Battery cell, preparation method of battery cell, starting battery of fuel engine and vehicle

Abstract

The invention relates to the field of lithium ion batteries, and discloses a battery cell, a preparation method of the battery cell, a starting battery of a fuel engine and a vehicle. The battery cell provided by the invention comprises a positive electrode and a negative electrode, the positive electrode comprises lithium iron phosphate composite particles, and the lithium iron phosphate composite particles comprise more than two kinds of lithium iron phosphate particles with the particle size of D50; the negative electrode comprises graphite composite particles, and the graphite composite particles comprise more than two kinds of graphite particles with the particle size of D50. The lithium iron phosphate battery provided by the invention not only has excellent low-temperature high-rate discharge performance, but also has excellent high-temperature cycle performance.

Description

Technical Field

[0001] The invention relates to the field of lithium ion batteries, and in particular to a battery cell and a preparation method thereof, a starting battery for a fuel engine, and a vehicle. Background Art

[0002] The normal temperature of lithium iron phosphate batteries is 0-60℃. At -10℃, the battery capacity begins to decline sharply, decaying to about 50%, and at -20℃, there is only 20-40% of the capacity. When the temperature exceeds 60℃, the side reaction between lithium ions and electrolyte intensifies, causing capacity decline.

[0003] In order to solve the problem that the poor low-temperature performance of lithium iron phosphate limits its application, the currently commonly used method is to improve the low-temperature performance at the electrode level, such as shortening the Li+ transmission path and reducing impedance by increasing the amount of conductive agent and reducing surface density.

[0004] The related technologies often fail to take into account the low-temperature high-rate discharge performance and high-temperature cycle performance required by the starter battery.

[0005] Therefore, there is an urgent need for a lithium iron phosphate battery and a preparation process thereof that can simultaneously have excellent low-temperature high-rate discharge performance and high-temperature cycle performance. Summary of the invention

[0006] The purpose of the present invention is to overcome the problem that lithium iron phosphate batteries in the prior art cannot simultaneously have excellent low-temperature high-rate discharge performance and high-temperature cycle performance, and to provide a battery cell and a preparation method thereof, a starting battery for a fuel engine, and a vehicle.

[0007] In order to achieve the above-mentioned object, the first aspect of the present invention provides a battery cell, wherein the battery cell comprises a positive electrode and a negative electrode;

[0008] The positive electrode comprises lithium iron phosphate composite particles, wherein the lithium iron phosphate composite particles comprise two or more D 50 Particle size of lithium iron phosphate particles;

[0009] The negative electrode comprises graphite composite particles, wherein the graphite composite particles comprise two or more D 50 Graphite particles of different particle sizes.

[0010] A second aspect of the present invention provides a method for preparing a battery cell, wherein the preparation method comprises:

[0011] Providing a positive electrode, wherein the positive electrode sheet includes lithium iron phosphate composite particles, and the lithium iron phosphate composite particles include lithium iron phosphate particles of two or more D50 particle sizes;

[0012] Providing a negative electrode, wherein the negative electrode sheet includes graphite composite particles, and the graphite composite particles include graphite particles of two or more D50 particle sizes;

[0013] The positive electrode and the negative electrode are assembled into a battery cell.

[0014] A third aspect of the present invention provides a starting battery for a fuel engine, comprising the battery cell provided by the present invention or the battery cell prepared by the preparation method provided by the present invention.

[0015] A fourth aspect of the present invention provides a vehicle, comprising the battery cell provided by the present invention or the battery cell prepared by the preparation method provided by the present invention or the starting battery of the fuel engine provided by the present invention.

[0016] Through the above technical solution, the beneficial effects of the present invention are:

[0017] The present invention uses two or more different D 50 The lithium iron phosphate composite particles obtained by mixing lithium iron phosphate particles with different particle sizes are used as positive electrode active materials, and two or more different D 50 The mixing of graphite particles of different particle sizes as the negative electrode active material can reduce the tortuosity of the positive and negative electrodes, shorten the lithium ion transmission path, and thus reduce the lithium ion migration impedance, so that the prepared lithium iron phosphate battery can take into account both low-temperature high-rate discharge performance and high-temperature cycle performance.

[0018] In a preferred embodiment of the present invention, the OI value of the graphite particles is controlled by adjusting the particle size of the lithium iron phosphate particles and the ratio of lithium iron phosphate particles of different particle sizes, the particle size of the graphite particles and the ratio of graphite particles of different particle sizes, and the particle size and ratio of the hard carbon. The low-temperature and high-rate discharge performance of the battery can be further improved and the high-temperature performance of the battery can be further improved by adjusting the types and ratios of the electrolyte lithium salt and additives. When the prepared lithium iron phosphate battery reaches the end of its life (deteriorates to 80% SOH), it discharges at 5C for 2s at -30°C and 50% SOC, and the cut-off voltage can reach 2.62V. The low-temperature cold starting capability is much higher than that of the same type of lithium iron phosphate batteries. In addition, according to the requirements of the fuel engine for the starting battery, the same life as the 8-year warranty of the whole vehicle can be achieved, that is, the 8-year capacity retention rate is greater than or equal to 80%. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a -30°C cold start discharge curve of the soft-pack lithium iron phosphate battery prepared in Example 1 of the present invention;

[0020] Figure 2 It is a capacity retention curve of the soft-pack lithium iron phosphate battery prepared in Example 1 of the present invention after 1000 cycles at 60°C. DETAILED DESCRIPTION

[0021] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0022] A first aspect of the present invention provides a battery cell, wherein the battery cell comprises a positive electrode and a negative electrode;

[0023] The positive electrode comprises lithium iron phosphate composite particles, wherein the lithium iron phosphate composite particles comprise two or more D 50 Particle size of lithium iron phosphate particles;

[0024] The negative electrode comprises graphite composite particles, wherein the graphite composite particles comprise two or more D 50 Graphite particles of different particle sizes.

[0025] The present invention can reduce the tortuosity of the positive and negative electrodes, shorten the lithium ion transmission path, and further reduce the lithium ion migration impedance by mixing two or more lithium iron phosphate particles of different particle sizes as the positive electrode active material and mixing two or more graphite particles of different particle sizes as the negative electrode active material, so as to enable the prepared lithium iron phosphate battery to have excellent low-temperature high-rate discharge performance and a longer high-temperature cycle life.

[0026] Specifically, in this application, D 50 It can be understood as D n50 .

[0027] In some embodiments, the OI value of the graphite composite particles is less than or equal to 7. The OI value is an orientation index, which represents the degree of isotropic distribution of the graphite particles. The smaller the OI value, the more the graphite tends to be distributed perpendicular to the current collector, and the isotropy is high, which is beneficial to low temperature and rate performance. The use of graphite composite particles with an OI value of less than or equal to 7 can shorten the lithium ion diffusion path and increase the lithium ion migration rate, thereby improving the low temperature and high rate discharge performance of the prepared lithium iron phosphate battery. By measuring the graphite crystals by XRD, the peak intensity (I110) of the surface (110) and the peak intensity (I004) of the surface (004) can be obtained. The OI value refers to the ratio of I004 / I110.

[0028] In some embodiments, the negative electrode further includes hard carbon.

[0029] In some embodiments, the D of the hard carbon 50 The particle size is 3-10μm. Hard carbon has a high degree of disorder, multiple lithium insertion directions, and low diffusion resistance, which makes it suitable for large current discharge, but its conductivity is slightly poor. 50When the particle size meets this range, both power and high temperature performance can be taken into account. When the particle size is too large, the ion solid phase diffusion impedance increases, which is not conducive to the power performance. When the particle size is too small, the side reaction with the electrolyte increases, which is not conducive to high temperature performance. Specifically, the D 50 The particle size may be 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm and 10 μm, and any value in the range between any two of the above values.

[0030] In some embodiments, the weight ratio of the graphite composite particles to the hard carbon is (85-95):(5-15). Specifically, the weight ratio of the graphite composite particles to the hard carbon can be, for example, 85:15, 90:15, or 95:15.

[0031] By adjusting the combination of graphite particles of different particle sizes in the graphite composite particles, as well as the ratio of the graphite composite particles to hard carbon, a synergistic effect can be produced between the positive electrode active material and the negative electrode active material, thereby enabling the prepared lithium iron phosphate battery to have better low-temperature, high-rate discharge and high-temperature cycle performance.

[0032] In some embodiments, the graphite composite particles include graphite particles A and graphite particles B; preferably, the graphite composite particles are a mixture of graphite particles A and graphite particles B; wherein the graphite particles A and the graphite particles B are each D 50 The particle sizes are 5-7μm and 9-11μm respectively.

[0033] D 50 Graphite particles A and B with particle sizes of 5-7 μm and 9-11 μm, respectively, are compounded with hard carbon. The slightly larger interlayer spacing of hard carbon promotes the insertion and extraction of lithium ions, thereby improving the low-temperature rate performance and power performance of lithium iron phosphate batteries (generally, good rate performance and good power performance). 50 Graphite particles with particle sizes of 5-7μm and 9-11μm respectively can inhibit the side reaction between the electrolyte and the negative electrode active material at high temperature, thereby taking into account the high temperature cycle performance.

[0034] In some embodiments, based on the total weight of the graphite composite particles, the contents of the graphite particles A and the graphite particles B in the graphite composite particles are 25-75 wt % and 25-75 wt %, respectively.

[0035] In some embodiments, the lithium iron phosphate composite particles include two or more of lithium iron phosphate particles A, lithium iron phosphate particles B, lithium iron phosphate particles C, and lithium iron phosphate particles D; wherein the lithium iron phosphate particles A, lithium iron phosphate particles B, lithium iron phosphate particles C, and lithium iron phosphate particles D are each D 50 The particle sizes are 1-3μm, 5-7μm, 9-11μm and 18-20μm respectively.

[0036] In some embodiments, the lithium iron phosphate composite particles include lithium iron phosphate particles A and lithium iron phosphate particles B, and also include lithium iron phosphate particles C and / or lithium iron phosphate particles D; preferably, based on the total weight of the lithium iron phosphate composite particles, the total content of the lithium iron phosphate particles A and the lithium iron phosphate particles B is less than or equal to 75wt%.

[0037] In some embodiments, the lithium iron phosphate composite particles are a mixture of lithium iron phosphate particles A, lithium iron phosphate particles B, and lithium iron phosphate particles C. 50 A mixture of lithium iron phosphate particles A, lithium iron phosphate particles B and lithium iron phosphate particles C with particle sizes of 1-3 μm, 5-7 μm and 9-11 μm respectively is used as a positive electrode active material. Particles of different sizes are accumulated in a certain space to construct a multi-dimensional pore structure.

[0038] In some embodiments, based on the total weight of the lithium iron phosphate composite particles, the contents of the lithium iron phosphate particles A, lithium iron phosphate particles B and lithium iron phosphate particles C in the lithium iron phosphate composite particles are 10-30wt%, 20-40wt% and 30-70wt%, respectively.

[0039] By adjusting the combination and mixing ratio of lithium iron phosphate particles of different particle sizes in the lithium iron phosphate composite particles, the porosity and tortuosity of the positive electrode can be adjusted, and the diffusion of lithium ions can be promoted, thereby further improving the low-temperature, high-rate discharge and high-temperature cycle performance of the prepared lithium iron phosphate battery.

[0040] In some embodiments, the double-sided density of the positive electrode is 200-250 g / m 2 , compacted density is 1.8-3g / cm 3 .

[0041] Specifically, the double-sided density of the positive electrode can be understood as the sum of the surface densities of the positive electrode material layers on both sides in the thickness direction of the positive electrode. For example, the surface density of the positive electrode material layer on one side in the thickness direction of the positive electrode is 100 g / m 2 The surface density of the positive electrode material layer on the other side of the positive electrode in the thickness direction is 100 g / m 2 The double-sided density of the positive electrode is 200g / m 2 .

[0042] In some embodiments, the double-sided density of the negative electrode is 80-110 g / m 2 , compacted density is 1-2g / cm 3 .

[0043] Specifically, the double-sided density of the negative electrode can be understood as the sum of the surface densities of the negative electrode material layers on both sides in the thickness direction of the negative electrode. For example, the surface density of the negative electrode material layer on one side in the thickness direction of the negative electrode is 50 g / m2 The surface density of the negative electrode material layer on the other side of the negative electrode in the thickness direction is 50 g / m 2 The double-sided density of the negative electrode is 100g / m 2 .

[0044] In some embodiments, the battery cell further includes an electrolyte, and the electrolyte includes a lithium salt.

[0045] In some embodiments, in the battery cell, the injection coefficient of the electrolyte is 3.8-6 g / Ah.

[0046] In some embodiments, the lithium salt includes lithium hexafluorophosphate and at least one selected from lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(oxalatoborate) and lithium difluorooxalatoborate; preferably, it is a mixture of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide and lithium difluorooxalatoborate.

[0047] In some embodiments, in the lithium salt, the weight ratio of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide and lithium difluorooxalatoborate is (5-10):(2-5):(0.5-1).

[0048] By selecting the appropriate type and proportion of lithium salt, the SEI impedance of the lithium iron phosphate battery can be reduced and high-temperature side reactions can be inhibited, thereby taking into account both high and low temperature performance.

[0049] In some embodiments, the content of the lithium salt in the electrolyte is 0.8-1.2 mol / L.

[0050] In some embodiments, the electrolyte further includes an additive, wherein the additive is selected from at least one of vinylene carbonate (VC), fluoroethylene carbonate (FEC) and diethylene sulfate (DTD); preferably, the additive is a mixture of vinylene carbonate and fluoroethylene carbonate.

[0051] In some embodiments, preferably, the content of the additive in the electrolyte is 1.5-5 mol / L.

[0052] In some embodiments, in the additive, the weight ratio of vinylene carbonate to fluoroethylene carbonate is (1.5-3.5):(2-5).

[0053] By selecting the appropriate type and proportion of electrolyte additives, the high-temperature performance of the prepared lithium iron phosphate battery can be further improved, thereby taking into account both low-temperature high-rate discharge and high-temperature life performance.

[0054] In some embodiments, the electrolyte further includes a solvent, and the solvent is selected from at least one of ethylene carbonate, diethyl carbonate, ethyl methyl carbonate and dimethyl carbonate; preferably a mixed solution of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate.

[0055] In some embodiments, in the solvent, the weight ratio of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate is (2-4): (2-4): (3-5).

[0056] In some embodiments, the positive electrode includes a positive electrode current collector and a positive electrode material layer disposed on the positive electrode current collector; wherein, based on the total weight of the positive electrode material layer, the content of the lithium iron phosphate composite particles in the positive electrode material layer is 90-94wt%.

[0057] In some embodiments, the negative electrode includes a negative electrode current collector and a negative electrode material layer disposed on the negative electrode current collector; wherein, based on the total weight of the negative electrode material layer, the total content of the graphite composite particles and hard carbon in the negative electrode material layer is 90-95wt%.

[0058] In some embodiments, based on the total weight of the negative electrode material layer, the content of the hard carbon in the negative electrode material layer is less than or equal to 15 wt %.

[0059] In some embodiments, the battery cell further includes a separator.

[0060] The present invention has no particular limitation on the type of the diaphragm, and conventional diaphragms in the art may be selected, such as PP / PE / PP diaphragms; preferably, the porosity of the diaphragm is less than 45%.

[0061] In some embodiments, the separator has a thickness of 9-25 μm.

[0062] A second aspect of the present invention provides a method for preparing a battery cell, wherein the preparation method comprises:

[0063] A positive electrode is provided, the positive electrode comprising lithium iron phosphate composite particles, the lithium iron phosphate composite particles comprising two or more D 50 Particle size of lithium iron phosphate particles;

[0064] A negative electrode is provided, the negative electrode comprising graphite composite particles, the graphite composite particles comprising two or more D 50 Particle size of graphite particles;

[0065] The positive electrode and the negative electrode are assembled into a battery cell.

[0066] It is understandable that although the above preparation method only mentions assembling the positive electrode and the negative electrode into a battery cell, it does not mean that the battery cell only contains the positive electrode and the negative electrode. The battery cell may also include other known components such as a separator, an electrolyte and / or a solid electrolyte.

[0067] In some embodiments, the method for preparing a battery cell comprises: winding a positive electrode, a negative electrode and a separator into an electrode group, and injecting an electrolyte to prepare the battery cell; wherein,

[0068] The positive electrode comprises lithium iron phosphate composite particles, wherein the lithium iron phosphate composite particles comprise two or more D 50 Particle size of lithium iron phosphate particles;

[0069] The negative electrode comprises graphite composite particles, wherein the graphite composite particles comprise two or more D 50 Graphite particles of different particle sizes.

[0070] In the preparation method of the battery cell in the second aspect of the present invention, the types and amounts of the positive electrode, negative electrode, separator and electrolyte are exactly the same as those in the battery cell described in the first aspect of the present invention. In order to avoid repetition, the present invention will not be described in detail in the second aspect, and those skilled in the art should not understand it as a limitation of the present invention.

[0071] In some embodiments, the negative electrode further includes hard carbon.

[0072] In some embodiments, the positive electrode is obtained by coating a positive electrode slurry obtained by mixing a positive electrode powder and a solvent 1 on the surface of a positive electrode current collector; wherein, based on the total weight of the positive electrode powder, the content of the lithium iron phosphate composite particles is 90-94wt%. When the content of the lithium iron phosphate composite particles meets this range, sufficient battery energy density can be provided.

[0073] In some embodiments, the negative electrode is obtained by coating a negative electrode slurry obtained by mixing a negative electrode powder and a solvent 2 on the surface of a negative electrode current collector; wherein, based on the total weight of the negative electrode powder, the total content of the graphite composite particles and the hard carbon is 90-95wt%; wherein, based on the total weight of the graphite composite particles and the hard carbon, the content of the hard carbon is less than or equal to 15wt%. When the total content of the graphite composite particles and the hard carbon meets this range, a high rate performance of the battery can be provided.

[0074] Furthermore, based on the total weight of the graphite composite particles and the hard carbon, the content of the hard carbon is 8-12 wt %.

[0075] In the present invention, the positive electrode powder includes lithium iron phosphate composite particles, a conductive agent 1 and a binder 1; the negative electrode powder includes graphite composite particles, hard carbon, a conductive agent 2 and a binder 2.

[0076] In some embodiments, the method for preparing the positive electrode comprises: 50The lithium iron phosphate particles of different particle sizes are mixed to obtain the lithium iron phosphate composite particles; the lithium iron phosphate composite particles, a conductive agent 1, a binder 1 and a solvent 1 are mixed to obtain a positive electrode slurry, and then the positive electrode slurry is coated on the surface of a positive electrode current collector, and then rolled to obtain the positive electrode.

[0077] Preferably, the method for preparing the negative electrode comprises: 50 Graphite particles of different particle sizes are mixed to obtain the graphite composite particles; the graphite composite particles, hard carbon, a conductive agent 2, a binder 2 and a solvent 2 are mixed to obtain a negative electrode slurry, and then the negative electrode slurry is coated on the surface of a negative electrode collector, and then rolled to obtain the negative electrode.

[0078] In some embodiments, based on the total weight of the positive electrode powder, the content of the conductive agent 1 is less than or equal to 5wt%, preferably 3-5wt%; the content of the binder 1 is less than or equal to 5wt%, preferably 3-5wt%. When the contents of the conductive agent 1 and the binder 1 meet this range, the positive electrode can enhance the electronic conductivity, reduce the charge transfer impedance, and promote the rate performance and power performance of the battery under the premise that the positive electrode has a certain manufacturability.

[0079] In some embodiments, the conductive agent 1 is selected from at least one of carbon black (SP), carbon nanotubes (CNTs), graphene, Ketjen black and carbon nanofibers; preferably a mixture of carbon black and carbon nanotubes. The mixture of carbon black and carbon nanotubes is used as the conductive agent of the positive electrode, which can play a synergistic role with the lithium iron phosphate composite particles, provide a multi-dimensional conductive network of points and lines, enhance electronic conductivity, and is beneficial to the low temperature performance of the battery.

[0080] In some embodiments, based on the total weight of the positive electrode powder, the content of carbon black in the conductive agent 1 is 1-3 wt %, and the content of carbon nanotubes is 1.5-3 wt %.

[0081] In some embodiments, the binder 1 is selected from polyvinylidene fluoride (PVDF) and / or polytetrafluoroethylene (PTFE); preferably polyvinylidene fluoride.

[0082] Furthermore, the binder 1 is selected from polyvinylidene fluoride having a weight average molecular weight of 1 million to 1.2 million g / mol.

[0083] In some embodiments, based on 1 g of the positive electrode powder, the amount of the solvent 1 is 0.5-1 mL.

[0084] In some embodiments, the solvent 1 is N-methylpyrrolidone (NMP).

[0085] In some embodiments, based on the total weight of the negative electrode powder, the content of the conductive agent 2 is less than or equal to 5wt%, and the content of the binder 2 is less than or equal to 5wt%. When the contents of the conductive agent 2 and the binder 2 meet this range, the electronic conductivity of the negative electrode can be enhanced, the charge transfer impedance can be reduced, and the rate performance and power performance of the battery can be promoted under the premise that the negative electrode has a certain manufacturability.

[0086] In some embodiments, the conductive agent 2 is selected from at least one of carbon black (SP), carbon nanotubes (CNTs), graphene, Ketjen black and carbon nanofibers, preferably carbon black. Using carbon black as the conductive agent of the negative electrode can achieve the best balance between cost and conductive performance.

[0087] In some embodiments, the binder 2 is selected from one or two of sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR) and polyacrylic acid (PAA); preferably a mixture of sodium carboxymethyl cellulose and styrene-butadiene rubber; more preferably a mixture of sodium carboxymethyl cellulose and styrene-butadiene rubber with a weight average molecular weight of 300,000-500,000 g / mol.

[0088] Furthermore, in the binder 2, the weight ratio of sodium carboxymethyl cellulose and styrene butadiene rubber is (1-3): (2-4). Using the mixture of sodium carboxymethyl cellulose and styrene butadiene rubber in this weight ratio as the binder for the negative electrode can make the powder material evenly adhere together and firmly adhere to the current collector.

[0089] In some embodiments, based on 1 g of the negative electrode powder, the amount of the solvent 2 is 0.8-1.2 mL.

[0090] In some embodiments, the solvent 2 is water.

[0091] According to a preferred embodiment of the present invention, the method for preparing the battery cell comprises the following steps:

[0092] S1. Preparation of positive electrode: Based on the total weight of the positive electrode powder, 90-94wt% of lithium iron phosphate composite particles, 3-5wt% of conductive agent 1 and 3-5wt% of binder 1 are weighed according to the mass percentage to obtain positive electrode powder, and then solvent 1 is added (based on the amount of positive electrode powder being 1g, the amount of solvent 1 is 0.5-1mL), and the materials are mixed in a vacuum mixer to obtain positive electrode slurry, and after coating and rolling, a double-sided density of 200-250g / m 2 , compacted density is 1.8-3g / cm 3 The positive electrode of

[0093] Lithium iron phosphate composite particles are D 50A mixture of lithium iron phosphate particles A, lithium iron phosphate particles B and lithium iron phosphate particles C with particle sizes of 1-3 μm, 5-7 μm and 9-11 μm respectively; based on the total weight of the lithium iron phosphate composite particles, the contents of the lithium iron phosphate particles A, the lithium iron phosphate particles B and the lithium iron phosphate particles C in the lithium iron phosphate composite particles are 10-30wt%, 20-40wt% and 30-70wt% respectively.

[0094] S2. Preparation of negative electrode: Based on the total weight of the negative electrode powder, weigh 90-95wt% of a mixture of graphite composite particles and hard carbon according to the mass percentage (based on the total weight of the mixture of graphite composite particles and hard carbon, the content of hard carbon is less than or equal to 15wt%, and the D of hard carbon is less than or equal to 15wt%). 50 The negative electrode powder is prepared by mixing a conductive agent 2 with a particle size of 3-10 μm, a conductive agent 2 with a particle size of 5 wt% or less, and a binder 2 with a particle size of 5 wt% or less, and then adding a solvent 2 (the amount of the solvent 2 is 0.8-1.2 mL based on the amount of the negative electrode powder being 1 g), and mixing the materials by a vacuum mixer to obtain a negative electrode slurry. After coating and rolling, a double-sided density of 80-110 g / m 2 , compacted density is 1-2g / cm 3 The negative electrode;

[0095] Graphite composite particles are D 50 A mixture of graphite particles A and graphite particles B having particle sizes of 5-7 μm and 9-11 μm, respectively; the OI values ​​of the graphite particles A and the graphite particles B are both less than or equal to 7; based on the total weight of the graphite composite particles, the contents of the graphite particles A and the graphite particles B are 25-75 wt% and 25-75 wt%, respectively;

[0096] S3. Preparation of electrolyte: Based on the total weight of the electrolyte, weigh 0.8-1.2 mol / L of lithium salt, 1.5-5 mol / L of additive and solvent 3 and mix them; wherein the lithium salt is a mixture of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide and lithium difluorooxalatoborate in a weight ratio of (5-10):(2-5):(0.5-1); the additive is a mixture of vinylene carbonate and fluoroethylene carbonate in a weight ratio of (1.5-3.5):(2-5); and solvent 3 is a mixture of vinyl carbonate, dimethyl carbonate and ethyl methyl carbonate in a weight ratio of (2-4):(2-4):(3-5).

[0097] S4. Winding the positive electrode, the negative electrode and the separator into an electrode group, and injecting an electrolyte with an injection coefficient of 3.8-6 g / Ah to obtain the battery cell.

[0098] A third aspect of the present invention provides a starting battery for a fuel engine, wherein the battery comprises the battery cell provided by the present invention or the battery cell prepared by the preparation method provided by the present invention.

[0099] A fourth aspect of the present invention provides a vehicle, comprising the battery cell provided by the present invention or the battery cell prepared by the preparation method provided by the present invention or the starting battery of the fuel engine provided by the present invention.

[0100] The present invention will be described in detail below through examples and comparative examples. In the following examples and comparative examples, unless otherwise specified, all are conventional methods; the reagents and materials used, unless otherwise specified, can be obtained from commercial channels. The determination methods involved in each example and comparative example are as follows:

[0101] Cold starting voltage test method: The battery is stored at 65°C and 100% SOC until it deteriorates to 80% SOH, then discharged at 5C for 2s at -30°C and 50% SOC, and the cut-off voltage is recorded.

[0102] 60℃ cycle capacity retention rate test method: Perform 1C charge and discharge cycle test at 60℃ and 82.5%-100% SOC conditions for 1000 cycles, record the capacity of each cycle, and calculate the retention rate relative to the initial capacity.

[0103] The following examples are used to illustrate the preparation of soft-pack lithium iron phosphate batteries.

[0104] Example 1

[0105] S1. Preparation of positive electrode: Based on the total weight of the positive electrode powder, 92wt% of lithium iron phosphate composite particles, 4.5wt% of conductive agent 1 (2wt% of SP + 2.5wt% of CNTs) and 3.5wt% of binder 1 (PVDF with a weight average molecular weight of 1.1 million g / mol) were weighed according to the mass percentage to obtain positive electrode powder, and then solvent 1 (NMP, based on the amount of positive electrode powder as 1g, the amount of NMP is 0.7mL) was added, and the positive electrode slurry was obtained by mixing with a vacuum mixer. After coating and rolling, a double-sided density of 220g / m 2 , compacted density is 2.2g / cm 3 The positive electrode of

[0106] Lithium iron phosphate composite particles are D 50 A mixture of lithium iron phosphate particles A, lithium iron phosphate particles B and lithium iron phosphate particles C with particle sizes of 2μm, 6μm and 10μm respectively; based on the total weight of the lithium iron phosphate composite particles, the contents of the lithium iron phosphate particles A, lithium iron phosphate particles B and lithium iron phosphate particles C in the lithium iron phosphate composite particles are 20wt%, 30wt% and 50wt% respectively.

[0107] S2. Preparation of negative electrode: Based on the total weight of the negative electrode powder, weigh 92 wt% of a mixture of graphite composite particles and hard carbon according to the mass percentage (based on the total weight of the mixture of graphite composite particles and hard carbon, the content of hard carbon is 10 wt%, and the D of hard carbon is 10 wt%). 50 The negative electrode powder is prepared by adding 4wt% of conductive agent 2 (SP) and 4wt% of binder 2 (1.5wt% CMC with a weight average molecular weight of 400,000 g / mol and 2.5wt% SBR), and then adding solvent 2 (deionized water, based on the amount of negative electrode powder as 1g, the amount of deionized water is 1mL). The negative electrode slurry is obtained by mixing with a vacuum mixer, and the double-sided density of 95g / m is obtained after coating and rolling. 2 , compacted density is 1.4g / cm 3 The negative electrode;

[0108] Graphite composite particles are D 50 A mixture of graphite particles A and graphite particles B having particle sizes of 6 μm and 10 μm, respectively; the OI values ​​of graphite particles A and graphite particles B are 5 and 4.5, respectively, and the OI value of the graphite composite particles is 4.6; based on the total weight of the graphite composite particles, the contents of graphite particles A and graphite particles B are 25 wt% and 75 wt%, respectively.

[0109] S3. Preparation of electrolyte: Based on the total weight of the electrolyte, weigh 1.1 mol / L of lithium salt, 4.5 mol / L of additive and solvent 3 and mix them; wherein the lithium salt is a mixture of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide and lithium difluorooxalatoborate in a weight ratio of 7:3:1; the additive is a mixture of VC and FEC in a weight ratio of 2.5:3; and solvent 3 is a mixture of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate in a weight ratio of 3:3:4.

[0110] S4. The positive electrode, the negative electrode and the separator (purchased from Enjie New Material Technology Co., Ltd., PP / PE / PP separator, porosity of 40%, thickness of 15 μm) are wound into a pole group, and the electrolyte is injected with an injection coefficient of 6 g / Ah to obtain the battery cell, and the battery cell is packaged to obtain a 20Ah soft-pack lithium iron phosphate battery.

[0111] Depend on Figure 1 From the -30℃ cold start discharge curve of the soft-pack lithium iron phosphate battery, it can be seen that the voltage is 2.627V and the temperature rise is 0.9℃ when discharged at 5C for 2s, indicating that the soft-pack lithium iron phosphate battery has excellent low-temperature starting performance.

[0112] Depend on Figure 2From the capacity retention rate curve of the soft-pack lithium iron phosphate battery after 1000 cycles at 60°C, it can be seen that the capacity retention rate of the soft-pack lithium iron phosphate battery prepared in this embodiment is 95.25% after 1000 cycles, indicating that the soft-pack lithium iron phosphate battery has very excellent high-temperature cycle performance.

[0113] Example 2

[0114] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the positive electrode active material lithium iron phosphate composite particles were D 50 Lithium iron phosphate particles A and D with a particle size of 2 μm 50 A mixture of lithium iron phosphate particles B with a particle size of 6 μm. Specifically, in S1, based on the total weight of the lithium iron phosphate composite particles, the contents of lithium iron phosphate particles A and lithium iron phosphate particles B in the lithium iron phosphate composite particles are 54.3 wt% and 45.7 wt%, respectively. A soft-pack lithium iron phosphate battery is prepared.

[0115] Example 3

[0116] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the positive electrode active material lithium iron phosphate composite particles were D 50 Lithium iron phosphate particles A and D with a particle size of 2 μm 50 A mixture of lithium iron phosphate particles D with a particle size of 20 μm. Specifically, in S1, based on the total weight of the lithium iron phosphate composite particles, the contents of the lithium iron phosphate particles A and the lithium iron phosphate particles D in the lithium iron phosphate composite particles are 60 wt % and 40 wt %, respectively. A soft-pack lithium iron phosphate battery is prepared.

[0117] Example 4

[0118] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the positive electrode active material lithium iron phosphate composite particles were D 50 A mixture of lithium iron phosphate particles B, lithium iron phosphate particles C and lithium iron phosphate particles D having particle sizes of 6 μm, 10 μm and 20 μm, respectively. Specifically, in S1, based on the total weight of the lithium iron phosphate composite particles, the contents of the lithium iron phosphate particles B, lithium iron phosphate particles C and lithium iron phosphate particles D in the lithium iron phosphate composite particles are 40 wt%, 50 wt% and 10 wt%, respectively. A soft-pack lithium iron phosphate battery is prepared.

[0119] Example 5

[0120] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the positive electrode active material lithium iron phosphate composite particles were D 50A mixture of lithium iron phosphate particles A, lithium iron phosphate particles B, lithium iron phosphate particles C and lithium iron phosphate particles D with particle sizes of 2 μm, 6 μm, 10 μm and 20 μm respectively. Specifically, in S1, based on the total weight of the lithium iron phosphate composite particles, the contents of lithium iron phosphate particles A, lithium iron phosphate particles B, lithium iron phosphate particles C and lithium iron phosphate particles D in the lithium iron phosphate composite particles are 15wt%, 25wt%, 50wt% and 10wt% respectively. A soft-pack lithium iron phosphate battery is prepared.

[0121] Example 6

[0122] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the positive electrode active material lithium iron phosphate composite particles were D 50 A mixture of lithium iron phosphate particles A, lithium iron phosphate particles B and lithium iron phosphate particles D with particle sizes of 2 μm, 6 μm and 20 μm respectively. Specifically, in S1, based on the total weight of the lithium iron phosphate composite particles, the contents of the lithium iron phosphate particles A, lithium iron phosphate particles B and lithium iron phosphate particles D in the lithium iron phosphate composite particles are 21.7 wt%, 50 wt% and 28.3 wt% respectively. A soft-pack lithium iron phosphate battery is prepared.

[0123] Example 7

[0124] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the proportions of lithium iron phosphate particles A, lithium iron phosphate particles B and lithium iron phosphate particles C in the positive electrode active material lithium iron phosphate composite particles were different. Specifically, in S1, based on the total weight of the lithium iron phosphate composite particles, the contents of lithium iron phosphate particles A, lithium iron phosphate particles B and lithium iron phosphate particles C in the lithium iron phosphate composite particles were 5wt%, 10wt% and 85wt%, respectively. A soft-pack lithium iron phosphate battery was prepared.

[0125] Example 8

[0126] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the ratio of the graphite composite particles and the hard carbon in the negative electrode active material was different. Specifically, in S2, the content of the hard carbon was 5wt% based on the total weight of the mixture of the graphite composite particles and the hard carbon. A soft-pack lithium iron phosphate battery was prepared.

[0127] Example 9

[0128] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the ratio of graphite particles A and graphite particles B in the negative electrode active material was different. Specifically, in S2, based on the total weight of the graphite composite particles, the contents of graphite particles A and graphite particles B were 80wt% and 20wt%, respectively. A soft-pack lithium iron phosphate battery was prepared.

[0129] Example 10

[0130] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that D 50 Hard carbon with a particle size of 15 μm. A soft-pack lithium iron phosphate battery was prepared.

[0131] Embodiment 11

[0132] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that graphite particles A and graphite particles B with OI values ​​of 9 and 8 were selected as the negative electrode active material, and the OI value of the mixed graphite composite particles was 8.3. A soft-pack lithium iron phosphate battery was prepared.

[0133] Example 12

[0134] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that in the negative electrode active material, the graphite composite particles were D 50 Graphite particles C and D with a particle size of 3 μm 50 A mixture of graphite particles D with a particle size of 15 μm. Specifically, in S2, the OI values ​​of graphite particles C and graphite particles D are 5 and 6 respectively, and the OI value of the graphite composite particles is 5.4; based on the total weight of the graphite composite particles, the contents of graphite particles C and graphite particles D are 25wt% and 75wt% respectively. A soft-pack lithium iron phosphate battery is prepared.

[0135] Example 13

[0136] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the type of lithium salt in the electrolyte was different. Specifically, in S3, the lithium salt was a mixture of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide in a weight ratio of 2:1. A soft-pack lithium iron phosphate battery was prepared.

[0137] Embodiment 14

[0138] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the type of lithium salt in the electrolyte was different. Specifically, in S3, the lithium salt was a mixture of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium difluorooxalatoborate and lithium dioxalatoborate in a weight ratio of 35:10:8:1. A soft-pack lithium iron phosphate battery was prepared.

[0139] Embodiment 15

[0140] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the weight ratio of lithium hexafluorophosphate, lithium bisfluorosulfonyl imide and lithium difluorooxalate borate in the lithium salt was different. Specifically, in S3, the lithium salt was a mixture of lithium hexafluorophosphate, lithium bisfluorosulfonyl imide and lithium difluorooxalate borate in a weight ratio of 3:7:1. A soft-pack lithium iron phosphate battery was prepared.

[0141] Example 16

[0142] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the type of additive in the electrolyte was different. Specifically, in S3, the additive was a mixture of VC, DTD and FEC in a weight ratio of 2:1:1.5. A soft-pack lithium iron phosphate battery was prepared.

[0143] Embodiment 17

[0144] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the weight ratio of VC to FEC in the additive was different. Specifically, the weight ratio of VC to FEC was 3:1. A soft-pack lithium iron phosphate battery was prepared.

[0145] Embodiment 18

[0146] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the type of conductive agent 1 was different when preparing the positive electrode. Specifically, in S1, "4.5wt% conductive agent 1 (2wt% SP+1.5wt% CNTs+1wt% graphene)" was used to replace "4.5wt% conductive agent 1 (2wt% SP+2.5wt% CNTs)". A soft-pack lithium iron phosphate battery was obtained.

[0147] Embodiment 19

[0148] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the type of conductive agent 2 was different when preparing the negative electrode. Specifically, in S2, "4 wt % conductive agent 2 (CNTs)" was used to replace "4 wt % conductive agent 2 (SP)". A soft-pack lithium iron phosphate battery was obtained.

[0149] Embodiment 20

[0150] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the positive electrode active material was D 50 Lithium iron phosphate particles A and D with a particle size of 2 μm 50 A mixture of lithium iron phosphate particles B with a particle size of 6 μm; the negative electrode active material does not contain hard carbon, which is D 50 Graphite particles A and D with a particle size of 6 μm 50 A mixture of graphite particles B with a particle size of 10 μm. Specifically, in S1, based on the total weight of the lithium iron phosphate composite particles, the contents of the lithium iron phosphate particles A and the lithium iron phosphate particles B are 54.3wt% and 45.7wt%, respectively; in S2, based on the total weight of the graphite composite particles, the contents of the graphite particles A and the graphite particles B are 25wt% and 75wt%, respectively. A soft-pack lithium iron phosphate battery is prepared.

[0151] Embodiment 21

[0152] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the negative electrode active material did not contain hard carbon and was D 50 Graphite particles A and D with a particle size of 6 μm 50 A mixture of graphite particles B with a particle size of 10 μm. Specifically, in S2, based on the total weight of the graphite composite particles, the contents of graphite particles A and graphite particles B are 25 wt% and 75 wt%, respectively. A soft-pack lithium iron phosphate battery is prepared.

[0153] Embodiment 22

[0154] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the positive electrode active material was D 50 Lithium iron phosphate particles A and D with a particle size of 2 μm 50 A mixture of lithium iron phosphate particles D with a particle size of 20 μm; the negative electrode active material does not contain hard carbon, which is D 50 Graphite particles A and D with a particle size of 6 μm 50 A mixture of graphite particles B with a particle size of 10 μm. Specifically, in S1, based on the total weight of the lithium iron phosphate composite particles, the contents of the lithium iron phosphate particles A and the lithium iron phosphate particles D are 60wt% and 40wt% respectively; in S2, based on the total weight of the graphite composite particles, the contents of the graphite particles A and the graphite particles B are 25wt% and 75wt% respectively. A soft-pack lithium iron phosphate battery is prepared.

[0155] Embodiment 23

[0156] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the positive electrode active material was D 50 A mixture of lithium iron phosphate particles B, lithium iron phosphate particles C and lithium iron phosphate particles D with particle sizes of 6 μm, 10 μm and 20 μm respectively; the negative electrode active material does not contain hard carbon, which is D 50 Graphite particles A and D with a particle size of 6 μm 50 A mixture of graphite particles B with a particle size of 10 μm. Specifically, in S1, based on the total weight of the lithium iron phosphate composite particles, the contents of the lithium iron phosphate particles B, lithium iron phosphate particles C and lithium iron phosphate particles D are 40wt%, 50wt% and 10wt% respectively; in S2, based on the total weight of the graphite composite particles, the contents of the graphite particles A and graphite particles B are 25wt% and 75wt% respectively. A soft-pack lithium iron phosphate battery is prepared.

[0157] Embodiment 24

[0158] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the positive electrode active material was D 50A mixture of lithium iron phosphate particles A, lithium iron phosphate particles B, lithium iron phosphate particles C and lithium iron phosphate particles D with particle sizes of 2 μm, 6 μm, 10 μm and 20 μm respectively; the negative electrode active material does not contain hard carbon, which is D 50 Graphite particles A and D with a particle size of 6 μm 50 A mixture of graphite particles B with a particle size of 10 μm. Specifically, in S1, based on the total weight of the lithium iron phosphate composite particles, the contents of lithium iron phosphate particles A, lithium iron phosphate particles B, lithium iron phosphate particles C and lithium iron phosphate particles D are 15wt%, 25wt%, 50wt% and 10wt% respectively; in S2, based on the total weight of the graphite composite particles, the contents of graphite particles A and graphite particles B are 25wt% and 75wt% respectively. A soft-pack lithium iron phosphate battery is prepared.

[0159] Comparative Example 1

[0160] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the positive electrode active material was D 50 Lithium iron phosphate particles A with a particle size of 2 μm, and negative electrode active material D 50 Graphite particles A with a particle size of 6 μm. Specifically, in S1, based on the total weight of the positive electrode powder, 92wt% of lithium iron phosphate particles A, 4.5wt% of conductive agent 1 (2wt% of SP + 2.5wt% of CNTs) and 3.5wt% of binder 1 were weighed according to the mass percentage to obtain the positive electrode powder; in S2, based on the total weight of the negative electrode powder, 92wt% of graphite particles A (OI value of 5), 4wt% of conductive agent 2 (SP) and 4wt% of binder 2 (1.5wt% CMC with a weight average molecular weight of 400,000 g / mol and 2.5wt% SBR) were weighed according to the mass percentage to obtain the negative electrode powder. A soft-pack lithium iron phosphate battery was obtained.

[0161] Comparative Example 2

[0162] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the positive electrode active material was D 50 Lithium iron phosphate particles B with a particle size of 6 μm, and the negative electrode active material is D 50Graphite particles A with a particle size of 6 μm. Specifically, in S1, based on the total weight of the positive electrode powder, 92wt% of lithium iron phosphate particles B, 4.5wt% of conductive agent 1 (2wt% SP + 2.5wt% CNTs) and 3.5wt% of binder 1 were weighed according to the mass percentage to obtain the positive electrode powder; in S2, based on the total weight of the negative electrode powder, 92wt% of graphite particles A (OI value of 5), 4wt% of conductive agent 2 (SP) and 4wt% of binder 2 (1.5wt% CMC with a weight average molecular weight of 400,000 and 2.5wt% SBR) were weighed according to the mass percentage to obtain the negative electrode powder. A soft-pack lithium iron phosphate battery was obtained.

[0163] Comparative Example 3

[0164] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the positive electrode active material was D 50 Lithium iron phosphate particles C with a particle size of 10 μm, and the negative electrode active material is D 50 Graphite particles A with a particle size of 6 μm. Specifically, in S1, based on the total weight of the positive electrode powder, 92wt% of lithium iron phosphate particles C, 4.5wt% of conductive agent 1 (2wt% of SP + 2.5wt% of CNTs) and 3.5wt% of binder 1 were weighed according to the mass percentage to obtain the positive electrode powder; in S2, based on the total weight of the negative electrode powder, 92wt% of graphite particles A (OI value of 5), 4wt% of conductive agent 2 (SP) and 4wt% of binder 2 (1.5wt% CMC with a weight average molecular weight of 400,000 g / mol and 2.5wt% SBR) were weighed according to the mass percentage to obtain the negative electrode powder. A soft-pack lithium iron phosphate battery was obtained.

[0165] Comparative Example 4

[0166] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the positive electrode active material was D 50 The particle size of lithium iron phosphate particles D is 20 μm, and the negative electrode active material is D 50 Graphite particles A with a particle size of 6 μm. Specifically, in S1, based on the total weight of the positive electrode powder, 92wt% of lithium iron phosphate particles D, 4.5wt% of conductive agent 1 (2wt% of SP + 2.5wt% of CNTs) and 3.5wt% of binder 1 were weighed according to the mass percentage to obtain the positive electrode powder; in S2, based on the total weight of the negative electrode powder, 92wt% of graphite particles A (OI value of 5), 4wt% of conductive agent 2 (SP) and 4wt% of binder 2 (1.5wt% CMC with a weight average molecular weight of 400,000 g / mol and 2.5wt% SBR) were weighed according to the mass percentage to obtain the negative electrode powder. A soft-pack lithium iron phosphate battery was obtained.

[0167] Comparative Example 5

[0168] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the positive electrode active material was D 50 Lithium iron phosphate particles A with a particle size of 2 μm, and negative electrode active material D 50 Graphite particles B with a particle size of 10 μm. Specifically, in S1, based on the total weight of the positive electrode powder, 92wt% of lithium iron phosphate particles A, 4.5wt% of conductive agent 1 (2wt% of SP + 2.5wt% of CNTs) and 3.5wt% of binder 1 were weighed according to the mass percentage to obtain the positive electrode powder; in S2, based on the total weight of the negative electrode powder, 92wt% of graphite particles B (OI value of 4.5), 4wt% of conductive agent 2 (SP) and 4wt% of binder 2 (1.5wt% CMC with a weight average molecular weight of 400,000 g / mol and 2.5wt% SBR) were weighed according to the mass percentage to obtain the negative electrode powder. A soft-pack lithium iron phosphate battery was obtained.

[0169] Comparative Example 6

[0170] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the positive electrode active material was D 50 Lithium iron phosphate particles B with a particle size of 6 μm, and the negative electrode active material is D 50 Graphite particles B with a particle size of 10 μm. Specifically, in S1, based on the total weight of the positive electrode powder, 92wt% of lithium iron phosphate particles B, 4.5wt% of conductive agent 1 (2wt% of SP + 2.5wt% of CNTs) and 3.5wt% of binder 1 were weighed according to the mass percentage to obtain the positive electrode powder; in S2, based on the total weight of the negative electrode powder, 92wt% of graphite particles B (OI value of 4.5), 4wt% of conductive agent 2 (SP) and 4wt% of binder 2 (1.5wt% CMC with a weight average molecular weight of 400,000 g / mol and 2.5wt% SBR) were weighed according to the mass percentage to obtain the negative electrode powder. A soft-pack lithium iron phosphate battery was obtained.

[0171] Comparative Example 7

[0172] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the positive electrode active material was D 50 Lithium iron phosphate particles C with a particle size of 10 μm, and the negative electrode active material is D 50Graphite particles B with a particle size of 10 μm. Specifically, in S1, based on the total weight of the positive electrode powder, 92wt% of lithium iron phosphate particles C, 4.5wt% of conductive agent 1 (2wt% of SP + 2.5wt% of CNTs) and 3.5wt% of binder 1 were weighed according to the mass percentage to obtain the positive electrode powder; in S2, based on the total weight of the negative electrode powder, 92wt% of graphite particles B (OI value of 4.5), 4wt% of conductive agent 2 (SP) and 4wt% of binder 2 (1.5wt% CMC with a weight average molecular weight of 400,000 g / mol and 2.5wt% SBR) were weighed according to the mass percentage to obtain the negative electrode powder. A soft-pack lithium iron phosphate battery was obtained.

[0173] Comparative Example 8

[0174] A soft-pack lithium iron phosphate battery was prepared according to the method of Example 1, except that the positive electrode active material was D 50 The particle size of lithium iron phosphate particles D is 20 μm, and the negative electrode active material is D 50 Graphite particles B with a particle size of 10 μm. Specifically, in S1, based on the total weight of the positive electrode powder, 92wt% of lithium iron phosphate particles D, 4.5wt% of conductive agent 1 (2wt% of SP + 2.5wt% of CNTs) and 3.5wt% of binder 1 were weighed according to the mass percentage to obtain the positive electrode powder; in S2, based on the total weight of the negative electrode powder, 92wt% of graphite particles B (OI value of 4.5), 4wt% of conductive agent 2 (SP) and 4wt% of binder 2 (1.5wt% CMC with a weight average molecular weight of 400,000 g / mol and 2.5wt% SBR) were weighed according to the mass percentage to obtain the negative electrode powder. A soft-pack lithium iron phosphate battery was obtained.

[0175] Test Case

[0176] The cold starting voltage and the capacity retention rate after 1000 cycles of the soft-pack lithium iron phosphate batteries prepared in each embodiment and comparative example were measured.

[0177] The results are shown in Table 1.

[0178] Table 1

[0179]

[0180]

[0181] From the results in Table 1, it can be seen that the lithium iron phosphate batteries provided by the present invention in Examples 1-24 can have excellent high and low temperature performance; while the positive electrode active materials in Comparative Examples 1-8 are all single D 50 The lithium iron phosphate particles have a particle size of 1.5-1.5 mm, and the negative electrode active material is a single D 50Compared with Examples 1-24, the low-temperature cold start and / or life of the battery are significantly reduced. This shows that by mixing two or more lithium iron phosphate particles with different particle sizes as the positive electrode active material and mixing two or more graphite particles with different particle sizes as the negative electrode active material, the prepared lithium iron phosphate battery can have both good low-temperature high-rate discharge performance and long high-temperature cycle life.

[0182] In addition, only small particles (D 50 Compared with Example 1, the cold starting voltage of lithium iron phosphate particles A and lithium iron phosphate particles B with a particle size of less than or equal to 7 μm has no significant change, but the high temperature cycle capacity retention rate is significantly reduced, indicating that the positive electrode active material contains only D 50 The lithium iron phosphate particles with a particle size of less than or equal to 7 μm have a higher cold starting voltage of the battery, but a lower high temperature cycle capacity retention rate; Examples 3-6 respectively change the different D 50 The combination of lithium iron phosphate particles of different particle sizes, Example 7 changes the ratio of lithium iron phosphate particles A, lithium iron phosphate particles B and lithium iron phosphate particles C in the lithium iron phosphate composite particles, Example 8 changes the ratio of graphite composite particles and hard carbon, Example 9 changes the ratio of graphite particles A and graphite particles B, and Example 10 changes the ratio of hard carbon D 50 Example 11 changes the OI value of the graphite composite particles, and Example 12 changes the different D 50 The invention discloses a combination of graphite particles of different particle sizes, examples 13-15 respectively change the type and component ratio of lithium salt in the electrolyte, examples 16 and 17 respectively change the type and component ratio of additives in the electrolyte, examples 18 and 19 respectively change the type of positive and negative electrode conductive agents, example 21 does not contain hard carbon in the negative electrode active material, examples 20, 22-24 do not contain hard carbon in the negative electrode active material, and the different D in the positive electrode active material are changed. 50 The combination of lithium iron phosphate particles with different particle sizes shows that the cold starting voltage or high temperature cycle capacity retention rate of the obtained battery is reduced to a certain extent compared with Example 1, indicating that the different D 50 The compounding method of lithium iron phosphate particles with different particle sizes, different D 50 The compounding method of graphite particles of different particle sizes, the proportion of hard carbon, and the D 50 The particle size and OI value of the graphite composite particles, the types of positive and negative electrode conductive agents, and the types and component ratios of lithium salts and additives in the electrolyte jointly affect the cold starting voltage and high-temperature cycle capacity retention rate of the manufactured battery. The positive and negative electrode conductive agents, electrolyte and positive and negative electrode materials cooperate with each other to further improve the cold starting performance and high-temperature life of the manufactured battery, and can take into account excellent high and low temperature performance.

[0183] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A battery cell, It is characterized in that The battery cell comprises a positive electrode and a negative electrode; The positive electrode comprises lithium iron phosphate composite particles, wherein the lithium iron phosphate composite particles comprise two or more D 50 Particle size of lithium iron phosphate particles; The negative electrode comprises graphite composite particles, wherein the graphite composite particles comprise two or more D 50 Graphite particles of different particle sizes.

2. The battery cell according to claim 1, It is characterized in that The OI value of the graphite composite particles is less than or equal to 7.

3. The battery cell according to claim 1 or 2, It is characterized in that The negative electrode further includes hard carbon.

4. The battery cell according to claim 3, It is characterized in that The hard carbon D 50 The particle size is 3-10μm.

5. The battery cell according to claim 3 or 4, It is characterized in that The weight ratio of the graphite composite particles to the hard carbon is (85-95):(5-15).

6. The battery cell according to any one of claims 1 to 5, It is characterized in that The graphite composite particles include graphite particles A and graphite particles B; wherein the graphite particles A have a 50 The particle size is 5-7 μm, and the graphite particles B and D 50 The particle size is 9-11μm.

7. The battery cell according to any one of claim 6, It is characterized in that Based on the total weight of the graphite composite particles, in the graphite composite particles, the contents of the graphite particles A and the graphite particles B are 25-75wt% and 25-75wt%, respectively.

8. The battery cell according to any one of claims 1 to 7, It is characterized in that The lithium iron phosphate composite particles include two or more of lithium iron phosphate particles A, lithium iron phosphate particles B, lithium iron phosphate particles C and lithium iron phosphate particles D; wherein the lithium iron phosphate particles A, the lithium iron phosphate particles B, the lithium iron phosphate particles C and the lithium iron phosphate particles D are respectively 50 The particle sizes are 1-3μm, 5-7μm, 9-11μm and 18-20μm respectively.

9. The battery cell according to claim 8, It is characterized in that Based on the total weight of the lithium iron phosphate composite particles, the total content of the lithium iron phosphate particles A and the lithium iron phosphate particles B is less than or equal to 75 wt %.

10. The battery cell according to claim 8 or 9, It is characterized in that The lithium iron phosphate composite particles are a mixture of lithium iron phosphate particles A, lithium iron phosphate particles B and lithium iron phosphate particles C; Preferably, based on the total weight of the lithium iron phosphate composite particles, in the lithium iron phosphate composite particles, the contents of the lithium iron phosphate particles A, the lithium iron phosphate particles B and the lithium iron phosphate particles C are 10-30wt%, 20-40wt% and 30-70wt% respectively.

11. The battery cell according to any one of claims 1 to 10, It is characterized in that The battery cell also includes an electrolyte, and the electrolyte includes a lithium salt; Preferably, the lithium salt includes lithium hexafluorophosphate and at least one selected from lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(oxalatoborate) and lithium difluorooxalatoborate; Preferably, the lithium salt is a mixture of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide and lithium difluorooxalatoborate; Preferably, in the lithium salt, the weight ratio of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide and lithium difluorooxalatoborate is (5-10): (2-5): (0.5-1); Preferably, the content of the lithium salt in the electrolyte is 0.8-1.2 mol / L.

12. The battery cell according to any one of claims 1 to 11, It is characterized in that The battery core further includes an electrolyte, and the electrolyte further includes an additive, wherein the additive is selected from at least one of vinylene carbonate, fluoroethylene carbonate and vinyl sulfate; Preferably, the additive is a mixture of vinylene carbonate and fluoroethylene carbonate; Preferably, the content of the additive in the electrolyte is 1.5-5 mol / L; Preferably, in the additive, the weight ratio of vinylene carbonate to fluoroethylene carbonate is (1.5-3.5):(2-5).

13. The battery cell according to any one of claims 1 to 12, It is characterized in that The battery cell further comprises an electrolyte, and the electrolyte further comprises a solvent, wherein the solvent is selected from at least one of ethylene carbonate, diethyl carbonate, ethyl methyl carbonate and dimethyl carbonate; Preferably, the solvent is a mixture of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate; Preferably, in the solvent, the weight ratio of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate is (2-4): (2-4): (3-5).

14. The battery cell according to any one of claims 1 to 13, It is characterized in that The positive electrode comprises a positive electrode current collector and a positive electrode material layer disposed on the positive electrode current collector; wherein, based on the total weight of the positive electrode material layer, the content of the lithium iron phosphate composite particles in the positive electrode material layer is 90-94wt%; Preferably, the negative electrode comprises a negative electrode current collector and a negative electrode material layer disposed on the negative electrode current collector; wherein, based on the total weight of the negative electrode material layer, the total content of the graphite composite particles and hard carbon in the negative electrode material layer is 90-95wt%; Preferably, based on the total weight of the graphite composite particles and the hard carbon, the content of the hard carbon is less than or equal to 15 wt %.

15. A method for preparing a battery cell, It is characterized in that The preparation method comprises: A positive electrode is provided, the positive electrode comprising lithium iron phosphate composite particles, the lithium iron phosphate composite particles comprising two or more D 50 Particle size of lithium iron phosphate particles; A negative electrode is provided, the negative electrode comprising graphite composite particles, the graphite composite particles comprising two or more D 50 Particle size of graphite particles; The positive electrode and the negative electrode are assembled into a battery cell.

16. The preparation method according to claim 15, It is characterized in that The preparation method of the positive electrode comprises: 50 The lithium iron phosphate particles with different particle sizes are mixed to obtain the lithium iron phosphate composite particles; the lithium iron phosphate composite particles, the conductive agent 1, the binder 1 and the solvent 1 are mixed to obtain a positive electrode slurry, and then the positive electrode slurry is coated on the surface of the positive electrode current collector, and then rolled to obtain the positive electrode; Preferably, the method for preparing the negative electrode comprises: 50 Graphite particles of different particle sizes are mixed to obtain the graphite composite particles; the graphite composite particles, hard carbon, a conductive agent 2, a binder 2 and a solvent 2 are mixed to obtain a negative electrode slurry, and then the negative electrode slurry is coated on the surface of a negative electrode collector, and then rolled to obtain the negative electrode.

17. A starting battery for a fuel engine, It is characterized in that The invention comprises the battery cell described in any one of claims 1 to 14 or the battery cell prepared by the preparation method described in claim 15 or 16.

18. A vehicle, It is characterized in that It comprises the battery cell described in any one of claims 1 to 14, or the battery cell prepared by the preparation method described in claim 15 or 16, or the starting battery for the fuel engine described in claim 17.

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