A lithium iron phosphate polymer battery and a preparation method thereof

Abstract

The application belongs to the technical field of lithium ion batteries, and particularly relates to a lithium iron phosphate polymer battery and a preparation method thereof. The battery comprises a positive electrode sheet, a diaphragm coated with an electrolyte and a metal lithium negative electrode sheet which are sequentially stacked. The positive electrode is porous carbon-coated lithium iron phosphate, and the electrolyte comprises lithium tetrafluoroaluminate and polyethylene oxide. The battery uses lithium tetrafluoroaluminate and polyethylene oxide as the electrolyte. Lithium tetrafluoroaluminate continuously reacts with metal lithium and gradually forms in-situ grown Al-Li alloy by lithiation, so as to inhibit lithium deposition and lithium dendrite formation. Meanwhile, a stable SEI film is formed in the reaction process, so as to reduce lithium dendrite formation and improve energy density. The porous carbon-coated lithium iron phosphate is used as the positive electrode. The carbon coating can improve the conductivity of the lithium iron phosphate and reduce the internal resistance of the system, so as to enhance the energy density. The porous structure of the carbon can increase the reaction channel of lithium ions and enhance the energy density.

Description

[0001] This case is a divisional application based on the invention patent filed on January 4, 2024, with application number 202410015389.0 and titled "High Energy Density Lithium Iron Phosphate Polymer Battery and Preparation Method Thereof". Technical Field

[0002] This invention belongs to the field of lithium-ion battery technology, specifically relating to a lithium iron phosphate polymer battery and its preparation method. Background Technology

[0003] With the continuous development of technology, society is placing higher demands on electrochemical energy storage systems. Traditional lithium-ion batteries, due to their liquid electrolyte composition and battery structure, struggle to simultaneously improve both safety and energy density, thus hindering their further development. Compared to liquid lithium-ion batteries, polymer lithium-ion batteries offer advantages such as high thermal stability, strong mechanical strength, and chemical stability, making them promising candidates for the combined use of high-energy positive electrodes and metallic lithium negative electrodes. This allows them to achieve a balance between high energy density and high safety, making polymer batteries a promising research direction for novel electrochemical energy storage devices. However, the low conductivity of polymer electrolytes and the instability of the electrolyte / electrode interface hinder their widespread application.

[0004] The electrolyte, negative electrode, and positive electrode in polymer batteries still need improvement. Electrolytes in polymer batteries include several types such as polyether, polycarbonate, polyurethane, and polysiloxane. Polyethylene oxide (PEO), a polyether-based electrolyte, has been the most studied, but it suffers from low conductivity, low lithium-ion transference number, narrow electrochemical window, and poor thermodynamic stability. (The last sentence appears to be incomplete and possibly refers to a specific battery capacity of 3860 mAh / g.) -1 High theoretical specific capacity, -3.04 V vs. low oxidation potential of standard hydrogen electrode, and 0.534 g / cm³. -3 Low-volume-density lithium metal, used as an anode, significantly improves battery energy density. However, studies have shown that when polymer-based solid electrolytes are matched with lithium anodes to assemble solid-state lithium batteries, problems such as severe lithium deposition and lithium dendrite growth occur during long-term charge-discharge cycles. Lithium iron phosphate (LiFePO4), as the primary cathode material, has become one of the most widely commercialized materials due to its low cost, excellent safety performance, and stable operating voltage platform. However, it also has limitations. -9 S·cm -1 Low electrical conductivity, 10 -3 ~10 -6 cm 2 ·s -1 The low ion diffusion rate of LiFePO4 limits its development in polymer batteries.

[0005] Chinese invention patent application number 202310690101.5 discloses a method for preparing a polymer interface-modified inorganic solid electrolyte. This method improves the conductivity of the electrolyte by adding oxides and simultaneously incorporates ceramic sheets to create a sandwich structure that inhibits lithium dendrite growth. However, this method requires the addition of multiple additives to simultaneously achieve both improved electrolyte conductivity and inhibition of lithium dendrite growth. The process is complex, requires additional steps, and does not consider the formation of the SEI (solid electrolyte interface).

[0006] Chinese invention patent application number 201710874217.9 discloses a method for preparing carbon-coated LiFePO4 / CNTs composite cathode material for lithium-ion batteries. This method involves adding cyclodextrin-modified carbon nanotubes to the cathode slurry to improve the conductivity of lithium iron phosphate and provide lithium-ion channels during charging and discharging. However, this preparation process is complex, requires the use of multiple carbon sources, and the pore size of the carbon nanotubes is difficult to control, resulting in unstable product performance. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a lithium iron phosphate polymer battery and its preparation method, which can solve the problem of lithium dendrite growth piercing the electrolyte.

[0008] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: a method for preparing a lithium iron phosphate polymer battery, including the step of preparing a positive electrode sheet, wherein the method for preparing the positive electrode sheet includes: dissolving lithium iron phosphate and polyvinylpyrrolidone in ethanol and stirring, and then sequentially performing drying, carbonization and ammonia etching to obtain porous carbon-coated lithium iron phosphate; using the porous carbon-coated lithium iron phosphate as the positive electrode active material to prepare the positive electrode sheet; wherein the mass of the polyvinylpyrrolidone is 2 to 8% of the mass of lithium iron phosphate.

[0009] Another technical solution adopted in this invention is: a lithium iron phosphate polymer battery prepared by the above-mentioned preparation method of lithium iron phosphate polymer battery.

[0010] The beneficial effects of this invention are as follows: The lithium iron phosphate polymer battery of this invention solves the problems of battery short circuits and low electrolyte conductivity caused by lithium dendrite growth piercing the electrolyte through a porous carbon-coated lithium iron phosphate positive electrode, an electrolyte containing lithium tetrafluoroaluminate and polyethylene oxide, and a lithium metal negative electrode sheet. Using lithium tetrafluoroaluminate and polyethylene oxide as the electrolyte, when it comes into contact with the lithium metal negative electrode sheet, the lithium tetrafluoroaluminate continuously reacts with the lithium metal and is gradually lithiated to form an in-situ grown Al-Li alloy. This alloy is a good electronic conductor and can suppress lithium deposition and the formation of lithium dendrites. Simultaneously, LiF (lithium fluoride) is also formed during the reaction, forming a stable SEI film to reduce the formation of lithium dendrites. This allows the negative electrode-electrolyte interface to remain stable under high current density, thereby improving energy density. It also uses porous carbon-coated lithium iron phosphate as the positive electrode. The carbon coating can improve its conductivity, reduce the internal resistance of the system, effectively reduce the polarization caused by the internal resistance, and improve its voltage platform, thereby enhancing the energy density. The porous structure of carbon can increase the reaction channels of lithium ions and enhance the energy density. Attached Figure Description

[0011] Figure 1 The diagram shown is a schematic representation of the structure of the negative electrode after contact reaction with metallic lithium in a specific embodiment of the present invention.

[0012] Figure 2 The diagram shown is a schematic diagram of the carbon-coated lithium iron phosphate structure of Embodiment 2 of the present invention.

[0013] Figure 3 The figure shows the charge-discharge curves of the finished batteries of Embodiment 2 and Comparative Examples 1 to 3 of the present invention.

[0014] Figure 4 The figure shows the cycle capacity retention curves of the finished batteries of Embodiment 2 and Comparative Example 1 of the present invention. Detailed Implementation

[0015] To explain in detail the technical content, objectives, and effects of the present invention, the following description is provided in conjunction with the embodiments and accompanying drawings.

[0016] A lithium iron phosphate polymer battery includes a positive electrode, a separator coated with an electrolyte, and a lithium metal negative electrode stacked sequentially. The positive electrode is lithium iron phosphate coated with porous carbon, and the electrolyte includes lithium tetrafluoroaluminate and polyethylene oxide.

[0017] As can be seen from the above description, the beneficial effects of this invention are as follows: This invention selects lithium metal as the negative electrode, which is lightweight and has a high theoretical capacity. Using lithium tetrafluoroaluminate and polyethylene oxide as the electrolyte, when they come into contact with the lithium metal negative electrode, the lithium tetrafluoroaluminate forms a three-dimensional structure with the lithium metal. The two continuously react and are gradually lithiated to form an in-situ grown Al-Li alloy. The Al-Li alloy is a good electronic conductor and can suppress lithium deposition and the formation of lithium dendrites. Simultaneously, LiF is also formed during the reaction, forming a stable SEI film (see...). Figure 1 This reduces the formation of lithium dendrites, thereby ensuring the stability of the negative electrode-electrolyte interface under high current density and improving energy density.

[0018] Lithium tetrafluoroaluminate and lithium iron phosphate in the electrolyte undergo an interfacial reaction. Under high pressure, the lattice O of lithium tetrafluoroaluminate decomposes. 2- It will transform into a strong oxidizing agent, O. 2- 2. The formation of aluminum oxide at the interface with lithium tetrafluoroaluminate leads to increased interfacial impedance; when polyethylene oxide in the electrolyte is matched with lithium iron phosphate, it is prone to oxidative decomposition. This invention uses porous carbon-coated lithium iron phosphate as the positive electrode, which effectively avoids interfacial reactions, reduces the internal resistance of the system, reduces polarization caused by internal resistance, increases its voltage platform, thereby enhancing energy density, and also reduces the side effects of polyethylene oxide oxidative decomposition caused by interfacial reactions. The porous structure of carbon increases the reaction channels for lithium ions, enhancing energy density. The combined effect of the specific positive electrode material, the electrolyte-coated separator, and the lithium metal negative electrode effectively achieves the goal of improving energy density.

[0019] Furthermore, lithium tetrafluoroaluminate accounts for 5-20% of the total mass of the electrolyte.

[0020] Another technical solution adopted in this invention is: the preparation method of the above-mentioned lithium iron phosphate polymer battery includes the following steps:

[0021] S1: Lithium iron phosphate and polyvinylpyrrolidone are dissolved in ethanol and stirred to obtain a mixture. The mixture is dried and then carbonized and etched sequentially to obtain a positive electrode active material. A positive electrode sheet is prepared using the positive electrode active material.

[0022] S2: Mix lithium tetrafluoroaluminate and polyethylene oxide and ball mill to obtain an electrolyte, and coat the electrolyte onto both sides of the diaphragm;

[0023] S3: The positive electrode, the electrolyte-coated separator, and the lithium metal negative electrode are stacked in sequence to obtain a lithium iron phosphate polymer battery.

[0024] As can be seen from the above description, the preparation method of the present invention is simple and easy to operate. In the preparation process, an environmentally friendly solvent, ethanol, is selected as the solvent. Compared with directly using polyvinylpyrrolidone as the solvent, it not only has lower pollution but also accelerates the mixing rate and saves preparation time and cost.

[0025] Furthermore, the stirring time in S1 is 10~13h.

[0026] Furthermore, the carbonization temperature in S1 is 500~650℃. Preferably, the carbonization temperature in S1 is 560~620℃.

[0027] As described above, if the carbonization temperature is too low, the graphitization of the coated carbon will be insufficient, affecting electrochemical properties such as conductivity, and also prolonging the etching time and resulting in lower efficiency. If the carbonization temperature is too high, the sample is prone to agglomeration, which will increase the internal resistance of the battery, affecting the battery's cycle and capacity performance, and also affecting electrochemical performance. It will also lead to structural collapse during subsequent etching. Therefore, the carbonization temperature in S1 is limited to 500~650℃.

[0028] Furthermore, the preparation method of lithium tetrafluoroaluminate is as follows: aluminum nitrate, ammonium fluoride and lithium nitrate are dissolved in deionized water and heated and stirred. The mixture after heating and stirring is dried and then calcined to obtain lithium tetrafluoroaluminate.

[0029] Furthermore, the heating and stirring process specifically involves stirring at 75~85℃ for 5.5~6.5 hours.

[0030] As can be seen from the above description, insufficient stirring temperature and time will lead to the failure of lithium tetrafluoroaluminate preparation, while excessively high temperature may cause the decomposition of raw materials and the generation of impurities. Therefore, the specific heating and stirring time is limited to 75~85℃ for 5.5~6.5h.

[0031] Furthermore, the molar ratio of aluminum nitrate, ammonium fluoride, and lithium nitrate is 1:3 to 5:1.

[0032] As can be seen from the above description, changes in the molar ratio of aluminum nitrate, ammonium fluoride, and lithium nitrate may cause differences in their purity and generate more impurities.

[0033] Furthermore, the mass of polyvinylpyrrolidone is 2 to 8% of the mass of lithium iron phosphate.

[0034] As can be seen from the above description, by controlling the amount of polyvinylpyrrolidone added to adjust the carbon layer thickness, a suitable lithium iron phosphate compaction density can be obtained only when the carbon layer thickness is within a specific range, thereby improving the energy density.

[0035] Embodiment 1 of the present invention is a method for preparing the positive electrode sheet of a lithium iron phosphate polymer battery, comprising the following steps: dissolving 100g of lithium iron phosphate and 2-8g of polyvinylpyrrolidone in anhydrous ethanol and stirring for 12h to obtain a mixture; drying the mixture in an oven and then carbonizing it at 600℃ to form carbon-coated lithium iron phosphate (see...). Figure 2 Carbon-coated lithium iron phosphate was placed in a CVD tube and etched to form a porous carbon structure under ammonia gas. The ammonia gas was introduced for 15 minutes at a rate of 20 mL / min to obtain the positive electrode active material. The prepared carbon-coated lithium iron phosphate, PVDF (polyvinylidene fluoride), and conductive carbon black were added to N-methylpyrrolidone at a mass ratio of 8:1:1 and stirred to obtain a positive electrode slurry. The positive electrode slurry was coated onto carbon-coated aluminum foil to obtain the positive electrode sheet.

[0036] Embodiment 2 of the present invention is a method for preparing a lithium iron phosphate polymer battery, the steps of which are as follows:

[0037] S1: The positive electrode sheet of Example 1 is used.

[0038] S2: Dissolve 89.99g aluminum nitrate, 148.15g ammonium fluoride and 68.95g lithium nitrate in 500 mL deionized water and stir in an oil bath at 80°C for 6 hours. Dry the heated and stirred mixture in an oven at 70°C. The resulting powder is then calcined at 400°C for 6 hours in an Ar atmosphere to prepare lithium tetrafluoroaluminate.

[0039] S3: Mix 5~20g of lithium tetrafluoroaluminate and 80~95g of polyethylene oxide and ball mill them in a ball mill at 100rpm for 2h to obtain the electrolyte.

[0040] S4: Apply electrolyte to both sides of the diaphragm to a thickness of 30μm;

[0041] S5: The lithium metal anode sheet is an ultra-thin lithium strip from Zhongneng Lithium Industry. The positive electrode sheet, the separator coated with electrolyte, and the lithium metal anode sheet are stacked in sequence, and then the process is carried out in sequence by spot welding, encapsulation, baking, standing, hot pressing formation, degassing and capacity testing to make sample 1 to 6 finished batteries (see Table 1). The performance of the finished batteries (lithium iron phosphate polymer batteries) is tested, and the test results are shown in Table 2.

[0042] The testing method is as follows:

[0043] Battery internal resistance testing is performed using an internal resistance tester on the cells after capacity grading.

[0044] The test method for battery capacity and energy density is as follows: Step 1, charge the formed 0.5C cell with constant current and constant voltage to 3.65V and let it stand for 5 minutes; Step 2, discharge the 0.5C cell with constant current to 2.5V and let it stand for 5 minutes; Step 3, charge the 5C cell with constant current and constant voltage to 3.65V and let it stand for 5 minutes; Step 4, discharge the 0.5C cell with constant current to 2.5V and let it stand for 5 minutes; Step 5, charge the 5C cell with constant current and constant voltage to 3.65V and let it stand for 5 minutes. The test is complete.

[0045] Battery cycle test method: Step 1, initial discharge of individual battery cells; Step 2, charge individual battery cells at a constant power of 0.5×n×Prcn to the charging termination voltage of 3.65V, and let stand for 30 minutes; Step 3, discharge individual battery cells at a constant power of 0.5×n´×Prdn' to the discharge termination voltage of 2.5V, and let stand for 30 minutes; Step 4, repeat steps 2~3.

[0046] Table 1

[0047]

[0048] Table 2

[0049]

[0050] Comparative Example 1 of the present invention: A positive electrode and an electrolyte were prepared using conventional methods, and the preparation steps are as follows:

[0051] S1: Add N-methylpyrrolidone to lithium iron phosphate, PVDF and conductive carbon black in a mass ratio of 8:1:1 and stir to obtain a positive electrode slurry. Coat the positive electrode slurry onto carbon-coated aluminum foil to obtain a positive electrode sheet.

[0052] S2: Using PEO as the electrolyte, the electrolyte is coated on both sides of the diaphragm with a thickness of 30μm;

[0053] S4: The positive electrode, the electrolyte-coated separator, and the lithium metal negative electrode are stacked in sequence, and then the battery is manufactured by spot welding, encapsulation, baking, standing, hot pressing, degassing, and capacity testing.

[0054] Comparative Example 2 of the present invention is as follows:

[0055] The only difference between Comparative Example 2 and Example 2 is that the mass of polyvinylpyrrolidone is 10% of the mass of lithium iron phosphate.

[0056] Comparative Example 3 of the present invention is as follows:

[0057] The only difference between Comparative Example 3 and Example 2 is that lithium tetrafluoroaluminate accounts for 30% of the total mass of the electrolyte.

[0058] The sample 2 from Example 2 and the finished batteries prepared in Comparative Examples 1-3 were subjected to charge-discharge tests. The test results are shown in the figure. Figure 3 ,Depend on Figure 3 It can be seen that the finished battery prepared by the preparation method of the example has the highest capacity.

[0059] The finished batteries prepared from Sample 2 in Example 2 and Comparative Example 1 were subjected to battery cycle tests. The test results are shown in [Figure 1]. Figure 4 ,Depend on Figure 4 Up to the point of conclusion, the finished battery prepared by the method of the example was stable, while the comparative example showed a sharp decline, with lithium dendrites generated during the process affecting the cycle stability.

[0060] Example 3 of the present invention is a lithium iron phosphate polymer battery prepared by the preparation method of Example 2.

[0061] Embodiment four of the present invention is a method for preparing a lithium iron phosphate polymer battery, the steps of which are as follows:

[0062] S1: Dissolve 100g of lithium iron phosphate and 2g of polyvinylpyrrolidone in anhydrous ethanol and stir for 10h to obtain a mixture. Dry the mixture in an oven and then carbonize it at 560℃ to form carbon-coated lithium iron phosphate. Place the carbon-coated lithium iron phosphate in a CVD tube and etch a porous carbon structure under ammonia gas. The ammonia gas is introduced for 15min at a rate of 20mL / min to obtain the positive electrode active material. Add the prepared carbon-coated lithium iron phosphate, PVDF (polyvinylidene fluoride), and conductive carbon black to N-methylpyrrolidone at a mass ratio of 8:1:1 and stir to obtain a positive electrode slurry. Coat the positive electrode slurry onto carbon-coated aluminum foil to obtain the positive electrode sheet.

[0063] S2: Dissolve 89.99g aluminum nitrate, 111.11g ammonium fluoride and 68.95g lithium nitrate in 500mL deionized water and stir in an oil bath at 75℃ for 5.5h. Dry the heated and stirred mixture in an oven at 70℃. The dried powder needs to be calcined at 400℃ for 6h in an Ar atmosphere to prepare lithium tetrafluoroaluminate.

[0064] S3: Mix 80g of lithium tetrafluoroaluminate and 20g of polyethylene oxide and ball mill them in a ball mill at 100rpm for 2h to obtain the electrolyte.

[0065] S4: Apply electrolyte to both sides of the diaphragm to a thickness of 20μm;

[0066] S5: The positive electrode, the electrolyte-coated separator, and the lithium metal negative electrode are stacked in sequence, and then the battery is successively spot-welded, packaged, baked, left to stand, hot-pressed, degassed, and capacity-tested to finally produce the finished battery.

[0067] Example 5 of the present invention is a method for preparing a lithium iron phosphate polymer battery, the steps of which are as follows:

[0068] S1: Dissolve 100g of lithium iron phosphate and 8g of polyvinylpyrrolidone in anhydrous ethanol and stir for 13h to obtain a mixture. Dry the mixture in an oven and then carbonize it at 620℃ to form carbon-coated lithium iron phosphate. Place the carbon-coated lithium iron phosphate in a CVD tube and etch a porous carbon structure under ammonia gas. The ammonia gas is introduced for 15min at a rate of 20mL / min to obtain the positive electrode active material. Add the prepared carbon-coated lithium iron phosphate, PVDF (polyvinylidene fluoride), and conductive carbon black to N-methylpyrrolidone in a mass ratio of 8:1:1 and stir to obtain a positive electrode slurry. Coat the positive electrode slurry onto carbon-coated aluminum foil to obtain the positive electrode sheet.

[0069] S2: Dissolve 89.99g aluminum nitrate, 185.18g ammonium fluoride and 68.95g lithium nitrate in 500mL deionized water and stir in an oil bath at 85℃ for 6.5h. Dry the heated and stirred mixture in an oven at 70℃. The dried powder needs to be calcined at 400℃ for 6h in an Ar atmosphere to prepare lithium tetrafluoroaluminate.

[0070] S3: Mix 95g of polyethylene oxide and 5g of lithium tetrafluoroaluminate and ball mill them in a ball mill at 100rpm for 2h to obtain the electrolyte.

[0071] S4: Apply electrolyte to both sides of the diaphragm to a thickness of 40μm;

[0072] S5: The positive electrode, the electrolyte-coated separator, and the lithium metal negative electrode are stacked in sequence, and then the battery is successively spot-welded, packaged, baked, left to stand, hot-pressed, degassed, and capacity-tested to finally produce the finished battery.

[0073] In summary, the lithium iron phosphate polymer battery provided by this invention effectively improves energy density through the combined action of a positive electrode made of a specific material, a separator coated with electrolyte, and a lithium metal negative electrode. This battery and its preparation method have the following advantages:

[0074] 1. Select lithium metal, which is lightweight and has a high theoretical capacity, as the negative electrode.

[0075] 2. Using lithium tetrafluoroaluminate and polyethylene oxide as electrolytes, when they come into contact with the lithium metal anode, lithium tetrafluoroaluminate will form a three-dimensional structure with the lithium metal to grow an Al-Li alloy with good conductivity, which can suppress lithium deposition and the formation of lithium dendrites; at the same time, a stable SEI film will also be formed to reduce the formation of lithium dendrites and improve energy density.

[0076] 3. Using porous carbon-coated lithium iron phosphate as the positive electrode can effectively avoid interfacial reactions, reduce the internal resistance of the system, reduce polarization caused by internal resistance, increase its voltage platform and thus enhance energy density, and also reduce the side effects of polyethylene oxide oxidation decomposition caused by interfacial reactions.

[0077] 4. The carbon in the positive electrode has a porous structure, which can increase the reaction channels of lithium ions and enhance the energy density.

[0078] 5. The preparation method of this battery is simple and easy to operate. The environmentally friendly solvent ethanol is selected as the solvent during the preparation process, which can accelerate the mixing rate.

[0079] 6. During the battery fabrication process, the carbon layer thickness is adjusted by controlling the amount of polyvinylpyrrolidone added to obtain a suitable lithium iron phosphate compaction density, thereby improving the energy density.

[0080] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent modifications made based on the content of the present invention specification and drawings, or direct or indirect applications in related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for preparing a lithium iron phosphate polymer battery, characterized in that, Includes the following steps: S1: Lithium iron phosphate and polyvinylpyrrolidone are dissolved in ethanol and stirred to obtain a mixture. The mixture is dried and then subjected to carbonization and ammonia etching to obtain porous carbon-coated lithium iron phosphate. Porous carbon-coated lithium iron phosphate is used as the positive electrode active material to prepare a positive electrode sheet; the mass of polyvinylpyrrolidone is 2-8% of the mass of lithium iron phosphate. S2: Mix lithium tetrafluoroaluminate and polyethylene oxide and ball mill to obtain an electrolyte, and coat the electrolyte onto both sides of the diaphragm; S3: The positive electrode, the electrolyte-coated separator, and the lithium metal negative electrode are stacked in sequence to obtain a lithium iron phosphate polymer battery.

2. The method for preparing a lithium iron phosphate polymer battery according to claim 1, characterized in that, The carbonization temperature is 560~620℃.

3. The method for preparing a lithium iron phosphate polymer battery according to claim 1, characterized in that, The ammonia gas was introduced for 15 minutes during the ammonia etching process, and the introduction rate was 20 mL / min.

4. The method for preparing a lithium iron phosphate polymer battery according to claim 1, characterized in that, The preparation method of the lithium tetrafluoroaluminate is as follows: aluminum nitrate, ammonium fluoride and lithium nitrate are dissolved in deionized water and heated and stirred. The mixture after heating and stirring is dried and then calcined to obtain lithium tetrafluoroaluminate.

5. The method for preparing a lithium iron phosphate polymer battery according to claim 4, characterized in that, The stirring temperature for heating and stirring is 75~85℃, and the stirring time is 5.5~6.5h.

6. The method for preparing a lithium iron phosphate polymer battery according to claim 4, characterized in that, The molar ratio of aluminum nitrate, ammonium fluoride, and lithium nitrate is 1:3 to 5:

1.

7. The method for preparing a lithium iron phosphate polymer battery according to claim 1, characterized in that, The lithium tetrafluoroaluminate accounts for 5-20% of the total mass of the electrolyte.

8. A lithium iron phosphate polymer battery prepared by the method of any one of claims 1-7.

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