A low-expansion lithium iron phosphate pole piece, a lithium iron phosphate battery and a preparation method thereof

Abstract

The application discloses a kind of low expansion lithium iron phosphate pole piece, including positive current collector;The upper and lower sides of positive current collector surface are respectively coated with at least one positive active material layer;Or, the upper side or lower side of positive current collector surface is coated with at least one positive active material layer;Wherein, positive active material layer includes lithium iron phosphate particles meeting the first material condition, lithium iron phosphate particles meeting the second material condition, conductive agent, binder, dispersing agent and inorganic filler.In addition, the application also discloses a kind of lithium iron phosphate battery, and discloses a kind of low expansion lithium iron phosphate pole piece preparation method, a kind of lithium iron phosphate battery preparation method.The application provides a kind of low expansion lithium iron phosphate pole piece and lithium iron phosphate battery and preparation method, and its design science, the expansion rate of this lithium iron phosphate pole piece is low, can effectively improve the cycle life and safety performance of battery, has great practical significance.

Description

Technical Field

[0001] This invention relates to the field of lithium iron phosphate battery technology, and in particular to a low-expansion lithium iron phosphate electrode, a lithium iron phosphate battery, and a method for preparing them. Background Technology

[0002] Lithium iron phosphate batteries have advantages such as high safety, long service life and low cost, and are widely used in electric vehicles such as passenger cars, commercial vehicles, buses and logistics vehicles. However, the energy density of the battery systems made from them is relatively low compared to lithium nickel cobalt manganese batteries, which limits their market share.

[0003] Currently, there are three methods to improve the energy density of battery systems: 1. Lightweighting of system structural components; 2. Improving the integration efficiency of battery cells into the system; 3. Improving the energy density of individual battery cells. Among these, increasing the proportion of active materials in the individual battery cell by thinning the current collector, increasing the proportion of active materials in the electrode formulation, increasing the electrode areal density, and increasing the electrode compaction density can improve the energy density of the individual battery cell.

[0004] The compaction density of commonly used lithium iron phosphate electrodes is between 2.1 and 2.4 g / cm³. 3 With increased compaction density, the gravimetric energy density of a single battery cell can be increased from 125Wh / kg to 165Wh / kg. There is still significant room for improvement in the gravimetric energy density of lithium iron phosphate battery cells. Currently, vehicle manufacturers are beginning to propose market targets of 175Wh / kg to 190Wh / kg, therefore, there is a need to develop cells with a density of 2.4–2.7 g / cm³. 3 Compacted density lithium iron phosphate electrode sheets.

[0005] However, as the compaction density of lithium iron phosphate (LFP) electrodes increases, after electrode drying, battery drying, and electrolyte injection charging and discharging, the LFP electrodes expand significantly compared to the designed thickness, with an expansion rate of 5-10% after being made into fresh batteries (i.e., batteries that have not undergone charge-discharge cycles). This expansion of the LFP electrodes further causes the electrode assembly to expand, resulting in significant compressive stress on the electrode assembly. Excessive compressive stress at certain locations on the electrode assembly (such as the corners of wound electrode assemblies and the bottom corners of stacked electrode assemblies) makes lithium-ion intercalation difficult and may even lead to lithium plating, affecting the battery's cycle life and safety performance. Summary of the Invention

[0006] The purpose of this invention is to address the technical deficiencies of existing technologies by providing a low-expansion lithium iron phosphate electrode, a lithium iron phosphate battery, and a method for their preparation.

[0007] Therefore, the present invention provides a low-expansion lithium iron phosphate electrode, including a positive current collector;

[0008] The upper and lower surfaces of the positive electrode current collector are each coated with at least one layer of positive electrode active material.

[0009] Alternatively, the upper or lower surface of the positive electrode current collector is coated with at least one layer of positive electrode active material.

[0010] The positive electrode active material layer includes lithium iron phosphate particles that meet the first material conditions, lithium iron phosphate particles that meet the second material conditions, a conductive agent, a binder, a dispersant, and an inorganic filler.

[0011] In the positive electrode active material layer, the mass ratio between lithium iron phosphate particles that meet the first material conditions, lithium iron phosphate particles that meet the second material conditions, conductive agent, binder, dispersant and inorganic filler is (4~48):(50~94):(0.4~5):(1.5~5):(0.05~1):(0.05~1).

[0012] Preferably, the lithium iron phosphate particles that meet the first material conditions include lithium iron phosphate particles with a primary particle structure and lithium iron phosphate particles with a secondary particle structure.

[0013] Among them, the secondary particle size distribution of lithium iron phosphate particles that meet the first material condition is: D min ≥0.05μm, D 10 = (0.8±0.1)μm, D 50 = (9.0±2.0)μm, D 90 = (20.0±2)μm, D max ≤35μm;

[0014] The primary particle size of the lithium iron phosphate particles that meet the first material condition is 0.05–0.5 μm.

[0015] Preferably, the lithium iron phosphate particles that meet the second material conditions include lithium iron phosphate particles with a primary particle structure and lithium iron phosphate particles with a secondary particle structure.

[0016] Among them, the secondary particle size distribution of lithium iron phosphate particles that meet the second material conditions is: D min ≥0.2μm, D 10 = (0.5±0.1)μm, D 50 = (1.6±0.5)μm, D 90 = (3.7±1.0)μm, D max ≤7.0μm;

[0017] The primary particle size of the lithium iron phosphate particles that meet the second material conditions is 0.1–7.0 μm.

[0018] Preferably, the mass ratio of lithium iron phosphate particles meeting the first material condition to lithium iron phosphate particles meeting the second material condition is (4-48):(50-94).

[0019] Preferably, the conductive agent includes at least one of carbon black, multi-walled carbon nanotubes, single-walled carbon nanotubes, graphene, conductive graphite, and carbon fiber.

[0020] Adhesives, including at least one of fluoropolymers, nitrile polymers, and carboxylic acid derivatives;

[0021] Fluoropolymers include at least one of polyvinylidene fluoride, poly(vinylidene fluoride-tetrafluoroethylene), poly(vinylidene fluoride-hexafluoropropylene), polytetrafluoroethylene, and poly(tetrafluoroethylene-hexafluoropropylene);

[0022] Nitrile polymers, including at least one of polyacrylonitrile, acrylonitrile-butadiene copolymer, nitrile butadiene rubber and hydrogenated nitrile butadiene rubber;

[0023] Carboxylic acid derivatives, including at least one of polyacrylate, polyacrylate, acrylate-acrylate-acrylonitrile copolymer, styrene-butadiene rubber and carboxymethyl cellulose salt;

[0024] The dispersant is a styrene-acrylate copolymer;

[0025] Inorganic fillers include at least one of alumina, bauxite, silica, and montmorillonite.

[0026] In addition, the present invention also provides a lithium iron phosphate battery, including a positive electrode sheet as described above, which is a low-expansion lithium iron phosphate electrode sheet.

[0027] In addition, the present invention also provides a method for preparing a low-expansion lithium iron phosphate electrode as described above, comprising the following steps:

[0028] The first step is slurry preparation: Lithium iron phosphate particles that meet the first material conditions, lithium iron phosphate particles that meet the second material conditions, conductive agent, binder, dispersant and inorganic filler are mixed according to a preset mass ratio and stirred evenly. Then, they are dissolved in a solvent for dispersion to obtain positive electrode slurry.

[0029] The second step is coating: the positive electrode slurry is uniformly coated on the surface of the positive electrode current collector to obtain a positive electrode sheet with a positive electrode active material layer on the surface.

[0030] The third step is rolling: the dried positive electrode sheet with a positive active material layer on the surface is rolled until the compaction density of the positive active material meets the preset compaction density value range.

[0031] Step 4, First drying: Place the rolled positive electrode sheet into an oven and perform a drying operation for a preset first duration under a preset first temperature range and a preset first pressure range.

[0032] Step 5, stamping: Cut the first dried positive electrode sheet to the size required for the finished positive electrode sheet to obtain the stamped positive electrode sheet;

[0033] Step 6, Second drying: Place the punched positive electrode sheet into an oven and perform a drying operation for a preset second duration under a preset second temperature range and a preset second pressure range to finally obtain the finished positive electrode sheet.

[0034] Preferably, in the first step, the preset mass ratio between the lithium iron phosphate particles meeting the first material conditions, the lithium iron phosphate particles meeting the second material conditions, the conductive agent, the binder, the dispersant, and the inorganic filler is: (4-48): (50-94): (0.4-5): (1.5-5): (0.05-1): (0.05-1).

[0035] Preferably, in the second step, when the positive electrode slurry is uniformly coated on the upper or lower surface of the positive electrode current collector, the surface density of the coating on one side is 10.0–25.0 mg / cm³. 2 ;

[0036] In the second step, when the positive electrode slurry is uniformly coated on the upper and lower surfaces of the positive electrode current collector, the double-sided coating surface density is 20.0–50.0 mg / cm³. 2 ;

[0037] In the third step, the dried positive electrode sheet with a positive active material layer on its surface is hot-rolled using the rolling rollers on the existing rolling mill.

[0038] The temperature of the rolling roller is 90–180℃;

[0039] In the third step, the preset compaction density range is 2.4 g / cm³. 3 ~2.7g / cm 3 ;

[0040] In the fourth step, during the first drying operation, the preset first temperature range is 80 to 130°C, the preset first pressure range is -50 to -90 kPa, and the preset first duration is 4 to 24 hours.

[0041] In the sixth step, during the second drying operation, the preset second temperature range is 80 to 130°C, the preset second pressure range is -50 to -90 kPa, and the preset second duration is 4 to 24 hours.

[0042] Furthermore, the present invention also provides a method for preparing a lithium iron phosphate battery, comprising the following steps:

[0043] Step 1, stacking: The low-expansion lithium iron phosphate electrode sheet as described in any one of claims 1-5 is used as the positive electrode sheet and assembled into an electrode assembly with the graphite negative electrode sheet and the separator in the form of stacking.

[0044] The second step is assembly: the electrode tabs of the electrode assembly are welded to the battery cover through the adapter piece, and then the electrode assembly is wrapped with a protective sleeve to obtain the assembly.

[0045] The third step is perimeter welding: the assembly and bottom pad are placed into the battery case, and the battery cover is welded to the battery case to produce a semi-finished battery.

[0046] Step 4, third drying: Place the semi-finished batteries in an oven to dry them thoroughly;

[0047] Step 5, electrolyte injection: Inject the electrolyte into the above-mentioned semi-finished battery;

[0048] Step 6, Formation: The semi-finished battery after liquid injection is charged and formed;

[0049] Step 7, let stand, add liquid and seal: Let the semi-finished battery stand, then add liquid and seal it;

[0050] Step 8, Capacity testing and aging: After the existing capacity testing and aging process, the finished lithium iron phosphate battery is obtained.

[0051] As can be seen from the technical solutions provided by the present invention above, compared with the prior art, the present invention provides a low-expansion lithium iron phosphate electrode, a lithium iron phosphate battery, and a preparation method thereof. Its design is scientific, and the lithium iron phosphate electrode has a low expansion rate, which can effectively improve the cycle life and safety performance of the battery, and has significant practical significance. Attached Figure Description

[0052] Figure 1a A schematic diagram of the positive electrode active material layer on the upper side of a low-expansion lithium iron phosphate electrode sheet provided by the present invention;

[0053] Figure 1b A schematic diagram of the positive electrode active material layer on the underside of a low-expansion lithium iron phosphate electrode sheet provided by the present invention;

[0054] Figure 2 A flowchart illustrating a method for preparing a low-expansion lithium iron phosphate electrode provided by the present invention;

[0055] Figure 3 A schematic diagram of a three-dimensional network structure formed by a binder for a low-expansion lithium iron phosphate electrode provided by the present invention;

[0056] Figure 4 This is a schematic diagram illustrating the change in crosslinking points of the binder in a low-expansion lithium iron phosphate electrode provided by the present invention. Detailed Implementation

[0057] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0058] See Figure 1a , Figure 1b The present invention provides a low-expansion lithium iron phosphate electrode (which is a positive electrode), including a positive current collector 5 (e.g., aluminum foil or carbon-coated aluminum foil);

[0059] The upper and lower surfaces of the positive electrode current collector 5 are respectively coated with at least one layer of positive electrode active material;

[0060] Alternatively, the upper or lower surface of the positive electrode current collector 5 is coated with at least one layer of positive electrode active material.

[0061] The positive electrode active material layer includes lithium iron phosphate particles that meet the first material conditions, lithium iron phosphate particles that meet the second material conditions, a conductive agent, a binder 3, a dispersant, and an inorganic filler 4.

[0062] In the positive electrode active material layer, the mass ratio between lithium iron phosphate particles that meet the first material conditions (i.e., including first lithium iron phosphate particles 1), lithium iron phosphate particles that meet the second material conditions (i.e., including second lithium iron phosphate particles 2), conductive agent, binder 3, dispersant and inorganic filler 4 is (4~48):(50~94):(0.4~5):(1.5~5):(0.05~1):(0.05~1).

[0063] It should be noted that, for the present invention, by controlling the particle size distribution and usage amount of lithium iron phosphate particles that meet the first material conditions and the second material conditions, the particle packing density in the electrode can be increased, thereby reducing electrode expansion.

[0064] In specific implementation, lithium iron phosphate particles that meet the first material conditions specifically include first lithium iron phosphate particles 1;

[0065] Lithium iron phosphate particles that meet the second material conditions specifically include second lithium iron phosphate particles 2.

[0066] It should be noted that both the first lithium iron phosphate particle 1 and the second lithium iron phosphate particle 2 are positive electrode active materials.

[0067] In this invention, specifically, lithium iron phosphate particles that meet the first material conditions include lithium iron phosphate particles with a primary particle structure and lithium iron phosphate particles with a secondary particle structure.

[0068] Among them, the particle size distribution of secondary particles (i.e., lithium iron phosphate particles with secondary particle structure) in lithium iron phosphate particles that meet the first material conditions is: D min (Minimum diameter) ≥ 0.05 μm, D 10 = (0.8±0.1)μm, D 50 = (9.0±2.0)μm, D 90 = (20.0±2)μm, D max ≤35μm;

[0069] The primary particles (i.e., lithium iron phosphate particles with a primary particle structure) in the lithium iron phosphate particles that meet the first material conditions have a particle size of 0.05 to 0.5 μm.

[0070] In this invention, specifically, lithium iron phosphate particles that meet the second material conditions include lithium iron phosphate particles with a primary particle structure and lithium iron phosphate particles with a secondary particle structure.

[0071] Among them, the particle size distribution of secondary particles (i.e., lithium iron phosphate particles with secondary particle structure) in lithium iron phosphate particles that meet the second material conditions is: D min ≥0.2μm, D 10 = (0.5±0.1)μm, D 50 = (1.6±0.5)μm, D 90 = (3.7±1.0)μm, D max ≤7.0μm;

[0072] The primary particles (i.e., lithium iron phosphate particles with a primary particle structure) in the lithium iron phosphate particles that meet the second material conditions have a particle size of 0.1 to 7.0 μm.

[0073] In this invention, specifically, the mass ratio of lithium iron phosphate particles meeting the first material condition to lithium iron phosphate particles meeting the second material condition is (4-48):(50-94).

[0074] In this invention, specifically, the conductive agent includes at least one of carbon black, multi-walled carbon nanotubes, single-walled carbon nanotubes, graphene, conductive graphite, and carbon fiber.

[0075] In this invention, specifically, the adhesive 3 includes at least one of fluoropolymers, nitrile polymers, and carboxylic acid derivatives;

[0076] Fluoropolymers include at least one of polyvinylidene fluoride, poly(vinylidene fluoride-tetrafluoroethylene), poly(vinylidene fluoride-hexafluoropropylene), polytetrafluoroethylene, and poly(tetrafluoroethylene-hexafluoropropylene);

[0077] Nitrile polymers, including at least one of polyacrylonitrile, acrylonitrile-butadiene copolymer, nitrile butadiene rubber and hydrogenated nitrile butadiene rubber;

[0078] Carboxylic acid derivatives, including at least one of polyacrylate, polyacrylate, acrylate-acrylate copolymer, acrylate-acrylate-acrylonitrile copolymer, styrene-butadiene rubber and carboxymethyl cellulose salt.

[0079] In this invention, specifically, the dispersant is a styrene-acrylate copolymer, with the acrylate segment serving as the anchoring segment and the styrene segment serving as the solubilizing segment.

[0080] It should be noted that the anchoring segments of the dispersant can adsorb onto the surface of the active material particles, preventing the dispersant from detaching from the active material. The solvation segments of the dispersant contain sterically hindered groups, which can dissolve and disperse in the slurry solvent, preventing particle agglomeration. Through the action of the anchoring and solvation segments, the dispersant can improve the dispersion effect of the slurry, which is beneficial to particle accumulation in the electrode, reduces the interparticle packing forces, and thus reduces electrode expansion.

[0081] In this invention, specifically, the inorganic filler 4 includes at least one of alumina, boehmite, silica and montmorillonite.

[0082] In addition, the present invention also provides a lithium iron phosphate battery, which includes a positive electrode sheet as described above, a low-expansion lithium iron phosphate electrode sheet.

[0083] To prepare the low-expansion lithium iron phosphate electrode provided by the present invention, see [link to relevant documentation]. Figure 2 This invention provides a method for preparing low-expansion lithium iron phosphate electrodes, comprising the following steps:

[0084] Step 1, Slurry preparation: Lithium iron phosphate particles that meet the first material conditions (specifically, first lithium iron phosphate 1), lithium iron phosphate particles that meet the second material conditions (e.g., second lithium iron phosphate 2), conductive agent, binder and dispersant are mixed according to a preset mass ratio and stirred evenly. Then, they are dissolved in a solvent (specifically N-methylpyrrolidone) for dispersion to obtain positive electrode slurry. By adjusting the amount of solvent, the solid content of the positive electrode slurry is adjusted to 63±5% or the viscosity of the positive electrode slurry is adjusted to 8000±2000cp.

[0085] In the first step, specifically, the preset mass ratio between lithium iron phosphate particles that meet the first material conditions (specifically, first lithium iron phosphate 1), lithium iron phosphate particles that meet the second material conditions (e.g., second lithium iron phosphate 2), conductive agent, binder, dispersant and inorganic filler is: (4~48):(50~94):(0.4~5):(1.5~5):(0.05~1):(0.05~1).

[0086] It should be noted that, in the first step of this invention, by controlling the ratio of raw materials in the pulping process and using a dispersant, the positive electrode slurry can be evenly dispersed and the lithium iron phosphate particles can be highly packed.

[0087] The second step is coating: the positive electrode slurry is uniformly coated on the surface of the positive electrode current collector to obtain a positive electrode sheet with a positive electrode active material layer on the surface.

[0088] In the second step, specifically, when the positive electrode slurry is uniformly coated on the upper or lower surface of the positive electrode current collector (e.g., carbon-coated aluminum foil), the surface density of the coating on one side is 10.0–25.0 mg / cm³. 2 The preferred concentration is 14.0–18.0 mg / cm³. 2 .

[0089] In the second step, specifically, when the positive electrode slurry is uniformly coated on the upper and lower surfaces of the positive electrode current collector, the double-sided coating surface density is 20.0–50.0 mg / cm³. 2 The preferred concentration is 28.0–36.0 mg / cm³. 2 .

[0090] It should be noted that, in this invention, electrode expansion is related to areal density. By controlling the areal density range, the electrode can be controlled at a low expansion level.

[0091] The third step is rolling: the dried positive electrode sheet with a positive active material layer on the surface is rolled until the compaction density of the positive active material meets the preset compaction density value range.

[0092] In the third step, the preset compaction density range is 2.4 g / cm³. 3 ~2.7g / cm 3 .

[0093] In the third step, specifically, the dried positive electrode sheet with a positive active material layer on its surface is hot-rolled using the rolling rollers on an existing rolling mill.

[0094] The temperature of the rolling roller is 90–180℃, preferably 110–150℃.

[0095] It should be noted that, in this invention, hot rolling with a rolling roller can improve the adhesion between the binder and the current collector, eliminate internal stress in the electrode slurry layer, and reduce electrode expansion.

[0096] Step 4, First drying: Place the rolled positive electrode sheet into an oven and perform a drying operation for a preset first duration under a preset first temperature range and a preset first pressure range.

[0097] In the fourth step, specifically, during the first drying operation, the preset first temperature range is 80~130℃, the preset first pressure range is -50~-90kPa, and the preset first time is 4~24h; preferably, the preset first temperature range is 100~120℃, the preset first time is 8~16h, and the preset first pressure range is -70~-90kPa.

[0098] Step 5, stamping (i.e. cutting): The positive electrode sheet after the first drying is cut to the size required for the finished positive electrode sheet to obtain the stamped positive electrode sheet;

[0099] Step 6, Second drying: Place the punched positive electrode sheet into an oven and perform a drying operation for a preset second duration under a preset second temperature range and a preset second pressure range to finally obtain the finished positive electrode sheet.

[0100] In the sixth step, specifically in the second drying operation, the preset second temperature range is 80~130℃, the preset second pressure range is -50~-90kPa, and the preset second duration is 4~24h; preferably: the preset second temperature range is 100~120℃, the preset second duration is 8~16h, and the preset second pressure range is -70~-90kPa.

[0101] In this invention, specifically, lithium iron phosphate particles that meet the first material conditions include lithium iron phosphate particles with a primary particle structure and lithium iron phosphate particles with a secondary particle structure.

[0102] Among them, the particle size distribution of secondary particles (i.e., lithium iron phosphate particles with secondary particle structure) in lithium iron phosphate particles that meet the first material conditions is: D min (Minimum diameter) ≥ 0.05 μm, D 10 = (0.8±0.1)μm, D 50 = (9.0±2.0)μm, D 90 = (20.0±2)μm, D max ≤35μm;

[0103] The primary particles (i.e., lithium iron phosphate particles with a primary particle structure) in the lithium iron phosphate particles that meet the first material conditions have a particle size of 0.05 to 0.5 μm.

[0104] In this invention, in the first step, specifically, the lithium iron phosphate particles that meet the second material conditions include lithium iron phosphate particles with a primary particle structure and lithium iron phosphate particles with a secondary particle structure.

[0105] Among them, the particle size distribution of secondary particles (i.e., lithium iron phosphate particles with secondary particle structure) in lithium iron phosphate particles that meet the second material conditions is: D min ≥0.2μm, D10 = (0.5±0.1)μm, D 50 = (1.6±0.5)μm, D 90 = (3.7±1.0)μm, D max ≤7.0μm;

[0106] The primary particles (i.e., lithium iron phosphate particles with a primary particle structure) in the lithium iron phosphate particles that meet the second material conditions have a particle size of 0.1 to 7.0 μm.

[0107] In this invention, in the first step, specifically, the mass ratio of lithium iron phosphate particles that meet the first material conditions to lithium iron phosphate particles that meet the second material conditions is (4-48):(50-94).

[0108] In this invention, in the first step, specifically, the conductive agent includes at least one of carbon black, multi-walled carbon nanotubes, single-walled carbon nanotubes, graphene, conductive graphite, and carbon fiber.

[0109] In this invention, in the first step, specifically, the adhesive includes at least one of fluoropolymers, nitrile polymers, and carboxylic acid derivatives;

[0110] Fluoropolymers include at least one of polyvinylidene fluoride, poly(vinylidene fluoride-tetrafluoroethylene), poly(vinylidene fluoride-hexafluoropropylene), polytetrafluoroethylene, and poly(tetrafluoroethylene-hexafluoropropylene);

[0111] Nitrile polymers, including at least one of polyacrylonitrile, acrylonitrile-butadiene copolymer, nitrile butadiene rubber and hydrogenated nitrile butadiene rubber;

[0112] Carboxylic acid derivatives, including at least one of polyacrylate, polyacrylate, acrylate-acrylate copolymer, acrylate-acrylate-acrylonitrile copolymer, styrene-butadiene rubber and carboxymethyl cellulose salt.

[0113] In this invention, in the first step, specifically, the dispersant is a styrene-acrylate copolymer, with the acrylate segment serving as the anchoring segment and the styrene segment serving as the solubilizing segment.

[0114] like Figure 3As shown, in this invention, through the first drying operation and the second drying operation, i.e., through a two-step heating method, chemical cross-linking points 31 are formed between the binder 3 itself, physical cross-linking points and / or chemical cross-linking points 32 are formed between the binder 3 and the first lithium iron phosphate particle 1 and / or the second lithium iron phosphate particle 2, and physical cross-linking points and / or chemical cross-linking points 33 are formed between the binder 3 and the inorganic filler 4. This makes the binder 3 form a three-dimensional network structure, which is beneficial to improve mechanical strength, enhance resistance to electrolyte swelling, and form a binding force on the lithium iron phosphate particles, reducing the expansion of lithium iron phosphate particles caused by charging and discharging, thereby reducing electrode expansion.

[0115] In this invention, a method for preparing a lithium iron phosphate battery using the aforementioned low-expansion lithium iron phosphate electrode is also provided, comprising the following steps:

[0116] The first step is stacking: The previously prepared low-expansion lithium iron phosphate electrode sheet is used as the positive electrode sheet and assembled into an electrode assembly with the graphite negative electrode sheet and the separator in a stacked form.

[0117] The second step is assembly: the electrode tabs of the electrode assembly are welded to the battery cover through the adapter piece, and then the electrode assembly is wrapped with a protective sleeve to obtain the assembly.

[0118] It should be noted that the positive tab is connected to the positive terminal on the battery cover via a positive adapter, and the negative tab is connected to the negative terminal on the battery cover via a negative adapter.

[0119] The third step is perimeter welding: the assembly and bottom pad are placed into the battery case, and the battery cover is welded to the battery case to form a semi-finished battery.

[0120] Step 4, third drying: Place the semi-finished batteries in an oven to dry them thoroughly;

[0121] In the fourth step, specifically, the semi-finished battery is dried at a temperature of 95℃ and a pressure of -90kPa for 32 hours.

[0122] Step 5, electrolyte injection: Inject the electrolyte into the above-mentioned semi-finished battery.

[0123] Step 6, Formation: The semi-finished battery after liquid injection is charged and formed;

[0124] Step 7, let stand, add electrolyte and seal: Let the semi-finished battery stand, and then add electrolyte (i.e. add electrolyte) and seal it.

[0125] Step 8, Capacity testing and aging: After the existing capacity testing and aging process, the finished lithium iron phosphate battery is obtained.

[0126] It should be noted that, in this invention, after the lithium iron phosphate electrode sheets undergo stacking, assembly, and perimeter welding before being assembled into the battery, a third drying operation further forms physical and / or chemical cross-linking points between the binder itself, between the binder and the lithium iron phosphate particles, and between the binder and the inorganic filler. Figure 4 The increased number of crosslinking points further enables the binder to form a three-dimensional network structure, improving mechanical strength, enhancing resistance to electrolyte swelling, and forming a binding force on lithium iron phosphate particles, reducing the expansion of lithium iron phosphate particles caused by charging and discharging, thereby further reducing electrode expansion.

[0127] To better understand the technical solution of the present invention, the following specific embodiments will be used to illustrate the technical solution of the present invention.

[0128] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0129] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0130] Comparative example.

[0131] The existing methods for preparing lithium iron phosphate electrodes and lithium iron phosphate batteries are described in detail below:

[0132] I. Preparation of lithium iron phosphate slurry, by weight ratio (parts);

[0133] 1. Mix 2 parts of conductive carbon black powder and 33.3 parts of polyvinylidene fluoride (PVDF) adhesive solution with an effective solid content of 6% evenly in a pulping tank.

[0134] 2. Add 48 parts of lithium iron phosphate B powder to the adhesive solution in the slurry mixing tank where conductive carbon black and binder are mixed. Specifically, lithium iron phosphate B powder refers to secondary particles with a particle size distribution of D. min =0.26μm, D 10 =0.53μm, D 50 =1.26μm, D 90 =2.83μm, D max =6.31μm lithium iron phosphate particles.

[0135] 3. Add 48 parts of lithium iron phosphate B powder and N-methylpyrrolidone (NMP) solvent to the well-mixed slurry in the pulping tank to adjust the solid content of the slurry in the pulping tank to 58.0%, and then discharge the material.

[0136] II. Preparation of lithium iron phosphate electrode sheets;

[0137] 1. Coating: The lithium iron phosphate slurry prepared in the above steps is coated onto the carbon-coated aluminum foil current collector. The specifications of the carbon-coated aluminum foil are (13+1+1) μm, where 13 μm is the thickness of the aluminum foil and 1 μm is the thickness of the carbon coating layer on both sides of the aluminum foil. The carbon coating layer is distributed on both sides of the aluminum foil. The areal density of the double-sided coating of the electrode material is 42.0 mg / cm³. 2 .

[0138] 2. Rolling: At room temperature, the thickness of the rolled lithium iron phosphate electrode sheet is 181μm.

[0139] 3. Punching: The lithium iron phosphate electrode sheets are cut into the electrode sheets required for battery manufacturing using a punch.

[0140] III. Preparation of Lithium Iron Phosphate Batteries;

[0141] 1. Stacking: The lithium iron phosphate electrode prepared in the above steps is stacked with the graphite negative electrode and the separator to form an electrode assembly.

[0142] 2. Assembly: Weld the above-mentioned electrode assembly to the adapter plate, battery cover, etc., cover the electrode assembly with a protective sleeve, and assemble.

[0143] 3. Peripheral welding of the casing: Place the above-mentioned assembly and bottom gasket into the battery casing, and weld the battery cover to the battery casing around the periphery to produce a semi-finished battery.

[0144] 4. Third drying: Place the above semi-finished batteries into an oven and dry them for 24 hours at a temperature of 90℃ and a pressure of -80kPa.

[0145] 5. Electrolyte injection: Inject the electrolyte into the above-mentioned semi-finished battery.

[0146] 6. Formation: The semi-finished battery after liquid injection is charged and formed.

[0147] 7. Settling, adding liquid, and sealing: After formation, the semi-finished battery is left to stand, then liquid is added and sealed.

[0148] 8. Capacity testing and aging are performed to obtain lithium iron phosphate batteries.

[0149] Example 1.

[0150] The preparation method of the low-expansion lithium iron phosphate electrode and the preparation method of the lithium iron phosphate battery provided by the present invention are described in detail below:

[0151] I. Preparation of lithium iron phosphate slurry, by weight ratio (parts);

[0152] 1. Mix 4.8 parts of lithium iron phosphate A powder, 90.5 parts of lithium iron phosphate B powder, 2 parts of conductive carbon black powder, 2 parts of polyvinylidene fluoride (PVDF) powder, 0.5 parts of dispersant and 0.2 parts of boehmite (as inorganic filler) evenly in a slurry tank.

[0153] Specifically, lithium iron phosphate A powder refers to secondary particles with a particle size distribution of D. min =0.08μm, D 10 =0.82μm, D 50 =10.58μm, D 90 =20.23μm, D max The lithium iron phosphate particles with a diameter of 30.36 μm are lithium iron phosphate particles that meet the first material condition.

[0154] Lithium iron phosphate B powder specifically refers to secondary particles with a particle size distribution of D. min =0.26μm, D 10 =0.53μm, D 50 =1.26μm, D 90 =2.83μm, D max The lithium iron phosphate particles with a diameter of 6.31 μm are lithium iron phosphate particles that meet the second material conditions.

[0155] 2. Add the solvent N-methylpyrrolidone (NMP) to the mixed dry powder in the pulping tank to adjust the solid content of the pulp in the pulping tank to 74.0%;

[0156] 3. Add N-methylpyrrolidone (NMP) solvent to the pulping tank to adjust the solid content of the pulp in the pulping tank to 68.0%.

[0157] 4. Add N-methylpyrrolidone (NMP) solvent to the pulping tank to adjust the solid content of the pulp in the pulping tank to 60.0%, and then discharge the pulp.

[0158] II. Preparation of lithium iron phosphate electrode sheets;

[0159] 1. Coating: The lithium iron phosphate slurry prepared in the above steps is coated onto the carbon-coated aluminum foil current collector. The specifications of the carbon-coated aluminum foil are (13+1+1) μm, where 13 μm is the thickness of the aluminum foil and 1 μm is the thickness of the carbon coating layer on both sides of the aluminum foil. The carbon coating layer is distributed on both sides of the aluminum foil. The areal density of the double-sided coating of the electrode material is 36.0 mg / cm³. 2 .

[0160] 2. Rolling: At room temperature, the thickness of the rolled lithium iron phosphate electrode sheet is 157μm.

[0161] 3. First drying: Dry for 10 hours at a temperature of 100℃ and a pressure of -90kPa.

[0162] 4. Punching: The lithium iron phosphate electrode sheets are cut into the electrode sheets required for battery manufacturing using a punch.

[0163] 5. Second drying: Dry for 12 hours at a temperature of 110℃ and a pressure of -90kPa.

[0164] III. Preparation of Lithium Iron Phosphate Batteries;

[0165] 1. Stacking: The lithium iron phosphate electrode prepared in the above steps is stacked with the graphite negative electrode and the separator to form an electrode assembly.

[0166] 2. Assembly: Weld the above-mentioned electrode assembly to the adapter plate, battery cover, etc., cover the electrode assembly with a protective sleeve, and assemble.

[0167] 3. Peripheral welding of the casing: Place the above-mentioned assembly and bottom gasket into the battery casing, and weld the battery cover to the battery casing around the periphery to produce a semi-finished battery.

[0168] 4. Third drying: Place the above semi-finished batteries into an oven and dry them for 32 hours at a temperature of 95℃ and a pressure of -90kPa.

[0169] 5. Electrolyte injection: Inject the electrolyte into the above-mentioned semi-finished battery.

[0170] 6. Formation: The semi-finished battery after liquid injection is charged and formed.

[0171] 7. Settling, adding liquid, and sealing: After formation, the semi-finished battery is left to stand, then liquid is added and sealed.

[0172] 8. Capacity testing and aging are performed to obtain lithium iron phosphate batteries.

[0173] Example 2.

[0174] The preparation method of the low-expansion lithium iron phosphate electrode and the preparation method of the lithium iron phosphate battery provided by the present invention are described in detail below:

[0175] I. Preparation of lithium iron phosphate slurry, by weight ratio (parts);

[0176] 1. Mix 9.5 parts of lithium iron phosphate A powder, 85.8 parts of lithium iron phosphate B powder, 2 parts of conductive carbon black powder, 2 parts of polyvinylidene fluoride (PVDF) powder, 0.5 parts of dispersant and 0.2 parts of boehmite (i.e., as inorganic filler) evenly in a slurry tank.

[0177] Specifically, lithium iron phosphate A powder refers to secondary particles with a particle size distribution of D. min =0.08μm, D 10 =0.82μm, D 50 =10.58μm, D 90 =20.23μm, D max The lithium iron phosphate particles with a diameter of 30.36 μm are lithium iron phosphate particles that meet the first material condition.

[0178] Lithium iron phosphate B powder specifically refers to secondary particles with a particle size distribution of D. min =0.26μm, D10 =0.53μm, D 50 =1.26μm, D 90 =2.83μm, D max The lithium iron phosphate particles with a diameter of 6.31 μm are lithium iron phosphate particles that meet the second material conditions.

[0179] 2. Add the solvent N-methylpyrrolidone (NMP) to the mixed dry powder in the pulping tank to adjust the solid content of the pulp in the pulping tank to 74.0%.

[0180] 3. Add N-methylpyrrolidone (NMP) solvent to the pulping tank to adjust the solid content of the pulp in the pulping tank to 68.0%.

[0181] 4. Add N-methylpyrrolidone (NMP) solvent to the pulping tank to adjust the solid content of the pulp in the pulping tank to 60.0%, and then discharge the pulp.

[0182] II. Preparation of lithium iron phosphate electrode sheets;

[0183] 1. Coating: The lithium iron phosphate slurry prepared in the above steps is coated onto the carbon-coated aluminum foil current collector. The specifications of the carbon-coated aluminum foil are (13+1+1) μm, where 13 μm is the thickness of the aluminum foil and 1 μm is the thickness of the carbon coating layer on both sides of the aluminum foil. The carbon coating layer is distributed on both sides of the aluminum foil. The areal density of the double-sided coating of the electrode material is 36.0 mg / cm³. 2 .

[0184] 2. Rolling: At room temperature, the thickness of the rolled lithium iron phosphate electrode sheet is 157μm.

[0185] 3. First drying: Dry for 10 hours at a temperature of 100℃ and a pressure of -90kPa.

[0186] 4. Punching: The lithium iron phosphate electrode sheets are cut into the electrode sheets required for battery manufacturing using a punch.

[0187] 5. Second drying: Dry for 12 hours at a temperature of 110℃ and a pressure of -90kPa.

[0188] III. Preparation of Lithium Iron Phosphate Batteries;

[0189] 1. Stacking: The lithium iron phosphate electrode prepared in the above steps is stacked with the graphite negative electrode and the separator to form an electrode assembly.

[0190] 2. Assembly: Weld the above-mentioned electrode assembly to the adapter plate, battery cover, etc., cover the electrode assembly with a protective sleeve, and assemble.

[0191] 3. Peripheral welding of the casing: Place the above-mentioned assembly and bottom gasket into the battery casing, and weld the battery cover to the battery casing around the periphery to produce a semi-finished battery.

[0192] 4. Third drying: Place the above semi-finished batteries into an oven and dry them for 32 hours at a temperature of 95℃ and a pressure of -90kPa.

[0193] 5. Electrolyte injection: Inject the electrolyte into the above-mentioned semi-finished battery.

[0194] 6. Formation: The semi-finished battery after liquid injection is charged and formed.

[0195] 7. Settling, adding liquid, and sealing: After formation, the semi-finished battery is left to stand, then liquid is added and sealed.

[0196] 8. Capacity testing and aging are performed to obtain lithium iron phosphate batteries.

[0197] Example 3.

[0198] The preparation method of the low-expansion lithium iron phosphate electrode and the preparation method of the lithium iron phosphate battery provided by the present invention are described in detail below:

[0199] I. Preparation of lithium iron phosphate slurry, by weight ratio (parts);

[0200] 1. Mix 14.3 parts of lithium iron phosphate A powder, 81.0 parts of lithium iron phosphate B powder, 2 parts of conductive carbon black powder, 2 parts of polyvinylidene fluoride (PVDF) powder, 0.5 parts of dispersant and 0.2 parts of boehmite (i.e., as inorganic filler) evenly in a slurry tank.

[0201] Specifically, lithium iron phosphate A powder refers to secondary particles with a particle size distribution of D. min =0.08μm, D 10 =0.82μm, D 50 =10.58μm, D 90 =20.23μm, D max The lithium iron phosphate particles with a diameter of 30.36 μm are lithium iron phosphate particles that meet the first material condition.

[0202] Lithium iron phosphate B powder specifically refers to secondary particles with a particle size distribution of D. min =0.26μm, D 10 =0.53μm, D 50 =1.26μm, D 90 =2.83μm, D max The lithium iron phosphate particles with a diameter of 6.31 μm are lithium iron phosphate particles that meet the second material conditions.

[0203] 2. Add the solvent N-methylpyrrolidone (NMP) to the mixed dry powder in the pulping tank to adjust the solid content of the pulp in the pulping tank to 74.0%.

[0204] 3. Add N-methylpyrrolidone (NMP) solvent to the pulping tank to adjust the solid content of the pulp in the pulping tank to 68.0%.

[0205] 4. Add N-methylpyrrolidone (NMP) solvent to the pulping tank to adjust the solid content of the pulp in the pulping tank to 60.0%, and then discharge the pulp.

[0206] II. Preparation of lithium iron phosphate electrode sheets;

[0207] 1. Coating: The lithium iron phosphate slurry prepared in the above steps is coated onto the carbon-coated aluminum foil current collector. The specifications of the carbon-coated aluminum foil are (13+1+1) μm, where 13 μm is the thickness of the aluminum foil and 1 μm is the thickness of the carbon coating layer on both sides of the aluminum foil. The carbon coating layer is distributed on both sides of the aluminum foil. The areal density of the double-sided coating of the electrode material is 36.0 mg / cm³. 2 .

[0208] 2. Rolling: At room temperature, the thickness of the rolled lithium iron phosphate electrode sheet is 157μm.

[0209] 3. First drying: Dry for 10 hours at a temperature of 100℃ and a pressure of -90kPa.

[0210] 4. Punching: The lithium iron phosphate electrode sheets are cut into the electrode sheets required for battery manufacturing using a punch.

[0211] 5. Second drying: Dry for 12 hours at a temperature of 110℃ and a pressure of -90kPa.

[0212] III. Preparation of Lithium Iron Phosphate Batteries;

[0213] 1. Stacking: The lithium iron phosphate electrode prepared in the above steps is stacked with the graphite negative electrode and the separator to form an electrode assembly.

[0214] 2. Assembly: Weld the above-mentioned electrode assembly to the adapter plate, battery cover, etc., cover the electrode assembly with a protective sleeve, and assemble.

[0215] 3. Peripheral welding of the casing: Place the above-mentioned assembly and bottom gasket into the battery casing, and weld the battery cover to the battery casing around the periphery to produce a semi-finished battery.

[0216] 4. Third drying: Place the above semi-finished batteries into an oven and dry them for 32 hours at a temperature of 95℃ and a pressure of -90kPa.

[0217] 5. Electrolyte injection: Inject the electrolyte into the above-mentioned semi-finished battery.

[0218] 6. Formation: The semi-finished battery after liquid injection is charged and formed.

[0219] 7. Settling, adding liquid, and sealing: After formation, the semi-finished battery is left to stand, then liquid is added and sealed.

[0220] 8. Capacity testing and aging are performed to obtain lithium iron phosphate batteries.

[0221] Example 4.

[0222] The preparation method of the low-expansion lithium iron phosphate electrode and the preparation method of the lithium iron phosphate battery provided by the present invention are described in detail below:

[0223] I. Preparation of Lithium Iron Phosphate Slurry, by weight ratio (parts)

[0224] 1. Mix 19.0 parts of lithium iron phosphate A powder, 76.3 parts of lithium iron phosphate B powder, 2 parts of conductive carbon black powder, 2 parts of polyvinylidene fluoride (PVDF) powder, 0.5 parts of dispersant and 0.2 parts of boehmite (i.e., as inorganic filler) evenly in a slurry tank.

[0225] Specifically, lithium iron phosphate A powder refers to secondary particles with a particle size distribution of D. min =0.08μm, D 10 =0.82μm, D 50 =10.58μm, D 90 =20.23μm, D max The lithium iron phosphate particles with a diameter of 30.36 μm are lithium iron phosphate particles that meet the first material condition.

[0226] Lithium iron phosphate B powder specifically refers to secondary particles with a particle size distribution of D. min =0.26μm, D 10 =0.53μm, D 50 =1.26μm, D 90 =2.83μm, D max The lithium iron phosphate particles with a diameter of 6.31 μm are lithium iron phosphate particles that meet the second material conditions.

[0227] 2. Add the solvent N-methylpyrrolidone (NMP) to the mixed dry powder in the pulping tank to adjust the solid content of the pulp in the pulping tank to 74.0% dispersion.

[0228] 3. Add N-methylpyrrolidone (NMP) solvent to the pulping tank to adjust the solid content of the pulp in the pulping tank to 68.0% dispersion.

[0229] 4. Add N-methylpyrrolidone (NMP) solvent to the pulping tank to adjust the solid content of the pulp in the pulping tank to 60.0%, and then discharge the pulp.

[0230] II. Preparation of lithium iron phosphate electrode sheets;

[0231] 1. Coating: The lithium iron phosphate slurry prepared in the above steps is coated onto the carbon-coated aluminum foil current collector. The specifications of the carbon-coated aluminum foil are (13+1+1) μm, where 13 μm is the thickness of the aluminum foil and 1 μm is the thickness of the carbon coating layer on both sides of the aluminum foil. The carbon coating layer is distributed on both sides of the aluminum foil. The areal density of the double-sided coating of the electrode material is 36.0 mg / cm³. 2 .

[0232] 2. Rolling: At room temperature, the thickness of the rolled lithium iron phosphate electrode sheet is 157μm.

[0233] 3. First drying: Dry for 10 hours at a temperature of 100℃ and a pressure of -90kPa.

[0234] 4. Punching: The lithium iron phosphate electrode sheets are cut into the electrode sheets required for battery manufacturing using a punch.

[0235] 5. Second drying: Dry for 12 hours at a temperature of 110℃ and a pressure of -90kPa.

[0236] III. Preparation of Lithium Iron Phosphate Batteries;

[0237] 1. Stacking: The lithium iron phosphate electrode prepared in the above steps is stacked with the graphite negative electrode and the separator to form an electrode assembly.

[0238] 2. Assembly: Weld the above-mentioned electrode assembly to the adapter plate, battery cover, etc., cover the electrode assembly with a protective sleeve, and assemble.

[0239] 3. Peripheral welding of the casing: Place the above-mentioned assembly and bottom gasket into the battery casing, and weld the battery cover to the battery casing around the periphery to produce a semi-finished battery.

[0240] 4. Third drying: Place the above semi-finished batteries into an oven and dry them for 32 hours at a temperature of 95℃ and a pressure of -90kPa.

[0241] 5. Electrolyte injection: Inject the electrolyte into the above-mentioned semi-finished battery.

[0242] 6. Formation: The semi-finished battery after liquid injection is charged and formed.

[0243] 7. Settling, adding liquid, and sealing: After formation, the semi-finished battery is left to stand, then liquid is added and sealed.

[0244] 8. Capacity testing and aging are performed to obtain lithium iron phosphate batteries.

[0245] Example 5.

[0246] The preparation method of the low-expansion lithium iron phosphate electrode and the preparation method of the lithium iron phosphate battery provided by the present invention are described in detail below:

[0247] I. Preparation of lithium iron phosphate slurry, by weight ratio (parts);

[0248] 1. Mix 23.8 parts of lithium iron phosphate A powder, 71.5 parts of lithium iron phosphate B powder, 2 parts of conductive carbon black powder, 2 parts of polyvinylidene fluoride (PVDF) powder, 0.5 parts of dispersant and 0.2 parts of boehmite (i.e., as inorganic filler) evenly in a slurry tank.

[0249] Specifically, lithium iron phosphate A powder refers to secondary particles with a particle size distribution of D. min =0.08μm, D 10 =0.82μm, D 50 =10.58μm, D 90 =20.23μm, D max The lithium iron phosphate particles with a diameter of 30.36 μm are lithium iron phosphate particles that meet the first material conditions.

[0250] Lithium iron phosphate B powder, specifically referring to secondary particles with a particle size distribution of D. min =0.26μm, D 10 =0.53μm, D 50 =1.26μm, D 90 =2.83μm, D max The lithium iron phosphate particles with a diameter of 6.31 μm are lithium iron phosphate particles that meet the second material conditions.

[0251] 2. Add the solvent N-methylpyrrolidone (NMP) to the mixed dry powder in the pulping tank to adjust the solid content of the pulp in the pulping tank to 74.0%.

[0252] 3. Add N-methylpyrrolidone (NMP) solvent to the pulping tank to adjust the solid content of the pulp in the pulping tank to 68.0%.

[0253] 4. Add N-methylpyrrolidone (NMP) solvent to the pulping tank to adjust the solid content of the pulp in the pulping tank to 60.0%, and then discharge the pulp.

[0254] II. Preparation of lithium iron phosphate electrode sheets;

[0255] 1. Coating: The lithium iron phosphate slurry prepared in the above steps is coated onto the carbon-coated aluminum foil current collector. The specifications of the carbon-coated aluminum foil are (13+1+1) μm, where 13 μm is the thickness of the aluminum foil and 1 μm is the thickness of the carbon coating layer on both sides of the aluminum foil. The carbon coating layer is distributed on both sides of the aluminum foil. The areal density of the double-sided coating of the electrode material is 36.0 mg / cm³. 2 .

[0256] 2. Rolling: At room temperature, the thickness of the rolled lithium iron phosphate electrode sheet is 157μm.

[0257] 3. First drying: Dry for 10 hours at a temperature of 100℃ and a pressure of -90kPa.

[0258] 4. Punching: The lithium iron phosphate electrode sheets are cut into the electrode sheets required for battery manufacturing using a punch.

[0259] 5. Second drying: Dry for 12 hours at a temperature of 110℃ and a pressure of -90kPa.

[0260] III. Preparation of Lithium Iron Phosphate Batteries;

[0261] 1. Stacking: The lithium iron phosphate electrode prepared in the above steps is stacked with the graphite negative electrode and the separator to form an electrode assembly.

[0262] 2. Assembly: Weld the above-mentioned electrode assembly to the adapter plate, battery cover, etc., cover the electrode assembly with a protective sleeve, and assemble.

[0263] 3. Peripheral welding of the casing: Place the above-mentioned assembly and bottom gasket into the battery casing, and weld the battery cover to the battery casing around the periphery to produce a semi-finished battery.

[0264] 4. Third drying: Place the above semi-finished batteries into an oven and dry them for 32 hours at a temperature of 95℃ and a pressure of -90kPa.

[0265] 5. Electrolyte injection: Inject the electrolyte into the above-mentioned semi-finished battery.

[0266] 6. Formation: The semi-finished battery after liquid injection is charged and formed.

[0267] 7. Settling, adding liquid, and sealing: After formation, the semi-finished battery is left to stand, then liquid is added and sealed.

[0268] 8. Capacity testing and aging are performed to obtain lithium iron phosphate batteries.

[0269] Example 6.

[0270] The preparation method of the low-expansion lithium iron phosphate electrode and the preparation method of the lithium iron phosphate battery provided by the present invention are described in detail below:

[0271] I. Preparation of lithium iron phosphate slurry, by weight ratio (parts);

[0272] 1. Mix 9.5 parts of lithium iron phosphate A powder, 85.8 parts of lithium iron phosphate B powder, 2 parts of conductive carbon black powder, 2 parts of polyvinylidene fluoride (PVDF) powder, 0.5 parts of dispersant and 0.2 parts of boehmite (i.e., as inorganic filler) evenly in a slurry tank.

[0273] Specifically, lithium iron phosphate A powder refers to secondary particles with a particle size distribution of D. min =0.08μm, D 10 =0.82μm, D 50 =10.58μm, D 90=20.23μm, D max The lithium iron phosphate particles with a diameter of 30.36 μm are lithium iron phosphate particles that meet the first material condition.

[0274] Lithium iron phosphate B powder specifically refers to secondary particles with a particle size distribution of D. min =0.26μm, D 10 =0.53μm, D 50 =1.26μm, D 90 =2.83μm, D max The lithium iron phosphate particles with a diameter of 6.31 μm are lithium iron phosphate particles that meet the second material conditions.

[0275] 2. Add the solvent N-methylpyrrolidone (NMP) to the mixed dry powder in the pulping tank to adjust the solid content of the pulp in the pulping tank to 74.0%.

[0276] 3. Add N-methylpyrrolidone (NMP) solvent to the pulping tank to adjust the solid content of the pulp in the pulping tank to 68.0%.

[0277] 4. Add N-methylpyrrolidone (NMP) solvent to the pulping tank to adjust the solid content of the pulp in the pulping tank to 60.0%, and then discharge the pulp.

[0278] II. Preparation of lithium iron phosphate electrode sheets;

[0279] 1. Coating: The lithium iron phosphate slurry prepared in the above steps is coated onto the carbon-coated aluminum foil current collector. The specifications of the carbon-coated aluminum foil are (13+1+1) μm, where 13 μm is the thickness of the aluminum foil and 1 μm is the thickness of the carbon coating layer on both sides of the aluminum foil. The carbon coating layer is distributed on both sides of the aluminum foil. The areal density of the double-sided coating of the electrode material is 36.0 mg / cm³. 2 .

[0280] 2. Rolling: At a high temperature of 100℃, the thickness of the rolled lithium iron phosphate electrode sheet is 157μm.

[0281] 3. First drying: Dry for 10 hours at a temperature of 100℃ and a pressure of -90kPa.

[0282] 4. Punching: The lithium iron phosphate electrode sheets are cut into the electrode sheets required for battery manufacturing using a punch.

[0283] 5. Second drying: Dry for 12 hours at a temperature of 110℃ and a pressure of -90kPa.

[0284] III. Preparation of Lithium Iron Phosphate Batteries;

[0285] 1. Stacking: The lithium iron phosphate electrode prepared in the above steps is stacked with the graphite negative electrode and the separator to form an electrode assembly.

[0286] 2. Assembly: Weld the above-mentioned electrode assembly to the adapter plate, battery cover, etc., cover the electrode assembly with a protective sleeve, and assemble.

[0287] 3. Peripheral welding of the casing: Place the above-mentioned assembly and bottom gasket into the battery casing, and weld the battery cover to the battery casing around the periphery to produce a semi-finished battery.

[0288] 4. Third drying: Place the above semi-finished batteries into an oven and dry them for 32 hours at a temperature of 95℃ and a pressure of -90kPa.

[0289] 5. Electrolyte injection: Inject the electrolyte into the above-mentioned semi-finished battery.

[0290] 6. Formation: The semi-finished battery after liquid injection is charged and formed.

[0291] 7. Settling, adding liquid, and sealing: After formation, the semi-finished battery is left to stand, then liquid is added and sealed.

[0292] 8. Capacity testing and aging are performed to obtain lithium iron phosphate batteries.

[0293] Example 7.

[0294] The preparation method of the low-expansion lithium iron phosphate electrode and the preparation method of the lithium iron phosphate battery provided by the present invention are described in detail below:

[0295] I. Preparation of lithium iron phosphate slurry, by weight ratio (parts);

[0296] 1. Mix 9.5 parts of lithium iron phosphate A powder, 85.8 parts of lithium iron phosphate B powder, 2 parts of conductive carbon black powder, 2 parts of polyvinylidene fluoride (PVDF) powder, 0.5 parts of dispersant and 0.2 parts of boehmite (i.e., as inorganic filler) evenly in a slurry tank.

[0297] Specifically, lithium iron phosphate A powder refers to secondary particles with a particle size distribution of D. min =0.08μm, D 10 =0.82μm, D 50 =10.58μm, D 90 =20.23μm, D max The lithium iron phosphate particles with a diameter of 30.36 μm are lithium iron phosphate particles that meet the first material condition.

[0298] Lithium iron phosphate B powder specifically refers to secondary particles with a particle size distribution of D. min =0.26μm, D 10 =0.53μm, D 50 =1.26μm, D 90 =2.83μm, D max The lithium iron phosphate particles with a diameter of 6.31 μm are lithium iron phosphate particles that meet the second material conditions.

[0299] 2. Add the solvent N-methylpyrrolidone (NMP) to the mixed dry powder in the pulping tank to adjust the solid content of the pulp in the pulping tank to 74.0%.

[0300] 3. Add N-methylpyrrolidone (NMP) solvent to the pulping tank to adjust the solid content of the pulp in the pulping tank to 68.0%.

[0301] 4. Add N-methylpyrrolidone (NMP) solvent to the pulping tank to adjust the solid content of the pulp in the pulping tank to 60.0%, and then discharge the pulp.

[0302] II. Preparation of lithium iron phosphate electrode sheets;

[0303] 1. Coating: The lithium iron phosphate slurry prepared in the above steps is coated onto the carbon-coated aluminum foil current collector. The specifications of the carbon-coated aluminum foil are (13+1+1) μm, where 13 μm is the thickness of the aluminum foil and 1 μm is the thickness of the carbon coating layer on both sides of the aluminum foil. The carbon coating layer is distributed on both sides of the aluminum foil. The areal density of the double-sided coating of the electrode material is 36.0 mg / cm³. 2 .

[0304] 2. Rolling: At a high temperature of 110℃, the thickness of the rolled lithium iron phosphate electrode sheet is 157μm.

[0305] 3. First drying: Dry for 10 hours at a temperature of 100℃ and a pressure of -90kPa.

[0306] 4. Punching: The lithium iron phosphate electrode sheets are cut into the electrode sheets required for battery manufacturing using a punch.

[0307] 5. Second drying: Dry for 12 hours at a temperature of 110℃ and a pressure of -90kPa.

[0308] III. Preparation of Lithium Iron Phosphate Batteries;

[0309] 1. Stacking: The lithium iron phosphate electrode prepared in the above steps is stacked with the graphite negative electrode and the separator to form an electrode assembly.

[0310] 2. Assembly: Weld the above-mentioned electrode assembly to the adapter plate, battery cover, etc., cover the electrode assembly with a protective sleeve, and assemble.

[0311] 3. Peripheral welding of the casing: Place the above-mentioned assembly and bottom gasket into the battery casing, and weld the battery cover to the battery casing around the periphery to produce a semi-finished battery.

[0312] 4. Third drying: Place the above semi-finished batteries into an oven and dry them for 32 hours at a temperature of 95℃ and a pressure of -90kPa.

[0313] 5. Electrolyte injection: Inject the electrolyte into the above-mentioned semi-finished battery.

[0314] 6. Formation: The semi-finished battery after liquid injection is charged and formed.

[0315] 7. Settling, adding liquid, and sealing: After formation, the semi-finished battery is left to stand, then liquid is added and sealed.

[0316] 8. Capacity testing and aging are performed to obtain lithium iron phosphate batteries.

[0317] Example 8.

[0318] The preparation method of the low-expansion lithium iron phosphate electrode and the preparation method of the lithium iron phosphate battery provided by the present invention are described in detail below:

[0319] I. Preparation of Lithium Iron Phosphate Slurry, by weight ratio (parts)

[0320] 1. Mix 9.5 parts of lithium iron phosphate A powder, 85.8 parts of lithium iron phosphate B powder, 2 parts of conductive carbon black powder, 2 parts of polyvinylidene fluoride (PVDF) powder, 0.5 parts of dispersant and 0.2 parts of boehmite (i.e., as inorganic filler) evenly in a slurry tank.

[0321] Specifically, lithium iron phosphate A powder refers to secondary particles with a particle size distribution of D. min =0.08μm, D 10 =0.82μm, D 50 =10.58μm, D 90 =20.23μm, D max The lithium iron phosphate particles with a diameter of 30.36 μm are lithium iron phosphate particles that meet the first material condition.

[0322] Lithium iron phosphate B powder specifically refers to secondary particles with a particle size distribution of D. min =0.26μm, D 10 =0.53μm, D 50 =1.26μm, D 90 =2.83μm, D max The lithium iron phosphate particles with a diameter of 6.31 μm are lithium iron phosphate particles that meet the second material conditions.

[0323] 2. Add the solvent N-methylpyrrolidone (NMP) to the mixed dry powder in the pulping tank to adjust the solid content of the pulp in the pulping tank to 74.0%.

[0324] 3. Add N-methylpyrrolidone (NMP) solvent to the pulping tank to adjust the solid content of the pulp in the pulping tank to 68.0%.

[0325] 4. Add N-methylpyrrolidone (NMP) solvent to the pulping tank to adjust the solid content of the pulp in the pulping tank to 60.0%, and then discharge the pulp.

[0326] II. Preparation of lithium iron phosphate electrode sheets;

[0327] 1. Coating: The lithium iron phosphate slurry prepared in the above steps is coated onto the carbon-coated aluminum foil current collector. The specifications of the carbon-coated aluminum foil are (13+1+1) μm, where 13 μm is the thickness of the aluminum foil and 1 μm is the thickness of the carbon coating layer on both sides of the aluminum foil. The carbon coating layer is distributed on both sides of the aluminum foil. The areal density of the double-sided coating of the electrode material is 36.0 mg / cm³. 2 .

[0328] 2. Rolling: At a high temperature of 120℃, the thickness of the rolled lithium iron phosphate electrode sheet is 157μm.

[0329] 3. First drying: Dry for 10 hours at a temperature of 100℃ and a pressure of -90kPa.

[0330] 4. Punching: The lithium iron phosphate electrode sheets are cut into the electrode sheets required for battery manufacturing using a punch.

[0331] 5. Second drying: Dry for 12 hours at a temperature of 110℃ and a pressure of -90kPa.

[0332] III. Preparation of Lithium Iron Phosphate Batteries;

[0333] 1. Stacking: The lithium iron phosphate electrode prepared in the above steps is stacked with the graphite negative electrode and the separator to form an electrode assembly.

[0334] 2. Assembly: Weld the above-mentioned electrode assembly to the adapter plate, battery cover, etc., cover the electrode assembly with a protective sleeve, and assemble.

[0335] 3. Peripheral welding of the casing: Place the above-mentioned assembly and bottom gasket into the battery casing, and weld the battery cover to the battery casing around the periphery to produce a semi-finished battery.

[0336] 4. Third drying: Place the above semi-finished batteries into an oven and dry them for 32 hours at a temperature of 95℃ and a pressure of -90kPa.

[0337] 5. Electrolyte injection: Inject the electrolyte into the above-mentioned semi-finished battery.

[0338] 6. Formation: The semi-finished battery after liquid injection is charged and formed.

[0339] 7. Settling, adding liquid, and sealing: After formation, the semi-finished battery is left to stand, then liquid is added and sealed.

[0340] 8. Capacity testing and aging are performed to obtain lithium iron phosphate batteries.

[0341] Example 9.

[0342] The preparation method of the low-expansion lithium iron phosphate electrode and the preparation method of the lithium iron phosphate battery provided by the present invention are described in detail below:

[0343] I. Preparation of lithium iron phosphate slurry, by weight ratio (parts);

[0344] 1. Mix 9.5 parts of lithium iron phosphate A powder, 85.8 parts of lithium iron phosphate B powder, 2 parts of conductive carbon black powder, 2 parts of polyvinylidene fluoride (PVDF) powder, 0.5 parts of dispersant and 0.2 parts of boehmite (i.e., as inorganic filler) evenly in a slurry tank.

[0345] Specifically, lithium iron phosphate A powder refers to secondary particles with a particle size distribution of D. min =0.08μm, D 10 =0.82μm, D 50 =10.58μm, D 90 =20.23μm, D max The lithium iron phosphate particles with a diameter of 30.36 μm are lithium iron phosphate particles that meet the first material condition.

[0346] Lithium iron phosphate B powder specifically refers to secondary particles with a particle size distribution of D. min =0.26μm, D 10 =0.53μm, D 50 =1.26μm, D 90 =2.83μm, D max The lithium iron phosphate particles with a diameter of 6.31 μm are lithium iron phosphate particles that meet the second material conditions.

[0347] 2. Add the solvent N-methylpyrrolidone (NMP) to the mixed dry powder in the pulping tank to adjust the solid content of the pulp in the pulping tank to 74.0%.

[0348] 3. Add N-methylpyrrolidone (NMP) solvent to the pulping tank to adjust the solid content of the pulp in the pulping tank to 68.0%.

[0349] 4. Add N-methylpyrrolidone (NMP) solvent to the pulping tank to adjust the solid content of the pulp in the pulping tank to 60.0%, and then discharge the pulp.

[0350] II. Preparation of lithium iron phosphate electrode sheets;

[0351] 1. Coating: The lithium iron phosphate slurry prepared in the above steps is coated onto the carbon-coated aluminum foil current collector. The specifications of the carbon-coated aluminum foil are (13+1+1) μm, where 13 μm is the thickness of the aluminum foil and 1 μm is the thickness of the carbon coating layer on both sides of the aluminum foil. The carbon coating layer is distributed on both sides of the aluminum foil. The areal density of the double-sided coating of the electrode material is 36.0 mg / cm³. 2 .

[0352] 2. Rolling: At a high temperature of 130℃, the thickness of the rolled lithium iron phosphate electrode sheet is 157μm.

[0353] 3. First drying: Dry for 10 hours at a temperature of 100℃ and a pressure of -90kPa.

[0354] 4. Punching: The lithium iron phosphate electrode sheets are cut into the electrode sheets required for battery manufacturing using a punch.

[0355] 5. Second drying: Dry for 12 hours at a temperature of 110℃ and a pressure of -90kPa.

[0356] III. Preparation of Lithium Iron Phosphate Batteries;

[0357] 1. Stacking: The lithium iron phosphate electrode prepared in the above steps is stacked with the graphite negative electrode and the separator to form an electrode assembly.

[0358] 2. Assembly: Weld the above-mentioned electrode assembly to the adapter plate, battery cover, etc., cover the electrode assembly with a protective sleeve, and assemble.

[0359] 3. Peripheral welding of the casing: Place the above-mentioned assembly and bottom gasket into the battery casing, and weld the battery cover to the battery casing around the periphery to produce a semi-finished battery.

[0360] 4. Third drying: Place the above semi-finished batteries into an oven and dry them for 32 hours at a temperature of 95℃ and a pressure of -90kPa.

[0361] 5. Electrolyte injection: Inject the electrolyte into the above-mentioned semi-finished battery.

[0362] 6. Formation: The semi-finished battery after liquid injection is charged and formed.

[0363] 7. Settling, adding liquid, and sealing: After formation, the semi-finished battery is left to stand, then liquid is added and sealed.

[0364] 8. Capacity testing and aging are performed to obtain lithium iron phosphate batteries.

[0365] The differences in key parameters and electrode expansion rates between the comparative example and the nine embodiments are shown in Tables 1 and 2.

[0366] Table 1. Differences in key parameters and electrode expansion rates between comparative examples and Examples 1-5;

[0367]

[0368]

[0369] In Table 1, the electrode expansion rate = the thickness of the positive electrode of the finished battery (30% SOC) / the thickness of the rolled electrode - 1;

[0370] Table 2. Differences in key parameters and electrode expansion rates between comparative examples and Examples 2, 6-10;

[0371]

[0372]

[0373] In Table 2, the electrode expansion rate = thickness of the positive electrode of the finished battery (30% SOC) / thickness of the rolled electrode - 1;

[0374] As can be seen from Tables 1 and 2, the electrode expansion rate of the embodiment is lower than that of the comparative embodiment, and is directly related to the two factors of the ratio of lithium iron phosphate raw material A and lithium iron phosphate raw material B and the temperature of the rolling roller. By adjusting the parameters of the above two factors, as well as the type of pulp raw material, the feeding sequence, and the electrode coating density, the expansion rate of lithium iron phosphate electrode can be significantly reduced.

[0375] The differences in electrode expansion rate, cycle performance, and negative electrode surface condition between the comparative example and the embodiment are shown in Tables 3 and 4.

[0376] Table 3 shows the differences in electrode expansion rate, cycle performance, and negative electrode surface condition between the comparative examples and Examples 1-5.

[0377] plan Electrode expansion rate % Battery cycle performance (25℃, 1C / 1C) Surface state of negative electrode Comparative Example 10.5 2089 times @ 80% Large-scale lithium plating Example 1 8.9 2311 times @ 80% Small amount of lithium plating Example 2 6.4 2832 times @ 80% Almost no lithium plating Example 3 7.0 2595 times @ 80% Slight lithium plating Example 4 7.6 2488 times @ 80% Slight lithium plating Example 5 9.6 2193 times @ 80% Small amount of lithium plating

[0378] Table 4. Differences in expansion rate, cycle performance and negative electrode surface condition between comparative example and example 2, 6-10;

[0379] plan Electrode expansion rate % Battery cycle performance Surface state of negative electrode Comparative Example 10.5 2089 times @ 80% Large-scale lithium plating Example 2 6.4 2832 times @ 80% Almost no lithium plating Example 6 5.1 3116 times @ 80% Lithium-free Example 7 3.2 3262 times @ 80% Lithium-free Example 8 1.9 3502 times @ 80% Lithium-free Example 9 2.5 3288 times @ 80% Lithium-free

[0380] As can be seen from Tables 3 and 4, the electrode expansion rate of the embodiment is lower than that of the comparative embodiment, and the cycle life of the battery is improved. The lithium plating state of the negative electrode is improved, thus enhancing the safety performance of the battery.

[0381] Compared with existing technologies, the low-expansion lithium iron phosphate electrode, lithium iron phosphate battery, and preparation method provided by the present invention have the following beneficial technical effects:

[0382] This invention reduces electrode expansion by controlling the particle size distribution and dosage of two active materials, lithium iron phosphate (LiFePO4) (i.e., LiFePO4 particles meeting the first material condition and LiFePO4 particles meeting the second material condition), using a dispersant to improve the dispersibility of the slurry, synergistically increasing the particle packing density in the electrode, limiting the coating surface density, reducing interparticle packing forces, and employing hot roller compaction to eliminate internal stress in the electrode slurry layer. Controlled heating creates physical and / or chemical cross-linking points between the binder itself, between the binder and LiFePO4 particles, and between the binder and inorganic fillers, increasing binder strength, enhancing resistance to electrolyte swelling, and creating a binding force on the LiFePO4 particles, reducing the expansion of LiFePO4 particles caused by charging and discharging, thereby further reducing electrode expansion. The low-expansion LiFePO4 electrode effectively improves the battery's cycle life and safety performance.

[0383] In summary, compared with the prior art, the low-expansion lithium iron phosphate electrode, lithium iron phosphate battery, and preparation method provided by the present invention are scientifically designed. The low expansion rate of the lithium iron phosphate electrode can effectively improve the cycle life and safety performance of the battery, which has significant practical significance.

[0384] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A low-expansion lithium iron phosphate electrode, characterized in that, Including the positive current collector; The upper and lower surfaces of the positive electrode current collector are each coated with at least one layer of positive electrode active material. Alternatively, the upper or lower surface of the positive electrode current collector is coated with at least one layer of positive electrode active material. The positive electrode active material layer includes lithium iron phosphate particles that meet the first material conditions, lithium iron phosphate particles that meet the second material conditions, a conductive agent, a binder, a dispersant, and an inorganic filler. In the positive electrode active material layer, the mass ratio of lithium iron phosphate particles meeting the first material condition, lithium iron phosphate particles meeting the second material condition, conductive agent, binder, dispersant and inorganic filler is (4~48):(50~94):(0.4~5):(1.5~5):(0.05~1):(0.05~1); Lithium iron phosphate particles that meet the first material condition include lithium iron phosphate particles with a primary particle structure and lithium iron phosphate particles with a secondary particle structure. Among them, the secondary particle size distribution of lithium iron phosphate particles that meet the first material condition is: D min ≥0.05μm, D 10 = (0.8 ± 0.1) μm, D 50 = (9.0 ± 2.0) μm, D 90 =(20.0±2)μm,D max ≤35μm; The primary particle size of the lithium iron phosphate particles that meet the first material condition is 0.05–0.5 μm; Lithium iron phosphate particles that meet the second material conditions include lithium iron phosphate particles with a primary particle structure and lithium iron phosphate particles with a secondary particle structure. Among them, the secondary particle size distribution of lithium iron phosphate particles that meet the second material conditions is: D min ≥0.2μm, D 10 = (0.5 ± 0.1) μm, D 50 = (1.6 ± 0.5) μm, D 90 = (3.7 ± 1.0) μm, D max ≤7.0μm; The primary particle size of the lithium iron phosphate particles that meet the second material conditions is 0.1–7.0 μm; The method for preparing the low-expansion lithium iron phosphate electrode includes the following steps: The first step is slurry preparation: Lithium iron phosphate particles that meet the first material conditions, lithium iron phosphate particles that meet the second material conditions, conductive agent, binder, dispersant and inorganic filler are mixed according to a preset mass ratio and stirred evenly. Then, they are dissolved in a solvent for dispersion to obtain positive electrode slurry. The second step is coating: the positive electrode slurry is uniformly coated on the surface of the positive electrode current collector to obtain a positive electrode sheet with a positive electrode active material layer on the surface. The third step is rolling: the dried positive electrode sheet with a positive active material layer on the surface is rolled until the compaction density of the positive active material meets the preset compaction density value range. Step 4, First drying: Place the rolled positive electrode sheet into an oven and perform a drying operation for a preset first duration under a preset first temperature range and a preset first pressure range. Step 5, stamping: Cut the first dried positive electrode sheet to the size required for the finished positive electrode sheet to obtain the stamped positive electrode sheet; Step 6, Second drying: Place the punched positive electrode sheet into an oven and perform a drying operation for a preset second duration under a preset second temperature range and a preset second pressure range to finally obtain the finished positive electrode sheet.

2. The low-expansion lithium iron phosphate electrode as described in claim 1, characterized in that, Conductive agents include at least one of carbon black, multi-walled carbon nanotubes, single-walled carbon nanotubes, graphene, conductive graphite, and carbon fiber. Adhesives, including at least one of fluoropolymers, nitrile polymers, and carboxylic acid derivatives; Fluoropolymers include at least one of polyvinylidene fluoride, poly(vinylidene fluoride-tetrafluoroethylene), poly(vinylidene fluoride-hexafluoropropylene), polytetrafluoroethylene, and poly(tetrafluoroethylene-hexafluoropropylene). Nitrile polymers, including at least one of polyacrylonitrile, acrylonitrile-butadiene copolymer, nitrile butadiene rubber and hydrogenated nitrile butadiene rubber; Carboxylic acid derivatives, including at least one of polyacrylate, polyacrylate, acrylate-acrylate-acrylonitrile copolymer, styrene-butadiene rubber and carboxymethyl cellulose salt; The dispersant is a styrene-acrylate copolymer; Inorganic fillers include at least one of alumina, bauxite, silica, and montmorillonite.

3. The low-expansion lithium iron phosphate electrode as described in claim 1, characterized in that, In the second step, when the positive electrode slurry is uniformly coated on the upper or lower surface of the positive electrode current collector, the surface density of the coating on one side is 10.0–25.0 mg / cm³. 2 ; In the second step, when the positive electrode slurry is uniformly coated on the upper and lower surfaces of the positive electrode current collector, the double-sided coating surface density is 20.0–50.0 mg / cm³. 2 ; In the third step, the dried positive electrode sheet with a positive active material layer on its surface is hot-rolled using the rolling rollers on an existing rolling mill; the temperature of the rolling rollers is 90-180℃. In the third step, the preset compaction density range is 2.4 g / cm³. 3 ~2.7g / cm 3 ; In the fourth step, during the first drying operation, the preset first temperature range is 80 to 130°C, the preset first pressure range is -50 to -90 kPa, and the preset first duration is 4 to 24 hours. In the sixth step, during the second drying operation, the preset second temperature range is 80 to 130°C, the preset second pressure range is -50 to -90 kPa, and the preset second duration is 4 to 24 hours.

4. A lithium iron phosphate battery, characterized in that, The positive electrode included therein is a low-expansion lithium iron phosphate electrode as described in any one of claims 1-3.

5. A method for preparing a lithium iron phosphate battery, characterized in that, Includes the following steps: Step 1, stacking: The low-expansion lithium iron phosphate electrode sheet as described in any one of claims 1-3 is used as the positive electrode sheet and assembled into an electrode assembly with the graphite negative electrode sheet and the separator in the form of stacking. The second step is assembly: the electrode tabs of the electrode assembly are welded to the battery cover through the adapter piece, and then the electrode assembly is wrapped with a protective sleeve to obtain the assembly. The third step is perimeter welding: the assembly and bottom pad are placed into the battery case, and the battery cover is welded to the battery case to produce a semi-finished battery. Step 4, third drying: Place the semi-finished batteries in an oven to dry them thoroughly; Step 5, electrolyte injection: Inject the electrolyte into the above-mentioned semi-finished battery; Step 6, Formation: The semi-finished battery after liquid injection is charged and formed; Step 7, let stand, add liquid and seal: Let the semi-finished battery stand, then add liquid and seal it; Step 8, Capacity testing and aging: After the existing capacity testing and aging process, the finished lithium iron phosphate battery is obtained.

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