Lithium secondary battery based on lithium iron manganese phosphate and applications thereof

Abstract

The present application provides a kind of lithium secondary battery based on lithium iron manganese phosphate and its application.The lithium secondary battery includes positive electrode, negative electrode and non-aqueous electrolyte;Wherein, including lithium iron manganese phosphate positive electrode active material in positive electrode, its general structure is represented as LiMn y Fe 1‑y PO4, wherein y represents the doping content of element Mn;The non-aqueous electrolyte includes nitrile pyridazine compound as component (A);The structure of the nitrile pyridazine compound of component (A) is shown in formula (I) as follows: wherein X represents a bond or a divalent organic group;R1, R2 and R3 respectively represent hydrogen atom or monovalent organic group independently;And the lithium secondary battery satisfies the following relationship: 4≤100a / (CW·y)≤15, wherein the a represents the mass percentage of the nitrile pyridazine compound of component (A) in the non-aqueous electrolyte;The CW represents the area density of positive electrode sheet, and the unit is mg / cm 3 .

Description

Technical Field

[0001] This invention belongs to the field of new energy and relates to a lithium secondary battery based on lithium manganese iron phosphate and its application. More specifically, it relates to a lithium secondary battery for power driving or energy storage and its application. Background Technology

[0002] With the continuous development of society and the gradual improvement of performance requirements, the mainstream battery systems currently used in the power battery field—lithium iron phosphate (LFP) batteries and ternary (NCM) lithium batteries—are facing their own bottleneck problems. Among them, the energy density of lithium iron phosphate has approached its limit and its low-temperature performance is poor, while ternary lithium batteries have poor safety performance and high cost. Lithium manganese iron phosphate (LMFP), as an upgraded version of lithium iron phosphate, has become a research hotspot for power lithium batteries due to its comprehensive advantages of high energy density, long life, high safety, and low cost.

[0003] LMFP is an olivine structure material obtained by mixing a certain amount of manganese into LFP. The introduction of manganese gives it a higher voltage platform and better low-temperature performance than LFP, but it also brings some characteristic problems. The most prominent one is that due to the Jan Taylor effect, severe manganese ion dissolution and deposition occur, which degrades its cycling / storage and power performance under high temperature / high pressure, thus limiting its widespread application.

[0004] Existing technologies are mostly focused on the modification of cathode materials, such as cathode surface coating, metal doping, and particle size adjustment.

[0005] A modified lithium manganese iron phosphate composite cathode material, its preparation method, and a battery have been reported. This method uses silicides such as zirconium silicide to modify the lithium manganese iron phosphate material, improving the structural and cycle stability of the composite cathode material. However, the organic layer has a weak confinement effect on dissolved manganese ions, thus limiting the improvement in battery cycle performance. Furthermore, the coating process is complex, time-consuming, and not conducive to mass production.

[0006] In addition, a single-atom-doped lithium manganese iron phosphate composite material has been reported, comprising a lithium manganese iron phosphate core and a carbon material shell layer doped with metal single atoms covering the outer surface of the core. Applying this single-atom-doped lithium manganese iron phosphate composite material to lithium-ion batteries can effectively improve the electrochemical performance of lithium-ion batteries, including rate performance, cycle performance, discharge specific capacity, and low-temperature performance. However, the type and content of metal doping are not ideal, failing to fully improve the intrinsic ionic conductivity and electronic conductivity of the material.

[0007] In summary, although a series of studies have been conducted to improve the manganese leaching problem in lithium iron phosphate-based secondary batteries, the research cannot be considered sufficient and there is still room for further improvement. Summary of the Invention

[0008] The problem the invention aims to solve

[0009] As mentioned above, although lithium iron manganese phosphate has many advantages as a cathode in lithium batteries, the Jahn-Teller effect (JTE) causes severe manganese ion dissolution and deposition, which degrades its cycling / storage and power performance under high temperature / high pressure, thus limiting its widespread application.

[0010] Existing technologies typically address these issues by modifying the cathode material, such as coating the cathode surface, metal doping, and adjusting particle size. However, these methods all have limitations, such as limited improvement in battery performance and high cost.

[0011] To address the aforementioned problems, this invention provides a lithium secondary battery based on manganese iron phosphate. By introducing a nitrile-based pyridazine compound with a specific structure into the non-aqueous electrolyte of the lithium secondary battery, the nitrile group and unsaturated nitrogen heterocycle in its structure effectively complex manganese ions, suppressing the problems caused by manganese dissolution and precipitation, thereby improving its cycle stability and storage stability under high temperature and high pressure conditions.

[0012] Furthermore, the present invention also provides a transportation or vehicle, or an energy storage device, wherein the vehicle or device includes a lithium secondary battery according to the present invention.

[0013] Solution for solving the problem

[0014] The present invention first provides a lithium secondary battery based on lithium manganese iron phosphate, wherein the lithium secondary battery includes a positive electrode, a negative electrode and a non-aqueous electrolyte;

[0015] The positive electrode includes lithium manganese iron phosphate as the positive electrode active material, whose general structural formula is LiMn. y Fe 1-y PO4, where y represents the doping content of element Mn;

[0016] The non-aqueous electrolyte includes nitrile pyridazine compounds as component (A).

[0017] The structure of the nitrile pyridazine compound of component (A) is shown in formula (I):

[0018]

[0019] Where X represents a linking bond or a divalent organic group;

[0020] R1, R2, and R3 each independently represent a hydrogen atom or a monovalent organic group;

[0021] Furthermore, the lithium secondary battery satisfies the following relationship: 4 ≤ 100a / (CW·y) ≤ 15.

[0022] Wherein, 'a' represents the mass percentage of the nitrile pyridazine compound (A) in the non-aqueous electrolyte;

[0023] CW represents the areal density of the positive electrode, with units of mg / cm³. 3 .

[0024] According to the lithium secondary battery of the present invention, the non-aqueous electrolyte further includes a non-aqueous solvent as component (B) and a lithium salt as component (C).

[0025] According to the lithium secondary battery of the present invention, the content of component (A) nitrile pyridazine compound is 0.1% to 2% by mass of the total mass of the non-aqueous electrolyte.

[0026] In the lithium secondary battery according to the present invention, X represents a linking bond or a substituted or unsubstituted hydrocarbon group.

[0027] In the lithium secondary battery according to the present invention, R1, R2 and R3 may be the same or different, and each time they appear, they independently represent at least one of the following: a hydrogen atom, a substituted or unsubstituted hydrocarbon group, a halogen-containing group, an ether-containing group, a nitrile-containing group, an isocyanate-containing group, and a heteroatom-containing cyclic group.

[0028] According to the lithium secondary battery of the present invention, the nitrile pyridazine compound of component (A) is selected from one or more combinations of the compounds shown in formulas (I-1) to (I-6) below:

[0029]

[0030]

[0031] According to the lithium secondary battery of the present invention, the non-aqueous solvent of component (B) is selected from one or more of cyclic carbonate solvents, linear carbonate solvents, ether solvents, ester solvents, and ketone solvents.

[0032] According to the lithium secondary battery of the present invention, wherein the component (C) lithium salt is selected from one or more salts formed by lithium ions and the following anions: PF6 - BF4 - Cl - ,Br - I - ClO4 - AsF6 - SiF6 2- AlCl4- B(C2O4)2 - CH3CO2 - CF3SO3 - N(CF3SO2)2 - N(FSO2)2 - C(CF2SO2)3 - C2BF2O4 - .

[0033] According to the lithium secondary battery of the present invention, the non-aqueous electrolyte further includes a functional additive as component (D); and / or, the value of y is in the range of 0.2≤y≤0.8; and the value of CW is in the range of 10≤CW≤30.

[0034] In addition, the present invention also provides a transportation or vehicle, or an energy storage device, wherein the vehicle or device includes a lithium secondary battery according to the present invention.

[0035] The effects of the invention

[0036] By implementing the above technical solution, the present invention can achieve at least the following technical effects:

[0037] 1) The lithium manganese iron phosphate-based secondary battery provided by the present invention introduces a nitrile pyridazine compound with a specific structure into the non-aqueous electrolyte of the lithium battery. On the one hand, the nitrile group and unsaturated nitrogen heterocycle in its structure can effectively complex manganese ions, suppressing the changes in the positive electrode structure and the damage to the negative electrode SEI film caused by manganese dissolution and precipitation. On the other hand, it can preferentially decompose to form a highly stable organic-inorganic composite interface film rich in inorganic Li3N, thereby improving the ion transport kinetics at the positive electrode / electrolyte interface and reducing the kinetic loss caused by the obstruction of ion transport during charging and discharging.

[0038] 2) The nitrile pyridazine compounds contain N with lone pair electrons that can bind to proton hydrogen, thus effectively eliminating residual water or HF byproducts generated during discharge in the cathode material and inhibiting the catalytic reaction of acidic harmful byproducts on manganese ion dissolution and interfacial side reactions.

[0039] 3) In lithium manganese iron phosphate (LFP) batteries, higher manganese content leads to a greater increase in voltage plateau and higher cathode areal density, resulting in higher energy density. However, this also leads to poorer kinetics of the cathode material and more severe capacity decay due to the dissolution of lithium manganese ions during charge and discharge. Therefore, LFP cathode materials with different manganese contents and cathode areal densities require different non-aqueous electrolytes. In some preferred embodiments, by limiting the relationship between the content of nitrile pyridazine compounds in the non-aqueous electrolyte and the manganese content and cathode areal density (100a / (CW·y)) within a certain range, better improvement can be achieved, giving the battery excellent cycle and storage performance. Detailed Implementation

[0040] The present invention will now be described in detail. The descriptions of the technical features described below are based on representative embodiments and specific examples of the present invention, but the present invention is not limited to these embodiments and specific examples. It should be noted that:

[0041] In this specification, the range of values ​​referred to as "value A to value B" refers to the range including the endpoint values ​​A and B.

[0042] In this specification, the numerical range indicated by "above" or "below" refers to the numerical range that includes the stated number.

[0043] In this specification, the word "may" has two meanings: to perform a certain process and not to perform a certain process.

[0044] In this specification, the terms "optional" or "optional" are used to indicate the use or omission of certain substances, components, procedures, application conditions, etc.

[0045] In this instruction manual, "normal temperature" or "room temperature" refers to an indoor ambient temperature of "23±2℃".

[0046] All unit names used in this manual are international standard unit names, and unless otherwise stated, the "%" used refers to weight or mass percentage content.

[0047] In this specification, the terms "substantially" and "essentially" are used to indicate that the standard deviation from the theoretical model, theoretical data, or target data is within a range of 2%, preferably 1%, and more preferably 0.8%.

[0048] In this specification, the terms “comprising” and / or “including” are used to indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0049] In this specification, references to "some specific / preferred embodiments," "other specific / preferred embodiments," "implementation," etc., refer to specific elements (e.g., features, structures, properties, and / or characteristics) related to that embodiment, which are included in at least one of the embodiments described herein and may or may not be present in other embodiments. Furthermore, it should be understood that these elements may be combined in any suitable manner in various embodiments.

[0050] This invention provides a lithium secondary battery based on manganese iron phosphate. By introducing a nitrile-based pyridazine compound with a specific structure into the non-aqueous electrolyte of the lithium battery, the nitrile group and unsaturated nitrogen heterocycle in the compound effectively complex manganese ions, suppressing the problems caused by manganese dissolution and precipitation, thereby improving its cycle stability and storage stability under high temperature and high pressure conditions.

[0051] This invention is based on the following insights:

[0052] Lithium iron manganese phosphate (LFP) suffers from severe manganese ion dissolution and deposition due to the Jan Taylor effect, especially when free acid is present in the electrolyte. Free acid readily corrodes the material, causing manganese ions to dissolve from the crystal lattice. These dissolved manganese ions then migrate through the electrolyte to the negative electrode side, undergoing reduction reactions on the surface and precipitating out, damaging the SEI film. This increases the consumption of active lithium and the impedance of the negative electrode SEI film, ultimately affecting the battery's cycle life. Furthermore, high voltage exacerbates the oxidative decomposition of the electrolyte at the positive electrode, producing acidic byproducts that further increase manganese ion dissolution, rapidly deteriorating the battery's cycle performance. Therefore, the deterioration of LFP's cycle / storage and power performance under high temperature / high pressure limits its widespread application. Existing technologies such as positive electrode surface coating, metal doping, and particle size adjustment each have their own problems, offering limited improvement in manganese dissolution and incurring high costs.

[0053] Among numerous methods to improve manganese leaching, optimizing the electrolyte composition is the most economical and effective. The inventors of this invention believe that, theoretically, introducing compounds with highly polar groups and polynitrogen heterocyclic structures into the non-aqueous electrolyte of lithium batteries could potentially solve the aforementioned problems. Therefore, through further experimentation, nitrile-based pyridazine compounds were found to be suitable in terms of solubility, electronic effects, and complexing ability. On the one hand, the nitrile groups and unsaturated nitrogen heterocycles in their structure can effectively complex manganese ions, inhibiting changes in the positive electrode structure and damage to the negative electrode SEI film caused by manganese leaching and precipitation. On the other hand, they can preferentially decompose to form a highly stable organic-inorganic composite interface film rich in inorganic Li3N, thereby improving ion transport kinetics at the positive electrode / electrolyte interface and reducing kinetic losses due to impaired ion transport during charging and discharging. Furthermore, the nitrogen atoms in the nitrile pyridazine compounds containing lone pair electrons can bind to protonated hydrogen atoms, thus effectively eliminating residual water or HF byproducts generated during discharge in the cathode material and inhibiting the catalytic reactions of acidic harmful byproducts on manganese ion dissolution and interfacial side reactions. Through the optimization of the electrolyte composition of this invention, problems caused by manganese dissolution and precipitation can be effectively improved, thereby enhancing its cycling stability and storage stability under high temperature and high pressure conditions.

[0054] <First Aspect>

[0055] The first aspect of the present invention provides a lithium secondary battery based on lithium manganese iron phosphate, the lithium secondary battery comprising a positive electrode, a negative electrode and a non-aqueous electrolyte.

[0056] (Non-aqueous electrolyte)

[0057] The non-aqueous electrolyte of the present invention includes a nitrile pyridazine compound as component (A). Other commonly used components in electrolytes include, for example, a non-aqueous solvent as component (B) and a lithium salt as component (C).

[0058] Furthermore, without limitation, various optional functional additives may be used in the non-aqueous electrolyte, provided that they do not impede the realization of the technical effects of the present invention.

[0059] Component (A)

[0060] The component (A) of this invention is a nitrile pyridazine compound. The nitrile group and unsaturated nitrogen heterocycle in this compound can not only effectively complex manganese ions, but also neutralize acidic components that may be generated in the electrolyte during operation. In addition, it can more easily decompose to form a highly stable organic-inorganic composite interface film rich in inorganic Li3N.

[0061] The structure of component (A), a nitrile pyridazine compound, is shown in formula (Ⅰ):

[0062]

[0063] Where X represents a linking bond or a divalent organic group.

[0064] There are no particular restrictions on such divalent linking groups, and examples include substituted or unsubstituted hydrocarbon groups.

[0065] In some specific implementations, the hydrocarbon group can be one or more combinations of chain-like aliphatic hydrocarbon groups, alicyclic hydrocarbon groups, or aromatic hydrocarbon groups.

[0066] The aliphatic hydrocarbon group can be a straight-chain structure or an aliphatic hydrocarbon group with one or more branches having 1 to 20 carbon atoms, preferably 1 to 10, such as methylene, 1,2-ethylene, etc.; the alicyclic hydrocarbon group can have a 5- to 8-membered alicyclic structure; the aromatic hydrocarbon group can have a carbon-aromatic structure formed by 6 to 12 carbon atoms, such as a benzene ring group, etc. The substituent can be a halogen, preferably an F atom.

[0067] In some preferred embodiments of the present invention, the divalent organic group is selected from the substituted or unsubstituted aliphatic hydrocarbon groups described above.

[0068] Furthermore, R1, R2, and R3 may be the same or different, and each time they appear, they independently represent at least one of the following: a hydrogen atom, a substituted or unsubstituted hydrocarbon group, a halogen-containing group, an ether-containing group, a nitrile-containing group, an isocyanate-containing group, or a heteroatom-containing cyclic group.

[0069] Such hydrocarbon groups can be substituted or unsubstituted aliphatic hydrocarbon groups, alicyclic hydrocarbon groups, or aromatic hydrocarbon groups.

[0070] The aliphatic hydrocarbon group can be a straight-chain structure or a hydrocarbon group with one or more branches having 1 to 10 carbon atoms. Preferably, it has 1 to 6 carbon atoms, such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, etc. The alicyclic hydrocarbon group can have a carbon ring with 5 to 8 carbon atoms. The aromatic hydrocarbon group can be a carbon aromatic structure with 6 to 12 carbon atoms, such as phenyl.

[0071] The substituent can be a halogen, preferably an F atom.

[0072] Furthermore, there are no particular limitations on the halogen-containing groups, ether-containing groups, nitrile-containing groups, isocyanate-containing groups, and heteroatom-containing cyclic groups. In particular, for heteroatom-containing cyclic groups, the ring structure contains one or more heteroatoms, which can be one or a combination of oxygen, nitrogen, and sulfur atoms. The ring can be a 5- or 6-membered aliphatic heterocycle or an aromatic heterocycle, etc.

[0073] Furthermore, the nitrile pyridazine compounds may be selected from one or more combinations of compounds shown in the following structural formulas:

[0074]

[0075] Component (B)

[0076] The present invention does not particularly limit the type of non-aqueous solvent for component (B), as long as it is a non-aqueous solvent commonly used as a non-aqueous electrolyte.

[0077] In some specific embodiments, the non-aqueous solvent may be selected from one or more of cyclic carbonate solvents, linear carbonate solvents, ether solvents, ester solvents, and ketone solvents.

[0078] The cyclic carbonate solvents can be selected from ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC), etc.; the linear carbonate solvents can be selected from diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), methyl trifluoroethyl carbonate (FEMC), bis(2,2,2-trifluoroethyl) carbonate (DFDEC), etc.; the esters The solvent can be selected from methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, methyl butyrate, ethyl butyrate, and methyl neopentanoate, etc.; the ether solvent can be selected from dibutyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, ethylene glycol dimethyl ether (DME), ethylene glycol diethyl ether (DEE), dioxane (DX), dioxane (DOL), etc.; the ketone solvent can be selected from polymethyl vinyl ketone, etc. These non-aqueous solvents can be used alone or in mixtures of two or more.

[0079] In some preferred embodiments, the non-aqueous solvent may be selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethyl acetate, propyl acetate, ethyl propionate, propyl propionate, methyl butyrate, and ethyl butyrate.

[0080] Component (C)

[0081] The present invention does not particularly limit the type of lithium salt used in component (C), and it can be a lithium salt commonly used in the art. In some specific embodiments, the lithium salt may be selected from one or more salts formed by lithium ions and the following anions: PF6 - BF4 - Cl - ,Br - I - ClO4 - AsF6 - SiF6 2- AlCl4 - B(C2O4)2 - CH3CO2 - CF3SO3 - N(CF3SO2)2 - N(FSO2)2 - C(CF2SO2)3 - C2BF2O4 - .

[0082] In some preferred embodiments, the lithium salt may be selected from lithium hexafluorophosphate (LiPF6).

[0083] Other functional additives (D)

[0084] There are no particular limitations in principle on other functional additives that can be used in the non-aqueous electrolyte of the present invention. For example, the use of some additives can promote film formation.

[0085] Such additives include ethylene carbonate (VC), lithium difluorophosphate (LiPO2F2), fluoroethylene carbonate (FEC), boron-containing additives, sulfur-containing additives, or oxalate-containing additives. The boron-containing additives can be selected from lithium tetrafluoroborate (LiBF4), trimethylsilyl phosphate (TMSP), trimethylsilyl borate ester (TMSB), etc.; the sulfur-containing additives can be selected from 1,3-propanesulfonate lactone (1,3-PS), 1,4-butanesulfonate lactone (1,4-BS), 2,4-butanesulfonate lactone (2,4-BS), 1,3-propenesulfonate lactone (PST), vinyl sulfate (DTD), methylene disulfonate (MMDS), vinyl sulfite (ES), etc.; the oxalate-containing additives can be selected from lithium difluorooxalate borate (LiDFOB), lithium bis(oxalate borate) borate (LiBOB), lithium tetrafluorooxalate phosphate (LiTFOP), lithium difluorobis(oxalate phosphate) (LiDFOP), etc. These additives can be used alone or in mixtures of two or more.

[0086] Composition of non-aqueous electrolyte

[0087] In this invention, the content of nitrile pyridazine compounds in component (A) is, in principle, related to the manganese content and areal density of the cathode material in the lithium battery described below. That is, the optimal content of nitrile pyridazine compounds required to match lithium iron phosphate cathode materials with different manganese contents and cathode sheets with different coating weights is different.

[0088] In some specific embodiments, the content of component (A), a nitrile pyridazine compound, is 0.1% to 2% of the total mass of the non-aqueous electrolyte. For example, 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, etc.

[0089] There are no particular limitations in principle regarding the lithium salt of component (C). In some specific embodiments of the present invention, the concentration of the lithium salt in the non-aqueous electrolyte can be 0.8 to 1.5 mol / L, for example, 0.9 mol / L, 1 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, etc.

[0090] There are no particular restrictions on the amount of other functional additives (D) in principle, and they can be selected as needed.

[0091] (positive electrode)

[0092] The positive electrode of this invention includes a current collector and a positive electrode active material. The positive electrode active material includes lithium manganese iron phosphate, whose general structural formula is LiMn. y Fe 1-y PO4, where y represents the molar ratio of manganese to iron. There is no particular limitation on the value of y; in some specific implementations, y can be 0.2 to 0.8, for example, 0.3, 0.4, 0.5, 0.6, 0.7, etc.

[0093] Furthermore, without affecting the realization of the technical effects of the present invention, the positive electrode active material may also include other components.

[0094] There is no particular limitation on the areal density of the lithium manganese iron phosphate cathode; it can be selected according to needs. In some specific embodiments, it can be 10–30 mg / cm³. 2 For example, 15mg / cm 2 20mg / cm 2 25mg / cm 2 wait.

[0095] (negative electrode)

[0096] There are no particular restrictions on the negative electrode of the battery in principle. It can be any negative electrode commonly used in the field, including a current collector and a negative electrode active material. Typically, the negative electrode active material can be a carbon material, a silicon material, or a mixture thereof.

[0097] (Battery composition)

[0098] In this invention, the lithium secondary battery satisfies the following relationship: 4≤100a / (CW·y)≤15.

[0099] Wherein, 'a' represents the mass percentage of the nitrile pyridazine compound (A) in the non-aqueous electrolyte; for example, if the content of component (A) is x%, then the value of 'a' is x. CW represents the areal density of the positive electrode, expressed in mg / cm³. 3 ;y represents the doping content of element Mn, that is, the molar ratio of manganese to iron.

[0100] Because higher manganese content in lithium manganese iron phosphate leads to a greater increase in voltage plateau and higher cathode areal density, resulting in higher energy density, but also worse cathode material kinetics and more severe capacity decay due to the dissolution of lithium manganese ions during charge and discharge, lithium manganese iron phosphate cathode materials with different manganese contents and cathode areal densities need to be matched with different contents of nitrile pyridazine compounds to improve cycle and energy storage performance. When the above relationship is satisfied, lithium batteries can exhibit excellent cycle and energy storage performance.

[0101] (Other components of the battery)

[0102] The lithium secondary battery of the present invention can also use a separator. In principle, there are no particular restrictions on the separator used; it can be used or not. That is, the lithium secondary battery of the present invention can be a lithium secondary battery with a separator or a lithium secondary battery without a separator. When using a separator, it can be a separator commonly used in the art. Preferably, a separator with high moisture retention capacity for the electrolyte solution and low resistance to electrolyte ion transfer can be used. In the case of no separator, the non-aqueous electrolyte of the present invention can be used in combination with other solid electrolytes, wherein the solid electrolyte can also act as a separator.

[0103] <Second aspect>

[0104] A second aspect of the invention provides a transportation or vehicle, or an energy storage device, which includes a lithium secondary battery according to the first aspect, for such transportation or vehicle, such as a passenger car, SUV, or SUV.

[0105] Example

[0106] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0107] Examples 1-12 and Comparative Examples 1-7

[0108] The lithium secondary battery was prepared according to the raw material formula in Table 1. In addition, the electrolyte, besides the nitrile pyridazine compounds shown in Table 1, also included 1 mol / L lithium salt LiPF6, non-aqueous solvent EC / EMC / DEC = 30 / 50 / 20 (mass ratio), and other functional additives 1.5 wt% VC and 1 wt% PS.

[0109] Preparation of lithium-ion secondary batteries:

[0110] i) Cathode preparation: The cathode active material LiMn... 0.6 Fe 0.4 PO4, conductive agent (SP), single-walled carbon nanotubes (SWNT), and binder polyvinylidene fluoride (PVDF) were added to an appropriate amount of N-methyl-2-pyrrolidone (NMP) solvent at a mass ratio of 94.5:2:2:1.5 and stirred thoroughly until the solid content was controlled to 68%, thus obtaining a positive electrode mixture slurry. Subsequently, the formed positive electrode slurry was coated onto aluminum foil, and after drying, it was roll-cut to form a positive electrode sheet.

[0111] ii) Negative electrode preparation: Graphite, conductive agent (SP), thickener CMC, and binder SBR are added to an appropriate amount of deionized water in a mass ratio of 95:2:1:2 and stirred thoroughly. The solid content is controlled to 55% to obtain a negative electrode mixture slurry. Subsequently, the formed negative electrode slurry is coated onto copper foil and then dried and die-cut by roll forming to form a negative electrode sheet.

[0112] iii) Battery assembly: The positive electrode, negative electrode and separator are stacked in the order of negative electrode, separator and positive electrode, and then the tabs are welded and the cells are encapsulated with aluminum-plastic film to obtain a soft-pack dry cell. Finally, the non-aqueous electrolyte prepared in each embodiment is injected into the cell to prepare a lithium-ion battery with a capacity of 5Ah.

[0113] The obtained lithium secondary batteries were subjected to subsequent performance tests, and the test results are shown in Table 2.

[0114] Performance testing

[0115] The performance test was conducted as follows, and the test results are shown in Table 2.

[0116] (1) High temperature cycling: At 45℃, the packaged lithium-ion battery is cycled after formation and capacity testing. It is charged at 1C constant current to 4.25V, and then charged at constant voltage until the current is 0.05C. After resting for half an hour, it is discharged at 1C constant current to obtain the initial capacity of the battery in the first cycle and recorded as C1. Thereafter, the charging and discharging cycle is repeated in the manner of CCCV / DC in the first cycle. The capacity after the 200th cycle is recorded as C200. The capacity retention rate (%) after 200 cycles of high temperature cycling is C200 / C1×100%. The DC internal resistance (DCR) of the cell at 50% SOC is tested and recorded before and after the high temperature cycling by the following current pulse method.

[0117] (2) DCR Test: At 25℃, the cell is charged at a constant current of 0.5C to 4.25V before or after high-temperature cycling, then charged at a constant voltage to a current of 0.05C. After resting for 1 hour, it is discharged at 0.5C for 1 hour (50% SOC), and then rested for another hour. The voltage V1 at the end is recorded. Then, it is discharged at 2C for 10 seconds, and the voltage V2 at the end is recorded. Then, the DCR of the cell before or after cycling is (V1-V2) / (I 1C -I 0.1C The unit is mΩ, and the DCR growth rate after cycling is (DCR after cycling - DCR). 循环前 ) / DCR 循环前 ×100%.

[0118] (3) High temperature storage: Charge the cells after capacity division to 100% SOC, and record its capacity as C0. Then, place the fully charged battery in a 55℃ oven for 30 days and test the cell capacity after storage as C1. Then, the capacity retention rate (%) after 30 days of storage at 55℃ is (C1-C0) / C0×100%.

[0119] Table 1 Formulations of Examples and Comparative Examples

[0120]

[0121] Table 2 Performance test results of the examples and comparative examples

[0122]

[0123] As shown in Table 2, a comparison between Examples 1-12 and Comparative Examples 1-7 demonstrates that introducing nitrile-based pyridazine compounds with specific structures can effectively improve the cycle and storage performance of the battery. Furthermore, by controlling the relationship between the content of nitrile-based pyridazine compounds and the manganese content and the positive electrode areal density (100a / (CW·y)) in the non-aqueous electrolyte within a certain range, even better improvement can be achieved, thereby further enhancing the cycle and storage performance of the battery.

[0124] It should be noted that although the technical solution of the present invention has been described with specific examples, those skilled in the art will understand that the present invention should not be limited thereto.

[0125] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or technical improvements to the embodiments in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A lithium secondary battery based on lithium manganese iron phosphate, characterized in that, The lithium secondary battery includes a positive electrode, a negative electrode, and a non-aqueous electrolyte; The positive electrode includes lithium manganese iron phosphate as the positive electrode active material, whose general structural formula is LiMn. y Fe 1-y PO4, where y represents the doping content of element Mn; The non-aqueous electrolyte includes nitrile pyridazine compounds as component (A); The structure of the nitrile pyridazine compound of component (A) is shown in formula (I): Where X represents a linking bond or a divalent organic group; R1, R2, and R3 each independently represent a hydrogen atom or a monovalent organic group; Furthermore, the lithium secondary battery satisfies the following relationship: 4 ≤ 100a / (CW·y) ≤ 15. Wherein, 'a' represents the mass percentage of the nitrile pyridazine compound (A) in the non-aqueous electrolyte; CW represents the areal density of the positive electrode, with units of mg / cm³. 3 .

2. The lithium secondary battery according to claim 1, characterized in that, The non-aqueous electrolyte also includes a non-aqueous solvent as component (B) and a lithium salt as component (C).

3. The lithium secondary battery according to claim 1 or 2, characterized in that, The content of component (A) nitrile pyridazine compound is 0.1% to 2% by mass of the total mass of the non-aqueous electrolyte.

4. The lithium secondary battery according to any one of claims 1 to 3, characterized in that, X represents a linking bond or a substituted or unsubstituted hydrocarbon group.

5. The lithium secondary battery according to any one of claims 1 to 4, characterized in that, R1, R2, and R3 may be the same or different, and each time they appear, they independently represent at least one of the following: a hydrogen atom, a substituted or unsubstituted hydrocarbon group, a halogen-containing group, an ether-containing group, a nitrile-containing group, an isocyanate-containing group, and a heteroatom-containing cyclic group.

6. The lithium secondary battery according to any one of claims 1 to 5, characterized in that, The nitrile pyridazine compound of component (A) is selected from one or more combinations of the compounds shown in formulas (I-1) to (I-6) below:

7. The lithium secondary battery according to claim 2, characterized in that, The non-aqueous solvent of component (B) is selected from one or more of cyclic carbonate solvents, linear carbonate solvents, ether solvents, ester solvents, and ketone solvents.

8. The lithium secondary battery according to claim 2, characterized in that, The lithium salt of component (C) is selected from one or more salts formed by lithium ions and the following anions: PF6 - BF4 - Cl - ,Br - I - ClO4 - AsF6 - SiF6 2- AlCl4 - B(C2O4)2 - CH3CO2 - CF3SO3 - N(CF3SO2)2 - N(FSO2)2 - C(CF2SO2)3 - C2BF2O4 - .

9. The lithium secondary battery according to any one of claims 1 to 8, characterized in that, The non-aqueous electrolyte also includes functional additives as component (D); and / or, the value of y is in the range of 0.2≤y≤0.8; the value of CW is in the range of 10≤CW≤30.

10. A means of transportation or a vehicle, or an energy storage device, characterized in that, The tool or device includes a lithium secondary battery according to any one of claims 1-9.

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