Lithium manganese iron phosphate secondary battery and device

Abstract

The invention discloses a lithium manganese iron phosphate secondary battery and device, and relates to the technical field of lithium manganese iron phosphate batteries. The lithium manganese iron phosphate secondary battery comprises an electrolyte and a positive pole piece, wherein the positive pole piece comprises a lithium manganese iron phosphate positive pole material; the electrolyte comprises an additive and an organic solvent, the additive comprises vinylene carbonate, and the mass of the vinylene carbonate accounts for 0.8%-1.7% of the total mass of the electrolyte; the organic solvent comprises ethylene carbonate, and the mass of the ethylene carbonate is 10-19% of the total mass of the electrolyte; the mass content of the fluorine element in the electrolyte is 7.5%-12.75%; the mass content of fluoroethylene carbonate in the electrolyte is less than 10 ppm, and the mass content of boron is less than 10 ppm. According to the invention, the contents of vinylene carbonate, ethylene carbonate and total fluorine element in the electrolyte are accurately controlled, and meanwhile, the contents of fluoroethylene carbonate and boron element are strictly limited to be less than a preset threshold value, so that the dissolution of manganese ions is effectively inhibited, and the cycle life of the battery at high voltage and high temperature is remarkably prolonged.

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 secondary battery and device. BACKGROUND

[0002] In the current energy storage field, lithium manganese iron phosphate (LMFP) cathode materials have attracted widespread attention due to their high energy density and cost-effectiveness. Compared with lithium iron phosphate (LFP), LMFP has a higher voltage platform, which not only improves the energy density of the battery, but also helps to more accurately estimate the state of charge (SOC) of the battery, thereby improving the accuracy of the battery management system (BMS) and the safety of the battery. However, the cycle stability of LMFP under high-voltage charging and high-temperature environments is a technical challenge, because manganese ions (Mn 2+ ) are easily dissolved under high-voltage charging and high-temperature environments, which in turn leads to rapid degradation of battery performance.

[0003] To solve the problem of cycle stability of LMFP under high-voltage charging and high-temperature, the traditional method includes adding various additives such as fluoroethylene carbonate (FEC) and boron-containing compounds in the electrolyte, which are believed to inhibit manganese dissolution and form a LiF-rich SEI film on the negative electrode surface, thereby improving the cycle life of the battery.

[0004] Therefore, the industry generally believes that FEC or boron-containing compounds are essential for LMFP and are the preferred additives for improving the cycle stability of the battery. However, even with the addition of the above additives, the cycle life of LMFP under high-voltage charging and high-temperature is still lower than that of lithium iron phosphate cathode batteries.

[0005] 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 common knowledge. SUMMARY

[0006] The application provides a lithium manganese iron phosphate secondary battery and device, which at least solves the technical problem of poor cycle stability of the prior art lithium manganese iron phosphate secondary battery under high-voltage and high-temperature conditions, leading to rapid degradation of battery performance.

[0007] In order to achieve the above-mentioned purpose, in a first aspect, the present application provides a lithium iron manganese phosphate secondary battery, comprising an electrolyte and a positive electrode sheet, the positive electrode sheet comprising a lithium iron manganese phosphate positive electrode material; the electrolyte comprising an additive, an organic solvent, the additive comprising vinylene carbonate, the mass of the vinylene carbonate being 0.8% to 1.7% of the total mass of the electrolyte; the organic solvent comprising ethylene carbonate, the mass of the ethylene carbonate being 10% to 19% of the total mass of the electrolyte; the mass content of fluorine in the electrolyte being 7.5% to 12.75%; the mass content of fluoroethylene carbonate in the electrolyte being less than 10 ppm, and the mass content of boron being less than 10 ppm.

[0008] Preferably, the additive further comprises a nitrogen-containing additive, the mass of the nitrogen-containing additive being less than 1% of the total mass of the electrolyte; more preferably, less than 0.5%; even more preferably, less than 0.2%.

[0009] 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.

[0010] Preferably, the additive does not comprise fluoroethylene carbonate.

[0011] Preferably, the additive does not comprise a boron-containing additive.

[0012] Preferably, the additive does not comprise a nitrogen-containing additive.

[0013] Preferably, the mass ratio of the ethylene carbonate and the vinylene carbonate in the electrolyte is 8 to 19.

[0014] Preferably, the additive further comprises one or more of vinylene sulfate, lithium difluorophosphate, tris(trimethylsilyl) phosphate, and vinylene ethylene carbonate.

[0015] Preferably, the organic solvent further comprises one or more of propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate.

[0016] Preferably, the electrolyte further comprises an electrolyte salt, the electrolyte salt being lithium hexafluorophosphate, the mass of the lithium hexafluorophosphate being 10% to 17% of the total mass of the electrolyte.

[0017] Preferably, the fluorine in the electrolyte is provided only by the lithium hexafluorophosphate.

[0018] Preferably, the mass of the ethylene carbonate is 12% to 19% of the total mass of the electrolyte.

[0019] Preferably, the lithium iron manganese phosphate positive electrode 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% to 3% of the total mass of the lithium iron manganese phosphate positive electrode material.

[0020] Preferably, the lithium iron manganese phosphate secondary battery further comprises a negative electrode sheet, 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.

[0021] Preferably, the lithium iron manganese phosphate secondary battery further comprises a separator, and the separator material is polyethylene or polypropylene.

[0022] Preferably, the lithium iron manganese phosphate secondary battery has a capacity retention rate of >80% after 2200 cycles at a 4.25V charging cutoff voltage and 45℃, and a capacity retention rate of >80% after 700 cycles at a 4.25V charging cutoff voltage and 60℃.

[0023] In a second aspect, the application further provides a power consumption device comprising the lithium iron manganese phosphate secondary battery.

[0024] The application has the following beneficial effects: 1. The application precisely controls the content of vinylene carbonate, ethylene carbonate and total fluorine elements in the electrolyte, while strictly limiting the content of fluorinated ethylene carbonate and boron elements to be less than a preset threshold, thereby effectively inhibiting the dissolution of manganese ions and significantly improving the cycle life of the battery under high voltage and high temperature. The lithium iron manganese phosphate secondary battery of the application has a capacity retention rate of >80% after 2200 cycles at 4.25V / 45℃, and a capacity retention rate of >80% after 700 cycles at 4.25V / 60℃.

[0025] 2. The application discards the traditional fluorinated ethylene carbonate and boron-containing additives, reduces the antagonistic effect between electrolyte components, thereby improving the uniformity of the SEI film, reducing the internal resistance of the battery, and improving the overall performance of the battery.

[0026] 3、The application controls the type of additives in the electrolyte, controls the total fluorine content to make the system in a low HF generation critical state, thereby reducing the corrosion of the electrolyte to the electrode interface, effectively avoiding the risk of thermal runaway caused by the concentrated release of manganese ions at high temperature, so that the battery can also maintain good electrical performance under high temperature conditions, significantly improving the safety performance of the battery under high pressure and high temperature conditions.

[0027] 4、The application simplifies the additive system, thereby avoiding the addition of expensive fluoroethylene carbonate and boron-containing additives, reducing the production cost of the electrolyte, and improving the performance of the battery, achieving dual optimization of cost and performance. BRIEF DESCRIPTION OF DRAWINGS

[0028] 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, and 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, and for those skilled in the art, other related drawings can also be obtained without creative labor on the basis of these drawings.

[0029] Figure 1 The cycle performance curve of the battery of the application embodiment 1 at 45 DEG C, 4.25V charge cut-off voltage; Figure 2 The cycle performance curve of the battery of the application embodiment 1 at 60 DEG C, 4.25V charge cut-off voltage. DETAILED DESCRIPTION

[0030] In the application, the orientation words such as "up, down, left, right" are generally understood in connection with the orientation shown in the drawings and the actual application, unless otherwise stated.

[0031] 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 indicated technical features. Therefore, the features defined with "first", "second" can explicitly or implicitly include one or more features. In the description of the application, the meaning of "multiple" is two or more, unless otherwise specifically limited.

[0032] In the present disclosure, unless specifically stated and limited otherwise, a first feature is "on" or "under" a second feature can mean that the first and second features are in direct contact, or that the first and second features are in indirect contact with an intervening medium. Also, a first feature "over", "above", and "on top of" a second feature can mean that the first feature is directly above or obliquely above the second feature, or simply means that the first feature is horizontally higher than the second feature. A first feature "under", "below", and "underneath" a second feature can mean that the first feature is directly below or obliquely below the second feature, or simply means that the first feature is horizontally lower than the second feature.

[0033] The endpoints of the ranges and any values disclosed herein are not limited to the precise values recited as implicitly split into this precise range unless otherwise indicated. For values which are ostensibly dichotomous, e.g., male or female, each member of the dichotomy is also therefore individually and explicitly disclosed. For values which are stated as being "greater than" or "less than" a stated value, the endpoint of the range is also expressly stated. Any numerical values include increments of one unit, e.g., 1, 2, 3, etc. Any numerical range includes increments of one unit, e.g., 1-6 means 1, 2, 3, 4, 5, or 6. Any numerical range includes increments of 0.1, e.g., 1-1.2 means 1, 1.1, or 1.2, and so forth. Any numerical range includes increments of 0.01, and so forth. Any numerical range includes increments of 0.001, and so forth. Any numerical range includes increments of 0.0001, and so forth. Any numerical range includes increments of 1%, 0.1%, 0.01%, and so forth. Any numerical range includes increments of 1 / 1000th, 1 / 10,000th, 1 / 1,000,000th, and so forth. Any numerical range includes increments of 1%, 0.1%, 0.01%, and so forth. Any numerical range includes increments of 1 / 1000th, 1 / 10,000th, 1 / 1,000,000th, and so forth. The terms "about" and "approximately" used herein denote an interval of accuracy of ±1%, ±10%, ±0.1%, ±0.01%, ±1 / 1000th, ±1 / 10,000th, ±1 / 1,000,000th, etc.

[0034] The present disclosure provides a lithium iron manganese phosphate secondary battery, comprising an electrolyte and a positive electrode sheet, the positive electrode sheet comprising a lithium iron manganese phosphate positive electrode material; the electrolyte comprising an additive, an organic solvent, the additive comprising vinylene carbonate, the mass of the vinylene carbonate being 0.8%-1.7% of the total mass of the electrolyte; the organic solvent comprising ethylene carbonate, the mass of the ethylene carbonate being 10%-19% of the total mass of the electrolyte; the mass content of fluorine in the electrolyte being 7.5%-12.75%; the mass content of fluoroethylene carbonate in the electrolyte being less than 10 ppm, and the mass content of boron being less than 10 ppm.

[0035] Preferably, the mass of the vinylene carbonate is 0.8%-1.7% of the total mass of the electrolyte, which can be 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, and any value between them.

[0036] Vinylene carbonate (VC) is a new type of organic film-forming additive and overcharge protection additive for lithium ion batteries, which has good high and low temperature performance and anti-gas expansion function, and can improve the capacity and cycle life of the battery.

[0037] Preferably, the mass of the ethylene carbonate is 10%-19% of the total mass of the electrolyte, which can be 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, and any value between them. More preferably, the mass of the ethylene carbonate is 12%-19% of the total mass of the electrolyte.

[0038] Ethylene carbonate (EC) is an excellent organic solvent with good performance, which can dissolve various polymers and can be used as an excellent solvent for lithium battery electrolyte.

[0039] Preferably, the mass content of fluorine in the electrolyte is 7.5% to 12.75%, which can be 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, 10.5%, 11%, 11.5%, 12%, 12.5%, 12.75% and any value between them.

[0040] Preferably, the mass content of fluorinated ethylene carbonate in the electrolyte is less than 10 ppm. More preferably, the additive does not contain fluorinated ethylene carbonate.

[0041] Fluorinated ethylene carbonate (FEC) is one of the most widely used organic film-forming additives at present, which is reported to be an additive that can effectively inhibit the dissolution of manganese iron lithium phosphate. Current research shows that it can on the one hand stabilize the positive electrode interface and reduce the migration of Mn 2+ to the negative electrode, and on the other hand can be preferentially reduced on the surface of the negative electrode (such as graphite or silicon-carbon material) to form a SEI film rich in LiF, so it is considered to be able to effectively improve the high-voltage stability and improve the cycle life of the battery.

[0042] Preferably, the mass content of boron in the electrolyte is less than 10 ppm. More preferably, the additive does not contain a boron-containing additive.

[0043] Boron-containing additives, such as LiBOB (lithium bisoxalate borate) and LiODFB (lithium difluoro oxalate borate), are also reported to be able to improve the cycle life of manganese iron lithium phosphate positive electrode batteries. The possible principles include complexing metal ions, constructing stable CEI / SEI and inhibiting HF corrosion, etc.

[0044] The present application unexpectedly found that when the content of fluorine in the organic solvent, the content of fluorine in the vinylene carbonate of the additive and the content of fluorine in the entire electrolyte solution are all at a low level, an unexpectedly high high-temperature and high-voltage cycle efficiency is obtained. On the contrary, the additional addition of fluorinated ethylene carbonate or boron-containing additives commonly used in the prior art will cause a sharp decline in cycle life.

[0045] The electrolyte composition of the present application makes the battery have excellent high-temperature and high-voltage cycle efficiency, and the possible reason is that in the manganese iron lithium phosphate system, the addition amount of vinylene carbonate (VC) is controlled at 0.8% to 1.7%, and the addition amount of ethylene carbonate (EC) is controlled at 10% to 19%, and a dense CEI film is formed through the synergistic effect of the molecular level electrochemical behavior and the solvation environment, thereby directly intercepting Mn 2+dissolution. Specifically, ethylene carbonate (EC) as a high dielectric constant solvent, preferentially participates in the formation of lithium ion solvation sheath, leading to partial VC being expelled to the positive electrode interface due to low solvation ability. In the manganese iron phosphate lithium high-voltage system (charge cut-off voltage 4.25V) of the present application, the positive electrode surface is in a strong oxidation environment, and the dissolved Mn 2+ Local catalytic active sites are formed on the positive electrode surface, and the vinyl group (-CH=CH2) of VC and the carbonyl group (C=O) of EC copolymerize to form a cross-linked network structure, forming a dense CEI film, the intermolecular chain gap of which can physically block the diffusion of Mn 2+ , and the carbonyl oxygen atoms enriched in the film layer chemically anchor the dissolved Mn 2+ .

[0046] Further, VC can initiate an effective ring-opening polymerization reaction when the mass content is greater than 0.8%, and below this amount, the polymerization is discontinuous and easy to cause the CEI film porosity to be too large, affecting the Mn 2+ blocking effect, and the mass content of VC is less than 1.7% to avoid excessive VC self-polymerization causing the film layer to be too thick and the internal resistance to increase sharply; EC can optimize the structure of the lithium ion solvation sheath when the mass content is greater than 10% to promote the directional reaction of VC at the positive electrode interface, and the mass content of EC is less than 19% to avoid the competitive decomposition of solvent molecules destroying the density of the CEI film.

[0047] Further, under high temperature conditions, the VC derivative in the CEI film will undergo a glass transition, and after the molecular chain segment rearranges, a more dense topological structure is formed, effectively improving the blocking ability.

[0048] Further, vinylene carbonate (VC) at the negative electrode interface can also form an SEI film through a two-electron reduction ring-opening polymerization, which is completely independent of the formation of the positive electrode CEI film, and its vinyl group (-CH=CH2) is catalyzed by the graphite surface to undergo a free radical reaction to generate a cross-linked network containing a polyvinyl carbonate skeleton ( -[CH2-CH2-O-CO-O]-) and Li2CO3 grains. This SEI film can effectively inhibit the continuous decomposition of the electrolyte, and its chemical stability can resist structural relaxation at high temperatures.

[0049] Further, the mass content of fluorine in the electrolyte is controlled at 7.5% to 12.75% to balance the LiPF6 dissociation reaction (LiPF6 LiF+PF5) and the PF5 hydrolysis path (PF5+H2O→POF3+2HF), dynamically suppressing the HF concentration to a critical safety threshold. The lower limit of the mass content of fluorine is 7.5% to ensure sufficient PF6 -The ion inhibits the dissociation of LiPF6, and the upper limit of the mass content of fluorine element is 12.75%, which blocks the water absorption multiplication effect of LiPF6 and reduces the generation of HF from the source; meanwhile, the low HF environment makes the carbonyl-Mn 2+ The coordination bond binding energy remains stable, guarantees the anchoring performance of manganese, and prevents the loss of Li2CO3 binder phase, which leads to the increase of the porosity of the negative electrode SEI film.

[0050] When the electrolyte is additionally added with the fluorinated ethylene carbonate or the boron-containing additive commonly used in the prior art, the cycle life is sharply reduced, and the possible reason is that the FEC and the boron-containing additive will occupy the negative electrode SEI film formation site, and competitive decomposition of the FEC and the boron-containing additive with VC will lead to uneven SEI film structure and increased internal resistance. Meanwhile, the FEC and the boron-containing additive will form Mn 2+ The heat-unstable complex will release Mn 2+ at high temperatures, and the sharply increased Mn 2+ On the one hand, the CEI film anchoring site is saturated, which leads to the rapid increase of the porosity of the CEI film, and on the other hand, the MnCO3 / MnF2 insulating layer is generated along the gap of the SEI film, which seriously blocks the ion channel. In addition, the HBF4 generated by the decomposition of the boron-containing additive will be hydrolyzed, which leads to the rapid increase of the HF content, the dissolution of the interface binder phase, and finally the collapse of the CEI / SEI synergy, which leads to the sharp reduction of the cycle life.

[0051] Preferably, the additive further comprises a nitrogen-containing additive, and the mass of the nitrogen-containing additive is less than 1% of the total mass of the electrolyte, which can be 0, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1% and any value between them. More preferably, less than 0.5%; more preferably, less than 0.2%.

[0052] Preferably, the nitrogen-containing additive comprises at least one of a nitrile compound, a nitrogen-containing lithium salt, pyridine, and an amide.

[0053] Preferably, 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. More preferably, the nitrile compound comprises at least one of 1-(2-cyanoethyl)pyrrole (CP), adiponitrile (ADN), and succinonitrile (SN).

[0054] Most preferably, the additive does not contain a nitrogen-containing additive.

[0055] The mass content of the nitrogen-containing additive is controlled at a low level or even not added at all, which helps to ensure the long cycle stability of the battery under high voltage and high temperature conditions. The possible reason is that in the electrolyte system constructed in the application, vinylene carbonate (VC) as the dominant film-forming additive can form a thin and dense interface film (CEI / SEI) with ethylene carbonate (EC) and other solvents at the interface of the positive and negative electrodes, which has excellent ion conductivity and structural stability. The interface film can effectively inhibit the dissolution of manganese ions, reduce the continuous decomposition of the electrolyte, and maintain a low interface impedance. If excessive nitrogen-containing additives are introduced, they have strong coordination ability and reduction potential, which may compete with VC for reduction reaction, interfere with the VC-dominated film-forming process, and result in the formation of a mixed interface film with uneven structure, high impedance or poor mechanical properties. Such a mixed film not only increases the lithium ion migration energy barrier and reduces the rate performance, but also may not effectively block the migration of manganese ions and the penetration of electrolyte due to its loose structure or poor chemical stability, thereby accelerating the capacity decay of the battery, especially under high temperature and high voltage conditions.

[0056] Preferably, the mass ratio of ethylene carbonate (EC) to vinylene carbonate (VC) in the electrolyte is 8-19, which can be 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 and any value between them.

[0057] The mass ratio of ethylene carbonate (EC) to vinylene carbonate (VC) in the electrolyte is controlled at 8-19, mainly to achieve the best balance between battery performance and stability. If the content of VC is too high and the content of EC is too low, although it can promote the formation of a denser negative solid electrolyte interface film, the thick SEI film will increase the interface impedance, which will lead to capacity decay and rate performance decline. On the contrary, if the content of EC is too high and the content of VC is too low, the SEI film may not be stable enough to effectively inhibit the continuous decomposition of the electrolyte, resulting in shortened cycle life and reduced capacity retention. Therefore, the ratio design can coordinate the solvation ability of EC and the film-forming properties of VC to form a stable and low-impedance SEI film while ensuring the ionic conductivity, thereby improving the cycle stability, rate performance and high-temperature performance of the battery.

[0058] Preferably, the additive further includes one or more of vinyl sulfite (DTD), lithium difluorophosphate (LiPO2F2), tris(trimethylsilyl) phosphate (TMSP) and vinyl ethylene carbonate (VEC).

[0059] Preferably, the organic solvent further includes one or more of propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl ethyl carbonate (EMC), methyl propyl carbonate (MPC) and ethyl propyl carbonate (EPC).

[0060] Preferably, the electrolyte further comprises an electrolyte salt, and the electrolyte salt is lithium hexafluorophosphate, and the mass of the lithium hexafluorophosphate accounts for 10% to 17% of the total mass of the electrolyte.

[0061] Further, the electrolyte of the present application preferably comprises lithium hexafluorophosphate (LiPF6) as an electrolyte salt, and the mass of the lithium hexafluorophosphate accounts for 10% to 17% of the total mass of the electrolyte. This design is based on the excellent comprehensive performance of lithium hexafluorophosphate in conventional carbonate solvent systems. LiPF6 is the most mature and widely used electrolyte salt in currently commercialized lithium ion batteries. It has high solubility and ionic conductivity in commonly used organic carbonate solvents (such as EC, DMC, DEC, EMC), and can form stable interface films (SEI / CEI) with graphite negative electrodes and high-voltage positive electrode materials. These characteristics are crucial to ensure the high-rate charge-discharge performance, long cycle life and safety of the battery.

[0062] Preferably, the fluorine element in the electrolyte is only provided by the lithium hexafluorophosphate.

[0063] Further, the fluorine element is limited to only come from the lithium hexafluorophosphate, which aims to accurately control the fluorine content and form in the electrolyte, avoid unpredictable side reactions and complex interface chemical interference that may be caused by the introduction of other fluorine-containing lithium salts or additives, and ensure that the specific fluorides (such as LiF) produced by the decomposition of lithium hexafluorophosphate play a core role in building a stable and uniform electrode / electrolyte interface, thereby optimizing the overall electrochemical performance of the battery.

[0064] Preferably, the lithium iron manganese phosphate positive electrode 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% to 3% of the total mass of the lithium iron manganese phosphate positive electrode material.

[0065] In the present application, in the chemical formula of the lithium iron manganese phosphate base, 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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 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 the doping element, and the material structure is optimized by 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.

[0071] 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 material, for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, and any value between them. In the present application, the carbon content meeting the above range can balance the conductivity and specific surface area of the positive electrode material, thereby improving the electrochemical performance, compaction density and interface stability of the positive electrode material.

[0072] In the present application, the lithium iron manganese phosphate secondary battery further comprises a negative electrode sheet, and 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.

[0073] 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, 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.

[0074] In some embodiments, the negative electrode tab further includes a 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.

[0075] In the present application, a separator is provided between the positive electrode tab and the negative electrode tab to prevent short circuiting. The material and shape of the separator that can be used 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 includes a polymer or an inorganic substance formed of a material stable to the electrolyte solution of the present application, or the like.

[0076] For example, the separator 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.

[0077] 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.

[0078] 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.

[0079] 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).

[0080] Preferably, the capacity retention of the lithium iron manganese phosphate secondary battery is >80% after 2200 cycles at a 4.25V charge cut-off voltage and 45℃, and >80% after 700 cycles at a 4.25V charge cut-off voltage and 60℃.

[0081] Due to the innovation of the technical solution, it can be widely applied in electric vehicles, energy storage systems, portable electronic devices and other application fields. With the development of technology, the energy density and cycle life of batteries in these fields are increasingly demanding. The technical solution can effectively improve the energy density and cycle life of lithium iron manganese phosphate batteries, meeting the performance requirements of batteries in these fields. At the same time, the development of electric vehicles also puts forward higher requirements for the high-temperature stability and cycle life of batteries under high-voltage platform. The application of the technical solution can improve the endurance and safety of electric vehicles, promoting the development of electric vehicles. In addition, as an important part of new energy, energy storage systems also have high requirements for the high-temperature stability and cycle life of batteries. The application of the technical solution can improve the energy storage efficiency and reliability of energy storage systems, promoting the development of new energy. Therefore, the technical solution has broad market demand and good application prospect.

[0082] The application also provides a power utilization device comprising the lithium iron manganese phosphate secondary battery.

[0083] In some embodiments, the power utilization device includes but is not limited to electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, energy storage systems, etc. In order to meet the high power and high energy density requirements of the power utilization device for lithium iron manganese phosphate secondary batteries, battery packs or battery modules can be used.

[0084] In other embodiments, the power utilization device can be a mobile phone, tablet computer, notebook computer, etc. The power utilization device usually requires thinning, and lithium iron manganese phosphate secondary batteries can also be used as power sources.

[0085] The application will be described in detail below by way of examples, which are exemplary and do not constitute any limitation on the application.

[0086] 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 the positive electrode material lithium iron manganese phosphate, the negative electrode material, and the electrolyte. The positive electrode material is LiFe 0.4 Mn 0.6 PO4 / C, the content of C is 1.5%; the negative electrode material is graphite.

[0087] Step 2: coat the positive electrode material and the negative electrode material on aluminum foil and copper foil respectively to make positive electrode sheets and negative electrode sheets.

[0088] Step 3: Assembling the battery by using the positive electrode sheet, the negative electrode sheet, the separator and the electrolyte.

[0089] The compositions of the electrolytes in the following examples and comparative examples are shown in Table 1.

[0090] Table 1

[0091] According to Table 1, the mass content of fluorine element in the electrolyte in all examples is between 7.5% and 12.75%, wherein the mass content of fluorine element in the electrolyte of Example 1 is 12.75%, the mass content of fluorine element in the electrolyte of Example 3 is 7.5%, the mass content of fluorine element in the electrolyte of Example 8 is about 10% (both LiPF6 and LiPO2F2 in Example 8 provide fluorine element), and the mass content of fluorine element in the electrolyte of all other examples is 9.375%.

[0092] The mass content of fluorine element in the electrolyte of Comparative Example 1 is about 9.7%, the mass content of fluorine element in the electrolyte of Comparative Example 3 is about 22.5%, and the mass content of fluorine element in the electrolyte of all other comparative examples is 9.375%.

[0093] Test Example The lithium manganese iron phosphate secondary batteries prepared by using the electrolytes of the examples and comparative examples were subjected to the following verification tests. 1. First cycle performance test: The test was carried out under the condition of a test thermostat at 45°C, and the test steps were as follows: S1, 1C constant current charging to 4.25V, then constant voltage charging to the cutoff current 0.05C, and standing for 30 minutes; S2, 1C constant current discharging to 2.5V, and standing for 30 minutes; S3, repeating steps S1 and S2, and the cycle number was 2200.

[0094] 2. Second cycle performance test: The test was carried out under the condition of a test thermostat at 60°C, and the test steps were as follows: S1, 1C constant current charging to 4.25V, then constant voltage charging to the cutoff current 0.05C, and standing for 30 minutes; S2, 1C constant current discharging to 2.5V, and standing for 30 minutes; S3, repeating steps S1 and S2, and the cycle number was 700.

[0095] The cycle test results are shown in Table 2, and the specific test method of the battery capacity can refer to GB / T 31486-2015. The capacity retention rate after 2200 cycles at 45℃ is calculated according to the following formula: discharge capacity after 2200 cycles / initial cycle discharge capacity x 100%.

[0096] The capacity retention rate after 700 cycles at 60℃ is calculated according to the following formula: discharge capacity after 700 cycles / initial cycle discharge capacity x 100%.

[0097] Table 2

[0098] From the cycle test results of the examples in Table 2, when the mass content of VC, the mass content of EC, and the mass content of fluorine element are in the preferred values, and when the fluorine element is only provided by lithium hexafluorophosphate, it is beneficial to improve the capacity retention rate of the battery under high temperature and high voltage.

[0099] From the test results of the comparative examples and the examples, when the fluorinated ethylene carbonate or the boron-containing additive commonly used in the prior art is additionally added to the electrolyte of the application, it will cause a sharp decline in cycle life, and the attenuation is additional obvious when the boron-containing additive is added. At the same time, when the content of VC, EC or fluorine element exceeds the preset range, the battery performance will also deteriorate.

[0100] Specifically, Figure 1 The cycle performance curve of the battery of Example 1 under the condition of 45℃ and 4.25V charging cutoff voltage is shown. As shown in the figure, the capacity retention rate of the battery after 2353 cycles is 81.56%, and the capacity retention rate after 2200 cycles is 83.08%.

[0101] Figure 2 The cycle performance curve of the battery of Example 1 under the condition of 60℃ and 4.25V charging cutoff voltage is shown. As shown in the figure, the capacity retention rate of the battery after 750 cycles is 79.44%, and the capacity retention rate after 700 cycles is 82.05%.

[0102] Figure 1 And Figure 2 It is directly verified that the battery of the application has excellent cycle stability under the condition of high temperature and high voltage. The capacity retention rate after 2200 cycles at 45℃ is still higher than 80%, and the capacity retention rate after 700 cycles at 60℃ is also higher than 80%. This shows that by accurately controlling the composition of the electrolyte (such as the content of VC, EC, and fluorine), and strictly limiting the content of FEC and boron-containing additives, the application effectively suppresses the dissolution of manganese ions, significantly improves the cycle life of the battery under harsh conditions, and achieves the expected technical effect.

[0103] 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 manganese iron phosphate secondary battery, characterized in that: The lithium iron manganese phosphate secondary battery comprises an electrolyte and a positive electrode plate, wherein the positive electrode plate comprises a lithium iron manganese phosphate positive electrode material; The electrolyte includes an additive, wherein the additive includes vinylene carbonate, and the mass of the vinylene carbonate is 0.8% to 1.7% of the total mass of the electrolyte; The electrolyte further includes an organic solvent, wherein the organic solvent includes ethylene carbonate, and the mass of the ethylene carbonate is 10% to 19% of the total mass of the electrolyte; The mass content of fluorine in the electrolyte is 7.5% to 12.75%; The mass content of fluoroethylene carbonate in the electrolyte is less than 10 ppm, and the mass content of boron is less than 10 ppm.

2. The lithium manganese iron phosphate secondary battery according to claim 1, characterized in that The additives also include nitrogen-containing additives, and the mass of the nitrogen-containing additives is less than 1% of the total mass of the electrolyte.

3. The lithium manganese iron phosphate secondary battery according to claim 2, characterized in that: The mass of the nitrogen-containing additive is less than 0.5% of the total mass of the electrolyte.

4. The lithium manganese iron phosphate secondary battery according to claim 3, characterized in that: The mass of the nitrogen-containing additive is less than 0.2% of the total mass of the electrolyte.

5. The lithium manganese iron phosphate secondary battery according to claim 2, characterized in that: The nitrogen-containing additive includes at least one of a nitrile compound, a nitrogen-containing lithium salt, pyridine, and an amide; the nitrile compound includes at least one of a dinitrile, a phosphazene compound, a fluorine-containing nitrile compound, and a silicon-containing nitrile compound.

6. The lithium manganese iron phosphate secondary battery according to claim 1, characterized in that: The additive does not contain fluoroethylene carbonate; and / or, the additives do not include boron-containing additives; And / or, the additives do not include nitrogen-containing additives.

7. The lithium manganese iron phosphate secondary battery according to claim 1, characterized in that: The mass ratio of ethylene carbonate to vinylene carbonate in the electrolyte is 8-19.

8. The lithium manganese iron phosphate secondary battery according to claim 1, characterized in that: The additive further comprises one or more of vinyl sulfate, lithium difluorophosphate, tris(trimethylsilyl)phosphate, and vinyl ethylene carbonate.

9. The lithium manganese iron phosphate secondary battery according to claim 1, characterized in that: The organic solvent further comprises one or more of propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate.

10. The lithium manganese iron phosphate secondary battery according to claim 1, characterized in that: The electrolyte further includes an electrolyte salt, which is lithium hexafluorophosphate. The mass of the lithium hexafluorophosphate accounts for 10% to 17% of the total mass of the electrolyte.

11. The lithium manganese iron phosphate secondary battery according to claim 10, characterized in that: The fluorine element in the electrolyte is provided only by lithium hexafluorophosphate.

12. The lithium manganese iron phosphate secondary battery according to claim 1, characterized in that: The mass of ethylene carbonate is 12% to 19% of the total mass of the electrolyte.

13. The lithium manganese iron phosphate secondary battery according to claim 1, characterized in that: The lithium iron manganese phosphate positive electrode material includes a lithium iron manganese phosphate matrix and a carbon coating layer coated on the surface of the lithium iron manganese 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 material.

14. The lithium manganese iron phosphate secondary battery according to claim 1, characterized in that: The lithium manganese iron phosphate secondary battery further includes a negative electrode plate, which includes a negative electrode active material. 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.

15. The lithium manganese iron phosphate secondary battery according to claim 1, characterized in that: The lithium manganese iron phosphate secondary battery further includes a diaphragm, and the diaphragm material is polyethylene or polypropylene.

16. The lithium manganese iron phosphate secondary battery according to any one of claims 1 to 15, characterized in that: The capacity retention rate of the lithium manganese iron phosphate secondary battery is greater than 80% after 2200 cycles at a charge cut-off voltage of 4.25V and 45°C, and the capacity retention rate of the lithium manganese iron phosphate secondary battery is greater than 80% after 700 cycles at a charge cut-off voltage of 4.25V and 60°C. 17 . An electric device comprising a lithium iron manganese phosphate secondary battery, comprising the lithium iron manganese phosphate secondary battery according to claim 1 .

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