (LiNi 0.5 Mn 1.5 O 4 ), and lithium manganese iron phosphate (LiMnFePO 4 ) are mentioned.

[0078] Preferably, the anode 5 comprises a bare current collector. By bare current collector is meant in the context of the present invention a current collector having no layer deposited thereon, such as a graphite layer. The current collector may comprise or consist essentially of one or more of copper, nickel, nickel-plated copper, titanium, stainless steel.

[0079] The stacked battery cell 1 can optionally include a battery separator membrane 4 between the anode 5 and the cathode 2. The battery separator membrane can include at least one layer of a porous separator membrane. For example, the battery separator membrane can be a single layer membrane or can include two or more, for example three, layers attached to each other. Each layer of the battery separator membrane can include or consist essentially of known materials, polypropylene, polyethylene (PE), aromatic polyamides (aramids), or fluorinated compounds such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE). These can be loaded with ceramics as desired. For example, the battery separator can include a PE layer sandwiched between two layers of ceramic-loaded PVDF. Preferably, at least one of the separator materials is chemically inert. However, while the porous separator is preferably capable of absorbing the liquid electrolyte, chemically inert materials often do not wet easily. For this purpose, the membrane material can be subjected to a surface treatment or coating process such as spray coating, dip coating or plasma coating (atmospheric pressure plasma or low pressure plasma). Alternatively, the battery separator membrane can be a ceramic material.

[0080] Preferably, the metal of the metal salt and the metal of the anode-free battery cell are the same. Preferably, the metal salt is a lithium salt, a magnesium salt, an aluminum salt or a sodium salt and the battery cell is a anode-free lithium metal battery cell, anode-free magnesium metal battery cell, anode-free aluminum metal battery cell or anode-free sodium metal battery cell.

[0081] example Anode-free battery cells were fabricated to evaluate the charge / discharge and cycle life properties of anode-free lithium metal battery cells. To reduce the risk of contamination during fabrication, the two-electrode cells were fabricated in a dry room with a dew point of -60°C.

[0082] A two-electrode battery cell according to FIG. 1 was made in which the electrolyte was according to the present invention and had the following composition: 8.00 g LiFSI (lithium salt), 6.00 g DME (first solvent), 6.00 g 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (second solvent), and 0.14 g of the additive 1,4-diiodoperfluorobutane.

[0083] A reference battery cell was also prepared in which the electrolyte composition was 8.00 g LiFSI (lithium salt), 6.00 g DME (first solvent), and 6.00 g 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (second solvent), without adding any perfluorinated compound containing at least one halogen atom as an additive.

[0084] In both cells, the cathode was an LFP cathode, the anode was a bare current collector, a 15 μm thick copper foil, and the separator was a tri-layer membrane including a PE layer sandwiched between two ceramic-loaded PVDF layers. Both cells contained 60 μl of electrolyte. The cells were vacuum sealed.

[0085] The resulting thin-film battery cell according to the present invention and the reference thin-film battery cell were subjected to a first charge-discharge-charge sequence and repeated charge / discharge cycles at 25° C. In the first charge of the sequence, the battery was discharged to 3.6 V at 1 mA (0.13 mA / cm 2 The battery cell was charged to 2.8 V (1 mA / cm) at 7.56 mA for the discharge sequence. 2 The battery cell was discharged to 3.6 V (0.2 mA / cm) at 1.52 mA for the second charge in the sequence. 2 ) the battery cells were charged.

[0086] The specific discharge capacity was measured for each charge / discharge cycle, and the results are shown in Figure 2. Figure 2 shows the specific discharge capacity as a function of the number of charge / discharge cycles up to 155 cycles for the battery cell (10) according to the present invention and the reference battery cell (20). Figure 2 clearly shows that the specific discharge capacity of the battery cell (10) according to the present invention is always equal to or greater than the capacity of the reference battery cell (20), and is often even greater. [Explanation of symbols]

[0087] 1 Battery Cell 2 Cathode 3 Electrolytes 4. Separator 5 Anode 10 Profile of a battery cell containing the electrolyte composition according to the present invention 20 Profiles of battery cells containing reference electrolyte compositions

Claims

1. 1. An electrolyte composition for an anode-free metal battery cell, comprising: the metal is an alkali metal, an alkaline earth metal, or a metal from Group IIIa of the periodic table; The electrolyte composition comprises: A first solvent; a salt of an alkali metal, an alkaline earth metal, or a metal from Group IIIa of the periodic table; and an additive, the salt is soluble in the first solvent; the concentration of the salt in the electrolyte composition is 2M to 3M; the additive is a perfluorinated organic compound containing at least one halogen atom other than fluorine; the halogen is chlorine, bromine or iodine; The additive has the formula (I): XCF 2 -(CF 2 ) x -CF 3-y Y y (I) is a perfluoroalkyl halide conforming to the formula where X is Cl, Br or I; Y is Cl, Br or I; x is 0 to 10; y is 0 to 3; X and Y are I, The additive is 1,4-diiodoperfluorobutane. Electrolyte composition comprising:

2. The metal is lithium (Li) or magnesium (Mg), the salt is a lithium salt or a magnesium salt, The battery cell is a lithium metal battery cell or a magnesium metal battery cell. The electrolyte composition according to claim 1 .

3. The electrolyte composition contains the additive in an amount of 0.01 to 20% by weight based on the total weight of the electrolyte composition. The electrolyte composition according to claim 1 .

4. The first solvent is selected from the group consisting of carbonates, ethers, nitriles, sulfones, sulfoxides, esters, and ionic liquids. The electrolyte composition according to any one of claims 1 to 3.

5. The first solvent is an ionic liquid, The ionic liquid comprises a cation and an anion, the cation comprises or consists essentially of pyrrolidinium, piperidinium, imidazolium, cyclic saturated ammonium, or combinations thereof; The anion comprises or consists essentially of bis(fluorosulfonyl)imide (FSI), bis(trifluoromethanesulfonyl)imide (TFSI), or a combination thereof.

5. The electrolyte composition according to claim 4.

6. The electrolyte composition further comprises a second solvent, The second solvent is a hydrofluoroether or a partially fluorinated alkyl ether. The electrolyte composition according to any one of claims 1 to 3.

7. The electrolyte composition further comprises a second solvent, The second solvent is a hydrofluoroether or a partially fluorinated alkyl ether.

5. The electrolyte composition according to claim 4.

8. A metal battery cell (1) having a free anode, The electrolyte composition (3) according to any one of claims 1 to 3, The battery cell according to claim 1, wherein the metal is an alkali metal, an alkaline earth metal, or a metal from Group IIIa of the periodic table.

9. A metal battery cell (1) having a free anode, The electrolyte composition (3) according to claim 4, The battery cell according to claim 1, wherein the metal is an alkali metal, an alkaline earth metal, or a metal from Group IIIa of the periodic table.

10. The battery cell is a lithium metal battery cell or a magnesium metal battery cell. The battery cell according to claim 8 .

11. The battery cell is a lithium metal battery cell or a magnesium metal battery cell. The battery cell according to claim 9 .

12. A secondary battery cell. The battery cell according to claim 8 .

13. A secondary battery cell. The battery cell according to claim 9 .

14. The anode (5) comprises a bare current collector The battery cell according to claim 8 .

15. The anode (5) comprises a bare current collector. The battery cell according to claim 9 .

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