Ultralow-temperature lithium iron phosphate battery

Abstract

The invention relates to the field of lithium iron phosphate batteries, in particular to an ultralow-temperature lithium iron phosphate battery which comprises a positive plate, a negative plate, a diaphragm and electrolyte. The positive plate forms an inner and outer double-layer gradient porosity structure through a 3D printing technology, and adopts a'point-line-plane 'three-dimensional conductive network to optimize the electron / ion transmission efficiency at low temperature. The anode active material is prepared into nanoscale lithium iron phosphate through a hydrothermal method and a carbon coating process, and the lithium ion diffusion path is shortened. A carbon-based 3D nanowire array is designed on the surface of the negative plate, and a B / N co-doped graphite matrix is combined, so that the fast charging performance is improved, and the cycle life is prolonged. The diaphragm adopts a polyimide / nano aluminum oxide composite base material and a temperature response polymer coating. The electrolyte is composed of esters, ethers, nitrile solvents and a fluoro additive, and the esters, ethers and nitrile solvents and the fluoro additive synergistically reduce viscosity and form a stable SEI film. According to the invention, the capacity retention ratio and the conductive efficiency of the battery in an ultra-low temperature environment are obviously improved.

Description

Technical Field

[0001] The present invention relates to the field of lithium iron phosphate batteries, and in particular to an ultra-low temperature lithium iron phosphate battery. Background Art

[0002] Lithium iron phosphate battery is a lithium-ion battery that uses lithium iron phosphate as the positive electrode material and carbon as the negative electrode material. It is highly favored in the battery field due to its advantages such as high operating voltage, high energy density, long cycle life, good safety performance, low self-discharge rate and no memory effect.

[0003] During the charging process of a lithium iron phosphate battery, some lithium ions are released from the lithium iron phosphate and transferred to the negative electrode through the electrolyte, where they are embedded in the negative electrode's carbon material. Simultaneously, electrons are released from the positive electrode and travel through the external circuit to the negative electrode, maintaining the balance of the chemical reaction. During the discharge process, lithium ions are released from the negative electrode and travel through the electrolyte to the positive electrode. Simultaneously, electrons are released from the negative electrode and travel through the external circuit to the positive electrode, providing energy to the outside world.

[0004] However, the capacity and charge-discharge performance of existing lithium iron phosphate batteries will show obvious attenuation in low temperature environments. The main reasons are: 1. The olivine crystal structure of lithium iron phosphate only provides a one-dimensional lithium ion diffusion channel. At low temperatures, the migration speed of lithium ions slows down, resulting in reduced positive electrode activity. In addition, its electronic conductivity is poor, and polarization is prone to occur, further reducing the capacity; 2. At low temperatures, the viscosity of the electrolyte increases, and the lithium ion migration impedance increases. During charging, metallic lithium is easily precipitated to form dendrites, which not only consumes reversible lithium ions, but may also pierce the diaphragm and cause short circuit risks. In addition, low temperatures will aggravate the internal polarization of the battery, resulting in a decrease in the discharge platform voltage and a reduction in actual available energy. Summary of the Invention

[0005] In order to solve the aforementioned technical problems, the present invention provides an ultra-low temperature lithium iron phosphate battery, which is used to solve the performance degradation problem of traditional lithium iron phosphate batteries in low temperature environments, and is specifically achieved through the following technical solutions.

[0006] The present invention discloses an ultra-low temperature lithium iron phosphate battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte. The positive electrode sheet, the separator and the negative electrode sheet are stacked in sequence and encapsulated in a battery casing, and the electrolyte is filled in the battery casing.

[0007] The positive electrode sheet includes a positive electrode active material, a conductive agent and an adhesive. The positive electrode sheet has an inner and outer double-layer gradient porosity structure, with the inner layer porosity of 30% to 40% and the outer layer porosity of 60% to 70%; The negative electrode sheet comprises a negative electrode active material, a conductive agent, a thickener and a binder. The surface of the negative electrode sheet is provided with a carbon-based 3D nanowire array structure, the nanowire diameter is 10nm to 20nm, and the nanowire length is 3μm to 5μm. The diaphragm comprises a diaphragm base material and a coating layer, the diaphragm base material comprises polyimide and nano-aluminum oxide, the diaphragm base material is a porous material with a porosity of 50%, the coating layer has a thickness of 10-20 microns, and the coating layer is a temperature-responsive polymer. The electrolyte comprises a solvent, a lithium salt and an additive, the concentration of the lithium salt is 0.8-1.5 mol / L, the solvent is a mixture of esters, ethers and nitriles, the additive is a mixture of esters and tris(pentafluorophenyl)borane, and the additive accounts for 1-5% of the total mass of the electrolyte.

[0008] Preferably, the mass ratio of the positive electrode active material, the conductive agent and the binder in the positive electrode sheet is 90-95:3-15:1-5.

[0009] Preferably, the conductive agent in the positive electrode sheet and the negative electrode sheet comprises a point-shaped conductive agent, a one-dimensional linear conductive agent and a two-dimensional planar conductive agent.

[0010] Preferably, the binder in the positive electrode sheet is one or more of polyvinylidene fluoride, polyacrylic acid, polyaniline and polystyrene sulfonate.

[0011] Preferably, the mass ratio of the negative electrode active material, the conductive agent, the thickening agent and the binder is 90-98:1-5:1-5:1-5.

[0012] Preferably, the thickening agent in the negative electrode sheet is sodium carboxymethyl cellulose, and the binder in the negative electrode sheet is a macromolecular material with a specially modified low glass transition temperature.

[0013] Preferably, the mass ratio of polyimide to nano-aluminum oxide in the diaphragm base material is 1-2:1-2.

[0014] Preferably, the mass ratio of esters, ethers and nitriles in the electrolyte solvent is 8-15:3-8:2-6.

[0015] Preferably, the preparation of the positive electrode active material comprises the following steps: S1. Weigh raw materials in a stoichiometric ratio of Li:Fe:P=1:1:1, dissolve them in deionized water, add ascorbic acid and stir until completely dissolved, adjust the pH of the solution to 2-4, and prepare a mixed solution; S2. Transfer the mixed solution to a reaction kettle, seal it, heat it to 160-180℃, and react for 12-24 hours. After the reaction is completed, naturally cool it to room temperature, collect the precipitate, wash and dry it to obtain lithium iron phosphate precursor powder; S3. Grind the lithium iron phosphate precursor powder to a fine powder, dry it, and obtain nano lithium iron phosphate powder; S4. Place a carbon source in ethanol, add nano lithium iron phosphate powder, stir and dry to form a uniform precursor mixture; S5, the precursor mixture is placed in a boat, heated by a tube furnace, carbon-coated lithium iron phosphate is prepared to realize the positive electrode active material.

[0016] Preferably, the preparation of the negative electrode active material comprises the following steps: S1, the high-purity petroleum coke particles and coal tar are mixed and stirred uniformly, then pressed into a shape, and high-temperature graphitization treatment is performed to form an isotropic graphite matrix; S2, the graphite matrix is mixed with stearic acid in a certain proportion and ground into a nano-sized slurry, and spray drying is performed to form secondary particles; S3, a metal oxide nano layer is deposited on the surface of the secondary particles to form a graphitized precursor; S4, boracic acid and urea are doped in the graphitized precursor, and high-temperature pyrolysis is performed to co-dope to form a negative electrode active material.

[0017] After the above technical solutions are adopted, the application has the following beneficial effects: 1. The positive electrode double-layer gradient porosity structure shortens the lithium ion transmission path, the composite conductive agent constructs a three-dimensional network, and the low-viscosity solvent combination of the electrolyte improves the ion migration rate and capacity retention rate in a low-temperature environment, and improves the low-temperature performance of the battery.

[0018] 2. The nanoscale lithium iron phosphate carbon-coated positive electrode reduces the internal resistance, the 3D nanowire array and B / N doped negative electrode enhance the electronic conductivity, and the conductivity and rate performance of the battery are improved.

[0019] 3. The high-temperature graphitization of the negative electrode graphite matrix reduces impurities, the metal oxide nano layer inhibits volume expansion, the carbon-coated positive electrode structure is stable, and the capacity decay is reduced, thereby effectively prolonging the cycle life.

[0020] 4. The temperature-responsive polymer of the separator closes the pores to block thermal runaway at high temperatures, and the fluorinated additive in the electrolyte catalyzes the decomposition of flammable components and inhibits combustion, thereby significantly reducing the risk of battery explosion. DETAILED DESCRIPTION

[0021] The features and exemplary embodiments of various aspects of the application will be described in detail below, in order to make the purpose, technical scheme and advantages of the application more clear and apparent, the application will be further described in detail below. It should be understood that the specific embodiments described herein are configured to explain the application and are not configured to limit the application. The application can be implemented without some of these specific details for those skilled in the art. The following description of the embodiments is merely to provide a better understanding of the application by showing examples of the application.

[0022] An embodiment of the present invention provides an ultra-low temperature lithium iron phosphate battery, comprising a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte. The positive electrode sheet, the separator and the negative electrode sheet are stacked in sequence and encapsulated inside a battery casing, and the electrolyte is filled inside the battery casing.

[0023] The positive electrode sheet is constructed of positive electrode active material, conductive agent and adhesive in a mass ratio of 90~95:3~15:1~5, and the positive electrode sheet is set to an inner and outer double-layer gradient porosity structure through 3D printing technology. The inner layer porosity is 30%~40%, and the outer layer porosity is 60%~70%. This structure can shorten the lithium ion transmission path while enhancing the electrolyte wettability, thereby having a positive impact on the capacity retention rate of lithium iron phosphate batteries in low temperature environments.

[0024] Among them, the conductive agent in the positive electrode sheet includes a point-shaped conductive agent, a one-dimensional linear conductive agent and a two-dimensional surface conductive agent. The point-shaped conductive agent is one or more of carbon black particles and activated carbon particles, the one-dimensional linear conductive agent is one or more of carbon fiber and carbon nanotubes, and the two-dimensional surface conductive agent is one or more of graphene and single-layer MoS2 / graphene heterostructure, and the mass ratio of the point-shaped conductive agent, the one-dimensional linear conductive agent and the two-dimensional surface conductive agent is 2~5:7~10:3~6.

[0025] In low-temperature environments, the performance of lithium iron phosphate batteries is limited mainly due to the increased viscosity of the electrolyte, which hinders ion migration and reduces the conductivity of the electrode material.

[0026] By combining point-, line-, and surface-shaped conductive agents, a three-dimensional "point-line-surface" conductive network can be constructed, significantly improving electron / ion transport efficiency at low temperatures. Point-shaped conductive agents fill gaps between active materials, quickly establishing local conductive contact points; line-shaped conductive agents form long-range conductive bridges across multiple particles in a long fiber structure; and surface-shaped conductive agents coat lithium iron phosphate particles with ultra-thin sheets, reducing the tortuosity of the lithium ion diffusion path.

[0027] The three work together to form a multi-dimensional conductive network, which can maintain conductive stability through multiple paths even when shrinking at low temperatures or when the electrolyte is partially frozen.

[0028] The adhesive in the positive electrode sheet is one or more of polyvinylidene fluoride, polyacrylic acid, polyaniline and polystyrene sulfonate.

[0029] The preparation of the positive electrode active material includes the following steps: S1. Weigh the raw materials according to the stoichiometric ratio of Li:Fe:P=1:1:1, dissolve them in deionized water, add ascorbic acid and stir until completely dissolved, adjust the pH of the solution to 2-4, and prepare a mixed solution.

[0030] In the above process, it is necessary to add an excess of 5% to 10% lithium source to the raw materials to make up for the possible loss of lithium raw materials in subsequent high-temperature environments. Excessive addition can ensure the optimal ratio of lithium, iron and phosphorus in the positive electrode sheet.

[0031] Among the weighed raw materials, the lithium source is lithium carbonate or lithium hydroxide, the iron source is ferrous oxalate, and the phosphorus source is ammonium dihydrogen phosphate.

[0032] In the above steps, ammonia or dilute sulfuric acid is used to adjust the pH of the solution, and the pH is adjusted to 2-4 to maintain the Fe 2+ It improves the stability of the solution, optimizes the dispersibility of the solution, inhibits the hydrolysis and precipitation of metal ions, and provides a stable system for subsequent hydrothermal reactions.

[0033] The molar ratio of ascorbic acid to Fe is 1:1. Ascorbic acid acts as a reducing agent to prevent Fe 2+ Oxidized to Fe 3+ , thereby maintaining the electrochemical activity of the material.

[0034] S2. Transfer the mixed solution to a reactor, seal it, heat it to 160° C. to 180° C., react for 12 to 24 hours, and naturally cool it to room temperature after the reaction. Collect the precipitate, wash and dry it to obtain lithium iron phosphate precursor powder.

[0035] During the above process, a stainless steel reactor lined with polytetrafluoroethylene was used to complete the above reaction to prevent the mixed liquid from corroding the reactor; the precipitate was collected by centrifugation and washed alternately with deionized water and ethanol three times, and finally dried at 80°C in a vacuum environment for 12 hours to obtain lithium iron phosphate precursor powder.

[0036] This process promotes the formation of lithium iron phosphate nuclei under high temperature conditions, obtaining an olivine structure with high crystallinity and uniform morphology; the longer reaction cycle ensures sufficient grain growth and avoids agglomeration caused by too small size; the nanoscale precursor powder prepared by this step can effectively shorten the lithium ion diffusion path.

[0037] S3. Grind the lithium iron phosphate precursor powder into fine powder and then dry it to obtain nano lithium iron phosphate powder.

[0038] In the above steps, a ball mill is used to grind and refine the lithium iron phosphate precursor powder, with a ball-to-material ratio of 10:1, a ball mill speed of 300-400 rpm, and a ball milling time of 6-12 hours. After the ball milling is completed, the powder and the grinding balls are separated with a sieve, and the powder is dried at 80°C.

[0039] In this process, the particle size is physically ground to the nanometer level to increase the specific surface area, increase the lithium ion diffusion rate, and improve the rate performance. In addition, nanoscale particles reduce the pores between particles, which can increase the electrode compaction density and volume energy density to a certain extent.

[0040] S4, the carbon source is placed in ethanol, lithium iron phosphate nano powder is added, and stirring and drying are performed to form a uniform precursor mixture.

[0041] The carbon source is one or more of glucose, sucrose or citric acid, and the carbon source mass is 5% to 15% of the total mass of the precursor mixture.

[0042] The stirring time is 2 hours, and the drying temperature is 80°C.

[0043] In the above step, the carbon source is uniformly dispersed on the surface of lithium iron phosphate by dissolving the carbon source in ethanol to form a continuous coating layer, and stirring and drying ensure that the carbon source is in full contact with the active material, avoiding local carbon content that is too high or too low.

[0044] In addition, the carbon coating layer can enhance the electrical conductivity, reduce the internal resistance of the battery, and inhibit the agglomeration during the sintering process of the particles, maintaining the nanoscale dispersibility.

[0045] S5, the precursor mixture is placed in a ceramic boat, heated by a tube furnace, and carbon-coated lithium iron phosphate is prepared to obtain a positive electrode active material.

[0046] In the above step, the ceramic boat needs to be placed in the constant temperature zone of the tube furnace, and inert gas is introduced, and the temperature is raised to 600°C to 700°C at a rate of 5°C / min and held for 2 to 4 hours, and then naturally cooled.

[0047] In this process, high temperature causes the carbon source to decompose to form a conductive carbon layer coated on the surface of lithium iron phosphate, and at the same time, the carbon network fixes the crystal structure, reducing the volume expansion during the cyclic charge and discharge.

[0048] In addition, the outer carbon layer after high-temperature sintering can isolate the corrosion of the electrolyte, the inner carbon network can improve the electron conduction, and the carbon layer enhances the wettability of the electrolyte, thereby improving the discharge capacity in a low-temperature environment.

[0049] As a further explanation of the present application, the negative electrode sheet is constructed from a negative electrode active material, a conductive agent, a thickening agent and a binder in a mass ratio of 90 to 98: 1 to 5: 1 to 5: 1 to 5, and the surface of the negative electrode sheet is provided with a carbon-based 3D nanowire array structure, the nanowire diameter is 10 to 20 nm, and the nanowire length is 3 to 5 μm. The structure is optimized by spatial topology and interface engineering, so that the negative electrode sheet has high energy density, long cycle life and excellent fast charging performance.

[0050] The conductive agent in the negative electrode sheet includes a point-shaped conductive agent, a one-dimensional linear conductive agent, and a two-dimensional planar conductive agent, the point-shaped conductive agent is one or more of carbon black particles and activated carbon particles, the one-dimensional linear conductive agent is one or more of carbon fibers and carbon nanotubes, the two-dimensional planar conductive agent is one or more of graphene and single-layer MoS2 / graphene heterostructures, and the mass ratio of the point-shaped conductive agent, the one-dimensional linear conductive agent, and the two-dimensional planar conductive agent is 2-5:7-10:3-6.

[0051] The thickening agent in the negative electrode sheet is sodium carboxymethyl cellulose, and the binder in the negative electrode sheet is a low glass transition temperature macromolecular material specially modified, preferably one or more of an acrylate copolymer, a cross-linked composite adhesive, a modified butadiene-styrene rubber latex, a polyurethane, and a fluorinated binder.

[0052] The carbon-based 3D nanowire array structure on the surface of the negative electrode sheet is prepared by guiding nanowire growth on the surface of the negative electrode sheet through a physical vapor deposition method.

[0053] The preparation of the negative electrode active material includes the following steps: S1, high-purity petroleum coke particles and coal tar are uniformly mixed and stirred, and then are pressed into a shape for high-temperature graphitization treatment to form an isotropic graphite matrix.

[0054] In the above step process, the particle size of the petroleum coke particles is less than or equal to 20 pm, the mass ratio of the petroleum coke particles and the coal tar is 7-9:2-4, the pressing process uses cold isostatic pressing technology, and the pressing is performed under a pressure of 120 MPa-150 MPa.

[0055] The temperature condition for the high-temperature graphitization treatment is 2800-3000 DEG C, and the graphitization process is performed in an inert atmosphere.

[0056] This step converts amorphous carbon into ordered layered graphite structure by high-temperature treatment of petroleum coke and coal tar mixture, reduces the interlayer spacing, optimizes the lithium ion insertion channel, and in addition, the high-temperature environment can volatilize impurities such as sulfur and oxygen, improve the purity of the material, reduce the side reaction, the resistivity after graphitization is reduced, and the electronic conductivity is significantly improved, thereby effectively reducing the internal resistance of the battery.

[0057] The high-temperature graphitization forms a high-crystallinity isotropic graphite matrix with excellent electrical conductivity and thermal stability, which can effectively improve the initial coulombic efficiency and cycle life of the negative electrode material.

[0058] S2, the graphite matrix is mixed with stearic acid in a certain proportion to grind into a nanoscale slurry, and is spray dried to form secondary particles.

[0059] In the above step, a ball mill is used to prepare the nanoscale slurry, and the mixing mass ratio of the graphite matrix and stearic acid is 95-100:3-5.

[0060] In the nanoscale slurry, the particle size is 50-200 nm, and the secondary particle size is 5-10 μm.

[0061] In this step, the stearic acid acts as a surfactant to improve the dispersibility of the graphite matrix and reduce the probability of agglomeration. In addition, during the spray drying process, the solvent is rapidly volatilized to form secondary particles with a porous interior and a dense surface, thereby increasing the specific surface area, effectively shortening the lithium ion diffusion path, and enhancing the electrolyte wettability and rate capability due to the porous structure.

[0062] S3, depositing a metal oxide nanolayer on the surface of the secondary particles to form a graphitized precursor.

[0063] In the above step, a sol-gel method is used to complete the surface deposition, and the thickness of the metal oxide nanolayer is 5-10 nm.

[0064] In the above step, a sol-gel method is used to complete the surface deposition, and the thickness of the metal oxide nanolayer is 5-10 nm.

[0065] In the above step, a sol-gel method is used to complete the surface deposition, and the thickness of the metal oxide nanolayer is 5-10 nm.

[0066] In addition, the metal oxide as a lithium ion conductor can accelerate the interface ion transmission, thereby improving the low-temperature performance.

[0067] S4, doping boric acid and urea into the graphitized precursor, and pyrolyzing the co-doping to form a negative electrode active material.

[0068] In the above step, boric acid is used as a B source, and urea is used as an N source to realize B / N co-doping at a temperature of 1000-1200°C.

[0069] In the above step, boric acid is used as a B source, and urea is used as an N source to realize B / N co-doping at a temperature of 1000-1200°C.

[0070] In the above step, boric acid is used as a B source, and urea is used as an N source to realize B / N co-doping at a temperature of 1000-1200°C.

[0071] As a further explanation of the present application, the separator includes a separator substrate and a coating layer, the separator substrate includes polyimide and nano-aluminum oxide, the separator substrate is a porous material with a porosity of 50%, and the coating layer is a temperature-responsive polymer with a thickness of 10-20 μm. The mass ratio of polyimide to nano-aluminum oxide in the diaphragm substrate is 1-2:1-2.

[0072] The temperature-responsive polymer is one or more of poly-N-isopropyl acrylamide, polyimide and polyacrylonitrile.

[0073] The polyimide and nano-aluminum oxide composite substrate has good temperature resistance, so that the substrate has little thermal shrinkage during operation, thereby forming a stable porous framework; in addition, the temperature-responsive polymer such as poly-N-isopropyl acrylamide coated on the substrate quickly closes the pores at high temperature, blocks the lithium ion transmission path, prevents the spread of thermal runaway, and opens multiple pores at low temperature, thereby significantly increasing the porosity and increasing the contact area with the electrolyte and the conductivity efficiency.

[0074] In addition, the substrate structure with a porosity of 50% provides uniform ion channels, and the nano-Al2O3 enhances the electrolyte wettability and reduces the interface impedance, thereby further improving the conductivity.

[0075] As a further explanation of the application, the electrolyte includes a solvent, a lithium salt and an additive, the concentration of the lithium salt is 0.8-1.5 mol / L, the solvent is a mixture of esters, ethers and nitriles, and the additive is a mixture of esters and tris(pentafluorophenyl)borane, and the additive accounts for 1%-5% of the total mass of the electrolyte.

[0076] The ester in the electrolyte solvent is one or more of propylene carbonate, ethylene carbonate and dimethyl carbonate, the ether in the electrolyte solvent is one or more of 1,3-dioxolane, ethylene glycol dimethyl ether and tetraglycol dimethyl ether, and the nitrile in the electrolyte solvent is one or more of acetonitrile and glutaronitrile.

[0077] The ester in the additive is diethyl fluoromalonate, and the mass ratio of the ester to tris(pentafluorophenyl)borane in the additive is 2-3:1-2.

[0078] The lithium salt in the electrolyte is lithium hexafluorophosphate.

[0079] The mass ratio of the ester, ether and nitrile in the electrolyte solvent is 8-15:3-8:2-6.

[0080] The ester in the electrolyte reduces the solvent viscosity and freezing point, thereby enhancing the lithium ion migration rate at low temperature; the ether solvent has low viscosity and high flowability, thereby reducing the ion transmission resistance of the electrolyte at low temperature; the nitrile solvent with high dielectric constant enhances the lithium salt dissociation ability, and its low melting point effectively prevents low-temperature crystallization, thereby maintaining the uniformity of the electrolyte, so that the low-temperature ion conductivity can be synergistically improved by reasonable proportioning.

[0081] And, diethyl fluoromalonate and tri(pentafluorophenyl)borane can generate a dense and low-impedance SEI film on the negative electrode surface, reduce active lithium loss, and thus improve the cycle capacity retention rate.

[0082] In addition, the fluorine atoms in diethyl fluoromalonate can capture free radicals to inhibit electrolyte combustion chain reactions, and tri(pentafluorophenyl)borane can reduce the risk of thermal runaway by catalytically decomposing flammable components, both of which synergistically effectively reduce the flammable risk of the battery.

[0083] In order to facilitate further understanding of the present application, several embodiments and comparative examples of the present application are given below: Example 1 Step 1: Preparation of positive electrode active material The raw materials were weighed in a stoichiometric ratio of Li:Fe:P = 1:1:1, dissolved in deionized water, ascorbic acid was added in a molar ratio of 1:1 with Fe, and stirred until completely dissolved. The pH of the solution was adjusted to 3 to form a mixed solution.

[0084] The mixed solution was transferred to a stainless steel reaction kettle with a polytetrafluoroethylene liner, sealed, and heated to 170°C. The reaction was carried out for 18 hours, and after the reaction was completed, the precipitate was collected and washed with deionized water and ethanol alternately 3 times. The lithium iron phosphate precursor powder was dried at 80°C under vacuum for 12 hours.

[0085] The lithium iron phosphate precursor powder was ground in a ball mill at a ball-to-material ratio of 10:1. The ball mill was operated at a speed of 300 rpm, and after 8 hours of grinding, the powder was separated by a screen and dried at 80°C to obtain nano lithium iron phosphate powder.

[0086] The glucose was weighed in a mass ratio of glucose: nano lithium iron phosphate powder = 3:17, placed in ethanol, and the nano lithium iron phosphate powder was added. After stirring for 2 hours, the mixture was dried at 80°C to obtain a precursor mixture.

[0087] The precursor mixture was placed in a porcelain boat, which was placed in the constant temperature zone of a tube furnace. Inert gas was introduced, and the temperature was raised to 650°C at a rate of 5°C / min and held for 3 hours before natural cooling to obtain the positive electrode active material.

[0088] Step 2: Preparation of positive electrode sheet The positive electrode active material, carbon black particles, carbon nanotubes, single-layer MoS2 / graphene heterostructure, and polyaniline were mixed in a mass ratio of 90:2:7:3:5, and a 3D printer was used to print an inner-outer double-layer gradient porosity structure with an inner porosity of 30% and an outer porosity of 60% to obtain the positive electrode sheet.

[0089] Step 3: Preparation of negative electrode active material High-purity petroleum coke particles with a particle size of 20 μm or less and coal tar pitch were mixed at a mass ratio of 8:5, stirred uniformly, and then pressed into a shape under a pressure of 140 MPa. The pressed product was placed in a graphitization furnace, inert gas was introduced, and the temperature was raised to 2800°C to perform high-temperature graphitization treatment, thereby obtaining a graphite matrix.

[0090] The graphite matrix was mixed with stearic acid at a mass ratio of 95:4, ground into a nano-sized slurry with a particle size of 100 nm, and spray-dried to form secondary particles with a particle size of 10 μm.

[0091] An Al2O3 nano-layer with a thickness of 5 nm was deposited on the surface of the secondary particles to form a graphitized precursor.

[0092] The graphitized precursor, boric acid, and urea were mixed at a mass ratio of 1:1.5:0.6, and then pyrolyzed at a temperature of 1000°C to form a co-doped negative electrode active material.

[0093] Step 4: Preparation of a negative electrode sheet The negative electrode active material, carbon black particles, carbon nanotubes, single-layer MoS2 / graphene heterostructure, sodium carboxymethyl cellulose, and cross-linked composite binder were mixed at a mass ratio of 90:1:2:1:5:5, and then pressed into a shape under a pressure of 150 MPa.

[0094] Carbon-based 3D nanowires were guided to grow on the surface of the negative electrode sheet by physical vapor deposition, and then a nanowire array with a diameter of 20 nm and a length of 5 μm was prepared on the surface of the negative electrode sheet.

[0095] Step 5: Preparation of a separator Polyimide and nano-alumina were mixed at a mass ratio of 1:1, and then fully stirred to obtain a uniform mixture. A separator substrate with a porosity of 50% was prepared by 3D printing technology. Poly N-isopropyl acrylamide with a thickness of 15 μm was flow-coated on the separator substrate, and then dried at 50°C for 8 hours to obtain a battery separator.

[0096] Step 6: Preparation of an electrolyte Ethylene carbonate, ethylene glycol dimethyl ether, and acetonitrile were mixed at a mass ratio of 10:7:5, and then lithium hexafluorophosphate was added to the mixture until the concentration of lithium hexafluorophosphate in the mixture was 1.2 mol / L.

[0097] Diethyl fluoromalonate and tris(pentafluorophenyl)borane were mixed at a mass ratio of 2:1 to prepare an additive. The additive was added to the above mixture until the additive accounted for 4% of the total mass, thereby preparing an electrolyte.

[0098] Step 7: Assembly After the positive plate, the separator and the negative plate are sequentially stacked and packaged inside the battery shell, the electrolyte is used to fill the internal space of the battery shell, and the ultra-low temperature lithium iron phosphate battery is completed.

[0099] Example 2 This embodiment is based on example 1, adjusting the component ratio in the positive plate, specifically: Step 2: Preparation of positive plate According to the mass ratio of positive active material: carbon black particles: carbon nanotubes: single-layer MoS2 / graphene heterostructure: polyaniline = 90:2:8:5:5, mix uniformly, print by 3D printer to prepare inner and outer double-layer gradient porosity structure with inner porosity of 30% and outer porosity of 60%, as positive plate.

[0100] The remaining steps are the same as example 1.

[0101] Example 3 This embodiment is based on example 1, adjusting the component ratio in the negative plate, specifically: Step 4: Preparation of negative plate According to the mass ratio of negative active material: carbon black particles: carbon nanotubes: single-layer MoS2 / graphene heterostructure: sodium carboxymethyl cellulose: cross-linked composite adhesive = 90:0.5:2.5:1.5:5:5, stir uniformly, and press form under the condition of 150MPa pressure.

[0102] On the surface of the negative plate, the growth of carbon-based 3D nanowires is guided by physical vapor deposition method, and then nanowire array with a diameter of 20nm and a length of 5μm is prepared on the surface of the negative plate.

[0103] The remaining steps are the same as example 1.

[0104] Example 4 This embodiment is based on example 1, adjusting the component ratio and coating thickness in step 5, specifically: Step 5: Preparation of separator Polyimide and nano-alumina are fully stirred and mixed uniformly according to the mass ratio of 1:2, and 3D printing technology is used to prepare a separator substrate with a porosity of 50%. The thickness of the poly N-isopropyl acrylamide is 10μm, and it is dried at 50℃ for 8 hours to prepare the battery separator.

[0105] The remaining steps are the same as example 1.

[0106] Example 5 This embodiment is based on Example 1, adjusting step 6, adjusting the component ratio and lithium salt concentration in the electrolyte, specifically: Step 6: Configure electrolyte Mix ethylene carbonate, ethylene glycol dimethyl ether and acetonitrile in a ratio of 13:5:3, add lithium hexafluorophosphate to the mixed solution, and the concentration of lithium hexafluorophosphate in the mixed solution is 1.0 mol / L.

[0107] Prepare an additive by uniformly mixing diethyl fluoromalonate and tris(pentafluorophenyl)borane in a mass ratio of 2:1, add the additive to the above mixed solution until the additive accounts for 4% of the total mass, and prepare the electrolyte.

[0108] The remaining steps are the same as Example 1.

[0109] Comparative Example 1 This comparative example is based on Example 1, adjusting step 1, and the positive active material is no longer nano-treated, specifically: Step 1: Prepare positive active material Weigh the raw materials according to the stoichiometric ratio Li:Fe:P=1:1:1, dissolve them in deionized water, add ascorbic acid in a molar ratio of 1:1 with Fe, stir until completely dissolved, adjust the solution pH to 3, and prepare the mixed solution.

[0110] Transfer the mixed solution to a stainless steel reaction kettle lined with polytetrafluoroethylene, seal it, heat to 170°C, and react for 18 hours. After the reaction is complete, naturally cool to room temperature, collect the precipitate, wash it with deionized water and ethanol alternately 3 times, and dry it in a vacuum environment at 80°C for 12 hours to obtain lithium iron phosphate precursor powder.

[0111] Weigh the glucose in a mass ratio of glucose: lithium iron phosphate precursor powder = 3:17, place it in ethanol, add nano lithium iron phosphate powder, stir for 2 hours, and dry it at 80°C to obtain a precursor mixture.

[0112] Place the precursor mixture in a porcelain boat, place the porcelain boat in the constant temperature zone of the tube furnace, introduce inert gas, heat to 650°C at a rate of 5°C / min and keep for 3 hours, and then naturally cool to obtain the positive active material.

[0113] The remaining steps are the same as Example 1.

[0114] Comparative Example 2 This comparative example is based on Example 1, adjusting step 2, and no longer using the inner-outer double-layer gradient porosity structure, specifically: Step 2: Prepare the positive electrode sheet According to the mass ratio of positive active material: carbon black particles: carbon nanotubes: single-layer MoS2 / graphene heterostructure: polyaniline = 90:2:7:3:5, mix uniformly, and cut into positive electrode sheets under the condition of 100 MPa pressure.

[0115] The remaining steps are the same as in Example 1.

[0116] Comparative Example 3 This comparative example is based on Example 1, and step 3 is adjusted, and co-doping with boric acid and urea is not used. Specifically: Step 3: Preparation of negative active material High-purity petroleum coke particles with a particle size of less than or equal to 20 μm and coal tar are mixed and stirred uniformly according to a mass ratio of 8:5, and then pressed and formed under the condition of 140 MPa pressure, and then placed in a graphitization furnace, inert gas is introduced, and heated to 2800°C for high-temperature graphitization treatment to obtain a graphite matrix.

[0117] The graphite matrix and stearic acid are mixed and ground into a nano-level slurry with a particle size of 100 nm according to a mass ratio of 95:4, and spray dried to form secondary particles with a particle size of 10 μm.

[0118] An Al2O3 nano-layer with a thickness of 5 nm is deposited on the surface of the secondary particles to form a negative active material.

[0119] The remaining steps are the same as in Example 1.

[0120] Comparative Example 4 This comparative example is based on Example 1, and step 4 is adjusted, and no carbon-based 3D nanowires are arranged on the surface of the negative electrode sheet. Specifically: Step 4: Preparation of negative electrode sheet According to the mass ratio of negative active material: carbon black particles: carbon nanotubes: single-layer MoS2 / graphene heterostructure: carboxymethyl cellulose sodium: cross-linked composite adhesive = 90:1:2:1:5:5, stir uniformly, and press and form under the condition of 150 MPa pressure as a negative electrode sheet.

[0121] The remaining steps are the same as in Example 1.

[0122] Comparative Example 5 This comparative example is based on Example 1, and step 5 is adjusted, and poly-N-isopropyl acrylamide is no longer used to coat the separator substrate. Specifically: Step 5: Preparation of separator Polyimide and nano-alumina are mixed and stirred uniformly according to a mass ratio of 1:1, and a separator with a porosity of 50% is prepared using 3D printing technology.

[0123] The remaining steps are the same as Example 1.

[0124] Comparative Example 6 This comparative example is based on Example 1, adjusting Step 6, not using diethyl fluoromalonate as an additive, specifically: Step 6: Preparation of electrolyte Ethylene carbonate, ethylene glycol dimethyl ether and acetonitrile are uniformly mixed in a ratio of 10:7:5, and lithium hexafluorophosphate is added to the mixed solution, so that the concentration of lithium hexafluorophosphate in the mixed solution is 1.2 mol / L.

[0125] Tris(pentafluorophenyl)borane is added as an additive to the above mixed solution, accounting for 4% of the total mass, to form an electrolyte.

[0126] The remaining steps are the same as Example 1.

[0127] The lithium iron phosphate batteries in the above examples and comparative examples are respectively tested for performance, and the test results are as follows:

[0128] Compared with Example 1, Example 2 increases the proportion of carbon nanotubes and MoS2 / graphene in the positive electrode, resulting in an increase in electron transmission efficiency, which leads to an improvement in charge and discharge performance. However, the outer porosity is not adjusted accordingly, which leads to a slight decrease in low-temperature electrolyte infiltration, affecting the low-temperature performance.

[0129] Compared with Example 1, the proportion of negative electrode conductive agent in Example 3 is optimized, resulting in a decrease in internal resistance and an increase in cycle life, but the charge and discharge performance is slightly sacrificed.

[0130] Compared with Example 1, the proportion of nano-alumina in the separator in Example 4 is increased, resulting in an increase in electrolyte wettability and an optimization of low-temperature performance. At the same time, nano-alumina slightly increases the rigidity of the base material, resulting in a slight decrease in cycle life.

[0131] Compared with Example 1, the proportion of ethers in the electrolyte in Example 5 is reduced, resulting in a decrease in low-temperature ionic conductivity, and the decrease in lithium salt concentration further leads to an increase in internal resistance, resulting in a decrease in all performance.

[0132] Compared with Example 1, the positive electrode in Comparative Example 1 is not nano-sized, resulting in a long lithium ion diffusion path, a significant decrease in capacity and rate performance, and a further increase in active material agglomeration, resulting in a faster cycle decay.

[0133] Compared with Example 1, the gradient porosity structure is cancelled in Comparative Example 2, resulting in uneven electrolyte infiltration, which hinders ion migration at low temperatures. In addition, the low inner porosity also leads to a decrease in capacity utilization.

[0134] Comparative Example 3, compared with Example 1, does not use co-doping for the negative electrode sheet, which results in high lithium ion insertion barrier, thus causing severe volume expansion in the cycle, and capacity attenuation due to active material shedding.

[0135] Comparative Example 4, compared with Example 1, cancels the 3D nanowire array of the negative electrode sheet, which makes the negative electrode electron conduction path single, and causes the charge-discharge performance to decline, and the risk of thermal runaway due to local polarization to slightly increase.

[0136] Comparative Example 5, compared with Example 1, does not have a temperature-responsive coating on the separator, which results in low capacity retention rate due to insufficient porosity at low temperature, and the porosity cannot be closed at high temperature, thus causing the thermal runaway temperature to significantly decrease.

[0137] Comparative Example 6, compared with Example 1, cancels the fluorinated additive, which causes poor SEI film stability and severe active lithium loss, and the lack of fluorinated additive results in the inability to effectively inhibit flammable components, thus increasing the risk of thermal runaway.

[0138] In accordance with the above embodiments of the present application, these embodiments do not exhaustively describe all the details, nor limit the present application to only the specific embodiments. Obviously, according to the above description, many modifications and changes can be made. The present specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present application, so that those skilled in the art can well utilize the present application and make modifications and uses on the basis of the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. An ultra-low temperature lithium iron phosphate battery, comprising a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, wherein the positive electrode sheet, the separator, and the negative electrode sheet are stacked in sequence and encapsulated in a battery housing, and the electrolyte is filled in the battery housing, characterized in that: The positive electrode sheet includes a positive electrode active material, a conductive agent and an adhesive. The positive electrode sheet has an inner and outer double-layer gradient porosity structure, with the inner layer porosity of 30% to 40% and the outer layer porosity of 60% to 70%; The negative electrode sheet comprises a negative electrode active material, a conductive agent, a thickener and a binder. The surface of the negative electrode sheet is provided with a carbon-based 3D nanowire array structure, the nanowire diameter is 10nm to 20nm, and the nanowire length is 3μm to 5μm. The diaphragm includes a diaphragm substrate and a coating layer, the diaphragm substrate includes polyimide and nano-aluminum oxide, the diaphragm substrate is a porous material with a porosity of 50%, the coating layer has a thickness of 10 to 20 μm, and the coating layer is a temperature-responsive polymer; The electrolyte includes a solvent, a lithium salt and an additive. The concentration of the lithium salt is 0.8 to 1.5 mol / L. The solvent is a mixture of esters, ethers and nitriles. The additive is a mixture of esters and tris(pentafluorophenyl)borane. The additive accounts for 1% to 5% of the total mass of the electrolyte.

2. The ultra-low temperature lithium iron phosphate battery according to claim 1, characterized in that: The mass ratio of the positive electrode active material, the conductive agent and the adhesive in the positive electrode sheet is 90-95:3-15:1-5.

3. The ultra-low temperature lithium iron phosphate battery according to claim 1, characterized in that: The conductive agents in the positive electrode sheet and the negative electrode sheet include point-shaped conductive agents, one-dimensional line-shaped conductive agents and two-dimensional surface-shaped conductive agents.

4. The ultra-low temperature lithium iron phosphate battery according to claim 1, characterized in that: The binder in the positive electrode sheet is one or more of polyvinylidene fluoride, polyacrylic acid, polyaniline and polystyrene sulfonate.

5. The ultra-low temperature lithium iron phosphate battery according to claim 1, characterized in that: The mass ratio of the negative electrode active material, the conductive agent, the thickener and the binder is 90-98:1-5:1-5:1-5.

6. The ultra-low temperature lithium iron phosphate battery according to claim 1, characterized in that: The thickener in the negative electrode sheet is sodium carboxymethyl cellulose, and the binder in the negative electrode sheet is a specially modified macromolecular material with a low glass transition temperature.

7. The ultra-low temperature lithium iron phosphate battery according to claim 1, characterized in that: The mass ratio of polyimide to nano-aluminum oxide in the diaphragm substrate is 1-2:1-2.

8. The ultra-low temperature lithium iron phosphate battery according to claim 1, characterized in that: The mass ratio of esters, ethers and nitriles in the electrolyte solvent is 8-15:3-8:2-6.

9. The ultra-low temperature lithium iron phosphate battery according to claim 1, characterized in that: The preparation of the positive electrode active material comprises the following steps: S1. Weigh the raw materials according to the stoichiometric ratio of Li:Fe:P=1:1:1, dissolve them in deionized water, add ascorbic acid and stir until completely dissolved, adjust the pH of the solution to 2-4, and prepare a mixed solution; S2. Transfer the mixed solution to a reactor, seal it, heat it to 160-180° C., react for 12-24 hours, cool it naturally to room temperature after the reaction, collect the precipitate, wash and dry it to obtain lithium iron phosphate precursor powder; S3, grinding and refining the lithium iron phosphate precursor powder and drying it to obtain nano lithium iron phosphate powder; S4, placing a carbon source in ethanol, adding nano-lithium iron phosphate powder, stirring and drying to form a uniform precursor mixture; S5. Place the precursor mixture in a porcelain boat and heat it in a tube furnace to achieve carbon coating of the lithium iron phosphate to obtain a positive electrode active material.

10. The ultra-low temperature lithium iron phosphate battery according to claim 1, characterized in that: The preparation of the negative electrode active material comprises the following steps: S1, mixing high-purity petroleum coke particles and coal tar pitch uniformly, pressing and molding, and performing high-temperature graphitization treatment to form an isotropic graphite matrix; S2, mixing the graphite matrix and stearic acid in proportion, grinding them into a nano-scale slurry, and spray drying them to form secondary particles; S3, depositing a metal oxide nanolayer on the surface of the secondary particles to form a graphitized precursor; S4. Boric acid and urea are added to the graphitized precursor, and the resulting mixture is pyrolyzed at high temperature to form a negative electrode active material.

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