Lithium iron phosphate battery and preparation method thereof

Abstract

The invention provides a lithium iron phosphate battery and a preparation method thereof, the lithium iron phosphate battery comprises: (1) a gradient structure positive plate, the positive plate comprises a current collector, an inorganic solid electrolyte interface layer arranged on the current collector, and a positive active material layer, the positive active material layer comprises an inner layer and an outer layer, the inner layer is close to the inorganic solid electrolyte interface layer, the outer layer is close to the diaphragm, the compaction density of the inner layer is larger than that of the outer layer, and the conductive agent content of the inner layer is smaller than that of the outer layer; (2) a negative plate; (3) a diaphragm; and (4) performing in-situ polymerization to form a solid electrolyte phase. The gradient structure design is introduced into the positive plate, that is, the inner layer adopts relatively high compaction density and relatively low conductive agent content, and the outer layer adopts relatively low compaction density and relatively high conductive agent content, so that the electronic conductivity and the ion transmission efficiency can be effectively considered.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, specifically to a lithium iron phosphate battery and its preparation method. Background Technology

[0002] Lithium-ion batteries are widely used in power batteries, energy storage systems, and consumer electronics due to their high energy density, long cycle life, and environmental friendliness. With the rapid development of the new energy industry, higher requirements are being placed on the safety, energy density, and cycle performance of lithium-ion batteries.

[0003] Currently, liquid electrolyte lithium-ion batteries have advantages in energy density and cost, but they also pose safety risks such as flammability, leakage, and poor interface stability. Solid-state electrolyte batteries, while offering high safety, suffer from low ionic conductivity, high interfacial impedance, and complex fabrication processes, hindering large-scale application. Therefore, developing novel battery systems that combine high energy density, safety, and good cycle performance has become a research hotspot.

[0004] Against this backdrop, how to improve the ionic conductivity and overall stability of batteries while ensuring high energy density is a core technical problem that urgently needs to be solved. Summary of the Invention

[0005] To address the aforementioned technical problems, this application provides a lithium iron phosphate battery and its preparation method.

[0006] The first aspect of this application provides a lithium iron phosphate battery, comprising: (1) a gradient structure positive electrode, the positive electrode comprising a current collector, an inorganic solid electrolyte interface layer disposed on the current collector, and a multilayer positive electrode active material layer, the multilayer positive electrode active material layer comprising an inner layer and an outer layer, wherein the compaction density of the inner layer is greater than that of the outer layer, and the conductive agent content of the inner layer is less than that of the outer layer; (2) a negative electrode; (3) a separator; and (4) a solid electrolyte phase formed by in-situ polymerization.

[0007] Furthermore, the total thickness of the positive electrode active material layer is 20-100 μm, wherein the thickness ratio of the inner layer to the outer layer is 1:5 – 5:1, and the thickness of the inorganic solid electrolyte interface layer is 0.5-3 μm.

[0008] Furthermore, the inorganic solid electrolyte interface layer comprises at least one of lithium aluminum titanium phosphate, lithium aluminum germanium phosphate, or lithium lanthanum zirconium oxide.

[0009] Furthermore, the compaction density of the inner layer is 2.4-2.8 g / cm³. 3 The conductive agent content of the inner layer is 1-3 wt%; the compaction density of the outer layer is 2.0-2.4 g / cm³. 3The conductive agent content accounts for 4-8 wt% of the outer layer.

[0010] The second aspect of this application provides a method for preparing the above-mentioned lithium iron phosphate battery, comprising the following steps: (1) coating an inorganic solid electrolyte interface layer on a current collector and drying and shaping it; (2) sequentially coating a positive electrode active material layer on the interface layer to form an inner layer and an outer layer, wherein the compaction density of the inner layer is greater than that of the outer layer, and the conductive agent content of the inner layer is less than that of the outer layer; (3) assembling a positive electrode sheet, a separator and a negative electrode sheet to form a stack; (4) injecting an electrolyte containing polymer monomers, allowing it to stand and soak before heating and polymerization to form a solid electrolyte phase.

[0011] Furthermore, the heating polymerization temperature is 60-100℃, and the time is 1-4 hours.

[0012] Furthermore, the electrolyte comprises an organic solvent and a lithium salt, wherein the organic solvent is ethylene carbonate and dimethyl carbonate.

[0013] Furthermore, the lithium salt is at least one of LiTFSI and lithium hexafluorophosphate.

[0014] Furthermore, the polymeric monomer is selected from polyethylene glycol diacrylate, diethylene glycol diacrylate, trimethylolpropane triacrylate, polyethylene glycol dimethacrylate, or combinations thereof.

[0015] Furthermore, the content of the polymeric monomer accounts for 5-8 wt% of the electrolyte.

[0016] The present invention has the following beneficial effects: The gradient structure cathode design enables efficient synergistic transport of electrons and ions, improving the rate performance and energy density of the battery.

[0017] The inorganic solid electrolyte interface layer significantly reduces the cathode / electrolyte interface impedance, enhancing interface stability and cycle life.

[0018] In-situ polymerized solid electrolytes improve battery safety and interface contact, making them suitable for applications requiring high safety and high power. Attached Figure Description

[0019] Figure 1 A flowchart illustrating the preparation method of a lithium iron phosphate battery provided in this embodiment of the disclosure. Detailed Implementation

[0020] To facilitate understanding of this application, a more complete description will be provided below. This application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of this application.

[0021] This disclosure provides a lithium iron phosphate battery, comprising: (1) Gradient structure positive electrode, the positive electrode includes a current collector, an inorganic solid electrolyte interface layer disposed on the current collector, and a positive electrode active material layer, the positive electrode active material layer includes an inner layer and an outer layer, the inner layer is close to the inorganic solid electrolyte interface layer, the outer layer is close to the separator, and the compaction density of the inner layer is greater than the compaction density of the outer layer, and the conductive agent content of the inner layer is less than the conductive agent content of the outer layer; (2) negative electrode; (3) separator; (4) solid electrolyte phase formed by in-situ polymerization.

[0022] This invention employs a gradient structure design in the positive electrode, with the inner layer using a higher compaction density and a lower conductive agent content, while the outer layer uses a lower compaction density and a higher conductive agent content. This effectively balances electronic conductivity and ion transport efficiency. The high compaction density in the inner layer helps improve energy density and mechanical stability, while the low conductive agent content reduces ineffective components and improves the utilization rate of active materials. The low compaction density and high conductive agent content in the outer layer enhance electron pathways, reduce overall electrode impedance, and improve rate performance. By introducing an inorganic solid electrolyte interface layer between the current collector and the positive electrode active material, the interface impedance is significantly reduced, enhancing the interface stability between the positive electrode and the electrolyte and suppressing side reactions. An electrolyte containing polymeric monomers is injected into the battery cell and then polymerized by heating to form a continuously distributed solid electrolyte phase, effectively improving the interfacial contact between the electrolyte and the electrode and overall safety. This synergistic structural optimization allows the battery to maintain excellent capacity retention and safety performance under high-rate charge-discharge and long-cycle conditions.

[0023] In some preferred embodiments, the positive electrode active material is lithium iron phosphate. Lithium iron phosphate has excellent thermal stability and cycle life, making it suitable for high-safety solid-state battery systems. Specifically, the lithium iron phosphate can be carbon-coated lithium iron phosphate or doped lithium iron phosphate. These types of lithium iron phosphate have good electronic conductivity and structural stability, which helps to improve the cycle life and rate performance of the battery.

[0024] In some preferred embodiments, the inorganic solid electrolyte interface layer comprises at least one of lithium aluminum titanium phosphate, lithium aluminum germanium phosphate, or lithium lanthanum zirconium oxide. These inorganic interface materials possess high ionic conductivity and good chemical stability, which helps to reduce interfacial impedance and improve interfacial compatibility.

[0025] In some preferred embodiments, the compaction density of the inner layer is 2.4-2.8 g / cm³. 3 The conductive agent content of the inner layer is 1-3 wt%; the compaction density of the outer layer is 2.0-2.4 g / cm³. 3The conductive agent content in the outer layer is 4-8 wt%. By controlling the compaction density and the conductive agent content, the gradient optimization of the electrode structure is achieved, improving the synergistic transport efficiency of ions and electrons. Specifically, the inner layer compaction density can be selected from 2.4 g / cm³. 3 2.6 g / cm 3 2.8 g / cm 3 Alternatively, it can be any range of compacted density between two values, with the outer layer compacted density selected from 2.0 g / cm³. 3 2.2 g / cm 3 2.4 g / cm 3 Or any two values ​​of compaction density; the content of the inner conductive agent can be selected from 1wt%, 2wt%, 3wt% or any two values ​​of content, and the content of the outer conductive agent can be selected from 4wt%, 6wt%, 8wt% or any two values ​​of content.

[0026] In some preferred embodiments, this application provides a method for preparing the above-mentioned lithium iron phosphate battery, comprising the following steps: (1) coating an inorganic solid electrolyte interface layer on a current collector and drying and shaping it; (2) sequentially coating multiple layers of positive electrode active material on the interface layer to form an inner layer and an outer layer, wherein the compaction density of the inner layer is greater than that of the outer layer, and the conductive agent content of the inner layer is less than that of the outer layer; (3) assembling a positive electrode sheet, a separator and a negative electrode sheet to form a stacked body; (4) injecting an electrolyte containing polymeric monomers, allowing it to stand and soak before heating and polymerization to form a solid electrolyte phase.

[0027] This method, by optimizing the cathode structure and in-situ polymerization process, can achieve efficient interfacial contact between the solid electrolyte and the electrode, thereby improving the overall safety and cycle performance of the battery.

[0028] This method pre-constructs an inorganic solid electrolyte interface layer on the current collector surface, followed by layering and coating with positive electrode active material slurries of different ratios to form a gradient structure with different compaction densities. Combined with in-situ polymerization technology, this method prepares a solid electrolyte phase, significantly improving electrode interface stability and ion transport efficiency. Specifically, the polymerization temperature can be 60℃, 70℃, 80℃, 90℃, 100℃, or any range between two values, and the polymerization time can be 1 hour, 2 hours, 3 hours, 4 hours, or any range between two values.

[0029] In some preferred embodiments, the electrolyte comprises an organic solvent and a lithium salt, wherein the organic solvent is vinylene carbonate and dimethyl carbonate. The lithium salt is at least one selected from LiTFSI and lithium hexafluorophosphate. The above electrolyte is compatible with various in-situ polymerized monomers, improving the ionic conductivity and chemical stability of the electrolyte phase. Specifically, battery-grade ethylene carbonate (EC) and dimethyl carbonate (DMC) can be mixed in a 1:1 volume ratio to prepare the electrolyte. This ratio balances the low-temperature performance and high-temperature stability of the electrolyte, improves the solubility and transport efficiency of lithium ions, and is suitable for in-situ polymerized solid-state electrolyte systems.

[0030] In some preferred embodiments, the polymeric monomer is selected from polyethylene glycol diacrylate, diethylene glycol diacrylate, trimethylolpropane triacrylate, polyethylene glycol dimethacrylate, or combinations thereof. Specifically, the content of the polymeric monomer may be 5-8 wt% of the electrolyte.

[0031] like Figure 1 As shown in the figure, this disclosure provides a flowchart of the above-described method 100 for preparing a lithium iron phosphate battery, the method comprising: Step S101: Coat the current collector with an inorganic solid electrolyte interface layer, and dry and set it. Preferably, the current collector is aluminum foil, and the inorganic solid electrolyte interface layer can be selected from materials such as lithium aluminum titanium phosphate, lithium aluminum germanium phosphate, and lithium lanthanum zirconium oxide, with a preferred thickness of 0.5-3 μm. The coating method can be roller coating, blade coating, or spray coating. The drying temperature is preferably 80-120℃, and the drying time is 1-10 hours to ensure the uniformity and density of the interface layer.

[0032] Step S102: A positive electrode active material layer is sequentially coated on the interface layer to form an inner layer and an outer layer, wherein the compaction density of the inner layer is greater than that of the outer layer, and the conductive agent content of the inner layer is less than that of the outer layer. Preferably, the positive electrode active material is lithium iron phosphate, which can be carbon-coated lithium iron phosphate or doped lithium iron phosphate.

[0033] After coating, the compaction density can be adjusted using a roller pressing process. The preferred compaction density for the inner layer is 2.4-2.8 g / cm³. 3 The preferred value is 2.4 g / cm³. 3 2.6 g / cm 3 2.8 g / cm 3 Alternatively, the compaction density range between any two values; the preferred compaction density of the outer layer is 2.0-2.4 g / cm³. 3 The preferred value is 2.0 g / cm³. 3 2.2 g / cm 3 2.4 g / cm3 Or the range of compacted density between any two values.

[0034] The content of the inner conductive agent is 1-3 wt%, preferably 1 wt%, 2 wt%, 3 wt%, or any two of these values; the content of the outer conductive agent is 4-8 wt%, preferably 4 wt%, 6 wt%, 8 wt%, or any two of these values.

[0035] Preferably, the thickness of the inner layer of the positive electrode active material layer is 10-50 μm, and the thickness of the outer layer is 10-50 μm, wherein the thickness ratio of the inner layer to the outer layer is 1:5 – 5:1, preferably 2:3 – 3:2; preferably, the total thickness of the positive electrode active material layer is 20-100 μm.

[0036] Step S103: Assemble the positive electrode, separator, and negative electrode to form a stacked body; Preferably, the separator can be a polyolefin microporous separator or a ceramic-coated polyolefin microporous separator with a thickness of 12-25 μm. The negative electrode can be a conventional lithium-ion battery negative electrode material such as graphite or silicon-carbon. In other embodiments, the assembly method can be a winding type.

[0037] Step S104: Inject an electrolyte containing polymerizable monomers, allow it to stand and soak, then heat to polymerize, forming a solid electrolyte phase.

[0038] Preferably, the electrolyte comprises an organic solvent and a lithium salt, and a polymeric monomer comprising 5-8 wt% of the electrolyte. More preferably, the organic solvent is vinylene carbonate and dimethyl carbonate (volume ratio 1:1); the lithium salt is at least one of LiTFSI and lithium hexafluorophosphate; and the polymeric monomer is selected from polyethylene glycol diacrylate, diethylene glycol diacrylate, trimethylolpropane triacrylate, polyethylene glycol dimethacrylate, or combinations thereof.

[0039] After injection, the preferred time for standing soaking is 3-15 hours to ensure that the electrolyte fully penetrates the electrode structure.

[0040] In the heating polymerization step, the heating temperature can be 60℃, 70℃, 80℃, 90℃, 100℃, or any range between two values, and the polymerization time can be 1 hour, 2 hours, 3 hours, 4 hours, or any range between two values. The preferred condition is 2 hours at 80℃. Heating can be performed using an oven, hot plate, or other temperature-controlled device to ensure complete polymerization of the monomers and the formation of a continuous, dense solid electrolyte phase within the electrode voids.

[0041] The following specific embodiments further illustrate the lithium iron phosphate battery and its preparation method as disclosed in this disclosure.

[0042] Example 1: This example provides a lithium iron phosphate battery and its preparation method. (1) Preparation of positive electrode First, carbon-coated lithium iron phosphate (LFP-C, D50=1.0 μm, primary particle size approximately 150 nm, grade DY-X1, purchased from Defang Nano) was selected as the positive electrode active material. LFP-C, conductive graphite (purchased from Shanshan Co., Ltd., model SS-100), and polyvinylidene fluoride (PVDF, model HSV900, purchased from Arkema) were mixed according to the inner and outer layer formulations, respectively. N-methylpyrrolidone (NMP, electronic grade, purchased from BASF) was added as a solvent, and the mixture was stirred at low speed for 2 hours using a planetary ball mill to obtain uniform inner and outer layer positive electrode slurries.

[0043] On both sides of a 15 μm thick aluminum foil current collector (purchased from Nanshan Aluminum), NASICON-structured lithium aluminum titanium phosphate (LATP, an inorganic solid electrolyte, purchased from Jiuwu Hi-Tech) was uniformly coated using a scraping method. The LATP slurry was made by mixing LATP powder and PVDF at a mass ratio of 95:5, using NMP as a solvent, and after thorough dispersion, it was scraped onto the aluminum foil surface, controlling the dry film thickness to be 1.5 μm. After coating, it was dried in a vacuum oven at 80℃ for 6 hours to obtain a shaped LATP interface layer aluminum foil.

[0044] Subsequently, inner and outer positive electrode active material layers were sequentially coated onto the LATP surface. The inner layer (closer to the LATP side) contained 2.0 wt% conductive agent, 95 wt% LFP-C, and 3 wt% PVDF. After coating, the material was vacuum dried at 80°C for 6 hours, and then compacted using a calender to control the compaction density at 2.6 g / cm³. 3 The outer layer (near the separator) of the positive electrode active material layer contains 6.0 wt% conductive agent, 91 wt% LFP-C, and 3 wt% PVDF. After coating, it is vacuum dried at 80℃ for 6 hours, then calendered, with the compaction density controlled at 2.2 g / cm³. 3 The final product is a gradient structure positive electrode sheet with an inner high-density / low-conductivity agent layer (20 μm thick) and an outer low-density / high-conductivity agent layer (30 μm thick), resulting in a total gradient structure positive electrode sheet with a positive electrode active material layer thickness of 50 μm. All processes were performed in a cleanroom environment to ensure that the electrode sheet was free of impurities and dust.

[0045] (2) Preparation of negative electrode sheet Graphite (BTR-12, a product of BTR-Y), conductive agent (acetylene black, purchased from Cabot), and PVDF (HSV900) were mixed at a mass ratio of 94:3:3, with NMP as the solvent. The mixture was ball-milled and dispersed for 2 hours to obtain a uniform slurry. The slurry was then uniformly coated onto a 12 μm thick copper foil current collector (Nanshan Aluminum) using a blade coating method. The coating was vacuum dried at 80℃ for 6 hours and then calendered to a uniform thickness to obtain a graphite anode sheet. The active material layer of the graphite anode sheet had a thickness of 60 μm.

[0046] (3) Diaphragm The membrane is made of polyethylene (PE) microporous membrane with a thickness of 16 μm (purchased from Shenzhen Xingyuan Material). The membrane has a pore size of about 80 nm and a porosity of about 45%, and has good ion permeability and mechanical strength.

[0047] (4) Preparation of pre-mixed and injection systems The monomer used for polymerization is polyethylene glycol diacrylate (PEGDA, molecular weight 700, purchased from Aladdin). The electrolyte organic solvent is ethylene carbonate (EC, battery grade, BASF) and dimethyl carbonate (DMC, battery grade, BASF) in a volume ratio of 1:1. The lithium salt is lithium hexafluorophosphate (LiPF6, battery grade, Tinci Materials) with a concentration of 1.0 mol / L. 6 wt% PEGDA monomer is added based on the total mass of the electrolyte, and the mixture is thoroughly stirred to prepare a blend for injection. This blend is subsequently used for injection into the battery cell and in-situ polymerization under heating conditions to form a continuously distributed solid electrolyte phase.

[0048] (5) Stacking and assembly The electrodes are stacked in the order of "positive electrode / separator / negative electrode," with the positive electrode sheet, separator, and negative electrode sheet stacked alternately to form a multi-layer stack. The stack is then fitted into an aluminum-plastic film housing, nickel strip tabs are inserted, and the layers are ultrasonically welded to a seal, resulting in the preliminary cell structure.

[0049] (6) Liquid injection, infiltration and in-situ polymerization The PEGDA-containing electrolyte prepared in step (4) was injected into the battery cell using a vacuum injection method to ensure that the electrolyte fully wets the electrode plates and separator pores. The cell was allowed to stand at room temperature for 12 hours to ensure uniform electrolyte distribution. Subsequently, the battery cell was heated at 80 °C for 2 hours to promote in-situ polymerization of PEGDA monomers, forming a dense and continuous solid electrolyte phase. After cooling, the preparation of the lithium iron phosphate battery was completed.

[0050] Example 2 The difference between this embodiment and Example 1 is that the polymer monomer is diethylene glycol diacrylate (DEGDA, Aladdin Chemicals), while the remaining components and process steps are exactly the same as in Example 1.

[0051] Example 3 The difference between this embodiment and Embodiment 1 is that the inorganic solid electrolyte interface layer is lithium aluminum germanium phosphate (LAGP, purchased from Kramar) with a NASICON structure and a thickness controlled to be 2.0 μm.

[0052] Example 4 The difference between this embodiment and Embodiment 1 is that the positive electrode active material is doped lithium iron phosphate (LiMg). 0.02 Fe 0.98 PO4 (D50=0.5μm, purchased from Defang Nano), the remaining components and process steps are the same as in Example 1.

[0053] Example 5 This embodiment further optimizes the positive electrode gradient structure: the inner layer compaction density is 2.8 g / cm³. 3 The conductive agent content is 1 wt%; the outer layer compaction density is 2.0 g / cm³. 3 The conductive agent content is 8 wt%. The remaining components and process steps are the same as in Example 1.

[0054] Example 6 The difference between this embodiment and Embodiment 1 is that the diaphragm is made of polyethylene microporous membrane with a thickness of 12 μm (Xingyuan material), while the other components and process steps are the same.

[0055] Example 7 The difference between this embodiment and Embodiment 1 is that the in-situ polymerization step is performed by heating at 90 °C for 3 hours, while the remaining components and process steps are the same.

[0056] Example 8 The difference between this embodiment and Example 1 is that the lithium salt in the electrolyte is lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, Tinci Materials), with a concentration of 1.0 mol / L. The other components and process steps are the same.

[0057] Example 9 The difference between this embodiment and Embodiment 1 is that the polymer monomer is trimethylolpropane triacrylate (TMPTA, Aladdin Chemicals), while the remaining components and process steps are the same.

[0058] Example 10 The difference between this embodiment and Embodiment 1 is that the thickness of the inorganic solid electrolyte interface layer is 2.5 μm, while the other components and process steps are the same.

[0059] Comparative Example 1 This comparative example does not include an inorganic solid electrolyte interface layer. The remaining positive electrode structure, negative electrode sheet, separator, and electrolyte are the same as in Example 1.

[0060] Comparative Example 2 The positive electrode in this comparative example does not employ a gradient structure; the compaction density of both the inner and outer layers and the content of the conductive agent are both 2.4 g / cm³. 3 And 4 wt%. The remaining components and process steps are the same as in Example 1.

[0061] The lithium batteries of the examples and comparative examples were tested using the following performance testing methods: Cycle life test: At 25°C, charge and discharge cycles were performed at a 1C rate, and the capacity retention rate was recorded as a function of the number of cycles.

[0062] Rate performance test: At 25℃, charge and discharge were performed at rates of 0.2C, 0.5C, 1C, 2C and 5C respectively, and the discharge capacity at each rate was recorded.

[0063] Capacity testing method: Constant current charge-discharge method was used. At the test rate, the charging cut-off voltage was 4.2 V and the discharging cut-off voltage was 2.5 V. Cyclic testing was conducted at room temperature (25 ℃), and the capacity of the first cycle, the capacity after 100 cycles, and the capacity retention rate were recorded. Coulombic efficiency was also monitored.

[0064] Interfacial impedance testing method: Interfacial impedance was measured using AC impedance spectroscopy. The testing instrument was a Bio-Logic VMP3 multichannel electrochemical workstation, with the frequency range set from 0.01 Hz to 1 MHz and the AC perturbation amplitude set to 5 mV. After the battery was allowed to stand for 12 hours, the test was conducted at 25 ℃. The initial value of the interfacial impedance and its change after cycling were obtained by fitting a Nyquist plot.

[0065] Table 1 Test results of the examples and comparative examples As shown in Table 1, all examples exhibited high initial capacity, excellent cycle retention, and low interfacial impedance, especially maintaining high capacity output at high rates (5C). Example 4, using doped lithium iron phosphate, further improved capacity and rate performance. Example 5, with a larger inner and outer layer gradient and further optimized electron and ion channels, showed a slight improvement in rate performance. Examples 3 and 8, by replacing the inorganic solid electrolyte and lithium salt, respectively, showed a slight decrease in performance, but were still superior to the comparative examples.

[0066] Comparative Example 1 lacked an inorganic solid electrolyte interface layer, resulting in a significant increase in interface impedance and a decrease in cycle and rate performance. Comparative Example 2 did not employ a gradient structure, leading to an imbalance in electron and ion transport on the electrodes, and its rate and cycle performance were both lower than those of the examples.

[0067] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0068] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A lithium iron phosphate battery, characterized in that, include: (1) Gradient structure positive electrode, the positive electrode includes a current collector, an inorganic solid electrolyte interface layer disposed on the current collector, and a positive electrode active material layer, the positive electrode active material layer includes an inner layer and an outer layer, and the compaction density of the inner layer is greater than that of the outer layer, and the conductive agent content of the inner layer is less than that of the outer layer; (2) negative electrode; (3) separator; (4) solid electrolyte phase formed by in-situ polymerization.

2. The lithium iron phosphate battery according to claim 1, characterized in that, The total thickness of the positive electrode active material layer is 20-100 μm, wherein the thickness ratio of the inner layer to the outer layer is 1:5 – 5:1, and the thickness of the inorganic solid electrolyte interface layer is 0.5-3 μm.

3. The lithium iron phosphate battery according to claim 1, characterized in that, The inorganic solid electrolyte interface layer comprises at least one of lithium aluminum titanium phosphate, lithium aluminum germanium phosphate, or lithium lanthanum zirconium oxide.

4. The lithium iron phosphate battery according to claim 1, characterized in that, The compaction density of the inner layer is 2.4-2.8 g / cm³. 3 The conductive agent content of the inner layer is 1-3 wt%; the compaction density of the outer layer is 2.0-2.4 g / cm³. 3 The conductive agent content accounts for 4-8 wt% of the outer layer.

5. A method for preparing a lithium iron phosphate battery according to any one of claims 1-4, characterized in that, The process includes the following steps: (1) coating an inorganic solid electrolyte interface layer on the current collector and drying it; (2) coating a positive electrode active material layer on the interface layer in sequence to form an inner layer and an outer layer, wherein the compaction density of the inner layer is greater than that of the outer layer and the conductive agent content of the inner layer is less than that of the outer layer; (3) assembling the positive electrode sheet, the separator and the negative electrode sheet to form a stack; (4) injecting an electrolyte containing polymer monomers, allowing it to stand and soak before heating and polymerizing to form a solid electrolyte phase.

6. The preparation method according to claim 5, characterized in that, The heating polymerization is carried out at a temperature of 60-100℃ for 1-4 hours.

7. The preparation method according to claim 5, characterized in that, The electrolyte comprises an organic solvent and a lithium salt, wherein the organic solvent is ethylene carbonate and dimethyl carbonate.

8. The preparation method according to claim 7, characterized in that, The lithium salt is at least one of LiTFSI and lithium hexafluorophosphate.

9. The preparation method according to claim 5, characterized in that, The polymer monomers are selected from polyethylene glycol diacrylate, diethylene glycol diacrylate, trimethylolpropane triacrylate, polyethylene glycol dimethacrylate, or combinations thereof.

10. The preparation method according to claim 5, characterized in that, The content of the polymeric monomer is 5-8 wt% of the electrolyte.

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