Method for testing internal resistance characteristics of lithium iron manganese phosphate positive electrode material

Abstract

This invention proposes a method for testing the internal resistance characteristics of lithium manganese iron phosphate cathode materials, relating to the technical field of battery testing. The method includes: activating and capacitating the battery cell; performing the first discharge cycle on the cell and collecting the discharge voltage-capacity data; determining three test points based on the discharge voltage-capacity data: a first characteristic point, a first offset point, and a second offset point; determining the voltage and depth of discharge (DOD) values ​​corresponding to the three test points; calculating the DC internal resistance (DCR) values ​​at the three test points and taking the arithmetic mean as the DCR value for the first-cycle transition zone; after cyclic aging and discharging the cell for n cycles, discharging the cell to the voltages corresponding to the three test points, and calculating the DCR value for the transition zone after n aging cycles; this invention employs a three-point constant voltage test in the transition zone, effectively eliminating the abnormal increase in DC internal resistance measurement values ​​caused by voltage drift.

Description

Technical Field

[0001] This invention relates to the technical field of battery testing, and in particular to a method for testing the internal resistance characteristics of lithium manganese iron phosphate cathode material. Background Technology

[0002] At present, new energy power batteries are mainly divided into ternary lithium batteries and lithium iron phosphate batteries. In a battery, DC internal resistance is the ratio of the change in battery voltage to the corresponding change in discharge current under working conditions. It can be measured by battery testing equipment. DC internal resistance directly affects the battery's power performance and heat generation, and can be used for battery life assessment and fault diagnosis.

[0003] For example, application number CN202411239304.3, entitled "An Evaluation Method and Apparatus for Evaluating the Internal Resistance Distribution Characteristics of a Battery," can evaluate a battery through its internal resistance distribution characteristics. In the prior art, when testing the DC internal resistance of a battery, the battery is usually adjusted to a fixed state of charge (the method adjusts the sample SOC to the value specified by the manufacturer; if the manufacturer does not provide an SOC value, the sample SOC is adjusted to 50%), and then pulse discharge is performed to calculate the DCR (DC internal resistance measurement value). Due to the special characteristics of lithium iron phosphate batteries, there is a "manganese-iron transition region" in its material. The test value obtained by conventional testing methods is close to or equal to the resistance value of the manganese-iron transition region. As the transition region ages, the resistance value of this region will continue to increase. Therefore, the conventional DCR value cannot reflect the true battery resistance value. Summary of the Invention

[0004] The technical problem to be solved by this invention is that existing constant-capacity SOC testing methods cannot accurately reflect the increase of DC internal resistance measurement value in the material transition region of lithium iron phosphate batteries during cycling. This invention provides a testing method for the internal resistance characteristics of lithium manganese iron phosphate cathode materials to solve this problem.

[0005] Based on the technical problems existing in the background art, the present invention proposes a method for testing the internal resistance characteristics of lithium manganese iron phosphate cathode material, including the following steps:

[0006] S1: Activate and capacitate the battery cells;

[0007] S2: Perform the first pulse discharge on the cell and collect the discharge voltage-capacity data of the cell within the full discharge depth (DOD) range of the first cycle;

[0008] S3: Based on the discharge voltage-capacity data, determine three test points: the first characteristic point corresponding to the center of the transition zone, the first offset point where the discharge depth DOD value shifts in different directions based on the first characteristic point, and the second offset point; determine the voltage and discharge depth DOD value corresponding to the three test points.

[0009] S4: Discharge the cell to the voltage corresponding to the three test points respectively, calculate the DC internal resistance measurement value (DCR) of the three test points, and take the arithmetic average value as the DCR value of the first transition zone;

[0010] S5: After cyclic aging and discharging the cell for n cycles, discharge the cell to the voltage corresponding to the three test points in S3, calculate the DCR value of the three test points after aging, and take the arithmetic average as the DCR value of the transition zone after n aging cycles.

[0011] Compared with existing technologies, the method for testing the internal resistance characteristics of lithium manganese iron phosphate cathode material proposed in this invention, using the above-mentioned technical solution, achieves the following technical effects:

[0012] Based on the internal resistance characteristics of lithium manganese iron phosphate material, this invention effectively eliminates the abnormal increase in DCR value in the transition region caused by voltage drift by using a three-point constant voltage mode to lock the test points in the transition region, thereby improving the accuracy of DCR value testing in the transition region. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the DCR value test curve of the battery discharge according to the present invention;

[0014] Figure 2 The DOD-capacity differential curve for battery discharge at 0.05C;

[0015] Figure 3 This is a comparison of the stability of the transition region and non-transition region between Example 1 and Comparative Example 1;

[0016] Figure 4 This is a comparison of the stability of the transition region and non-transition region between Example 2 and Comparative Example 2;

[0017] Figure 5 This is the normalized curve for the battery discharge curve. Detailed Implementation

[0018] Example 1

[0019] Please refer to Figures 1-5 This invention proposes a method for testing the internal resistance characteristics of lithium manganese iron phosphate cathode materials, comprising the following steps:

[0020] S1: Activate and capacitate the battery cell; the ambient temperature for capacitating the battery cell is 25℃-30℃, and 2-3 charge-discharge cycles are performed at a rate of 0.02-0.33C and a voltage range of 2.5-4.25V; the initial rated capacity of the battery cell is determined by capacitating the battery cell.

[0021] Specifically, at an ambient temperature of 25°C, 3-5 charge-discharge cycles are performed within the voltage range of 2.5V-4.25V at any rate of 0.1C, 0.2C, 0.33C, 0.5C or 1C, and the final discharge capacity is taken as the initial rated capacity of the cell.

[0022] S2: Perform the first cycle discharge on the cell, and collect the discharge voltage-capacity data of the cell over the full depth of discharge DOD range of the first cycle.

[0023] S3: Determine three test points according to the discharge voltage-capacity data, which are respectively the first feature point corresponding to the center of the transition region, the first offset point and the second offset point whose depth of discharge DOD values are offset in different directions based on the first feature point; determine the voltage and depth of discharge DOD values corresponding to the three test points;

[0024] Specifically, differential processing is performed on the discharge voltage-capacity curve to obtain a depth of discharge DOD-capacity differential curve; the peak and valley positions in the differential curve are identified, the first feature point is determined by referring to the discharge voltage-capacity curve, the point position corresponding to each peak and valley position of the differential curve on the discharge voltage-capacity curve is found, and the change speed on the left and right of the point position in the discharge voltage-capacity curve is observed. If the curve descends in a bent manner on the left and right of the point position in the discharge voltage-capacity curve, the point position is the first feature point, and the first feature voltage of the first feature point and the corresponding depth of discharge DOD value of x% are recorded.

[0025] Further, based on the first feature point, offset k% to the left to obtain the first offset point with a depth of discharge DOD value of x%-k%, and offset y% to the right to obtain the second offset point with a depth of discharge DOD value of x%+y%, where both k and y are positive numbers, and satisfy k%+y%=10%, the value range of k is 0<k<5, and the value range of y is 5<y<10; the first offset voltage and the second offset voltage corresponding to the first offset point and the second offset point are determined respectively.

[0026] S4: Discharge the cell to the voltages corresponding to the three test points respectively, calculate the measured direct current internal resistance DCR values of the three test points, and take the arithmetic average as the DCR value of the transition region in the first cycle.

[0027] S5: After the cell is subjected to n cycles of aging discharge, discharge the cell to the voltages corresponding to the three test points in step S3, calculate the DCR values of the three test points after aging, and take the arithmetic average as the DCR value of the transition region after n times of aging;

[0028] Specifically, the battery cell is subjected to cyclic aging discharge up to the nth cycle. The aged battery cell is then discharged to the first characteristic voltage, the first offset voltage, and the second offset voltage, and pulse discharge is performed at each voltage point. The voltage response and current value during the pulse discharge process are recorded, and the DCR value corresponding to each voltage point is calculated. The specific parameters of the pulse discharge are: pulse duration is 10 seconds, and the test environment temperature is 45℃.

[0029] The DCR test for the non-transition region of the battery cell includes:

[0030] For the DCR test of the non-transition zone of the first cycle of the battery cell: the battery cell is discharged to the preset non-transition zone DOD point, pulse discharge is performed, voltage changes are collected and the DCR value of each DOD point is calculated;

[0031] For DCR testing of the non-transition zone of the battery cell after n cycles of aging: Based on the cell capacity after aging, discharge to the preset non-transition zone DOD point, perform pulse discharge, and calculate the DCR value at each point.

[0032] The non-transition zone includes discharge depth (DOD) points with discharge depths of 0%-30% and 70%-100%.

[0033] The method for obtaining DCR values ​​includes the following steps:

[0034] Sa: Discharge the cell to the target discharge depth DOD with a small current I1, and record the end voltage U0 after a resting time T1.

[0035] Sb: Discharge at a high-rate current I2 for 10 seconds and record the discharge termination voltage U. 10 ;

[0036] Sc: According to the formula DCR(10s)=(U 10 -U0) / I2 is obtained by calculation.

[0037] Among them, the high-rate current I2 > the low-rate current I1; and the value of current I2 is fixed as the current value corresponding to the initial rated capacity of the cell, and remains unchanged when the nth cell is tested in step S5.

[0038] Example 2

[0039] Lap 1 Test

[0040] Battery activation treatment: Select 5 LiMn0.6Fe0.4PO4 lithium manganese iron phosphate batteries with good consistency from the production line after electrolyte injection. Set the temperature to 25℃ and the voltage range of 2.5-4.25V. Charge to 3.8V at a rate of 0.02C; charge to 4.25V at a rate of 0.2C and maintain the voltage at 0.05C; age for 48 hours; discharge to 2.5V at 0.33C; cycle charge and discharge at 0.33C 2.5-4.25V for 2-3 times.

[0041] To determine the capacity of the battery: After activation, let the battery stand at 45°C for 3 hours; charge it at a constant current rate of 0.33C to 4.25V, charge it at a constant voltage rate to 0.05C and let it stand for 1 hour; discharge it at a constant current rate of 0.33C to 2.5V, record the capacity, and repeat this cycle three times to determine the capacity.

[0042] The battery was subjected to the first pulse discharge using the data from the last cycle of the constant capacity test. Then, the battery was charged and discharged at a low rate of 0.05C. The discharge voltage-capacity curve was obtained. The discharge voltage-capacity curve was differentiated to obtain the discharge depth DOD-capacity differential curve.

[0043] Identify the peaks and valleys in the differential curve, and determine the first characteristic point a based on the discharge voltage-capacity curve. Record the first characteristic voltage of the first characteristic point and the corresponding discharge depth DOD value x%. Determine the voltages of the corresponding first characteristic point (x%), first offset point (x%-3%), and second offset point (x%+7%).

[0044] Based on the constant volume data, the DCR values ​​of the voltage at three locations in the non-transition region and the transition region of the first cycle were tested.

[0045] 100th lap test

[0046] The test battery was left to stand at 45℃ for 3 hours and then subjected to 1C charge-discharge aging in the 2.5-4.25V range.

[0047] After cycle aging, the battery was left to stand at 45°C for 3 hours; then charged at a constant current of 0.33C to 4.25V, and charged at a constant voltage of 0.05C to end; left to stand for 1 hour, and discharged at a constant current of 0.33C to 2.5V, and the capacity was recorded; the cycle was repeated three times, and the discharge capacity of the last cycle was used as the benchmark for adjusting the SOC capacity of the aged battery.

[0048] The cycle-aged battery was left to stand at 45℃ for 3 hours; then charged at a constant current of 0.33C to 4.25V, and then charged at a constant voltage of 4.25V to 0.05C; the battery was then discharged sequentially to the following points: 80% SOC, the voltage corresponding to the first offset point (x%-3%), the voltage corresponding to the first characteristic point (x%), the voltage corresponding to the second offset point (x%+7%), and the 20% SOC point. At each point, the battery was left to stand for 1 hour, discharged at 2C for 10 seconds, and then left to stand for 1 hour before charging at 1.5C for 10 seconds. The corresponding voltage and current were recorded.

[0049] DCR value test at different lap counts:

[0050] Test on the nth lap

[0051] In the non-transition region, the battery is adjusted to different depths of discharge (DOD) based on its capacity. Pulse discharge is performed at different DODs, and the voltage and current changes during the pulse discharge process are collected. The DCR values ​​at different DODs are then calculated using the voltage and current data. For the same DOD, the average DCR values ​​of five batteries at this location are taken.

[0052] In the transition zone, based on the voltages of the first characteristic point (x%), the first offset point (x%-3%), and the second offset point (x%+7%) at the first cycle position, the cyclically aged battery is discharged to the corresponding voltage to complete the pulse discharge. The voltage changes during the battery pulse discharge process are collected, and the DCR values ​​at the three points in the transition zone are calculated using voltage and current. The average of the three points is recorded as the DCR value of the transition zone for aging n cycles. For the DCR value at the same discharge depth, the average of the DCR values ​​of 5 batteries at this point needs to be taken.

[0053] The voltage DCR values ​​at the three test locations in the transition region and in the non-transition region are shown in Table 1:

[0054] Table 1. Test values ​​of voltage DCR at three test locations in the transition region and outside the non-transition region in Example 2.

[0055]

[0056] Example 3

[0057] Set the DOD of the first feature point to x%, the DOD of the first offset point to x%-4%, and the DOD of the second offset point to x%+6%, and then obtain the DCR value again.

[0058] Table 2. Test values ​​of voltage DCR at three test locations in the transition region and outside the non-transition region in Example 3.

[0059]

[0060] Comparative Example 1

[0061] Corresponding to the 64 manganese iron material in the above embodiment 2, unlike the scheme of the present invention, the DCR value of the transition zone with different number of turns is determined by the 70% SOC constant volume mode to determine the test points.

[0062] Table 3. Test values ​​of DCR in the non-transition zone and transition zone of Comparative Example 1

[0063]

[0064] Comparative Example 2

[0065] Corresponding to the 55 manganese iron material in the above embodiment 3, unlike the scheme of the present invention, the DCR value of the transition zone with different number of turns is determined by the 60% SOC constant volume mode to determine the test points.

[0066] Table 4. Test values ​​of DCR in the non-transition and transition regions of Comparative Example 2

[0067]

[0068] contrast Figure 3 , Figure 4 As shown in Tables 1, 2, 3, and 4, the DC internal resistance (DCR) test using the conventional constant-capacity adjustable SOC is relatively accurate in the plateau region, but the error is large in the transition region. The data shows that for the transition region, using the conventional method, the measured value and growth rate of 55% manganese iron material after 400 cycles are 93.28 mΩ and 78.66%, respectively. However, using the improved three-point constant voltage mode, the measured value and growth rate are 63.88 mΩ and 22.35%, respectively. The increase in the average value of the three points in the transition region is not much different from that in the plateau region. A similar situation is observed for 64% manganese iron material.

[0069] from Figure 5 The normalized discharge curves of lithium manganese iron phosphate (LFP) show that voltage drift occurs continuously at the test points in the constant-capacity mode during the transition region. This is a characteristic of LFP material and the reason for the inaccuracy of conventional measurement methods. The comparison demonstrates that the improved method accurately reflects the growth of the LFP transition region during cycling.

[0070] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for testing internal resistance characteristics of a lithium iron manganese phosphate cathode material, characterized in that, Includes the following steps: S1: Activate and capacitate the battery cells; S2: Perform the first discharge cycle on the battery cell and collect the discharge voltage-capacity data within the full discharge depth (DOD) range of the battery cell during the first cycle; S3: Based on the discharge voltage-capacity data, determine three test points: the first characteristic point corresponding to the center of the transition zone, the first offset point where the discharge depth DOD value shifts in different directions based on the first characteristic point, and the second offset point; determine the voltage and discharge depth DOD value corresponding to the three test points. S4: Discharge the battery cell to the voltage corresponding to the three test points respectively, calculate the DC internal resistance measurement value (DCR) of the three test points, and take the arithmetic average value as the DCR value of the first transition zone. S5: After cyclic aging and discharging the cell for n cycles, discharge the cell to the voltage corresponding to the three test points in S3, calculate the DCR value of the three test points after aging, and take the arithmetic average as the DCR value of the transition zone after n aging cycles.

2. The method for testing the internal resistance characteristics of lithium manganese iron phosphate cathode material according to claim 1, characterized in that: For the DCR test of the non-transition zone of the first cycle of the battery cell: the battery cell is discharged to the preset non-transition zone DOD point, pulse discharge is performed, voltage changes are collected and the DCR value of each DOD point is calculated; For DCR testing of the non-transition zone of the battery cell after n cycles of aging: Based on the cell capacity after aging, discharge to the preset non-transition zone DOD point, perform pulse discharge, and calculate the DCR value at each point. 3.The method for testing the internal resistance characteristics of a lithium iron manganese phosphate cathode material according to claim 1, characterized in that, In step S1, at 25°C, the battery cell is charged and discharged 3-5 times within a voltage range of 2.5V-4.25V at any rate of 0.1C, 0.2C, 0.33C, 0.5C or 1C. The capacity of the last discharge cycle is taken as the initial rated capacity of the battery cell. 4.The method for testing the internal resistance characteristics of a lithium iron manganese phosphate cathode material according to claim 1, characterized in that, In step S2, the DCR value under the first discharge depth DOD of the cell is measured and recorded to obtain the discharge voltage-capacity curve. In step S3, the discharge voltage-capacity curve is differentiated to obtain the discharge depth DOD-capacity differential curve. By identifying the peak and valley positions in the differential curve, the first feature point of the discharge voltage-capacity curve is determined. The first feature voltage of the first feature point and the corresponding discharge depth DOD value x are recorded.

5. The method for testing the internal resistance characteristics of a lithium iron manganese phosphate cathode material according to claim 4, characterized in that, In step S3, based on the first feature point, shift left by k% to obtain the first offset point with a discharge depth DOD value of x%-k%, and shift right by y% to obtain the second offset point with a discharge depth DOD value of x%+y%. 6.The method for testing the internal resistance characteristics of a lithium iron manganese phosphate cathode material according to claim 1, characterized in that, The non-transition region includes discharge depth (DOD) points with discharge depths of 0%-30% and 70%-100%.

7. The method for testing the internal resistance characteristics of a lithium iron manganese phosphate cathode material according to claim 1, characterized in that, The method for obtaining DCR values ​​includes the following steps: Sa: Discharge the cell to the target discharge depth DOD with a small current I1, and record the end voltage U0 after a resting time T1. Sb: discharge at a large current I2for 10 seconds, and record the discharge termination voltage U 10 ; Sc: calculated according to the formula DCR(10s) = (U 10 -U0) / I2. 8.The method for testing the internal resistance characteristics of a lithium iron manganese phosphate cathode material according to claim 2, characterized in that, The specific parameters of the pulse discharge are: pulse duration is 10 seconds, and the test environment temperature is 45℃.

9. The method for testing the internal resistance characteristics of lithium manganese iron phosphate cathode material according to claim 7, characterized in that, The high-rate current I2 is greater than the low-rate current I1; and the value of current I2 is fixed as the current value corresponding to the initial rated capacity of the cell, and remains unchanged when the nth cell is tested in step S5.

10. The method for testing the internal resistance characteristics of lithium manganese iron phosphate cathode material according to claim 5, characterized in that, Both k and y are positive numbers, satisfying k% + y% = 10%, the value range of k is 0 < k < 5, and the value range of y is 5 < y < 10.

Images