Lithium iron manganese phosphate-based lithium secondary battery and application thereof

Abstract

The invention provides a lithium iron manganese phosphate-based lithium secondary battery and application thereof. The lithium secondary battery comprises a positive electrode, a negative electrode and a non-aqueous electrolyte, wherein the positive electrode comprises a lithium manganese iron phosphate positive electrode active material, the structural general formula of the lithium manganese iron phosphate positive electrode active material is shown as LiMnyFe1-yPO4, and y represents the doping content of the element Mn; the non-aqueous electrolyte comprises a nitrile group pyridazine compound serving as a component (A); the structure of the nitrile group pyridazine compound of the component (A) is as shown in a formula (I): # imgabs0 #, wherein X represents a connecting bond or a divalent organic group; r1, R2, and R3 each independently represent a hydrogen atom or a monovalent organic group; the lithium secondary battery satisfies the following relational expression: 4 < = 100a / (CW.y) < = 15, and a represents the mass percentage of the component (A) nitrile group pyridazine compound in the non-aqueous electrolyte; the CW represents the surface density of the positive pole piece, and the unit is mg / cm < 3 >.

Description

Technical Field

[0001] The present invention belongs to the field of new energy, and relates to a lithium iron manganese phosphate-based lithium secondary battery and its application, and more specifically, to a lithium secondary battery for power driving or energy storage and its application. Background Art

[0002] With the continuous development of society and the gradual increase in performance requirements, the mainstream battery systems used in the current power battery field - lithium iron phosphate (LFP) batteries and ternary (NCM) lithium batteries have their own bottleneck problems. Among them, the energy density of lithium iron phosphate has reached its limit and its low-temperature performance is poor, and the ternary lithium battery has poor safety performance and high cost. As an upgraded version of lithium iron phosphate, lithium manganese iron phosphate (LMFP) has the comprehensive advantages of high energy density, long life, high safety and low cost, and has become a hot spot in the current power lithium battery research.

[0003] LMFP is an olivine-structured material obtained by doping 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 of these is the serious dissolution and deposition of manganese ions due to the Jan-Taylor effect, which deteriorates its cycle / storage and power performance under high temperature / high pressure, limiting its promotion and application.

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

[0005] There have been reports disclosing a coated modified lithium manganese iron phosphate composite positive electrode material, a preparation method thereof and a battery. The method uses silicides such as zirconium silicide to coat and modify the lithium manganese iron phosphate material, thereby improving the structural stability and cycle stability of the positive electrode material. However, the organic layer has a weak confinement effect on the dissolved manganese ions, so the improvement in the battery cycle performance is limited, and the coating process is relatively complicated and time-consuming, which is not conducive to mass production.

[0006] In addition, there are reports that disclose a single-atom doped lithium manganese iron phosphate composite material, including a lithium manganese iron phosphate core and a carbon material shell layer doped with metal single atoms coated on the outer surface of the core. Applying the single-atom doped lithium manganese iron phosphate composite material to lithium-ion batteries can effectively improve the electrochemical properties of lithium-ion batteries, such as rate performance, cycle performance, discharge specific capacity and low-temperature performance. However, the type and content of metal doping are not ideal, and the intrinsic ionic conductivity and electronic conductivity of the material cannot be fully improved.

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

[0008] Problem that the invention aims to solve

[0009] As mentioned above, although lithium iron manganese phosphate as a positive electrode of lithium batteries has many advantages, due to the Jahn-Teller effect (JTE) caused by serious manganese ion dissolution and deposition, the cycle / storage and power performance under high temperature / high pressure deteriorate, limiting its promotion and application.

[0010] The prior art generally improves the above problems by modifying the positive electrode material, such as positive electrode surface coating, metal doping and particle size adjustment, etc. However, the above methods all have certain problems, such as the limited improvement of battery performance and high cost.

[0011] In order to solve the above problems, the present invention provides a lithium manganese iron phosphate-based lithium secondary battery, which introduces a nitrile pyridazine compound of a specific structure into the non-aqueous electrolyte in the lithium secondary battery. The nitrile group and unsaturated nitrogen heterocycle in the structure are used to effectively complex manganese ions, thereby inhibiting the problems caused by manganese dissolution and precipitation, thereby improving its cycle stability and storage stability under high temperature and high pressure conditions.

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

[0013] Solutions for solving problems

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

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

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

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

[0018]

[0019] Wherein, X represents a connecting bond or a divalent organic group;

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

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

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

[0023] The CW represents the surface density of the positive electrode sheet, and its unit is mg / cm 3 .

[0024] According to the lithium secondary battery of the present invention, the non-aqueous electrolyte further comprises 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 the nitrile pyridazine compound as component (A) is 0.1% to 2% by mass of the total mass of the non-aqueous electrolyte.

[0026] According to the lithium secondary battery of the present invention, wherein X represents a connecting bond or a substituted or unsubstituted hydrocarbon group.

[0027] According to the lithium secondary battery of the present invention, wherein R1, R2 and R3 are the same or different, and each time they appear, they independently represent at least one of a hydrogen atom, a substituted or unsubstituted hydrocarbon group, a halogen atom-containing group, an ether group-containing group, a nitrile group-containing group, an isocyanate 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 represented by the following formulas (I-1) to (I-6):

[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 comprises a functional additive as component (D); and / or the value range of y is 0.2≤y≤0.8; the value range of CW is 10≤CW≤30.

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

[0035] Effects of the Invention

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

[0037] 1) The lithium manganese iron phosphate-based lithium secondary battery provided by the present invention introduces a nitrile pyridazine compound of a specific structure into the non-aqueous electrolyte in the lithium battery. On the one hand, the nitrile group and unsaturated nitrogen heterocycle in the structure can be used to effectively complex manganese ions, thereby inhibiting the structural changes of the positive electrode 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 transfer kinetics at the positive electrode / electrolyte interface and reducing the kinetic loss caused by the obstruction of ion transfer during the charge and discharge process.

[0038] 2) The nitrile pyridazine compound contains a lone pair of electrons in N that can combine with proton hydrogen, thereby effectively eliminating the residual water or HF byproducts in the positive electrode material and generated during the discharge process, and inhibiting the catalytic reaction of acidic harmful byproducts on the dissolution of manganese ions and interface side reactions.

[0039] 3) The higher the manganese content in lithium manganese iron phosphate, the greater the voltage platform improvement, the greater the positive electrode surface density, the higher the energy density, but the corresponding positive electrode material has worse kinetics, and the more serious the capacity decay caused by the dissolution of manganese lithium ions during the charge and discharge process, therefore, different manganese contents and different positive electrode surface densities of lithium manganese iron phosphate positive electrode materials need to match 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 the positive electrode surface density 100a / (CW·y) within a certain range, a better improvement effect can be achieved, so that the battery has excellent cycle and storage performance. DETAILED DESCRIPTION

[0040] The following is a detailed description of the present invention. The following description of the technical features is based on representative embodiments and specific examples of the present invention, but the present invention is not limited to these embodiments and specific examples. It should be noted that:

[0041] In this specification, the numerical range expressed using "a numerical value A to a numerical value B" means a range including the endpoints numerical values ​​A and B.

[0042] In the present specification, a numerical range expressed using "above" or "below" means a numerical range including the number.

[0043] In this specification, the word "may" means both performing a certain process and not performing a certain process.

[0044] In this specification, the use of "optional" or "optional" indicates that certain substances, components, execution steps, application conditions and other factors are used or not used.

[0045] In this specification, the "normal temperature" or "room temperature" used means an indoor ambient temperature of "23±2°C".

[0046] In this specification, the unit names used are all international standard unit names, and unless otherwise stated, the "%" used indicates weight or mass percentage.

[0047] In the present specification, the use of “substantially” or “essentially” means that the standard deviation from a theoretical model, theoretical data or target data is within a numerical range of 2%, preferably 1%, and more preferably 0.8%.

[0048] In this specification, when the terms “include” and / or “comprise” are used, they indicate the existence of features, steps, operations, devices, components and / or their combinations.

[0049] In this specification, the references to "some specific / preferred embodiments", "other specific / preferred embodiments", "embodiments", etc., mean that the specific elements (e.g., features, structures, properties and / or characteristics) described in connection with the embodiments are included in at least one embodiment described herein, and may or may not exist in other embodiments. In addition, it should be understood that the elements may be combined in various embodiments in any suitable manner.

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

[0051] The present invention is based on the following insights:

[0052] Lithium manganese iron phosphate will cause serious dissolution and deposition of manganese ions due to the Jan-Taylor effect, especially when there is free acid in the electrolyte, the free acid is very easy to corrode the material, causing manganese ions to dissolve from the material lattice. The dissolved manganese ions will shuttle to the negative electrode side in the electrolyte, undergo a reduction reaction and precipitate on the negative electrode surface, damage the SEI film, thereby increasing the consumption of active lithium in the battery and the increase in the impedance of the negative electrode SEI film, thereby affecting the cycle life of the battery. In addition, under high pressure, the oxidative decomposition of the electrolyte at the positive electrode will be aggravated, acidic by-products will be produced, the dissolution amount of manganese ions will increase, and the cycle performance of the battery will be rapidly deteriorated. Therefore, the circulation / storage and power performance of lithium manganese iron phosphate deteriorate under high temperature / high pressure, limiting its promotion and application. The positive electrode surface coating, metal doping and particle size adjustment used in the prior art all have their own problems, and the improvement effect on manganese dissolution is limited and the cost is high.

[0053] Among the many methods for improving manganese dissolution, the optimization of electrolyte components is the most economical and has better effects in terms of economy and effect. The inventors of the present invention believe that, theoretically, the introduction of compounds with high polar groups and polynitrogen heterocyclic structures of specific structures into the non-aqueous electrolyte of lithium batteries is expected to solve the above problems. Therefore, through further attempts, it was found that nitrile pyridazine compounds are suitable in terms of solubility, electronic effect, complexing ability, etc. from many of the above compounds. On the one hand, the nitrile group and unsaturated nitrogen heterocycle in its structure can be used to effectively complex manganese ions, inhibit the changes in the positive electrode structure and the destruction of 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 transfer kinetics at the positive electrode / electrolyte interface and reducing the kinetic loss caused by the obstruction of ion transfer during the charge and discharge process. In addition, the nitrile pyridazine compound contains a lone pair of electrons in N that can combine with proton hydrogen, thereby effectively eliminating the residual water or HF byproducts in the positive electrode material and generated during the discharge process, and inhibiting the catalytic reaction of acidic harmful byproducts on the dissolution of manganese ions and interface side reactions. Through the optimization of the electrolyte components of the present invention, the problems caused by manganese dissolution and precipitation can be effectively improved, thereby improving its cycle stability and storage stability under high temperature and high pressure conditions.

[0054] <First aspect>

[0055] A first aspect of the present invention provides a lithium manganese iron phosphate-based lithium secondary battery, 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). In addition, other commonly used components in other electrolytes include a non-aqueous solvent as component (B) and a lithium salt as component (C).

[0058] In addition, without limitation, various optional functional additive components may be used in the non-aqueous electrolyte as long as they do not hinder the realization of the technical effects of the present invention.

[0059] Component (A)

[0060] Component (A) of the present invention is a nitrile pyridazine compound. The nitrile group and unsaturated nitrogen heterocycle in the 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 be more easily decomposed to form a highly stable organic-inorganic composite interface film rich in inorganic Li3N.

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

[0062]

[0063] Wherein, X represents a connecting bond or a divalent organic group.

[0064] Such a divalent linking group is not particularly limited in principle, and examples thereof include substituted or unsubstituted hydrocarbon groups.

[0065] In some specific embodiments, the hydrocarbon group may be a combination of one or more of a chain aliphatic hydrocarbon group, an alicyclic hydrocarbon group or an aromatic hydrocarbon group.

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

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

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

[0069] Such a hydrocarbon group may be a substituted or unsubstituted aliphatic hydrocarbon group, alicyclic hydrocarbon group or aromatic hydrocarbon group.

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

[0071] The substituent may be a halogen, and preferably a F atom.

[0072] Further, there is no particular restriction on the halogen-containing group, ether-containing group, nitrile-containing group, isocyanate-containing group, and heteroatom-containing cyclic group. In particular, the heteroatom-containing cyclic group has one or more heteroatoms in the ring structure, and the heteroatoms can be one or more of oxygen atoms, nitrogen atoms, and sulfur atoms. The ring can be a 5-membered or 6-membered aliphatic heterocycle or aromatic heterocycle.

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

[0074]

[0075] Component (B)

[0076] The present invention does not particularly limit the type of the non-aqueous solvent of 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 can be selected from one or more of cyclic carbonate solvents, linear carbonate solvents, ether solvents, ester solvents, and ketone solvents.

[0078] The cyclic carbonate solvent can be selected from ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC); the linear carbonate solvent can be selected from diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), methyl trifluoroethyl carbonate (FEMC), di(2,2,2-trifluoroethyl) carbonate (DFDEC); the ester solvent can be selected from diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), trifluoroethyl methyl carbonate (FEMC), di(2,2,2-trifluoroethyl) carbonate (DFDEC); The solvent may be selected from methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, and methyl pivalate, etc.; the ether solvent may 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), dioxolane (DOL), etc.; the ketone solvent may be selected from polymethyl vinyl ketone, etc. These non-aqueous solvents may be used alone or in the form of a mixture of two or more.

[0079] In some preferred embodiments, the non-aqueous solvent can 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 specifically limit the type of lithium salt of component (C), and it can be a lithium salt commonly used in the art. In some specific embodiments, the lithium salt can be selected from one or more salts formed by lithium ions and the following anions: PF6 - 、BF4 - , Cl - Br - ,I - 、ClO4 - 、AsF6 - 、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 added ingredients (D)

[0084] In principle, there is no particular limitation 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] Examples of such additives include vinyl ethylene carbonate (VC), lithium difluorophosphate (LiPO2F2), fluoroethylene carbonate (FEC), boron-containing additives, sulfur-containing additives or oxalate-containing additives. Among them, the boron-containing additive can be selected from lithium tetrafluoroborate (LiBF4), trimethylsilyl phosphate (TMSP), trimethylsilyl borate (TMSB), etc.; the sulfur-containing additive can be selected from 1,3-propane sultone (1,3-PS), 1,4-butane sultone (1,4-BS), 2,4-butane sultone (2,4-BS), 1,3-propylene sultone (PST), vinyl sulfate (DTD), methylene disulfonate (MMDS), vinyl sulfite (ES), etc.; the oxalate-containing additive can be selected from lithium difluorooxalatoborate (LiDFOB), lithium bisoxalatoborate (LiBOB), lithium tetrafluorooxalatophosphate (LiTFOP), lithium difluorobisoxalatophosphate (LiDFOP), etc. These additives can be used alone or in the form of a mixture of two or more.

[0086] Composition of non-aqueous electrolyte

[0087] In the present invention, the content of the nitrile pyridazine compound of the component (A) is, in principle, related to the manganese content in the positive electrode material and the positive electrode surface density in the lithium battery to be described below, that is, the optimal content of the nitrile pyridazine compound required to match the lithium iron manganese phosphate positive electrode material with different manganese contents and the positive electrode sheets with different coating weights is different.

[0088] In some specific embodiments, the content of the nitrile pyridazine compound of component (A) is 0.1% to 2% by mass 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 is no particular limitation on the lithium salt of component (C) in principle. In some specific embodiments of the present invention, the concentration of the lithium salt in the non-aqueous electrolyte may 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] In principle, there is no particular limitation on the amount of other functional additives (D), and the amount can be selected as needed.

[0091] (positive electrode)

[0092] The positive electrode of the present invention comprises a current collector and a positive electrode active material. The positive electrode active material comprises lithium iron manganese phosphate, and its general structural formula is LiMn y Fe 1-y PO4, wherein y represents the molar ratio of manganese element in manganese element and iron element. There is no particular limitation on the value of y. In some specific embodiments, y may be 0.2 to 0.8, such as 0.3, 0.4, 0.5, 0.6, 0.7, etc.

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

[0094] There is no particular limitation on the surface density of the lithium manganese iron phosphate positive electrode, and it can be selected as needed. In some specific embodiments, it can be 10 to 30 mg / cm 2 , for example 15mg / cm 2 , 20mg / cm 2 , 25mg / cm 2 wait.

[0095] (negative electrode)

[0096] In principle, there is no particular limitation on the negative electrode of the battery, and it can be any negative electrode commonly used in the art. Such a negative electrode includes 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 the present 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 of component (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 surface density of the positive electrode sheet, and its unit is mg / cm 3 ; y represents the doping content of the element Mn, that is, the molar ratio of the manganese element in the manganese element and the iron element.

[0100] Since the higher the manganese content in lithium manganese iron phosphate, the greater the voltage platform improvement, the greater the positive electrode surface density, the higher the energy density, but the corresponding positive electrode material kinetics are worse, and the capacity decay caused by the dissolution of manganese lithium ions during the charge and discharge process is more serious, so lithium manganese iron phosphate positive electrode materials with different manganese contents and different positive electrode surface densities need to be matched with different contents of nitrile pyridazine compounds to achieve improved cycle and energy storage performance. When the above relationship is met, the lithium battery can have 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 diaphragm. In principle, there is no particular restriction on the diaphragm of the battery, and the diaphragm may be used or not. That is, the lithium secondary battery of the present invention may be a lithium secondary battery with a diaphragm, or a lithium secondary battery without a diaphragm. When a diaphragm is used, it may be a diaphragm commonly used in the art. Preferably, a diaphragm with high moisture retention capacity for electrolyte solution and low resistance to electrolyte ion transfer may be used. In the absence of a diaphragm, the nonaqueous electrolyte of the present invention may be used in combination with other solid electrolytes, wherein the solid electrolyte may also act as a diaphragm.

[0103] <Second Aspect>

[0104] The second aspect of the present invention provides a transport or vehicle, or an energy storage device, wherein the tool or device includes the lithium secondary battery according to the first aspect, for such a transport or vehicle, such as a family car, an off-road vehicle or a sports utility vehicle.

[0105] Example

[0106] The embodiments of the present invention will be described in detail below in conjunction with the examples, but those skilled in the art will appreciate that the following examples are only used to illustrate the present invention and should not be considered to limit the scope of the present invention. If no specific conditions are specified in the examples, they are carried out according to normal conditions or the conditions recommended by the manufacturer. If the manufacturer is not specified for the reagents or instruments used, they are all conventional products that can be obtained commercially.

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

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

[0109] Preparation of lithium-ion secondary batteries:

[0110] i) Positive electrode preparation: The positive electrode active material LiMn 0.6 Fe 0.4 PO4, conductive agent (SP), single-walled carbon nanotube (SWNT) and binder polyvinylidene fluoride (PVDF) are added to an appropriate amount of N-methyl-2-pyrrolidone (NMP) solvent in a mass ratio of 94.5:2:2:1.5 and stirred thoroughly. The solid content is controlled to 68% to prepare a positive electrode mixture slurry. Thereafter, the formed positive electrode slurry is coated on aluminum foil and rolled and die-cut to form a positive electrode sheet after drying.

[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, and the solid content is controlled to 55% to prepare a negative electrode mixture slurry. Thereafter, the formed negative electrode slurry is coated on a copper foil, and the negative electrode sheet is formed by rolling and die-cutting after drying.

[0112] iii) Battery assembly: positive electrode sheet, negative electrode sheet and separator are stacked in order of negative electrode, separator and positive electrode, then the tabs are welded and packaged with aluminum-plastic film to obtain a soft-package dry cell, and 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 battery was subjected to subsequent performance tests, and the test results are shown in Table 2.

[0114] Performance Testing

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

[0116] (1) High temperature cycle: At 45°C, the packaged lithium-ion battery was subjected to a cycle test after the formation and capacity division steps. The battery was charged to 4.25V at a constant current of 1C, and then charged at a constant voltage until the current reached 0.05C. After standing for half an hour, the battery was discharged at a constant current of 1C to obtain the initial capacity of the first cycle of the battery and recorded as C1. Thereafter, the charging and discharging cycle was repeated in the CCCV / DC manner of the first cycle. The capacity after the 200th cycle of discharge was recorded as C200. The capacity retention rate after 200 cycles of high temperature cycling (%) = C200 / C1×100%. Before and after the high temperature cycling of the battery cell, the DC internal resistance DCR value of the battery cell at 50% SOC was tested and recorded by the following current pulse method.

[0117] (2) DCR test: At 25°C, charge the cell before or after high temperature cycling at a constant current of 0.5C to 4.25V, charge at a constant voltage to a current of 0.05C, leave it for 1h, then discharge it at 0.5C for 1h (50% SOC), leave it for another 1h, and record the voltage V1 at the end; then discharge it at 2C for 10s, and record the voltage V2 at the end. The DCR of the cell before or after cycling = (V1-V2) / (I 1C -I 0.1C ), the unit is mΩ, and the DCR growth rate after the cycle is (DCR cycle after – DCR 循环前 ) / DCR 循环前 ×100%.

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

[0119] Table 1 Formula table of embodiments and comparative examples

[0120]

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

[0122]

[0123] As can be seen from Table 2, the comparison between Examples 1 to 12 and Comparative Examples 1 to 7 shows that the introduction of a nitrile pyridazine compound with a specific structure can effectively improve the cycle and storage performance of the battery. Moreover, by controlling the relationship between the content of the nitrile pyridazine compound in the non-aqueous electrolyte and the manganese content and the positive electrode surface density 100a / (CW·y) within a certain range, a better improvement effect can be achieved, thereby further improving the cycle and storage performance of the battery.

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

[0125] The embodiments of the present invention have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The selection of terms used herein is intended to best explain the principles of the embodiments, practical applications, or technical improvements in the market, or to enable other persons of ordinary skill in the art to understand the embodiments disclosed herein.

Claims

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

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

3. The lithium secondary battery according to claim 1 or 2, characterized in that: The content of the nitrile pyridazine compound as component (A) 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: The 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: The R1, R2 and R3 are the same or different, and each time they appear, they independently represent at least one of a hydrogen atom, a substituted or unsubstituted hydrocarbon group, a halogen atom-containing group, an ether group-containing group, a nitrile group-containing group, an isocyanate 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 the component (A) is selected from one or more combinations of the compounds represented by the following formulas (I-1) to (I-6):

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 further comprises a functional additive as component (D); and / or, the value range of y is 0.2≤y≤0.8; the value range of CW is 10≤CW≤30.

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

Images