High-voltage lithium ion battery

Abstract

A high-voltage lithium ion battery, comprising a positive electrode, a negative electrode, and an electrolyte. Using a branched carboxylic ester-containing compound as an electrolyte solvent better satisfies the requirements of high-voltage battery systems, thereby significantly the stability of batteries under high temperature and high pressure, and improving the low-temperature cycle performance of the batteries.

Description

A high-voltage lithium ion battery TECHNICAL FIELD

[0001] The present application relates to a lithium ion battery, in particular to a high-voltage lithium ion battery. BACKGROUND

[0002] As one of the four parts of lithium ion battery, electrolyte has a significant impact on the performance of lithium ion battery. Compared with the widely used carbonate-based electrolyte (EC), carboxylate solvent has higher dielectric constant and smaller viscosity, and lower melting point than carbonate, so it is a good low-temperature solvent. However, the widely used carboxylate solvent is basically a small molecule solvent such as methyl formate, ethyl acetate, butyl acetate, methyl acetate and ethyl propionate, and is usually used as an auxiliary solvent in combination with carbonate and other solvents, and there is a lack of comprehensive research on carboxylate solvent and combined system. CN107666011A discloses an electrolyte system using carboxylate as the main solvent, and it is found that the performance of the electrolyte system formed by configuring the isomers with the same molecular weight is greatly different.

[0003] On the other hand, with the popularization and development of electrification technology, the energy density of electrochemical energy storage devices represented by lithium ion batteries is required to be higher and higher. Improving the energy density is a hot and difficult point of current research. One of the methods to improve the energy density is to increase the charge cut-off voltage of traditional cathode materials, for example, to increase the charge voltage of lithium cobaltate to 4.35V or 4.4V, and the capacity of the battery can be increased by about 15%. However, with the increase of working voltage and charge cut-off voltage, the oxidation activity of the cathode material increases, and the reaction between the cathode active material and the electrolyte also accelerates, resulting in serious gas swelling of the battery at high voltage, and the cycle performance decreases, which seriously restricts the performance of the cathode material. On the other hand, with the increase of voltage, the low-temperature charge-discharge performance of the battery is sharply decreased, especially the discharge capacity at-40° is significantly decreased.

[0004] Therefore, it is of great significance to develop an electrolyte suitable for high-voltage material system to improve the overall performance of the battery. The present application develops a high-voltage battery containing a new carboxylate solvent system, which can better solve the problems of the current high-voltage battery.

[0005] SUMMARY

[0006] To solve the above problems, the present application provides a lithium ion battery, which comprises a positive electrode, a negative electrode and an electrolyte, wherein the electrolyte comprises a solvent, the solvent comprises compound A and compound B, and the mass fraction of compound A in the solvent is denoted as w A , and the mass fraction of compound B in the solvent is w B ; the compound A satisfies the following structure:

[0007] wherein R1, R2 are independently selected from hydrocarbyl, fluorohydrocarbyl, the number of methyl groups in R1 is x, 2≤x≤5 and at least one carbon atom of the R1 backbone is attached to at least two methyl groups;

[0008] Compound B satisfies the following structural formula:

[0009] R and R' are methylene groups, and at least one H atom of at least one of R and R' is substituted with F, and the number of F atoms in R, R' is y, 1≤y≤4;

[0010] w A , w B , x, y satisfy the following relationship:

[0011] 1.5≤w A / w B ≤50;

[0012] 3≤x+y≤7;

[0013] Preferably, 2.4≤w A / w B ≤20;

[0014] Preferably, 3≤x+y≤5.

[0015] Preferably, the number of backbone carbon atoms of R1 is 1-3.

[0016] Preferably, the lithium ion battery comprises at least one positive electrode active material with a charge cut-off voltage≥4.5V.

[0017] Preferably, the negative electrode lithium intercalation cut-off potential is≤1.0V.

[0018] Preferably, the sum of the number of carbon atoms of R1 and R2 is≥4 and≤9.

[0019] Preferably, the number of backbone carbon atoms of R2 is 1-3.

[0020] Preferably, R2 is selected from one of methyl, ethyl.

[0021] Preferably, in compound A, at least one H atom in R1, R2 is substituted with F.

[0022] Preferably, in compound A, at least one H atom in R1 is substituted with F.

[0023] Preferably, the mixture of compound A and compound B accounts for 60-100% of the total mass of the solvent.

[0024] Preferably, the mixture of the compound A and the compound B accounts for 70-100% of the total mass of the solvent.

[0025] Preferably, the mixture of the compound A and the compound B accounts for 80-100% of the total mass of the solvent.

[0026] Preferably, the mixture of the compound A and the compound B accounts for 90-100% of the total mass of the solvent.

[0027] Preferably, the positive electrode active material is a mixture of multiple charge cut-off voltages ≥ 4.5V.

[0028] The present application forms an electrolyte solvent system by compounding two compounds with specific structures, unexpectedly produces good effects in a high-voltage battery system, and significantly improves the room temperature cycle performance and low-temperature charge-discharge performance of the high-voltage battery. DETAILED DESCRIPTION

[0029] The embodiment provides a high-voltage lithium ion battery, which comprises a positive electrode, a negative electrode and an electrolyte, the electrolyte comprises a solvent, the solvent comprises a compound A and a compound B, the mass fraction of the compound A in the solvent is denoted as w A , and the mass fraction of the compound B in the solvent is w B ; the compound A satisfies the following structure:

[0030] wherein, and the mass fraction of the A solvent is denoted as w A , the number of methyl groups in R1 is x, 2≤x≤5, and at least one carbon atom in the main chain of R1 is connected to at least two methyl groups;

[0031] The compound B satisfies the following structural formula:

[0032] R and R' are methylene groups, at least one H atom in at least one of R and R' is replaced by F, and the number of F atoms in R and R' is y, 1≤y≤4;

[0033] and the mass fraction of the B solvent is denoted as w B ;

[0034] 1.5≤w A / w B ≤50;

[0035] Preferably, 2.4≤w A / w B ≤20;

[0036] 3≤x+y≤7;

[0037] Preferably, 3≤x+y≤5.

[0038] Preferably, the number of main chain carbon atoms of R1 is 1-3.

[0039] Preferably, the mixture of compound A and compound B accounts for 50-100% of the total mass of the solvent.

[0040] Preferably, the mixture of compound A and compound B accounts for 60-100% of the total mass of the solvent.

[0041] Preferably, the mixture of compound A and compound B accounts for 70-100% of the total mass of the solvent.

[0042] Preferably, the mixture of compound A and compound B accounts for 80-100% of the total mass of the solvent.

[0043] Preferably, the mixture of compound A and compound B accounts for 90-100% of the total mass of the solvent.

[0044] It can be understood that, whether R1 or R2, the meaning of main chain is that when R1 or R2 contains a special functional group (such as alkenyl, alkynyl, cycloalkyl, etc., the special group has a conventional definition in organic chemistry, which is not described here), the carbon chain containing the special functional group is the main chain; if it does not contain a special functional group, the longest carbon chain is the main chain. It should be noted that, in order to facilitate description, the chain end methyl is not counted in the main chain length.

[0045] Taking compound R1C(O)OCH(CH3)C(CH3)3 as an example, wherein R2 is CH(CH3)C(CH3)3, according to the above counting method, -CH-C- is the main chain, and the number of main chain carbon atoms is 2, wherein -CH-C- is counted as the number of main chain carbon atoms, and the terminal methyl (CH3) is not counted as the number of main chain carbon atoms; similarly, the H in the terminal methyl is not counted as the number of main chain hydrogen atoms. The number of main chain hydrogen atoms in -CH-C- is 1.

[0046] It can be understood that when R2 is methyl, the number of main chain carbon atoms is 0.

[0047] Preferably, the sum of the number of carbon atoms of R1 and R2 is greater than or equal to 4 and less than or equal to 9.

[0048] It can be understood that the sum of the number of carbon atoms here does not refer to the number of main chain carbon atoms, but the number of all carbon atoms in R1 and R2, including the terminal methyl. Taking R1C(O)OCH(CH3)C(CH3)3 as an example, wherein R2 is CH(CH3)C(CH3)3, the number of carbon atoms is 6. When the number of carbon atoms of R1 and R2 is too large, it can cause an undesirable negative impact on the performance of the battery.

[0049] Preferably, the number of carbon atoms in the main chain of R2 is 1-3.

[0050] Preferably, R2 is selected from one of methyl, ethyl.

[0051] Preferably, in compound A, at least one H atom in R1, R2 is substituted by F.

[0052] Preferably, in compound A, at least one H atom in R1 is substituted by F.

[0053] As an embodiment, R1, R2 are independently selected from a hydrocarbon group or a fluorohydrocarbon group with a main chain of 1-3 carbon atoms.

[0054] As an embodiment, the number of hydrogen atoms in the main chain of R2 is less than or equal to 1.

[0055] Preferably, in compound A, at least one H atom in R1, R2 is substituted by F.

[0056] Preferably, in compound A, at least one H atom in R1 is substituted by F.

[0057] Preferably, in compound A, at least one H atom in R2 is substituted by F.

[0058] As an embodiment, R2 contains more than 2 methyl groups, and each methyl group has at least one H substituted by F.

[0059] As an embodiment, the number of hydrogen atoms in the main chain of R2 is less than or equal to 1.

[0060] It can be understood that, for compound A, F substitution is a known technology, and fluorine-substituted ester compounds have relatively better certain properties. Any fluorine substitution modification of compound A based on actual use needs without departing from the inventive concept of the present application shall be considered within the protection scope of the present application.

[0061] The number of hydrogen atoms in the main chain being less than or equal to 1 can be understood as, after removing the terminal methyl group, the number of hydrogen atoms connected to the carbon atoms in the main chain is less than or equal to 1. Still taking R1C(O)OCH(CH3)C(CH3)3 as an example, wherein R2 is CH(CH3)C(CH3)3, and the main chain is -CH-C, the number of hydrogen atoms in the main chain is 1.

[0062] As an embodiment, compound A accounts for 60-98% of the total mass of the solvent.

[0063] As a preferred embodiment, the compound A accounts for 70-98% of the total mass of the solvent.

[0064] Surprisingly, on the basis of meeting the ratio relationship of compound A and compound B of the present application, the performance of the electrolyte of the present application is improved with the increase of the content of compound B.

[0065] It can be understood that compound B has the effect of an additive in the electrolyte system, which has the effect of assisting film formation, providing physical and chemical stability, inhibiting redox reaction, enhancing solid electrolyte interface stability, improving battery charge and discharge performance, inhibiting mucus generation, etc. The above-mentioned cognitive mechanism does not hinder the use of compound B in the present application. It should be considered that the technical solution of using compound A and compound B in the ratio range defined in the present application is within the protection scope of the present application.

[0066] Preferably, the compound B accounts for more than 10% of the total mass of the solvent.

[0067] Only as a guess and not as a limitation on the scope of protection, when the added amount of compound B is greater than 10%, the F atom in it forms a more effective complex with compound A.

[0068] It can be understood that the solvent can contain a compound A satisfying the structural formula, or can be composed of two or more compounds A satisfying the related structural formula to form a mixture, i.e. composed of compound A1 and compound A2.

[0069] As an embodiment, the solvent includes two or more compounds A satisfying the structural formula.

[0070] As an embodiment, the solvent includes two or more compounds A satisfying the structural formula.

[0071] It can be understood that on the basis of meeting the composition and ratio relationship of compound A and compound B of the present application, the electrolyte solvent can contain other compounds not meeting the structural formula.

[0072] It can be understood that using the compound satisfying the relevant structural formula or the mixture of two or more compounds as the main solvent can significantly improve the battery performance, but the protection scope of the present application does not exclude the technical solutions using other carboxylate compounds not satisfying the structural formula, for example, using two compounds satisfying the structural formula, and the mass of the two compounds accounts for 80% of the total mass of the electrolyte solvent, and the electrolyte solvent further includes a carboxylate compound C with the number of hydrogen atoms in the R1 main chain being greater than 1, and the mass of the compound C accounts for 5% of the total mass of the electrolyte. The technical solution should still be considered as the protection scope of the present application. It can be understood that as long as the compound satisfying the mass ratio of the present application is used in the electrolyte solvent of the high-voltage lithium ion battery system, it should be considered as falling within the protection scope of the present application, and not all carboxylate compounds must use the compound satisfying the structural formula.

[0073] As an embodiment, the solvent further includes a cyclic carbonate, and the mass of the cyclic carbonate accounts for 0-20% of the total mass of the solvent.

[0074] As a preferred embodiment, the mass of the cyclic carbonate accounts for 0-10% of the total mass of the solvent.

[0075] As an embodiment, the solvent further includes a chain carbonate, and the mass of the chain carbonate accounts for 0-20% of the total mass of the solvent.

[0076] As a preferred embodiment, the mass of the chain carbonate accounts for 0-10% of the total mass of the solvent.

[0077] As an embodiment, the sum of the mass of the cyclic carbonate and the mass of the chain carbonate accounts for 0-20% of the total mass of the solvent; preferably, the sum of the mass of the cyclic carbonate and the mass of the chain carbonate accounts for 0-10% of the total mass of the solvent; preferably, the sum of the mass of the cyclic carbonate and the mass of the chain carbonate accounts for 0-5% of the total mass of the solvent, and the mass of the cyclic carbonate and the mass of the chain carbonate are not zero.

[0078] It can be understood that the high-voltage positive electrode material refers to the positive electrode active material with a relatively high working voltage. Generally, the positive electrode material with a charge cut-off voltage of 4.5 V or more is referred to as a high-voltage material. At present, the traditional carbonate-based electrolyte cannot be well adapted to the high-voltage system, and improving the oxidation resistance of the electrolyte is the main means. The present application does not have special requirements for the positive electrode material, and any known high-voltage positive electrode material can be used in the present application without deviating from the inventive concept of the present application.

[0079] Preferably, the charge cut-off voltage of the positive electrode material is greater than 4.5 V;

[0080] Further preferably, the charge cut-off voltage of the cathode material is greater than 4.6 V.

[0081] Particularly preferably, the charge cut-off voltage of the cathode material is greater than 4.8 V.

[0082] It can be understood that the cathode material of the present application can be a mixture or a pure substance, and when it is a pure substance, it can be a cathode active material with a charge cut-off voltage greater than 4.5 V. When it is a mixture, it can be a mixture of a plurality of cathode active materials with a charge cut-off voltage greater than 4.5 V, or a mixture of one or more cathode active materials with a charge cut-off voltage greater than 4.5 V and one or more cathode materials with a charge cut-off voltage less than 4.5 V.

[0083] As an embodiment, the cathode active material is a mixture, and at least one of the cathode active materials has a voltage plateau higher than 4.5 V.

[0084] As an embodiment, the cathode active material is a mixture, and at least one of the cathode active materials has a charge cut-off voltage greater than or equal to 4.6 V.

[0085] As an embodiment, the cathode active material is a mixture of a plurality of cathode active materials with a charge cut-off voltage greater than or equal to 4.6 V.

[0086] As an embodiment, the cathode active material is a pure substance.

[0087] It can be understood that a pure substance refers to containing only one cathode active material, and the presence of impurities and non-cathode active material additives in a single cathode active material system should not be understood as the cathode active material system being a mixture.

[0088] Preferably, the charge cut-off voltage of the cathode active material is greater than 4.8 V.

[0089] Preferably, the cathode active material with a charge cut-off voltage greater than 4.5 V accounts for more than 50 wt% of all cathode active materials in the cathode active material layer.

[0090] Preferably, the cathode active material with a charge cut-off voltage greater than 4.5 V accounts for more than 60 wt% of all cathode active materials in the cathode active material layer.

[0091] Preferably, the cathode active material with a charge cut-off voltage greater than 4.5 V accounts for more than 70 wt% of all cathode active materials in the cathode active material layer.

[0092] Preferably, the cathode active material with a charge cut-off voltage greater than 4.5 V accounts for more than 80 wt% of all cathode active materials in the cathode active material layer.

[0093] Preferably, the positive electrode active material having a charge cut-off voltage of greater than 4.5 V accounts for 90 wt% or more of all positive electrode active materials in the positive electrode active material layer.

[0094] Preferably, the positive electrode active material having a charge cut-off voltage of greater than 4.5 V accounts for 95 wt% or more of all positive electrode active materials in the positive electrode active material layer.

[0095] It is to be understood that, when a multi-layered electrode is used, as long as one layer in the entire positive electrode active material layer satisfies the above-mentioned proportion requirement, it is to be considered as falling within the scope of the present application. The above-mentioned positive electrode active material layer is to be understood as the active material layer containing the positive electrode active material having a charge cut-off voltage of greater than 4.5 V, and not the entire positive electrode active material layer.

[0096] By way of example only, and not by way of limitation, the positive electrode active material that operates at a potential of 4.5 V or more with respect to lithium is preferably a lithium-containing composite oxide. The lithium-containing composite oxide includes a spinel-type lithium-manganese composite oxide, an olivine-type lithium-containing manganese composite oxide, and an inverse spinel-type lithium-containing manganese composite oxide. Specifically, for example, Li a (M x Mn 2-x )O4, wherein 0.4 < x < 2 and 0 < a < 1.2, and M is at least one selected from the group consisting of Ni, Co, Fe, Cr, and Cu. Among these, from the viewpoint of safety, the spinel-type lithium-manganese composite oxide is preferred.

[0097] In addition, as the lithium-manganese composite oxide containing Mn, for example, a spinel-type manganese oxide represented by the following formula can be used.

[0098] Preferably, a lithium-manganese composite oxide represented by the following formula is preferably used. Li a (M x Mn 2-x-y Y y )(O 4-w Z w ) wherein 0.4 < x < 1.2, 0 < y, x + y < 2, 0 < a < 1.2, and 0 < w < 1; M is at least one selected from Co, Ni, Fe, Cr, and Cu; Y is at least one selected from Li, B, Na, Mg, Al, Ti, Si, K, and Ca; and Z is at least one selected from F and Cl.

[0099] Preferably, from the viewpoint of obtaining sufficient capacity and achieving a longer life, among these lithium-manganese composite oxides, a spinel-type compound represented by the following formula is more preferably used. LiNi x Mn 2-x-y A yO4wherein 0.4 < x < 0.6 and 0 < y < 0.3, and A represents at least one metal selected from Li, B, Na, Mg, Al, Ti and Si. Preferably, 0 < y < 0.2.

[0100] Preferably, examples of the other high-voltage positive electrode active material include olivine-type compounds represented by the following formula.

[0101] Li x MPO4F y wherein 0 < x < 2 and 0 < y < 1, and M is at least one selected from Co and Ni.

[0102] Preferably, examples of the positive electrode active material for which the charge cut-off voltage reaches 5 V include compounds represented by the following formula.

[0103] Li x [Li a M b Mn 1-a-b ]O2wherein 0 < x < 1, 0.05 < a < 0.3, and 0.1 < b < 0.4, and M is at least one selected from Ni, Co, Fe and Cr.

[0104] It can be understood that when the positive electrode active material consists of a mixture of the high-voltage positive electrode active material and the positive electrode active material for which the charge cut-off voltage is lower than 4.5 V, the kind of the positive electrode active material for which the charge cut-off voltage is lower than 4.5 V is not particularly limited in the present application, and any positive electrode active material for which the charge cut-off voltage is lower than 4.5 V can be used in the present application without departing from the concept of the present application, and for example, the positive electrode active material for which the charge cut-off voltage is lower than 4.5 V is one of a layered oxide positive electrode material, a spinel positive electrode and a polyanion positive electrode. For example, the layered oxide positive electrode (e.g., a rock-salt layered oxide) includes one or more lithium-based positive electrode active materials selected from the group consisting of LiCoO2(LCO), LiNi 1-x-y Co x Al y O2(wherein 0 < x < 1 and 0 < y < 1) and Li1 +x MO2(wherein M is one of Ni, Co and Al and 0 < x < 1); the spinel positive electrode includes one or more lithium-based positive electrode active materials selected from the group consisting of LiMn2O4(LMO) and LiNi x Mn 1.5 O4. The olivine-type cathode includes one or more lithium-based positive electrode active materials LiMPO4(wherein M is at least one of Fe, Ni and Co); it can be understood that the above-mentioned materials are merely illustrative examples and are not limiting to the scope of protection.

[0105] In one embodiment, one or more lithium-based positive active materials can optionally be coated (e.g., by LiNbO3and / or Al2O3) and / or can be doped (e.g., by magnesium (Mg)) and can also be coated with two positive active materials to form a core-shell structure, such as coating a high voltage positive material with a low voltage positive material, it is understood that modifications to the positive material or positive active material layer are within the scope of the present application. Further, in certain embodiments, one or more lithium-based positive active materials can optionally be mixed with one or more conductive agents that provide an electronic conduction path and / or at least one polymeric binder that improves the structural integrity of the positive electrode. For example, the positive active material layer can include greater than or equal to about 30 wt% to less than or equal to about 99 wt% of one or more lithium-based positive active materials; greater than or equal to about 0 wt% to less than or equal to about 30 wt% of a conductive agent; and greater than or equal to about 0 wt% to less than or equal to about 20 wt% of a binder, and in certain aspects, optionally greater than or equal to about 1 wt% to less than or equal to about 20 wt% of a binder. In the lithium-based positive active materials, the present application does not limit the proportion of high voltage positive materials, and adjustments to the amount of high voltage positive materials should be considered within the scope of the present application without departing from the inventive concept of the present application; as a preferred technical solution, the positive active material with a charge cut-off voltage greater than 4.5 V accounts for more than 50 wt% of all positive active materials; further preferably, the positive active material with a charge cut-off voltage greater than 4.5 V accounts for more than 70 wt% of all positive active materials; particularly preferably, the positive active material with a charge cut-off voltage greater than 4.5 V accounts for more than 90 wt% of all positive active materials.

[0106] The positive active material can optionally be mixed with a binder such as polytetrafluoroethylene (PTFE), sodium carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), nitrile rubber (NBR), styrene-ethylene-butylene-styrene copolymer (SEBS), styrene-butadiene-styrene copolymer (SBS), lithium polyacrylate (LiPAA), sodium polyacrylate (NaPAA), sodium alginate, lithium alginate, or combinations thereof. The conductive agent can include carbon-based materials, powdered nickel or other metal particles, or conductive polymers. Carbon-based materials can include particles of, for example, carbon black, graphite, acetylene black (e.g., KETCHEN™ black or DENKA™ black), carbon fibers and nanotubes, graphene, and the like. Examples of conductive polymers include polyaniline, polythiophene, polyacetylene, polypyrrole, and the like.

[0107] Surprisingly, the electrolyte system involved in the present application is particularly suitable for high-voltage battery systems, and also has good comprehensive performance for the negative electrode.

[0108] Preferably, the negative electrode lithium intercalation potential is ≤ 1.0 V.

[0109] It is known that for carboxylic acid ester solvents, branched chains can cause the electrolyte viscosity to be too high, thereby affecting battery performance. Surprisingly, the electrolyte system of the present application has good viscosity performance and maintains a good low-temperature capacity retention rate even under low-temperature conditions.

[0110] For compound A, an increase in the number of methyl groups increases the viscosity of the electrolyte, which adversely affects the battery. Surprisingly, when the number of methyl groups in R1 in the compound and the number of F atoms in compound B satisfy a certain relationship, it exhibits particular advantages in high-voltage systems, showing better results than the compound B and the linear carboxylic acid ester complex, which is significantly different from the fact that linear carboxylic acid esters exhibit better performance in the prior art, especially in non-high-voltage systems.

[0111] As a preferred embodiment, the structural formula of the compound B is selected from one of the following structural formulas:

[0112] Preferably, the electrolyte includes two or more compounds B that meet the structural formula.

[0113] It is understood that the electrolyte also includes a lithium salt, and the application does not have a particular requirement for the type of lithium salt, and any known type of lithium salt can be used in the application without departing from the inventive concept of the application, and only as an illustrative example, without limiting the scope of protection, and the appropriate lithium salt generally has an inert anion. A non-limiting list of lithium salts that are soluble in an organic solvent or mixture of organic solvents to form a non-aqueous liquid electrolyte solution includes lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium tetrachloroaluminate (LiAlCl4), lithium iodide (LiI), lithium bromide (LiBr), lithium thiocyanate (LiSCN), lithium tetrafluoroborate (LiBF4), lithium difluoro oxalato borate (LiBF2(C2O4)) (LiODFB), lithium tetraphenylborate (LiB(C6H5)4), lithium bis(oxalato)borate (LiB(C2O4)2) (LiBOB), lithium tetrafluoro oxalato phosphate (LiPF4(C2O4)) (LiFOP), lithium nitrate (LiNO3), lithium hexafluoroarsenate (LiAsF6), lithium triflate (LiCF3SO3), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) (LiN(CF3SO2)2), lithium bis(fluorosulfonyl)imide (LiN(FSO2)2) (LiFSI), and combinations thereof. In certain variations, the lithium salt is selected from lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) (LiN(CF3SO2)2), lithium bis(fluorosulfonyl)imide (LiN(FSO2)2) (LiFSI), lithium fluoroalkylphosphonate (LiFAP), lithium phosphate (Li3PO4), and combinations thereof.

[0114] In certain solutions, the electrolyte can include one or more lithium salts at a concentration greater than or equal to 1 M to less than or equal to about 2 M. In certain variations, for example, when the electrolyte has a lithium concentration greater than about 2 M or has an ionic liquid, the electrolyte can include one or more diluents, such as hydrofluoroethers (HFEs).

[0115] It is understood that the solvent can also include other solvents in addition to Compound A and Compound B, such as one or more of carbonates, sulfites, sulfonates, sulfones, ethers, organosilicon compounds, organoboron compounds, nitriles, ionic liquids, and phosphine nitrile compounds, and the use of other solvents in the solvent should still be considered within the scope of the present application without departing from the inventive concept of the present application. By way of example only and not limitation, the known carbonates can be cyclic carbonates, linear carbonates, by way of example only and not limitation, the cyclic carbonates can be one or more of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC); the linear carbonates can be one or more of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC). Aliphatic carboxylic acid esters (e.g., methyl formate, methyl acetate, methyl propionate), γ-lactones (e.g., γ-butyrolactone, γ-valerolactone), linear structure ethers (e.g., 1,2-dimethoxyethane (DME), 1-2-diethoxyethane, ethoxymethoxyethane), cyclic ethers (e.g., tetrahydrofuran, 2-methyltetrahydrofuran), 1,3-dioxolane (DOL), sulfur compounds (e.g., sulfolane), and combinations thereof.

[0116] Surprisingly, although the addition of carbonates in the electrolyte system of the present application is a possible embodiment, it does not increase the performance of the electrolyte and the battery comprising the electrolyte of the present application, on the contrary, it is advantageous not to add carbonates.

[0117] Preferably, the amount of carbonates added in the electrolyte is less than 10wt% of the total mass of the electrolyte;

[0118] Further preferably, the amount of carbonates added in the electrolyte is less than 5wt% of the total mass of the electrolyte.

[0119] Further preferably, the amount of carbonates added in the electrolyte is less than 5wt% of the total mass of the electrolyte.

[0120] More preferably, the amount of carbonates added in the electrolyte is less than 1wt% of the total mass of the electrolyte.

[0121] Particularly preferably, the amount of carbonates added in the electrolyte is less than 0.01wt% of the total mass of the electrolyte.

[0122] It is understood that the above-mentioned carbonates, which conform to the conventional definition in the art, refer to compounds in which the hydrogen atoms of the two hydroxyl groups (-OH) in the carbonate molecule are partially or completely replaced by alkyl groups (R3, R4). The general formula is R3O-CO-OH or R3O-CO-OR4. Fluorinated or other modified carbonates are not included.

[0123] As an embodiment, the electrolyte further comprises an additive, and the additive of the present application includes, but is not limited to, at least one of a film-forming additive, an overcharge-prevention additive, a flame-retardant additive, a conductive additive, and a wetting additive. It can be understood that the above-mentioned additives are only illustrative examples, and the use of related additives based on the needs of different functions should still be considered within the protection scope of the present application without deviating from the inventive concept of the present application.

[0124] The lithium ion battery further comprises a separator or a solid-state electrolyte membrane or a combination of both. The separator can be a microporous polymeric separator, such as a polyolefin, including polyolefins made from homopolymers (derived from a single monomer component) or heteropolymers (derived from more than one monomer component), which can be linear or branched. In certain aspects, the polyolefin can be polyethylene (PE), polypropylene (PP), or a blend of PE and PP, or a multi-layer structured porous membrane of PE and / or PP.

[0125] When the separator is a microporous polymeric separator, it can be a single layer or a multi-layer laminate. For example, in one embodiment, a single layer of polyolefin can form the entire microporous polymeric separator. As another embodiment, similar or different layers can be assembled to form the separator.

[0126] In addition, the separator can be mixed with a ceramic material, or its surface can be coated with a ceramic material. For example, the ceramic coating can include aluminum oxide (AI2O3), silicon dioxide (SiO2), or a combination thereof.

[0127] The solid-state electrolyte membrane can also serve as a separator to isolate the positive electrode and the negative electrode to prevent short circuiting of the battery. The present application does not have a specific requirement for the material system or structure of the solid-state electrolyte membrane, and any known solid-state electrolyte material can be used in the present application without deviating from the inventive concept of the present application, including oxide solid-state electrolytes, sulfide solid-state electrolytes, halide solid-state electrolytes, boride solid-state electrolytes, polymer solid-state electrolytes, or a combination thereof.

[0128] The present application does not have a specific requirement for the negative electrode, and any known negative electrode material or system can be used in the present application without deviating from the inventive concept of the present application. Generally, the negative electrode is formed of a lithium host material that can be used as a lithium ion battery negative electrode active material. The negative electrode active material is disposed in one or more layers, and in certain embodiments, the negative electrode can further include an electrolyte, such as a plurality of electrolyte particles.

[0129] The negative electrode may include a lithium-based negative electrode active material, which includes, for example, lithium metal and / or a lithium alloy. In certain embodiments, the negative electrode is a silicon-based negative electrode active material, which includes silicon, such as a silicon alloy, silicon oxide, or a combination thereof, which may also be mixed with graphite in some cases. In other embodiments, the negative electrode may include a carbonaceous-based negative electrode active material, which includes one or more of graphite, graphene, carbon nanotubes (CNTs), and combinations thereof. In another embodiment, the negative electrode includes one or more negative electrode active materials that accept lithium, such as lithium titanium oxide (Li4Ti5O 12 ), one or more transition metals (e.g., tin (Sn)), one or more metal oxides (e.g., vanadium oxide (V2O5), tin oxide (SnO), titanium dioxide (TiO2)), titanium niobium oxide (Ti x Nb y O z , where 0≤x≤2, 0≤y≤24 and 0≤z≤64), a metal alloy such as copper tin alloy (Cu6Sn5), and one or more metal sulfides such as iron sulfide (FeS).

[0130] Alternatively, the negative electrode active material in the negative electrode can be mixed with one or more conductive agents that provide an electronic conduction path and / or at least one polymer binder material that improves the structural integrity of the negative electrode. For example, alternatively, the binder can be poly (tetrafluoroethylene) (PTFE), sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), acrylonitrile-butadiene rubber (NBR), styrene ethylene butylene styrene copolymer (SEBS), styrene butadiene styrene copolymer (SBS), lithium polyacrylate (LiPAA), sodium polyacrylate (NaPAA), sodium alginate, lithium alginate or a combination thereof. The conductive agent may include carbon-based materials, powdered nickel or other metal particles or conductive polymers. Carbon-based materials may include particles such as carbon black, graphite, superP, acetylene black, carbon fibers and nanotubes, graphene, etc. Examples of conductive polymers include polyaniline, polythiophene, polyacetylene, polypyrrole, poly (3,4-ethylenedioxythiophene) polystyrene sulfonate, etc.

[0131] The negative electrode may include greater than or equal to about 50 weight % to less than or equal to about 97 weight % of a negative electrode active material, optionally greater than or equal to about 0 weight % to less than or equal to about 60 weight % of a solid electrolyte, optionally greater than or equal to about 0 weight % to less than or equal to about 15 weight % of a conductive material, and optionally greater than or equal to about 0 weight % to less than or equal to about 10 weight % of a binder.

[0132] In order to verify the experimental effect of this application, the following examples and comparative examples are designed:

[0133] Example

[0134] Electrolyte preparation: The electrolyte was prepared as follows: the electrolyte was prepared in a glove box (water content ≤0.01 ppm, oxygen content ≤0.01 ppm), and compound A, compound B, compound C (if present) were mixed according to the corresponding mass fraction to obtain a mixed organic solvent. Then the mixed organic solvent, fully dried LiPF6, lithium difluorophosphate (LiPO2F2) and 1,3-propanesultone (PS) were mixed according to a mass ratio of 85.5:13:0.5:1 to prepare the electrolyte, and the total weight of the electrolyte was 100 wt%.

[0135] Battery preparation: 1.5 wt% of binder PVDF, 2 wt% of conductive agent Super-P, 96.5 wt% of lithium nickel manganese oxide LNMO positive electrode material and an appropriate amount of dispersant N-methyl pyrrolidone were mixed to prepare a slurry with a solid content of 65 wt%, and then the slurry was uniformly grinded, coated, rolled, and cut to prepare a positive electrode sheet.

[0136] 97.5 wt% of graphite negative electrode material, 1.5 wt% of conductive agent Super-P, 0.5 wt% of sodium carboxymethyl cellulose (CMC) and 0.5 wt% of binder SBR were mixed with an appropriate amount of water, and then the mixture was uniformly grinded, coated, rolled, and cut to prepare a negative electrode sheet.

[0137] The electrolyte provided by the examples and comparative examples was assembled with the above-mentioned positive electrode sheet and the above-mentioned negative electrode sheet to prepare a lithium ion battery, and the ratio of the capacity of the negative electrode to the capacity of the positive electrode (N / P ratio) was designed to be 1.12, and the capacity was 1.2 Ah.

[0138] Performance test:

[0139] Room temperature cycle performance test: the charge and discharge potential range was 3.5V-4.9V, the charge process was constant current 1C to 4.9V, 4.9V constant voltage charge to the cutoff current ≤0.05C, 5 minutes of standing, 1C discharge to 3.5V, 5 minutes of standing; such cycle charging and discharging, the cycle performance at room temperature (25℃) was tested, and the cycle capacity retention rate after 50 cycles was recorded.

[0140] Low temperature capacity retention rate test: the above fresh battery was first charged and discharged at room temperature at a rate of 0.33C for one week, and the voltage range was 3.5-4.9V. The discharge capacity was recorded as Cap.25℃. Then, the battery was fully charged at 25℃ at a rate of 0.33C, and the charge cutoff voltage was 4.9V. The battery was then placed at -20℃ for 8h, so that the temperature of the battery was -20℃. The battery was then discharged at a rate of 0.33C, and the discharge cutoff voltage was 2.8V. The discharge capacity was recorded as Cap. -20℃, and the low temperature capacity retention rate was Cap. -20℃ / Cap. 25℃.

[0141] Table 1 Examples

[0142] Table 2 Comparative Example

[0143] From the above examples and comparative examples, it can be seen that when the number of methyl groups in compound A and the number of F atoms in compound B are in the range of 3-7, the electrolyte involved in the present application is compatible with the high-voltage positive electrode material system, and exhibits good performance. At the same time, as shown by comparing comparative examples 5 and 6, the solvent system formed by compounding the linear carboxylate solvent with compound B, even if the value of x+y falls within the protection scope of the present application, cannot achieve the effect of the present application. It is shown that unlike the traditional belief that branched chains have the defect of increased viscosity, in the high-voltage system, the specific branched structure of the present application and the F atoms in compound B produce a good synergistic effect, which can meet the performance requirements of the electrolyte for the high-voltage system.

[0144] In particular, within the numerical range of x+y = 3-5, the effect of the electrolyte of the present application is further improved, indicating that the methyl groups of compound A and the F atoms in compound B under the branched structure have a strong synergistic effect, making the electrolyte of the present application have excellent performance.

[0145] At the same time, as can be seen from comparative example 13 and other examples, the addition amount of compound B in the present application should not be too low, and when the addition amount is less than 5%, the effect gradually decreases.

[0146] And surprisingly, when a carbonate solvent is added to the electrolyte solvent system of the present application, the effect unexpectedly worsens, which is different from the synergistic effect of traditional linear carboxylate and carbonate.

[0147] From comparative examples 5-6, the traditional linear carboxylate has good low-temperature performance, but has poor compatibility with the high-voltage system. This is because compared to linear carboxylate, branched chains have poor viscosity performance, which will affect the low-temperature performance of the battery, which is consistent with the existing understanding of carboxylate.

[0148] Those skilled in the art will readily understand that the above description is only an example of the present application and is not intended to limit the present application, and any modifications, equivalent replacements and improvements made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A high voltage lithium ion battery comprising a positive electrode, a negative electrode, and an electrolyte, characterized in that: The electrolyte includes a solvent, the solvent includes compound A and compound B, and the mass fraction of compound A in the solvent is recorded as w A The mass fraction of compound B in the solvent is recorded as w B ; The compound A satisfies the following structure: wherein R1 and R2 are independently selected from hydrocarbon groups and fluorocarbon groups, the number of methyl groups in R1 is x, 2≤x≤5; and at least one carbon atom in the main chain of R1 is connected to at least two methyl groups; Compound B satisfies the following structural formula: R and R' are methylene, and at least one H atom in at least one of R and R' is substituted by F, and the number of F atoms in R and R' is y, 1≤y≤4; w A 、w B , x, y satisfy the following relationship: 1.5≤w A / In B ≤50; 3≤x+y≤7.

2. The lithium-ion battery according to claim 1, wherein The number of carbon atoms in the main chain of R1 is 1-3.

3. The lithium-ion battery according to claim 1, wherein The lithium-ion battery comprises at least one positive electrode active material with a charging cut-off voltage of ≥4.5V.

4. The lithium-ion battery according to claim 3, wherein The positive electrode active material with a charge cut-off voltage greater than 4.5V accounts for more than 50 wt % of all positive electrode active materials in the positive electrode active material layer.

5. The lithium-ion battery according to claim 1, wherein The negative electrode has a lithium insertion cutoff potential of ≤1.0V.

6. The lithium-ion battery according to claim 1, wherein The sum of the carbon numbers of R1 and R2 is greater than or equal to 4 and less than or equal to 9.

7. The lithium-ion battery according to claim 1, wherein The main chain carbon number of R2 is 1-3.

8. The lithium-ion battery according to claim 1, wherein The R2 is selected from one of methyl and ethyl. 9 . The lithium ion battery according to claim 1 , wherein in the compound A, at least one H atom in R1 and R2 is replaced by F.

10. The lithium-ion battery according to claim 1, wherein In the electrolyte, the added amount of carbonate is less than 5 wt % of the total mass of the electrolyte.

11. The lithium-ion battery according to claim 9, wherein In the electrolyte, the added amount of carbonate is less than 1 wt % of the total mass of the electrolyte.

12. The lithium-ion battery according to claim 10, wherein In the electrolyte, the added amount of carbonate is less than 0.01 wt % of the total mass of the electrolyte.

13. The lithium-ion battery according to claim 1, wherein The mixture of the compound A and the compound B accounts for 60-100% of the total mass of the solvent.

14. The lithium ion battery according to claim 12, wherein: The mixture of the compound A and the compound B accounts for 70-100% of the total mass of the solvent.

15. The lithium ion battery according to claim 13, wherein The mixture of the compound A and the compound B accounts for 80-100% of the total mass of the solvent.

16. The lithium ion battery according to claim 14, wherein The mixture of the compound A and the compound B accounts for 90-100% of the total mass of the solvent.

17. The lithium ion battery according to claim 1, wherein The compound B accounts for more than 10% of the total mass of the electrolyte solvent.

18. The lithium ion battery according to claim 1, wherein The structural formula of the compound B is selected from one of the following structural formulas:

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