Positive electrode material, positive plate and lithium ion battery
Patent Information
- Application Number
- CN202511051478.1
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-07-29
- Publication Date
- 2025-10-17
AI Technical Summary
Under low temperature conditions, the lithium ion extraction efficiency of lithium iron phosphate positive electrode materials decreases, resulting in reduced power performance and service life of lithium-ion batteries.
By controlling the specific surface area, compaction density and mixing ratio of single-crystal lithium iron phosphate and polycrystalline lithium iron phosphate in the positive electrode material, and combining it with a carbon coating layer, the diffusion performance of lithium ions in the positive electrode material is optimized.
In low temperature environments, lithium-ion batteries exhibit better power performance and maintain good cycle life in long-term use.
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Figure CN120809795A_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The present application relates to the technical field of batteries, and in particular to a positive electrode material, a positive electrode sheet and a lithium ion battery. BACKGROUND
[0002] As a positive electrode material of a lithium ion battery, lithium iron phosphate (LiFePO4) releases lithium ions in the lithium iron phosphate during the charging process, which are transmitted to the negative electrode through the electrolyte and embedded in the negative electrode carbon material. At the same time, electrons from the positive electrode reach the negative electrode through the external circuit to maintain charge balance. During discharging, lithium ions are released from the negative electrode, return to the positive electrode through the electrolyte, and at the same time, electrons flow from the negative electrode to the positive electrode through the external circuit, thereby providing energy to the outside world.
[0003] In some related technologies, when the ambient temperature is low, the lithium ion release efficiency of the lithium iron phosphate positive electrode is greatly reduced, the kinetics of lithium ions is poor, which in turn results in poor power performance of the battery, the migration speed of lithium ions is slowed down, and the formation of lithium dendrites is possible. The above problems greatly reduce the power and service life of the battery using lithium iron phosphate as the positive electrode material in a low temperature environment. SUMMARY
[0004] To solve at least one of the problems mentioned in the background, the present application provides a positive electrode material, a positive electrode sheet and a lithium ion battery to improve the power performance of the positive electrode material in a low temperature environment.
[0005] The specific technical solutions provided by the embodiments of the present application are as follows:
[0006] In a first aspect, a positive electrode material is provided, which includes lithium iron phosphate particles, the lithium iron phosphate particles including single crystal lithium iron phosphate and / or polycrystalline lithium iron phosphate, and the positive electrode material satisfies the following formula:
[0007]
[0008] The C1 represents the discharge gram capacity of the battery after the positive electrode material is assembled, mAh / g; the BET1 represents the specific surface area of the single crystal lithium iron phosphate, m 2 / g; the BET2 represents the specific surface area of the polycrystalline lithium iron phosphate, m 2 / g; the K represents the ratio between the mass of the polycrystalline lithium iron phosphate and the mass of the lithium iron phosphate particles; and the PD represents the compacted density of the lithium iron phosphate particles, g / cm 3 .
[0009] In a specific embodiment, the chemical formula of the single crystal lithium iron phosphate and / or the polycrystalline lithium iron phosphate is LiFe X Mn 1-XPO4, and X ranges from 0.2≤X≤1.
[0010] In one specific embodiment,
[0011] In one specific embodiment, the surface of the single-crystal lithium iron phosphate and / or the surface of the polycrystal lithium iron phosphate is coated with a carbon coating layer; wherein,
[0012] The mass fraction of the carbon coating layer is 0.1wt% to 3wt% based on the mass of the lithium iron phosphate particle with the carbon coating layer.
[0013] In one specific embodiment, the C1 ranges from 135mAh / g to 155mAh / g.
[0014] In one specific embodiment, the BET1 ranges from 10m 2 / g to 15m 2 / g.
[0015] In one specific embodiment, the BET2 ranges from 6m 2 / g to 10m 2 / g.
[0016] In one specific embodiment, the PD ranges from 1.90g / cm 3 to 2.75g / cm 3 .
[0017] In one specific embodiment, the PD ranges from 2.12g / cm 3 to 2.65g / cm 3 .
[0018] In one specific embodiment, the K ranges from 0 to 1.
[0019] In one specific embodiment, 0
[0020] In one specific embodiment, 0
[0021] In one specific embodiment, 0.1
[0022] In one specific embodiment, the tap density PD1 of the single-crystal lithium iron phosphate is 2.50 to 2.75g / cm 3 .
[0023] In one specific embodiment, the volume median particle size D v50 of the single-crystal lithium iron phosphate is 0.8 to 1.8μm.
[0024] In one specific embodiment, the compacted density PD2 of the polycrystalline lithium iron phosphate is 1.90-2.40 g / cm3. 3 .
[0025] In one specific embodiment, the volume median particle size D50 of the polycrystalline lithium iron phosphate is 3.5-9.5 μm. v50
[0026] In a second aspect, a positive electrode sheet is provided, which comprises the positive electrode material as described above.
[0027] In a third aspect, a lithium ion battery is provided, which comprises the positive electrode sheet as described above.
[0028] The embodiments of the present application have the following beneficial effects:
[0029] The embodiments of the present application provide a solution by controlling the positive electrode material to meet improve the electrochemical performance of the positive electrode material in a low-temperature environment: in a low-temperature environment, lithium ions in the positive electrode material can reach the negative electrode more quickly during charging, which promotes the lithium ion battery in which it is located to exhibit better low-temperature power performance, while ensuring that the lithium ion battery also has good cycle life during long-term use. BRIEF DESCRIPTION OF DRAWINGS
[0030] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.
[0031] Figure 1 shows a schematic diagram of the polycrystalline lithium iron phosphate according to the present application.
[0032] Figure 2 shows a schematic diagram of the polycrystalline lithium iron phosphate according to the present application. DETAILED DESCRIPTION
[0033] In order to make the purposes, technical solutions and advantages of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, not all. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.
[0034] The ranges disclosed herein are defined by their lower and upper endpoints, given that a range is defined by selecting a lower endpoint and an upper endpoint, the selected lower and upper endpoints define the boundaries of the particular range. Ranges defined by endpoints can be inclusive or exclusive of the endpoints, and can be arbitrarily combined, i.e., any lower endpoint can be combined with any upper endpoint to form a range. For example, if ranges of 60-120 and 80-110 are listed, it is understood that ranges of 60-110 and 80-120 are also contemplated. Also, if minimum range values of 1 and 2 are listed, and if maximum range values of 3, 4, and 5 are listed, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present disclosure, unless otherwise stated, a numerical range "a-b" indicates a shorthand way of describing each and every
[0035] In the present disclosure, all the embodiments and preferred embodiments mentioned herein can be combined with each other to form new technical solutions, unless otherwise specified.
[0036] In the present disclosure, all the technical features and preferred features mentioned herein can be combined with each other to form new technical solutions, unless otherwise specified.
[0037] In the present disclosure, all the steps mentioned herein can be performed in sequence or randomly, but preferably in sequence, unless otherwise specified. For example, the method comprises steps (a) and (b), which means that the method can comprise steps (a) and (b) performed in sequence, or steps (b) and (a) performed in sequence. For example, the method can further comprise step (c), which means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.
[0038] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.
[0039] The application provides a positive electrode material, which comprises lithium iron phosphate particles, wherein the lithium iron phosphate particles comprise single-crystal lithium iron phosphate and / or polycrystal lithium iron phosphate; and the positive electrode material satisfies the following formula:
[0040]
[0041] wherein C1 represents the discharge capacity of the battery after the positive electrode material is assembled, mAh / g; BET1 represents the specific surface area of the single-crystal lithium iron phosphate, m 2 / g; BET2 represents the specific surface area of the polycrystal lithium iron phosphate, m 2 / g; K represents the ratio between the mass of the polycrystal lithium iron phosphate and the mass of the lithium iron phosphate particles; and PD represents the tap density of the lithium iron phosphate particles, g / cm 3 .
[0042] Specifically, the single-crystal lithium iron phosphate is primary particles, please refer to Figure 1 The average particle size of the single-crystal lithium iron phosphate is 200-400 nm. The average particle size of the single-crystal lithium iron phosphate can be obtained by the following method: taking multiple SEM images with a magnification of 30000-500000 times, and randomly selecting not less than 5 SEM images. In each SEM image, randomly select more than 20 primary particles, measure the particle size of each primary particle, and calculate the average value.
[0043] The particle size of the primary particle refers to the length of the line segment that passes through the geometric center of the primary particle and whose endpoints are located on the edge of the primary particle.
[0044] Further, please refer to Figure 2 The polycrystal lithium iron phosphate is secondary particles composed of primary particles. The secondary particles have a spherical appearance. The average particle size of the primary particles in the polycrystal lithium iron phosphate is 50-200 nm. The measurement method of the average particle size of the primary particles in the polycrystal lithium iron phosphate is consistent with the measurement method of the primary particles of the single-crystal lithium iron phosphate, which will not be described here.
[0045] In a specific embodiment, the lithium iron phosphate particles are composed of single-crystal lithium iron phosphate and / or polycrystal lithium iron phosphate.
[0046] In a specific embodiment, the value of is 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85 or a range composed of any two of the above values; by controlling the specific surface area of the single-crystal lithium iron phosphate and the polycrystal lithium iron phosphate in the lithium iron phosphate particles, and the mixing ratio K of the two, the lithium ions can quickly reach the negative electrode during the charging process when the positive electrode material is in a low-temperature environment, thereby effectively improving the power performance of the lithium ion battery with the positive electrode material.
[0047] In one specific embodiment, the above positive electrode material satisfies:
[0048] Through the above configuration, when When greater than or equal to 76 and less than or equal to 84, the mixing ratio of single crystal lithium iron phosphate and polycrystal lithium iron phosphate, and the specific surface area after mixing reach a relatively optimized balance state, and under this condition, the lithium ions in the positive electrode material can maintain normal diffusion state in low temperature environment, thereby ensuring that the lithium ion battery prepared by the positive electrode material has good low temperature power performance.
[0049] In one specific embodiment, C1 in the embodiment represents the discharge gram capacity of the battery after assembly of the positive electrode material, wherein C1 is in the range of 135 mAh / g to 155 mAh / g. Specifically, the discharge gram capacity of the 1C of the button cell after assembly of the positive electrode material can be set to 135 mAh / g, 136 mAh / g, 137 mAh / g, 138 mAh / g, 139 mAh / g, 140 mAh / g, 141 mAh / g, 142 mAh / g, 143 mAh / g, 144 mAh / g, 145 mAh / g, 146 mAh / g, 147 mAh / g, 148 mAh / g, 149 mAh / g, 150 mAh / g, 151 mAh / g, 152 mAh / g, 153 mAh / g, 154 mAh / g, 155 mAh / g, or a range composed of any two of the above values. Through the above setting, since the size of the particles in the positive electrode material will directly affect the discharge gram capacity of the battery after assembly, if the particles of the positive electrode material are too large, the diffusion distance of lithium ions in the positive electrode material particles is longer, the polarization is greater, and the discharge gram capacity is lower. Larger particle size is not conducive to the migration of lithium ions in the positive electrode active material, especially under low temperature conditions, the migration rate will further decrease, the material kinetics is insufficient, and the battery power and low temperature performance are poor. If the particles of the positive electrode material are too small, the discharge gram capacity is too large, but in the battery system, the specific surface area of the small particles is larger, the contact area with the electrolyte is larger, and more impurities are generated in the electrochemical system, which is not conducive to the electrochemical reaction, leading to poor cycle performance and high temperature storage performance. At the same time, in the processing process, smaller particles will cause the slurry to become thinner, the viscosity rebound to become larger, the coating surface density to decrease, the coating surface density to be difficult to control, and the cycle performance and high temperature storage performance of the battery after assembly to be poor. Therefore, when mixing single crystal lithium iron phosphate and polycrystal lithium iron phosphate, the specific surface area of single crystal lithium iron phosphate and polycrystal lithium iron phosphate in the positive electrode material needs to be considered, so as to achieve the effect of considering the discharge gram capacity and ensuring the cycle performance of the battery.
[0050] It should be noted that C1 in the embodiment represents the 1C discharge gram capacity of the assembled button cell of the positive electrode material; the test content of the 1C discharge gram capacity of the assembled button cell of the positive electrode material includes: 0.1C charge-discharge one cycle, 0.33C charge-1C discharge one cycle; 0.1C charge-discharge one cycle, 0.1C charge-1C discharge one cycle; wherein the greater the 1C discharge gram capacity, the better the power performance of the positive electrode material, and the lithium ion is more easily released from the positive electrode material at a large rate, the size of the particles in the positive electrode material needs to be controlled to ensure the cycle performance of the battery in a low temperature environment.
[0051] Further, the specific surface area BET1 of the single crystal lithium iron phosphate is 10m 2 / g~15m 2 / g. Specifically, BET1 can be set to 10m 2 / g, 10.5m 2 / g, 11m 2 / g, 11.5m 2 / g, 12m 2 / g, 12.5m 2 / g, 13m 2 / g, 13.5m 2 / g, 14m 2 / g, 14.5m 2 / g, 15m 2 / g or a range composed of any two of the above values; by setting the above, the value of BET1 in the embodiment is controlled to make the positive electrode material have discharge gram capacity and battery cycle performance, and the battery exhibits good electrochemical performance.
[0052] Further, the specific surface area BET2 of the polycrystalline lithium iron phosphate can be 6m 2 / g~10m 2 / g. Specifically, BET2 can be set to 6m 2 / g, 6.5m 2 / g, 7m 2 / g, 7.5m 2 / g, 8m 2 / g, 8.5m 2 / g, 9m 2 / g, 9.5m 2 / g, 10m 2 / g or a range composed of any two of the above values; since there is a corresponding relationship between the specific surface area and the particle size of the positive electrode particles, the larger the specific surface area, the smaller the corresponding positive electrode particles, in the embodiment, the value of BET2 is controlled to make the particle size of the positive electrode material in the above range, the lithium ion battery has better low temperature power performance, and at the same time, the lithium ion battery also has good cycle service life.
[0053] In a specific embodiment, the compacted density PD of the lithium iron phosphate particles is in the range of 1.90 g / cm 3 ~2.75g / cm 3 Specifically, the PD of the lithium iron phosphate particles can be set to 1.90 g / cm 3 , 1.95g / cm 3 , 2.00g / cm 3 , 2.10g / cm 3 , 2.15g / cm 3 , 2.20g / cm 3 , 2.25g / cm 3 , 2.30g / cm 3 , 2.35g / cm 3 , 2.40g / cm 3 , 2.45g / cm 3 , 2.50g / cm 3 , 2.55g / cm 3 , 2.60g / cm 3 , 2.65g / cm 3 , 2.70g / cm 3 , 2.75g / cm 3 Or it can be a range consisting of any two of the above values. If the compaction density is too small, the mass of lithium iron phosphate per unit volume will decrease, the battery cell capacity will decrease, and it will become difficult to achieve a higher capacity for the battery cell. The greater the compaction density, the higher the capacity of the battery, the smaller the internal resistance, and the longer the cycle life; however, the compaction density cannot be too high, otherwise it will affect the penetration of the electrolyte and the diffusion of lithium ions. Therefore, the embodiment of the present application improves the low-temperature power performance of the corresponding lithium-ion battery by controlling the compaction density of lithium iron phosphate particles.
[0054] In a specific embodiment, the compaction density PD1 of single crystal lithium iron phosphate is selected from: 2.50 to 2.75 g / cm 3 For example, the value of PD1 can be 2.50 g / cm 3 , 2.55g / cm 3 , 2.60g / cm 3 , 2.65g / cm 3 , 2.70g / cm 3 , 2.75g / cm 3 Or selected from the range consisting of any two thereof.
[0055] In a specific embodiment, the compaction density PD2 of polycrystalline lithium iron phosphate is selected from: 1.90 to 2.40 g / cm 3 For example, the value of PD1 can be 1.90 g / cm 3, 1.95g / cm 3 , 2.00g / cm 3 , 2.05g / cm 3 , 2.10g / cm 3 , 2.15g / cm 3 , 2.20g / cm 3 , 2.25g / cm 3 , 2.30g / cm 3 , 2.35g / cm 3 , 2.40g / cm 3 Or selected from the range consisting of any two thereof.
[0056] In a specific embodiment, the ratio K of the mass of polycrystalline lithium iron phosphate to the sum of the mass of single crystal lithium iron phosphate and the mass of polycrystalline lithium iron phosphate is in the range of 0 to 1; specifically, K can be set to 0, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1, or a range consisting of any two of the above values. For example, when K is set to 0.1, the mass ratio of single crystal lithium iron phosphate to the sum of the mass of single crystal lithium iron phosphate and polycrystalline lithium iron phosphate is 0.9, and the mass ratio of polycrystalline lithium iron phosphate to the sum of the mass of single crystal lithium iron phosphate and polycrystalline lithium iron phosphate is 0.1. It should be noted that when K is set to 0, polycrystalline lithium iron phosphate is not added to the positive electrode material, that is, the positive electrode material only includes single crystal lithium iron phosphate; when K is set to 1, single crystal lithium iron phosphate is not added to the positive electrode material, that is, the positive electrode material only includes polycrystalline lithium iron phosphate; since the mixing ratio of single crystal lithium iron phosphate and polycrystalline lithium iron phosphate will affect the compaction density of the two substances, when the mixing ratio of polycrystalline lithium iron phosphate in the positive electrode material is low or high, the specific surface area of single crystal lithium iron phosphate and polycrystalline lithium iron phosphate is controlled within the corresponding range to achieve a balance in the compaction density of monocrystalline lithium iron phosphate particles, thereby ensuring the low-temperature power of the assembled battery and avoiding the reduction of lithium iron phosphate particle compaction, which in turn leads to a reduction in the electrode compaction density and affects the capacity performance of the battery.
[0057] In one embodiment, the lithium iron phosphate particles are composed of single crystal lithium iron phosphate and polycrystalline lithium iron phosphate. Accordingly, 0<K<1.
[0058] The chemical formula of the single crystal lithium iron phosphate and / or polycrystalline lithium iron phosphate in this embodiment is LiFe X Mn 1-XPO4, wherein X ranges from 0.2≤X≤1. Specifically, X can be set as 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 1.0 or a range between any two of the above values. By the above setting, when the chemical formula of the single crystal lithium iron phosphate or the polycrystal lithium iron phosphate is configured as any value within the above range of X, the corresponding positive electrode material can meet the performance requirement of the positive electrode material in a low temperature environment in the present embodiment.
[0059] In one specific embodiment, the single crystal lithium iron phosphate and / or the polycrystal lithium iron phosphate is coated with a carbon coating layer. It can be understood that the carbon coating layer can form a coating on part of the surface of the corresponding lithium iron phosphate particles.
[0060] Specifically, the mass fraction of the carbon coating layer is 0.1wt%, 0.5wt%, 1wt%, 1.5wt%, 2wt%, 2.5wt%, 3wt% or a range between any two of the above values. By the above setting, when the carbon coating layer accounts for any value in the mass fraction of the single crystal lithium iron phosphate and / or the polycrystal lithium iron phosphate, the performance requirement of the positive electrode material in a low temperature environment can be met.
[0061] It can be understood that when the single crystal lithium iron phosphate in the lithium iron phosphate particles has a carbon coating layer, the mass fraction of the carbon coating layer is 0.1wt% to 3wt% based on the mass of the single crystal lithium iron phosphate with the carbon coating layer.
[0062] Alternatively, when the polycrystal lithium iron phosphate in the lithium iron phosphate particles has a carbon coating layer, the mass fraction of the carbon coating layer is 0.1wt% to 3wt% based on the mass of the polycrystal lithium iron phosphate with the carbon coating layer.
[0063] Alternatively, when the single crystal lithium iron phosphate and the polycrystal lithium iron phosphate in the lithium iron phosphate particles both have a carbon coating layer, the mass fraction of the carbon coating layer is 0.1wt% to 3wt% based on the sum of the mass of the polycrystal lithium iron phosphate with the carbon coating layer and the mass of the single crystal lithium iron phosphate with the carbon coating layer.
[0064] Further, when the lithium iron phosphate particles include the single crystal lithium iron phosphate and the polycrystal lithium iron phosphate, the single crystal lithium iron phosphate and the polycrystal lithium iron phosphate can be first obtained and added into a ball milling jar, and a planetary ball mill is used to dry-mix the single crystal lithium iron phosphate and the polycrystal lithium iron phosphate. Under the condition that the rotation speed is 200r / min to 400r / min and the ball milling time is 100min to 140min, the lithium iron phosphate particles with uniform mixing are obtained.
[0065] Specifically, the rotation speed when the single-crystal lithium iron phosphate and the polycrystal lithium iron phosphate are dry mixed can be set to 200 r / min, 250 r / min, 300 r / min, 350 r / min, 400 r / min, or a range formed by any two of the above values; and the ball milling time can be set to 100 min, 110 min, 120 min, 130 min, 140 min, or a range formed by any two of the above values. Through the above settings, when the rotation speed when dry mixing is selected to be any value in the above range, and the ball milling time is selected to be any value in the above range, the positive electrode material in the embodiment can be prepared.
[0066] It should be noted that after the single-crystal lithium iron phosphate and the polycrystal lithium iron phosphate are dry mixed to obtain lithium iron phosphate particles, a small amount of lithium iron phosphate particles is taken and observed under a scanning electron microscope. If the two kinds of particles are uniformly distributed, it indicates that the mixed lithium iron phosphate particles are uniform.
[0067] In the embodiment, by controlling the mixing ratio, specific surface area, and compaction density of the single-crystal lithium iron phosphate and the polycrystal lithium iron phosphate, the lithium ions in the positive electrode material reach the negative electrode quickly during the charging process in a low-temperature environment, so that the lithium ion battery has better low-temperature power performance, and at the same time, the lithium ion battery also has a good cycle service life during long-term use.
[0068] Corresponding to the above embodiment, the application provides a positive electrode sheet, which comprises the positive electrode material as described above.
[0069] Corresponding to the above embodiment, the application provides a lithium ion battery, which comprises the positive electrode sheet as described above.
[0070] Specifically, the preparation steps of the lithium ion battery in the embodiment include: preparing a core by stacking the positive electrode sheet, a separator, and a negative electrode sheet through a Z-shaped stacking machine, and then obtaining the lithium ion battery through tab welding, side sealing of a soft package battery, top sealing, liquid injection, and pre-sealing.
[0071] Embodiment 1
[0072] Corresponding to the above embodiment, the application provides a lithium ion battery, and the preparation steps thereof include:
[0073] Step 1: The specific surface area of the single-crystal lithium iron phosphate and / or the polycrystal lithium iron phosphate is tested by a nitrogen adsorption specific surface area tester; and the compaction density of the lithium iron phosphate particles is tested by a Three-Think vertical and horizontal tester, so as to select the single-crystal lithium iron phosphate and / or the polycrystal lithium iron phosphate in the corresponding lithium iron phosphate particles.
[0074] The compacted density is measured as follows: 1.0 ± 0.0005 g of lithium iron phosphate particles are weighed and placed in a 13 mm diameter mold. The mold is then pressurized in stages: 10 kN, 20 kN, and 30 kN. Each stage is maintained for 10 seconds. Finally, the height h is measured after 30 kN is maintained for 10 seconds. The calculation is performed as follows: First, the powder volume V (V = πr²h) is calculated based on the height h, and then the compacted density (ρ = m / V).
[0075] Step 2: uniformly mix single crystal lithium iron phosphate and / or polycrystalline lithium iron phosphate according to a ratio K to obtain lithium iron phosphate particles;
[0076] Step 3: Use lithium iron phosphate particles as the positive electrode material, acetylene black as the conductive agent, and polyvinylidene fluoride as the binder in a mass ratio of 90:5:5 to make a positive electrode sheet, and use the lithium sheet as the negative electrode to assemble it into a 2430 button battery. At room temperature, with a voltage window of 2.0-3.75V, test on a blue electric test system according to the button charging process of 0.1C charge / 0.1C discharge and 0.33C charge / 1C discharge. The nominal specific capacity is 1C=162mAh / g; after the test, the 1C discharge gram capacity is calculated to be C1.
[0077] formula Please refer to Table 1 for the various parameters in .
[0078] Where C1 represents the discharge capacity of the button cell assembled with the positive electrode material at 1C, mAh / g; BET1 represents the specific surface area of single crystal lithium iron phosphate, m 2 / g; BET2 represents the specific surface area of polycrystalline lithium iron phosphate, m 2 / g; K represents the ratio of the mass of polycrystalline lithium iron phosphate to the mass of lithium iron phosphate particles; PD represents the compacted density of lithium iron phosphate particles, g / cm 3 .
[0079] Example 2 to Example 9
[0080] The difference from Example 1 is that the specific surface area or the mixing ratio K value of single-crystal lithium iron phosphate and polycrystalline lithium iron phosphate is adjusted. Please refer to Table 1 for details.
[0081] Comparative Example 1 to Comparative Example 5
[0082] The difference from Example 1 is that the specific surface area or the mixing ratio K value of single-crystal lithium iron phosphate and polycrystalline lithium iron phosphate is adjusted. Please refer to Table 1 for details.
[0083] It should be noted that the lithium iron phosphate particles in the above embodiments and comparative examples are all LiFePO4; and the single crystal lithium iron phosphate and polycrystalline lithium iron phosphate in the corresponding embodiments and comparative examples all have a carbon coating layer.
[0084] Among them, the mass fraction of the carbon coating layer of the single-crystalline lithium iron phosphate and the polycrystalline lithium iron phosphate is 1.12wt% based on the mass of the single-crystalline lithium iron phosphate with a carbon coating layer; the mass fraction of the carbon coating layer of the polycrystalline lithium iron phosphate is 1.12wt% based on the mass of the polycrystalline lithium iron phosphate with a carbon coating layer.
[0085] The lithium iron phosphate particles in the above examples and comparative examples were used to prepare lithium ion batteries, and then a low-temperature DCR test was performed on the prepared lithium ion batteries.
[0086] The following first describes the preparation of lithium-ion batteries:
[0087] The above-mentioned lithium iron phosphate particles were used as the positive electrode material, and acetylene black was selected as the conductive agent, polyvinylidene fluoride was selected as the binder, and carbon nanotubes were selected as the additive. The positive electrode material, conductive agent, binder and additive were mixed in a mass ratio of 97:0.9:1.8:0.3, and N-methylpyrrolidone NMP was added to prepare a positive electrode slurry with a solid content of 55%. The positive electrode slurry was coated on the positive electrode current collector to form a surface density of 295g / m 2 The positive electrode.
[0088] Next, artificial graphite, conductive carbon black, styrene-butadiene rubber and lithium carboxymethyl cellulose were mixed in a mass ratio of 96.6:0.6:1.6:1.2 to prepare a negative electrode slurry with a solid content of 45%; the slurry was coated on the current collector to form a negative electrode slurry with a surface density of 130 g / m 2 The negative electrode.
[0089] Finally, the positive electrode sheet, negative electrode sheet, separator and electrolyte are prepared into a stacked core through a Z-shaped stacking machine, and then a lithium-ion battery is obtained through tab welding, soft-pack battery side sealing, top sealing, liquid injection and pre-sealing.
[0090] Furthermore, the test conditions for low temperature DCR are described as follows:
[0091] (1) The low-temperature discharge DCR test conditions include: at 25°C, the lithium-ion battery is first fully charged at 0.33C, then discharged at 0.33C to 90% SOC, then left to stand at -20°C for 1h to reach thermal equilibrium, and then rapidly discharged at 0.4C for 10s. The voltage difference before and after rapid discharge is divided by the discharge current to obtain the low-temperature discharge DCR of the battery at 90% SOC; and so on, data is recorded every 10% SOC, and the low-temperature discharge DCR at 90% SOC, 80% SOC, 70% SOC, 60% SOC, 50% SOC, 40% SOC, 30% SOC, 20% SOC, and 10% SOC are recorded respectively. The average value of DCR at each SOC is the low-temperature discharge DCR in Table 1.
[0092] (2) The low-temperature charging DCR test conditions include: at 25°C, the lithium-ion battery is first fully discharged at 0.33C, then charged to 10% SOC at 0.33C, then left to stand at -20°C for 1 hour to reach thermal equilibrium, and then fast-charged at 0.4C for 10S. The voltage difference before and after the fast charge is divided by the charging current to obtain the low-temperature charging DCR of the battery at 10% SOC. Similarly, data is recorded every 10% SOC, and the low-temperature charging DCR at 10% SOC, 20% SOC, 30% SOC, 40% SOC, 50% SOC, 60% SOC, 70% SOC, 80% SOC, and 90% SOC are recorded respectively. The average value of the DCR at each SOC is the low-temperature charging DCR in Table 1. Please refer to Table 1 for details.
[0093] Table 1
[0094]
[0095]
[0096] Combined with the test results in Table 1, it can be seen that when When the value is between 74 and 85, especially between 76 and 84, the lithium battery has a lower low-temperature DCR regardless of how the mixing ratio changes.
[0097] Furthermore, from the data results of Examples 1 to 3, it can be seen that when the K value gradually increases, the 1C discharge capacity C1 of the lithium iron phosphate particles gradually increases, and the compaction density PD of the lithium iron phosphate particles gradually decreases. Gradually increases; and when the blending ratio reaches a certain level, as shown in Examples 4-5, the decrease in compaction is greater than the increase in 1C gram capacity, The value will gradually decrease, the corresponding charging DCR and discharging DCR will become larger, and the corresponding low-temperature power performance will deteriorate.
[0098] Furthermore, from Examples 1 to 10, it can be seen that There is a certain correspondence between the charging DCR and the discharging DCR, that is, In the numerical range, The larger the value, the smaller the charge and discharge DCR, which means that the lithium battery has better low-temperature power performance.
[0099] In summary, by controlling the mixing ratio, specific surface area, and compaction density and other parameters of the single-crystal lithium iron phosphate and the polycrystal lithium iron phosphate, the lithium iron phosphate particles exhibit good low-temperature DCR performance in a low-temperature environment. Thus, the lithium ions in the positive electrode material reach the negative electrode quickly during the charging process, ensuring that the lithium ion battery has better low-temperature power performance, while also ensuring that the lithium ion battery has a good cycle service life during use.
[0100] Although the preferred embodiments in the application have been described, those skilled in the art can make other changes and modifications to the embodiments once they know the basic inventive concept. Therefore, the appended claims are intended to be interpreted as including all the preferred embodiments and all the changes and modifications falling within the scope of the embodiments in the application.
[0101] Obviously, various modifications and changes can be made to the present application by those skilled in the art without departing from the spirit and scope of the present application. Thus, it is intended that the present application also cover such modifications and changes as fall within the scope of the claims of the present application and their equivalents.
Claims
1. A positive electrode material, characterized in that The positive electrode material includes lithium iron phosphate particles, and the lithium iron phosphate particles include single-crystal lithium iron phosphate and / or polycrystalline lithium iron phosphate; the positive electrode material satisfies the following formula: Wherein, C1 represents the discharge capacity of the battery after the positive electrode material is assembled, mAh / g; BET1 represents the specific surface area of the single crystal lithium iron phosphate, m 2 / g; BET2 represents the specific surface area of the polycrystalline lithium iron phosphate, m 2 / g; K represents the ratio of the mass of the polycrystalline lithium iron phosphate to the mass of the lithium iron phosphate particles; PD represents the compacted density of the lithium iron phosphate particles, g / cm 3 .
2. The positive electrode material according to claim 1, characterized in that The chemical formula of the single crystal lithium iron phosphate and / or the polycrystalline lithium iron phosphate is LiFe X Mn 1-X PO4, the range of X is 0.2≤X≤1.
3. The positive electrode material according to claim 1, characterized in that The surface of the single crystal lithium iron phosphate and / or the polycrystalline lithium iron phosphate is coated with a carbon coating layer; wherein, Based on the mass of the lithium iron phosphate particles having the carbon coating layer, the mass fraction of the carbon coating layer is 0.1 wt % to 3 wt %.
4. The positive electrode material according to any one of claims 1 to 3, characterized in that The range of C1 is 135 mAh / g to 155 mAh / g.
5. The positive electrode material according to any one of claims 1 to 3, characterized in that The range of the BET1 is 10m 2 / g~15m 2 / g.
6. The positive electrode material according to any one of claims 1 to 3, characterized in that The range of the BET2 is 6m 2 / g~10m 2 / g.
7. The positive electrode material according to any one of claims 1 to 3, characterized in that The PD range is 1.90 g / cm 3 ~2.75g / cm 3 .
8. The positive electrode material according to any one of claims 1 to 3, characterized in that The range of K is 0-1.
9. A positive electrode sheet, characterized in that: The positive electrode sheet comprises the positive electrode material according to any one of claims 1 to 8.
10. A lithium ion battery, characterized in that: The lithium-ion battery comprises the positive electrode sheet according to claim 9.