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

Abstract

The application discloses a kind of lithium manganese iron phosphate positive electrode material and its preparation method, positive pole piece and secondary battery, belong to lithium manganese iron phosphate battery technical field, lithium manganese iron phosphate positive electrode material includes lithium manganese iron phosphate positive electrode active material and the solid electrolyte interface film on the surface of lithium manganese iron phosphate positive electrode active material, solid electrolyte interface film contains C, N, F, O element, the relative content of C element in solid electrolyte interface film and the relative content of N element The ratio of element is (0.5~2):1.The solid electrolyte interface film of the application significantly inhibits the decomposition of manganese ion elution and electrolyte under high voltage / high temperature Side reaction, fundamentally reduces gas production, so that battery is not bloated after high temperature condition cycle Phenomenon, greatly improve the safety and reliability of battery.

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 the internal pressure of the battery, 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 purpose, in a first aspect, 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, the solid electrolyte interface film contains C, N, F and O elements, and the ratio of the relative content of C element to the relative content of N element in the solid electrolyte interface film is (0.5-2):1.

[0009] Preferably, the ratio of the relative content of C element to the relative content of N element in the solid electrolyte interface film is (0.8-1.5):1.

[0010] Preferably, the ratio of the sum of the relative content of C element and N element to the sum of the relative content of F element and O element in the solid electrolyte interface film is (1-10):1. More preferably, it is (2-6):1.

[0011] Preferably, the ratio of the relative content of C element to the relative content of F element in the solid electrolyte interface film is (1.5-15):1. More preferably, it is (2-7):1.

[0012] Preferably, the ratio of the relative content of N element to the relative content of F element in the solid electrolyte interface film is (1.5-15):1. More preferably, it is (2-7):1.

[0013] Preferably, the thickness of the solid electrolyte interface film is 2-20 nm.

[0014] Preferably, in the solid electrolyte interface film, the mass percentage content of C element is 30%-50%, the mass percentage content of N element is 30%-50%, the mass percentage content of F element is 2%-10%, and the mass percentage content of O element is 2%-10%, based on the total mass of the solid electrolyte interface film being 100%.

[0015] Preferably, the lithium iron manganese phosphate positive electrode active material comprises a lithium iron manganese phosphate base and a carbon coating layer coated on the surface of the lithium iron manganese phosphate base; the chemical formula of the lithium iron manganese 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.

[0016] In a second aspect, the present application further provides a preparation method for preparing the lithium iron manganese phosphate positive electrode material of the first aspect, comprising: preparing a lithium iron manganese phosphate positive electrode active material into a positive electrode sheet, and an electrolyte to form a lithium iron manganese phosphate secondary battery for electrochemical activation treatment, so as to form a solid electrolyte interface film on the surface of the lithium iron manganese phosphate positive electrode active material, and the electrochemical activation treatment is at least one cycle of activation treatment; the electrolyte comprises a nitrogen-containing additive.

[0017] Preferably, the rate of the electrochemical activation treatment is 0.05-0.5C, and the temperature of the electrochemical activation treatment is 35-45℃.

[0018] Preferably, the mass of the nitrogen-containing additive is 0.2%-3% of the total mass of the electrolyte.

[0019] Preferably, the nitrogen-containing additive comprises at least one of a nitrile compound, a nitrogen-containing lithium salt, pyridine, and an amide; the nitrile compound comprises at least one of a dinitrile, a phosphorus-containing nitrile compound, a fluorine-containing nitrile compound, and a silicon-containing nitrile compound.

[0020] In a third aspect, the present application further provides a positive electrode sheet comprising the lithium iron manganese phosphate positive electrode material of the first aspect or the lithium iron manganese phosphate positive electrode material prepared by the preparation method of the second aspect.

[0021] In a fourth aspect, the present application further provides a lithium iron manganese phosphate secondary battery comprising the positive electrode sheet of the third aspect, and further comprising a negative electrode sheet and an electrolyte.

[0022] Preferably, the negative electrode sheet comprises a negative electrode active material, and the negative electrode active material comprises at least one of a silicon-based material, a carbon-based material, a tin-based material, a phosphorus-based material, and metallic lithium.

[0023] Preferably, the electrolyte comprises a nitrogen-containing additive.

[0024] Preferably, the mass of the nitrogen-containing additive is 0.2%-3% of the total mass of the electrolyte.

[0025] Preferably, the nitrogen-containing additive comprises at least one of a nitrile compound, a nitrogen-containing lithium salt, pyridine, and an amide; the nitrile compound comprises at least one of a dinitrile, a phosphorus-containing nitrile compound, a fluorine-containing nitrile compound, and a silicon-containing nitrile compound.

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

[0027] Preferably, the electrolyte further comprises an organic solvent, and the organic solvent comprises at least one of a cyclic carbonate and a 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 further comprises other additives, and the other additives comprise at least one of vinylene carbonate, fluoroethylene carbonate, vinyl sulfate, methylene methane disulfonate, and tris(trimethylsilyl) phosphate.

[0029] Preferably, the lithium iron manganese phosphate secondary battery further comprises a separator, and the separator material is polyethylene or polypropylene.

[0030] Preferably, the thickness expansion rate of the lithium iron manganese phosphate secondary battery is less than 3% after 700 cycles at 60℃.

[0031] In a fifth aspect, the application further provides a power consumption device comprising the lithium iron manganese phosphate secondary battery, which comprises the lithium iron manganese phosphate secondary battery of the fourth aspect.

[0032] The application has the following beneficial effects:

[0033] 1. The application forms a solid electrolyte interface film (CEI film) with a carbon / nitrogen element ratio of 0.5-2 on the surface of the lithium iron manganese phosphate active positive electrode material, which has a dense film layer structure and stable composition, significantly inhibits the dissolution of manganese ions (Mn 2+ ) and the decomposition side reaction of electrolyte under high voltage / high temperature, fundamentally reduces the generation of gas (such as CO, CO2, H2, C2H4, etc.), and makes the battery basically have no swelling phenomenon after cycling under specific high temperature conditions, greatly improving the safety and reliability of the battery.

[0034] 2. The CEI film of the application has a thin layer structure rich in carbon and nitrogen and low in fluorine and oxygen, has excellent chemical stability and mechanical strength, can effectively anchor the dissolved manganese ions and block the electrolyte decomposition side reaction, significantly improves the interface stability of the battery under harsh working conditions (such as high temperature and overcharge), and thus prolongs the service life of the battery.

[0035] 3. The application has wide compatibility with lithium iron manganese phosphate positive electrode materials, negative electrode materials and separator materials, and can realize accurate regulation and control of the CEI film through electrolyte formula and electrochemical activation treatment, and has good industrial application universality. BRIEF DESCRIPTION OF DRAWINGS

[0036] 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.

[0037] Figure 1 EDS line scan analysis diagram of the lithium iron manganese phosphate positive electrode material before cycling provided for the embodiment 1 of the application;

[0038] Figure 2 TEM image of the CEI of the lithium iron manganese phosphate positive electrode after cycling provided for the embodiment 1 of the application;

[0039] Figure 3 EDS element mapping diagram of the lithium iron manganese phosphate positive electrode after cycling provided for the embodiment 1 of the application;

[0040] Figure 4 Fig. 6 is an EDS element distribution diagram of the lithium iron manganese phosphate positive electrode after cycling provided in Embodiment 1 of the present application, Figure 4 Fig. 7 is an EDS element line scanning data diagram of the lithium iron manganese phosphate positive electrode after cycling provided in Embodiment 1 of the present application;

[0041] Figure 5 Fig. 8 is an XRD diffraction pattern of the lithium iron manganese phosphate positive electrode after cycling provided in Embodiment 1 of the present application. DETAILED DESCRIPTION

[0042] In the present application, the orientation words such as "upper", "lower", "left", "right" used without the opposite indication generally refer to the orientation shown in the accompanying drawings and the actual application.

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

[0044] In the present application, unless otherwise specifically defined and limited, the first feature "on" or "under" 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 first feature "above", "over" and "on" 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 first feature "below", "under" and "under" 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 values should be construed as being open-ended to the extent that the language of the range or value limits itself in conjunction with the description specific to that range or value. For numeric values, the endpoints of each range, the endpoints of each range and individual point values, and individual point values can be combined with each other to obtain one or more new numeric ranges, which should be considered as specifically disclosed herein. Among them, the terms "optional" and "optional" mean that it can be included or not included (or can be included or not included).

[0046] 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 contains C, N, F and O elements. The ratio of the relative content of C element to the relative content of N element in the solid electrolyte interface film is (0.5-2):1.

[0047] The present application forms a solid electrolyte interface film (CEI film) with a carbon / nitrogen element ratio of 0.5-2 on the surface of the lithium iron manganese phosphate positive electrode active material. The film layer structure is dense and the composition is stable, which significantly inhibits the dissolution of manganese ions (Mn 2+ ) and the decomposition side reaction of the electrolyte at high voltage / high temperature, fundamentally reduces the generation of gas (such as CO, CO2, H2, C2H4, etc.), and makes the battery not to swell after cycling under certain high temperature conditions, greatly improving the safety and reliability of the battery.

[0048] It should be noted that in the lithium iron manganese phosphate positive electrode material protected by the present scheme, a solid electrolyte interface film (CEI film) containing C, N, F, and O elements is provided, and the relative content ratio of C element to N element is controlled to be between (0.5-2):1. Whether the CEI film is formed by chemical methods (such as liquid self-growth) or physical methods (such as vapor deposition, magnetron sputtering, etc.), as long as the interface film structure with the above-mentioned element composition and ratio relationship is finally formed on the surface of the positive electrode active material, it shall belong to the protection scope of the present application. The specific film forming process can be realized by referring to the prior art.

[0049] It should be noted that the solid electrolyte interface film contains C, N, F, and O 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 sample surface. This mode can provide element distribution information along a line. By continuous measurement along a line, the chemical composition change 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 accounting for the total signal strength of all elements, which reflects the local proportion of the element at that position. Based on this measurement guide, 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 C element to the relative content of N element in the solid electrolyte interface film is (0.5-2):1, such as (0.5:1), (0.6:1), (0.7:1), (0.8:1), (0.9:1), (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), (2:1) and any value therebetween.

[0051] More preferably, the ratio of the relative content of C element to the relative content of N element in the solid electrolyte interface film is (0.8-1.5):1.

[0052] The ratio of the relative content of carbon element to the relative content of nitrogen element in the CEI film is set to (0.5-2):1 (preferably 0.8-1.5:1), which can effectively inhibit the swelling phenomenon of the battery, and the possible principle is as follows: by adjusting the carbon-nitrogen ratio close to 1:1, a dense and stable interface layer rich in carbon-nitrogen polymer network is formed on the surface of the positive electrode. Under this ratio, carbon and nitrogen atoms can cooperatively construct a stable conjugated structure, in which the lone pair electrons of nitrogen atoms can effectively anchor the dissolved Mn 2+ ions, significantly inhibiting the catalytic decomposition of electrolyte by manganese ions; at the same time, the high chemical inertness of the carbon-nitrogen polymer network can physically block the direct contact of electrolyte with high-activity positive electrode material, thereby reducing the chain oxidation side reactions caused by manganese dissolution at high voltage / high temperature from the source, and finally achieving the purpose of inhibiting the swelling of the battery. If the carbon-nitrogen ratio is too low (<0.5), excessive nitrogen may cause the film layer to crosslink excessively and crack; if the carbon-nitrogen ratio is too high (>2), the anchoring ability of manganese ions and the electrolyte blocking performance of the carbon chain-dominated structure will decrease, and long-term anti-swelling effect cannot be achieved.

[0053] Preferably, the ratio of the sum of the relative contents of C element and N element to the sum of the relative contents of F element and O element in the solid electrolyte interface film is (1-10):1, such as (1:1), (1.5:1), (2:1), (2.5:1), (3:1), (3.5:1), (4:1), (4.5:1), (5:1), (5.5:1), (6:1), (6.5:1), (7:1), (7.5:1), (8:1), (8.5:1), (9:1), (9.5:1), (10:1), and any value therebetween.

[0054] More preferably, the ratio of the sum of the relative contents of C element and N element to the sum of the relative contents of F element and O element in the solid electrolyte interface film is (2-6):1.

[0055] Such setting can effectively inhibit the swelling phenomenon of the battery, and the possible principle is as follows: by forming an interface film structure with high carbon-nitrogen content and low fluorine-oxygen content, the present application preferentially constructs a continuous and dense network mainly composed of carbon-nitrogen polymer. High proportion of carbon-nitrogen components have excellent chemical stability and mechanical toughness, which can effectively block the contact of electrolyte with high-activity positive electrode material, and at the same time, nitrogen atoms in carbon-nitrogen polymer can anchor dissolved manganese ions through coordination, inhibiting their catalytic decomposition of electrolyte; while limiting fluorine-oxygen elements (mainly derived from lithium salt decomposition products such as LiF, Li x PO y F zThe content of the inorganic substance can avoid the problems of increased interface film brittleness, loose structure or ion transmission channel blockage caused by excessive inorganic substance, so as to maintain the structural integrity of the interface film in long-term circulation, and fundamentally reduce the gas generation phenomenon caused by continuous decomposition of the electrolyte.

[0056] Preferably, the ratio of the relative content of C element to the relative content of F element in the solid electrolyte interface film is (1.5-15):1, such as (1.5:1), (2:1), (2.5:1), (3:1), (3.5:1), (4:1), (4.5:1), (5:1), (5.5:1), (6:1), (6.5:1), (7:1), (7.5:1), (8:1), (8.5:1), (9:1), (9.5:1), (10:1), (10.5:1), (11:1), (11.5:1), (12:1), (12.5:1), (13:1), (13.5:1), (14:1), (14.5:1), (15:1) and any value therebetween.

[0057] More preferably, the ratio of the relative content of C element to the relative content of F element in the solid electrolyte interface film is (2-7):1.

[0058] This design significantly enhances the chemical stability and structural integrity of the CEI film, thereby more effectively inhibiting electrolyte decomposition and manganese ion leaching, and fundamentally improving the high-temperature performance and anti-gassing ability of the battery. The possible principle is as follows: the carbon element can be derived from the electrochemical polymerization of organic solvents or additives, forming a polymer network with good toughness and ion conductivity, while the fluorine element mainly comes from the decomposition products (such as LiF) of lithium salts (such as LiPF6), providing a high interface energy and high-strength inorganic protective layer. By controlling the C / F ratio within this range, the optimal ratio of organic and inorganic phases in the CEI film can be achieved, with sufficient carbon chain structure to ensure the continuity and flexibility of the film layer, avoiding rupture due to volume changes; the appropriate amount of fluorinated inorganic substance fills the organic network, enhancing the mechanical strength and electrochemical inertness of the film, and together forming a dense and stable barrier.

[0059] Preferably, the ratio of the relative content of N element to the relative content of F element in the solid electrolyte interface film is (1.5-15):1, which can be (1.5:1), (2:1), (2.5:1), (3:1), (3.5:1), (4:1), (4.5:1), (5:1), (5.5:1), (6:1), (6.5:1), (7:1), (7.5:1), (8:1), (8.5:1), (9:1), (9.5:1), (10:1), (10.5:1), (11:1), (11.5:1), (12:1), (12.5:1), (13:1), (13.5:1), (14:1), (14.5:1), (15:1) and any value therebetween.

[0060] More preferably, the ratio of the relative content of N element to the relative content of F element in the solid electrolyte interface film is (2-7):1.

[0061] This design builds a composite protective layer with excellent kinetic performance and stable mechanical barrier properties by regulating the chemical composition of the interface film, thereby effectively inhibiting manganese ion dissolution and improving the high-temperature cycle stability of the battery. The possible principle is as follows: nitrogen element can be derived from the oxidative decomposition and polymerization of nitrogen-containing substances in the system, and the formed nitrogen-rich polymer network not only can strongly chelate dissolved Mn 2+ ions through the lone pair of electrons of nitrogen atoms to inhibit their catalytic decomposition of electrolyte, but also provides a good interface ion transmission channel; and the fluorine element is mainly derived from the decomposition products (such as LiF) of lithium salts (such as LiPF6), which constitutes a high-modulus inorganic barrier and gives the CEI film a higher mechanical strength to resist particle volume changes and physically block electrolyte erosion. Maintaining the N / F ratio in this range can achieve the optimal synergy of organic chelation phase and inorganic shielding phase, and sufficient nitrogen component ensures efficient manganese ion fixation ability and moderate interface lithium ion conductivity, while the appropriate amount of fluorine component is interspersed in the organic network to enhance the compactness and structural rigidity of the film layer, and together form a stable and ion-conducting multifunctional interface layer.

[0062] Preferably, the thickness of the solid electrolyte interface film is 2-20 nm, which can be 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 12 nm, 14 nm, 16 nm, 17 nm, 18 nm, 19 nm, 20 nm and any value between them. Such setting can effectively suppress 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. Controlling the thickness of the solid electrolyte interface film in this interval can form a complete amorphous dense layer, completely covering the surface defects of the positive electrode material, physically isolating the electrolyte from direct contact with the high-activity manganese-lithium interface, and inhibiting the generation of solvent oxidation gas; the thin layer structure can maintain a low lithium ion diffusion energy barrier, avoid ion migration delay and interface impedance surge caused by excessive thickness, thereby stably inhibiting gas generation in long-term cycling.

[0063] It should be noted that the thickness of the solid electrolyte interface film can be measured by transmission electron microscopy (TEM).

[0064] Preferably, in the solid electrolyte interface film, the mass percentage of C element is 30%-50%, the mass percentage of N element is 30%-50%, the mass percentage of F element is 2%-10%, and the mass percentage of O element is 2%-10%, based on the total mass of the solid electrolyte interface film.

[0065] Preferably, the manganese iron phosphate lithium positive electrode active material comprises a manganese iron phosphate lithium matrix and a carbon coating layer coated on the surface of the manganese iron phosphate lithium matrix; the chemical formula of the manganese iron phosphate lithium matrix 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 manganese iron phosphate lithium positive electrode active material.

[0066] In the present application, in the chemical formula of the manganese iron phosphate lithium 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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 doping 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.

[0072] 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 constructs a stable CEI film with nitrogen-containing additives, thereby inhibiting manganese dissolution and electrolyte decomposition gas from the source.

[0073] The present application explicitly indicates that, no matter whether the CEI film is formed by a chemical method (such as liquid-phase self-growth) or a physical method (such as vapor deposition, magnetron sputtering), as long as an interface film structure meeting specific element composition and proportion relationship is finally formed on the surface of the positive 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, which 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 the dissolution of manganese ions and the decomposition of electrolyte, and significantly improving the high-temperature cycle stability and anti-gas performance of the battery.

[0074] The preparation method comprises: preparing the lithium manganese iron phosphate positive active material into a positive electrode sheet, and combining with an electrolyte to form a lithium manganese iron phosphate secondary battery for electrochemical activation treatment, so as to form a solid electrolyte interface film on the surface of the lithium manganese iron phosphate positive active material, and the electrochemical activation treatment is at least one cycle of activation treatment; and the electrolyte comprises a nitrogen-containing additive.

[0075] Preferably, the rate of the electrochemical activation treatment is 0.05-0.5C, and the temperature of the electrochemical activation treatment is 35-45℃.

[0076] It can be understood that the electrochemical activation treatment comprises a selective oxidation and decomposition reaction of the nitrogen-containing additive in the electrolyte on the surface of the lithium manganese iron phosphate positive active material, and the decomposition product cooperates with other components of the electrolyte to form a thin-layer solid-state electrolyte interface film (CEI film) with specific element proportion in situ.

[0077] In some embodiments, the electrochemical activation treatment can comprise: charging at a temperature of 35-45℃, for example 40℃, a pressure of 150-250kgf, for example 210kgf, 0.05C current to 3.6V and standing for 15-45min, for example 30min, then charging at 0.1C to 3.95V and standing for 15-45min, for example 30min, and then charging at 0.5C to 4.25V.

[0078] Preferably, the mass of the nitrogen-containing additive is 0.2% to 3% of the total mass of the electrolyte, which can be 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3%, and any value therebetween. The present application precisely controls the concentration of the additive according to the adopted positive electrode material, induces selective oxidative decomposition of the electrolyte on the surface of the positive electrode in the electrochemical activation treatment, and forms a dense CEI film with a target carbon-nitrogen ratio and thickness.

[0079] Preferably, the nitrogen-containing additive includes at least one of a nitrile compound, a nitrogen-containing lithium salt, pyridine, and an amide.

[0080] Further, the nitrile compound includes at least one of a dinitrile, a phosphorus-containing nitrile compound, a fluorine-containing nitrile compound, and a silicon-containing nitrile compound.

[0081] Optionally, the nitrile compound includes at least one of succinonitrile, adiponitrile, glutaronitrile, trifluoroacetonitrile, pentafluoropropionitrile, and trimethylsilylacetonitrile.

[0082] Optionally, the amide includes at least one of N-methylacetamide, N,N-dimethylformamide, N-methylpyrrolidone, and ε-caprolactam.

[0083] Optionally, the nitrogen-containing lithium salt includes at least one of lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium nitrate (LiNO3).

[0084] Optionally, the pyridine includes at least one of pyridine, 4-tert-butylpyridine, 2,6-dimethylpyridine, and vinylpyridine.

[0085] The present application also provides a positive electrode tab including the above-mentioned lithium iron manganese phosphate positive electrode material prepared by any other method not provided by the present application or the lithium iron manganese phosphate positive electrode material prepared by the preparation method provided by the present application.

[0086] The present application also provides a lithium iron manganese phosphate secondary battery including the above-mentioned positive electrode tab, and further including a negative electrode tab and an electrolyte.

[0087] In the present application, the negative electrode tab includes a negative electrode active material, and the negative electrode active material includes at least one of a silicon-based material, a carbon-based material, a tin-based material, a phosphorus-based material, and metallic lithium.

[0088] In some embodiments, the silicon-based material includes at least one of silicon, silicon alloy, silicon oxide, and silicon carbide. In some embodiments, the carbon-based material includes at least one of graphite, soft carbon, hard carbon, carbon nanotube, and graphene. In some embodiments, the tin-based material includes at least one of tin, tin oxide, and tin alloy. In some embodiments, the phosphorus-based material includes phosphorus and / or phosphorus compound.

[0089] In some embodiments, the negative electrode tab further includes a negative current collector, the negative current collector including: 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.

[0090] It should be noted that the foregoing has mentioned that, no matter whether the CEI film is formed by a chemical method (such as liquid-phase self-growth) or a physical method (such as vapor deposition, magnetron sputtering, etc.), as long as an interface film structure meeting the above element composition and proportion relationship 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 in the present application, because the nitrogen source is not necessarily provided by the nitrogen-containing additive in the electrolyte, therefore at this time the present application does not have special limitations on the components and contents of the electrolyte, 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 in the present application, the electrolyte should include the nitrogen-containing additive, and the specific components and contents have been described in detail in the foregoing preparation method, and will not be described here.

[0091] In the present application, the electrolyte includes a lithium salt, and the lithium salt includes lithium hexafluorophosphate (LiPF6).

[0092] In the present application, the electrolyte further includes an organic solvent, and the organic solvent includes at least one of 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).

[0093] In the present application, 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).

[0094] In some embodiments, a separator film is provided between the positive electrode tab and the negative electrode tab to prevent short circuiting. The material and shape of the separator film usable in the embodiments of the present application are not particularly limited and can be any of the techniques disclosed in the prior art. In some embodiments, the separator film includes a polymer or inorganic substance formed of a material stable to the electrolyte of the present application, etc.

[0095] For example, the separator film can include a base material layer and a surface treatment layer. The base material layer is a non-woven fabric, a film, or a composite film having a porous structure, and the material of the base material 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.

[0096] A surface treatment layer is provided on at least one surface of the base material layer, and the surface treatment layer can be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic substance.

[0097] The inorganic layer includes inorganic particles and a binder, and 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.

[0098] The polymer layer includes a polymer, and the material of the polymer includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride, or poly(vinylidene fluoride-hexafluoropropylene).

[0099] Preferably, the thickness expansion rate of the lithium manganese iron phosphate secondary battery is less than 3% after 700 cycles at 60°C.

[0100] Since the technical solution provides an innovative solution to the outgassing problem of lithium iron manganese phosphate secondary batteries, it has wide application prospects in application fields such as electric vehicles, mobile communication devices, and home energy storage systems. With the increasing demand for battery safety, the technical solution can effectively reduce the risk of lithium battery self-ignition, improve battery safety, and meet the market demand for high-safety batteries. At the same time, the rapid development of electric vehicles and mobile communication devices has higher requirements for the energy density and cycle stability of batteries. The technical solution significantly improves the energy density and cycle stability of batteries 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, home energy storage systems have high requirements for the safety and stability of batteries. The application of the technical solution can improve the safety and stability of energy storage systems and promote the development of new energy. Therefore, the technical solution has broad market demand and good application prospects.

[0101] The application also provides a power consumption device comprising a lithium iron manganese phosphate secondary battery.

[0102] In some embodiments, the power consumption 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 power consumption device for lithium iron manganese phosphate secondary batteries, a battery pack or a battery module can be used.

[0103] In other embodiments, the power consumption device can be a mobile phone, a tablet computer, a notebook computer, etc. The power consumption device usually requires thinning, and a lithium iron manganese phosphate secondary battery can also be used as a power source.

[0104] The application will be described in detail below through examples, which are exemplary and do not constitute any limitation on the application.

[0105] The specific preparation method of the lithium iron manganese phosphate secondary battery in the following examples and comparative examples is as follows:

[0106] Step 1: Prepare lithium iron manganese phosphate positive electrode material, negative electrode material graphite, and electrolyte.

[0107] 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.

[0108] Step 3: Assemble the positive electrode sheets, negative electrode sheets, separators, and electrolyte into a battery.

[0109] In the examples and comparative examples, ethylene carbonate (EC) and methyl ethyl carbonate (EMC) were mixed at 15:85, lithium hexafluorophosphate (LiPF6) was added, the mass fraction of lithium hexafluorophosphate was 1.2 mol / L, and after the lithium salt was completely dissolved, the nitrogen-containing additive and FEC with the mass fraction shown in Table 1 were added and stirred uniformly as the electrolyte.

[0110] In the examples and comparative examples, the chemical formula of the lithium manganese iron phosphate matrix was LiFe 0.4 Mn 0.6 PO4 / C, the mass fraction of the carbon coating layer in the lithium manganese iron phosphate positive electrode material was 1.5%.

[0111] In the examples and comparative examples, the electrochemical activation treatment conditions were: temperature 40℃, pressure 210kgf, 0.05C current charging to 3.6V and standing for 30min, then 0.1C charging to 3.95V and standing for 30min, and then 0.5C charging to 4.25V.

[0112] Table 1

[0113]

[0114] Test Example

[0115] 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 Table 2:

[0116] The specific measurement method and steps of the relative content of C, N, F and O elements in the solid electrolyte interface film are as follows: first, the lithium manganese iron phosphate positive electrode material was subjected to EDS line scanning analysis 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 scanning analysis not only verifies the clarity of the positive electrode material structure, but also provides direct evidence for the formation of the CEI interface.

[0117] Subsequently, the element distribution of the positive electrode tab after the cycle is measured, specifically, the positive electrode tab 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 region 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 C, N, F and O is recorded synchronously; then, the EDS mapping mode is switched to, and two-dimensional element distribution imaging is performed on the selected region; the X-ray intensity count of each element is converted into the relative content by a quantitative analysis software (such as the EDS analysis module attached to the TEM). The content of Fe and Mn elements is lower than the baseline error value, and the content of C and N elements is lower than the baseline error value, as the starting point and the ending point of the CEI film, respectively, the thickness of the CEI film is measured, and the average of the relative content of each element in the CEI film is calculated as the relative content of the element in the CEI film.

[0118] The determination standard of whether to swell is: 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 by using a PPG soft package battery thickness tester and recorded as a. The above lithium ion battery is 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, take out and clean the surface, and the thickness of the battery at this time is recorded as b. The thickness expansion rate of the battery is measured, and the calculation formula of the thickness expansion rate is: (b-a) / a×100%.

[0119] Table 2

[0120]

[0121] It can be known from the analysis of examples 1 to 7 that within the process parameter range of the nitrogen-containing additive provided by the application, such as the type, content, and electrochemical activation treatment temperature, a solid electrolyte interface film (CEI film) with suitable element ratio and suitable thickness can be formed on the surface of the lithium manganese iron phosphate positive electrode material through electrochemical activation treatment. The film layer structure is dense and the composition is stable, which can effectively inhibit the dissolution of manganese ions and the decomposition of electrolyte, so that the thickness expansion rate of all example batteries is less than 3% after 700 cycles at 60°C, showing excellent high-temperature anti-swelling performance and cycle stability.

[0122] From the comparison of the examples and the comparative examples, it can be seen that when the electrolyte does not contain nitrogen-containing additives, the content of the nitrogen-containing additives is too low or too high, the key parameters such as the proportion of each element of the formed CEI film obviously deviate from the range required by the application, the structure of the CEI film is loose, the composition is uneven, the interface side reaction cannot be effectively inhibited, and serious swelling occurs after the battery is cycled.

[0123] As shown in Figure 2 , in order to explore the influence of the electrolyte on the derived interface structure of the lithium manganese iron phosphate positive electrode, the transmission electron microscopy characterization of the lithium manganese iron phosphate positive electrode after cycling is further carried out. Figure 2 The results show that the surface of the lithium manganese iron phosphate positive electrode after cycling is covered by the CEI film formed by the spontaneous decomposition of the electrolyte on the surface, and the thickness is about 5.3 nm. The internal region after cycling still maintains the layered crystal structure of lithium manganese iron phosphate, which indicates that the material maintains good structural stability. In addition, the surface of the material is coated with a single-layer amorphous structure, which does not decompose at high voltage during the cycling process, proving that the electrolyte spontaneously decomposes to form a thin and uniform CEI.

[0124] As shown in Figure 3 , the characteristics of the CEI on the surface of the positive electrode after cycling can be directly revealed by EDS mapping analysis. Figure 3 The results show that in addition to the distribution of main elements Mn, Fe and P, there are obvious signals of foreign elements such as C, F and N on the surface, and the content distribution of C and N is basically consistent.

[0125] Through TEM image and line scan data analysis, the surface of the lithium manganese iron phosphate positive electrode active particles cycled in the electrolyte has a clear CEI layer, as shown in Figure 4 , combined with EDS line scan analysis, it can be found that the signals of Fe, Mn, P and O elements are concentrated in the interior of the particles, and the signals of C and N elements corresponding to the edge of the particles, i.e. the outside of the material, after two cycles, indicating that the decomposition products of the electrolyte constitute the main components of the CEI.

[0126] According to the XRD analysis of Figure 5 , the change of the phase structure of the positive electrode material during the cycling process is explored. The results show that the peak position after cycling corresponds to the standard diffraction peak of PDF #13-0336, indicating that the sample is a pure phase. In addition, the diffraction peak is clear, the sample has good crystallinity, and no additional diffraction peak is observed, indicating that the lattice structure of lithium manganese iron phosphate is not damaged, the structure of lithium manganese iron phosphate is maintained, and the dissolution of Mn is inhibited. The results show that the material maintains good structural stability, and there is no vacancy and defect caused by the loss of transition metals such as Mn.

[0127] The preferred embodiments of the present application are described in detail above, 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 that each technical feature is combined in any other suitable manner. These simple modifications and combinations should also be considered as disclosed by the present application and fall within the protection scope of the present application.

Claims

1. A lithium iron manganese phosphate secondary battery, characterized by, The lithium manganese iron phosphate secondary battery comprises a positive electrode sheet, a negative electrode sheet and an electrolyte. The lithium manganese iron phosphate positive electrode material 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, wherein the solid electrolyte interface film is obtained by electrochemical activation treatment of the lithium manganese iron phosphate secondary battery. The solid electrolyte interface film contains C, N, F and O elements. The ratio of the relative content of C element to the relative content of N element in the solid electrolyte interface film is (0.5-2):

1. The ratio of the sum of the relative content of C element and N element to the sum of the relative content of F element and O element in the solid electrolyte interface film is (1-10):

1. The ratio of the relative content of C element to the relative content of F element in the solid electrolyte interface film is (1.5-15):

1. The ratio of the relative content of N element to the relative content of F element in the solid electrolyte interface film is (1.5-15):

1. The thickness of the solid electrolyte interface film is 2-20 nm.

2. The lithium iron manganese phosphate secondary battery according to claim 1, characterized by The ratio of the relative content of C element to the relative content of N element in the solid electrolyte interface film is (0.8-1.5):

1.

3. The lithium iron manganese phosphate secondary battery according to claim 1, characterized by The ratio of the sum of the relative content of C element and N element to the sum of the relative content of F element and O element in the solid electrolyte interface film is (2-6):

1.

4. The lithium iron manganese phosphate secondary battery according to claim 1, characterized by The ratio of the relative content of C element to the relative content of F element in the solid electrolyte interface film is (2-7):

1.

5. The lithium iron manganese phosphate secondary battery according to claim 1, characterized by The ratio of the relative content of N element to the relative content of F element in the solid electrolyte interface film is (2-7):

1.

6. The lithium iron manganese phosphate secondary battery according to claim 1, characterized by 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 iron manganese phosphate base 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, 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 manganese iron phosphate positive electrode active material.

7. The lithium iron manganese phosphate secondary battery according to claim 1, characterized by, The negative electrode active material comprises at least one of silicon-based material, carbon-based material, tin-based material, phosphorus-based material and metallic lithium.

8. The lithium iron manganese phosphate secondary battery according to claim 1, characterized by, The electrolyte comprises a nitrogen-containing additive.

9. The lithium iron manganese phosphate secondary battery according to claim 8, characterized in that, The mass of the nitrogen-containing additive accounts for 0.2%-3% of the total mass of the electrolyte.

10. The lithium iron manganese phosphate secondary battery according to claim 8, characterized by The nitrogen-containing additive comprises at least one of nitrile compound, nitrogen-containing lithium salt, pyridine and amide; the nitrile compound comprises at least one of dinitrile, phosphorus-containing nitrile compound, fluorine-containing nitrile compound and silicon-containing nitrile compound.

11. The lithium iron manganese phosphate secondary battery according to claim 1, characterized in that, The electrolyte comprises lithium salt, and the lithium salt comprises lithium hexafluorophosphate.

12. The lithium manganese iron phosphate secondary battery according to claim 1, wherein the electrolyte comprises organic solvent, and 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.

13. The lithium manganese iron phosphate secondary battery according to claim 1, wherein the electrolyte comprises other additive, and the other additive comprises at least one of vinylene carbonate, fluoroethylene carbonate, vinyl sulfate, methylene methane disulfonate and tris(trimethylsilyl) phosphate.

14. The lithium iron manganese phosphate secondary battery according to claim 1, characterized by, The lithium iron manganese phosphate secondary battery further comprises a separator, and a material of the separator is polyethylene or polypropylene.

15. The lithium iron manganese phosphate secondary battery according to any one of claims 1 to 14, characterized by, The lithium iron manganese phosphate secondary battery has a thickness expansion rate of less than 3% after 700 cycles at 60 DEG C.

16. A production method for producing the lithium iron manganese phosphate secondary battery according to any one of claims 1 to 15, characterized by, Comprise: The lithium iron manganese phosphate positive active material is prepared into a positive electrode sheet, and an electrolyte is used to form a lithium iron manganese phosphate secondary battery for electrochemical activation treatment, so as to form a solid electrolyte interface film on the surface of the lithium iron manganese phosphate positive active material, and the electrochemical activation treatment is at least one cycle of activation treatment. The electrolyte comprises a nitrogen-containing additive.

17. The method of claim 16, wherein the method further comprises, The rate of the electrochemical activation treatment is 0.05-0.5 C, and the temperature of the electrochemical activation treatment is 35-45 DEG C.

18. The method of claim 16, wherein, The mass of the nitrogen-containing additive is 0.2%-3% of the total mass of the electrolyte.

19. The method of claim 16, wherein, The nitrogen-containing additive comprises at least one of a nitrile compound, a nitrogen-containing lithium salt, pyridine and an amide; and the nitrile compound comprises at least one of a dinitrile, a phosphorus-containing nitrile compound, a fluorine-containing nitrile compound and a silicon-containing nitrile compound. 20.A power consumption device comprising a lithium iron manganese phosphate secondary battery, which comprises the lithium iron manganese phosphate secondary battery according to any one of claims 1 to 15 or the lithium iron manganese phosphate secondary battery prepared by the preparation method according to any one of claims 16 to 19.

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