A lithium-ion battery

Abstract

To overcome the problem of high self-discharge in existing lithium manganese iron phosphate batteries, this application provides a lithium-ion battery, including a positive electrode, a separator, and a non-aqueous electrolyte. The positive electrode includes a positive electrode material layer containing a positive electrode active material, which includes lithium manganese iron phosphate or a mixture containing lithium manganese iron phosphate. The non-aqueous electrolyte includes a boron-containing additive, which includes at least one of lithium difluorooxalate borate, lithium dioxalate borate, and tris(trimethylsilane)borate. The lithium-ion battery satisfies the following conditions: 0.2 ≤ 10 × p × m / d ≤ 50, and 0.01 ≤ m ≤ 2, 20 ≤ p ≤ 50, 12 ≤ d ≤ 40; where m is the mass percentage of the boron-containing additive in the non-aqueous electrolyte, in %; p is the porosity of the separator, in %; and d is the areal density of the positive electrode, in mg / cm³. 2 The lithium-ion battery provided in this application combines high energy density with excellent high-temperature storage performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of energy storage, and particularly relates to a lithium ion battery. Background Art

[0002] As a kind of chemical power source, lithium ion batteries can achieve efficient conversion between chemical energy and electrical energy, and are currently the most widely used energy storage devices. Lithium ion batteries generally use lithium nickel cobalt manganese oxide or lithium iron phosphate as the positive electrode material. The former has relatively high specific capacity and voltage platform, but its thermal stability is poor, there is a risk of oxygen release, and there are relatively large potential safety hazards; while lithium iron phosphate has an olivine structure, its structure has high stiffness, excellent thermal stability, and very prominent safety performance. However, the specific capacity and discharge platform of lithium iron phosphate are relatively low, resulting in its inability to be adapted to systems with high requirements for energy density.

[0003] Introducing redox couples of transition metals with higher potentials into the olivine structure of lithium iron phosphate is a feasible solution to achieve high safety and high energy density systems. Manganese has a high crustal reserve, and the radii of Mn 2+ and Fe 2+ are close, and they can be mutually soluble in any proportion. Compared with Fe 2+ / Fe 3+ (3.2V vs.Li + / Li), Mn 2+ / Mn 3+ (4.1V vs.Li + / Li), the redox potential has been significantly improved. Therefore, lithium manganese iron phosphate (LMFP, LiFe x Mn 1-x PO4, 0<x<1) solid solution materials are considered to be an upgraded version of lithium iron phosphate positive electrode materials.

[0004] However, one defect of the lithium manganese iron phosphate battery system is large self-discharge. Especially at high temperatures, manganese ions and iron ions are easily dissolved out, which in turn catalyze the decomposition of lithium salts and corrode the negative electrode / electrolyte interface film (SEI) and the positive electrode / electrolyte interface film (CEI), so that the passivation film cannot fully play the role of blocking electron conduction; on the other hand, the dissolved metal ions are reduced and deposited at the negative electrode. After multiple cycles, not only does it block the diaphragm and affect lithium ion transmission, but it even pierces the diaphragm to cause micro-short circuits and greater self-discharge. After disassembling a battery with large self-discharge, many black spots containing metal elements can be found on the diaphragm, which are caused by the deposition of metal impurities and decomposition products of non-aqueous electrolyte / interface film in the diaphragm. Large self-discharge will not only lead to a reduction in the usable capacity and service life of the battery, but the inconsistency of its self-discharge will also cause differences in the capacity retention rate of the batteries in the battery pack after storage, resulting in serious loss of the combined battery capacity and may cause overcharging or over-discharging of some batteries during the charge and discharge process, increasing the safety risk. Summary of the Invention

[0005] To address the technical problem of high self-discharge in existing lithium manganese iron phosphate batteries, this application provides a lithium-ion battery.

[0006] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows:

[0007] The present invention provides a lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. The positive electrode includes a positive electrode material layer containing a positive electrode active material, wherein the positive electrode active material comprises lithium manganese iron phosphate or a mixed material containing lithium manganese iron phosphate. The non-aqueous electrolyte includes a boron-containing additive, wherein the boron-containing additive comprises at least one of lithium difluorooxalate borate, lithium dioxalate borate, and tris(trimethylsilane)borate.

[0008] The lithium-ion battery meets the following conditions:

[0009] 0.2≤10×p×m / d≤50, and 0.01≤m≤2, 20≤p≤50, 12≤d≤40;

[0010] Where m is the mass percentage of boron-containing additives in the non-aqueous electrolyte, in %;

[0011] p represents the porosity of the membrane, expressed in %;

[0012] d represents the areal density of the positive electrode sheet on one side, in mg / cm³. 2 .

[0013] Preferably, the lithium-ion battery meets the following conditions:

[0014] 1≤10×p×m / d≤15.

[0015] Preferably, after the lithium-ion battery is fully charged and stored at 60°C for 30 days, the area of ​​the black spot region on the surface of the separator is less than 3%, based on the total surface area of ​​the separator being 100%.

[0016] Preferably, the mass percentage m% of the boron-containing additive in the non-aqueous electrolyte is 0.1 ≤ m ≤ 1.

[0017] Preferably, the porosity p% of the diaphragm is 30 ≤ p ≤ 42.

[0018] Preferably, the areal density d mg / cm² of the positive electrode is... 2 The value is 15≤d≤25.

[0019] Preferably, the diaphragm includes a substrate layer and a surface coating disposed on at least one side of the substrate, the surface coating including at least one of inorganic particles or organic gel.

[0020] Preferably, the positive electrode material layer further includes a positive electrode conductive agent and a positive electrode binder. Based on the total mass of the positive electrode material layer as 100%, the mass percentage of the positive electrode binder is 0.5% to 3%, and the mass percentage of the positive electrode conductive agent is 0.5% to 3%.

[0021] Preferably, the non-aqueous electrolyte further includes auxiliary additives, which include at least one of cyclic sulfate compounds, sulfonate compounds, cyclic carbonate compounds, phosphate compounds, borate compounds, and nitrile compounds;

[0022] Based on the total mass of the non-aqueous electrolyte as 100%, the mass content of the auxiliary additive is 0.01% to 30%.

[0023] Preferably, the cyclic sulfate compound is selected from vinyl sulfate, propylene sulfate, methyl vinyl sulfate, etc. At least one of them;

[0024] The sulfonyl lactone compound is selected from at least one of 1,3-propanesulfonyl lactone, 1,4-butanesulfonyl lactone, 1,3-propenesulfonyl lactone, and methylene disulfonate.

[0025] The cyclic carbonate compounds are selected from vinylene carbonate, ethylene ethylene carbonate, methylene carbonate, fluoroethylene carbonate, trifluoromethyl ethylene carbonate, difluoroethylene carbonate, or other compounds with the following structure.

[0026] At least one of the compounds shown in Formula 2:

[0027]

[0028] In structural formula 2, R 21 R 22 R 23 R 24 R 25 R 26 Each is independently selected from one of the following: hydrogen atom, halogen atom, or C1-C5 group;

[0029] The phosphate ester compound is selected from at least one of tris(trimethylsilane) phosphate, tris(trimethylsilane) phosphite, or the compound shown in structural formula 3:

[0030]

[0031] In structural formula 3, R 31 R 32 R 33Each is independently selected from C1-C5 saturated hydrocarbon groups, unsaturated hydrocarbon groups, halohydrocarbon groups, and -Si(C m H 2m+1 )3, m is a natural number from 1 to 3, and R 31 R 32 R 33 At least one of them is an unsaturated hydrocarbon group;

[0032] The borate esters are selected from tris(triethylsilane) borate esters;

[0033] The nitrile compound is selected from at least one of butadionitrile, glutaronitrile, ethylene glycol bis(propionitrile) ether, hexanetrionitrile, adiponitrile, heptanonitrile, octadionitrile, nonadionitrile, and sebaconitrile.

[0034] Beneficial effects:

[0035] The lithium-ion battery provided in this application uses lithium manganese iron phosphate or a mixture of materials containing lithium manganese iron phosphate as the positive electrode. A boron-containing additive is added to the non-aqueous electrolyte. This boron-containing additive inhibits the dissolution of metal ions from the positive electrode, inhibits the transport of metal ions to the negative electrode and their reduction and deposition on the negative electrode surface, inhibits the decomposition of the non-aqueous electrolyte, and inhibits the corrosion of the SEI and CEI films by the HF acid generated during decomposition. The boron-containing additive content (m), the single-sided areal density (d) of the positive electrode sheet, and the separator porosity (p) meet the condition 0.2 ≤ 10 × p × m / d ≤ 50. The boron-containing additive and the separator synergistically control the area of ​​the black spot region on the separator to be less than 3% of the total separator area after high-temperature storage testing, which is beneficial for extending battery life. Simultaneously, it ensures that the battery has low impedance, thereby reducing battery polarization during charging and discharging, allowing for more complete charging and discharging within the cutoff voltage range and improving the initial capacity of the battery. Ultimately, the battery achieves both high energy density and excellent high-temperature storage performance. The lithium-ion battery provided in this application improves the initial capacity, high-temperature storage performance, service life, safety and practicality of the battery, and solves the self-discharge problem of lithium manganese iron phosphate batteries. Detailed Implementation

[0036] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0037] This invention provides a lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. The positive electrode includes a positive electrode material layer containing a positive electrode active material, which includes lithium manganese iron phosphate or a mixed material containing lithium manganese iron phosphate. The non-aqueous electrolyte includes a boron-containing additive, which includes at least one of lithium difluorooxalate borate, lithium dioxalate borate, and tris(trimethylsilane)borate. The separator includes a substrate and a surface coating coated on one side of the substrate, which includes at least one of inorganic particles or an organic gel.

[0038] The lithium-ion battery meets the following conditions:

[0039] 0.2≤10×p×m / d≤50, and 0.01≤m≤2, 20≤p≤50, 12≤d≤40;

[0040] Where m is the mass percentage of boron-containing additives in the non-aqueous electrolyte, in %;

[0041] p represents the porosity of the membrane, expressed in %;

[0042] d represents the areal density of the positive electrode sheet on one side, in mg / cm³. 2 .

[0043] Lithium-ion batteries using lithium manganese iron phosphate as the positive electrode active material experience several issues during high-temperature storage. The dissolution of metal ions not only catalyzes the decomposition of lithium salts and solvents in the non-aqueous electrolyte, leading to an increase in HF acid content and accelerating the decomposition of the positive and negative electrode interfacial films, but also reduces to metal impurity particles at the negative electrode, clogging or puncturing the separator and causing micro-short circuits and self-discharge. Furthermore, the decomposition products of metal impurities, non-aqueous electrolyte, and interfacial films deposit on the separator surface and interior, forming visible black spots. These black spots are particularly pronounced when the separator porosity and permeability are low, resulting in larger and more easily formed abnormal areas, thus reducing battery capacity and lifespan. To address this issue… Through extensive research, the inventors discovered that by using lithium manganese iron phosphate or a mixture of materials containing lithium manganese iron phosphate in the positive electrode and adding boron-containing additives to the non-aqueous electrolyte, the boron-containing additives can capture dissolved metal ions on the surface of the positive electrode and complex with them to form a network-structured surface film. This surface film blocks further dissolution of metal ions, preventing corrosion and decomposition of the positive / electrolyte interface (CEI) and negative / electrolyte interface (SEI). Simultaneously, boron ions combine with phosphorus ions more readily than hydrogen ions, reducing the content of hydrofluoric acid and thus minimizing its damage to the CEI film on the positive electrode surface and the SEI film on the negative electrode surface. Furthermore, limiting the porosity (p) of the separator not only ensures normal lithium ion transport during battery charging and discharging but also reduces the reduction and deposition of metal ions on the negative electrode surface, preventing blockage or puncture of the separator. In synergy with the boron-containing additives, this reduces the battery's micro-short circuit and self-discharge problems. The areal density d of the positive electrode, the porosity p of the separator, and the boron-containing additive m satisfy the relationship 0.2≤10×p×m / d≤50. This not only fully utilizes the aforementioned effects of the boron-containing additive and the separator, effectively suppressing the dissolution of metal ions from the positive electrode, inhibiting the transport of metal ions to the negative electrode and their reduction and deposition on the negative electrode surface, but also suppresses the decomposition of non-aqueous electrolytes and prevents the corrosion of the SEI and CEI films by the HF acid generated during decomposition. In synergy with the separator, it controls the area of ​​the black spot region on the separator to be less than 3% of the total separator area after high-temperature storage testing, which is beneficial to extending the battery's lifespan. At the same time, it also ensures that the battery has low impedance, thereby reducing battery polarization during charging and discharging, allowing the battery to charge and discharge more completely within the cutoff voltage range and improving the battery's initial capacity. Ultimately, this allows the battery to achieve both high energy density and excellent high-temperature storage performance, solving the self-discharge problem of lithium manganese iron phosphate batteries.

[0044] Specifically, the mass percentage (m%) of boron-containing additives in the non-aqueous electrolyte is in the range of 0.01% ≤ m% ≤ 2%. The boron ions in the boron-containing additives readily combine with the fluoride ions to form BF bonds, thereby removing hydrofluoric acid and reducing its damage to the CEI and SEI films. Furthermore, the boron bonds readily complex with metal ions to form insoluble substances. These insoluble substances form a network-structured surface film on the positive electrode surface. This network-structured surface film blocks further dissolution of metal ions, reducing the blockage or puncture of the separator by metal impurities. This reduces micro-short circuits and self-discharge caused by metal impurities blocking or puncturing the separator. Simultaneously, the network-structured surface film prevents the dissolution of metal ions and hinders the decomposition of lithium salts, thus preventing the corrosion and decomposition of the positive electrode / electrolyte interface (CEI) and negative electrode / electrolyte interface (SEI), improving the battery's high-temperature storage performance.

[0045] The mass percentage (m%) of boron-containing additives in non-aqueous electrolytes can be 0.01%, 0.05%, 0.08%, 0.1%, 0.2%, 0.3%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2.0%, etc., as long as the mass percentage (m%) of boron-containing additives is within the range of 0.01% ≤ m% ≤ 2%. If the content of boron-containing additives is greater than 2%, excessive boron-containing additives will increase the viscosity of the non-aqueous electrolyte and form an excessively thick interfacial film on the positive and negative electrode surfaces, affecting the conduction rate of lithium ions in the liquid phase and interfacial film, increasing battery impedance and polarization. Excessive polarization will lead to incomplete charging and discharging of the battery, thereby reducing the initial capacity of the battery. If the content of boron-containing additives is less than 0.01%, it is difficult to form a network-structured surface film on the positive electrode surface, which cannot effectively prevent the dissolution of metal ions and thus cannot play a good positive electrode protection role. This will affect the stability of the battery positive electrode in non-aqueous electrolytes and cannot significantly improve the cycle life of the battery.

[0046] In a preferred embodiment, the mass percentage (m%) of the boron-containing additive in the non-aqueous electrolyte is 0.1% ≤ m% ≤ 1%. Within this preferred range, the mass percentage (m%) of the boron-containing additive results in a more stable and resilient network-structured surface film formed by complexing with dissolved metal ions on the positive electrode surface. This inhibits the dissolution of metal ions and effectively prevents the decomposition of the passivation film on both the positive and negative electrode surfaces. Furthermore, the combination with F ions further reduces the damage to the passivation film, leading to better high-temperature storage performance of the battery.

[0047] Specifically, during battery charging and discharging, the separator plays a crucial role in allowing lithium ions to pass freely while blocking metal impurity ions. A separator porosity (p%) within the range of 20% ≤ p% ≤ 50% not only ensures normal lithium ion transport during charging and discharging but also reduces the reduction and deposition of metal ions on the negative electrode surface, preventing blockage or puncture of the separator. This reduces micro-short circuits and self-discharge. If the separator porosity is below 20%, it is difficult for the separator to retain sufficient non-aqueous electrolyte, reducing the battery's electrolyte content and affecting the lithium ion transport rate. This, in turn, affects the continuous repair effect of the non-aqueous electrolyte on the passivation film, increasing battery polarization and impedance, and reducing the initial battery capacity. If the separator porosity is greater than 50%, although it can improve the battery's electrolyte content and the wetting effect of the non-aqueous electrolyte, it is difficult to prevent metal ions from transporting to the negative electrode side and damaging the SEI film. It is also more susceptible to puncture by metal impurities, leading to micro-short circuits and more severe self-discharge.

[0048] The porosity p% of the diaphragm can be 20%, 25%, 28%, 30%, 32%, 35%, 37%, 39%, 40%, 42%, 45%, 50%, etc., as long as the porosity p% of the diaphragm is in the range of 20%≤p%≤50%.

[0049] In some preferred embodiments, the porosity p% of the separator is 30% ≤ p% ≤ 42%. Within this preferred range, the separator can better prevent metal ions from transporting to the negative electrode side and damaging the SEI film, further reducing battery micro-short circuits and self-discharge. More preferably, the porosity p% of the separator is 35% ≤ p% ≤ 40%.

[0050] Specifically, the areal density d mg / cm² of the positive electrode is... 2 Controlled at 12 mg / cm 2 ≤d mg / cm 2 ≤40mg / cm 2 Within this range, d can take values ​​of 12, 13, 15, 16, 18, 20, 22, 24, 25, 27, 30, 35, or 40. The areal density of the positive electrode is controlled at 12 mg / cm³. 2 ≤d mg / cm 2 ≤40mg / cm 2 Within this range, a more complete and stable interface film (CEI film) forms at the positive electrode interface, blocking the dissolution of metal ions, reducing battery impedance, minimizing polarization, and improving battery energy density and high-temperature storage performance. If the areal density of one side of the positive electrode sheet is less than 12 mg / cm³... 2 A decrease in the active material content of the positive electrode leads to a decrease in battery energy density, which fails to meet user needs and is detrimental to commercial applications; if the areal density of one side of the positive electrode is higher than 40 mg / cm³... 2Non-aqueous electrolytes are difficult to wet, which increases the battery interface impedance, polarization, and initial battery capacity, thus affecting the battery's cycle performance and energy density.

[0051] In some preferred embodiments, the areal density d mg / cm² of the positive electrode sheet is... 2 15 mg / cm 2 ~25mg / cm 2 Within this range, the positive electrode is easily wetted by non-aqueous electrolytes. During charging and discharging, the battery polarization is small and the impedance is low, which increases the initial capacity of the battery. It also helps to form a CEI film of appropriate thickness at the interface of the positive electrode, which helps to improve the cycle performance and high-temperature storage performance of the battery.

[0052] In lithium-ion batteries, the areal density of the positive electrode is d mg / cm³. 2 At 12 mg / cm 2 ≤d mg / cm 2 ≤40mg / cm 2 Within the specified range, the mass percentage (m%) of boron-containing additives in the non-aqueous electrolyte is 0.01% ≤ m% ≤ 2%, and the membrane porosity (p%) is 20% ≤ p% ≤ 50%. Simultaneously, it must meet the condition of 0.2 ≤ 10 × p × m / d ≤ 50. This suppresses self-discharge and decomposition of the non-aqueous electrolyte / positive and negative electrode interface film during high-temperature storage. The battery also exhibits low impedance and high energy density, ultimately achieving a balance between high initial capacity and energy density, excellent high-temperature storage capacity retention, and after high-temperature storage, the proportion of abnormal black spots on the membrane to the total membrane area is less than 3%, improving battery lifespan, practicality, and safety.

[0053] If the relationship 10×p×m / d < 0.2, it indicates that the boron-containing additive content is too low, the separator porosity is too low, or the areal density of one side of the positive electrode is too high. If the boron-containing additive is too low, it cannot effectively remove hydrofluoric acid and cannot form an insoluble network film structure with metal ions. The interface films on the positive and negative electrode surfaces are easily corroded and decomposed, reducing the battery's high-temperature storage performance and increasing the area of ​​black spots on the separator. If the separator porosity is too low, it will hinder lithium-ion shuttle and greatly increase battery impedance and polarization, thus leading to battery performance degradation during use. If the surface density of the positive electrode is too high on one side, it will be difficult for the non-aqueous electrolyte to wet, thus preventing the formation of a complete and stable interface film after the first charge. The battery impedance will also increase, thereby reducing the initial capacity and energy density of the battery. In addition, the incomplete interface film is more difficult to suppress the dissolution of metal ions and block electron transport at high temperatures. Therefore, when stored at high temperatures, metal ions are reduced to particles on the surface of the negative electrode, which can easily puncture the separator, causing micro-short circuits and causing the capacity retention rate to decay faster.

[0054] If the relationship 10×p×m / d > 50, it indicates excessive boron additives, excessively high separator porosity, or excessively low areal density on one side of the positive electrode. While excessive boron additives can help reduce the damage to the interfacial film caused by hydrofluoric acid and prevent the dissolution of metal ions, excessive boron additives can also lead to excessively thick film layers after the positive and negative electrodes are formed, resulting in a significant increase in impedance and affecting the battery's initial capacity and power performance. Excessively high separator porosity, while beneficial for lithium ion transport between the positive and negative electrodes, also makes it easier for harmful substances such as metal ions to pass through the separator and be reduced at the negative electrode interface. The reduced metal impurities at the negative electrode can easily clog / puncture the separator, causing micro-short circuits. Furthermore, it can easily catalyze the decomposition of the SEI film, and the continuous damage and repair of the SEI film during high-temperature storage can lead to a rapid increase in impedance. If the areal density on one side of the positive electrode is too low, it will result in insufficient active material in the positive electrode, reducing the battery's energy density and hindering its practical application.

[0055] In some preferred embodiments, the lithium-ion battery satisfies the following condition: 1 ≤ 10 × p × m / d ≤ 15. In preferred embodiments, the hydrofluoric acid content is further reduced, and the surface film formed by the complexation of boron-containing additives and dissolved metal ions has a stronger barrier effect, improves the stability of the positive and negative electrode interface film, and results in higher energy density and better high-temperature storage performance.

[0056] In some embodiments, after the lithium-ion battery is fully charged and stored at 60°C for 30 days, the area of ​​the black spot region on the surface of the separator is less than 3%, based on the total surface area of ​​the separator being 100%.

[0057] The battery prepared using the technical solution of this application can effectively suppress the dissolution of metal ions from the positive electrode and the transport of metal ions to the negative electrode for reduction and deposition, prevent the decomposition of the positive and negative electrode interface film, and the separator has appropriate porosity. Therefore, it can control the area of ​​the black spot region on the separator to be less than 3% of the total area of ​​the separator after the battery is stored at high temperature, which is beneficial to extending the battery life.

[0058] In some embodiments, the non-aqueous electrolyte comprises an organic solvent, which includes at least one of cyclic carbonates, linear carbonates, carboxylic acid esters, sulfones, ethers, and nitriles.

[0059] In some embodiments, the ether solvent includes cyclic ethers or chain ethers, preferably chain ethers with 3 to 10 carbon atoms and cyclic ethers with 3 to 6 carbon atoms. The cyclic ether may be, but is not limited to, at least one of 1,3-dioxolane (DOL), 1,4-dioxolane (DX), crown ethers, tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-CH3-THF), and 2-trifluoromethyltetrahydrofuran (2-CF3-THF). The chain ether may be, but is not limited to, at least one of dimethoxymethane, diethoxymethane, ethoxymethoxymethane, ethylene glycol di-n-propyl ether, ethylene glycol di-n-butyl ether, diethylene glycol dimethyl ether, 1,3-dioxolane, and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether. Because chain ethers have high solvation ability with lithium ions and can improve ion dissociation, dimethoxymethane, diethoxymethane, and ethoxymethoxymethane, which have low viscosity and can impart high ionic conductivity, are particularly preferred. One ether compound can be used alone, or two or more can be used in any combination and ratio. There are no particular limitations on the amount of ether compound added; it is arbitrary as long as it does not significantly impair the lithium-ion battery effect of the present invention. Typically, the volume ratio is 1% or more, preferably 2% or more, and more preferably 3% or more, when the non-aqueous solvent volume ratio is 100%. Furthermore, the volume ratio is typically 30% or less, preferably 25% or less, and more preferably 20% or less. When two or more ether compounds are used in combination, the total amount of ether compounds should meet the above-mentioned range. When the amount of ether compound added is within the above-mentioned preferred range, it is easy to ensure the improved ionic conductivity effect resulting from the increased lithium-ion dissociation degree and reduced viscosity of the chain ether. In addition, when the negative electrode active material is a carbon material, the phenomenon of co-intercalation between chain ethers and lithium ions can be suppressed, thus enabling the input-output characteristics and charge-discharge rate characteristics to reach an appropriate range.

[0060] In some embodiments, the nitrile solvent may be, but is not limited to, at least one of acetonitrile, glutaronitrile, and malononitrile.

[0061] In some embodiments, the carbonate solvent includes cyclic carbonates or chain carbonates. Cyclic carbonates may specifically include, but are not limited to, at least one of vinylene carbonate, ethylene carbonate (EC), propylene carbonate (PC), γ-butyrolactone (GBL), butylene carbonate (BC), and butenyl carbonate. Chain carbonates may specifically include, but are not limited to, at least one of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and dipropyl carbonate (DPC). The content of cyclic carbonates is not particularly limited and is arbitrary within a range that does not significantly impair the performance of the lithium-ion battery of this invention. However, when using a single cyclic carbonate, its lower limit relative to the total volume of the non-aqueous electrolyte solvent is typically 3% or more by volume, preferably 5% or more. By setting this range, a decrease in conductivity due to a decrease in the dielectric constant of the non-aqueous electrolyte can be avoided, making it easier to achieve good high-current discharge characteristics, stability relative to the negative electrode, and cycle characteristics of the non-aqueous electrolyte battery. Furthermore, the upper limit is typically 90% or less by volume, preferably 85% or less, and more preferably 80% or less. Setting this range improves the oxidation / reduction resistance of the non-aqueous electrolyte, thereby contributing to improved stability during high-temperature storage. The content of the chain carbonate is not particularly limited, but relative to the total amount of solvent in the non-aqueous electrolyte, it is typically 15% or more by volume, preferably 20% or more, and more preferably 25% or more. Additionally, it is typically 90% or less by volume, preferably 85% or less, and more preferably 80% or less. By keeping the chain carbonate content within the above range, it is easier to achieve an appropriate viscosity for the non-aqueous electrolyte, suppressing the decrease in ionic conductivity, and thus contributing to achieving a good range of output characteristics for the non-aqueous electrolyte battery. When using two or more chain carbonates in combination, the total amount of chain carbonate should satisfy the above range.

[0062] In some embodiments, fluorine-containing chain carbonates (hereinafter referred to as "fluorinated chain carbonates") are also preferably used. There is no particular limitation on the number of fluorine atoms in a fluorinated chain carbonate as long as it is 1 or more, but it is generally 6 or less, preferably 4 or less. When a fluorinated chain carbonate has multiple fluorine atoms, these fluorine atoms can be bonded to the same carbon atom or to different carbon atoms. Examples of fluorinated chain carbonates include dimethyl fluorinated carbonate derivatives, methyl ethyl fluorinated carbonate derivatives, and diethyl fluorinated carbonate derivatives.

[0063] Carboxylic acid ester solvents include cyclic carboxylic acid esters and / or chain carbonates. Examples of cyclic carboxylic acid esters include at least one of γ-butyrolactone, γ-valerolactone, and δ-valerolactone. Examples of chain carbonates include at least one of methyl acetate (MA), ethyl acetate (EA), propyl acetate (EP), butyl acetate, propyl propionate (PP), butyl propionate, methyl propionate, ethyl propionate, methyl butyrate, methyl isobutyrate, methyl trimethylacetate, and ethyl trimethylacetate.

[0064] In some embodiments, the sulfone solvent includes cyclic sulfones and chain sulfones. Preferably, in the case of cyclic sulfones, it is typically a compound with 3 to 6 carbon atoms, more preferably 3 to 5 carbon atoms; in the case of chain sulfones, it is typically a compound with 2 to 6 carbon atoms, more preferably 2 to 5 carbon atoms. There are no particular limitations on the amount of sulfone solvent added, and it is arbitrary within a range that does not significantly impair the performance of the lithium-ion battery of the present invention. Relative to the total amount of solvent in the non-aqueous electrolyte, it is typically 0.3% or more by volume, preferably 0.5% or more by volume, more preferably 1% or more by volume. Furthermore, it is typically 40% or less by volume, preferably 35% or less by volume, more preferably 30% or less by volume. When using two or more sulfone solvents in combination, the total amount of sulfone solvent should satisfy the above range. When the amount of sulfone solvent added is within the above range, a non-aqueous electrolyte with excellent high-temperature storage stability is tended to be obtained.

[0065] In a preferred embodiment, the solvent is a mixture of cyclic carbonates and chain carbonates.

[0066] In some embodiments, the additive further includes auxiliary additives, which include at least one of cyclic sulfate compounds, sulfonyl lactone compounds, cyclic carbonate compounds, phosphate compounds, borate compounds, and nitrile compounds;

[0067] Based on the total mass of the non-aqueous electrolyte as 100%, the amount of the auxiliary additive is 0.01% to 30%.

[0068] In some embodiments, the cyclic sulfate compounds include vinyl sulfate, propylene sulfate, and methyl vinyl sulfate. At least one of them;

[0069] The sulfonyl lactone compound is selected from at least one of 1,3-propanesulfonyl lactone, 1,4-butanesulfonyl lactone, 1,3-propenesulfonyl lactone, and methylene disulfonate.

[0070] The cyclic carbonate compound is selected from at least one of vinylene carbonate, ethylene ethylene carbonate, fluoroethylene carbonate, or the compound shown in structural formula 2:

[0071]

[0072] In structural formula 2, R 21 R 22 R 23 R 24 R 25 R 26 Each is independently selected from one of the following: hydrogen atom, halogen atom, or C1-C5 group;

[0073] The phosphate ester compound is selected from at least one of tris(trimethylsilane) phosphate, tris(trimethylsilane) phosphite, or the compound shown in structural formula 3:

[0074]

[0075] In structural formula 3, R 31 R 32 R 33 Each is independently selected from C1-C5 saturated hydrocarbon groups, unsaturated hydrocarbon groups, halohydrocarbon groups, and -Si(C m H 2m+1 )3, m is a natural number from 1 to 3, and R 31 R 32 R 33 At least one of them is an unsaturated hydrocarbon group;

[0076] The borate esters are selected from tris(triethylsilane) borate esters;

[0077] In a preferred embodiment, the unsaturated phosphate compound may be at least one of the following: triargyl phosphate, diallyl methyl phosphate, diallyl ethyl phosphate, diallyl propyl phosphate, diallyl trifluoromethyl phosphate, diallyl-2,2,2-trifluoroethyl phosphate, diallyl-3,3,3-trifluoropropyl phosphate, diallyl hexafluoroisopropyl phosphate, triallyl phosphate, diallyl methyl phosphate, diallyl ethyl phosphate, diallyl propyl phosphate, diallyl trifluoromethyl phosphate, diallyl-2,2,2-trifluoroethyl phosphate, diallyl-3,3,3-trifluoropropyl phosphate, and diallyl hexafluoroisopropyl phosphate.

[0078] The nitrile compound is selected from at least one of butadionitrile, glutaronitrile, ethylene glycol bis(propionitrile) ether, hexanetrionitrile, adiponitrile, heptanonitrile, octadionitrile, nonadionitrile, and sebaconitrile.

[0079] In other embodiments, the additive may also include other additives that can improve battery performance: for example, additives that improve battery safety performance, such as flame retardant additives like fluorophosphates and cyclophosphonitriles, or overcharge prevention additives like tert-amylbenzene and tert-butylbenzene.

[0080] It should be noted that, unless otherwise specified, the amount of any one of the optional substances in the additives added to the non-aqueous electrolyte is generally less than 10%, preferably 0.1-5%, and more preferably 0.1% to 2%. Specifically, the amount of any one of the optional substances in the additives can be 0.05%, 0.08%, 0.1%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5%, 2.8%, 3%, 3.2%, 3.5%, 3.8%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 7.8%, 8%, 8.5%, 9%, 9.5%, or 10%.

[0081] In some embodiments, when the additive is selected from fluoroethylene carbonate, the amount of fluoroethylene carbonate added is 0.05% to 30% based on 100% of the total mass of the non-aqueous electrolyte.

[0082] In some embodiments, the non-aqueous electrolyte further includes lithium salts, such as LiPF6, LiPO2F2, LiSbF6, LiAsF6, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, LiN(SO2C2F5)2, LiN(SO2F)2, LiCl, LiBr, LiI, LiClO4, and LiB. 10 Cl 10 At least one of the following: LiAlCl4, lithium chloroborane, lithium difluorodioxane phosphate, lithium lower aliphatic carboxylic acids having four or fewer carbon atoms, lithium tetraphenylborate, and lithium imino.

[0083] Under the above conditions, adding LiPF6 as the main lithium salt to the non-aqueous electrolyte, along with the aforementioned auxiliary lithium salts, can improve the stability of the non-aqueous electrolyte and further enhance the high-temperature performance of the battery.

[0084] In the non-aqueous electrolyte, the concentration of the lithium salt is 0.5–3.5 mol / L. Preferably, the concentration of the lithium salt in the non-aqueous electrolyte is 0.8–2 mol / L.

[0085] In some embodiments, the bifacial areal density of the positive electrode is 20 mg / cm³. 2 ~70mg / cm 2 The areal density of the negative electrode is 10 mg / cm³.2 ~35mg / cm 2 More preferably, the bifacial areal density of the positive electrode is 30 mg / cm³. 2 ~50mg / cm 2 The areal density of the negative electrode is 15 mg / cm³. 2 ~30mg / cm 2 .

[0086] The bifacial density of the positive electrode and the bifacial density of the negative electrode are within the above range, ensuring that the positive and negative electrodes are sufficiently wetted with non-aqueous electrolyte, forming a stable interface film at the positive and negative electrode interface, reducing impedance and minimizing battery polarization, which is beneficial to improving the battery's initial capacity, energy density and high-temperature storage performance.

[0087] In some embodiments, the compaction density of the positive electrode sheet is 2.0 g / cm³. 3 ~4.4g / cm 3 The compaction density of the negative electrode sheet is 1.0 g / cm³. 3 ~2.0g / cm 3 More preferably, the compaction density of the positive electrode sheet is 2.3 g / cm³. 3 ~4.2g / cm 3 The compaction density of the negative electrode sheet is 1.4 g / cm³. 3 ~1.8g / cm 3 .

[0088] Within the aforementioned compaction density range, non-aqueous electrolytes can effectively wet the positive and negative electrodes, reducing battery impedance and polarization resistance, resulting in better performance for lithium-ion batteries. If the electrode compaction density is too high, the non-aqueous electrolyte will have difficulty wetting, increasing ion transport impedance and polarization, thus reducing the battery's initial capacity, energy density, and cycle performance. Conversely, if the electrode compaction density is too low, the adhesion between the active material layer and the substrate will be reduced, making it easier for the coating to detach from the substrate surface during battery cycling, affecting cycle performance, reducing battery life, and also decreasing the battery's energy density.

[0089] In some embodiments, the positive electrode material layer further includes a positive electrode conductive agent and a positive electrode binder. Based on the total mass of the positive electrode material layer (100%), the mass percentage of the positive electrode binder is 0.5% to 3%, and the mass percentage of the positive electrode conductive agent is 0.5% to 3%.

[0090] The positive electrode conductive agent includes at least one of conductive carbon black, conductive carbon spheres, conductive graphite, SP, conductive carbon fiber, carbon nanotubes, graphene, or reduced graphene oxide. Preferably, carbon nanotubes, SP, or conductive graphite are used. More preferably, the conductive agent is carbon nanotubes, and the content of carbon nanotubes is 0.1% to 2% of the total weight of the positive electrode material layer; more preferably, the content of carbon nanotubes is 0.5% to 2% of the total weight of the positive electrode material layer.

[0091] The positive electrode binder includes at least one of the following: polyvinylidene fluoride (PVDF), copolymers of PVDF, polytetrafluoroethylene (PTFE), copolymers of PVDF-hexafluoropropylene, copolymers of tetrafluoroethylene-hexafluoropropylene, copolymers of tetrafluoroethylene-perfluoroalkyl vinyl ethers, copolymers of ethylene-tetrafluoroethylene, copolymers of PVDF-tetrafluoroethylene, copolymers of PVDF-trifluoroethylene, copolymers of PVDF-trichloroethylene, copolymers of PVDF-fluorinated vinylides, copolymers of PVDF-hexafluoropropylene-tetrafluoroethylene, thermoplastic polyimide, thermoplastic resins such as polyethylene and polypropylene; acrylic resins; sodium carboxymethyl cellulose; polyvinyl butyral; ethylene-vinyl acetate copolymer; polyvinyl alcohol; and styrene-butadiene rubber.

[0092] The positive electrode active material includes lithium manganese iron phosphate or a mixture containing lithium manganese iron phosphate. The structural formula of lithium manganese iron phosphate is LiMn. x Fe y PO4, where 0 < x < 1, 0 < y < 1, and x + y = 1. Mixed materials containing lithium manganese iron phosphate include substances formed by doping lithium manganese iron phosphate with transition metals, or mixtures of lithium manganese iron phosphate doped with other lithium-containing compounds, such as lithium nickel cobalt manganese oxide. The structural formula of substances formed by doping lithium manganese iron phosphate with transition metals is LiM. z Mn x＇ Fe y＇ PO4, where 0 < x' < 1, 0 < y' < 1, 0 < z < 1, x' + y' + z = 1; M is a transition metal.

[0093] In some embodiments, the positive electrode material layer is obtained by blending positive electrode active material, positive electrode conductive agent, and positive electrode binder.

[0094] In some embodiments, the positive electrode sheet further includes a positive current collector, and the positive electrode material layer is formed on the surface of the positive current collector.

[0095] The positive current collector is selected from a metallic material that can conduct electrons. Preferably, the positive current collector includes at least one of Al, Ni, tin, copper, and stainless steel. In a more preferred embodiment, the positive current collector is selected from aluminum foil.

[0096] In some embodiments, the negative electrode sheet includes a negative electrode material layer, the negative electrode material layer includes a negative electrode active material, and the negative electrode active material is selected from at least one of silicon-based negative electrode and carbon-based negative electrode.

[0097] In a preferred embodiment, the carbon-based negative electrode may include graphite, hard carbon, soft carbon, graphene, mesophase carbon microspheres, etc. The graphite includes, but is not limited to, one or more of natural graphite, artificial graphite, amorphous carbon, carbon-coated graphite, graphite-coated graphite, and resin-coated graphite. The natural graphite may be flake graphite, flaky graphite, soil graphite, and / or graphite particles obtained by using these graphites as raw materials and subjecting them to spheroidization, densification, etc. The artificial graphite may be obtained by graphitizing organic materials such as coal tar pitch, heavy crude oil from coal, atmospheric residue, heavy crude oil from petroleum, aromatic hydrocarbons, nitrogen-containing cyclic compounds, sulfur-containing cyclic compounds, polystyrene, polyvinyl chloride, polyvinyl alcohol, polyacrylonitrile, polyvinyl butyral, natural polymers, polyphenylene sulfide, polyphenylene ether, furfuryl alcohol resin, phenolic resin, and imide resin at high temperatures. The amorphous carbon can be amorphous carbon particles obtained by heat treatment at a temperature range (400℃~2200℃) using easily graphitizable carbon precursors such as tar and pitch as raw materials, or amorphous carbon particles obtained by heat treatment using difficult-to-graphitize carbon precursors such as resin as raw materials. The carbon-coated graphite can be obtained by mixing natural graphite and / or artificial graphite with carbon precursors such as tar, pitch, and resin (organic compounds), and heat treatment at a temperature range (400℃~2300℃) at a temperature range of 400℃ to 2300℃. The obtained natural graphite and / or artificial graphite are used as the core graphite, and amorphous carbon is used to coat it to obtain a carbon-graphite composite. The carbon-graphite composite can be in the form where the entire or part of the surface of the core graphite is coated with amorphous carbon, or it can be in the form of multiple primary particles composited using carbon derived from the aforementioned carbon precursors as a binder. Alternatively, carbon-graphite composites can be obtained by reacting hydrocarbon gases such as benzene, toluene, methane, propane, and volatile aromatic compounds with natural and / or artificial graphite at high temperatures, causing carbon to deposit on the graphite surface. The graphite-coated graphite can be obtained by mixing natural and / or artificial graphite with carbon precursors of easily graphitized organic compounds such as tar, asphalt, and resin, and subjecting the mixture to one or more heat treatments at approximately 2400–3200°C. Using the resulting natural and / or artificial graphite as the core graphite, and coating the entire or part of the surface of the core graphite with graphitized materials, graphite-coated graphite can be obtained. The resin-coated graphite can be obtained by mixing natural and / or artificial graphite with resin, drying at a temperature below 400°C, and using the resulting natural and / or artificial graphite as the core graphite, coating the core graphite with resin, etc. The aforementioned organic compounds, such as tar and asphalt resin, can be listed as carbonizable organic compounds selected from coal-based heavy crude oil, direct-flow heavy crude oil, decomposed petroleum heavy crude oil, aromatic hydrocarbons, N-ring compounds, S-ring compounds, polystyrene, organic synthetic polymers, natural polymers, thermoplastic resins, and thermosetting resins.

[0098] The silicon-based anode includes at least one of silicon material, silicon oxide, silicon-carbon composite material, and silicon alloy material; the carbon-based anode includes at least one of graphite, hard carbon, soft carbon, graphene, and mesophase carbon microspheres; the lithium-based anode includes at least one of metallic lithium or lithium alloy. Specifically, the lithium alloy may be at least one of lithium-silicon alloy, lithium-sodium alloy, lithium-potassium alloy, lithium-aluminum alloy, lithium-tin alloy, and lithium-indium alloy. The tin-based anode includes at least one of tin, tin-carbon, tin-oxygen, and tin metal compounds.

[0099] In some embodiments, the negative electrode material layer further includes a negative electrode binder and a negative electrode conductive agent, and the negative electrode active material, the negative electrode binder and the negative electrode conductive agent are blended to obtain the negative electrode material layer.

[0100] The selectable ranges of the negative electrode adhesive and negative electrode conductive agent are the same as those of the positive electrode adhesive and positive electrode conductive agent, and will not be repeated here.

[0101] In some embodiments, the negative electrode sheet further includes a negative electrode current collector, and the negative electrode material layer is formed on the surface of the negative electrode current collector.

[0102] The negative electrode current collector is selected from a metallic material that can conduct electrons. Preferably, the negative electrode current collector includes at least one of Al, Ni, tin, copper, and stainless steel. In a more preferred embodiment, the negative electrode current collector is selected from copper foil.

[0103] In some embodiments, the separator includes a substrate layer and a surface coating disposed on at least one surface of the substrate layer. The substrate layer includes, but is not limited to, single-layer PP (polypropylene), single-layer PE (polyethylene), double-layer PP / PE, double-layer PP / PP, and triple-layer PP / PE / PP. The surface coating is disposed on at least one surface of the separator, and the surface coating includes at least one of inorganic particles and organic gels.

[0104] Inorganic particles include ceramic particles, such as one or a mixture of two or more of zirconium dioxide, aluminum oxide, titanium dioxide, silicon dioxide, barium sulfate, barium titanate, calcium carbonate, magnesium oxide, zinc oxide, silicon carbide, and boron nitride. Organic gels include organic polymers selected from at least one of vinylidene fluoride-hexafluoropropylene copolymer, polyvinyl chloride, polyoxyethylene, polyacrylonitrile, polymethyl methacrylate, polyamide, polyhexanediol, polyurethane, polyethylene terephthalate, polybutylene terephthalate, polystyrene, polybutadiene, polytetrafluoroethylene, polysulfide rubber, styrene-butadiene rubber, styrene-butadiene-styrene block copolymer, or ethylene-vinyl acetate copolymer.

[0105] In some embodiments, the thickness of the surface coating is 5 μm to 30 μm. By providing a surface coating with a thickness of 5 to 30 μm, the mechanical strength and puncture resistance of the separator can be effectively improved, thereby enhancing the safety performance of the lithium-ion battery.

[0106] The present invention will be further illustrated by the following examples.

[0107] Example 1

[0108] This embodiment is used to illustrate a lithium-ion battery disclosed in this invention.

[0109] 1) Preparation of positive electrode sheet

[0110] The positive electrode active material LiMn was mixed at a mass ratio of 97:1.5:1.5. 0.6 Fe 0.4 PO4, conductive carbon black Super-P, and binder polyvinylidene fluoride (PVDF) were dispersed in N-methyl-2-pyrrolidone (NMP) to obtain a positive electrode slurry. The slurry was uniformly coated on both sides of an aluminum foil, and after drying, calendering, and vacuum drying, aluminum leads were welded on using an ultrasonic welder to obtain the positive electrode sheet. The specific areal density of the coating on one side of the positive electrode sheet is shown in Table 1.

[0111] 2) Preparation of negative electrode sheet

[0112] Weigh out each component according to the following mass ratio: graphite (FSN-1) as the negative electrode active material, Super-P as the conductive carbon black, styrene-butadiene rubber (SBR) as the binder, and carboxymethyl cellulose (CMC).

[0113] CMC was added to pure water at a solid content of 1.5% and stirred thoroughly to prepare a transparent CMC adhesive solution. Conductive carbon (Super P) was added to the CMC adhesive solution and stirred thoroughly to prepare a conductive adhesive. Graphite was then added and stirred thoroughly to obtain the desired negative electrode slurry. The prepared negative electrode slurry was uniformly coated onto copper foil, and then dried, rolled, die-cut, or slit to obtain the negative electrode sheet. The compacted density of the negative electrode sheet was 1.0 g / cm³. 3 ~2.0g / cm 3 Within the range.

[0114] 3) Preparation of non-aqueous electrolyte

[0115] Ethylene carbonate (EC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) were mixed in a mass ratio of EC:DEC:EMC = 1:1:1. Lithium hexafluorophosphate (LiPF6) was added to a molar concentration of 1.1 mol / L, and then additives were added according to Table 1. The amount of additives was calculated as a percentage of the total mass of the non-aqueous electrolyte.

[0116] 4) Preparation of lithium-ion battery cells

[0117] A separator is placed between the positive and negative electrode plates. Then, the sandwich structure consisting of the positive electrode plate, negative electrode plate, and separator is wound up. The wound body is then placed in an aluminum foil packaging bag and vacuum baked at 75°C for more than 48 hours to obtain the battery cell to be injected with electrolyte. The coating thickness on the separator surface is in the range of 5μm to 30μm, and the porosity of the separator is shown in Table 1.

[0118] 5) Electrolyte injection and formation of battery cells

[0119] In a glove box with the dew point controlled below -40°C, the non-aqueous electrolyte prepared above was injected into the battery cell, vacuum sealed, and left to stand for 72 hours. Then, the first charge was performed according to the following steps: 0.05C constant current charging for 180 min, 0.1C constant current charging for 120 min, 0.2C constant current charging for 120 min, followed by a second vacuum sealing, and then a full charge at 0.2C (100% SOC). After resting at room temperature for 72 hours, a full discharge at 0.2C (0% SOC) was performed.

[0120] Examples 2-25 and Comparative Examples 1-18

[0121] Examples 2-25 and Comparative Examples 1-18 are used to illustrate the battery and its preparation method disclosed in this invention, including most of the operational steps in Example 1. The difference lies in the specific selection or content of the positive electrode active material type, separator porosity, positive electrode sheet areal density, boron-containing additive, and other additives as shown in Table 1. In Examples 1-25, the boron-containing additive added to the non-aqueous electrolyte is lithium difluorooxalate borate (LiDFOB).

[0122] Table 1. Battery parameter design for Examples 1-25 and Comparative Examples 1-18

[0123]

[0124]

[0125] Performance testing

[0126] The lithium-ion batteries prepared in Examples 1-25 and Comparative Examples 1-18 were subjected to the following performance tests: High-temperature storage performance test:

[0127] Before the high-temperature storage test, the lithium-ion batteries prepared in the examples and comparative examples were charged and discharged at a 1C rate for three cycles within the charge / discharge cutoff voltage range. The discharge capacity of the last cycle was recorded as the battery capacity before the test, C1. Finally, the batteries were charged to full capacity at a 1C rate. The test batteries were placed in an oven at 60°C for 30 days. After the high-temperature storage was completed, the batteries were discharged at a 1C rate to the cutoff voltage range, and the discharged capacity was recorded as the battery capacity after the test, C2.

[0128] High-temperature storage capacity retention rate is calculated using the following formula:

[0129] Capacity retention rate after 30 days of storage at 60℃ = C2 / C1 × 100%.

[0130] Method for testing the area of ​​abnormal black spots on the diaphragm after high-temperature storage test:

[0131] The test battery was placed in a 60°C oven for 30 days. After high-temperature storage, the battery was disassembled in a nitrogen atmosphere in a glove box to obtain the battery separator. The white area was the normal area, and the black spots were the abnormal areas (they may appear in different shades of gray depending on their density). After taking pictures with a high-magnification (20x or more) microscope, the different areas were analyzed. The white area was abstracted into a circle, and the area of ​​the black spot abnormal area was obtained by statistically analyzing the gray difference.

[0132] The formula for calculating the area percentage of the dark spot region is: Area of ​​the abnormal dark spot region / Total area of ​​the septum × 100%.

[0133] Initial battery capacity

[0134] The lithium-ion batteries prepared in the examples and comparative examples were charged at a 1C rate and discharged at a 1C rate to obtain the first discharge capacity, which is recorded as the initial capacity of the battery.

[0135] The results obtained from the tests of Examples 1-25 and Comparative Examples 1-18 are filled in Table 2.

[0136] Table 2. Electrical performance test results of Examples 1-25 and Comparative Examples 1-18

[0137]

[0138]

[0139] As shown in Tables 1 and 2, when the positive electrode uses lithium manganese iron phosphate or a mixture containing lithium manganese iron phosphate, and boron-containing additives are added to the non-aqueous electrolyte, and the porosity p of the separator, the areal density d of the positive electrode sheet, and the mass percentage m of the boron-containing additives in the non-aqueous electrolyte meet the preset conditions of 0.2 ≤ 10 × p × m / d ≤ 50, the resulting lithium-ion battery exhibits high initial capacity, high capacity retention at high temperatures, and a separator black plate area of ​​less than 3% after 30 days of storage at 60℃. This indicates that the boron-containing additives in the non-aqueous electrolyte can inhibit the dissolution of metal ions from the positive electrode and inhibit metal ion transfer. The electrolyte is transported to the negative electrode and reduced and deposited on the negative electrode surface, inhibiting the decomposition of non-aqueous electrolyte and suppressing the corrosion of SEI and CEI films by HF acid generated during decomposition. The boron-containing additive works synergistically with the separator to control the area of ​​black spots on the separator to be less than 3% of the total separator area after high-temperature storage testing, which is beneficial to extending the battery life. At the same time, it can also ensure that the battery has low impedance, thereby reducing the polarization of the battery during charging and discharging, making the battery charge and discharge more completely within the cutoff voltage range and improving the initial capacity of the battery. Ultimately, the battery achieves both high energy density and excellent high-temperature storage performance.

[0140] As shown in Examples 1-25 and Comparative Examples 1, 5-10, 13, 15, and 17, even if the porosity p of the separator, the areal density d of the positive electrode, and the mass percentage m of the boron-containing additive in the non-aqueous electrolyte meet the preset condition 0.2 ≤ 10 × p × m / d ≤ 50, the p, d, and m values ​​do not meet their range limits. This results in low initial capacity and high-temperature storage capacity retention of the lithium-ion battery, and the black spot area of ​​the separator after high-temperature storage is generally greater than 3%. As shown in Comparative Examples 11-12, 14, and 18, although the p, d, and m values ​​meet their range limits, they do not meet the preset condition 0.2 ≤ 10 × p × m / d ≤ 50. This also results in low initial capacity and high-temperature storage capacity retention of the lithium-ion battery, and the black spot area of ​​the separator after high-temperature storage is generally greater than 3%. This indicates that the p, d, and m values ​​have a strong correlation in improving the high-temperature storage performance and initial capacity of the battery.

[0141] The test results of Examples 1-17 show that when the preferred condition 1≤10×p×m / d≤15 is further met, it is beneficial to further improve the initial capacity of the battery, the high-temperature storage capacity retention rate, and further reduce the area of ​​black spots on the separator after high-temperature storage, thus solving the problem of high battery self-discharge. Examples 18-25 involve different positive electrode active materials. The positive electrode is selected as lithium manganese iron phosphate or a mixed material containing lithium manganese iron phosphate. Boron-containing additives are added to the non-aqueous electrolyte. At the same time, when the porosity p of the separator, the single-sided areal density d of the positive electrode sheet, and the mass percentage m of the boron-containing additives in the non-aqueous electrolyte meet the preset condition 0.2≤10×p×m / d≤50, the resulting lithium-ion battery has a high initial capacity, a high high-temperature storage capacity retention rate, and a separator black spot area of ​​less than 3% after 30 days of storage at 60°C.

[0142] Examples 26-29

[0143] The difference between Examples 26-29 and Example 1 lies in the type of boron-containing additive added to the non-aqueous electrolyte; otherwise, they are the same as Example 1. Specific battery parameters are shown in Table 3, and electrical performance test results are shown in Table 4. The boron-containing additive includes at least one of lithium difluorooxalate borate (LiDFOB), lithium dioxalate borate (LiBOB), and tris(trimethylsilane)borate (TMSB).

[0144] Table 3. Battery parameter design for Examples 1 and 26-29.

[0145]

[0146] Table 4. Electrical performance test results for Examples 1 and 26-29

[0147]

[0148] Tables 3 and 4 show that when different types of boron-containing additives are added to the non-aqueous electrolyte, the porosity p of the separator, the areal density d of the positive electrode, and the mass percentage m of the boron-containing additives in the non-aqueous electrolyte all meet the preset condition 0.2≤10×p×m / d≤50, they have similar effects. The high-temperature storage capacity retention rate and initial capacity of the resulting batteries are basically similar, and the proportion of black spots on the separator after high-temperature storage is also the same as that on the substrate. This indicates that the relationship provided in this application is suitable for different types of boron-containing additives.

[0149] Examples 30-32

[0150] The difference between Examples 30-32 and Example 1 is that other additives are added to the non-aqueous electrolyte. The types and contents of the other additives are shown in Table 5. The rest is the same as Example 1. The electrical performance test results are shown in Table 6.

[0151] Table 5. Battery parameter design for Examples 1 and 30-32.

[0152]

[0153] Table 6. Electrical performance test results for Examples 1 and 30-32

[0154]

[0155] As shown in Tables 5 and 6, when the types of positive and negative active materials are the same, adding VC (ethylene carbonate), PS (1,3-propane sulfonyl lactone), or FEC (fluoroethylene carbonate) to the boron-containing additives can further improve the battery's cycle performance and initial capacity. This is presumably because the boron-containing additives and the aforementioned additives jointly participate in the formation of the passivation film on the surface of the positive and negative active materials, resulting in a more stable and denser passivation film. This effectively reduces the dissolution of metal ions, reduces the reduction and deposition of metal ions at the negative electrode, improves the battery's high-temperature storage performance, further reduces micro-short circuits and self-discharge, and reduces the area of ​​black spots on the separator after high-temperature storage. More preferably, among the aforementioned additives, it can be seen that using boron-containing additives in combination with VC additives in non-aqueous electrolytes results in the most significant improvement in battery cycle performance and a lower proportion of black spots on the separator.

[0156] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A lithium-ion battery, characterized in that, The device includes a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. The positive electrode includes a positive electrode material layer containing a positive electrode active material, which includes lithium manganese iron phosphate or a mixed material containing lithium manganese iron phosphate. The negative electrode includes a negative electrode material layer containing a negative electrode active material, which is selected from at least one of silicon-based negative electrodes and carbon-based negative electrodes. The non-aqueous electrolyte includes a boron-containing additive, which includes at least one of lithium difluorooxalate borate, lithium dioxalate borate, and tris(trimethylsilane)borate. The lithium-ion battery meets the following conditions: 1≤10×p×m / d≤15, and 0.1≤m≤0.8, 30≤p≤42, 12≤d≤40; Wherein, m is the mass percentage of boron-containing additives in the non-aqueous electrolyte, expressed in % %. p represents the porosity of the membrane, expressed in % . d represents the areal density of the positive electrode sheet on one side, in mg / cm³. 2 .

2. The lithium-ion battery according to claim 1, characterized in that, After the lithium-ion battery is fully charged and stored at 60°C for 30 days, the area of ​​the black spot region on the surface of the separator is less than 3%, based on the total surface area of ​​the separator being 100%.

3. The lithium-ion battery according to claim 1, characterized in that, The areal density d mg / cm² of the positive electrode is... 2 The value is 15≤d≤25.

4. The lithium-ion battery according to claim 1, characterized in that, The diaphragm includes a substrate layer and a surface coating disposed on at least one side of the substrate, the surface coating including at least one of inorganic particles or organic gel.

5. The lithium-ion battery according to claim 1, characterized in that, The positive electrode material layer further includes a positive electrode conductive agent and a positive electrode binder. Based on the total mass of the positive electrode material layer as 100%, the mass percentage of the positive electrode binder is 0.5% to 3%, and the mass percentage of the positive electrode conductive agent is 0.5% to 3%.

6. The lithium-ion battery according to claim 1, characterized in that, The non-aqueous electrolyte also includes auxiliary additives, which include at least one of cyclic sulfate compounds, sulfonyl lactone compounds, cyclic carbonate compounds, phosphate compounds, borate compounds, and nitrile compounds. Based on the total mass of the non-aqueous electrolyte as 100%, the mass content of the auxiliary additive is 0.01% to 30%.

7. The lithium-ion battery according to claim 6, characterized in that, The cyclic sulfate compounds are selected from vinyl sulfate, propylene sulfate, methyl vinyl sulfate, etc. , At least one of them; The sulfonyl lactone compound is selected from at least one of 1,3-propanesulfonyl lactone, 1,4-butanesulfonyl lactone, 1,3-propenesulfonyl lactone, and methylene disulfonate. The cyclic carbonate compound is selected from at least one of vinylene carbonate, ethylene ethylene carbonate, methylene carbonate, fluoroethylene carbonate, trifluoromethyl ethylene carbonate, difluoroethylene carbonate, or the compound shown in structural formula 2. Structural Formula 2 In structural formula 2, R 21 R 22 R 23 R 24 R 25 R 26 Each is independently selected from one of the following: hydrogen atom, halogen atom, or C1-C5 group; The phosphate ester compound is selected from at least one of tris(trimethylsilane) phosphate, tris(trimethylsilane) phosphite, or the compound shown in structural formula 3: Structural Formula 3 In structural formula 3, R 31 R 32 R 33 Each is independently selected from C1-C5 saturated hydrocarbon groups, unsaturated hydrocarbon groups, halohydrocarbon groups, and -Si(C m H 2m+1 )3, m is a natural number from 1 to 3, and R 31 R 32 R 33 At least one of them is an unsaturated hydrocarbon group; The borate esters are selected from tris(triethylsilane) borate esters; The nitrile compound is selected from at least one of butadionitrile, glutaronitrile, ethylene glycol bis(propionitrile) ether, hexanetrionitrile, adiponitrile, heptanonitrile, octadionitrile, nonadionitrile, and sebaconitrile.

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