lithium-ion batteries

Abstract

The present invention belongs to the field of new energy and relates to a lithium ion battery, which includes a positive electrode, a negative electrode and an electrolyte, wherein the electrolyte includes: a fluorinated cyclic carbonate, lithium difluorophosphate and a nitrogen-containing organic Lewis base compound, the negative electrode includes a negative electrode active material layer, the negative electrode active material layer includes Si element, and the lithium ion battery satisfies the following formula (1): 0.35≤A*W1*(W2+W3) / S≤42 (1) In the formula (1), W1 is the mass percentage of the fluorinated cyclic carbonate in the electrolyte; W2 is the mass percentage of the nitrogen-containing organic Lewis base compound in the electrolyte; W3 is the mass percentage of the lithium difluorophosphate in the electrolyte; A is the filling coefficient of the lithium ion battery, that is, the ratio of the total mass of the electrolyte in the lithium ion battery to the capacity of the lithium ion battery, in units of g / Ah; S is the mass percentage of the silicon element in the negative electrode active material.

Description

Technical Field

[0001] The present invention belongs to the field of new energy and relates to a lithium ion secondary battery that can be used in a power device or an energy storage device. Background Art

[0002] Lithium-ion batteries have been widely used in electric vehicles and energy storage grids. With the expansion of application areas and the increase in market demand, the development of lithium-ion batteries with higher energy density has become a current research hotspot and an urgent need for industrial development.

[0003] Currently, traditional graphite anode materials are unable to meet market demand due to their low theoretical specific capacity (372 mAh / g). Silicon-based materials, with their ultra-high theoretical specific capacity (4200 mAh / g), are considered promising candidates for anode materials in next-generation, high-energy-density lithium-ion batteries. However, silicon-based materials undergo significant volume expansion during charge and discharge, causing the SEI membrane to rupture, exposing new active silicon surfaces. These newly exposed silicon surfaces react with the electrolyte, leading to continuous electrolyte decomposition and consequent degradation of the battery's cycling performance.

[0004] To address the issues of volume expansion and resulting interface damage in silicon-based anodes, research in academia and industry has focused on structural design of silicon-based anode materials and on controlling the electrode-electrolyte interface through electrolytes. The complex electrode-electrolyte interface that forms on electrodes is one of the most common obstacles hindering the promising future of lithium-ion battery technology.

[0005] Furthermore, it has been found that to address the problem of poor stability of the silicon-based electrode material / electrolyte interface, introducing electrolyte additives into the electrolyte to promote the formation of the SEI film and thus prevent the decomposition of the electrolyte is a simple and cost-effective method.

[0006] For example, reference 1 reviews the research progress of silicon-based negative electrode electrolyte additives for lithium-ion batteries, mainly summarizing the use of some additives to date.

[0007] However, although these additives have certain advantages, they usually also have some inevitable disadvantages. Therefore, how to combine several different additives to obtain the desired performance has become a research hotspot.

[0008] Increasing the operating voltage is also an effective means of increasing battery energy density. However, high voltage conditions accelerate the oxidative decomposition of the electrolyte and the continuous degradation of the cathode material structure. Furthermore, the reaction of persistent trace amounts of water (H2O) in the electrolyte with lithium hexafluorophosphate (LiPF6) produces corrosive hydrofluoric acid (HF), which can easily lead to the dissolution of transition metal (TM) ions in the ternary cathode material, thus shortening the lifespan of the lithium-ion battery.

[0009] Therefore, in order to meet the requirements of high capacity and high safety, the exploration of lithium secondary batteries, especially lithium secondary batteries containing silicon negative electrodes, still needs to be further deepened.

[0010] References:

[0011] Reference 1: Research Progress of Silicon-Based Anode Electrolyte Additives for Lithium-Ion Batteries: Challenges and Prospects, Chen Shanshan, Energy Storage Materials and Devices, January 2024 Summary of the Invention

[0012] Problems to be solved by the invention

[0013] The present invention is mainly aimed at lithium secondary batteries that use silicon-containing negative electrodes for the purpose of high capacity. As mentioned above, lithium-ion batteries containing silicon-carbon composite materials can significantly improve energy density, and the energy density increases with the increase of silicon content in the negative electrode active layer. However, the silicon-carbon composite material has a significant volume expansion during the process of lithium insertion and extraction, which leads to pulverization and rupture of silicon particles, and at the same time causes consumption and rupture of the SEI film. The direct contact between the newly exposed silicon surface and the electrolyte will cause the continuous generation of the SEI film, resulting in the consumption of active lithium and electrolyte, thereby reducing the battery cycle life. This phenomenon is particularly prominent under high-temperature operation / storage conditions.

[0014] While the use of additives has been proposed to address these issues, the combination of specific additives remains challenging, given the advantages and disadvantages of each additive. Furthermore, minimizing the use of additives (in terms of type and amount) is a crucial issue for industrial production, driven by compatibility and cost considerations.

[0015] In view of the above technical problems, the present invention proposes to mix specific types of additives (fluorinated cyclic carbonates, lithium difluorophosphate and nitrogen-containing organic Lewis base compounds) in high-capacity lithium-ion batteries with silicon negative electrodes, and establish a specific correlation between the dosage of various additives and the silicon content in the negative electrode and the battery filling coefficient. This improves or improves the high-temperature cycling and high-temperature storage performance of lithium-ion batteries with silicon negative electrodes while increasing the battery energy density, and effectively reduces the initial DCR.

[0016] Solutions for solving problems

[0017] It has been found that the above technical problems can be solved by implementing the following technical solutions:

[0018] The present invention mainly provides a lithium ion battery comprising a positive electrode, a negative electrode and an electrolyte, wherein the electrolyte comprises: a fluorinated cyclic carbonate, lithium difluorophosphate and a nitrogen-containing organic Lewis base compound.

[0019] The negative electrode includes a negative electrode active material layer, wherein the negative electrode active material layer includes Si element.

[0020] Furthermore, the lithium-ion battery satisfies the following condition (1):

[0021] 0.35≤A*W1*(W2+W3) / S≤42 (1)

[0022] In the formula (1),

[0023] W1 is the mass percentage of the fluorinated cyclic carbonate in the electrolyte, in wt%;

[0024] W2 is the mass percentage of the nitrogen-containing organic Lewis base compound in the electrolyte, in wt%;

[0025] W3 is the mass percentage of lithium difluorophosphate in the electrolyte, in wt%;

[0026] A is the filling coefficient of the lithium-ion battery, that is, the ratio of the total mass of the electrolyte inside the lithium-ion battery to the capacity of the lithium-ion battery, in g / Ah;

[0027] S is the mass percentage of silicon element in the negative electrode active material layer, and the unit is wt%.

[0028] In some specific embodiments, the formula (1) satisfies one or more of the following conditions:

[0029] 5≤W1≤20;

[0030] 0.2≤W2≤2;

[0031] 0.2≤W3≤1;

[0032] 2≤A≤4;

[0033] 5≤S≤30.

[0034] In some specific embodiments, the negative electrode active material layer includes Si element and C element.

[0035] In some specific embodiments, the fluorinated cyclic carbonate includes ethylene carbonate substituted with one or more fluorine atoms and / or propylene carbonate substituted with one or more fluorine atoms.

[0036] In some specific embodiments, the nitrogen-containing organic Lewis base compound includes one or more of an amide compound or an imide compound, and optionally, the amide compound or the imide compound has a fluorine atom substitution.

[0037] In some specific embodiments, the amide compound includes one or more of an aliphatic amide or an aromatic amide; and the imide compound includes a cyclic imide.

[0038] In some specific embodiments, in the negative electrode active material layer: the Si element is derived from silicon alone, silicon oxide, or silicon alloy; and the C element is derived from graphite or carbon.

[0039] In some specific embodiments, the electrolyte further comprises a lithium salt and an organic solvent.

[0040] In some specific embodiments, the positive electrode active material in the positive electrode includes Li a Ni x Co y Mn z A b O2 ternary material; wherein, 0.9≤a≤1.1, 0.5≤x<1, x+y+z=1, 0≤b≤0.1, and A is selected from at least one of Al, Zr, Ti, Mo, W, Zn, Cu, Cr, Mg, Fe, Nb, and B.

[0041] Furthermore, the lithium-ion battery described in the present invention may be at least one of a power battery and an energy storage battery.

[0042] Effects of the Invention

[0043] By implementing the above technical solution, the present invention can achieve the following technical effects:

[0044] ① The present invention uses lithium-ion batteries containing silicon, especially silicon-carbon composite materials, which can significantly improve energy density, and the energy density increases with the increase of silicon content in the negative electrode active layer.

[0045] ② The present invention mixes specific types of additives (fluorinated cyclic carbonate, lithium difluorophosphate and nitrogen-containing organic Lewis base compound) and establishes a specific correlation between the amount of various additives and the silicon content in the negative electrode and the battery filling coefficient. This improves the battery energy density while improving or improving the high-temperature cycling and high-temperature storage performance of lithium-ion batteries containing silicon negative electrodes, while effectively reducing the initial DCR (Direct Current Internal Resistance). DETAILED DESCRIPTION

[0046] The following is a detailed description of the present invention. The following description of the technical features is based on representative embodiments and specific examples of the present invention, but the present invention is not limited to these embodiments and specific examples. It should be noted that:

[0047] In this specification, a numerical range expressed using "a numerical value A to a numerical value B" means a range including the endpoints A and B.

[0048] In this specification, the numerical range expressed using "above" or "below" means a numerical range including the number.

[0049] In this specification, the use of "may" includes both the meaning of performing a certain process and the meaning of not performing a certain process.

[0050] In this specification, the use of "optional" or "optional" indicates that certain substances, components, execution steps, application conditions and other factors are used or not used.

[0051] In this specification, "normal temperature" or "room temperature" refers to an indoor ambient temperature of "23±2°C".

[0052] In this specification, the unit names used are all international standard unit names, and unless otherwise stated, the "%" used indicates weight or mass percentage.

[0053] In this specification, the use of “substantially” or “essentially” means that the standard deviation from a theoretical model, theoretical data or target data is within a numerical range of 2%, preferably 1%, and more preferably 0.8%.

[0054] When the terms “include” and / or “comprising” are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or their combinations.

[0055] References throughout this specification to "some specific / preferred embodiments," "other specific / preferred embodiments," "embodiments," etc., mean that the particular elements (e.g., features, structures, properties, and / or characteristics) described in connection with the embodiments are included in at least one embodiment described herein and may or may not be present in other embodiments. Furthermore, it should be understood that the elements may be combined in any suitable manner in various embodiments.

[0056] The present invention mainly provides a lithium ion secondary battery which has improved energy density, improved high temperature cycle and high temperature storage performance, and reduced initial DCR.

[0057] The present invention is mainly based on the following insights:

[0058] For lithium secondary batteries containing silicon anodes, structural contraction and expansion caused by temperature fluctuations is a major issue affecting battery performance. This volume change makes it difficult to form a stable SEI (Solid Electrolyte Interface) film on the anode surface. Furthermore, the present invention has discovered that this problem can be addressed by combining a limited number of specific additives and establishing a specific correlation between the additive content, the silicon content in the anode, and the battery's filling coefficient.

[0059] Specifically, the main challenge is to mitigate the impact of high-temperature volume changes in the negative electrode material on SEI stability. Therefore, the first consideration is the use of fluorinated cyclic carbonates, primarily due to their ability to effectively form a stable SEI film on the silicon negative electrode surface. Furthermore, the SEI film possesses a certain degree of elasticity, making it particularly adaptable to volume expansion during charge and discharge, thereby improving the stability and cycling performance of lithium-ion batteries.

[0060] However, on the one hand, it has also been found that fluorinated cyclic carbonates are unstable under high temperature conditions. Under high temperature conditions, they easily react with Lewis acids such as PF5 in the electrolyte to produce corrosive substances such as HF, thereby causing the dissolution of the positive electrode transition metal and causing damage to the structure of the positive electrode material. On the other hand, during the cycle process, it will cause battery gas production and increase interfacial impedance.

[0061] Furthermore, in order to make up for the above-mentioned shortcomings of fluorinated cyclic carbonates, it is conceivable to use nitrogen-containing organic Lewis base compounds, especially amide compounds or imide compounds that are optionally fluorinated. In these compounds, the N atoms containing lone pairs of electrons can effectively combine with the Lewis acid in the electrolyte to achieve the capture of HF and PF5, thereby inhibiting the dissolution of transition metal elements in the positive electrode material caused by the decomposition of fluorinated cyclic carbonates, greatly improving the high-temperature performance of the battery, and enabling the lithium-ion battery to have high-voltage tolerance.

[0062] However, it was further discovered that with the use of nitrogen-containing organic Lewis base compounds, although they can have a high-temperature stabilization effect on the SEI film and electrodes, these substances also lead to an increase in the internal resistance of the formed SEI.

[0063] Therefore, from the perspective of the high ionic conductivity and low internal resistance of the formed SEI film, the further use of lithium difluorophosphate (LiPO2F2) as a film-forming additive can form a highly ionic conductive SEI film (rich in LiF and PO compounds) at the negative electrode, which not only improves the stability of the SEI film but also reduces the interfacial film impedance. Although lithium difluorophosphate generally has limited solubility in electrolytes, it has been found that even under limited solubility conditions, it is sufficient to significantly improve the ionic conductivity characteristics of the SEI film under the above additive environment.

[0064] Ultimately, the present invention was completed by selecting a combination of the three additives mentioned above and establishing a specific correlation between the additive content, the silicon content in the negative electrode, and the battery filling coefficient. As a result, the lithium-ion battery of the present invention can improve the battery's high-temperature cycling and high-temperature storage performance while increasing the battery's energy density, while effectively reducing the initial DCR. Furthermore, the battery of the present invention can better meet or exceed the performance or test levels specified in documents such as GB38031-2025.

[0065] lithium-ion batteries

[0066] The lithium-ion secondary battery described in the present invention can be a power battery, that is, a battery used to provide power to transportation or vehicles, or a secondary battery used in energy storage equipment such as wind power, hydropower, solar power or traditional petrochemical energy power.

[0067] The lithium-ion secondary battery of the present invention may include, as a single unit, components such as a positive electrode, a negative electrode, an electrolyte, and a separator. Furthermore, the lithium-ion secondary battery of the present invention may be a non-aqueous electrolyte lithium-ion battery or a semi-solid lithium-ion battery having a certain non-aqueous electrolyte.

[0068] positive electrode

[0069] The positive electrode includes a current collector and a positive electrode active material layer. In principle, there is no particular limitation on the current collector of the positive electrode. For example, it can be copper or aluminum, preferably aluminum.

[0070] The positive electrode active material layer may include a positive electrode active material, a binder, and optionally a conductive agent, an auxiliary agent, and the like.

[0071] In principle, there is no particular limitation on the positive electrode active material, and examples thereof include:

[0072] Layered transition metal oxides mainly include LiMO2 (M=Co, Ni, Mn) positive electrode materials, lithium-rich manganese-based positive electrode materials, nickel cobalt manganese oxide (NCM) and nickel cobalt aluminum oxide (NCA) ternary positive electrode materials.

[0073] Preferably, the positive electrode active material may include Li a Ni x Co y Mn z A b O2 ternary material; wherein, 0.9≤a≤1.1, 0.5≤x<1, x+y+z=1, 0≤b≤0.1, and A is selected from at least one of Al, Zr, Ti, Mo, W, Zn, Cu, Cr, Mg, Fe, Nb, and B.

[0074] From the perspective of high capacity, ternary layered transition metal oxide positive electrode materials are preferred, as they have higher specific capacity and can better meet the requirements of high energy density.

[0075] Furthermore, for positive electrode active materials, especially NCM and NCA, the key to improving the discharge capacity of ternary positive electrode materials is to increase the proportion of nickel content. Therefore, in a further preferred embodiment of the present invention, the positive electrode active material may include ternary positive electrode materials with medium nickel (mainly 5 series such as NCM523), medium-high nickel (mainly 6 series such as NCM613, NCM622) and high nickel (mainly 8 series such as NCM811).

[0076] Furthermore, other components in the positive electrode active material layer are generally not limited. For example, they may be binders, conductive agents, and additives commonly used in the art. Examples of such binders include fluorinated polyolefins, acrylates, and cellulose-based binders; examples of conductive agents include carbon nanotubes and conductive metal particles; and examples of additives include thickeners and dispersing agents.

[0077] negative electrode

[0078] The negative electrode includes a current collector and a negative electrode active material layer. In principle, there is no particular limitation on the current collector of the negative electrode. For example, it can be copper or aluminum, preferably copper.

[0079] The negative electrode active material layer may include a negative electrode active material, a binder, and optionally an auxiliary agent.

[0080] The negative electrode active material contains Si element from the viewpoint of increasing the battery capacity. The source of Si element is not particularly limited, and may be, for example, silicon element, silicon oxide, or silicon alloy.

[0081] In order to achieve both good capacity and good dimensional stability, the negative electrode active material further includes element C. The source of the element C is not particularly limited, and may be, for example, carbon material or graphite.

[0082] In some preferred embodiments of the present invention, the negative electrode active material includes C element and Si element, that is, the preferred negative electrode active material includes a silicon-carbon composite material.

[0083] Furthermore, other components in the negative electrode active material layer, such as a binder and other auxiliary agents, can be selected from conventional components available in the art.

[0084] Regarding the Si content in the negative electrode active material layer, a higher Si content in the negative electrode active material layer increases the energy density of the battery, but also faces the problem of significant volume expansion. Therefore, it is necessary to reasonably control the Si content. In some preferred embodiments, the Si content in the negative electrode active material layer can generally be 5-30% by mass, more preferably 10-25% by mass, and examples thereof include 8% by mass, 12% by mass, 14% by mass, 16% by mass, 18% by mass, 20% by mass, 22% by mass, 28% by mass, etc.

[0085] diaphragm

[0086] In some specific embodiments of the present invention, the lithium-ion battery further uses a separator.

[0087] The separator may include a porous membrane and optionally a modified layer formed on one or both major surfaces of the porous membrane.

[0088] In principle, there is no particular limitation on the porous membrane layer. For example, it can be olefin (PP, PE), polyamide, etc. Such a porous membrane layer can be composed of one or more organic resin layers.

[0089] In principle, there is no particular limitation on the modified layer, and for example, the modified layer may include organic heat-resistant particles, inorganic particles, a conductive agent, a lithium supplement, a binder, and the like.

[0090] The porosity of the separator may generally be 20 to 60% by volume, preferably 30 to 46% by volume, and the thickness of the separator may generally be 30 μm or less.

[0091] electrolyte

[0092] The electrolyte of the present invention comprises, in addition to commonly used solvents and lithium salts, at least three additives: fluorinated cyclic carbonate, lithium difluorophosphate, and a nitrogen-containing organic Lewis base compound, and optionally, may also include other types of additives.

[0093] (Non-aqueous organic solvent)

[0094] The present invention does not particularly limit the type of the non-aqueous organic solvent as long as it is a non-aqueous organic solvent commonly used as a non-aqueous electrolyte solution.

[0095] In some specific embodiments, the non-aqueous organic solvent can be selected from one or more of cyclic carbonate solvents, linear carbonate solvents, ether solvents, ester solvents, nitrile solvents and ketone solvents.

[0096] The cyclic carbonate solvent can be selected from ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC); the linear carbonate solvent can be selected from diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), trifluoroethyl methyl carbonate (FEMC), di(2,2,2-trifluoroethyl) carbonate (DFDEC); the ester solvent can be selected from The non-aqueous solvents may be selected from methyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, and methyl pivalate; the ether solvent may be selected from dibutyl ether, tetrahydrofuran (THF), 2-methyltetrahydrofuran, ethylene glycol dimethyl ether (DME), ethylene glycol diethyl ether (DEE), dioxane (DX), and dioxolane (DOL); the nitrile solvent may include acetonitrile; and the ketone solvent may be selected from polymethyl vinyl ketone. These non-aqueous solvents may be used alone or in mixtures of two or more.

[0097] In some preferred embodiments, the non-aqueous organic solvent may be selected from at least two of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl formate (MA), ethyl acetate (EA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB) and propyl butyrate (PB).

[0098] (lithium salt)

[0099] The present invention does not particularly limit the type of lithium salt that can be used, and it can be a lithium salt commonly used in the art (a lithium salt different from the additive described below).

[0100] In some specific embodiments, the lithium salt can be selected from one or more salts formed by lithium ions and the following anions: PF6 - 、BF4 - 、Cl - Br - , I - 、ClO4 - 、AsF6 - 、CH3CO2 - CF3SO3 - 、N(CF3SO2)2 - 、N(FSO2)2 - 、C(CF2SO2)3 - 、C2BF2O4 - wait.

[0101] In some preferred embodiments, the lithium salt may be a combination of one or more selected from lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).

[0102] In principle, there is no particular limitation on the content of the lithium salt. In some specific embodiments of the present invention, from the perspective of controlling electrolyte viscosity and cost, the concentration of the lithium salt in the non-aqueous electrolyte can be 0.8 mol / L to 5 mol / L, preferably 1 mol / L to 1.5 mol / L, for example, 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, 3.5 mol / L, 4 mol / L, 4.5 mol / L, etc.

[0103] (additive)

[0104] The additives of the present invention include at least three essential additive components: fluorinated cyclic carbonate, lithium difluorophosphate and nitrogen-containing organic Lewis base compound.

[0105] The fluorinated cyclic carbonate may be a cyclic carbonate substituted with one or more fluorine atoms.

[0106] The cyclic carbonate can generally be ethylene carbonate or propylene carbonate. In some preferred embodiments, the fluorinated cyclic carbonate of the present invention can be monofluorinated ethylene carbonate (FEC) and / or monofluorinated propylene carbonate (FPC). More preferably, the fluorinated cyclic carbonate is monofluorinated ethylene carbonate.

[0107] The use of fluorinated cyclic carbonates can form a tough SEI film on the negative electrode, especially on the Si surface, thereby suppressing the damage of the SEI film caused by the negative electrode volume expansion under high temperature conditions. In some specific embodiments, the fluorinated cyclic carbonate can form a highly cross-linked polymer through free radical polymerization and defluorination, producing a resilient SEI layer that can withstand the stress of charge and discharge cycles.

[0108] For nitrogen-containing organic Lewis bases, their main function is to stabilize fluorinated cyclic carbonates, which are unstable at high temperatures. Under high temperature conditions, they easily react with Lewis acids such as PF5 in the electrolyte to produce corrosive substances such as HF, thereby causing the dissolution of the positive electrode transition metal and destroying the structure of the positive electrode material.

[0109] The nitrogen-containing organic Lewis base of the present invention can effectively combine with excess Lewis acid in the electrolyte through the nitrogen atom containing a lone pair of electrons, thereby capturing by-products such as HF and PF5, thereby reducing the dissolution of transition metal elements in the positive electrode material and greatly improving the high-temperature storage performance of the battery.

[0110] The nitrogen-containing organic Lewis base of the present invention preferably includes one or more of an amide compound or an imide compound, and optionally, the amide compound or the imide compound has a fluorine atom substitution.

[0111] As for the amide compound, it may be an aliphatic or aromatic amide compound.

[0112] In some preferred embodiments, the amide compound can be an amide formed by an acyl group having 2 to 12 carbon atoms and -NR2. The acyl moiety is a linear or branched saturated or unsaturated aliphatic structure, or has a benzene ring structure. Furthermore, one or more hydrogen atoms in the acyl moiety may be substituted with fluorine atoms. R can be H or an alkyl group having 5 or fewer carbon atoms (e.g., 1, 2, or 3).

[0113] The imide compound may be at least one of an aliphatic or aromatic imide compound and a cyclic imide compound. Preferably, the imide compound at least includes a cyclic imide compound.

[0114] In some preferred embodiments, the aliphatic or aromatic imide compound can be an acyclic imide formed by an acyl group having 2 to 12 carbon atoms and -NR'-. The acyl moieties can be the same or different and can be a linear or branched saturated or unsaturated aliphatic structure, or have a benzene ring structure. Furthermore, one or more hydrogen atoms in the acyl moiety can be substituted with fluorine atoms. R' can be H or an alkyl group having 5 or fewer carbon atoms (e.g., 1, 2, or 3). Preferably, R' is an alkyl group.

[0115] In other preferred embodiments, the cyclic imide compound can be a cyclic imide formed by a double-headed acyl group having 2 to 12 carbon atoms and -NR''-, wherein the acyl group is a linear or branched saturated or unsaturated aliphatic structure, or has a benzene ring structure. R'' can be H or an alkyl group having 5 or fewer carbon atoms (e.g., 1, 2, or 3). Preferably, R'' is an alkyl group.

[0116] In some further preferred embodiments of the present invention, the nitrogen-containing organic Lewis base comprises at least one of the following compounds:

[0117]

[0118] In principle, there is no particular restriction on the amounts of the three essential additive components, namely, the fluorinated cyclic carbonate, lithium difluorophosphate, and nitrogen-containing organic Lewis base compound, used in the electrolyte, as long as the relationship of the general formula (1) described above is satisfied.

[0119] In some specific embodiments of the present invention, the percentage of the fluorinated cyclic carbonate in the electrolyte is represented by W1 mass %, and the preferred W1 is 5 to 20, more preferably 8 to 18, and can be exemplified by 6, 10, 12, 14, 16, 19, etc.

[0120] In some other specific embodiments, the percentage of the nitrogen-containing organic Lewis base compound in the electrolyte is represented by W2 mass %, and the preferred W2 is 0.2~2, more preferably 0.3~1.8, and examples thereof include 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, etc.

[0121] In some other specific embodiments, the percentage of lithium difluorophosphate in the electrolyte is represented by W3 mass %, and the preferred W3 is 0.2-1, more preferably 0.3-0.9, and examples thereof include 0.4, 0.5, 0.6, 0.7, 0.8, etc.

[0122] Among them, when the content W1 of fluorocarbonate in the electrolyte is too low, it may not be possible to effectively form a stable and elastic solid electrolyte membrane on the surface of the negative electrode active material layer, and it is difficult to suppress the huge volume expansion of the silicon-based material during the charging and discharging process, which will cause the battery cycle life to decline and even cause the battery to dive; when the content W1 of fluorocarbonate is too high, it may cause battery gas production and increase interfacial impedance during the cycle process. At the same time, under high temperature conditions, it is easy to react with Lewis acids such as PF5 in the electrolyte to produce defluorination reactions to produce corrosive substances such as HF, thereby causing the dissolution of the positive electrode transition metal and the destruction of the positive electrode material structure, thereby worsening the battery cycle performance.

[0123] When the content W2 of the nitrogen-containing organic Lewis base compound in the electrolyte is too low, the protective film formed on the surface of the positive electrode may be unstable, and the HF generated in the electrolyte may not be effectively captured, causing the dissolution of the positive electrode transition metal and affecting the high-temperature storage performance of the battery; when the content W2 of the nitrogen-containing organic Lewis base compound is too high, an excessively thick interface film may be formed on the surface of the positive electrode, increasing the resistance of lithium ions passing through the interface.

[0124] In addition, when the content W3 of lithium difluorophosphate in the electrolyte is too low, it may not be possible to effectively form an interfacial film with high ionic conductivity and excellent density on the surface of the negative electrode, and it may not be possible to effectively slow down the increase in interfacial impedance caused by the decomposition of the electrolyte; due to the limited solubility of lithium difluorophosphate in carbonate electrolyte, it is easy to precipitate from the electrolyte when the content is too high, resulting in uneven lithium insertion and extraction during the cycle, affecting the transmission of lithium ions.

[0125] Furthermore, in addition to the above-mentioned additives, various other additives known in the art may be used in the electrolyte of the present invention, as long as they do not hinder the achievement of the technical effects of the present invention.

[0126] Examples of such other additives include vinyl ethylene carbonate (VC), 2-cyanoethyltriethoxysilane (TEOSCN), sulfur-containing additives, and oxalate-containing additives. The sulfur-containing additives can be selected from 1,3-propane sultone (PS), 1,4-butane sultone (1,4-BS), 2,4-butane sultone (2,4-BS), 1,3-propene sultone (PST), vinyl sulfate (DTD), methylene methanedisulfonate (MMDS), and vinyl sulfite (ES). The oxalate-containing additives can be selected from lithium difluorooxalatoborate (LiDFOB), lithium bis(oxalatoborate) (LiBOB), lithium tetrafluorooxalatophosphate (LiTFOP), and lithium difluorobis(oxalatophosphate) (LiDFOP). These additives can be used alone or in mixtures of two or more.

[0127] Lithium-ion battery characteristics

[0128] The present invention combines the above three specific additives and establishes a specific correlation between the content of these additives and the silicon content in the negative electrode and the battery filling coefficient. As a result, the lithium-ion battery of the present invention can improve the battery energy density while improving the high-temperature cycle and high-temperature storage performance of the battery, while effectively reducing the initial DCR.

[0129] Specifically, the lithium-ion battery of the present invention satisfies the following condition (1):

[0130] 0.35≤A*W1*(W2+W3) / S≤42 (1)

[0131] In the formula (1),

[0132] W1 is the mass percentage of the fluorinated cyclic carbonate in the electrolyte, in wt%;

[0133] W2 is the mass percentage of the nitrogen-containing organic Lewis base compound in the electrolyte, in wt%;

[0134] W3 is the mass percentage of lithium difluorophosphate in the electrolyte, in wt%;

[0135] A is the filling coefficient of the lithium-ion battery, that is, the ratio of the total mass of the electrolyte to the capacity of the lithium-ion battery, in g / Ah;

[0136] S is the mass percentage of silicon element in the negative electrode active material layer, and the unit is wt%.

[0137] When the value of A*W1*(W2+W3) / S is too small, the battery filling coefficient may be too low or the content of additives added to the electrolyte may be too low, which will significantly affect the battery's cycle life and storage performance. A low battery filling coefficient cannot fully wet the electrode and separator, resulting in a large internal resistance of the battery. At the same time, it cannot support the electrolytic consumption of the battery during long cycles, resulting in a decrease in the battery's cycle performance and kinetic performance. If the amount of additives added is too small, a stable interface film cannot be formed on the positive and negative electrode surfaces, making it difficult to effectively inhibit the volume expansion of the silicon-based negative electrode and the dissolution of the positive electrode transition metal during the cycle, further deteriorating the battery's cycle performance and storage performance, and the improvement in high-temperature performance and initial DCR is not obvious. When the value of A*W1*(W2+W3) / S is too large, the battery filling coefficient may be too high or the content of additives added to the electrolyte may be too high, which will also affect the battery's cycle performance. A high battery filling coefficient not only reduces the battery's energy density, but also excessive electrolyte increases interfacial side reactions, increases battery gas production, and further deteriorates the battery's cycle performance. Alternatively, this may indicate that the Si content in the negative electrode active material layer is low, which may be insufficient for increasing the capacity.

[0138] Furthermore, considering the high capacity characteristics and high temperature stability, the lithium ion battery of the present invention preferably satisfies the following condition (1a):

[0139] 1≤A*W1*(W2+W3) / S≤40 (1a)

[0140] Lithium-ion battery usage

[0141] A usable lithium-ion secondary battery can be obtained by assembling the various components and components described above (other auxiliary components or components may also be used if necessary) according to a common assembly method in the art.

[0142] For such a battery, as far as its single unit is concerned, its form can be one or more of a cylindrical battery, a sheet battery, a plate battery, and a block battery.

[0143] Furthermore, in some specific embodiments, the battery of the present invention appears and is used in the form of a single unit. In other specific embodiments, the battery of the present invention can be used in parallel or in series in any number of scales, such as a battery pack or battery group formed by packaging several battery cells.

[0144] Example

[0145] The embodiments of the present invention will be described in detail below with reference to the examples, but it will be understood by those skilled in the art that the following examples are merely illustrative of the present invention and should not be construed as limiting the scope of the invention. Where specific conditions are not specified in the examples, the methods were performed according to conventional conditions or the conditions recommended by the manufacturer. Where the manufacturers of the reagents or instruments are not specified, they are all conventional products that can be obtained commercially.

[0146] Example 1

[0147] Positive electrode sheet: The positive electrode active material Li (Ni 0.8 Co 0.1 Mn 0.1 )O2, conductive ultrafine carbon powder (SP), single-walled carbon nanotubes (SWNTs), and a binder, polyvinylidene fluoride (PVDF), are added to an appropriate amount of N-methyl-2-pyrrolidone (NMP) solvent and thoroughly stirred to prepare a positive electrode mixture slurry. The resulting positive electrode slurry is then coated onto aluminum foil, dried, and roll-pressed and die-cut to form positive electrode sheets.

[0148] Negative electrode sheet: The negative electrode silicon-carbon composite material, conductive agent ultrafine carbon powder (SP), thickener CMC, and binder SBR are added to an appropriate amount of deionized water in a mass ratio of 95:2:1:2 and thoroughly stirred to prepare the negative electrode mixture slurry. The resulting negative electrode slurry is then coated on copper foil, dried, roll-pressed, and die-cut to form the negative electrode sheet.

[0149] Electrolyte: At room temperature, a non-aqueous electrolyte was prepared in an argon-protected glove box with a moisture content of less than 0.1 ppm. The organic solvents ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a mass ratio (EC: PC: EMC: DEC = 10:20:50:20). Then, 12.5% ​​mass fraction of lithium hexafluorophosphate (LiPF6) was gradually added to the mixed solvent with continuous stirring. To the above mixed solution, 0.5% of lithium difluorophosphate, 0.5% of compound 1, and 10% of fluoroethylene carbonate, based on the total mass of the electrolyte, were added to obtain the final electrolyte.

[0150] Preparation of lithium-ion batteries: The prepared negative electrode sheets, separators, and positive electrode sheets are stacked in order, then the tabs are welded and encapsulated with aluminum-plastic film to obtain soft-package dry cells, which are placed in an oven at 90±2°C and baked for 18 hours. Finally, the electrolyte prepared above is injected into the cell, and the cells are allowed to stand, undergo formation, and undergo capacity separation operations to complete the preparation of the lithium-ion battery. The battery's injection coefficient is 2.8g / Ah.

[0151] Examples 2 to 30 :

[0152] The relationships satisfied by the type and amount of additives in the electrolyte, the mass percentage of the silicon element, and the filling coefficient of the battery are shown in Examples 2 to 30 in Table 1.

[0153] Comparative Examples 1 to 5 :

[0154] The relationships satisfied by the type and amount of additives in the electrolyte, the mass percentage of the silicon element, and the filling coefficient of the battery are shown in Comparative Examples 1 to 5 in Table 1.

[0155] Table 1 Battery parameters of Examples and Comparative Examples

[0156]

[0157]

[0158] The performance of the lithium-ion batteries obtained in the above examples and comparative examples was tested, and the specific test methods and results are as follows:

[0159] (Cyclic performance test)

[0160] At 45±2°C, the battery was charged at a constant current of 1C to 4.25V, then charged at a constant voltage to a current of 0.05C. After standing for half an hour, it was discharged at a constant current of 1C to 2.8V. The initial discharge capacity of the first week of the battery cycle was obtained and recorded as C0. Thereafter, the charge and discharge cycles were repeated in the above manner. The capacity after the 500th week of discharge was recorded as C500. The capacity retention rate (%) after 500 weeks of cycling at room temperature = C500 / C0×100%.

[0161] (High temperature storage performance test)

[0162] At 60±2°C, the battery was charged at a constant current of 1C to 4.25V, then charged at a constant voltage to a current of 0.05C. After standing for half an hour, it was discharged at a constant current of 1C to 2.8V to obtain the initial discharge capacity of the battery. The battery was then charged at a constant current and constant voltage of 1C to 4.25V with a cutoff current of 0.05C. After fully charged and stored at 60°C for 30 days, the battery was discharged at a constant current of 1C to 2.8V to obtain the discharge capacity after 30 days of storage. High-temperature storage capacity retention = discharge capacity after 30 days / initial discharge capacity * 100%.

[0163] (DCR test at room temperature)

[0164] At 25±2°C, charge the battery at a constant current of 1C to 4.25V, then charge it at a constant voltage to a current of 0.05C. Discharge it at 1C for 30 minutes (50% SOC), then pulse-discharge it at a constant current of 2C for 10 seconds. The voltage before the 10-second pulse discharge (V0) and the voltage after the 10-second pulse discharge (V1) were recorded. DCR = (voltage before pulse discharge (V0) – voltage after pulse discharge (V1)) / discharge current.

[0165] Table 2 Performance test results of lithium ion batteries prepared in Examples and Comparative Examples

[0166]

[0167] Based on the tests of the above examples and comparative examples, the examples satisfying general formula (1) of the present invention can achieve good high-temperature cycling and high-temperature storage performance, while also having a low initial DCR. However, the comparative examples that do not satisfy general formula (1) have insufficient high-temperature cycling performance, high-temperature storage performance, and / or initial DCR performance.

[0168] It should be noted that, although the technical solutions of the present invention are described with specific examples, those skilled in the art will appreciate that the present invention should not be limited thereto.

[0169] While various embodiments of the present invention have been described above, the above descriptions are intended to be illustrative, non-exhaustive, and not limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is selected to best explain the principles of the embodiments, their practical applications, or technological improvements in the marketplace, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A lithium-ion battery comprising a positive electrode, a negative electrode and an electrolyte, characterized in that: The electrolyte comprises: a lithium salt, an organic solvent and an electrolyte additive, wherein the electrolyte additive comprises a fluorinated cyclic carbonate, lithium difluorophosphate and a nitrogen-containing organic Lewis base compound, and the lithium salt is different from the lithium difluorophosphate. The negative electrode includes a negative electrode active material layer, wherein the negative electrode active material layer includes Si element. Furthermore, the lithium-ion battery satisfies the following condition (1): 0.35≤A*W1*(W2+W3) / S≤42 (1) In the formula (1), 5≤W1≤20； 0.2≤W2≤2； 0.2≤W3≤1； 2≤A≤4； 5≤S≤30， W1 is the mass percentage of the fluorinated cyclic carbonate in the electrolyte, in wt%; W2 is the mass percentage of the nitrogen-containing organic Lewis base compound in the electrolyte, in wt%; W3 is the mass percentage of lithium difluorophosphate in the electrolyte, in wt%; A is the filling coefficient of the lithium-ion battery, that is, the ratio of the total mass of the electrolyte inside the lithium-ion battery to the capacity of the lithium-ion battery, in g / Ah; S is the mass percentage of Si element in the negative electrode active material layer, and the unit is wt%.

2. The lithium-ion battery according to claim 1, wherein The negative electrode active material layer includes Si element and C element.

3. The lithium-ion battery according to claim 1 or 2, characterized in that The fluorinated cyclic carbonate includes ethylene carbonate substituted with one or more fluorine atoms and / or propylene carbonate substituted with one or more fluorine atoms.

4. The lithium-ion battery according to claim 1 or 2, characterized in that The nitrogen-containing organic Lewis base compound includes one or more of an amide compound or an imide compound.

5. The lithium-ion battery according to claim 4, characterized in that The amide compound or the imide compound has a fluorine atom substitution.

6. The lithium-ion battery according to claim 4, characterized in that The amide compound includes one or more of aliphatic amides or aromatic amides; and the imide compound includes cyclic imide.

7. The lithium-ion battery according to claim 2, characterized in that In the negative electrode active material layer, the Si element is derived from silicon, silicon oxide or silicon alloy; and the C element is derived from graphite or carbon.

8. The lithium-ion battery according to claim 1 or 2, characterized in that The positive electrode active material in the positive electrode includes Li a Ni x Co y Mn z A b O2 ternary material; wherein, 0.9≤a≤1.1, 0.5≤x<1, x+y+z=1, 0≤b≤0.1, and element A in the ternary material is selected from at least one of Al, Zr, Ti, Mo, W, Zn, Cu, Cr, Mg, Fe, Nb, and B.

9. The lithium-ion battery according to claim 1 or 2, characterized in that The lithium-ion battery is at least one of a power battery and an energy storage battery.

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