Lithium manganese iron phosphate positive electrode material and preparation method thereof, positive electrode plate and secondary battery

Abstract

The invention discloses a lithium iron manganese phosphate positive electrode material and a preparation method thereof, a positive electrode plate and a secondary battery, and belongs to the technical field of lithium iron manganese phosphate batteries, the lithium iron manganese phosphate positive electrode material comprises a lithium iron manganese phosphate positive electrode active material and a solid electrolyte interface film located on the surface of the lithium iron manganese phosphate positive electrode active material, the solid electrolyte interface film is of a double-layer structure, a first CEI layer close to the surface of the lithium manganese iron phosphate positive electrode material comprises elements of P, F, N, C and O, and a second CEI layer far away from the surface of the lithium manganese iron phosphate positive electrode material comprises elements of C, N and F; the ratio of the relative content of the N element to the relative content of the F element in the first CEI layer is (1.5-3): 1; and the ratio of the relative content of the N element to the relative content of the F element in the second CEI layer is not less than 3. According to the double-layer CEI membrane disclosed by the invention, the dissolution of manganese ions and the decomposition of the electrolyte are remarkably inhibited through the synergistic construction of the water-based adhesive and the electrolyte, and the problem of battery gas expansion is solved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium manganese iron phosphate batteries, and particularly relates to a lithium manganese iron phosphate positive electrode material, a preparation method thereof, a positive electrode sheet and a secondary battery. BACKGROUND

[0002] With the rapid development of new energy technology, lithium manganese iron phosphate (LMFP) secondary batteries have become a research hotspot due to their high energy density and low cost. However, the swelling problem of the batteries during cycling, especially under high temperature or overcharge conditions, seriously restricts their safety and service life. Swelling is manifested as a significant increase in the internal pressure of the battery, which can lead to deformation, rupture of the battery shell, and even cause serious safety accidents such as thermal runaway and fire.

[0003] Existing research reveals that the core reason for the swelling of lithium manganese iron phosphate batteries is that during the charging and discharging process, the redox side reactions of the electrolyte itself (especially the solvent and lithium salt) continuously produce a large amount of gas (such as CO, CO2, H2, C2H4, etc.). These gases continue to accumulate inside the battery, eventually leading to a sharp rise in internal pressure, i.e., the swelling phenomenon occurs.

[0004] Currently, in order to reduce the swelling problem of lithium manganese iron phosphate batteries, the main measures include improving battery materials, optimizing battery structure design and adjusting electrolyte formula. For example, by using high-stability positive electrode materials and high-safety electrolytes, the side reactions of the battery during charging and discharging can be reduced to a certain extent, thereby reducing gas production. However, these methods often cannot completely prevent the occurrence of swelling, especially under high temperature or overcharge conditions, the safety problem of the battery is still prominent.

[0005] Therefore, it is of great practical significance and application value to develop a new type of lithium manganese iron phosphate secondary battery anti-swelling technology scheme.

[0006] It should be noted that this part of the application only provides background technology related to the application, and does not necessarily constitute prior art or public knowledge. SUMMARY

[0007] The application provides a lithium manganese iron phosphate positive electrode material, a preparation method thereof, a positive electrode sheet and a secondary battery, which at least solves the swelling problem of the prior art lithium manganese iron phosphate secondary battery.

[0008] In order to achieve the above-mentioned purpose, in a first aspect, the present application provides a lithium iron manganese phosphate positive electrode material, which comprises a lithium iron manganese phosphate positive electrode active material and a solid electrolyte interface film on the surface of the lithium iron manganese phosphate positive electrode active material, the solid electrolyte interface film has a double-layer structure, the first CEI layer close to the surface of the lithium iron manganese phosphate positive electrode active material comprises P, F, N, C and O elements, and the second CEI layer away from the surface of the lithium iron manganese phosphate positive electrode active material comprises C, N and F elements; the ratio of the relative content of N element to the relative content of F element in the first CEI layer is (1.5-3):1; and the ratio of the relative content of N element to the relative content of F element in the second CEI layer is not less than 3.

[0009] More preferably, the ratio of the relative content of N element to the relative content of F element in the second CEI layer is (3-15):1.

[0010] Preferably, the ratio of the relative content of C element to the relative content of N element in the second CEI layer is (0.6-1.5):1. More preferably, it is (0.8-1.3):1.

[0011] Preferably, the ratio of the relative content of C element to the relative content of N element in the first CEI layer is (0.5-1):1. More preferably, it is (0.5-0.78):1.

[0012] Preferably, the ratio of the relative content of C element to the relative content of F element in the second CEI layer is not less than 2. More preferably, it is (2.5-15):1.

[0013] Preferably, the thickness of the solid electrolyte interface film is 1-20 nm.

[0014] Preferably, the thickness ratio of the first CEI layer to the second CEI layer is (0.2-4):1. More preferably, it is (0.5-2):1.

[0015] Preferably, in the first CEI layer, the mass percentage content of C element is 20%-30%, the mass percentage content of N element is 30%-40%, the mass percentage content of F element is 10%-20%, and the mass percentage content of O element is 20%-30%, based on the total mass of the first CEI layer being 100%, and the total of the mass percentage contents of C element, N element, F element and O element is >90%.

[0016] Preferably, in the second CEI layer, the mass percentage content of C element is 30%-40%, the mass percentage content of N element is 30%-40%, and the mass percentage content of F element is 5%-15%, based on the total mass of the second CEI layer being 100%.

[0017] Preferably, the lithium manganese iron phosphate positive electrode active material comprises a lithium manganese iron phosphate base and a carbon coating layer coated on the surface of the lithium manganese iron phosphate base; the chemical formula of the lithium manganese iron phosphate base is Li a Fe x Mn y M j PO q ; wherein M comprises at least one of Al, Mg, Cu, Co, Ni, Nb, V, Ti, La; 0.8≤a≤1.2, 0.1≤x≤0.9, 0.1≤y≤0.9, 0≤j≤0.1, 4≤q≤5; the mass of the carbon coating layer accounts for 0.5% to 3% of the total mass of the lithium manganese iron phosphate positive electrode active material.

[0018] In a second aspect, the present application further provides a preparation method for preparing the lithium manganese iron phosphate positive electrode material of the first aspect, comprising: preparing the lithium manganese iron phosphate positive electrode active material into a positive electrode sheet, and performing electrochemical activation treatment on the lithium manganese iron phosphate secondary battery composed of the electrolyte and the negative electrode sheet, so as to form a solid electrolyte interface film on the surface of the lithium manganese iron phosphate positive electrode active material, and the electrochemical activation treatment is at least one cycle of activation treatment; the negative electrode sheet comprises a negative electrode active material and a binder, and the binder contains a nitrogen-containing compound, and the mass of the nitrogen-containing compound accounts for 0.5% to 5% of the mass of the negative electrode active material.

[0019] Preferably, the rate of the electrochemical activation treatment is 0.05 to 0.5C, and the temperature of the electrochemical activation treatment is 35 to 45℃.

[0020] Preferably, the nitrile compound comprises at least one of polyacrylonitrile, succinonitrile, adiponitrile, and benzonitrile.

[0021] Preferably, the amine compound comprises at least one of polyethyleneimine and polyacrylamide.

[0022] In a third aspect, the present application further provides a positive electrode sheet comprising the lithium manganese iron phosphate positive electrode material of the first aspect or the lithium manganese iron phosphate positive electrode material prepared by the preparation method of the second aspect.

[0023] In a fourth aspect, the present application further provides a lithium manganese iron phosphate secondary battery comprising the positive electrode sheet of the third aspect, and further comprising a negative electrode sheet and an electrolyte.

[0024] Preferably, the electrolyte comprises a nitrogen-containing compound.

[0025] More preferably, the electrolyte does not comprise a nitrogen-containing compound.

[0026] Preferably, the electrolyte comprises a lithium salt, and the lithium salt comprises lithium hexafluorophosphate.

[0027] Preferably, the electrolyte comprises an organic solvent, the organic solvent comprises at least one of cyclic carbonate and chain carbonate, the cyclic carbonate comprises at least one of ethylene carbonate and propylene carbonate, and the chain carbonate comprises at least one of dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate.

[0028] Preferably, the electrolyte comprises an additive, the additive comprises at least one of vinylene carbonate, fluoroethylene carbonate, vinyl sulfate, methylene methane disulfonate and tris(trimethylsilyl) phosphate.

[0029] Preferably, the negative electrode tab comprises a negative active material and a binder, the binder contains a nitrogen-containing compound, and the mass of the nitrogen-containing compound is 0.5% to 5% of the mass of the negative active material.

[0030] Preferably, the nitrile compound comprises at least one of polyacrylonitrile, succinonitrile, adiponitrile and benzonitrile.

[0031] Preferably, the amine compound comprises at least one of polyethyleneimine and polyacrylamide.

[0032] Preferably, the negative active material comprises at least one of silicon-based material, carbon-based material, tin-based material, phosphorus-based material and lithium metal.

[0033] Preferably, the lithium iron manganese phosphate secondary battery further comprises a separator, and the separator material is polyethylene or polypropylene.

[0034] In a fifth aspect, the application further provides a power consumption device comprising the lithium iron manganese phosphate secondary battery of the fourth aspect.

[0035] The application has the following beneficial effects: 1. By introducing the nitrogen-containing compound into the aqueous binder of the negative electrode tab, the electrolyte cooperates to form a solid electrolyte interface film (CEI film) with a double-layer structure on the surface of the lithium iron manganese phosphate positive active material. The first CEI layer of the CEI film contains P, F, N, C and O elements, the second CEI layer is rich in C, N and F elements, and the relative content ratio of N and F elements in each layer is controlled respectively. The structure effectively anchors the dissolved Mn ions through the carbon-nitrogen polymer network, fundamentally inhibits the migration of manganese ions to catalyze the decomposition of the electrolyte, significantly reduces the gas generation in high-temperature cycling, and completely solves the problem of swelling. 2+ 2. By accurately controlling the total thickness of the CEI film and the thickness ratio of the first CEI layer and the second CEI layer, the efficient transmission of lithium ions is ensured, and the direct contact of the electrolyte with the high-activity positive electrode material is physically blocked, the solvent oxidation chain reaction is inhibited, and the battery still maintains the interface integrity and structural stability at high voltage.

[0036] 2. By accurately controlling the total thickness of the CEI film and the thickness ratio of the first CEI layer and the second CEI layer, the efficient transmission of lithium ions is ensured, and the direct contact of the electrolyte with the high-activity positive electrode material is physically blocked, the solvent oxidation chain reaction is inhibited, and the battery still maintains the interface integrity and structural stability at high voltage.

[0037] 3、The application further optimizes the chemical stability and mechanical toughness of the CEI film, enhances the anchoring ability of manganese ions and the barrier effect on the electrolyte, thereby significantly improving the cycle life and high-temperature safety of the battery, by regulating the mass percentage of each element in the first CEI layer and the second CEI layer, especially the specific ratio of C, N and F elements.

[0038] 4、The preparation method of the application forms a double-layer CEI film on the surface of the positive electrode in situ through electrochemical activation treatment, without the need to add nitrogen-containing compounds in the electrolyte, and the nitrogen-containing compounds in the negative electrode binder migrate to the positive electrode interface to participate in the construction of the outer layer of the nitrogen-rich CEI, realizing the full-battery collaborative anti-gassing path from negative electrode modification to positive electrode interface regulation.

[0039] 5、The scheme is suitable for various negative electrode materials such as silicon-based, carbon-based and tin-based, and has no special restrictions on the carbon coating layer, doping elements and conventional components of the manganese iron lithium phosphate positive electrode, has wide applicability and good process compatibility, and can be widely applied to power batteries, energy storage systems and consumer electronic devices, meeting the stringent safety requirements of high-energy-density batteries. BRIEF DESCRIPTION OF DRAWINGS

[0040] In order to more clearly illustrate the technical solutions of the embodiments of the application, the following will briefly introduce the drawings needed to be used in the embodiments. It should be understood that the following drawings only show some embodiments of the application, and therefore should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can also be obtained without creative labor on the basis of these drawings.

[0041] Figure 1 EDS line scan analysis diagram of the manganese iron lithium phosphate positive electrode material before cycling provided in Embodiment 1 of the application; Figure 2 EDS element mapping diagram of the manganese iron lithium phosphate positive electrode after cycling provided in Embodiment 1 of the application; Figure 3 TEM image of the CEI of the manganese iron lithium phosphate positive electrode after cycling provided in Embodiment 1 of the application, Figure 3 EDS element line scan data diagram of the manganese iron lithium phosphate positive electrode after cycling provided in Embodiment 1 of the application; Figure 4XPS spectrum test results of the negative graphite electrode of the embodiment 1 of the present application before and after etching. Wherein, a is the XPS spectrum full spectrum of the surface of the graphite after cycling; b is the XPS spectrum Mn2p fine spectrum of the surface of the graphite after cycling; c is the XPS spectrum Fe2p fine spectrum of the surface of the graphite after cycling; d is the XPS spectrum full spectrum of the surface of the graphite after cycling after etching; e is the XPS spectrum Mn2p fine spectrum of the surface of the graphite after cycling after etching; f is the XPS spectrum Fe2p fine spectrum of the surface of the graphite after cycling after etching; Figure 5 The transmission characterization map of the SEI generated on the surface of the graphite negative electrode after cycling of the embodiment 1 of the present application; Figure 6 The TEM image and EDS element mapping of the graphite negative electrode after cycling of the embodiment 1 of the present application. DETAILED DESCRIPTION

[0042] In the present application, the orientation words such as "upper", "lower", "left", "right" are generally understood in connection with the orientation shown in the drawings and the actual application, unless otherwise stated.

[0043] In addition, the terms "first", "second" are only for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined with "first", "second" can explicitly or implicitly include one or more of the features. In the description of the present application, the meaning of "multiple" is two or more, unless otherwise specifically limited.

[0044] In the present application, unless otherwise specifically stated and limited, the "on" or "under" of the first feature to the second feature can be that the first and second features are in direct contact, or the first and second features are indirectly in contact through an intermediate medium. Moreover, the "over", "above" and "on" of the first feature to the second feature can be that the first feature is directly above or obliquely above the second feature, or only indicates that the horizontal height of the first feature is higher than that of the second feature. The "under", "below" and "under" of the first feature to the second feature can be that the first feature is directly below or obliquely below the second feature, or only indicates that the horizontal height of the first feature is less than that of the second feature.

[0045] The endpoints of the ranges and any values disclosed herein are not limited to the precise values stated. The endpoints of the ranges and the values are approximate values and should be understood to include values approximately near these ranges and values within these ranges. For ranges, the endpoints are included within the ranges, and the endpoints and individual points within the ranges can be combined with other endpoints and individual points to create new ranges within the scope of the present application. The terms "optional", "optionally" mean that the subsequently described event or circumstance can or can not occur or exist (or can or can not be present).

[0046] The application provides a lithium manganese iron phosphate positive electrode material, which comprises a lithium manganese iron phosphate positive electrode active material and a solid electrolyte interface film on the surface of the lithium manganese iron phosphate positive electrode active material.

[0047] The application forms a solid electrolyte interface film (CEI film) with a double-layer structure on the surface of the lithium manganese iron phosphate positive electrode active material by introducing a nitrile compound into the water-based binder of the negative electrode sheet and cooperating with the electrolyte. 2+ The CEI film is formed by the nitrile compound and the electrolyte, and the first CEI layer of the CEI film comprises P, F, N, C and O elements, and the second CEI layer is rich in C, N and F elements.

[0048] It should be noted that the lithium manganese iron phosphate positive electrode material provided in the present application has a solid electrolyte interface film (CEI film) with a double-layer structure, the first CEI layer close to the surface of the lithium manganese iron phosphate positive electrode material comprises P, F, N, C and O elements, and the second CEI layer away from the surface of the lithium manganese iron phosphate positive electrode material comprises C, N and F elements.

[0049] It should be noted that the solid electrolyte interface film contains multiple elements, and the relative content of these elements can be measured by transmission electron microscopy (TEM) combined with energy dispersive X-ray spectroscopy (EDS) technology. In the line scan mode, EDS scans a line on the surface of the sample. This mode can provide element distribution information along a line. By continuous measurement along a line, the chemical composition changes at different positions on the line can be obtained, including element types and relative contents. In the EDS line scan mode, the relative content refers to the signal strength of a certain element at the detection point as a percentage of the total signal strength of all elements, reflecting the local proportion of the element at that position. Based on this measurement guideline, those skilled in the art should be able to determine how to detect the relative content of each element in the CEI film and calculate the ratio between elements.

[0050] Preferably, the ratio of the relative content of N element to the relative content of F element in the first CEI layer is (1.5~3):1, which can be (1.5:1), (1.6:1), (1.7:1), (1.8:1), (1.9:1), (2:1), (2.1:1), (2.2:1), (2.3:1), (2.4:1), (2.5:1), (2.6:1), (2.7:1), (2.8:1), (2.9:1), (3:1) and any value between them.

[0051] Preferably, the ratio of the relative content of N element to the relative content of F element in the second CEI layer is not less than 3, which can be 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5, 20, 20 or more larger values and any value between them.

[0052] More preferably, the ratio of the relative content of N element to the relative content of F element in the second CEI layer is (3~15):1.

[0053] The application controls the relative content ratio of N and F elements in the first CEI layer to be (1.5-3):1, and controls the ratio of N and F in the second CEI layer to be not less than 3, constructs a double-layer CEI structure with a functional gradient distribution, and thus significantly improves the interface stability and anti-gas performance of the lithium iron manganese phosphate battery. The possible principle is as follows: the moderate N / F ratio in the first CEI layer ensures that the layer is rich in inorganic components such as P and O, and has a certain nitrogen doping, thereby enhancing the interface bonding force and lithium ion transmission efficiency; and the higher N / F ratio in the second CEI layer forms a nitrogen-rich outer layer mainly composed of carbon-nitrogen polymers, and through the lone pair electrons of nitrogen atoms, a stable coordination bond is formed to effectively anchor manganese ions and block their migration into the electrolyte to catalyze the decomposition reaction. This double-layer synergistic mechanism not only physically blocks the contact between the electrolyte and the high-activity positive electrode material, but also chemically inhibits the dissolution and catalytic effect of manganese ions, thereby fundamentally reducing the occurrence of gas side reactions and solving the problem of battery swelling under high temperature and high voltage conditions. 2+ The application controls the relative content ratio of N and F elements in the first CEI layer to be (1.5-3):1, and controls the ratio of N and F in the second CEI layer to be not less than 3, constructs a double-layer CEI structure with a functional gradient distribution, and thus significantly improves the interface stability and anti-gas performance of the lithium iron manganese phosphate battery. The possible principle is as follows: the moderate N / F ratio in the first CEI layer ensures that the layer is rich in inorganic components such as P and O, and has a certain nitrogen doping, thereby enhancing the interface bonding force and lithium ion transmission efficiency; and the higher N / F ratio in the second CEI layer forms a nitrogen-rich outer layer mainly composed of carbon-nitrogen polymers, and through the lone pair electrons of nitrogen atoms, a stable coordination bond is formed to effectively anchor manganese ions and block their migration into the electrolyte to catalyze the decomposition reaction. This double-layer synergistic mechanism not only physically blocks the contact between the electrolyte and the high-activity positive electrode material, but also chemically inhibits the dissolution and catalytic effect of manganese ions, thereby fundamentally reducing the occurrence of gas side reactions and solving the problem of battery swelling under high temperature and high voltage conditions.

[0054] Preferably, the ratio of the relative content of C element to the relative content of N element in the second CEI layer is (0.6-1.5):1, which can be (0.6:1), (0.65:1), (0.7:1), (0.75:1), (0.8:1), (0.85:1), (0.9:1), (0.95:1), (1:1), (1.05:1), (1.1:1), (1.15:1), (1.2:1), (1.25:1), (1.3:1), (1.35:1), (1.4:1), (1.45:1), (1.5:1) and any value therebetween.

[0055] More preferably, the ratio of the relative content of C element to the relative content of N element in the second CEI layer is (0.8-1.3):1.

[0056] Preferably, the ratio of the relative content of C element to the relative content of N element in the first CEI layer is (0.5-1):1, which can be (0.5:1), (0.55:1), (0.6:1), (0.65:1), (0.7:1), (0.75:1), (0.8:1), (0.85:1), (0.9:1), (0.95:1), (1:1) and any value therebetween.

[0057] More preferably, the ratio of the relative content of C element to the relative content of N element in the first CEI layer is (0.5-0.78):1.

[0058] The application further optimizes the chemical composition and structural stability of the double-layer CEI film by controlling the ratio of C and N elements in the first CEI layer to be (0.5-1):1 and the ratio of C and N in the second CEI layer to be (0.6-1.5):1, thereby synergistically enhancing the anchoring ability of manganese ions and the blocking effect on the electrolyte. The possible principle is as follows: the appropriate carbon-nitrogen ratio in the first CEI layer helps to form a stable interface layer with carbon as the skeleton and nitrogen doping. This structure not only ensures the transmission efficiency of lithium ions but also enhances the mechanical strength and electronic shielding capacity of the inner layer, effectively inhibiting the penetration of the electrolyte to the surface of the positive active material; the carbon-nitrogen ratio close to 1:1 in the second CEI layer promotes the formation of a nitrogen-rich carbon polymer network. The nitrogen species in this structure can provide abundant coordination sites, form strong coordination bonds with dissolved Mn 2+ and realize efficient chemical capture of manganese ions, thereby fundamentally preventing the migration of manganese ions to the electrolyte to trigger the catalytic decomposition reaction. The differential design of the carbon-nitrogen ratio from the inside to the outside of the CEI film realizes a functional gradient transition from physical blocking to chemical anchoring, not only improving the compactness and stability of the interface film but also significantly reducing the byproduct gas during high-temperature and high-voltage cycling, thereby completely solving the problem of battery swelling.

[0059] Preferably, the ratio of the relative content of C elements to the relative content of F elements in the second CEI layer is not less than 2, which can be 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5, 20, or a larger value than 20, and any value therebetween.

[0060] More preferably, the ratio of the relative content of C elements to the relative content of F elements in the second CEI layer is (2.5-15):1.

[0061] Preferably, the ratio of the relative content of C elements to the relative content of F elements in the first CEI layer is (1-2.5):1, which can be (1:1), (1.1:1), (1.2:1), (1.3:1), (1.4:1), (1.5:1), (1.6:1), (1.7:1), (1.8:1), (1.9:1), (2.0:1), (2.1:1), (2.2:1), (2.3:1), (2.4:1), (2.5:1), and any value therebetween.

[0062] More preferably, the ratio of the relative content of C elements to the relative content of F elements in the first CEI layer is (1.2-2):1.

[0063] The present application controls the ratio of the relative content of C and F elements in the first CEI layer to be (1-2.5):1, and controls the ratio of C and F in the second CEI layer to be no less than 2, finely controls the chemical composition and functional distribution of the double-layer interface film, thereby synergistically improving the ion conductivity, structural stability and manganese ion anchoring capacity. The possible principle is as follows: the lower carbon-fluorine ratio in the first CEI layer makes it rich in inorganic fluoride components such as LiF produced by electrolyte decomposition, which has high ion conductivity and interface energy, can construct an efficient lithium ion transmission channel and effectively block the direct contact of electrolyte and active material, and inhibit solvent oxidation and decomposition; the higher carbon-fluorine ratio in the second CEI layer indicates that the layer is mainly composed of organic carbon polymers and has little fluorine content, and the stable carbon skeleton structure provides good mechanical toughness and chemical inertness, which can physically cover the inner layer and firmly anchor the manganese ions permeated from the inner layer, preventing further migration to the electrolyte. This gradient design of decreasing fluorine content and increasing carbon content from the inside to the outside realizes the functional ordered transition of the CEI film from high ion conductivity to high mechanical barrier and chemical anchoring, not only guarantees the kinetic performance and structural integrity of the interface, but also inhibits the electrolyte decomposition gas caused by manganese ion dissolution and catalysis from the source, completely solving the problem of battery swelling.

[0064] Preferably, the thickness of the solid electrolyte interface film is 1-20 nm, which can be 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 12 nm, 14 nm, 16 nm, 18 nm, 20 nm and any value therebetween. Such setting can effectively inhibit the swelling phenomenon of the battery, and the possible principle is as follows: by optimizing the film layer thickness to the nanometer level continuous coverage interval, while ensuring effective barrier to electrolyte decomposition and manganese ion dissolution, the lithium ion transmission resistance is minimized. The present application controls the thickness of the solid electrolyte interface film in the appropriate interval, which can form a complete amorphous dense layer, completely covering the surface defects of the positive electrode material, physically isolating the direct contact of electrolyte and high-activity manganese-iron lithium interface, and inhibiting solvent oxidation gas generation; at the same time, the thin layer structure can maintain a low lithium ion diffusion energy barrier, avoiding ion migration delay and interface impedance surge caused by excessive thickness, thereby stably inhibiting gas generation in long-term cycling.

[0065] Preferably, the thickness ratio of the first CEI layer and the second CEI layer is (0.2-4):1, which can be (0.2:1), (0.3:1), (0.4:1), (0.5:1), (0.6:1), (0.7:1), (0.8:1), (0.9:1), (1:1), (1.2:1), (1.3:1), (1.4:1), (1.5:1), (1.6:1), (1.7:1), (1.8:1), (1.9:1), (2:1), (2.5:1), (3:1), (3.5:1), (4:1), and any value therebetween.

[0066] More preferably, the thickness ratio of the first CEI layer and the second CEI layer is (0.5-2):1.

[0067] This design can effectively suppress the swelling phenomenon of the battery, and the possible principle is as follows: the inner layer is dominated by a fluorine-rich inorganic phase, which forms a dense ion channel with LiF / Li x PO y F z and other components to ensure efficient transmission of lithium ions; the outer layer relies on high nitrogen content to form a flexible carbon-nitrogen polymer network, which has abundant nitrile group nitrogen lone pair electrons and Mn 2+ forms a strong coordination bond, completely locking out the dissolved ions; and a suitable thickness ratio range ensures that the inner and outer layers are positively coupled, the inorganic phase of the inner layer inhibits the penetration of electrolyte to the positive active surface (blocking the solvent oxidation chain reaction), and the polymer network of the outer layer physically covers the inner layer and chemically anchors the manganese ions.

[0068] Preferably, in the first CEI layer, the mass percentage of C element is 20%-30%, the mass percentage of N element is 30%-40%, the mass percentage of F element is 10%-20%, and the mass percentage of O element is 20%-30%, and the total of the mass percentages of C element, N element, F element and O element is >90%, based on the total mass of the first CEI layer being 100%.

[0069] Preferably, in the second CEI layer, the mass percentage of C element is 30%-40%, the mass percentage of N element is 30%-40%, and the mass percentage of F element is 5%-15%, based on the total mass of the second CEI layer being 100%.

[0070] Preferably, the lithium manganese iron phosphate positive active material comprises a lithium manganese iron phosphate base and a carbon coating layer coated on the surface of the lithium manganese iron phosphate base; the chemical formula of the lithium manganese iron phosphate base is Li a Fe x Mn y M j PO q; wherein M comprises at least one of Al, Mg, Cu, Co, Ni, Nb, V, Ti, La; 0.8≤a≤1.2, 0.1≤x≤0.9, 0.1≤y≤0.9, 0≤j≤0.1, 4≤q≤5; the mass of the carbon coating layer accounts for 0.5%~3% of the total mass of the lithium iron manganese phosphate positive electrode active material.

[0071] In the present application, in the chemical formula of the lithium iron manganese phosphate matrix, 0.8≤a≤1.2, for example, 0.8, 0.85, 0.9, 0.95, 1, 1.05, 1.1, 1.15, 1.2, and any value in the range between any two of them.

[0072] In the present application, in the chemical formula of the lithium iron manganese phosphate matrix, 0.1≤x≤0.9, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and any value in the range between any two of them.

[0073] In the present application, in the chemical formula of the lithium iron manganese phosphate matrix, 0.1≤y≤0.9, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and any value in the range between any two of them.

[0074] In the present application, in the chemical formula of the lithium iron manganese phosphate matrix, 0≤j≤0.1, for example, 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, and any value in the range between any two of them.

[0075] In the present application, in the chemical formula of the lithium iron manganese phosphate matrix, 4≤q≤5, for example, 4, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 5, and any value in the range between any two of them.

[0076] The present application reasonably regulates the values of a, x, y, j and q in the chemical formula of the lithium iron manganese phosphate matrix, thereby effectively improving the electrochemical performance of the lithium iron manganese phosphate positive electrode active material. Specifically, adjusting the value of a can control the number of lithium vacancies in the lattice, thereby affecting lithium ion diffusion kinetics and structural stability. Adjusting the value of x can control the iron content, thereby affecting the voltage platform and electronic conductivity. Adjusting the value of y can control the manganese content, thereby determining the high voltage platform. Adjusting the value of j can control the content of doped elements, and optimize the material structure through bulk doping or surface modification. Adjusting the value of q can control the ratio of phosphorus to oxygen, maintain the stability of the olivine structure, and regulate the lattice energy.

[0077] Preferably, the mass of the carbon coating layer accounts for 0.5% to 3% of the total mass of the lithium iron manganese phosphate positive electrode active material, for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, and any value between them. The present application optimizes the balance between carbon layer coverage and conductivity, and cooperates with the nitrogen-containing additive to build a stable CEI film, thereby inhibiting manganese dissolution and electrolyte decomposition gas from the source.

[0078] The present application clearly indicates that, regardless of whether the CEI film is formed by a chemical method (such as liquid self-growth) or a physical method (such as vapor deposition, magnetron sputtering), as long as the interface film structure with specific element composition and proportion relationship is finally formed on the surface of the positive electrode active material, it belongs to the protection scope of the present application, but the present application still particularly provides an electrochemical activation method for preparing the lithium manganese iron phosphate positive material. This method not only has simple process, low cost, and is suitable for large-scale industrial production, but also can ensure the structural compactness, chemical stability and interface compatibility of the CEI film, thereby fundamentally inhibiting manganese ion dissolution and electrolyte decomposition, and significantly improving the high-temperature cycle stability and anti-gas performance of the battery.

[0079] The preparation method comprises: preparing the lithium manganese iron phosphate positive electrode active material into a positive electrode sheet, and performing electrochemical activation treatment with an electrolyte and a negative electrode sheet to form a solid electrolyte interface film on the surface of the lithium manganese iron phosphate positive electrode active material, the electrochemical activation treatment being at least one cycle of activation treatment; the negative electrode sheet comprises a negative electrode active material and a binder, and the binder contains a nitrogen-containing compound.

[0080] Preferably, the mass of the nitrogen-containing compound is 0.5% to 5% of the mass of the negative electrode active material, which can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, and any value between them.

[0081] The present application can effectively inhibit the gas swelling phenomenon of the battery by precisely controlling the addition amount of the nitrogen-containing compound to be 0.5% to 5% of the mass of the negative electrode active material. The possible principle is as follows: the low limit addition (≥0.5%) ensures that sufficient nitrogen-containing substances are dissolved and migrated from the negative electrode binder to the positive electrode interface to provide nitrogen-containing free radicals (such as ·CN) through oxidative cleavage during the formation process, and copolymerize with the carbonate solvents (such as EC / DMC) in the electrolyte to build a nitrogen-rich outer layer; under this proportion, the migrated nitrogen-containing substances can not only fully participate in the formation of a stable polymer network such as a polynitrile network, but also can be anchored by the nitrogen lone pair electrons and Mn 2+ strong coordination, without destroying the electrochemical window of the electrolyte.

[0082] Preferably, the rate of the electrochemical activation treatment is 0.05 to 0.5 C, and the temperature of the electrochemical activation treatment is 35 to 45℃.

[0083] It can be understood that the electrochemical activation treatment includes a selective oxidative decomposition reaction of the nitrogen-containing compound in the binder on the surface of the lithium iron manganese phosphate positive electrode material, and the decomposition products cooperate with other components of the electrolyte to form a thin layer of solid-state electrolyte interface film (CEI film) in situ.

[0084] In some embodiments, the electrochemical activation treatment can include: 0.05C current charging to 3.6V at a temperature of 35℃-45℃, for example 40℃, and a pressure of 150kgf-250kgf, for example 210kgf, standing for 15-60min, for example 40min, then 0.1C charging to 3.95V, standing for 15-60min, for example 40min, and then 0.5C charging to 4.25V.

[0085] Preferably, the binder is an aqueous binder.

[0086] Preferably, the nitrogen-containing compound includes at least one of a nitrile compound, an amine compound, and a sugar compound, and more preferably, the nitrogen-containing compound is a nitrile compound.

[0087] Optionally, the nitrile compound includes at least one of polyacrylonitrile, succinonitrile, adiponitrile, and benzonitrile.

[0088] Optionally, the amine compound includes at least one of polyethyleneimine and polyacrylamide.

[0089] The application also provides a positive electrode tab, which includes the lithium iron manganese phosphate positive electrode material prepared by any of the above methods other than the method provided by the application or the lithium iron manganese phosphate positive electrode material prepared by the method provided by the application.

[0090] The application also provides a lithium iron manganese phosphate secondary battery, which includes any of the above positive electrode tabs, and further includes a negative electrode tab and an electrolyte.

[0091] In the application, the electrolyte includes a lithium salt, an organic solvent, and an additive.

[0092] In the application, the electrolyte can include a nitrogen-containing compound. Although the preferred scheme is that the electrolyte does not include a nitrogen-containing compound, it is still technically possible to provide a nitrogen source through this approach to participate in the formation of the positive electrode interface film. The nitrogen-containing compound in the electrolyte can migrate to the positive electrode interface during the activation process of the battery, undergo oxidative decomposition, and participate in the construction of the CEI film, especially the second CEI layer rich in C and N elements, thereby to some extent assisting in inhibiting the dissolution of manganese ions and the decomposition of the electrolyte.

[0093] Optionally, when the electrolyte includes a nitrogen-containing compound, the nitrogen-containing compound is provided by a nitrogen-containing additive, and the mass of the nitrogen-containing additive is not more than 0.2% of the total mass of the electrolyte.

[0094] Optionally, the nitrogen-containing additive comprises at least one of a nitrile compound, a nitrogen-containing lithium salt, a pyridine, an amide; the nitrile compound comprises at least one of a dinitrile, a phosphorus-containing nitrile compound, a fluorine-containing nitrile compound, a silicon-containing nitrile compound.

[0095] Optionally, the nitrile compound comprises at least one of succinonitrile, adiponitrile, glutaronitrile, trifluoroacetonitrile, pentafluoropropionitrile, trimethylsilylacetonitrile.

[0096] Optionally, the amide comprises at least one of N-methylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, ε-caprolactam.

[0097] Optionally, the nitrogen-containing lithium salt comprises at least one of lithium bisfluorosulfonylimide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium nitrate (LiNO3).

[0098] Optionally, the pyridine comprises at least one of pyridine, 4-tert-butylpyridine, 2,6-dimethylpyridine, vinylpyridine.

[0099] More preferably, the electrolyte does not comprise a nitrogen-containing compound, which is more conducive to the formation of a double-layer CEI film, and the possible principle is as follows: the nitrogen-free environment of the electrolyte can ensure that the migration-polymerization path of the nitrogen-containing compound from the negative electrode binder is highly controllable, and the nitrogen-containing group in the nitrogen-containing compound, such as the nitrile group (-C≡N), is directionally oxidized and polymerized under high voltage at the positive electrode to generate a nitrogen-containing radical and copolymerize with an organic solvent in the electrolyte, such as a carbonate, to build a coordination network that anchors Mn 2+ .

[0100] Further, when the electrolyte deliberately excludes a nitrogen-containing compound, a double-layer CEI film can be better formed. Specifically, at the initial stage of electrochemical activation treatment, the lithium salt is preferentially decomposed at the positive high-voltage interface to generate inorganic components containing P / F, which rapidly deposit to form a dense inorganic inner layer (first CEI layer) due to strong polarity and high surface energy; the migration-lagged nitrogen-containing compound needs to be driven by the electric field to pass through the electrolyte to reach the positive electrode interface, and under the action of time difference and concentration gradient, it occurs oxidative polymerization outside the already formed inorganic layer to form an organic polymer outer layer rich in N (second CEI layer) by copolymerizing with a carbonate solvent, and the spatiotemporal isolation of this nitrogen source supply path and the cascading differences in reaction kinetics jointly drive the spontaneous assembly of the double-layer structure; conversely, if the electrolyte contains nitrogen, the nitrogen-containing components in the electrolyte will compete with the migration nitrogen-containing groups, destroying the time sequence and spatial isolation of the step-by-step deposition, which may cause a certain interweaving of the double-layer structure of the CEI film.

[0101] Further, in the technical solution of the negative electrode aqueous binder providing nitrogen-containing groups, the conjectural principle of the first CEI layer (near the positive electrode side) still containing N elements is as follows: on the one hand, although the P / F-containing inorganic matter generated by the decomposition of the lithium salt preferentially deposits to form the inner layer skeleton at the initial stage of formation, part of the nitrogen-containing compounds can reach the positive electrode interface and participate in the early reaction due to the faster migration rate or local concentration fluctuation, and the nitrogen-containing fragments generated by the partial oxidation and cracking of the nitrogen-containing compounds under high pressure co-deposit with the active phosphorus / f luorine species generated by the decomposition of the lithium salt, or are physically wrapped by the inorganic network, so as to be doped into the first CEI layer, while another part of the nitrogen polymerization reaction still occurs on the outside and forms a nitrogen-rich outer layer. The doping of the first CEI layer N element can actually enhance the function of the inner layer, and the nitrogen atom can improve the interfacial bonding force through coordination with the transition metal, and the nitrogen-containing polar group (such as -C≡N) can optimize the transport kinetics of lithium ions at the grain boundary of the inorganic phase (LiF, etc.); on the other hand, the nitrogen-containing species can also have an interfacial chemical reaction with the surface of the lithium manganese iron phosphate, and enter the lithium manganese iron phosphate through the interfacial bonding-gradual diffusion mode, resulting in part of the nitrogen gathering in the first layer, and the EDS spectrum also confirms that there is nitrogen element in the positive electrode material after the electrochemical reaction.

[0102] In the present application, the lithium salt includes lithium hexafluorophosphate (LiPF6).

[0103] In the present application, the organic solvent includes a cyclic carbonate and a chain carbonate; the cyclic carbonate includes at least one of ethylene carbonate (EC) and propylene carbonate (PC), and the chain carbonate includes at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC).

[0104] Preferably, the electrolyte further includes other additives, and the other additives include at least one of vinylene carbonate (VC), fluoroethylene carbonate (FEC), vinyl sulfate (DTD), methylene methane disulfonate (MMDS), and tris (trimethylsilyl) phosphate (TMSP).

[0105] It should be noted that the aforementioned content has mentioned that, no matter whether the CEI film is formed by a chemical method (such as liquid self-growth) or a physical method (such as vapor deposition, magnetron sputtering, etc.), as long as an interface film structure meeting the element composition and proportion relationship described above is finally formed on the surface of the positive active material, it shall belong to the protection scope of the present application. When the lithium manganese iron phosphate secondary battery includes the lithium manganese iron phosphate positive material prepared by any other method not provided by the present application, because the nitrogen source is not necessarily provided by the nitrile compound in the aqueous binder, therefore at this time the present application does not have special restrictions on the components and contents of the negative electrode additive, and the prior art can be referred to. When the lithium manganese iron phosphate secondary battery includes the lithium manganese iron phosphate positive material prepared by the preparation method provided by the present application, the binder should include a nitrogen-containing compound, and the specific components and contents have been described in detail in the aforementioned preparation method, and will not be described here.

[0106] In the present application, the negative active material includes at least one of a silicon-based material, a carbon-based material, a tin-based material, a phosphorus-based material, and lithium metal.

[0107] In some embodiments, the silicon-based material includes at least one of silicon, a silicon alloy, a silicon oxide compound, and a silicon carbon compound. In some embodiments, the carbon-based material includes at least one of graphite, soft carbon, hard carbon, a carbon nanotube, and graphene. In some embodiments, the tin-based material includes at least one of tin, a tin oxide, and a tin alloy. In some embodiments, the phosphorus-based material includes phosphorus and / or a phosphorus compound.

[0108] In some embodiments, the negative electrode sheet further includes a negative current collector, and the negative current collector includes a copper foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, a polymer substrate coated with a conductive metal, or any combination thereof.

[0109] In some embodiments, a separator film is provided between the positive electrode sheet and the negative electrode sheet to prevent short circuiting. The material and shape of the separator film that can be used in the embodiments of the present application are not particularly limited and can be any of the technologies disclosed in the prior art. In some embodiments, the separator film includes a polymer or an inorganic substance formed of a material stable to the electrolyte of the present application, etc.

[0110] For example, the separator film can include a substrate layer and a surface treatment layer. The substrate layer is a non-woven fabric, a film, or a composite film having a porous structure, and the material of the substrate layer includes at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Specifically, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric, or a polypropylene-polyethylene-polypropylene porous composite film can be used.

[0111] The surface treatment layer can be a polymer layer or an inorganic layer, or a mixed polymer and inorganic layer.

[0112] The inorganic layer includes inorganic particles and a binder. The inorganic particles include at least one of alumina, silica, magnesia, titania, hafnia, tin oxide, ceria, nickel oxide, zinc oxide, calcium oxide, zirconia, yttria, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. The binder includes at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene.

[0113] The polymer layer includes a polymer. The polymer includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride, or poly(vinylidene fluoride-hexafluoropropylene).

[0114] The present application also provides an electric device including the lithium manganese iron phosphate secondary battery.

[0115] In some embodiments, the electric device includes, but is not limited to, an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an energy storage system, etc. To meet the high power and high energy density requirements of the electric device, a battery pack or a battery module can be used.

[0116] In other embodiments, the electric device can be a mobile phone, a tablet computer, a notebook computer, etc. The electric device usually requires thin and light design, and a lithium manganese iron phosphate secondary battery can be used as a power source.

[0117] The technical solution provides an innovative solution for the anti-gas problem of the lithium iron manganese phosphate secondary battery, and therefore has a wide application prospect in application fields such as electric vehicles, mobile communication devices, and household energy storage systems. With the increasing demand for battery safety in society, the technical solution can effectively reduce the risk of lithium battery self-ignition, improve the safety of the battery, and meet the market demand for high-safety batteries. At the same time, the rapid development of electric vehicles and mobile communication devices puts forward higher requirements for the energy density and cycle stability of the battery. The technical solution significantly improves the energy density and cycle stability of the battery by optimizing the positive electrode material and electrolyte formula, which helps to promote the development of these industries. In addition, as an important part of new energy, household energy storage systems also have high requirements for the safety and stability of the battery. The application of the technical solution can improve the safety and stability of the energy storage system and promote the development of new energy. Therefore, the technical solution has broad market demand and good application prospect.

[0118] The application will be described in detail below through examples, which are exemplary and do not constitute any limitation on the application.

[0119] The specific preparation method of the lithium iron manganese phosphate secondary battery in the following examples and comparative examples is as follows: Step 1: Prepare lithium iron manganese phosphate positive electrode material, negative electrode material, and electrolyte.

[0120] Step 2: Coat the positive electrode material and the negative electrode material on aluminum foil and copper foil respectively to form positive electrode sheets and negative electrode sheets.

[0121] Step 3: Assemble the positive electrode sheets, negative electrode sheets, separator, and electrolyte into a battery.

[0122] In the examples and comparative examples, in an argon-filled glove box, ethylene carbonate (EC) and methyl ethyl carbonate (EMC) were mixed at a ratio of 15:85, lithium hexafluorophosphate (LiPF6) was added, the mass concentration of lithium hexafluorophosphate was 1.2 mol / L, 2% DTD and 1% FEC were added to the electrolyte, and the types and mass fractions of the nitrogen-containing additives in the electrolyte are shown in Table 1.

[0123] In the examples and comparative examples, the chemical formula of the lithium iron manganese phosphate matrix is LiFe 0.4 Mn 0.6 The mass fraction of the carbon-coated layer of LiFePO4 / C in the lithium iron manganese phosphate positive electrode material was 1.5%.

[0124] In the examples and comparative examples, the negative electrode active material was graphite, and the types and mass fractions of the nitrogen-containing compounds in the binder are shown in Table 1.

[0125] In the examples and comparative examples, the electrochemical activation treatment conditions are: temperature 40℃, pressure 210kgf, 0.05C current charging to 3.6V standing for 40min, then 0.1C charging to 3.95V standing for 40min, and then 0.5C charging to 4.25V.

[0126] Table 1

[0127] Test Example The lithium manganese iron phosphate secondary batteries prepared in the examples and comparative examples were subjected to the following verification tests, and the specific test results are shown in Tables 2 to 3: The specific measurement method and steps of the mass content of each element in the solid electrolyte interface film are as follows: First, the EDS line scan analysis was performed on the lithium manganese iron phosphate positive electrode material to calibrate the baseline, Figure 1 The element distribution characteristics of the carbon-coated lithium manganese iron phosphate positive electrode material are clearly shown. Figure 1 The results show that the Fe, Mn, P and O element signals are uniformly distributed within the particles, which is consistent with the chemical composition of the phosphate lattice; and the C element signal is obviously enhanced at the outer edge of the particles, forming a uniform distribution characteristic, indicating that there is a continuous carbon coating layer on the surface of the positive electrode material. Since the positive electrode material does not contain N, it can be determined that the N content range belongs to the baseline error category. Figure 1 The carbon layer signal is highly consistent with the particle outline, the thickness is uniform, and there is no obvious fracture or missing area, indicating that the carbon layer is uniform and dense, and this line scan analysis not only verifies the clarity of the positive electrode material structure, but also provides direct evidence for the formation of the CEI interface.

[0128] Subsequently, the element distribution of the positive electrode sheet after the cycle is measured. Specifically, the lithium manganese iron phosphate positive electrode sheet after the cycle is taken out from the battery, repeatedly washed with dimethyl carbonate (DMC) to remove residual electrolyte, and then dried in an argon environment. Then, the target positive electrode particle is cut, a sample containing the cross section of the CEI film is prepared, and transferred to a TEM special copper mesh. The CEI film area is located under TEM observation, and the EDS line scan mode is started: the scanning path is set in the direction perpendicular to the surface of the particle (from the inside of the particle to the electrolyte side), the element signal is collected with a step of 0.048 nm, and the characteristic X-ray intensity of each element is recorded synchronously. Then switch to the EDS mapping mode, and perform two-dimensional element distribution imaging on the selected area. The X-ray intensity count of each element is converted into relative content by quantitative analysis software (such as the EDS analysis module attached to TEM). Then the ratio between the relative contents of each element can be calculated. Selecting the content of Fe and Mn elements and the content of C and N elements both lower than the baseline error value as the starting point and the ending point of the whole CEI film respectively, to measure the thickness of the CEI film, and calculate the average of the relative content of each element in the CEI film as the relative content of the element in the CEI film.

[0129] The swelling criterion is as follows: the battery is discharged at 0.5C to 2.5V at 25°C, charged to 4.45V at 0.5C, and then charged to 0.05C at 4.45V. The thickness of the battery at this time is measured using a PPG soft pack battery thickness gauge and recorded as a. The lithium ion battery is then charged to 4.25V at 1C constant current and constant voltage at 60°C, and then discharged to 2.5V at 1C constant current. After 700 cycles, the battery is taken out and the surface is cleaned. The thickness of the battery at this time is recorded as b. The thickness swelling rate of the battery is calculated as follows: (b-a) / a x 100%.

[0130] Battery first coulomb efficiency test: At 25°C, the lithium ion secondary battery is charged at 1C constant current to 4.25V, and then charged at constant voltage to a current less than 0.05C, and the initial charge capacity is recorded. After standing for 5 minutes, the battery is discharged at 1C constant current to 2.5V, and the initial discharge capacity is recorded. The first coulomb efficiency of the lithium ion secondary battery = initial discharge capacity / initial charge capacity x 100%.

[0131] Battery cycle capacity retention rate test: At 60°C, the lithium ion battery is charged at 1C constant current to 4.25V, and then charged at constant voltage to 0.05C at 4.25V, and then discharged at 1C constant current to 2.5V. After 700 cycles of charging and discharging, the capacity retention rate after the 700th cycle at 25°C is calculated according to the following formula: discharge capacity after the 700th cycle / first cycle discharge capacity x 100%.

[0132] Table 2

[0133] Table 3

[0134] According to the results of Examples 1 to 7, it can be seen that adding an appropriate amount of nitrogen-containing compound in the negative electrode binder and preferably a nitrogen-free electrolyte can successfully build a double-layer CEI film with a specific element ratio and a nanoscale thickness on the surface of the lithium manganese iron phosphate positive electrode. The film layer structure is uniform and dense, which can significantly inhibit the dissolution of manganese ions and the decomposition of electrolyte, so that the thickness expansion rate of the battery is less than 2.5% after high-temperature cycling, and the cycle capacity retention rate is more than 80% after 700 cycles, effectively improving the first coulombic efficiency, solving the battery swelling problem and improving the cycle stability.

[0135] As can be seen from the comparison of the examples and the comparative examples, the preparation method of the present application (i.e. adding an appropriate amount of nitrogen-containing compound in the negative electrode binder and the electrolyte containing no nitrogen or only a very low amount of nitrogen) can successfully build a CEI film with a specific element ratio and a double-layer structure on the surface of the positive electrode, which can significantly inhibit the dissolution of manganese ions and the decomposition of electrolyte, thereby effectively reducing the thickness expansion rate of the battery, improving the first coulombic efficiency and the high-temperature cycle capacity retention rate. The comparative examples cannot form a complete double-layer CEI film due to the lack of nitrogen-containing compound or excessive nitrogen source in the electrolyte, resulting in serious deterioration of the battery performance, especially the prominent swelling problem.

[0136] More specifically, a variety of test analyses were performed on the lithium manganese iron phosphate positive electrode material of Example 1 to further illustrate the technical effects of the present application.

[0137] As shown in Figure 2 , the influence of the electrolyte on the element distribution of the lithium manganese iron phosphate positive electrode interface was analyzed by further electrochemically cycling the positive electrode material. As shown in Figure 2 , the EDS element mapping results of the carbon-coated lithium manganese iron phosphate positive electrode after multiple cycles showed that Fe, Mn, P and O still maintained uniform distribution inside the particles, indicating that the crystal framework of the material had high stability. The C element was continuously distributed on the surface of the particles, indicating that part of the carbon coating layer was not significantly damaged during long-term cycling. In addition, N and F elements were uniformly distributed on the surface of the particles, which was speculated to be formed by the decomposition of the nitrogen-containing components in the aqueous glue and the electrolyte to form CEI, and the distribution was uniform, which reflected the reaction compatibility of the electrolyte and the carbon material interface. These changes indicate that the electrolyte plays a positive role in maintaining the structural stability of the positive electrode material after multiple cycles, as well as the interface chemical stability during the cycling process.

[0138] The high-resolution TEM image of the lithium manganese iron phosphate positive electrode after multiple cycles in the electrolyte is shown in Figure 3As shown, the active particle surface still maintains a continuous and dense CEI layer, which is uniformly distributed in thickness without obvious cracks or peeling phenomenon, with a thickness of about 4 nm, and is tightly attached to the surface of the carbon coating layer, indicating that the carbon layer effectively stabilizes the interface structure in the cycling process in the electrolyte environment. According to the EDS line scan analysis, it can be seen that the Fe, Mn, P and O element signals are concentrated in the particle interior, and the F and P elements are significantly enhanced after the C and N element signals, which correspond to the outside of the carbon layer. The F and a small amount of P element signals are consistent with the position of the CEI region in TEM, indicating that these elements come from the interface film formed by the decomposition products of the electrolyte and its deposition.

[0139] The interface stability of the positive electrode material has an important influence on the formation of the negative electrode solid electrolyte interface film (SEI). The manganese iron phosphate positive electrode may release dissolved metal ions (such as Mn 2+ , Fe 2+ , etc.) during the cycling process, which migrate to the negative electrode surface through the electrolyte, promote the non-uniform growth and structural damage of the SEI film, and may cause the structural complexity of the negative electrode SEI film and reduce its stability. The structural stability and surface chemical properties of the positive electrode material indirectly affect the formation environment of the negative electrode SEI, and therefore, the design and modification of the positive electrode play a key role in the stability of the negative electrode SEI. In order to further prove the influence of the carbon compound coated manganese iron phosphate positive electrode on the electrolyte interface of the graphite negative electrode, the graphite negative electrode side and the carbon compound coated manganese iron phosphate positive electrode in the functional electrolyte were subjected to experimental characterization.

[0140] According to the XPS spectral analysis of Figure 4 , neither in the full spectrum of SEI nor in the fine spectrum of Mn2p and Fe2p, Mn and Fe elements were observed. In particular, the electrode interface was also etched for 30 s, and the results after etching proved that the pure graphite surface covered by SEI also had no signal of Mn and Fe elements. This consistency of the results before and after etching completely proves that there is no Mn and Fe element on the entire graphite negative electrode (including SEI). This is exactly the effective control means implemented on the positive electrode side, which makes the Mn and Fe in the positive electrode not obviously dissolved, not migrated to the negative electrode surface and deposited.

[0141] According to the transmission electron microscopy of Figure 5 , the microstructure of the SEI formed on the surface of the graphite negative electrode after cycling was further analyzed. The results show that the SEI after cycling uniformly covers the surface of the graphite electrode and maintains a thickness of about 13 nm. Since it is the transmission electron microscopy characterization result after multiple cycles, it indicates that the SEI film generated by the electrolyte on the graphite surface inhibits the side reaction of the continuous decomposition of the electrolyte, thereby effectively adapting to the volume change of the electrode in the charging and discharging process, and further providing important protection for the long-term cycling stability of the graphite negative electrode.

[0142] On the negative side, transition metals are usually deposited on the negative electrode and act as catalysts for the decomposition of the electrolyte. Here, the extent of dissolution of the transition metal is described by elemental mapping of the deposits on the negative electrode surface. According to Figure 6 The EDS elemental mapping analysis in the middle shows that there is almost no Mn element on the graphite surface, which corresponds to the XPS results, and is related to less Mn loss of the positive material in the electrolyte.

[0143] The above describes the preferred embodiments of the present application in detail, but the present application is not limited thereto. Within the technical concept of the present application, various simple modifications can be made to the technical solutions of the present application, including the combination of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as disclosed by the present application and belong to the protection scope of the present application.

Claims

1. A lithium manganese iron phosphate positive electrode material, characterized in that: The lithium iron manganese phosphate positive electrode material includes a lithium iron manganese phosphate positive electrode active material and a solid electrolyte interface film located on the surface of the lithium iron manganese phosphate positive electrode active material, wherein the solid electrolyte interface film has a double-layer structure, wherein a first CEI layer close to the surface of the lithium iron manganese phosphate positive electrode material includes P, F, N, C, and O elements, and a second CEI layer away from the surface of the lithium iron manganese phosphate positive electrode material includes C, N, and F elements; The ratio of the relative content of the N element to the relative content of the F element in the first CEI layer is (1.5-3):1; the ratio of the relative content of the N element to the relative content of the F element in the second CEI layer is not less than 3.

2. The lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that The ratio of the relative content of the N element to the relative content of the F element in the second CEI layer is (3-15):

1.

3. The lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that The ratio of the relative content of the C element to the relative content of the N element in the second CEI layer is (0.6-1.5):

1.

4. The lithium iron manganese phosphate positive electrode material according to claim 3, characterized in that The ratio of the relative content of the C element to the relative content of the N element in the second CEI layer is (0.8-1.3):

1.

5. The lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that The ratio of the relative content of the C element to the relative content of the N element in the first CEI layer is (0.5-1):

1.

6. The lithium iron manganese phosphate positive electrode material according to claim 5, characterized in that The ratio of the relative content of the C element to the relative content of the N element in the first CEI layer is (0.5-0.78):

1.

7. The lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that The ratio of the relative content of the C element to the relative content of the F element in the second CEI layer is not less than 2.

8. The lithium iron manganese phosphate positive electrode material according to claim 7, characterized in that The ratio of the relative content of the C element to the relative content of the F element in the second CEI layer is (2.5-15):

1.

9. The lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that The ratio of the relative content of the C element to the relative content of the F element in the first CEI layer is (1-2.5):

1.

10. The lithium iron manganese phosphate positive electrode material according to claim 9, characterized in that The ratio of the relative content of the C element to the relative content of the F element in the first CEI layer is (1.2-2):

1.

11. The lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that The thickness of the solid electrolyte interface film is 1-20 nm.

12. The lithium manganese iron phosphate positive electrode material according to claim 1 or 11, characterized in that The thickness ratio of the first CEI layer to the second CEI layer is (0.2-4):

1.

13. The lithium manganese iron phosphate positive electrode material according to claim 12, characterized in that The thickness ratio of the first CEI layer to the second CEI layer is (0.5-2):

1.

14. The lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that Based on the total mass of the first CEI layer being 100%, in the first CEI layer: the mass percentage of the C element is 20% to 30%, the mass percentage of the N element is 30% to 40%, the mass percentage of the F element is 10% to 20%, the mass percentage of the O element is 20% to 30%, and the sum of the mass percentages of the C element, the N element, the F element, and the O element is greater than 90%; And / or, based on the total mass of the second CEI layer being 100%, in the second CEI layer: the mass percentage of the C element is 30%-40%, the mass percentage of the N element is 30%-40%, and the mass percentage of the F element is 5%-15%.

15. The lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that The lithium manganese iron phosphate positive electrode active material includes a lithium manganese iron phosphate matrix and a carbon coating layer coated on the surface of the lithium manganese iron phosphate matrix; The chemical formula of the lithium manganese iron phosphate matrix is ​​Li a Fe x Mn y M j PO q ; Wherein, M includes at least one of Al, Mg, Cu, Co, Ni, Nb, V, Ti, and La; 0.8≤a≤1.2, 0.1≤x≤0.9, 0.1≤y≤0.9, 0≤j≤0.1, and 4≤q≤5; The mass of the carbon coating layer accounts for 0.5% to 3% of the total mass of the lithium manganese iron phosphate positive electrode active material.

16. A method for preparing the lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 15, characterized in that: include: The lithium iron manganese phosphate positive electrode active material is prepared into a positive electrode sheet, and the positive electrode sheet is combined with an electrolyte and a negative electrode sheet to form a lithium iron manganese phosphate secondary battery, and an electrochemical activation treatment is performed to form a solid electrolyte interface film on the surface of the lithium iron manganese phosphate positive electrode active material, wherein the electrochemical activation treatment is an activation treatment for at least one cycle; The negative electrode plate includes a negative electrode active material and a binder. The binder contains a nitrogen-containing compound, and the mass of the nitrogen-containing compound is 0.5% to 5% of the mass of the negative electrode active material.

17. The preparation method according to claim 16, characterized in that The electrochemical activation treatment has a magnification of 0.05-0.5°C and a temperature of 35-45°C.

18. The preparation method according to claim 16, characterized in that The adhesive is a water-based adhesive; the nitrogen-containing compound is selected from at least one of nitrile compounds, amine compounds, and sugar compounds.

19. The preparation method according to claim 18, characterized in that The nitrile compound includes at least one of polyacrylonitrile, succinonitrile, adiponitrile, and benzonitrile; And / or, the amine compound includes at least one of polyethyleneimine and polyacrylamide.

20. A positive electrode plate, characterized in that: The positive electrode plate includes the lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 15 or the lithium iron manganese phosphate positive electrode material prepared by the preparation method according to any one of claims 16 to 19.

21. A lithium manganese iron phosphate secondary battery, characterized in that: It includes the positive electrode sheet as described in claim 20, and also includes a negative electrode sheet and an electrolyte.

22. The lithium manganese iron phosphate secondary battery according to claim 21, characterized in that: The electrolyte includes a nitrogen-containing compound.

23. The lithium manganese iron phosphate secondary battery according to claim 21, characterized in that: The electrolyte does not include nitrogen-containing compounds.

24. The lithium manganese iron phosphate secondary battery according to claim 22 or 23, characterized in that: The electrolyte includes a lithium salt, and the lithium salt includes lithium hexafluorophosphate; And / or, the electrolyte includes an organic solvent, the organic solvent includes a cyclic carbonate and a chain carbonate; the cyclic carbonate includes at least one of ethylene carbonate and propylene carbonate, and the chain carbonate includes at least one of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; And / or, the electrolyte includes an additive, and the additive includes at least one of vinylene carbonate, fluoroethylene carbonate, vinyl sulfate, methylene methanedisulfonate, and tris(trimethylsilyl)phosphate.

25. The lithium manganese iron phosphate secondary battery according to claim 21, characterized in that: The negative electrode plate includes a negative electrode active material and a binder. The binder contains a nitrogen-containing compound, and the mass of the nitrogen-containing compound is 0.5% to 5% of the mass of the negative electrode active material.

26. The lithium manganese iron phosphate secondary battery according to claim 25, characterized in that: The adhesive is a water-based adhesive; the nitrogen-containing compound is selected from at least one of nitrile compounds, amine compounds, and sugar compounds.

27. The lithium manganese iron phosphate secondary battery according to claim 26, characterized in that: The nitrile compound includes at least one of polyacrylonitrile, succinonitrile, adiponitrile, and benzonitrile; And / or, the amine compound includes at least one of polyethyleneimine and polyacrylamide.

28. The lithium manganese iron phosphate secondary battery according to claim 25, characterized in that: The negative electrode active material includes at least one of silicon-based materials, carbon-based materials, tin-based materials, phosphorus-based materials, and metallic lithium.

29. The lithium manganese iron phosphate secondary battery according to claim 21, characterized in that: The lithium manganese iron phosphate secondary battery further includes a diaphragm, and the diaphragm material is polyethylene or polypropylene. 30 . An electric device comprising a lithium iron manganese phosphate secondary battery, comprising the lithium iron manganese phosphate secondary battery according to claim 21 .

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