A lithium iron phosphate battery

Abstract

The application belongs to the technical field of batteries, and particularly relates to a high-safety lithium iron phosphate battery. The lithium iron phosphate battery comprises a positive electrode sheet and a negative electrode sheet; the positive electrode sheet comprises lithium iron phosphate; a ratio of a curve area between a voltage of 4.5-5.5 V and a curve area between a voltage of 3.3-5.5 V in an overcharge curve obtained by charging the lithium iron phosphate battery at a constant current of 1000 mA is a; a content of Fe element in the negative electrode sheet is b ppm; a x 10 4 / b is 0.5-89. Compared with the prior art, the application improves the safety of the battery while taking into account the fast-charging performance by comprehensively controlling the relationship between a and b.

Description

Technical Field

[0001] This invention belongs to the field of battery technology, and in particular relates to a high-safety lithium iron phosphate battery. Background Technology

[0002] Lithium-ion batteries, as core components of various electric drive products, possess advantages such as high operating voltage, long cycle life, no memory effect, and environmental friendliness, and are widely used in electric vehicles, portable electronic products, drones, and other fields. The most important concern is that batteries, while ensuring safe use, can withstand high-rate charging and discharging and have a long service life.

[0003] Lithium iron phosphate batteries have many advantages, including high energy density, long lifespan, and environmental friendliness, making them one of the mainstream batteries in new energy vehicles and energy storage systems. Their basic principle is that ions are transferred between two electrodes via an electrolyte, creating a potential difference between the positive and negative electrodes, thereby generating current.

[0004] During use, lithium iron phosphate batteries may generate gases due to various factors, such as overcharging. Oxygen is the primary gas produced, originating from the decomposition of the electrolyte. Hydrogen and carbon dioxide may also be generated under certain conditions, but in relatively smaller quantities. If these gases generated inside the battery are not addressed promptly, they can lead to increased internal pressure and temperature, thus affecting the overall performance of the battery. In extreme cases, they may even cause safety accidents such as battery meltdown or explosion. Summary of the Invention

[0005] In view of this, the technical problem to be solved by the present invention is to provide a high-safety lithium iron phosphate battery that also has good fast-charging performance.

[0006] This invention provides a lithium iron phosphate battery, comprising a positive electrode and a negative electrode; the positive electrode comprises lithium iron phosphate; the ratio of the area under the overcharge curve between 4.5 and 5.5 V to the area under the overcharge curve between 3.3 and 5.5 V obtained by charging the lithium iron phosphate battery with a constant current of 1000 mA is 'a'; the Fe element content in the negative electrode is 'b' ppm; a × 10 4 / b ranges from 0.5 to 89.

[0007] Preferably, the a×10 4 / b is 3~20.

[0008] Preferably, a is 0.1~0.5; and / or, 50≤b≤3000.

[0009] Preferably, a is 0.18~0.27; and / or, 100 <b≤1000。

[0010] Preferably, the average particle size Dv50 of the lithium iron phosphate is 0.5~1.4 μm.

[0011] Preferably, the lithium iron phosphate is further doped with a doping element; the doping element is a transition metal element; and the doping amount of the doping element is 2000~6000 ppm.

[0012] Preferably, the surface of the lithium iron phosphate is further provided with a coating layer; the coating layer is a carbon layer; the thickness of the carbon layer is 10~100 nm.

[0013] Preferably, the positive electrode sheet includes a positive current collector and a positive active material layer attached to at least one surface of the positive current collector; the powder resistivity of the positive active material layer is 5~50 Ω·cm.

[0014] Preferably, the ratio of the negative electrode capacity to the positive electrode capacity of the lithium iron phosphate battery, N / P, is 1 to 1.03.

[0015] Preferably, the negative electrode sheet includes a negative electrode active material; the negative electrode active material is selected from one or more of graphite, silicon, silicon-oxygen materials and silicon-carbon materials; the average particle size Dv50 of the negative electrode active material is 5~20 μm.

[0016] Preferably, the lithium iron phosphate battery further includes an electrolyte; the electrolyte includes an organic solvent and a film-forming additive; the organic solvent includes a cyclic carbonate; the mass of the cyclic carbonate is 20% to 40% of the mass of the organic solvent;

[0017] And / or, the film-forming additive includes one or more of propylene carbonate, vinylene carbonate, and fluoroethylene carbonate; the mass of the film-forming additive is 1.5% to 4% of the mass of the electrolyte.

[0018] Preferably, the lithium iron phosphate battery further includes a separator; the separator includes a base film and a coating disposed on at least one surface of the base film; the coating is aluminum oxide; the thickness of the coating is 1~4 μm.

[0019] Preferably, the porosity of the diaphragm is 35% to 50%.

[0020] This invention provides a high-safety lithium iron phosphate battery, comprising a positive electrode and a negative electrode; the positive electrode comprises lithium iron phosphate; the ratio of the area under the overcharge curve between 4.5 and 5.5 V to the area under the overcharge curve between 3.3 and 5.5 V obtained by charging the lithium iron phosphate battery with a constant current of 1000 mA is 'a'; the Fe element content in the negative electrode is 'b' ppm; a × 10 4 / b ranges from 0.5 to 89. Compared with existing technologies, this invention improves battery safety while maintaining fast charging performance by comprehensively controlling the relationship between a and b. Detailed Implementation

[0021] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0022] This invention provides a high-safety lithium iron phosphate battery, comprising a positive electrode and a negative electrode; the positive electrode comprises lithium iron phosphate; the ratio of the area under the overcharge curve between 4.5 and 5.5 V to the area under the overcharge curve between 3.3 and 5.5 V obtained by charging the lithium iron phosphate battery with a constant current of 1000 mA is 'a'; the Fe element content in the negative electrode is 'b' ppm; a × 10 4 / b ranges from 0.5 to 89.

[0023] In this invention, the battery overcharge curve refers to the curve showing that when the battery voltage reaches a set upper limit during the charging process, continued charging causes the battery voltage to continue to rise. This curve is typically used to monitor the battery's charging status and prevent overcharging. 'a' represents the area between 4.5 and 5.5V and the area between 3.3 and 5.5V in the overcharge curve obtained by charging a lithium iron phosphate battery with constant current or constant power. The horizontal axis of the overcharge curve represents time in seconds, and the vertical axis represents voltage. The area between 4.5 and 5.5V in the overcharge curve refers to the area enclosed by the curve and the vertical axis 4.5–5.5V; the area between 3.3 and 5.5V in the overcharge curve refers to the area enclosed by the curve and the vertical axis 3.3–5.5V.

[0024] This invention improves battery safety while maintaining fast-charging performance by comprehensively controlling the relationship between a and b. A higher a / b ratio leads to severe overcharging, increased internal temperature and voltage, and oxidation and decomposition of the electrolyte (e.g., LiPF6) to produce acidic substances like HF. This corrodes metal oxides in the positive electrode material, releasing magnetic impurities like Fe. These magnetic impurities catalyze electrolyte decomposition, producing gases (e.g., CO2, O2), exacerbating cell swelling and gas production. Insufficient Fe content in the negative electrode results in uneven SEI film formation, repeated rupture and regeneration during cycling, accelerated electrolyte decomposition and gas production, increased risk of thermal runaway, and poor battery safety. Conversely, a lower a / b ratio results in a smaller area ratio, fewer magnetic impurities in the positive electrode material (primarily less doping), and poorer Li ion diffusion rate, leading to poorer material kinetics. However, a higher Fe dissolution rate in the negative electrode catalyzes continuous SEI film formation, increasing interfacial impedance and reducing fast-charging performance.

[0025] In one specific embodiment provided by the present invention, optionally, the a×10 4 / b is 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 22, 25, 28, 30, 32, 35, 38, 40, 42, 45, 48, 50, 52, 55, 58, 60, 62, 65, 68, 70, 72, 75, 78, 80, 82, 85, 88, 89, or a range between any two of the above values.

[0026] Furthermore, in a specific embodiment provided by the present invention, the a×10 4 / b ranges from 3 to 26.

[0027] To ensure the performance of lithium iron phosphate batteries, the area under the overcharge curve between 4.5 and 5.5V, i.e., the value 'a', needs to be controlled. In this invention, 'a' is preferably 0.1 to 0.5. Within this range, on the one hand, the catalytic effect of magnetic impurities in the cathode material on electrolyte decomposition can be reduced, gas production can be decreased, and battery safety can be improved; on the other hand, lithium-ion transport can be promoted, improving the battery's fast-charging performance. Optionally, 'a' can be 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, or any two of the above values.

[0028] In one specific embodiment of the present invention, α is preferably 0.18 to 0.27; within this range, lithium iron phosphate batteries have better safety and fast charging performance.

[0029] In a specific embodiment of this invention, 'a' can be controlled by the NP ratio, the content of magnetic impurities in the positive electrode active material, and the coating uniformity of the positive electrode sheet. A higher NP ratio results in excess capacity in the negative electrode, making it easier for the positive electrode to reach a deep delithiation state at the end of charging, leading to a higher overcharge voltage plateau and a larger overcharge curve area ratio. Conversely, a lower NP ratio results in excess capacity in the positive electrode, reducing the amount of lithium delithiation during overcharging, limiting the voltage plateau rise, and decreasing the overcharge curve area ratio. Higher magnetic impurity content catalyzes electrolyte decomposition during charging and discharging, generating HF gas that corrodes the electrode material, increasing the overcharge curve area. Excessive areal density deviation leads to uneven distribution of active material in the localized areas of the electrode sheet. High areal density regions enter the overcharge state prematurely due to insufficient lithium-ion insertion, resulting in a sharp voltage plateau and a larger area ratio; low areal density regions, on the other hand, have a flatter voltage plateau and a smaller area ratio due to underutilized capacity.

[0030] Fe is an active material in lithium iron phosphate batteries, significantly affecting the battery's energy density and cycle life. Lithium iron phosphate (LiFePO4) may release trace amounts of phosphate ions (PO4) during cycling. 3- Iron migrates to the negative electrode surface through the electrolyte, forming a composite interface layer with Fe and Li. On the negative electrode surface, iron can synergistically interact with lithium and phosphorus to achieve efficient charging and discharging of the battery. During the battery manufacturing process, after formation and capacitance, a small amount of iron from the positive electrode material migrates and adsorbs onto the negative electrode surface. Additionally, some residual iron impurities may remain in the negative electrode material. The content of these impurities affects battery performance. If the Fe content in the negative electrode is too low, it may lead to uneven SEI film formation, repeated rupture and regeneration during cycling, accelerated electrolyte decomposition and gas production, and increased risk of thermal runaway. If the Fe content in the negative electrode is too high, it will catalyze the continuous formation of the SEI film, increasing the negative electrode interface impedance and reducing the battery's fast-charging performance. Controlling the Fe content in the negative electrode can improve battery safety and reduce Fe catalysis of the SEI film formation, thereby lowering the negative electrode interface impedance and improving the battery's fast-charging performance. Therefore, in this invention, b ppm is preferably 50 to 3000 ppm; optionally, b ppm is 50 ppm, 100 ppm, 200 ppm, 300 ppm, 400 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm, 1200 ppm, 1400 ppm, 1600 ppm, 1800 ppm, 2000 ppm, 2200 ppm, 2400 ppm, 2600 ppm, 2800 ppm, 3000 ppm or a range between any two of the above values.

[0031] In one specific embodiment of the present invention, the b ppm is preferably 100~1000 ppm; within this range, the lithium iron phosphate battery can have better performance.

[0032] Electrolytes (such as LiPF6) undergo oxidative decomposition to produce acidic substances like HF, which corrode metal oxides in the positive electrode material and release magnetic impurities such as Fe. These magnetic impurities catalyze the decomposition of the electrolyte to produce gases (such as CO2 and O2), exacerbating cell swelling and gas production. Therefore, this process needs to be controlled. In a specific embodiment of this invention, the Fe content in the negative electrode is controlled by adjusting the powder conductivity of the positive electrode active material layer through the doping element content and the coating layer thickness. Doping can stabilize the crystal structure of the material, and coating can reduce side reactions between the positive electrode active material and the electrolyte, thereby reducing iron dissolution. Increased positive electrode conductivity accelerates electron transport, leading to an increase in the positive electrode potential, which intensifies the oxidative dissolution of iron.

[0033] A lithium iron phosphate (LFP) battery is a battery that uses lithium iron phosphate as the positive electrode active material. Its positive electrode sheet includes the positive electrode active material, which comprises lithium iron phosphate. The particle size of lithium iron phosphate directly affects its electrochemical performance. Excessive particle size reduces the lithium-ion transport rate, leading to decreased fast-charging performance. Insufficient particle size increases side reactions with the electrolyte, increasing gas production and reducing battery safety. Therefore, particle size control is necessary. On one hand, this shortens the lithium-ion transport path within the active material, reducing diffusion resistance and thus improving the lithium-ion transport rate. On the other hand, it reduces the specific surface area of ​​the material, further decreasing the contact area between the material and the electrolyte, mitigating side reactions between the active material and the electrolyte, and reducing gas production. In this invention, the average particle size Dv50 of lithium iron phosphate is preferably 0.5~1.4 μm; optionally, the average particle size Dv50 of lithium iron phosphate is 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm or any two of the above values.

[0034] In one specific embodiment of the present invention, the magnetic impurity content in the positive electrode active material is preferably less than or equal to 10 ppm, more preferably 1 to 10 ppm; optionally, the magnetic impurity content in the positive electrode active material is 1 ppm, 2 ppm, 3 ppm, 4 ppm, 5 ppm, 6 ppm, 7 ppm, 8 ppm, 9 ppm, 10 ppm or any two of the above values.

[0035] In one specific embodiment of the present invention, doping can improve the structural stability of lithium iron phosphate, reduce structural changes during charge and discharge, thereby extending the cycle life of the battery. Furthermore, doping can also improve the electronic conductivity and ionic conductivity of lithium iron phosphate, enabling it to maintain excellent performance even at high charge and discharge rates. The doping element used in lithium iron phosphate doping is preferably a transition metal element; the transition metal element includes, but is not limited to, one or more of Mn, Co, Ni, Zn, and Ti, with Ti being preferred; the doping amount of the doping element in lithium iron phosphate doping is preferably 2000~6000 ppm; optionally, the doping amount of lithium iron phosphate doping is 2000 ppm, 3000 ppm, 4000 ppm, 5000 ppm, 6000 ppm, or any two of the above values.

[0036] In one specific embodiment of the present invention, the excessive growth and agglomeration of cathode material particles can be limited to a certain extent by coating modified lithium iron phosphate. Specifically, a layer of material with excellent conductivity is coated onto the surface of the lithium iron phosphate to construct an electron transport network, reducing the contact resistance between particles and improving the lithium ion mobility. The coating layer forms a dense protective film on the surface of the cathode particles, blocking direct contact between the electrolyte and the active material, reducing the oxidative decomposition of the electrolyte at the cathode interface under high voltage, and preventing gas generation. In this invention, carbon layer coating modified lithium iron phosphate is preferably used; the source of the carbon layer is preferably an organic carbon source, more preferably one or more of sucrose, glucose, and polyethylene glycol (PEG); the thickness of the carbon layer is preferably 10~100 nm; optionally, the thickness of the carbon layer is 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, or any two of the above values.

[0037] In this invention, the positive electrode sheet includes a positive current collector and a positive active material layer attached to at least one surface of the positive current collector. The resistivity of the positive active material layer directly affects the charge-discharge performance of the battery. If the powder resistivity of the positive active material layer is too high, electrons cannot reach the reaction interface in time during high-rate charge-discharge, which will severely limit the battery's fast-charging capability and high-current discharge performance. In this invention, the powder resistivity of the positive active material layer is preferably 5~50 Ω·cm; optionally, the powder resistivity of the positive active material layer is 5 Ω·cm, 10 Ω·cm, 15 Ω·cm, 20 Ω·cm, 25 Ω·cm, 30 Ω·cm, 35 Ω·cm, 40 Ω·cm, 45 Ω·cm, 50 Ω·cm, or any two of the above values.

[0038] In this invention, the resistivity of the positive electrode powder refers to the resistance value of the positive electrode material conductor per unit length on the cross-section, usually expressed in ohms per centimeter (Ω·cm). The detection method is as follows: 1) Pretreatment: The battery is discharged to 2.5V at 0.33C. The positive electrode sheet of the lithium-ion battery in the empty state is taken and soaked in dimethyl carbonate (DMC) solution at room temperature for 4 hours. After soaking, the electrode sheet is taken out and dried in a vacuum environment. The active material powder on the surface of the electrode sheet is scraped off with a ceramic knife; 2) The scraped powder is ball-milled at a speed of 300 r / min. The ball used for ball milling is zirconia ball with a diameter of 0.6 mm. The ball-to-material ratio is 5:1. The ball milling time is 20~40 min. After ball milling, it is placed on a four-probe resistivity tester (such as the ST2722 semiconductor powder resistivity tester of Suzhou Jingge Electronics Co., Ltd.) to test the powder resistivity. The specific operation method is as follows: Place the powder on the electrode and apply pressure to the powder with the other electrode. The pressure intensity is 10MPa. The powder resistivity value is then read.

[0039] In one specific embodiment of the present invention, the positive electrode sheet includes a positive current collector and a positive active material layer attached to at least one surface of the positive current collector; the positive active material layer includes a positive active material; the mass of the positive active material is preferably 94.5% to 98.5% of the mass of the positive active material layer; optionally, the mass of the positive material is 94.5%, 95%, 95.5%, 96%, 96.5%, 97%, 97.5%, 98%, 98.5% of the mass of the positive active material layer or a range between any two of the above values.

[0040] In one specific embodiment of the present invention, the positive electrode active material layer further includes a positive electrode binder and a positive electrode conductive agent; the positive electrode binder includes, but is not limited to, one or more of the binders polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), and polyacrylonitrile (PAN); the mass of the positive electrode binder is preferably 1.3% to 2.2% of the mass of the positive electrode active material layer; optionally, the mass of the positive electrode binder is 1.3%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, or 2% of the mass of the positive electrode active material layer. The positive electrode conductive agent is 0.0%, 2.1%, 2.2%, or any two of the above values; the positive electrode conductive agent includes, but is not limited to, one or more of conductive carbon black, acetylene black, carbon nanotubes, graphene, and carbon fiber materials; the mass of the positive electrode conductive agent is preferably 0.5% to 1.5% of the mass of the positive electrode active material layer; optionally, the mass of the positive electrode conductive agent is 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5% of the mass of the positive electrode active material layer, or any two of the above values.

[0041] In a specific embodiment of the present invention, the positive electrode conductive agent can be added by simultaneously adding a powdered conductive agent and a conductive slurry. Adding the conductive slurry allows the positive electrode active material layer to better form a conductive network. The conductive slurry can be any conductive slurry known to those skilled in the art and is not particularly limited. In this invention, it preferably includes N-methylpyrrolidone (NMP), a conductive agent, and a dispersant. The conductive agent can be any conductive agent known to those skilled in the art and is not particularly limited, including but not limited to graphene nanomaterials, carbon nanotubes, etc. The solid content of the conductive slurry is preferably 6%~7%, more preferably 6.05%~6.45%; the content of the conductive agent is preferably 4.5%~5.5%, more preferably 4.9%~5.1%; the average diameter of the carbon nanotubes is preferably 10~15 nm; and the viscosity of the conductive slurry is less than 3000 mPas. -1 .

[0042] In a specific embodiment of the present invention, the positive electrode active material layer preferably further includes a dispersant; the mass content of the dispersant in the positive electrode active material layer is preferably 0.05% to 2%; optionally, the mass content of the dispersant in the positive electrode active material layer is 0.05%, 0.1%, 0.5%, 1%, 1.5%, 2% or any two of the above values; the dispersant can be any dispersant well known to those skilled in the art, and there are no special limitations. In the present invention, polyvinylidene fluoride (PVDF) is preferred.

[0043] In one specific embodiment of the present invention, the positive current collector includes, but is not limited to, aluminum foil.

[0044] In a specific embodiment of the present invention, the areal density of the positive electrode sheet is preferably 350~450 g / m³. 2 Optionally, the areal density of the positive electrode is 350 g / m³. 2 360 g / m 2 380 g / m 2 400 g / m 2 420 g / m 2 450g / m 2 Or the range between any two of the above values.

[0045] In a specific embodiment of the present invention, the areal density deviation of the positive electrode sheet is preferably ±1% to ±3%; optionally, the areal density deviation of the positive electrode sheet is ±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%, or a range between any two of the above values.

[0046] In one specific embodiment of the present invention, the compaction density of the positive electrode sheet is preferably 2.45~2.65 g / cm³. 3 Optionally, the compaction density of the positive electrode sheet is 2.45 g / cm³. 3 2.50 g / cm 3 2.55 g / cm 3 2.60g / cm 3 2.65 g / cm 3 Or the range between any two of the above values.

[0047] The N / P ratio (Negative / Positive ratio) of a lithium-ion battery is a key design parameter. It refers to the ratio of the capacity of the negative electrode active material to that of the positive electrode active material in battery capacity design. It is typically used to ensure that the negative electrode active material has sufficient capacity to accept lithium ions released by the positive electrode active material during charging. By controlling the range of the N / P ratio, on the one hand, rapid lithium ion transport is ensured, improving the battery's fast-charging performance; on the other hand, lithium ions are prevented from precipitating on the negative electrode surface and forming lithium dendrites. Newly formed lithium dendrites will continuously break and reorganize the solid electrolyte interphase (SEI) film, consuming electrolyte and generating gas, thus reducing battery safety. In a specific embodiment of this invention, the N / P ratio of the negative electrode capacity to the positive electrode capacity of the lithium iron phosphate battery is preferably 1 to 1.03; optionally, the N / P ratio of the negative electrode capacity to the positive electrode capacity of the lithium iron phosphate battery is 1, 1.01, 1.02, 1.03, or any two of the above values.

[0048] The negative electrode is an important component of lithium iron phosphate; the negative electrode includes a negative electrode active material; in a specific embodiment provided by the present invention, the negative electrode active material is preferably one or more of graphite, silicon, silicon-oxygen materials and silicon-carbon materials, and more preferably graphite.

[0049] The particle size of the negative electrode active material has a significant impact on battery performance, therefore it is necessary to control its particle size. On the one hand, this shortens the transport path of lithium ions within the active material, reduces diffusion resistance, and thus improves the lithium ion transport rate. On the other hand, it reduces the specific surface area of ​​the material, thereby reducing the contact area between the material and the electrolyte, mitigating side reactions between the active material and the electrolyte, and reducing gas production. In a specific embodiment of the present invention, the average particle size Dv50 of the negative electrode active material is preferably 5~20 μm; optionally, the average particle size Dv50 of the negative electrode active material is 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 18 μm, 20 μm, or any two of the above values.

[0050] The negative electrode active material is located in the negative electrode active material layer of the negative electrode sheet; the mass of the negative electrode active material is preferably 96% to 96.5% of the mass of the negative electrode active material layer.

[0051] In a specific embodiment of the present invention, the negative electrode active material layer further includes a negative electrode conductive agent; the mass content of the negative electrode conductive agent in the negative electrode active material layer is preferably 0.5% to 1%; optionally, the mass content of the negative electrode conductive agent in the negative electrode active material layer is 0.5%, 0.6%, 1% or any two of the above values; the negative electrode conductive agent can be any negative electrode conductive agent well known to those skilled in the art, and there are no special limitations. In the present invention, conductive agent SP and / or acetylene black are preferred.

[0052] In a specific embodiment of the present invention, the negative electrode active material layer preferably further includes a negative electrode binder; the mass content of the negative electrode binder in the negative electrode active material layer is preferably 1.2% to 2.6%; optionally, the mass content of the negative electrode binder in the negative electrode active material layer is 1.2%, 1.5%, 1.8%, 2.0%, 2.2%, 2.4%, 2.6% or any two of the above values; the negative electrode binder can be any negative electrode binder known to those skilled in the art, and there are no special limitations. In the present invention, it is preferably one or more of polyacrylic acid (PAA), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR); the molecular weight of the PAA is preferably 30 to 100 W; optionally, the molecular weight of the PAA is 30 W, 40 W, 50 W, 60 W, 70 W, 80 W, 90 W, 100 W or any two of the above values; the particle size of the SBR is preferably 120 to 180 nm; optionally, the particle size of the SBR is 120 nm, 130 nm, or any two of the above values. nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, or any range between two of the above values.

[0053] In a specific embodiment of the present invention, the negative electrode active material layer preferably further includes a dispersant; the mass content of the dispersant in the negative electrode active material layer is preferably 0.4% to 1.8%; optionally, the mass content of the dispersant in the negative electrode active material layer is 0.4%, 0.6%, 0.8%, 1.0%, 1.2%, 1.4%, 1.6%, 1.8% or any two of the above values; the dispersant can be any dispersant well known to those skilled in the art, and there are no special limitations. In the present invention, polyvinylidene fluoride (PVDF) is preferred.

[0054] In one specific embodiment of the present invention, the negative electrode sheet further includes a negative electrode current collector; the negative electrode active material layer is disposed on at least one surface of the negative electrode current collector; the negative electrode current collector includes, but is not limited to, copper foil.

[0055] In a specific embodiment of the present invention, the areal density of the negative electrode sheet is preferably 180~230 g / m³. 2 Optionally, the areal density of the negative electrode sheet is 180 g / m³. 2 190 g / m 2 200 g / m 2 210 g / m 2 220 g / m 2 230g / m 2 Or the range between any two of the above values.

[0056] In one specific embodiment of the present invention, the compaction density of the negative electrode sheet is preferably 1.5~1.7 g / cm³. 3 Optionally, the compaction density of the negative electrode sheet is 1.5 g / cm³. 3 1.55 g / cm 3 1.6 g / cm 3 1.65 g / cm 3 1.7 g / cm 3 Or the range between any two of the above values.

[0057] As a key component of a battery, the electrolyte plays a significant role in conduction between the positive and negative electrodes, acting as an ionic conductor. The performance of the electrolyte and its contact with the positive and negative electrodes have a major impact on the overall performance of the battery.

[0058] In one specific embodiment of the present invention, the electrolyte includes additives; the additives include film-forming additives; the film-forming additives help form a stable SEI film on the negative electrode surface, reduce interfacial impedance, and improve battery efficiency; the film-forming additives are preferably one or more of propylene carbonate (PC), vinylene carbonate (VC), and fluoroethylene carbonate (FEC). However, excessive film-forming additives can cause side reactions with the electrolyte, resulting in increased gas production and reduced battery safety. Therefore, in the present invention, the mass of the film-forming additives is preferably 1.5% to 4% of the electrolyte mass; optionally, the mass of the film-forming additives is 1.5%, 2%, 2.5%, 3%, 3.5%, 4% of the electrolyte mass, or a range between any two of the above values.

[0059] In one specific embodiment of the present invention, the film-forming additive includes vinylene carbonate (VC) and fluoroethylene carbonate (FEC); the mass of the vinylene carbonate is preferably 1.5% to 4% of the electrolyte mass; optionally, the mass of the vinylene carbonate is 1.5%, 2%, 2.5%, 3%, 3.5%, 4% of the electrolyte mass or any two of the above values; the mass of the fluoroethylene carbonate is preferably 3% to 10% of the electrolyte mass; optionally, the mass of the fluoroethylene carbonate is 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10% of the electrolyte mass or any two of the above values.

[0060] Lithium salts play a conductive role in the electrolyte. In a specific embodiment of the present invention, the lithium salt can be any lithium salt well known to those skilled in the art, without any special limitations. The present invention preferably includes, but is not limited to, one or more of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium fluorosulfonyl (perfluorobutylsulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(oxalate-borate), lithium difluorooxalate-borate, lithium trifluoromethanesulfonate, and lithium tetrafluorooxalate phosphate. The concentration of lithium salt in the electrolyte is preferably 0.5~2 mol / L. Optionally, the concentration of lithium salt in the electrolyte is 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.2 mol / L, 1.5 mol / L, 2 mol / L, or any two of the above values.

[0061] According to the present invention, the electrolyte further includes an organic solvent; the organic solvent can be any organic solvent well known to those skilled in the art, and there are no special limitations. The main function of the organic solvent is to dissolve the lithium salt. In the present invention, it preferably includes carboxylic acid esters and carbonates; the carbonate solvent is preferably one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate; the carboxylic acid ester solvent is preferably one or more of methyl acetate, ethyl formate, and ethyl acetate. In the embodiments provided by the present invention, ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) in a mass ratio of 30:40:30 are used as an example for illustration.

[0062] According to the present invention, the battery preferably further includes a separator; the separator can be any separator well known to those skilled in the art, and there are no special limitations, and it can be at least one of glass fiber, non-woven fabric, polyethylene (PE), polypropylene (PP) and polyvinylidene fluoride; the surface of the separator can also be coated, and the coating can be an inorganic coating and / or an organic coating; wherein, the inorganic coating material includes at least one of alumina, silicon oxide, titanium oxide, magnesium oxide, zirconium oxide and boehmite; the organic coating includes at least one of aramid coating and polyvinylidene fluoride (PVDF) coating.

[0063] In one specific embodiment of the present invention, the separator includes a base film and a coating disposed on at least one surface of the base film; the coating is aluminum oxide; the thickness of the coating is preferably 1~4 μm. By controlling the coating thickness, on the one hand, the lithium ion transport path can be shortened, improving the fast charging performance of the battery; on the other hand, side reactions between the separator and the electrolyte can be reduced, lowering the risk of gas generation.

[0064] In one specific embodiment of the present invention, the porosity of the separator is preferably 35% to 50%; optionally, the porosity of the separator is 35%, 40%, 45%, 50%, or any two of the above values. By controlling the porosity, on the one hand, the lithium-ion transport rate can be improved, enhancing the fast-charging performance of the battery; on the other hand, side reactions between the positive and negative electrode materials and the electrolyte can be reduced, reducing gas production and improving battery safety.

[0065] The lithium iron phosphate battery provided by this invention can be prepared according to methods well known to those skilled in the art, and there are no special limitations. Specifically, it can be prepared according to the following steps:

[0066] S1) Positive electrode preparation: The positive electrode active material, positive electrode conductive agent, positive electrode binder and positive electrode conductive agent are dispersed in NMP to obtain a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector to obtain a double-sided coated positive electrode sheet; then it is rolled and cut to obtain the positive electrode sheet;

[0067] S2) Negative electrode preparation: The negative electrode active material, negative electrode conductive agent, negative electrode binder and dispersant are dispersed in deionized water to obtain a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector to obtain a double-sided coated electrode sheet; then it is rolled and cut to obtain the negative electrode sheet;

[0068] S3) Electrolyte preparation: Mix organic solvent with lithium salt to obtain electrolyte; additives may also be added as needed.

[0069] 4) Assembly and formation: The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. Then, they are wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a lithium iron phosphate battery is obtained.

[0070] In one specific embodiment of the present invention, the battery is prepared according to the following steps:

[0071] 1) Preparation of positive electrode sheet: The positive electrode active material (94.5%~98.5%), positive electrode binder (0.5%-1.5%), positive electrode conductive agent (1.3%~2.2%), and dispersant (0.05%~2%) are mixed evenly according to the mass ratio. NMP solvent is added, and the mixture is stirred under vacuum until the system is homogeneous to obtain a positive electrode slurry with a solid content of (50%~65%). The positive electrode slurry is coated on aluminum foil to obtain a double-sided coated positive electrode sheet. Then, it is rolled and cut to obtain the positive electrode sheet.

[0072] 2) Negative electrode preparation: The negative electrode active material (96.0%~96.5%), negative electrode conductive agent (0.5%~1%), dispersant (0.4%~1.8%) and negative electrode binder (1.2%~2.6%) are mixed evenly and dispersed in deionized water to obtain a negative electrode slurry; the negative electrode slurry is coated on copper foil to obtain a double-sided coated electrode; then it is rolled and cut to obtain a negative electrode sheet; the N / P ratio is mainly achieved by adjusting the negative electrode areal density to reach the design value.

[0073] 3) Electrolyte preparation: One or more of cyclic carbonates, chain carbonates, and chain carboxylic esters are mixed in a certain mass ratio, such as EC:EMC:DMC=30:40:30, to obtain an organic solvent. Then, one or more of fully dried lithium salts, lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), are dissolved in the mixed organic solvent in a certain molar ratio to prepare an electrolyte with a concentration of 1.0 mol / L. The electrolyte is allowed to stand in a refrigerator for 0.5 h, and additives, including vinylene carbonate (VC) and fluoroethylene carbonate (FEC), are added at amounts of 1.5%~4% and 3%~10%, respectively.

[0074] 4) Preparation of the separator: PP or PE is selected as the base film, and Al2O3 is selected as the coating.

[0075] 5) Assembly and formation: Stack the above positive electrode, separator, and negative electrode in sequence, so that the separator is between the positive and negative electrode to play a role in isolation, and then wind them to obtain a bare cell; place the bare cell in the outer packaging shell, dry it, inject electrolyte, and then go through vacuum sealing, standing, formation, shaping and other processes to obtain a lithium iron phosphate battery.

[0076] According to this invention, the positive electrode material can be commercially available or self-made, as long as it meets the above requirements, there are no special limitations. The parameters of the positive electrode active material can be controlled according to the following method:

[0077] Control of magnetic impurities: During LFP synthesis, ① the specifications of raw material impurities are strictly controlled; ② the demagnetization capability of the equipment reaches the industry level; ③ the cleanliness of the plant is controlled; and the crystallinity of the material is improved by selecting precursors with high vibration density and adjusting the sintering curve (increasing temperature, extending time, etc.).

[0078] Control of carbon layer thickness: carbon addition amount, carbonization temperature range and time adjustment, etc.; control of dopant content: doping ratio, precursor doping and back-end sintering doping.

[0079] To further illustrate the present invention, the following describes in detail a high-safety lithium iron phosphate battery provided by the present invention with reference to embodiments.

[0080] All reagents used in the following examples are commercially available.

[0081] Examples 1-16 and Comparative Examples 1-4

[0082] 1) Positive electrode active material: The core is doped lithium iron phosphate, with an average particle size Dv50 as shown in Table 1. The types and amounts of doping elements are shown in Table 1. The core is wrapped with a carbon layer with a thickness of 10~100 nm, as shown in Table 1. The carbon layer is obtained by pyrolysis of sucrose.

[0083] 2) Preparation of positive electrode sheet: The positive electrode active material, PVDF, SP, LB122-50 (NMP series conductive paste) and dispersant polyacrylic acid were mixed evenly at a mass ratio of 96.5%:2%:0.9%:0.5%:0.1%. NMP solvent was added, and the mixture was stirred under vacuum until homogeneous, obtaining a positive electrode paste with a solid content of 60%. This positive electrode paste was coated onto aluminum foil to obtain a double-sided coated positive electrode sheet. After rolling and cutting, the positive electrode sheet was obtained; the areal density was 420 g / m². 2 Compacted density 2.55 g / cm³ 3 .

[0084] 3) Negative electrode preparation: Artificial graphite (average particle size Dv50 as shown in Table 1) was mixed with SP (0.6%), carboxymethyl cellulose (CMC) (1.8%) (dispersant), and carboxylated styrene-butadiene latex (1.2%) (mass ratio) at a ratio of 96.4% to obtain a negative electrode slurry (solid content 52%). This negative electrode slurry was coated onto copper foil to obtain a double-sided coated electrode. The electrode was then rolled and cut to obtain the negative electrode sheet with an areal density of 190 g / m³. 2 Compacted density 1.6 g / cm³ 3 .

[0085] 4) Preparation of electrolyte: An organic solvent was prepared by mixing EC:EMC:DMC in a mass ratio of 30:40:30. Then, fully dried lithium salt lithium hexafluorophosphate (LiPF6) was dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1.0 mol / L. The electrolyte was then allowed to stand in a refrigerator for 0.5 h, and 4% vinylene carbonate (VC) was added as an additive.

[0086] 5) Preparation of the separating membrane:

[0087] PE is selected as the base membrane, Al2O3 is selected as the coating, and polyvinylidene fluoride (PVDF) is selected as the adhesive layer. The adhesive layer is set on one side of the base membrane, and the coating and adhesive layer are set on the other side in sequence. The membrane specification is 2+9+3+2 (adhesive layer + base membrane + coating + adhesive layer, μm).

[0088] 6) Assembly and formation:

[0089] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. Then, they are wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, standing at 45°C for 24 hours, formation (25°C / 0.5MPa clamp), and shaping, a lithium-ion battery is obtained.

[0090] ① Detection method for positive electrode surface density deviation

[0091] The battery was discharged at 0.33C to the lower limit voltage of 2.5V. Then, the empty battery was disassembled, and the positive electrode, negative electrode, and separator were removed. After vacuum drying at 80℃ for 12 hours, the areal density of different parts of the positive electrode was detected by X-ray diffraction to obtain its deviation. The results are shown in Table 2.

[0092] ② Methods for detecting the content of magnetic impurities in cathode materials

[0093] After discharging the secondary battery, it was disassembled. The resulting positive electrode sheets were soaked in DMC to remove electrolyte components and pre-remove byproducts. Then, 200 g of positive electrode active material powder was scraped from multiple positive electrode sheets and placed in a polypropylene sample container. A magnetic rod and 300 mL of ultrapure water were added. The sample container was sealed and placed on a ball mill at 80 rpm for 60 min. The magnetic rod was then retained, and the remaining sample was cleaned. Ultrapure water was added to the container to wash the magnetic rod. The magnetic rod was then transferred to a 250 mL beaker, soaked in ultrapure water, and ultrasonically cleaned. The water was then removed, and 60 mL of 10% hydrochloric acid solution was added and soaked for 30 min. The soaked solution was then placed in a centrifuge tube, and the solution temperature was recorded as -100. The Thermo Fisher Scientific iCAP PRO series ICP analyzer (RF power 1150W, nebulizer flow and auxiliary gas flow set to 0.5) was turned on. (L / min, rinsing time 30s), click "ICP-Expert" to enter the instrument interface, then click "Method" to select the magnetic material, then edit the sample information and save, then click Analyze. The accuracy of the standard curve test needs to be above 0.999. After the analysis is complete, read the data, rinse the injection system in 2% dilute nitric acid for 5 min, then rinse the injection system in distilled water for 5 min, and test the content of magnetic impurities in the solution -100, which is the content of magnetic impurities in the positive electrode material.

[0094] ③ Test method for voltage plateau area ratio:

[0095] Origin Integral Calculation: 1. The battery is discharged at 0.33C to the lower limit voltage of 2.5V. Then, the empty battery is disassembled, and the positive electrode, negative electrode, and separator are removed. After vacuum drying at 80℃ for 12 hours, they are reassembled into a single battery cell. Electrolyte formula: Lithium salt: LiPF6, concentration 1 mol / L; Solvent: EC:EMC:DMC = 30:40:30; Additives: Ethylene carbonate (VC) and fluoroethylene carbonate (FEC), added at 3.0% and 0.5% respectively. Afterwards, an overcharge test is performed on the battery at 25℃, charged from 3.65V to 5.5V at a 0.5C rate. The original charging data is obtained; open the charging data. 1. Copy the charging curve data for different voltage ranges from the charging curve and paste them into the Origin software. 2. Right-click on the first column and set it to X, right-click on the second column and set it to Y. Then select the X and Y columns, and click Analysis→Mathematics→Integrate at the top of the software. In the options that appear, find the baseline selection, select the baseline as 0 (i.e., the X-axis), and click OK. 3. Select the Y column data, click Trapeoidal at the top of the Origin software, and then perform calculus. 4. When the options appear, click OK to calculate, and the area between the curve and the baseline will be generated.

[0096] ④ Test method for Fe element content in the negative electrode:

[0097] Elemental Analysis (ICP): 1. Pretreatment: Discharge the battery at 0.33C to the lower limit voltage of 2.5V. Take the negative electrode sheet of the lithium-ion battery in its empty state and immerse it in dimethyl carbonate (DMC) solution at room temperature for 4 hours. After immersion, remove the negative electrode sheet and dry it in a vacuum environment. Scrape off the active material powder on the surface of the negative electrode sheet with a ceramic knife; 2. Weigh 1.0000 1. Place the active material powder in a polytetrafluoroethylene beaker, add 10 ml of aqua regia, and cover with a watch glass; 2. Digest at 230℃ for 15 min using a hot plate; 3. Add 20 mL of ultrapure water to the beaker and continue digesting at 230℃ for 15 min; 4. After cooling to room temperature, filter with filter paper; 5. Make up to 50 mL with pure water; 6. Place the sample in the sample tube of the elemental analyzer, start the instrument, select the parameter settings, set the iron wavelength to 259.94, and start the test; 7. Obtain the iron mass fraction, iron content Fe(%) = WFe / 10000, accurate to two decimal places, and multiply the obtained Fe content by one million to get the ppm value.

[0098] ⑤ Methods for testing fast charging time:

[0099] In step 6), when assembling the lithium-ion battery, a copper wire is added to the negative electrode side. After assembling the secondary battery, it is left to stand for 24 hours, then charged at 0.02C to 4.1V, then charged at a constant current of 0.33C to 4.25V, and then charged at a constant voltage to 0.05C. After standing for 10 minutes, the battery was discharged at a constant current of 0.33C to 2.5V to complete formation. Then, it was charged at a constant current of 0.33C to 4.25V, followed by constant voltage charging to 0.05C. After standing for 10 minutes, it was discharged at a constant current of 0.33C to 2.5V, completing one cycle of capacity determination. After two cycles, the copper wire was lithium-plated at a rate of 0.02C for 4 hours. Then, 10% of the capacity from the second cycle was charged at a rate of 0.33C, followed by constant current charging at 4C until the negative parameter potential reached 0 or the terminal voltage reached 4.25V. Then, a 0.2C gradient charging was performed with the cutoff conditions unchanged until the charging rate dropped to 0.33C or the capacity reached 80%. Finally, it was charged at a constant current and constant voltage of 0.33C to 4.25V. The test temperature was 25℃, and the equipment signal acquisition frequency was 100 ms. After the test, the SOC of the secondary battery from 10% to 80% was calculated. Required charging time t.

[0100] ⑥ Method for testing gas production:

[0101] Charge the battery at a constant current of 0.33C to the upper limit voltage of 3.65V, then charge at a constant voltage until the current is less than or equal to 0.05C. After full charging, test the battery volume using the water displacement method and record it as V0. Then, store the battery in a 60℃ oven for 15 days. After the battery temperature drops to room temperature (25℃), test the battery volume again using the water displacement method and record it as V1. Calculate the gas production at 60℃ using the following formula:

[0102] Gas production at 60℃ = (V1-V0) / battery capacity.

[0103] The specific method for testing battery volume using the water displacement method is as follows:

[0104] 1) Add an appropriate amount of pure water to the container and test its density ρ with a hydrometer and record the result;

[0105] 2) Place the aforementioned container on a balance and tare it (tareing must be performed before testing each battery cell).

[0106] 3) Submerge the battery cell body along with the tabs in pure water, ensuring that the battery cell does not contact the container wall. After stabilization, take a reading and record the data. Before the battery is placed in the oven for storage, this data is recorded as T0, and after the battery is placed in the oven for storage, this data is recorded as T1. The formula for calculating the volume of the battery is T / ρliquid. Then the difference between V1 and V0 is: V1-V0=T1 / ρliquid-T0 / ρliquid60°.

[0107] 4) Turn off the balance and seal the container to prevent the reagent from evaporating.

[0108] Table 1. Parameters of the positive and negative electrodes in Examples 1-16 and Comparative Examples 1-4

[0109]

[0110] Table 2 Performance test results of Examples 1-16 and Comparative Examples 1-4

[0111]

[0112] As can be seen from Table 2:

[0113] The battery described in this application has excellent fast-charging performance. The fast-charging time of the batteries in each embodiment is less than 28 minutes, and the amount of gas generated inside the battery is small. The amount of gas generated after storage at 60°C for 15 days is less than 5g. This is mainly due to the comprehensive control of the ratio 'a' of the area under the curve between 4.5 and 5.5V and the area under the curve between 3.3 and 5.5V in the overcharge curve obtained by charging the battery at a constant current of 1000 mA, and the Fe content 'b' in the negative electrode. That is, when a × 10 4 When / b is in the range of 0.5 to 89, the battery can achieve a balance between fast charging performance and safety performance. In contrast, the batteries obtained in Comparative Examples 1 to 4 are difficult to achieve the same effect.

[0114] Furthermore, when a×b is preferably within the range of 3 to 26, it not only avoids overcharging of the battery leading to increased internal temperature and the catalytic decomposition of the electrolyte by magnetic impurities, but also promotes the synergistic effect of iron with lithium and phosphorus to form a stable Fe-Li-P interface layer, reducing the SEI film impedance of the negative electrode. Therefore, the battery has a shorter fast-charging time and lower gas production at high temperatures.

[0115] Furthermore, the ratio 'a' of the area under the curve between 4.5 and 5.5V and the area under the curve between 3.3 and 5.5V in the overcharge curve obtained by charging the battery at a constant current of 1000 mA can be further optimized to be within the range of 0.18 to 0.27. At this point, the overcharge of the battery is appropriate, the oxidation and decomposition of the electrolyte is effectively controlled, the amount of gas generated inside the battery is further reduced, the lithium ion insertion / extraction rate is appropriate, and the fast charging time is also shorter. The change in the Fe content 'b' in the negative electrode of the battery will affect both fast charging performance and safety performance. When 'b' is further optimized to be within the range of 100 to 1000, the stability of the SEI film formation of the negative electrode can be guaranteed, the impedance can be reduced, and the side reactions between the negative electrode and the electrolyte can be reduced, thereby achieving better battery performance.

[0116] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A lithium iron phosphate battery, characterized in that, The battery includes a positive electrode and a negative electrode; the positive electrode includes lithium iron phosphate; the ratio of the area under the overcharge curve between 4.5 and 5.5V to the area under the overcharge curve between 3.3 and 5.5V obtained by charging the lithium iron phosphate battery with a constant current of 1000 mA is 'a'; the Fe content in the negative electrode is 'b' ppm; a × 10 4 / b is 0.5~89; The value of 'a' is 0.1~0.5; 50≤b≤3000; In the overcharge curve graph, the horizontal axis represents time in seconds, and the vertical axis represents voltage. The area under the curve between 4.5 and 5.5V in the overcharge curve refers to the area enclosed by the curve and the vertical axis between 4.5 and 5.5V. The area under the curve between 3.3 and 5.5V in the overcharge curve refers to the area enclosed by the curve and the vertical axis between 3.3 and 5.5V.

2. The lithium iron phosphate battery according to claim 1, characterized in that, The a×10 4 / b ranges from 3 to 26.

3. The lithium iron phosphate battery according to claim 1 or 2, characterized in that, The value of a is 0.18 to 0.27; and / or, 100 <b≤1000。 4. The lithium iron phosphate battery according to claim 1 or 2, characterized in that, The average particle size Dv50 of the lithium iron phosphate is 0.5~1.4 μm.

5. The lithium iron phosphate battery according to claim 1 or 2, characterized in that, The lithium iron phosphate is also doped with a doping element; the doping element is a transition metal element; the doping amount of the doping element is 2000~6000 ppm.

6. The lithium iron phosphate battery according to claim 5, characterized in that, The lithium iron phosphate is further provided with a coating layer on its surface; the coating layer is a carbon layer; the thickness of the carbon layer is 10~100 nm.

7. The lithium iron phosphate battery according to claim 1 or 2, characterized in that, The positive electrode sheet includes a positive current collector and a positive active material layer attached to at least one surface of the positive current collector; the powder resistivity of the positive active material layer is 5~50 Ω·cm.

8. The lithium iron phosphate battery according to claim 1, characterized in that, The ratio of the negative electrode capacity to the positive electrode capacity of the lithium iron phosphate battery, N / P, is 1~1.

03.

9. The lithium iron phosphate battery according to claim 8, characterized in that, The negative electrode sheet includes a negative electrode active material; the negative electrode active material is selected from one or more of graphite, silicon, silicon-oxygen materials and silicon-carbon materials; the average particle size Dv50 of the negative electrode active material is 5~20 μm.

10. The lithium iron phosphate battery according to claim 1, characterized in that, The lithium iron phosphate battery further includes an electrolyte; the electrolyte includes an organic solvent and a film-forming additive; the organic solvent includes a cyclic carbonate; the mass of the cyclic carbonate is 20% to 40% of the mass of the organic solvent; And / or, the film-forming additive includes one or more of propylene carbonate, vinylene carbonate, and fluoroethylene carbonate; the mass of the film-forming additive is 1.5% to 4% of the mass of the electrolyte.

11. The lithium iron phosphate battery according to claim 1, characterized in that, The lithium iron phosphate battery further includes a separator; the separator includes a base film and a coating disposed on at least one surface of the base film; the coating is aluminum oxide; the thickness of the coating is 1~4 μm.

12. The lithium iron phosphate battery according to claim 11, characterized in that, The porosity of the diaphragm is 35%~50%.

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