Preparation method of ultra-low temperature lithium iron phosphate power battery

Abstract

This invention provides a method for preparing an ultra-low temperature lithium iron phosphate power battery, comprising the following steps: S1: Dissolving an iron source, a phosphorus source, and a lithium source in a mixed solvent of polyol and water; mixing the wet-solid combination after liquid-solid separation with a low-carbon sugar; calcining under a protective atmosphere to obtain a positive electrode material. S2: Crushing and sieving needle coke raw material; performing low-temperature heat treatment followed by high-temperature graphitization; then liquid-phase coating; carbonization treatment; and finally, sieving to obtain a negative electrode material. S3: Adding lithium salt to a multi-electrode solvent to prepare an electrolyte. S4: Coating the positive electrode slurry onto a positive electrode current collector to obtain a positive electrode sheet. S5: Coating the negative electrode slurry onto a negative electrode current collector to obtain a negative electrode sheet. S6: Winding the positive electrode sheet, the negative electrode sheet, and the separator together, placing them into a casing, and injecting the electrolyte to obtain an ultra-low temperature lithium iron phosphate power battery. This invention can improve the electrochemical performance of power batteries in ultra-low temperature environments, enabling the application of power batteries in ultra-low temperature environments.

Description

[Technical Field]

[0002] This invention relates to the field of battery technology, and in particular to a method for preparing an ultra-low temperature lithium iron phosphate power battery. [Background Technology]

[0004] For lithium-ion battery cathode materials, conventional lithium iron phosphate (LFP) materials have a conductivity that is 1-2 exponentially lower than other cathode materials. This is especially true at ambient temperatures near or below 0°C, where the capacity, rate performance, and cycle life of LFP-based lithium-ion batteries show a significant decline compared to room temperature conditions. This limits the use of LFP-based lithium-ion batteries in cold regions and during low-temperature winters. Furthermore, while the high-temperature solid-state synthesis method for LFP is widely used in mass production due to its mature equipment, simple process, and ease of replication, the LFP material produced by this method exhibits poor isotropy, low conductivity, and a particle size (D50) typically between 1.0 and 2.5 μm. Therefore, the electrochemical properties of LFP materials and the resulting lithium-ion batteries are highly sensitive to temperature, with their capacity and discharge performance significantly affected by temperature.

[0005] For lithium-ion battery anode materials, artificial graphite anodes are usually used. These are obtained by crushing, shaping, screening, and graphitizing coal coke, needle coke, or petroleum coke. The D50 is between 10 and 25 μm. However, conventional graphite anodes have significant anisotropy in ion and electron conduction, which affects ion conduction and electrochemical reaction processes in ultra-low temperature environments, and thus affects charge and discharge performance in ultra-low temperature environments.

[0006] For lithium battery electrolytes, a ternary carbonyl organic solvent system of EC, DEC, and EMC is usually used, often with the addition of VC additives. However, at -33 to -38°C, the salt solubility of this type of electrolyte decreases and some solvent crystallizes, affecting the voltage rate.

[0007] Therefore, it is necessary to provide a novel method for preparing ultra-low temperature lithium iron phosphate power batteries to overcome the above-mentioned defects. [Summary of the Invention]

[0009] The purpose of this invention is to provide a method for preparing an ultra-low temperature lithium iron phosphate power battery, which can improve the electrochemical performance of the power battery in an ultra-low temperature environment and enable the power battery to be used in an ultra-low temperature environment.

[0010] To achieve the above objectives, the present invention provides a method for preparing an ultra-low temperature lithium iron phosphate power battery, comprising the following steps:

[0011] S1: Take iron source, phosphorus source, and lithium source and dissolve them in a mixed solvent of polyol and water. At the same time, take electrolytic graphite and mix it evenly in a mixed solvent of polyol and water. Adjust the pH value to the specified pH value range. After one reaction and two reactions, a nanocrystalline solid of electrolytic graphite and lithium iron phosphate is obtained. After liquid-solid separation, the wet solid combination after liquid-solid separation is mixed with low-carbon sugar. Then, it is calcined under an inert protective gas atmosphere to obtain a nanocrystalline carbon-coated lithium iron phosphate cathode material.

[0012] S2: The needle coke raw material is crushed and sieved to achieve particle shaping, then subjected to low-temperature heat treatment, followed by high-temperature graphitization, then size selection sieve, and then the graphitized needle coke is coated with a resin-based liquid carbon source in the liquid phase. After carbonization treatment, it is sieved again to obtain artificial graphite anode material with uniform hard carbon coating of secondary particles.

[0013] S3: Add lithium salt to a multi-component solvent to prepare an electrolyte.

[0014] S4: The positive electrode slurry, made from the positive electrode material obtained in step S1, is coated onto the positive electrode current collector to obtain the positive electrode sheet.

[0015] S5: The negative electrode slurry, made from the negative electrode material obtained in step S2, is coated onto the negative electrode current collector to form a negative electrode sheet.

[0016] S6: The positive electrode sheet obtained in step S4, the negative electrode sheet obtained in step S5, and the separator are wound and placed into the housing and injected with the electrolyte obtained in step S3 to obtain an ultra-low temperature lithium iron phosphate power battery.

[0017] In a preferred embodiment, step S3 further includes: using DMC, EP, and EC as solvents, and adding a low-temperature additive to the electrolyte, wherein the low-temperature additive is FEC and PS.

[0018] In a preferred embodiment, the EP is a linear carbonate; the composition of the electrolyte is EC:EP:DMC:FEC:PS of 5%~20%: 50%~75%: 5%~10%: 1%~5%: 1%~5%; and the concentration of lithium salt in the electrolyte is 1.20~1.45 mol / L.

[0019] In a preferred embodiment, step S3 includes: using EC, PC, and EMC as ternary solvents, and lithium hexafluorophosphate and lithium difluorosulfonylimide as mixed binary lithium salts to prepare an electrolyte, and adding a low-temperature additive and lithium difluorophosphate to the electrolyte, wherein the low-temperature additive is FEC and PS.

[0020] In a preferred embodiment, the electrolyte composition EC:EMC:PC:FEC:PS:LiPO2F2:VC is 15%~30%:50%~80%:5%~10%:1%~5%:1%~5%:0.5%~3.5%:1%~3.5%, and the total concentration of the mixed lithium salt of lithium hexafluorophosphate and lithium difluorosulfonylimide is 1.10~1.40 mol / L.

[0021] In a preferred embodiment, in step S4, the positive electrode slurry comprises: 0.5% ~ 2.0% CNT, 1.6% ~ 3.0% PVDF, 2.0% ~ 6.0% Super-Li active conductive carbon powder, and 90% ~ 95% nanocrystalline lithium iron phosphate positive electrode material.

[0022] In a preferred embodiment, in step S5, the composition of the negative electrode slurry is: 2.5% ~ 5.0% polyacrylonitrile, 2.0% ~ 6.0% Super-Li active conductive carbon powder, and 93% ~ 96.0% granular needle-shaped coke graphite negative electrode material.

[0023] In a preferred embodiment, in step S6, the diaphragm is a PE diaphragm, or a PP diaphragm, a PP and PE mixed composite stretched diaphragm, or a composite diaphragm with a ceramic or PVDF layer coated on a PE or PP diaphragm substrate, or a composite diaphragm with a ceramic or PVDF layer coated on a PP and PE mixed composite stretched diaphragm substrate, and the thickness of the diaphragm is 7~30μm.

[0024] In a preferred embodiment, in step S6, a plurality of spaced-apart positive electrode tabs are welded to one side of the positive electrode sheet, and a plurality of spaced-apart negative electrode tabs are welded to one side of the negative electrode sheet.

[0025] In a preferred embodiment, in step S6, before injecting the electrolyte, the positive and negative electrode sheets are vacuum baked in a contact high vacuum furnace to remove moisture.

[0026] In a preferred embodiment, in step S6, after injecting the electrolyte, negative pressure formation is performed to eliminate the gas generated during formation.

[0027] In a preferred embodiment, the surfaces of the positive current collector and the negative current collector are coated with a functional coating, which is prepared by dispersing nano-conductive graphite and carbon-coated particles.

[0028] Compared to existing technologies, the method for preparing an ultra-low temperature lithium iron phosphate power battery provided by this invention achieves conductivity of the lithium iron phosphate cathode at or near that at room temperature under ultra-low temperature conditions; the electrolyte retains good dissolving ability for lithium salts at ultra-low temperatures, maintaining a liquid state, and its conductivity, viscosity, and dielectric constant at ultra-low temperatures reach or approach those at room temperature; under high-rate discharge, the voltage platform of the lithium iron phosphate cathode system battery at ultra-low temperatures remains significantly higher than the discharge cutoff voltage or the lower limit voltage for terminating the operation of the electrical equipment; the ultra-fine powder lithium iron phosphate cathode slurry is effectively dispersed, and its anti-settling ability is improved; this method can solve the problem of lithium iron phosphate cathode... The isotropy of the cathode and graphite anode materials at low temperatures is addressed to ensure effective electrochemical reactions between the positive and negative electrodes under high-current charging and discharging conditions at ultra-low temperatures. This includes the effective and continuous steady-state conduction of ion migration, ion conduction, and electron conduction on the surfaces of both electrodes, as well as the oxidation reaction at the anode and the reduction reaction at the cathode. The ultra-low temperature lithium iron phosphate cathode system achieves a wide operating and storage range, meeting the requirements for both room temperature and ultra-low temperature environments. It overcomes the reverse effect of capillary action in ultrafine powders, removing adsorbed moisture from the surface of the ultrafine powders and controlling the moisture content of the electrode sheet before liquid injection. It achieves continuous discharge at high rates of 0.1 to 3C within a wide operating range of -43℃ to 55℃. It addresses the limitation of conventional batteries not being able to charge below 0℃, enabling battery charging at ambient temperatures from -20℃ to 0℃. [Attached Image Description]

[0030] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 A flowchart illustrating the preparation method of the ultra-low temperature lithium iron phosphate power battery provided by the present invention;

[0032] Figure 2 Flowchart for the preparation of secondary particle needle-shaped coke graphite anode material;

[0033] Figure 3 This is a process diagram of graphite modification for negative electrode materials.

[0034] Figure 4 This is a structural diagram of the core in the ultra-low temperature lithium iron phosphate power battery provided by the present invention;

[0035] Figure 5A flowchart illustrating the fabrication process of the ultra-low temperature lithium iron phosphate power battery provided by this invention;

[0036] Figure 6 The manufacturing process of nanocrystalline carbon-coated lithium iron phosphate provided by the present invention;

[0037] Figure 7 This is a SEM image of the nanocrystalline carbon-coated lithium iron phosphate prepared in this invention.

[0038] Figure 8 This is a TEM image of the nanocrystalline carbon-coated lithium iron phosphate prepared in this invention.

Detailed Implementation Methods

[0040] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0041] Please see Figure 1 This is a flowchart illustrating the preparation method of the ultra-low temperature lithium iron phosphate power battery provided by the present invention. The power battery prepared by the preparation method of the ultra-low temperature lithium iron phosphate power battery provided by the present invention can provide reliable high-power power to electrical equipment in harsh winter environments in high-altitude and cold regions, or in polar environments, or in high-altitude and cold environments, such as for starting locomotive internal combustion engines and starting engines.

[0042] The method for preparing an ultra-low temperature lithium iron phosphate power battery provided by the present invention includes the following steps:

[0043] S1: Iron, phosphorus, and lithium sources are dissolved in a mixed solvent of polyol and water. Simultaneously, electrolytic graphite is mixed evenly in the same mixed solvent and the pH is adjusted to a specified range. After a primary and secondary reaction, a nanocrystalline solid combining electrolytic graphite and lithium iron phosphate is obtained. Following liquid-solid separation, the wet-solid mixture is mixed with low-carbon sugars and then calcined under an inert protective atmosphere to obtain nanocrystalline carbon-coated lithium iron phosphate cathode material. Specifically, the main process for preparing nanocrystalline lithium iron phosphate cathode material involves using polyol and water as a mixed solvent. Lithium hydroxide (lithium source), ferrous sulfate (iron source), and phosphoric acid (phosphorus source) are mixed in a specific molar ratio under vacuum or inert gas conditions. A hydrothermal reaction is then carried out at 200℃±30℃ for 1.5~4.5 hours to obtain lithium iron phosphate product, which is then coated with monosaccharides or binary or ternary low-carbon sugars onto nanocrystalline carbon. Nanocrystalline lithium iron phosphate (LiFePO4) cathode materials were prepared using a hydrothermal process in liquid-phase synthesis and a nanolayer carbon coating technique. The resulting materials exhibited an electronic conductivity of 1-10. -1 S / cm, with an electronic conductivity 10 times higher than that of conventional solid-state lithium iron phosphate. -1 ~10 -2 The S / cm ratio is an order of magnitude higher, and the ion migration diameter is 1 / 18 to 1 / 10 of that of lithium iron phosphate ions produced by the traditional solid-state method.

[0044] The following is combined with Figure 6 The steps of S1 in this invention will be described in further detail. The preparation method of the nanocrystalline carbon-coated lithium iron phosphate cathode material in this embodiment includes the following steps:

[0045] Step S1-01: In a lithium hydroxide solution, a graphite die-cast electrode plate is used as the cathode and anode of an electrolytic cell to electrolyze the electrode plate to obtain electrochemically oxidized graphite block material. This electrolyzed graphite block material (electrolytic graphite) has good hydrophilicity and conductivity.

[0046] Step S1-02: Dissolve the iron source and phosphorus source in a mixed solvent of polyol and water to obtain an acid solution for synthesizing nanocrystalline carbon-coated lithium iron phosphate.

[0047] Step S1-03: Dissolve the lithium source in a mixed solvent of polyol and water to obtain an alkaline solution for synthesizing nanocrystalline carbon-coated lithium iron phosphate.

[0048] Step S1-04: Add the acid and alkali solutions to the primary reaction vessel, stir and mix evenly, and adjust to the specified pH range.

[0049] Step S1-05: A single reaction is carried out in a reactor to obtain a homogeneous primary suspension with co-precipitates.

[0050] Step S1-06: Transfer the primary suspension from S1-05 to a secondary reactor, place it under a protective atmosphere, and react it at high temperature and pressure in an aqueous solution of polyol to obtain a slurry in which electrolytic graphite and lithium iron phosphate are uniformly combined. The preferred temperature is 200℃±30℃, and the reaction time is 1.5~4.5 hours.

[0051] Step S1-07: Perform liquid-solid separation on the slurry of S1-06 to obtain a wet solid phase of electrolytic graphite and lithium iron phosphate nanocrystal combined solid.

[0052] Step S1-08: Mix the electrolytic graphite and lithium iron phosphate nanocrystals combined solid in wet solid phase S1-07 with one or more monosaccharides, disaccharides, or trisaccharides until homogeneous, and then compress into tablets.

[0053] Step S1-09: Calcination is carried out under a protective atmosphere and high temperature and pressure to obtain nanocrystalline carbon-coated lithium iron phosphate cathode material. The preferred calcination pressure is 10-25 MPa, the calcination temperature is 700-900°C, and the holding time is 1-3 hours.

[0054] Please see Figure 7 and Figure 8 , Figure 7 SEM image of the prepared nanocrystalline carbon-coated lithium iron phosphate. Figure 8 TEM image of the prepared carbon nanolayer coated lithium iron phosphate, by Figure 7 and Figure 8 It is evident that this process produces lithium iron phosphate particles with finer and more uniform distribution, and denser and more uniform carbon coating. This effectively solves the problem of low conductivity of lithium iron phosphate materials under ultra-low temperature conditions and achieves isotropic electrochemical performance. Furthermore, the hydrothermal method produces products with uniform phase, controllable nanoparticle size, adjustable crystal face growth, fewer defects, high uniformity, high purity, and lower magnetic impurities compared to the solid-state method.

[0055] S2: The needle-shaped coke raw material is crushed and sieved to achieve particle shaping, then subjected to low-temperature heat treatment, followed by high-temperature graphitization, and then size selection sieving. Next, a resin-based liquid-phase carbon source is used to liquid-phase coat the graphitized needle-shaped coke. After carbonization, size selection sieving is performed again to obtain a secondary particle artificial graphite anode material with uniform hard carbon coating. Specifically, the preparation process of the secondary particle needle-shaped coke graphite anode material is as follows: Figure 2 As shown, a liquid carbon source is used to mix with refracted small-particle needle-shaped coke graphite, and the mixture is stirred and granulated to obtain a secondary granular needle-shaped coke graphite precursor with uniform liquid carbon source coating and uniform particle size. The precursor is then subjected to high-temperature thermal pyrolysis (carbonization) to obtain a secondary granular needle-shaped coke graphite with a uniform hard carbon coating on the outer layer.

[0056] The modification process of graphite is as follows Figure 3 As shown, please refer to Figure 3a. Materials with rough surfaces exhibit higher electrochemical reactivity, increasing electrolyte consumption. Surface shaping or amorphous carbon coating can reduce active sites, while a dense internal structure and smooth surface structure can effectively extend battery cycle life. (Please refer to...) Figure 3 b. Large-particle graphite has a long lithium-ion diffusion distance but a small specific surface area, resulting in a small electrolyte wetting area. Conversely, small-particle graphite has a short lithium-ion diffusion distance but a large specific surface area, resulting in a large electrolyte wetting area. Using small-particle graphite can shorten the lithium-ion diffusion distance and increase the electrolyte wetting area, thereby effectively improving the material's rate and power performance. Furthermore, as... Figure 3 As shown in c, coating with amorphous carbon can significantly reduce the electrochemical reaction impedance of the material (see...). Figure 3 d) This method improves the power, low-temperature performance, and fast-charging performance of the material. The graphite obtained by this method possesses near-multidirectional ionic and electronic conductivity, maintaining excellent high-rate charge-discharge performance even at low temperatures. Furthermore, the graphite material, which is formed by re-granulating small-particle-diameter needle-shaped coke graphite, can shorten the ion migration path during the electrochemical reaction of charge-discharge, thereby improving the ability to charge and discharge at high currents.

[0057] The following is combined with Figure 2 The following is a further detailed description of step S2 of the present invention. The preparation method of the hard carbon-coated secondary particulate artificial graphite anode material in this embodiment includes the following steps:

[0058] Step S2-01: Particle shaping, which involves crushing the needle-shaped coke raw material to the target particle size, and then further screening it to separate large and small particles, thereby achieving a concentrated particle size distribution.

[0059] Step S2-02: Low-temperature heat treatment, heating to remove and eliminate volatile components from needle-shaped coke;

[0060] Step S2-03; High-temperature graphitization;

[0061] Step S2-04: The graphitized needle-shaped coke particles are further screened and then coated with a resin-based liquid carbon source in the liquid phase.

[0062] Step S2-05: The graphitized needle coke coated in liquid phase is carbonized to obtain artificial graphite crude product with secondary particles uniformly coated with hard carbon.

[0063] Step S2-06: Perform size selection and sieving to obtain secondary granular artificial graphite product with uniform hard carbon coating.

[0064] It should be noted that the above case only uses needle coke as the raw material for artificial graphite to illustrate the manufacturing process. In fact, the selection of artificial graphite raw materials in this invention includes one or more of coal coke, petroleum coke and needle coke.

[0065] S3: An electrolyte is prepared by adding lithium salt to a multi-component solvent. Specifically, the electrolyte is a lithium salt electrolyte with a concentration of DMC, EP, and EC in a ternary solvent system. The EP is chain-like, and straight-chain carboxyl acids are added as solvent components. Based on the principle of multiphase equilibrium, the electrolyte formulation is optimized to achieve its physicochemical properties of maintaining high salt solubility and conductivity at ultra-low melting points and ultra-low temperatures.

[0066] Understandably, the prerequisites for low-temperature electrolytes are a wide liquid range and high ionic conductivity, as well as low lithium-ion desolvation energy and low impedance film-forming energy. In other words, the multiphase cryogenic eutectic point characteristics of the electrolyte solvent are one of the decisive factors for low-temperature electrolytes. If the melting point is too high, the electrolyte is prone to crystallization at low temperatures, severely affecting the Li-ion content in the electrolyte. + Ion mobility and electrolyte conductivity. Ethyl carbonate (EC) is one of the main components of the electrolyte solvent, but its melting point is 36°C. At low temperatures, its solubility in the electrolyte decreases or even precipitates, significantly affecting the low-temperature performance of the battery. Therefore, by adding components with low melting points and low viscosity, the content of the solvent EC can be reduced, thereby effectively reducing the viscosity and eutectic point of the electrolyte at low temperatures and improving the electrolyte conductivity. Thus, by adding an appropriate amount of ethyl propionate (EP) to the electrolyte, with an EC:EP weight ratio in the range of 1 / 5 to 1 / 6, mixing the two solvents EC and EP, and adding 5% to 10% DMC, a ternary solvent with an ultra-low melting point and wide liquid range can be obtained, with a melting point below -50°C. To ensure that a large current can be achieved over a wide temperature range, the S3 electrolyte designed in this invention appropriately increases the concentration of lithium salt LiPO4, with a concentration range of 1.20 to 1.45 mol / L. By combining the selection and ratio control of the solvent and the adjustment of the lithium salt concentration, the electrolyte can maintain good fluidity and excellent conductivity at around -50°C. This low-melting-point electrolyte greatly improves the performance of the electrolyte at low temperatures. Furthermore, step S3 also includes:

[0067] Using DMC (dimethyl carbonate), EP (ethyl propionate), and EC (ethylene carbonate) as solvents, low-temperature additives, namely FEC (fluoroethylene carbonate) and PS (sulfite), are added to the electrolyte. Specifically, the SEI film formed by the electrolyte with added FEC has a higher LiF content than the SEI film formed by the electrolyte without added FEC. The resulting SEI film is denser and more stable, which is beneficial for reducing the impedance of the SEI film at low temperatures, thereby improving the low-temperature performance of the battery. Furthermore, the combined use of FEC and PS is beneficial for leveraging their low-temperature performance as co-solvents in organic electrolytes. This is because sulfite PS itself has excellent low-temperature performance, and both have good film-forming properties, which can compensate for the shortcomings of FEC, further improving the low-temperature performance of the electrolyte.

[0068] Furthermore, the weight percentages of each component in the S3 low-temperature electrolyte are as follows: EC 5%~20%, EP 50%~75%, DMC 5%~10%, FEC 1%~5%, PS 1%~5%, and the concentration of lithium salt LiPF6 is 1.20~1.45 mol / L.

[0069] In other embodiments, step S3 includes:

[0070] An electrolyte was prepared using EC, PC, and EMC as ternary solvents and lithium hexafluorophosphate and lithium difluorosulfonylimide as mixed binary lithium salts. Low-temperature additives and lithium difluorophosphate were added to the electrolyte. The low-temperature additives included FEC, PS, and VC (ethylene carbonate).

[0071] The electrolyte composition is EC:EMC:PC:FEC:PS:LiPO2F2:VC at 15%~30%:50%~80%:5%~10%:1%~5%:1%~5%:0.5%~3.5%:1%~3.5%, and the total concentration of the mixed lithium salt of lithium hexafluorophosphate and lithium difluorosulfonylimide is 1.10~1.40 mol / L.

[0072] Understandably, the electrolyte contains lithium bis(fluorosulfonyl)imide (LiFSI), a lithium salt that maintains good solubility and conductivity at low temperatures, and low-temperature additives. LiFSI has a slight corrosive effect on the positive electrode aluminum foil in electrolytes with high water content. The electrolyte obtained in this invention also contains lithium difluorophosphate (LiPO2F2), an additive that prevents LiFSI from corroding the positive electrode current collector aluminum foil. In step S3, the PC is a high-dielectric-constant, highly polar, low-melting-point cyclic carbonate with a melting point of -48℃, and the EMC is a low-viscosity, low-melting-point linear carbonate with a melting point of -53℃. This ensures that the electrolyte has a wide solvent range below -43℃ and good lithium salt solubility at low temperatures.

[0073] S4: The positive electrode slurry, made from the positive electrode material obtained in step S1, is coated onto the positive electrode current collector to form a positive electrode sheet. Specifically, the composition of the positive electrode slurry is: 0.5% ~ 2.0% CNT (carbon nanotubes), 1.6% ~ 3.0% PVDF (polyvinylidene fluoride), 2.0% ~ 6.0% Super-Li active conductive carbon powder, and 90% ~ 95% nanocrystalline lithium iron phosphate positive electrode material.

[0074] S5: The negative electrode slurry, made from the negative electrode material obtained in step S2, is coated onto the negative electrode current collector to form a negative electrode sheet. Specifically, the composition of the negative electrode slurry is: 2.5% ~ 5.0% polyacrylonitrile, 2.0% ~ 6.0% Super-Li active conductive carbon powder, and 93% ~ 96.0% granular needle-shaped coke graphite negative electrode material.

[0075] In this embodiment, the surfaces of the positive and negative current collectors are coated with a functional coating, which is prepared by dispersing nano-conductive graphite and carbon-coated particles. Understandably, using carbon-coated aluminum foil instead of conventional double-sided or single-sided smooth rolled aluminum foil as the battery current collector, and utilizing a functional coating to treat the aluminum foil current collector surface, is a groundbreaking technological innovation. Carbon coating involves uniformly and finely coating dispersed nano-conductive graphite and carbon-coated particles onto the aluminum / copper foil. It provides excellent static conductivity, collects the micro-current of the active material, thereby significantly reducing the contact resistance between the positive / negative electrode materials and the current collector, and improving their adhesion. This reduces the amount of binder used, enhances the adhesion between the active material and the current collector, protects the current collector from electrolyte corrosion, and ultimately significantly improves the overall performance of the battery.

[0076] S6: The positive electrode sheet obtained in step S4, the negative electrode sheet obtained in step S5, and the separator are wound and placed into the casing, and the electrolyte obtained in step S3 is injected to obtain an ultra-low temperature lithium iron phosphate power battery. Specifically, the separator is a high-strength, high-porosity separator, which is a PE (polyethylene) separator, a PP (polypropylene) separator, a PP and PE mixed composite stretched separator, or a composite separator with a ceramic or PVDF layer coated on a PE or PP separator substrate, or a composite separator with a ceramic or PVDF layer coated on a PP and PE mixed composite stretched separator substrate. The thickness of the separator is 7~30μm. Figure 4 As shown, the positive electrode, negative electrode, and separator are wound together to form a core. Several spaced-apart positive electrode tabs are welded to one side of the positive electrode, and several spaced-apart negative electrode tabs are welded to one side of the negative electrode. The assembly is carried out using a multi-tab die-cutting and multi-tab winding structure. The overall electrode structure can increase the electron conduction capability, and the multi-tab structure provides sufficient electron conduction area.

[0077] Furthermore, in order to ensure that each tab is neatly aligned on the winding cell after die-cutting, the spacing between each tab of each positive / negative electrode is calculated by computer simulation. Based on the initial circumference of the winding needle, the positive electrode, the negative electrode, and the separator thickness, the value of the electrode growth after each additional turn of winding is simulated and calculated. Based on this value, the spacing distance of each tab of the positive and negative electrodes is determined.

[0078] In this embodiment, the positive electrode, separator, and negative electrode are stacked and wound sequentially to form the battery cell; that is, the battery cell is formed by winding. In other embodiments, the battery cell can also be formed by stacking, that is, after the positive electrode, separator, and negative electrode are stacked, several positive electrode tabs overlap and are all welded to the total positive electrode tab to lead out the positive terminal of the battery, and several negative electrode tabs overlap and are all welded to the total negative electrode tab to lead out the negative terminal of the battery.

[0079] like Figure 5 As shown, the fabrication process of ultra-low temperature lithium iron phosphate power batteries is as follows:

[0080] The positive / negative electrode slurry is stirred and dispersed evenly; the positive / negative electrode slurry is coated onto the positive / negative current collector respectively; rolling; slitting; baking of positive / negative electrode sheets; die cutting; winding of positive, negative, and separator sheets; X-ray inspection; hot pressing; first short-circuit test; pre-welding of positive / negative electrode tabs; ultrasonic welding of cover plate and electrode tabs; casing; laser welding of cover plate and electrode tabs; second short-circuit test; helium mass spectrometry leak detection; laser marking; vacuum baking; moisture test; short-circuit sampling inspection; first automatic liquid injection. First high-temperature aging; negative pressure formation; second automatic liquid injection; second high-temperature aging; final degassing; final shaping; steel ball sealing; capacity testing; first constant-temperature aging; OCV1 / lmp1 test; second constant-temperature aging; OCV2 / lmp2 test; edge voltage test; third constant-temperature aging; OCV3 / lmp3 test; grouping and pairing; grouping verification; initial appearance inspection / thickness inspection; applying insulating pad; applying outer film to battery / applying heat shrink tubing; full inspection of appearance / thickness; packaging; warehousing.

[0081] This invention employs a special process flow for stirring and coating lithium-ion positive / negative electrode slurries and assembling secondary batteries using an ultra-low temperature lithium iron phosphate cathode system, thereby manufacturing high-performance ultra-low temperature lithium iron phosphate cathode system lithium-ion secondary batteries. In this embodiment, before injecting the electrolyte, a contact-type high-vacuum furnace is used for high-temperature, high-vacuum baking to remove moisture from the positive and negative electrode sheets. Furthermore, after injecting the electrolyte, negative pressure formation is introduced to eliminate the gases generated during formation. This process achieves dense SEI films for both positive and negative electrodes, ultra-low internal resistance, and stable, excellent low-temperature, high-rate discharge performance. The special contact-type vacuum baking technology removes trace amounts of moisture, eliminating the influence of trace moisture impurities. The special negative pressure formation process improves the film quality of the formed SEI film and eliminates the influence of gases generated during the formation process on SEI film formation.

[0082] The present invention provides a method for preparing ultra-low temperature lithium iron phosphate power batteries, preferably using a hydrothermal manufacturing process for lithium iron phosphate materials. This process produces lithium iron phosphate particles with finer and more uniform distribution, and denser and more uniform carbon coating, effectively solving the problem of low conductivity of lithium iron phosphate materials in ultra-low temperature environments and achieving isotropic electrochemical performance. Compared with lithium iron phosphate materials produced by traditional solid-state methods, hydrothermal nano-lithium iron phosphate materials have the following characteristics:

[0083] (1) Electronic conductivity: hydrothermal method: 1-10 -1 S / cm, while the traditional solid-phase synthesis method is: 10 -1 ~ 10 -2 The electronic conductivity of lithium iron phosphate materials produced by the hydrothermal method is one order of magnitude higher than that of lithium iron phosphate materials produced by the traditional solid-state synthesis method. Hydrothermal products also have better low-temperature and rate performance.

[0084] (2) The products produced by the hydrothermal method have uniform phases, controllable nanoparticle size, adjustable crystal growth, fewer defects, high uniformity, high purity, and lower magnetic impurities than those produced by the solid phase method.

[0085] (3) The battery cells made by the hydrothermal method have high consistency, fewer grades, low magnetic impurities, reduced self-discharge, and improved safety.

[0086] (4) The nano-sized ultrafine particles of lithium iron phosphate cathode material can also shorten the ion migration path of the charge and discharge reaction and improve the ability of high current charge and discharge. The primary particle diameter of the material obtained by this method is between 80 and 100 nm, and the coating layer is between 2 and 5 nm. In contrast, the primary particle diameter of the material obtained by the traditional solid-phase synthesis method is between 1000 and 1500 nm. The ion migration diameter of the lithium iron phosphate material produced by the hydrothermal method is 1 / 18 to 1 / 10 of that of the lithium iron phosphate material produced by the traditional solid-phase synthesis method.

[0087] Furthermore, the hydrothermal nano-lithium iron phosphate product consists of nanocrystalline primary particles, featuring ultra-low temperature performance, high rate capability, and high consistency. It is suitable for hybrid vehicle batteries, automotive start-stop and starter batteries, as well as energy storage frequency regulation batteries, off-grid energy storage batteries, and military special batteries. It can be widely used in various high and low temperature environments. Power batteries produced using this cathode have excellent high and low temperature performance, good rate capability, high specific capacity, long cycle life, and high safety.

[0088] Furthermore, the method for preparing an ultra-low temperature lithium iron phosphate power battery provided by this invention involves mixing a liquid-phase carbon source with refractory small-particle needle-shaped coke graphite, stirring and granulating the mixture to obtain a secondary granular needle-shaped coke graphite precursor with uniform liquid-phase carbon source coating and uniform particle size. This precursor is then subjected to high-temperature thermal decomposition to obtain secondary granular needle-shaped coke graphite with a uniform hard carbon coating on the outer layer. The graphite obtained by this method possesses near-multidirectional ionic and electronic conductivity, maintaining excellent high-rate charge-discharge performance even at low temperatures. This secondary granulation of small-particle needle-shaped coke graphite further shortens the ion migration path during the electrochemical reactions of charge and discharge, improving the ability to perform high-current charge and discharge.

[0089] The power battery prepared by the method of preparing ultra-low temperature lithium iron phosphate power battery provided by the present invention has the following advantages: (1) Excellent high rate performance: supports continuous charging of 0.1C~5C, supports continuous discharge of 0.1~5C, 6~8C pulse discharge, and the maximum surface temperature of 5C discharge does not exceed 55℃. (2) Discharge capacity in ultra-low temperature environments: 0.1C to 3C high-rate discharge capability in environments of -43℃ to -30℃; (3) Charging capability in ultra-low temperature environments: 0.1C to 0.5C charging capability in environments of -20℃; (4) Ultra-wide discharge operating range: -43℃ to 55℃, ultra-wide charging operating range: -20℃ to 55℃; (5) Ultra-long cycle life performance: more than 3000 cycles of 0.5C charge and discharge at room temperature (cut off at 80% initial capacity); more than 2000 cycles of 1C charge and discharge at room temperature (cut off at 80% initial capacity); more than 1600 cycles of 1C charge and 2C discharge at room temperature (cut off at 80% initial capacity); more than 1400 cycles of 1C charge and 3C discharge at room temperature (cut off at 80% initial capacity). (6) High safety, able to pass extreme safety tests such as heavy object impact, squeezing, high-altitude drop, overcharge and over-discharge, low-pressure storage, and seawater immersion.

[0090] The method for preparing an ultra-low temperature lithium iron phosphate power battery provided by this invention can achieve the following technical effects: the conductivity of the lithium iron phosphate cathode under ultra-low temperature conditions reaches or approaches the conductivity under normal temperature conditions; the electrolyte retains good dissolving ability of lithium salts under ultra-low temperature conditions, remains in a liquid state, and its conductivity under ultra-low temperature conditions reaches or approaches the conductivity, viscosity, and dielectric constant under normal temperature conditions; under high-rate discharge, the voltage platform of the lithium iron phosphate cathode system battery under ultra-low temperature conditions still needs to be much higher than the discharge cutoff voltage, or much higher than the lower limit voltage for the termination of operation of the electrical equipment; the ultra-fine powder lithium iron phosphate cathode slurry is effectively dispersed, and the anti-settling ability of the cathode slurry is improved; It solves the isotropic problem of lithium iron phosphate cathode materials and graphite anode materials at low temperatures, enabling effective electrochemical reactions between the positive and negative electrodes under high-current charging and discharging conditions at ultra-low temperatures. This includes the effective and continuous steady-state operation of ion migration, ion conduction, electron conduction on the surfaces of the positive and negative electrodes, oxidation reaction at the anode, and reduction reaction at the cathode. The ultra-low temperature lithium iron phosphate cathode system power battery has the ability to balance a wide operating temperature range and storage range, meeting the needs of both room temperature applications and ultra-low temperature environments. It overcomes the reverse effect of capillary action of ultrafine powders, achieving the removal of moisture adsorbed on the surface of ultrafine powders and controlling the moisture content of the electrode sheet before liquid injection. It has the ability to continuously discharge at high rates of 0.1 ~ 3C within a wide operating range of -43℃ to 55℃. It solves the problem that conventional batteries cannot be charged below 0℃, enabling charging at ambient temperatures of -20℃ to 0℃.

[0091] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for preparing an ultra-low temperature lithium iron phosphate power battery, characterized in that, Includes the following steps: S1: Take iron source, phosphorus source and lithium source and dissolve them in a mixed solvent of polyol and water. At the same time, take electrolytic graphite and mix it evenly in a mixed solvent of polyol and water. Adjust the pH value. After one reaction and two reactions, obtain a nano-combined solid of electrolytic graphite and lithium iron phosphate. After liquid-solid separation, mix the wet solid combination after liquid-solid separation with low-carbon sugar. Then calcine under an inert protective gas atmosphere to obtain a lithium iron phosphate cathode material coated with nanocrystalline carbon. S2: The needle coke raw material is crushed and sieved to achieve particle shaping, then subjected to low-temperature heat treatment, then high-temperature graphitization, then size selection sieve, and then the graphitized needle coke is coated with a resin-based liquid carbon source in the liquid phase. After carbonization, it is screened again to obtain artificial graphite anode material with uniform hard carbon coating of secondary particles. S3: Add lithium salt to a multi-component solvent to prepare an electrolyte; S4: The positive electrode slurry, made from the positive electrode material obtained in step S1, is coated onto the positive electrode current collector to obtain a positive electrode sheet. S5: The negative electrode slurry, made from the negative electrode material obtained in step S2, is coated onto the negative electrode current collector to obtain the negative electrode sheet. S6: The positive electrode sheet obtained in step S4, the negative electrode sheet obtained in step S5, and the separator are wound and placed into the housing and injected with the electrolyte obtained in step S3 to obtain an ultra-low temperature lithium iron phosphate power battery. Step S3 includes: DMC, EP, and EC are used as solvents, and a low-temperature additive, FEC and PS, is added to the electrolyte. The EP is a linear carbonate, and the composition of the electrolyte is EC:EP:DMC:FEC:PS of 5%~20%:50%~75%:5%~10%:1%~5%:1%~5%; the concentration of lithium salt in the electrolyte is 1.20~1.45 mol / L.

2. The method for preparing an ultra-low temperature lithium iron phosphate power battery as described in claim 1, characterized in that, In step S6, the diaphragm is a PE diaphragm, or a PP diaphragm, or a PP and PE mixed composite stretch diaphragm, or a composite diaphragm with a ceramic or PVDF layer coated on a PE or PP diaphragm substrate, or a composite diaphragm with a ceramic or PVDF layer coated on a PP and PE mixed composite stretch diaphragm substrate, and the thickness of the diaphragm is 7~30μm.

3. The method for preparing an ultra-low temperature lithium iron phosphate power battery as described in claim 1, characterized in that, In step S6, a number of spaced-apart positive electrode tabs are welded to one side of the positive electrode sheet, and a number of spaced-apart negative electrode tabs are welded to one side of the negative electrode sheet.

4. The method for preparing an ultra-low temperature lithium iron phosphate power battery as described in claim 1, characterized in that, In step S6, before injecting the electrolyte, a contact high vacuum furnace is used to vacuum bake the positive and negative electrode sheets to remove moisture.

5. The method for preparing an ultra-low temperature lithium iron phosphate power battery as described in claim 1, characterized in that, In step S6, after injecting the electrolyte, negative pressure formation is performed to eliminate the gas generated during formation.

6. The method for preparing an ultra-low temperature lithium iron phosphate power battery as described in claim 1, characterized in that, The surfaces of the positive current collector and the negative current collector are coated with a functional coating, which is prepared by dispersing nano-conductive graphite and carbon-coated particles.

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