Lithium ion secondary battery, battery module, battery pack, and electrical device

Abstract

This application relates to a lithium ion secondary battery, the electrolyte of which contains a salt with high thermal stability (M y+ ) x / y R1(SO2N ‑ ) x SO2R2, where M y+ is a metal ion, R1 and R2 are each independently a fluorine atom, an alkyl group having 1 to 20 carbon atoms, a fluoroalkyl group having 1 to 20 carbon atoms or a fluoroalkoxy group having 1 to 20 carbon atoms, and x is 1, 2 or 3, y is 1, 2 or 3; the mass percentage of the salt in the electrolyte is set as k2%; the temperature rise coefficient k1 of the positive electrode sheet satisfies 2.5 ≤ k1 ≤ 32, where k1 = Cw / Mc, Cw is the positive electrode material loading amount per unit area on either side surface of the positive electrode current collector loaded with the positive electrode material layer (mg / cm 2 ), Mc is the carbon content (%) of the positive electrode material layer; and the lithium ion secondary battery satisfies 0.34 ≤ k2 / k1 ≤ 8.

Description

Technical Field

[0001] The present application relates to the field of lithium-ion secondary batteries, and particularly to a lithium-ion secondary battery with high energy density, a battery module, a battery pack, and an electrical device. Background Art

[0002] Lithium-ion batteries have become the most popular energy storage systems due to their low cost, long life, and good safety, and are now widely used in fields such as pure electric vehicles, hybrid electric vehicles, and smart grids. However, current lithium-ion secondary batteries are difficult to meet people's higher demand for endurance. To solve the problem of "range anxiety" for electric vehicles, it is urgent to develop lithium-ion secondary batteries with higher energy density.

[0003] By increasing the discharge gram capacity of the active material per unit volume in the lithium-ion secondary battery, the energy density can be effectively increased. For example, by reducing the carbon content as a conductive agent on the electrode sheet or the carbon coating amount on the surface of the positive electrode material particles, the weight of the active material per unit volume can be increased, thereby increasing the discharge gram capacity of the active material per unit volume. In addition, by increasing the coating weight of the active material and reducing the usage ratio of the non-active substrate, the discharge gram capacity of the active material per unit volume can be further improved. However, while the above strategies achieve high energy density, they will cause a significant increase in the internal resistance of the battery cell. During high-rate charging, the battery cell generates a large amount of heat and thus is in a high-temperature condition. At this time, lithium salts in the electrolyte such as LiPF6 will accelerate decomposition to generate gases such as HF and PF5. These highly reactive components will accelerate the destruction of the SEI film, resulting in the exposure of the active substances in the electrolyte and the loss of the active material. And the repair process of the SEI film will continuously consume active lithium and the electrolyte, leading to further deterioration of the cycle performance and storage life of the battery cell.

[0004] Therefore, there is a need to increase the energy density of lithium-ion secondary batteries while ensuring the cycle and storage life of the battery cell. Summary of the Invention

[0005] The present application is made in view of the above problems, and its purpose is to provide a lithium-ion secondary battery to solve the problems that the battery cell with high energy density generates a large amount of heat during charging, resulting in electrolyte decomposition, and the cycle and storage life of the battery cell are poor.

[0006] To achieve the above object, in the first aspect of the present application, a lithium-ion secondary battery is provided, which has a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. The positive electrode sheet includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector. The positive electrode material layer contains a positive electrode active substance and carbon. Among them,

[0007] The electrolyte contains a lithium salt (M y+ ) x / yR1(SO2N - ) x SO2R2, wherein M y+ is a metal ion, R1 and R2 are each independently a fluorine atom, an alkyl group having 1 to 20 carbon atoms, a fluoroalkyl group having 1 to 20 carbon atoms, or a fluoroalkoxy group having 1 to 20 carbon atoms, and x is 1, 2 or 3, and y is 1, 2 or 3; the mass percentage of the lithium salt in the electrolyte is set to k2%;

[0008] The temperature rise coefficient k1 of the positive electrode sheet satisfies 2.5 ≤ k1 ≤ 32, where k1 = Cw / Mc, Cw is the positive electrode material loading per unit area on either side of the positive electrode current collector loaded with the positive electrode material layer (mg / cm 2 ), and Mc is the carbon content (%) of the positive electrode material layer;

[0009] Moreover, the lithium ion secondary battery satisfies 0.34 ≤ k2 / k1 ≤ 8.

[0010] By using an electrolyte containing a salt (M y+ ) x / y R1(SO2N - ) x SO2R2 and making the content of the salt (M y+ ) x / y (R1SO2N) x SO2R2 satisfy a specific relationship with the positive electrode material loading and carbon content of the positive electrode sheet, it is possible to improve the volumetric energy density of the battery cell while reducing the influence of high temperature on the electrolyte, thereby obtaining better battery cell performance and solving the problem that it is difficult to balance high energy density and long life.

[0011] In any embodiment, the lithium ion secondary battery satisfies at least one of the following conditions 1) to 3):

[0012] 1) 3.3 ≤ k1 ≤ 14.5;

[0013] 2) 0.48 ≤ k2 / k1 ≤ 7;

[0014] 3) 1 ≤ Mc ≤ 7.

[0015] By satisfying at least one of the above conditions 1) to 3), the energy density and / or charge-discharge performance and / or cycle life of the battery can be further improved.

[0016] In any embodiment, M y+ is selected from Li + , Na + , K + , Rb + , Cs+ and Mg 2+ and Ca 2+ and Ba 2+ and Al 3+ and Fe 2+ and Fe 3+ and Ni 2+ and Ni 3+ and at least one of them. Optionally, M y+ is selected from Li + and Na + and K + and Rb + and Cs + and at least one of them. By selecting the above cations, the power performance of the battery cell can be further improved.

[0017] In any embodiment, each of R1 and R2 is independently a fluorine atom, an alkyl group having 1 to 10 carbon atoms, a fluoroalkyl group having 1 to 10 carbon atoms, or a fluoroalkoxy group having 1 to 10 carbon atoms. Optionally, each of R1 and R2 is independently CH3, CF3 or F. By selecting the structures of R1 and R2, it helps to improve the cycle performance of the battery.

[0018] In any embodiment, the electrolyte of the lithium ion secondary battery further contains a low impedance additive. Optionally, the low impedance additive is at least one of fluorosulfonate NSO3F, difluorooxalate borate NDFOB, difluorophosphate NPO2F2, difluorodioxalate NDFOP, tris(trimethylsilyl) phosphate, and tris(trimethylsilyl) phosphite, where N is the metal ion of the salt, and can be optionally Li + and Na + and K + and Rb + and Cs + ; optionally, the low impedance additive is at least one of lithium fluorosulfonate, lithium difluorooxalate borate, tris(trimethylsilyl) phosphate, and lithium difluorophosphate. These low impedance additives can reduce the impedance of the protective film (SEI film) on the surfaces of the positive and negative electrode plates and mitigate the deterioration of battery performance caused by temperature rise.

[0019] In any embodiment, the mass percentage of the low impedance additive in the electrolyte is 0.1% to 10%, and can be optionally 0.2% to 5%. By selecting the mass percentage of the low impedance additive, the battery cycle performance can be further improved.

[0020] In any embodiment, the electrolyte of the lithium-ion secondary battery further contains a lithium salt that inhibits the corrosion of aluminum foil. Optionally, the lithium salt that inhibits the corrosion of aluminum foil is at least one selected from LiPF6, LiAsF6, and LiBF4. By adding the lithium salt that inhibits the corrosion of aluminum foil, the corrosion of aluminum foil can be inhibited, and the temperature rise inside the battery cell can be reduced.

[0021] In any embodiment, the mass percentage of the lithium salt that inhibits the corrosion of aluminum foil in the electrolyte is 0.1% to 10%, optionally 0.2% to 5%, and optionally 1% to 3%. By selecting the content of the lithium salt that inhibits the corrosion of aluminum foil, the cycle performance of the battery can be further improved.

[0022] In any embodiment, the total lithium salt content in the electrolyte of the lithium-ion secondary battery is in the range of 5 wt% - 50 wt%, optionally in the range of 5 wt% - 37 wt%, and optionally in the range of 5 wt% - 23 wt%, based on the total weight of the electrolyte. By selecting the total lithium salt content in the electrolyte, the cycle life and power performance of the lithium-ion secondary battery can be improved.

[0023] In any embodiment, the positive electrode active material in the lithium-ion secondary battery is selected from one or more of lithium transition metal oxides, lithium-containing phosphates with an olivine structure, and their respective modified compounds. Optionally, the lithium transition metal oxides are selected from lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, and lithium nickel cobalt manganese oxide LiNi a Co b Mn c O2, lithium nickel cobalt aluminum oxide, and their modified compounds.

[0024] The second aspect of the present application provides a battery module, which includes the lithium-ion secondary battery of the first aspect of the present application.

[0025] The third aspect of the present application provides a battery pack, which includes the battery module of the second aspect of the present application.

[0026] The fourth aspect of the present application provides an electrical device, which includes one or more of the lithium-ion secondary battery of the first aspect of the present application, the battery module of the second aspect of the present application, or the battery pack of the third aspect of the present application. Description of the Drawings

[0027] To more clearly illustrate the technical solutions of this application, the following will briefly introduce the drawings required for use in the embodiments of this application. Obviously, the following described drawings are only some embodiments of this application. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0028] Figure 1 It is a schematic diagram of a lithium-ion secondary battery according to an embodiment of this application.

[0029] Figure 2 is Figure 1 The exploded view of the lithium-ion secondary battery according to an embodiment of this application shown.

[0030] Figure 3 It is a schematic diagram of a battery module according to an embodiment of this application.

[0031] Figure 4 It is a schematic diagram of a battery pack according to an embodiment of this application.

[0032] Figure 5 is Figure 4 The exploded view of the battery pack according to an embodiment of this application shown.

[0033] Figure 6 It is a schematic diagram of a device using a lithium-ion secondary battery as a power source according to an embodiment of this application.

[0034] Description of the reference numerals

[0035] 1 Battery pack

[0036] 2 Upper box body

[0037] 3 Lower box body

[0038] 4 Battery module

[0039] 5 Lithium-ion secondary battery

[0040] 51 Housing

[0041] 52 Electrode assembly

[0042] 53 Cover plate Detailed implementation manners

[0043] For the sake of simplicity, this application specifically discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form a range not explicitly recited; and any lower limit can be combined with other lower limits to form a range not explicitly recited, and similarly any upper limit can be combined with any other upper limit to form a range not explicitly recited. In addition, each individually disclosed point or single numerical value itself can be used as a lower limit or an upper limit and combined with any other point or single numerical value or combined with other lower limits or upper limits to form a range not explicitly recited.

[0044] Lithium ion secondary battery

[0045] Generally, a lithium-ion secondary battery includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. During the charge and discharge process of the battery, active ions are intercalated and deintercalated back and forth between the positive electrode sheet and the negative electrode sheet. The separator is disposed between the positive electrode sheet and the negative electrode sheet and functions to isolate them. The electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet.

[0046] The lithium-ion secondary battery according to the first aspect of this application has a positive electrode sheet, a negative electrode sheet, a separator, and an electrolytic solution. The positive electrode sheet includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector. The positive electrode material layer contains a positive electrode active material and carbon. Among them,

[0047] the electrolytic solution contains a lithium salt (M y+ ) x / y R1(SO2N - ) x SO2R2, and its structural formula can be written as:

[0048]

[0049] wherein the M y+ is a metal ion, R1 and R2 are each independently a fluorine atom, an alkyl group having 1 to 20 carbon atoms, a fluoroalkyl group having 1 to 20 carbon atoms, or a fluoroalkoxy group having 1 to 20 carbon atoms, and x is 1, 2, or 3, and y is 1, 2, or 3; the mass percentage of the lithium salt in the electrolytic solution is set as k2%;

[0050] the temperature rise coefficient k1 of the positive electrode sheet satisfies 2.5 ≤ k1 ≤ 32, where k1 = Cw / Mc, Cw is the positive electrode material loading amount per unit area on either side surface of the positive electrode current collector loaded with the positive electrode material layer (mg / cm 2 ), and Mc is the carbon content (%) of the positive electrode material layer;

[0051] And, the lithium-ion secondary battery satisfies 0.34 ≤ k2 / k1 ≤ 8.

[0052] Among them, the carbon contained in the positive electrode material layer includes the carbon coated on the surface of the positive electrode active material and the carbon optionally contained as a conductive agent in the positive electrode slurry used to prepare the positive electrode material layer.

[0053] During the charging process, the temperature rise inside the battery cell mainly comes from the temperature rise of the positive electrode plate, and the temperature rise of the positive electrode plate is mainly related to the thickness and conductivity of the positive electrode plate. The thickness of the positive electrode plate is related to the positive electrode material loading amount Cw per unit area on either side surface of the positive electrode current collector loaded with the positive electrode material layer, and the conductivity is related to the carbon content Mc of the positive electrode material layer. Therefore, the temperature rise coefficient k1 of the positive electrode plate is defined as k1 = Cw / Mc. When the charging current is constant, the heat generation amount inside the battery cell is basically positively correlated with the temperature rise coefficient k1. If k1 is too small, the positive electrode material loading amount is too low, and the carbon content in the positive electrode material layer is too high, and the proportion of the non-active base material is large, which affects the reduction of the battery energy density; while if k1 is too large, the heat generation amount of the battery cell is too high, and the positive electrode material loading amount is too high and the carbon content in the positive electrode material layer is too low, which will lead to a low solid-phase lithium ion transmission rate and deteriorate the charge and discharge performance of the battery.

[0054] In addition, by adding (M y+ ) x / y R1(SO2N) x SO2R2 with high thermal stability to the electrolyte, the heat resistance coefficient of the electrolyte can be significantly improved, and the decomposition of the electrolyte at high temperature can be reduced, so that the battery has a long cycle life. However, the inventors found in the research that when the concentration of the high thermal stability salt is too high, it will lead to an increase in the viscosity of the electrolyte and a deterioration of the conductivity, and further lead to an increase in the internal resistance of the battery cell, while when the salt concentration is too low, the thermal stability of the electrolyte is insufficient and may decompose at high temperature, thus shortening the life of the battery cell. In addition, the inventors also found that the relationship between the mass percentage k2 of the thermal stability salt (M y+ ) x / y R1(SO2N) x SO2R2 in the electrolyte and the temperature rise coefficient k1 of the positive electrode plate has a great influence on the cycle life of the battery. When k2 and k1 satisfy the above relational formula, while ensuring that the battery has a high energy density, it can also ensure sufficient thermal stability of the electrolyte, inhibit the decomposition of the electrolyte, and ensure that the battery has a long cycle life.

[0055] In some embodiments, the lithium ion secondary battery satisfies: 3.3 ≤ k1 ≤ 14.5. By further selecting the value of k1, the energy density and / or charge and discharge performance of the battery can be further improved.

[0056] In some embodiments, the lithium ion secondary battery satisfies: 0.48 ≤ k2 / k1 ≤ 7. By further selecting the value of k2 / k1, the cycle life of the battery can be further improved.

[0057] In some embodiments, the lithium-ion secondary battery satisfies: 1% ≤ Mc% ≤ 7%. Optionally, the lithium-ion secondary battery satisfies: 1% < Mc% ≤ 5%. If the carbon content is too high, the following may occur: the proportion of the positive electrode active material per unit volume is low, thus reducing the battery energy density, and the surface area of the positive electrode material is too large, thus making it easy to absorb water and agglomerate, resulting in difficulty in processing the electrode sheet; if the carbon content is too low, the following may occur: the conductivity of the electrode sheet becomes poor, lithium metal plating may occur during charging, affecting the battery cycle life and posing a safety risk.

[0058] In some embodiments, the lithium-ion secondary battery satisfies: 18 ≤ Cw ≤ 32. If the loading amount of the positive electrode material Cw is too low, the proportion of the non-active substrate (such as the current collector) may be large, thus reducing the energy density. While if Cw is too large, the coating difficulty may become high, and the transport path of lithium ions in the direction perpendicular to the electrode sheet is greatly increased, resulting in difficulty for lithium ions to quickly escape from and embed into the active material, affecting the power performance of the battery cell.

[0059] In some embodiments, the M y+ is selected from at least one of Li + , Na + , K + , Rb + , Cs + , Mg 2+ , Ca 2+ , Ba 2+ , Al 3+ , Fe 2+ , Fe 3+ , Ni 2+ , and Ni 3+ . Optionally, M y+ is selected from at least one of Li + , Na + , K + , Rb + , and Cs + . The transference numbers of the above cations are larger, making the power performance of the battery cell better.

[0060] In some embodiments, each of R1 and R2 is independently a fluorine atom, an alkyl group having 1 to 10 carbon atoms, a fluoroalkyl group having 1 to 10 carbon atoms, or a fluoroalkoxy group having 1 to 10 carbon atoms. Optionally, each of R1 and R2 is independently CH3, CF3, or F. By selecting the structures of R1 and R2, the lithium-ion transference number can be increased and lithium ions can be more easily dissociated. Meanwhile, the viscosity of the electrolyte is small, making the electrolyte conductivity high, thus helping to improve the cycle performance of the battery.

[0061] In some embodiments, the electrolyte further contains a low-impedance additive. Optionally, the low-impedance additive is at least one of NSO3F (fluorosulfonate), NDFOB (difluorooxalate borate), NPO2F2 (difluorophosphate), NDFOP (difluorodioxalate), tris(trimethylsilyl) phosphate, and tris(trimethylsilyl) phosphite, where N is the metal ion of the salt, and can be optionally Li + , Na + , K + , Rb + , Cs + . Optionally, the low-impedance additive is at least one of LiSO3F (lithium fluorosulfonate), LiDFOB (lithium difluorooxalate borate), tris(trimethylsilyl) phosphate, and LiPO2F2 (lithium difluorophosphate). Optionally, the mass percentage of the low-impedance additive in the electrolyte is 0.1% to 10%, and can be optionally 0.2% to 5%. In addition to the temperature rise of the positive electrode sheet, the impedance of the protective film (SEI film) on the surfaces of the positive and negative electrode sheets also has a great influence on the internal temperature rise of the battery cell. When the impedance value of the SEI film increases, it will further cause the temperature rise of the battery cell during charging. Without wishing to be limited by theory, during the formation process, the low-impedance additive can be preferentially reduced on the surface of the negative electrode over the electrolyte solvent, forming a very low-impedance and dense protective film on the surface of the negative electrode, thereby inhibiting the reduction and decomposition of the solvent and other high-impedance additives in the electrolyte on the surface of the negative electrode, and thus reducing the impedance of the SEI film. Additionally, when the content of the low-impedance additive is within the above range, the viscosity of the electrolyte will not increase, and the conductivity of the electrolyte can be maintained.

[0062] In some embodiments, the electrolyte further contains a lithium salt that inhibits aluminum foil corrosion. Optionally, the lithium salt that inhibits aluminum foil corrosion is at least one selected from LiPF6, LiAsF6, and LiBF4. Optionally, the mass percentage of the lithium salt that inhibits aluminum foil corrosion in the electrolyte is 0.1% to 10%, can be optionally 0.2% to 5%, and can be optionally 1% to 3%. The lithium salt that inhibits aluminum foil corrosion can inhibit the corrosion of the aluminum foil, thereby inhibiting the increase in the impedance of the aluminum current collector, reducing the internal temperature rise of the battery cell, and improving the safety performance of the battery cell. When the content of the lithium salt that inhibits aluminum foil corrosion is within the above range, it can inhibit the corrosion of the aluminum foil and will not have an adverse effect on the high-temperature resistance performance of the electrolyte.

[0063] In some embodiments, the total lithium salt content in the electrolyte is in the range of 5 wt% - 50 wt%, optionally in the range of 5 wt% - 37 wt%, and optionally in the range of 5 wt% - 23 wt%, based on the total weight of the electrolyte. By selecting the total lithium salt content in the electrolyte, the cycle life and power performance of the lithium-ion secondary battery can be improved.

[0064] In some embodiments, the positive electrode active material is selected from one or more of lithium transition metal oxides, lithium-containing phosphates with an olivine structure, and their respective modified compounds. Examples of lithium transition metal oxides may include, but are not limited to, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide LiNi a Co b Mn c O2 (a + b + c = 1, a < 0.8), lithium nickel cobalt aluminum oxide, and their modified compounds. Examples of lithium-containing phosphates with an olivine structure may include, but are not limited to, lithium iron phosphate, a composite of lithium iron phosphate and carbon, lithium manganese phosphate, a composite of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite of lithium manganese iron phosphate and carbon, and their modified compounds. These materials can all be obtained commercially. The surface of the positive electrode active material may be coated with carbon.

[0065] In the present application, the carbon content Mc of the positive electrode material layer can be measured using equipment and methods known in the art. For example, it can be measured by scraping off the positive electrode material layer on the positive electrode current collector and then using the infrared absorption method, for example, referring to GB / T 20123-2006 Determination of total carbon and sulfur content in steel - Infrared absorption method after combustion in a high-frequency induction furnace.

[0066] [Electrolyte solution]

[0067] The electrolyte solution plays a role in conducting ions between the positive electrode plate and the negative electrode plate. The electrolyte solution includes an electrolyte salt and a solvent.

[0068] In the present application, the electrolyte salt can be a common electrolyte salt in lithium-ion secondary batteries, such as a lithium salt, including the lithium salts that can be used as the above-mentioned high thermal stability salts, lithium salts as low impedance additives, or lithium salts that inhibit aluminum foil corrosion. As an example, the electrolyte salt can be selected from one or more of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiAsF6 (lithium hexafluoroarsenate), LiFSI (lithium bis(fluorosulfonyl)imide), LiTFSI (lithium bis(trifluoromethanesulfonyl)imide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluorooxalate borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluorodioxalate phosphate), LiSO3F (lithium fluorosulfonate), NDFOP (difluorodioxalate), Li2F(SO2N)2SO2F, KFSI, CsFSI, Ba(FSI)2, and LiFSO2NSO2CH2CH2CF3.

[0069] There is no particular limitation on the type of the solvent, and it can be selected according to actual needs. In some embodiments, the solvent is a non-aqueous solvent. Optionally, the solvent may include one or more of chain carbonates, cyclic carbonates, and carboxylic acid esters. In some embodiments, the solvent may be selected from ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), tetrahydrofuran, sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0070] In some embodiments, other additives may optionally be included in the electrolyte. For example, the additives may include negative electrode film-forming additives, may also include positive electrode film-forming additives, and may further include additives that can improve certain battery performances, such as additives for improving the overcharge performance of the battery, additives for improving the high-temperature performance of the battery, and additives for improving the low-temperature performance of the battery. As an example, the additives are selected from at least one of cyclic carbonate compounds containing unsaturated bonds, halogen-substituted cyclic carbonate compounds, sulfate compounds, sulfite compounds, sultone compounds, disulfonic acid compounds, nitrile compounds, aromatic compounds, isocyanate compounds, phosphazene compounds, cyclic anhydride compounds, phosphite compounds, phosphate compounds, borate compounds, and carboxylic acid ester compounds.

[0071] [Positive electrode sheet]

[0072] The positive electrode sheet includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector, and the positive electrode material layer includes a positive electrode active material and carbon.

[0073] As an example, the positive electrode current collector has two surfaces opposite to each other in its own thickness direction, and the positive electrode material layer is provided on any one or both of the two opposite surfaces of the positive electrode current collector.

[0074] In the lithium-ion secondary battery of the present application, the positive current collector can be a metal foil or a composite current collector. For example, as the metal foil, aluminum foil can be used. The composite current collector can include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (such as aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0075] The positive electrode material layer optionally includes a conductive agent. However, the type of the conductive agent is not specifically limited, and those skilled in the art can select according to actual needs. As an example, the conductive agent for the positive electrode material can be selected from one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0076] The positive electrode material layer also optionally includes a binder. As an example, the binder can be one or several of styrene-butadiene rubber (SBR), water-based acrylic resin, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA), polyacrylic acid (PAA), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), and polyvinyl butyral (PVB).

[0077] In the present application, the positive electrode plate can be prepared according to methods known in the art. As an example, the carbon-coated positive electrode active material, conductive agent, and binder can be dispersed in a solvent (such as N-methylpyrrolidone (NMP)) to form a uniform positive electrode slurry; the positive electrode slurry is coated on the positive current collector, and after processes such as drying and cold pressing, the positive electrode plate is obtained.

[0078] [Negative Electrode Plate]

[0079] The negative electrode plate includes a negative current collector and a negative electrode material layer provided on at least one surface of the negative current collector, and the negative electrode material layer includes a negative electrode active material.

[0080] As an example, the negative current collector has two surfaces opposite to each other in its own thickness direction, and the negative electrode material layer is provided on any one or both of the two opposite surfaces of the negative current collector.

[0081] In the lithium-ion secondary battery of the present application, the negative electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, copper foil can be used. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (such as copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0082] In the lithium-ion secondary battery of the present application, the negative electrode material layer generally includes a negative electrode active material and optional binder, optional conductive agent, and other optional additives, and is usually formed by coating and drying a negative electrode slurry. The negative electrode slurry is usually formed by dispersing the negative electrode active material, optional conductive agent, binder, etc. in a solvent and stirring evenly. The solvent can be N-methylpyrrolidone (NMP) or deionized water.

[0083] The specific type of the negative electrode active material is not limited, and active materials known in the art that can be used for the negative electrode of a lithium-ion secondary battery can be used, and those skilled in the art can select according to actual needs. As an example, the negative electrode active material can be selected from one or more of graphite, soft carbon, hard carbon, mesophase carbon microspheres, carbon fibers, carbon nanotubes, elemental silicon, silicon oxides, silicon-carbon composites, and lithium titanate.

[0084] As an example, the conductive agent can be selected from one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0085] As an example, the binder can be selected from one or more of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0086] Other optional additives are, for example, thickeners (such as sodium carboxymethyl cellulose (CMC-Na), etc.).

[0087] [Separator membrane]

[0088] The lithium-ion secondary battery using an electrolyte further includes a separator. The separator is disposed between the positive electrode plate and the negative electrode plate and functions to isolate them. There is no particular limitation on the type of the separator in this application, and any well-known porous structure separator with good chemical stability and mechanical stability can be selected. In some embodiments, the material of the separator can be selected from one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0089] In some embodiments, the positive electrode plate, the negative electrode plate, and the separator can be made into an electrode assembly by a winding process or a stacking process.

[0090] In some embodiments, the lithium-ion secondary battery can include an outer package. The outer package can be used to encapsulate the above electrode assembly and electrolyte.

[0091] In some embodiments, the outer package of the lithium-ion secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the lithium-ion secondary battery can also be a soft package, such as a pouch soft package. The material of the soft package can be plastic, and examples of the plastic can include polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS), etc.

[0092] There is no particular limitation on the shape of the lithium-ion secondary battery in this application, and it can be cylindrical, square, or any other shape. For example, Figure 1 is a lithium-ion secondary battery 5 with a square structure as an example.

[0093] In some embodiments, referring to Figure 2 , the outer package can include a housing 51 and a cover plate 53. Among them, the housing 51 can include a bottom plate and side plates connected to the bottom plate, and the bottom plate and the side plates enclose a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode plate, the negative electrode plate, and the separator can form an electrode assembly 52 through a winding process or a stacking process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte infiltrates into the electrode assembly 52. The number of the electrode assemblies 52 included in the lithium-ion secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual requirements.

[0094] In some embodiments, the lithium-ion secondary batteries can be assembled into a battery module, and the number of the lithium-ion secondary batteries included in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0095] Figure 3This is the battery module 4 as an example. Refer to Figure 3 , in the battery module 4, multiple lithium-ion secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, they can also be arranged in any other way. Further, the multiple lithium-ion secondary batteries 5 can be fixed by fasteners.

[0096] Optionally, the battery module 4 can further include a housing having an accommodation space, and the multiple lithium-ion secondary batteries 5 are accommodated in the accommodation space.

[0097] In some embodiments, the above battery module can also be assembled into a battery pack, and those skilled in the art can select the number of battery modules included in the battery pack according to the application and capacity of the battery pack.

[0098] Figure 4 and Figure 5 This is the battery pack 1 as an example. Refer to Figure 4 and Figure 5 , in the battery pack 1, it can include a battery box and multiple battery modules 4 arranged in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form a closed space for accommodating the battery module 4. The multiple battery modules 4 can be arranged in the battery box in any way.

[0099] In addition, the present application also provides a device, and the device includes one or more of the lithium-ion secondary battery, battery module, or battery pack provided by the present application. The lithium-ion secondary battery, battery module, or battery pack can be used as the power source of the device or as the energy storage unit of the device. The device can be but is not limited to a mobile device (such as a mobile phone, laptop computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship, a satellite, an energy storage system, etc.

[0100] As the device, the lithium-ion secondary battery, battery module, or battery pack can be selected according to its usage requirements.

[0101] Figure 6 This is the device as an example. The device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the high power and high energy density requirements of the device for the lithium-ion secondary battery, a battery pack or a battery module can be adopted.

[0102] As another example of the device, it can be a mobile phone, a tablet computer, a laptop computer, etc. This device usually requires thin and light design, and a lithium-ion secondary battery can be used as the power source.

[0103] Embodiment

[0104] Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. If no specific techniques or conditions are indicated in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. If the manufacturer is not indicated in the reagents or instruments used, they are all conventional products that can be obtained commercially.

[0105] 1. Preparation of lithium-ion secondary batteries

[0106] (1) Preparation of positive electrode sheet

[0107] The carbon-coated lithium iron phosphate (LFP) as the positive electrode active material, the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) were dissolved in the solvent N-methylpyrrolidone (NMP) according to the weight ratio shown in Table 1, and the positive electrode slurry was obtained after being fully stirred and mixed. The carbon-coated lithium iron phosphate used had the carbon content shown in Table 1. Then, according to the Cw value to be achieved, the positive electrode slurry was evenly coated on the aluminum positive electrode collector, and then dried, cold pressed, and cut to obtain the positive electrode sheet.

[0108] (2) Preparation of negative electrode

[0109] The negative electrode active material artificial graphite, the conductive agent acetylene black, the binder styrene butadiene rubber (SBR), and the thickener sodium carboxymethyl cellulose (CMC-Na) were dissolved in the solvent deionized water at a weight ratio of 95:2:2:1, and mixed evenly to prepare the negative electrode slurry. Then, the negative electrode loading amount was determined according to the Cw value of the corresponding positive electrode, and the negative electrode slurry was evenly coated on the negative electrode current collector copper foil according to the negative electrode loading amount shown in Table 1, and after drying, the negative electrode sheet was obtained by cold pressing and slitting.

[0110] Table 1 (the embodiments or comparative examples using the same parameter values are listed in the same row)

[0111]

[0112] (3) Preparation of electrolyte

[0113] In an argon atmosphere glove box (H2O<0.1ppm, O2<0.1ppm), the organic solvent EC / EMC was evenly mixed in a volume ratio of 3 / 7, 3wt% of vinylene carbonate and the weight percentages of high thermal stability salts and optional additives shown in Table 2 were added, and stirred evenly to obtain the corresponding electrolyte, wherein the weight percentage of vinylene carbonate and the weight percentages shown in Table 2 were both based on the total weight of the obtained electrolyte.

[0114] (4) Preparation of lithium-ion secondary batteries

[0115] Using the positive electrode sheet prepared in the above (1) and the negative electrode sheet prepared in the above (2), with a polypropylene film as the separator, stack the positive electrode sheet, the separator, and the negative electrode sheet in sequence, so that the separator is between the positive and negative electrode sheets to play a role in isolation, and then wind to obtain an electrode assembly. Place the electrode assembly in a battery case, inject electrolyte after drying, and then through formation and standing, a lithium-ion secondary battery is prepared.

[0116] 2. Testing of Lithium-Ion Secondary Batteries

[0117] (1) Testing of Carbon Content in Positive Electrode Sheet

[0118] Testing Instrument: High-Frequency Infrared Carbon-Sulfur Analyzer, model HCS-140, commercially purchased from Shanghai Dekai Instrument Co., Ltd.

[0119] Refer to GB / T20123-2006 Determination of Total Carbon and Sulfur Content in Steel - Infrared Absorption Method after Combustion in High-Frequency Induction Furnace (conventional method). Testing Process: Scrape off the positive electrode material layer on the positive electrode current collector, heat and burn it in the above high-frequency furnace under oxygen-rich conditions, and oxidize the carbon contained in the material into carbon dioxide. After processing the generated gas, introduce it into the corresponding absorption cell, absorb the corresponding infrared radiation, and convert it into a corresponding digital signal via a detector. The obtained digital signal is sampled by a computer, linearly corrected, converted into a value proportional to the carbon dioxide concentration, and accumulated to obtain an accumulated value. Divide the obtained accumulated value by the sample weight, multiply by the correction factor, and deduct the blank to obtain the carbon percentage content Mc (%) in the sample.

[0120] (2) 45°C Cycle Performance Testing of Lithium-Ion Secondary Batteries

[0121] At 45°C, charge the lithium-ion secondary battery at a constant current of 1C to 3.65V, then charge it at a constant voltage of 3.65V until the current is less than 0.05C, and then discharge the lithium-ion battery at a constant current of 1C to 2.5V. This is one charge-discharge process. Repeat charging and discharging in this way, and calculate the number of cycles when the capacity retention rate of the lithium-ion battery is 80%. The 45°C cycle numbers of each example and comparative example are shown in Table 1.

[0122] (3) Volume Energy Density Testing

[0123] At 25°C, charge the lithium-ion battery at a constant current of 0.5C to 3.65V, then charge it at a constant voltage of 3.65V until the current is less than 0.05C, and then discharge it at 0.33C to 2.5V to obtain the discharge energy Q. Use a vernier caliper to measure the length, width, and height of the battery cell, calculate to obtain the volume V, and the volume energy density = Q / V

[0124] Table 2 shows Mc, Cw, k1, k2, the volume energy density, and the cycling performance at 45 °C of the lithium-ion batteries of the examples and comparative examples.

[0125]

[0126]

[0127] As can be seen from Table 1, k1 of Comparative Example 1 is too low, and correspondingly, the energy density of the battery is low. In Comparative Examples 2-3, by increasing the loading amount of the positive electrode material layer and reducing the carbon content, k1 increases, and correspondingly, the volume energy density of the battery is significantly improved. However, too high k1 results in a significant deterioration of the cycling life of the battery cell. Even though Comparative Example 3 uses a salt with high thermal stability and the value of k2 / k1 falls within the scope of the present application, the cycling life is still low. On the other hand, the values of k1 in Comparative Examples 4-5 fall within the scope of the present application, but k2 / k1 is too high or too low, so the cycling life is low.

[0128] In contrast, Examples 1-25 simultaneously use an electrolyte containing a lithium salt with high stability and a positive electrode sheet with a large loading amount of the positive electrode material layer and a low carbon content, enabling the battery cell to have both high energy density and long cycling life.

[0129] Comparing Example 1 with Example 2, it can be seen that the value of k2 / k1 in Example 1 is in the range of 0.48-7, thus obtaining better cycling performance than Example 2. Comparing Example 1 with Example 4, it can be seen that the value of k1 in Example 1 is in the range of 3.3-14.5, thus obtaining better cycling performance than Example 4.

[0130] As can be seen from Examples 8-24, by adding a low-impedance additive to the electrolyte, the cycling life of the battery can be further improved while ensuring that the battery has a high energy density.

[0131] As can be seen from Examples 16-21, adding a lithium salt that inhibits aluminum foil corrosion to the electrolyte can inhibit aluminum foil corrosion and reduce the increase in the impedance value of the aluminum foil, thereby further improving the cycling life of the battery. Moreover, when the mass percentage of the lithium salt that inhibits aluminum foil corrosion is in the range of 0.2%-5% (Examples 16, 19-20), the cycling life of the battery can be further improved.

[0132] Although the present application has been described with reference to the examples, various improvements can be made to it and components therein can be replaced with equivalents without departing from the scope of the present application. In particular, as long as there is no structural conflict, the various technical features mentioned in each example can be combined in any way. The present application is not limited to the specific examples disclosed in the text, but includes all technical solutions falling within the scope of the claims.

Claims

1. A lithium ion secondary battery, which has a positive electrode plate, a negative electrode plate, a separator, and an electrolyte. The positive electrode plate includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector. The positive electrode material layer contains a positive electrode active material and carbon, wherein, The electrolyte contains a salt with high thermal stability (M y+ ), x / y R1(SO2N - ), x SO2R2 and a low-impedance additive, where M y+ is a metal ion, R1 and R2 are each independently a fluorine atom, an alkyl group with 1-20 carbon atoms, a fluoroalkyl group with 1-20 carbon atoms, or a fluoroalkoxy group with 1-20 carbon atoms, and x is 1, 2, or 3, and y is 1, 2, or 3; the mass percentage of this salt in the electrolyte is set as k2%; the low-impedance additive is fluorosulfonate NSO3F, where N is the metal ion of the salt; The temperature rise coefficient k1 of the positive electrode plate satisfies 2.5 ≤ k1 ≤ 32, where k1 = Cw / Mc, Cw is the positive electrode material loading per unit area on either side surface of the positive electrode current collector loaded with the positive electrode material layer (mg / cm 2 ), and Mc is the carbon content (%) of the positive electrode material layer; moreover, the lithium ion secondary battery satisfies 0.34 ≤ k2 / k1 ≤ 8.

2. The lithium ion secondary battery according to claim 1, which satisfies at least one of the following conditions 1) to 3): 1)3.3≤k1≤14.5； 2) 0.48 ≤ k2 / k1 ≤ 7; 3) 1 ≤ Mc ≤ 7.

3. The lithium ion secondary battery according to claim 1 or 2, wherein, The said M y+ is selected from Li + , Na + , K + , Rb + , Cs + , Mg 2+ , Ca 2+ , Ba 2+ , Al 3+ , Fe 2+ , Fe 3+ , Ni 2+ , and Ni 3+ and at least one of them.

4. The lithium ion secondary battery according to claim 3, wherein, The M y+ is selected from Li + , Na + , K + , Rb + and Cs + and is at least one of them.

5. The lithium ion secondary battery according to claim 1, wherein, R1 and R2 are each independently a fluorine atom, an alkyl group having 1 to 10 carbon atoms, a fluoroalkyl group having 1 to 10 carbon atoms, or a fluoroalkoxy group having 1 to 10 carbon atoms.

6. The lithium ion secondary battery according to claim 5, wherein, R1 and R2 are each independently CH3, CF3, or F.

7. The lithium ion secondary battery according to claim 1, wherein, N is Li + , Na + , K + , Rb + or Cs + .

8. The lithium ion secondary battery according to claim 7, wherein the low impedance additive is lithium fluorosulfonate.

9. The lithium ion secondary battery according to claim 1, wherein, The mass percentage of the low impedance additive in the electrolyte is 0.1% to 10%.

10. The lithium ion secondary battery according to claim 9, wherein, The mass percentage of the low impedance additive in the electrolyte is 0.2% to 5%.

11. The lithium ion secondary battery according to claim 1, wherein, The electrolyte further contains a lithium salt for inhibiting aluminum foil corrosion, and the lithium salt for inhibiting aluminum foil corrosion is at least one selected from LiPF6, LiAsF6, and LiBF4.

12. The lithium ion secondary battery according to claim 11, wherein, The mass percentage of the lithium salt for inhibiting aluminum foil corrosion in the electrolyte is 0.1% to 10%.

13. The lithium ion secondary battery according to claim 12, wherein, The mass percentage of the lithium salt for inhibiting aluminum foil corrosion in the electrolyte is 0.2% to 5%.

14. The lithium ion secondary battery according to claim 13, wherein, The mass percentage of the lithium salt for inhibiting aluminum foil corrosion in the electrolyte is 1% to 3%.

15. The lithium ion secondary battery according to claim 1, wherein, The total lithium salt content in the electrolyte is in the range of 5 wt% - 50 wt%, based on the total weight of the electrolyte.

16. The lithium ion secondary battery according to claim 15, wherein, The total lithium salt content in the electrolyte is in the range of 5 wt% - 37 wt%, based on the total weight of the electrolyte.

17. The lithium ion secondary battery according to claim 16, wherein, The total lithium salt content in the electrolyte is in the range of 5 wt% - 23 wt%, based on the total weight of the electrolyte.

18. The lithium ion secondary battery according to claim 1, wherein, The positive electrode active material is selected from one or more of lithium transition metal oxides, lithium-containing phosphates with an olivine structure, and their respective modified compounds.

19. The lithium ion secondary battery according to claim 18, wherein, The lithium transition metal oxide is selected from one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide LiNi that satisfies a + b + c = 1 and a < 0.8 a Co b Mn c O2, lithium nickel cobalt aluminum oxide, and modified compounds thereof.

20. A battery module, which includes the lithium ion secondary battery according to any one of claims 1 - 19.

21. A battery pack, which includes the battery module according to claim 20.

22. An electrical device, which includes one or more selected from the lithium ion secondary battery according to any one of claims 1 - 19, the battery module according to claim 20, or the battery pack according to claim 21.

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