Negative electrode material and rate lithium precipitation detection method thereof

Abstract

The invention provides a negative electrode material and a rate lithium precipitation detection method thereof, and belongs to the technical field of lithium ion batteries. The dynamic index Q of the negative electrode material is more than 200 and less than 10000, Q meets the following relational expression: Q = S / (SOC x * y), y is the compaction density, g / cc, of the negative electrode material, and x is more than 0 and less than 1. SOC x is the capacity of the negative electrode material under the lithium precipitation rate, and the unit is%; s is the integral area of the lithium stripping peak in the differential curve of dV / dT-t. The method is mainly used for the rate lithium precipitation test of the negative electrode material, and the rate performance of the battery can be accurately judged.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and in particular relates to a negative electrode material and a method for detecting lithium deposition rate thereof. Background Art

[0002] Lithium-ion batteries have become a key energy storage device for portable electronic devices and electric vehicles due to their high energy density, long cycle life, and environmental friendliness. Currently, the negative electrode of commercial lithium-ion batteries is mainly composed of graphite-based carbon materials due to their low lithium insertion potential, stable voltage platform, good cycle stability, and low cost.

[0003] The development of lithium-ion batteries in electric vehicles has placed higher demands on their fast-charging capabilities. However, lithium plating is prone to occur during high-rate charge and discharge, reducing the battery's coulombic efficiency and rate performance. Furthermore, lithium deposition on the graphite surface can easily lead to battery capacity decay and threaten battery safety. Therefore, the lithium insertion kinetics of the negative electrode graphite material is the main factor limiting the battery's high-rate performance. Therefore, studying the mechanism of lithium plating in the negative electrode of lithium-ion batteries and proposing methods to inhibit lithium plating are of great significance to improving battery safety.

[0004] CN115097341B discloses a rate lithium analysis method, which can evaluate its lithium analysis performance by comparing the critical lithium analysis rates of different graphite materials. The larger the critical lithium analysis rate, the less likely the graphite material is to be lithium analyzed, and it has better fast charging performance, which is suitable for high rate batteries. Specifically, the button-type half-cell composed of a graphite material and a lithium sheet is activated-charge and discharge cycled, and the voltage change over time after each discharge phase is recorded, and differential processing is performed to obtain a voltage change rate curve, wherein the appearance of a lithium stripping peak represents lithium analysis, and the rate range of lithium analysis can be determined. However, this method only obtains the critical lithium analysis rate range by the peak position of the lithium stripping peak, and this method is used to compare the pros and cons of the rate performance of a material, which is not accurate enough, and the continuous lithium analysis process of the material cannot be analyzed, nor can the rate performance be accurately regulated. Summary of the Invention

[0005] The present invention provides a negative electrode material and a method for detecting lithium plating at a rate thereof. By simultaneously controlling the integrated area S of the lithium stripping peak a, the capacity SOC x, and the compaction density y of the negative electrode material, the negative electrode material can have optimal rate performance, thereby improving the kinetic performance. The present invention provides a negative electrode material, wherein the kinetic index Q of the negative electrode material satisfies 200<Q<10000, and Q satisfies the following relationship: Q= S / (SOC x*y), Wherein, y is the compaction density of the negative electrode material, g / cc; SOC x is the capacity of the negative electrode material at the lithium plating rate, unit is %; S is the integrated area of ​​the lithium stripping peak in the differential curve of dV / dT-t.

[0006] Furthermore, the method for obtaining the capacity SOC x and the integrated area S includes: The negative electrode material is subjected to a rate lithium deposition test, wherein one cycle is defined as discharge, rest, high rate charge, low rate charge, and rest; Cycle charge and discharge n times, and make n Vt curves of voltage versus time based on the change of voltage with time when the battery is at rest after discharge. Differentiate the Vt curves to get n dV / dT-t differential curves. Whether lithium deposition occurs is determined based on whether a lithium stripping peak appears in the differential curve of dV / dT-t. For the rate window where the lithium stripping peak appears, the integrated area S at that rate is calculated, and the capacity SOC x at that rate is measured. Among them, the differential curve of dV / dT-t has the unit of s on the horizontal axis and V / s on the vertical axis. The integral area S is the area when the vertical axis is magnified by 10 6 The value after.

[0007] Furthermore, the negative electrode material satisfies at least one of the following: 1) The compacted density y of the negative electrode material is 0.88-1.85 g / cc; 2) The capacity SOC x of the negative electrode material is 1-100%; 3) the integrated area S of the lithium stripping peak of the negative electrode material is 0-100000; 4) The negative electrode material includes at least one of silicon-based materials, graphite materials, hard carbon materials, soft carbon materials, and porous carbon materials. The present invention further provides a method for testing the rate lithium deposition of any of the above-mentioned negative electrode materials, comprising the following steps: (1) Assembling the above-mentioned negative electrode materials into a battery and conducting a rate lithium deposition test, wherein the battery is charged and discharged in a cycle consisting of discharge, rest, high-rate charging, low-rate charging, and rest, and the number of cycles is n; (2) monitoring the change of voltage over time during the rest period after discharge in each cycle in step (1), making n Vt curves of voltage over time, and differentiating the Vt curves to obtain n dV / dT-t differential curves; (3) judging whether lithium deposition occurs at the discharge rate corresponding to the dV / dT-t differential curve according to whether a lithium stripping peak appears in the dV / dT-t differential curve; If lithium deposition occurs, analyze the rate window where the lithium stripping peak appears and calculate the integrated area S at this rate, where S is the vertical axis magnified by 10 in the differential curve of dV / dT-t. 6 The capacity SOC x at the rate is measured, and the kinetic index Q=S / (SOC x*y), y is the compaction density of the negative electrode material, g / cc. The larger the Q value, the better the rate performance of the battery.

[0008] Furthermore, the rate lithium deposition test method satisfies at least one of the following: 1) The compacted density y of the negative electrode material is 0.88-1.85 g / cc; 2) The capacity SOC x of the negative electrode material is 1-100%; 3) the integrated area S of the lithium stripping peak of the negative electrode material is 0-100000; 4) The negative electrode material includes at least one of silicon-based materials, graphite materials, hard carbon materials, soft carbon materials, and porous carbon materials.

[0009] Furthermore, before performing the rate lithium deposition test step, the method further includes: performing a standard charge-discharge capacity first-efficiency test on the battery to screen batteries with a capacity and first-efficiency deviation of less than 5%; The steps of the standard buckle capacity first-effect test specifically include: cyclic charge and discharge according to the cycle of discharge, rest, high-rate charge, low-rate charge, and rest; Preferably, the discharge is performed at a discharge rate of 0.1C; Preferably, charging is performed at a charging rate of 0.1C and 0.05C respectively; Preferably, the voltage range is 0.005V-1.5V; Preferably, the number of charge and discharge cycles is 1 to 3 times; Preferably, the standing time is 0.2-1 h. Furthermore, in step (1), constant current and constant voltage charging is performed at the same rate, and stepwise constant current discharge is performed at different rates; Preferably, in step (1), the discharge rate is 0.1-5 C; Preferably, in step (1), the charging rate is 0.01-0.1C, more preferably, the charging rate is 0.05C or 0.1C; More preferably, in step (1), the discharge time is calculated according to the discharge rate of each discharge cycle, and the formula is as follows: x60; Where t is the discharge time, in minutes; C is the rated capacity of the battery; and C0 is the discharge rate.

[0010] Furthermore, in step (1), in the n cycles, starting from the first cycle, the discharge rate of each cycle increases successively, and the difference in discharge rate between every two adjacent cycles is greater than or equal to 0.1C; Preferably, in step (1), n ​​is a natural number, 2≤n≤30.

[0011] Furthermore, in step (2), in the Vt curve showing the voltage changing with time, the unit of time is s and the unit of voltage is V.

[0012] Furthermore, in step (3), the larger the Q value, the better the rate performance of the battery; Preferably, in step (3), when 200<Q<10000, the rate performance of the battery is excellent.

[0013] The present invention has the following advantages: The present invention proposes a negative electrode material and a method for detecting lithium deposition at a rate thereof. Through constant current charge and discharge tests, the lithium stripping peak a that appears in the static stage after lithium insertion at different rates, the rate window, the integral area S, and the capacity SOC x at the rate can all affect its rate performance. Among them, the rate window of the lithium stripping peak is the rate range of lithium deposition, which can be used to judge the fast charging performance of the material. The higher the rate at which the lithium stripping peak appears, the better the kinetic performance of the material; the larger the integral area S of the lithium stripping peak, the greater the polarization and the lower the kinetic performance of the material; the higher the capacity SOC x retention rate at each rate, the higher the rate performance. Based on this, the inventors of the present application use the compaction density y as a controllable factor, combined with the integral area S and the capacity SOC x at the rate, and propose to use the kinetic index Q=S / (SOC x*y) to judge the kinetic performance of the negative electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0015] Figure 1 The dV / dT-t differential curve obtained from the rate lithium deposition test in Example 1; Figure 2 The integrated area S corresponding to the lithium stripping peak where the lithium deposition window is located in the dV / dT-t differential curve obtained from the rate lithium deposition test in Example 1; Figure 3 The dV / dT-t differential curve obtained from the rate lithium deposition test in Example 2; Figure 4The integrated area S corresponding to the lithium stripping peak where the lithium deposition window is located in the dV / dT-t differential curve obtained from the rate lithium deposition test in Example 2; Figure 5 The dV / dT-t differential curve obtained from the rate lithium deposition test in Example 3; Figure 6 The integrated area S corresponding to the lithium stripping peak where the lithium deposition window is located in the dV / dT-t differential curve obtained from the rate lithium deposition test in Example 3; Figure 7 The dV / dT-t differential curve obtained from the rate lithium deposition test in Example 4; Figure 8 The integrated area S corresponding to the lithium stripping peak where the lithium deposition window is located in the dV / dT-t differential curve obtained from the rate lithium deposition test in Example 4; Figure 9 The dV / dT-t differential curve obtained from the rate lithium deposition test in Example 5; Figure 10 The integrated area S corresponding to the lithium stripping peak where the lithium deposition window is located in the dV / dT-t differential curve obtained from the rate lithium deposition test in Example 5; Figure 11 Comparison of the capacity SOC x retention rate of lithium-ion batteries assembled with materials of different compactions in Examples 1-5. DETAILED DESCRIPTION

[0016] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. The embodiments of the present invention and the features in the embodiments can be combined with each other unless there is a conflict.

[0017] Existing rate lithium deposition methods primarily compare the critical lithium deposition rates of different graphite materials to assess their lithium deposition performance. A higher critical lithium deposition rate indicates a graphite material that is less susceptible to lithium deposition and exhibits better fast-charging performance, making it suitable for high-rate batteries. This method primarily compares rate performance by comparing lithium deposition intervals. However, this method is not accurate and cannot analyze the material's ongoing lithium deposition process. On the one hand, an embodiment of the present invention provides a negative electrode material, wherein the kinetic index Q of the negative electrode material satisfies 200<Q<10000, and Q satisfies the following relationship: Q= S / (SOC x*y), Wherein, y is the compaction density of the negative electrode material, g / cc; SOC x is the capacity of the negative electrode material at the lithium plating rate, unit is %; S is the integrated area of ​​the lithium stripping peak in the differential curve of dV / dT-t.

[0018] In one embodiment of the present invention, the method for obtaining the capacity SOC x and the integrated area S includes: The negative electrode material is subjected to a rate lithium deposition test, wherein one cycle is defined as discharge, rest, high rate charge, low rate charge, and rest; Cycle charge and discharge n times, and make n Vt curves of voltage versus time based on the change of voltage with time when the battery is at rest after discharge. Differentiate the Vt curves to get n dV / dT-t differential curves. Whether lithium deposition occurs is determined based on whether a lithium stripping peak appears in the differential curve of dV / dT-t. For the rate window where the lithium stripping peak appears, the integrated area S at that rate is calculated, and the capacity SOC x at that rate is measured. Among them, the differential curve of dV / dT-t has the unit of s on the horizontal axis and V / s on the vertical axis. The integral area S is the area when the vertical axis is magnified by 10 6 The value after.

[0019] It should be noted that in the embodiments of the present invention, the absence of a lithium stripping peak means that dV / dT decreases continuously over time starting from the time of standing still after discharge. The presence of a lithium stripping peak means that dV / dT decreases continuously over time starting from the time of standing still after discharge, and then a lithium stripping peak appears, which first increases and then decreases. The negative electrode material provided in the embodiment of the present invention adopts a detection method for lithium extraction at a rate. When subjected to a constant current charge and discharge test, a lithium stripping peak a appears in the static stage after lithium insertion at different rates. The integral area S and capacity SOC x under the corresponding lithium extraction rate window can affect its rate performance, and the compaction density y is a controllable factor, which together with the integral area S and capacity SOC x regulates the rate performance of the graphite negative electrode material. Among them, when the compaction density y is 0.88-1.85 g / cc, the higher the rate window in which the lithium stripping peak a appears, the smaller the integral area S, and the higher the capacity SOC x retention rate at each rate. The three are combined to obtain the best kinetic performance, which is represented by the kinetic index Q. The kinetic index Q satisfies the formula Q=S / (SOC x*y), that is, Q is S divided by the product of SOC x and y, where 200<Q<10000. This method can accurately judge the rate performance of the battery. The larger the Q value, the better the rate performance of the battery.

[0020] In one embodiment of the present invention, Q satisfies the following relationship: Q = S / (SOC x*y), and the kinetic index Q satisfies 200 < Q < 10000, where Q is the final value obtained. For example, Q can be 200, 400, 600, 800, 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000, 10000, etc. In the embodiment of the present invention, the larger the Q value, the better the rate performance of the battery. In one embodiment of the present invention, the compacted density y of the negative electrode material is 0.88-1.85 g / cc. For example, the compacted density y can be 0.88 g / cc, 0.90 g / cc, 1.00 g / cc, 1.10 g / cc, 1.20 g / cc, 1.30 g / cc, 1.40 g / cc, 1.50 g / cc, 1.60 g / cc, 1.70 g / cc, 1.80 g / cc, 1.85 g / cc, etc. In the embodiment of the present invention, the unit of the compacted density y is g / cc (i.e., grams per cubic centimeter, g / cm³). The compacted density y is a controllable factor and varies depending on the material. In one embodiment of the present invention, the capacity SOC x of the negative electrode material is 1-100%. For example, the capacity SOC x can be 1%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, etc. In the embodiments of the present invention, the capacity SOC x refers to the battery state of charge, specifically the capacity at 0V of the discharge cycle at various rates divided by the capacity corresponding to the final voltage at the end of the discharge cycle, typically expressed as a percentage (%).

[0021] In one embodiment of the present invention, the integral area S of the lithium stripping peak of the negative electrode material is 0-100000. For example, the integral area S can be 0, 100, 500, 1000, 3000, 5000, 8000, 10000, 25000, 50000, 75000, 100000, etc.

[0022] In one embodiment of the present invention, the negative electrode material comprises at least one of a silicon-based material, a graphite material, a hard carbon material, a soft carbon material, and a porous carbon material. In a preferred embodiment of the present invention, the negative electrode material is a graphite material. Preferably, the graphite material comprises at least one of natural graphite, artificial graphite, or mesocarbon microbeads. In one embodiment of the present invention, n is a natural number, 2≤n≤30. For example, n can be 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, etc.

[0023] On the other hand, an embodiment of the present invention further provides a rate lithium deposition test method for the above-mentioned negative electrode material, comprising the following steps: (1) Assembling the above-mentioned negative electrode materials into a battery and conducting a rate lithium deposition test, wherein the battery is charged and discharged in a cycle consisting of discharge, rest, high-rate charging, low-rate charging, and rest, and the number of cycles is n; (2) monitoring the change of voltage over time during the rest period after discharge in each cycle in step (1), making n Vt curves of voltage over time, and differentiating the Vt curves to obtain n dV / dT-t differential curves; (3) judging whether lithium deposition occurs at the discharge rate corresponding to the dV / dT-t differential curve according to whether a lithium stripping peak appears in the dV / dT-t differential curve; If lithium deposition occurs, analyze the rate window where the lithium stripping peak appears and calculate the integrated area S at this rate, where S is the vertical axis magnified by 10 in the differential curve of dV / dT-t. 6 The capacity SOC x at the rate is measured, and the kinetic index Q=S / (SOC x*y), y is the compaction density of the negative electrode material, g / cc. The larger the Q value, the better the rate performance of the battery. In one embodiment of the present invention, the compacted density y of the negative electrode material is 0.88-1.85 g / cc. For example, the compacted density y can be 0.88 g / cc, 0.90 g / cc, 1.00 g / cc, 1.10 g / cc, 1.20 g / cc, 1.30 g / cc, 1.40 g / cc, 1.50 g / cc, 1.60 g / cc, 1.70 g / cc, 1.80 g / cc, 1.85 g / cc, etc.

[0024] In one embodiment of the present invention, the capacity SOC x of the negative electrode material is 1-100%. For example, the capacity SOC x can be 1%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, etc.

[0025] In one embodiment of the present invention, the integral area S of the lithium stripping peak of the negative electrode material is 0-100000. For example, the integral area S can be 0, 100, 500, 1000, 3000, 5000, 8000, 10000, 25000, 50000, 75000, 100000, etc.

[0026] In one embodiment of the present invention, the negative electrode material includes at least one of a silicon-based material, a graphite material, a hard carbon material, a soft carbon material, and a porous carbon material. In a preferred embodiment of the present invention, the negative electrode material is a graphite material, and the graphite material includes one or more of natural graphite, artificial graphite, or mesophase carbon microbeads. In one embodiment of the present invention, before performing the rate lithium plating test step, the method further includes: performing a standard charge-withdrawal capacity first-efficiency test to screen batteries with a capacity and first-efficiency deviation of less than 5%. In the embodiment of the present invention, the standard charge-discharge capacity first-effect test is a key evaluation method in the research and development and production of lithium-ion battery materials, and is mainly used to measure the capacity utilization and energy conversion efficiency (first effect) of battery materials (such as the negative electrode) in the first charge and discharge cycle.

[0027] In a preferred embodiment of the present invention, the steps of the standard buckle capacity first-effect test specifically include: performing cyclic charging and discharging according to the cycle of discharge, rest, high-rate charging, low-rate charging, and rest.

[0028] Preferably, in the standard capacity first-effect test, a discharge rate of 0.1C is used for discharge.

[0029] Preferably, in the standard capacity first-efficiency test, charging is performed at charging rates of 0.1C and 0.05C respectively.

[0030] Preferably, in the standard buckle capacitance first-effect test, the voltage range is 0.005V-1.5V, for example, 0.005V, 0.01V, 0.05V, 0.1V, 0.5V, 1.0V, and 1.5V.

[0031] Preferably, in the standard charge-discharge capacity first-effect test, the number of cycle charge and discharge is 1 to 3 times, for example, 1 time, 2 times, or 3 times.

[0032] Preferably, in the standard buckle capacitance first-effect test, the standing time is 0.2-1h, for example, 0.2h, 0.3h, 0.4h, 0.5h, 0.6h, 0.7h, 0.8h, 0.9h, and 1h. More preferably, the steps of the standard buckle capacitance first-effect test are carried out in the following order: 1. Constant current discharge: 0.1C to 0.005V; 2. Let it stand for 30 minutes; 3. Constant current charging: 0.1 C to 1.5 V; 4. Constant current charging: 0.05 C to 1.5 V; 5. Let stand for 30 minutes; 6. Constant current discharge: 0.1 C to 0.005 V; 7. Let stand for 30 minutes; 8. Constant current charging: 0.1 C to 1.5 V; 9. Constant current charging: 0.05 C to 1.5 V; 10. Let stand for 30 minutes.

[0033] In step (1) of the embodiment of the present invention, a cycle of "discharge-rest-high rate charging-low rate charging-rest" is adopted, and step-by-step charging is used to ensure complete delithiation, and finally ends with the discharge-rest step, further completing the lithium deposition.

[0034] In one embodiment of the present invention, in step (1), constant current and constant voltage charging is performed at the same rate, and constant current discharging is performed at different rates.

[0035] In a preferred embodiment of the present invention, in step (1), the discharge rate is 0.1-5 C. For example, it can be 0.1C, 0.5C, 0.75C, 1C, 1.25C, 1.375C, 1.5C, 1.75C, 2C, 2.5C, 3C, 3.5C, 4C, 4.5C, or 5C. In a preferred embodiment of the present invention, in step (1), the charging rate is 0.01-0.1 C. More preferably, the charging rate can be 0.05 C, 0.1 C, etc.

[0036] In one embodiment of the present invention, in step (1), in n cycles, starting from the first cycle, the discharge rate of each cycle increases successively, and the difference in discharge rate between every two adjacent cycles is greater than or equal to 0.1C.

[0037] In one embodiment of the present invention, in step (1), the discharge time is calculated according to the discharge rate of each cycle discharge stage, and the formula is as follows: x60; where, t: discharge time, unit: min; C: rated capacity of the battery; C0: discharge rate.

[0038] In one embodiment of the present invention, n is a natural number, 2≤n≤30. For example, n can be 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, etc.

[0039] Preferably, in step (1), the process of the rate lithium deposition test can be carried out in the following steps: 1. Constant current discharge: 0.5 C 2 h; 2. Let stand for 1 hour; 3. Constant current charging: 0.1 C to 1.5 V; 4. Constant current charging: 0.05 C to 1.5 V; 5. Let stand for 30 minutes; 6. Constant current discharge: 0.75 C 1 h 20 min; 7. Let stand for 1 hour; 8. Constant current charging: 0.1 C to 1.5 V; 9. Constant current charging: 0.05 C to 1.5 V; 10. Let stand for 30 minutes; 11. Constant current discharge: 1 C 1 h; 12. Let stand for 1 hour; 13. Constant current charging: 0.1 C to 1.5 V; 14. Constant current charging: 0.05 C to 1.5 V; 15. Let stand for 30 minutes; 16. Constant current discharge: 1.25 C 48 min; 17. Let stand for 1 hour; 18. Constant current charging: 0.1 C to 1.5 V; 19. Constant current charging: 0.05 C to 1.5 V; 20. Let stand for 30 minutes; 21. Constant current discharge: 1.5 C 40 min; 22. Let stand for 1 hour; 23. Constant current charging: 0.1 C to 1.5 V; 24. Constant current charging: 0.05 C to 1.5 V; 25. Let stand for 30 minutes; 26. Constant current discharge: 3C 20 min; 27. Let stand for 1 hour.

[0040] In step (2) of the embodiment of the present invention, the change of voltage over time during the static state after discharge is monitored during each cycle, and n Vt curves of voltage change over time are produced. After differentiating the Vt curves respectively, n dV / dT-t differential curves are obtained.

[0041] In one embodiment of the present invention, in step (2), in the Vt curve showing the voltage changing with time, the unit of time is second (s) and the unit of voltage is volt (V).

[0042] In step (3) of the embodiment of the present invention, the rate window is the discharge rate range in which the lithium stripping peak appears, the integral area S is the integral area at the discharge rate corresponding to lithium deposition, and the capacity SOC x is the battery state of charge at the discharge rate corresponding to lithium deposition. Combining the three, the kinetic index Q = S / (SOC x * y) value is set.

[0043] In one embodiment of the present invention, in step (3), the larger the Q value, the better the rate performance of the battery. In a preferred embodiment of the present invention, in step (3), when 200<Q<10000, the rate performance of the battery is excellent. The present invention will be described in detail below with reference to the embodiments.

[0044] Example 1 The rate lithium deposition test method of graphite negative electrode material includes the following steps: 1) Standard button capacity first-effect test: Prepare a graphite sample with a compaction density y of 0.88 g / cc and assemble four parallel button-type half-cells (electrode diameter 14 mm) at room temperature (25°C). Activate for two weeks (voltage range: 0.005-1.5V). After activation, all lithium on the resulting graphite material is released, and the battery voltage is 1.5V. Select at least three sets of parallel samples with a capacity and first-effect deviation of less than 5% to reduce error. Otherwise, retest; The steps for the first-effect test of standard buckled capacitance are as follows: 1. Constant current discharge: 0.1 C to 0.005 V; 2. Let it stand for 30 minutes; 3. Constant current charging: 0.1 C to 1.5 V; 4. Constant current charging: 0.05 C to 1.5 V; 5. Let stand for 30 minutes; 6. Constant current discharge: 0.1 C to 0.005 V; 7. Let stand for 30 minutes; 8. Constant current charging: 0.1 C to 1.5 V; 9. Constant current charging: 0.05 C to 1.5 V; 10. Let stand for 30 minutes; 2) The sample obtained by screening in step 1) is subjected to a rate lithium deposition test, wherein discharge, rest, high rate charge, low rate charge, and rest are regarded as one cycle, and cyclic charge and discharge are performed. The steps of the rate lithium deposition test are as follows: 1. Constant current discharge: 0.5 C 2 h; 2. Let stand for 1 hour; 3. Constant current charging: 0.1 C to 1.5 V; 4. Constant current charging: 0.05 C to 1.5 V; 5. Let stand for 30 minutes; 6. Constant current discharge: 0.75 C 1 h 20 min; 7. Let stand for 1 hour; 8. Constant current charging: 0.1 C to 1.5 V; 9. Constant current charging: 0.05 C to 1.5 V; 10. Let stand for 30 minutes; 11. Constant current discharge: 1 C 1 h; 12. Let stand for 1 hour; 13. Constant current charging: 0.1 C to 1.5 V; 14. Constant current charging: 0.05 C to 1.5 V; 15. Let stand for 30 minutes; 16. Constant current discharge: 1.25 C 48 min; 17. Let stand for 1 hour; 18. Constant current charging: 0.1 C to 1.5 V; 19. Constant current charging: 0.05 C to 1.5 V; 20. Let stand for 30 minutes; 21. Constant current discharge: 1.5 C 40 min; 22. Let stand for 1 hour; 23. Constant current charging: 0.1 C to 1.5 V; 24. Constant current charging: 0.05 C to 1.5 V; 25. Let stand for 30 minutes; 26. Constant current discharge: 3C 20 min; 27. Let stand for 1 hour; 3) In the rate lithium deposition test results obtained in step 2), the rest time (in seconds) and voltage data after the 0.5 C-3 C rate discharge are used to make n Vt curves of voltage versus time. After differentiating the Vt curves, n dV / dT-t differential curves are obtained (see Figure 1 ); 4) judging whether lithium stripping peaks appear in the above-mentioned dV / dT-t differential curve, whether lithium deposition occurs at the discharge rate corresponding to the dV / dT-t differential curve; Figure 1 It can be seen that the lithium deposition window is the <0.5 C range, and the integral area S under this rate is calculated (see Figure 2 ) and measured the capacity SOC x at this rate (see Figure 11 ), calculate the Q value according to the formula Q=S / (SOC x*y), see Table 1 for details. Figure 1 The dv / dt differential curve was obtained for the rate lithium deposition test, and the lithium deposition window was found to be <0.5 C range.

[0045] Figure 2 The integral area S corresponding to the lithium stripping peak in the lithium deposition window in the dv / dt differential curve is 5791, where S is the voltage magnification 10 6 The value after.

[0046] Example 2 The rate lithium deposition test method of graphite negative electrode material includes the following steps: Same as Example 1, except that the compaction of the graphite sample was 1.1 g / cc.

[0047] The differential curve of dV / dT-t is shown in Figure 3 ,Depend on Figure 3 It can be seen that the lithium deposition window is in the 1.5 C range, and the integral area S under this rate is calculated. Figure 4 , and measure the capacity SOC x at this rate. According to the formula Q=S / (SOC x*y), calculate the Q value, as shown in Table 1.

[0048] Figure 3 The dv / dt differential curve was obtained for the rate lithium deposition test, and the lithium deposition window was found to be <1.5 C.

[0049] Figure 4 The integrated area S corresponding to the lithium stripping peak where the lithium deposition window in the dv / dt differential curve is located is 11200 for the rate lithium deposition test.

[0050] Example 3 The rate lithium deposition test method of graphite negative electrode material includes the following steps: Same as Example 1, except that the compaction of the graphite sample was 1.3 g / cc.

[0051] The differential curve of dV / dT-t is shown in Figure 5 ,Depend on Figure 5 It can be seen that the lithium deposition window is in the 1.25 C range, and the integral area S under this rate is calculated. Figure 6 , and measure the capacity SOC x at this rate. According to the formula Q=S / (SOC x*y), calculate the Q value, as shown in Table 1.

[0052] Figure 5 The dv / dt differential curve was obtained for the rate lithium deposition test, and the lithium deposition window was found to be <1.25 C.

[0053] Figure 6 The integrated area S corresponding to the lithium stripping peak where the lithium deposition window in the dv / dt differential curve is located is 10846 for the rate lithium deposition test.

[0054] Example 4 The rate lithium deposition test method of graphite negative electrode material includes the following steps: The rate lithium deposition test method of graphite negative electrode material includes the following steps: Same as Example 1, except that the compaction of the graphite sample was 1.6 g / cc.

[0055] The differential curve of dV / dT-t is shown in Figure 7 ,Depend on Figure 7 It can be seen that the lithium deposition window is the <0.5 C range, and the integral area S under this rate is calculated. Figure 8 , and measure the capacity SOC x at this rate. According to the formula Q=S / (SOC x*y), calculate the Q value, as shown in Table 1. Figure 7 The dv / dt differential curve was obtained for the rate lithium deposition test, and the lithium deposition window was found to be <0.5 C range.

[0056] Figure 8 The integrated area S corresponding to the lithium stripping peak where the lithium deposition window in the dv / dt differential curve is located is 8301 for the rate lithium deposition test.

[0057] Example 5 The rate lithium deposition test method of graphite negative electrode material includes the following steps: Same as Example 1, except that the compaction of the graphite sample was 1.85 g / cc.

[0058] The differential curve of dV / dT-t is shown in Figure 9 ,Depend on Figure 9 It can be seen that the lithium deposition window is the <0.5 C range, and the integral area S under this rate is calculated. Figure 10 , and measure the capacity SOC x at this rate. According to the formula Q=S / (SOC x*y), calculate the Q value, as shown in Table 1.

[0059] Figure 9 The dv / dt differential curve was obtained for the rate lithium deposition test, and the lithium deposition window was found to be <0.5 C range.

[0060] Figure 10 The integrated area S corresponding to the lithium stripping peak where the lithium deposition window in the dv / dt differential curve is located is 4406 for the rate lithium deposition test.

[0061] The analysis is as follows in conjunction with Examples 1-5: observe Figure 1 、 3 , 5, 7, and 9 found that among Examples 1-5, Example 2 had the highest lithium precipitation rate window and the best rate performance when the compaction ratio was 1.1.

[0062] observe Figure 2 、 4 , 6, 8, and 10 found that the integrated area of ​​the lithium stripping peak where the lithium precipitation window is located in Examples 1-5 is similar in Examples 2 and 3, and similar in Examples 4 and 5, and generally shows a trend of gradually increasing with increasing compaction.

[0063] The capacity SOC x of the lithium-ion batteries assembled with the materials obtained in Examples 1-5 with different compactions was tested, where SOC x = the capacity at 0V in the discharge cycle at each rate / the capacity corresponding to the final voltage at the end of the discharge cycle. Figure 11 This is a comparison chart of the capacity SOC x retention rate of lithium-ion batteries assembled with materials with different compactions obtained in Examples 1-5.

[0064] Depend on Figure 11 It can be seen that among Examples 1-5, Examples 2 and 3 have the highest SOC x retention rate at each rate, and their rate performance is better than that of Examples 1, 4, and 5.

[0065] Further comparison reveals that while the lithium deposition windows for Examples 1, 4, and 5, y, are all <0.5°C (with a compacted density y of 0.88 g / cc, 1.6 g / cc, and 1.85 g / cc), the SOC x of 0.88 g / cc is approximately 20% higher than that of 1.6 g / cc and 1.85 g / cc. The resulting Q values ​​further differentiate these rate capabilities.

[0066] In Examples 2-3, although the SOC x retention curves for the 1.1 g / cc and 1.3 g / cc capacities are similar, the 1.1 g / cc lithium deposition window is at 1.5°C, corresponding to an SOC x of 23.03%, while the 1.3 g / cc lithium deposition window is at 1.25°C, corresponding to an SOC x of 43.24%. The difference in Q values ​​can further distinguish the rate performance of these cells.

[0067] For the values ​​obtained by Q = S / (SOC x * y), the Q values ​​of Examples 2 and 3, which have high rate performance, are higher than those of Examples 1, 4, and 5, which have low rate performance, as shown in Table 1. This can specifically distinguish the performance of Examples 1-5. Furthermore, preferably, when Q = S / (SOC x * y) > 200, the resulting negative electrode material has better rate performance. Table 1

[0068] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A negative electrode material, characterized in that The kinetic index Q of the negative electrode material satisfies 200<Q<10000, and Q satisfies the following relationship: Q= S / (SOC x*y), Wherein, y is the compaction density of the negative electrode material, g / cc; SOC x is the capacity of the negative electrode material at the lithium plating rate, unit is %; S is the integrated area of ​​the lithium stripping peak in the differential curve of dV / dT-t.

2. The negative electrode material according to claim 1, characterized in that The method for obtaining the capacity SOC x and the integrated area S includes: The negative electrode material is subjected to a rate lithium deposition test, wherein one cycle is defined as discharge, rest, high rate charge, low rate charge, and rest; Cycle charge and discharge n times, and make n Vt curves of voltage versus time based on the change of voltage with time when the battery is at rest after discharge. Differentiate the Vt curves to get n dV / dT-t differential curves. Whether lithium deposition occurs is determined based on whether a lithium stripping peak appears in the differential curve of dV / dT-t. For the rate window where the lithium stripping peak appears, the integrated area S at that rate is calculated, and the capacity SOC x at that rate is measured. Among them, the differential curve of dV / dT-t has the unit of s on the horizontal axis and V / s on the vertical axis. The integral area S is the area when the vertical axis is magnified by 10 6 The value after.

3. The negative electrode material according to claim 1, characterized in that The negative electrode material satisfies at least one of the following: 1) The compacted density y of the negative electrode material is 0.88-1.85 g / cc; 2) The capacity SOC x of the negative electrode material is 1-100%; 3) the integrated area S of the lithium stripping peak of the negative electrode material is 0-100000; 4) The negative electrode material includes at least one of silicon-based materials, graphite materials, hard carbon materials, soft carbon materials, and porous carbon materials.

4. A method for testing the rate of lithium deposition of a negative electrode material according to any one of claims 1 to 3, characterized in that: The steps include: (1) Assembling the above-mentioned negative electrode materials into a battery and conducting a rate lithium deposition test, wherein the battery is charged and discharged in a cycle consisting of discharge, rest, high-rate charging, low-rate charging, and rest, and the number of cycles is n; (2) monitoring the change of voltage over time during the rest period after discharge in each cycle in step (1), making n Vt curves of voltage over time, and differentiating the Vt curves to obtain n dV / dT-t differential curves; (3) judging whether lithium deposition occurs at the discharge rate corresponding to the dV / dT-t differential curve according to whether a lithium stripping peak appears in the dV / dT-t differential curve; If lithium deposition occurs, analyze the rate window where the lithium stripping peak appears and calculate the integrated area S at this rate, where S is the vertical axis magnified by 10 in the differential curve of dV / dT-t. 6 The capacity SOC x at the rate is measured, and the kinetic index Q=S / (SOC x*y), y is the compaction density of the negative electrode material, g / cc. The larger the Q value, the better the rate performance of the battery.

5. The rate lithium deposition test method according to claim 4, wherein: The rate lithium deposition test method satisfies at least one of the following: 1) The compacted density y of the negative electrode material is 0.88-1.85 g / cc; 2) The capacity SOC x of the negative electrode material is 1-100%; 3) the integrated area S of the lithium stripping peak of the negative electrode material is 0-100000; 4) The negative electrode material includes at least one of silicon-based materials, graphite materials, hard carbon materials, soft carbon materials, and porous carbon materials.

6. The rate lithium deposition test method according to claim 4, wherein: Before the rate lithium plating test step, the battery is also subjected to a standard charge-discharge capacity first-efficiency test to screen batteries with a capacity and first-efficiency deviation of less than 5%. The steps of the standard buckle capacity first-effect test specifically include: cyclic charge and discharge according to the cycle of discharge, rest, high-rate charge, low-rate charge, and rest; Preferably, the discharge is performed at a discharge rate of 0.1C; Preferably, charging is performed at a charging rate of 0.1C and 0.05C respectively; Preferably, the voltage range is 0.005V-1.5V; Preferably, the number of charge and discharge cycles is 1 to 3 times; Preferably, the standing time is 0.2-1 h.

7. The rate lithium deposition test method according to claim 4, wherein: In step (1), constant current and constant voltage charging is performed at the same rate, and stepwise constant current discharge is performed at different rates; Preferably, in step (1), the discharge rate is 0.1-5 C; Preferably, in step (1), the charging rate is 0.01-0.1C, more preferably, the charging rate is 0.05C or 0.1C; More preferably, in step (1), the discharge time is calculated according to the discharge rate of each discharge cycle, and the formula is as follows: x60； Where t is the discharge time, in minutes; C is the rated capacity of the battery; and C0 is the discharge rate.

8. The rate lithium deposition test method according to claim 4, wherein: In step (1), in n cycles, starting from the first cycle, the discharge rate of each cycle increases successively, and the difference in discharge rate between every two adjacent cycles is greater than or equal to 0.1C; Preferably, in step (1), n ​​is a natural number, 2≤n≤30.

9. The rate lithium deposition test method according to claim 4, wherein: In step (2), in the Vt curve showing the voltage changing with time, the unit of time is s and the unit of voltage is V.

10. The rate lithium deposition test method according to claim 4, wherein: In step (3), the larger the Q value, the better the rate performance of the battery; Preferably, in step (3), when 200<Q<10000, the rate performance of the battery is excellent.

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