Lithium-ion battery recovery processing method, charging / discharging device, and program

Abstract

To provide a recovery processing method for a lithium-ion battery, a charge and discharge device, and a program that give an excellent effect for recovering performance of a lithium-ion battery.SOLUTION: In a recovery processing method for a lithium-ion battery, a cycle including a first step and a second step is repeated a plurality of times. In the first step, the SOC of a lithium-ion battery is set to a first value S1 that is equal to or less than such a SOC value that the inclination of a SOC-voltage curve becomes the minimum value, by charging. In the second step, the SOC of the lithium-ion battery is set to a second value S2 smaller than the first value S1, by discharging.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates to a recovery processing method, a charge / discharge device, and a program for a lithium-ion battery. [Background technology]

[0002] In recent years, the CO 2 In order to reduce CO2 emissions, there is growing interest in electric vehicles, and the use of lithium-ion batteries for in-vehicle use is also being considered. The performance of lithium-ion batteries can deteriorate due to repeated charging and discharging. As a method for recovering the performance of lithium-ion batteries, a method of placing the lithium-ion battery under certain conditions has been proposed (see, for example, Patent Document 1 and Patent Document 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2000-277164 A [Patent Document 2] JP 2021-103646 A Summary of the Invention [Problem to be solved by the invention]

[0004] The above-mentioned techniques have not been found to be sufficiently effective in restoring the performance of lithium-ion batteries.

[0005] An object of the present invention is to provide a recovery processing method, a charge / discharge device, and a program for a lithium ion battery that are highly effective in recovering the performance of the lithium ion battery. [Means for solving the problem]

[0006] The recovery processing method, charge / discharge device, and program of the present invention are configured as follows. (1): A recovery processing method for a lithium ion battery according to one embodiment of the present invention is a recovery processing method for a lithium ion battery, which repeats a cycle including a first step of charging the lithium ion battery to a first value that is equal to or less than the SOC value at which the slope of an SOC-voltage curve has a minimum value, and a second step of discharging the lithium ion battery to a second value that is smaller than the first value, multiple times.

[0007] (2): In the above aspect (1), the first value is greater than 0% and is equal to or less than the SOC value at which the slope of the SOC-voltage curve is twice the minimum value.

[0008] (3): In the above embodiment (1) or (2), the first value is 15% or less.

[0009] (4): In any one of the embodiments (1) to (3), prior to the first step and the second step, it is determined whether or not there is a performance degradation of the lithium ion battery, and only if the performance degradation is confirmed, the first step and the second step are carried out.

[0010] (5): A charging / discharging device according to one embodiment of the present invention is electrically connected to a lithium-ion battery and includes a control unit for charging and discharging the lithium-ion battery, wherein the control unit repeats a cycle including a first step of charging the lithium-ion battery to a first value that is equal to or less than the SOC value at which the slope of an SOC-voltage curve has a minimum value, and a second step of discharging the lithium-ion battery to a second value that is smaller than the first value.

[0011] (6): A program according to one embodiment of the present invention causes a charging / discharging device electrically connected to a lithium ion battery to repeat a cycle including a first step of charging the lithium ion battery to a first value that is equal to or less than the SOC value at which the slope of an SOC-voltage curve is at a minimum, and a second step of discharging the lithium ion battery to a second value that is smaller than the first value. Effect of the Invention

[0012] According to the aspects (1) to (6), there are provided a recovery processing method, a charge / discharge device, and a program for a lithium ion battery, which are highly effective in recovering the performance of the lithium ion battery. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a perspective view of an example of a lithium ion battery. [Diagram 2] FIG. 2 is a configuration diagram of a charge / discharge device. [Diagram 3] 1 is a graph showing an example of a change in discharge capacity in a charge / discharge test. [Figure 4] 1 is a scanning electron microscope (SEM) image of the surface of a negative electrode of a lithium-ion battery at the start of a charge-discharge test. [Diagram 5] 1 is an SEM image of the surface of the negative electrode of a lithium-ion battery at the end of a charge-discharge test. [Figure 6] FIG. 2 is an explanatory diagram showing an example of a recovery processing method for a lithium ion battery according to an embodiment; [Figure 7] 1 is an example of an SOC-voltage curve. [Figure 8] This is an SEM image of the surface of the negative electrode of a lithium-ion battery after recovery treatment. [Figure 9] 1 is a graph showing test results. [Figure 10] 1 is a graph showing test results. [Figure 11] 1 is a graph showing test results. [Figure 12] 1 is a graph showing test results. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of a recovery processing method for a lithium ion battery, a charge / discharge device, and a program according to the present invention will be described with reference to the drawings.

[0015] [Lithium-ion battery] FIG. 1 is a perspective view of an example of a lithium ion battery. As shown in Fig. 1, the lithium ion battery 1 includes a laminate 2 including electrodes, an exterior body 4 that houses the laminate 2, and a lid body 5 that seals the exterior body 4. The exterior body 4 is, for example, a metal housing. The exterior body 4 or the lid body 5 is provided with a positive electrode terminal 6 and a negative electrode terminal 7 (see Fig. 2).

[0016] The laminate 2 includes a positive electrode 21, a negative electrode 22, and a separator 23. The separator 23 is interposed between the positive electrode 21 and the negative electrode 22. The positive electrode 21, the negative electrode 22, and the separator 23 are impregnated with an electrolyte.

[0017] The positive electrode 21 has a positive electrode current collector and a positive electrode active material layer. The positive electrode active material is, for example, a lithium composite oxide containing nickel, cobalt, etc. The lithium composite oxide is, for example, a lithium nickel composite oxide, a lithium cobalt composite oxide, a lithium manganese composite oxide, a lithium nickel cobalt composite oxide, a lithium nickel manganese composite oxide, a lithium nickel cobalt manganese composite oxide, etc.

[0018] The negative electrode 22 includes a negative electrode current collector and a negative electrode active material layer. The negative electrode active material is, for example, a carbon material such as graphite. The separator 23 is formed of a resin such as polyethylene (PE) or polypropylene (PP).

[0019] The electrolytic solution contains, for example, a non-aqueous solvent and a lithium salt (electrolyte). Examples of the non-aqueous solvent include ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC). Examples of the electrolyte include lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4 ) etc.

[0020] The lithium ion battery 1 is mounted on, for example, a vehicle.

[0021] [Charge / discharge device] FIG. 2 is a configuration diagram of the charge / discharge device 10 according to the embodiment. As shown in FIG. 2, the charging / discharging device 10 is electrically connected to a positive terminal 6 and a negative terminal 7 of the lithium ion battery 1. The charging / discharging device 10 includes a control unit 11. The control unit 11 can charge and discharge the lithium ion battery 1 according to a recovery processing method described later. The charging / discharging device 10 may include a power source for charging the lithium ion battery 1. If the charging / discharging device 10 does not include a power source, an external power source is used. The charging / discharging device 10 is mounted on, for example, a vehicle. The charging / discharging device 10 may also be mounted on a battery exchange device.

[0022] The control unit 11 is realized by, for example, a hardware processor such as a CPU (Central Processing Unit) executing a program (software). Some or all of these components may be realized by hardware (including circuitry) such as an LSI (Large Scale Integration), an ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or a GPU (Graphics Processing Unit), or may be realized by a combination of software and hardware. The program may be stored in advance in a storage device (storage device having a non-transient storage medium) such as an HDD or flash memory of the control unit 11, or may be stored in a removable storage medium such as a DVD or CD-ROM, and installed in the HDD or flash memory of the control unit 11 by mounting the storage medium (non-transient storage medium) in a drive device.

[0023] [Lithium-ion battery performance degradation due to repeated charging and discharging] Lithium-ion batteries can experience a decline in performance, such as discharge capacity, due to repeated charging and discharging. The cause of this decline in performance is, for example, lithium deposition (e.g., dendrites) on the surface of the negative electrode. This decline in performance is likely to occur in low-temperature environments (e.g., below 0°C).

[0024] FIG. 3 is a graph showing an example of the change in discharge capacity in a charge / discharge test. As shown in FIG. 3, the discharge capacity of the lithium-ion battery in this example gradually decreases as the number of charge / discharge cycles (repetitions) increases.

[0025] Figure 4 is a scanning electron microscope (SEM) image of the surface of the negative electrode of a lithium-ion battery at the start of a charge-discharge test, and Figure 5 is a SEM image of the surface of the negative electrode of a lithium-ion battery at the end of a charge-discharge test. As shown in Figures 4 and 5, it can be seen that needle-shaped crystals (dendrites) of lithium were formed on the surface of the negative electrode by repeated charging and discharging.

[0026] [How to recover a lithium-ion battery] A lithium ion battery whose performance has deteriorated can have its performance restored by the following method. The recovery treatment method described below can be performed by a charge / discharge device 10 (see FIG. 2).

[0027] 6 is an explanatory diagram showing an example of a recovery processing method according to an embodiment. The recovery processing method for a lithium ion battery is also simply called a "recovery processing method."

[0028] As shown in FIG. 6, the recovery method of this embodiment repeats a cycle including the following two steps multiple times. First step: The SOC of the lithium ion battery is set to an upper limit value S1 through charging. The upper limit value S1 is an example of a "first value." Second step: The SOC of the lithium ion battery is set to a lower limit value S2 by discharging. The lower limit value S2 is an example of a "second value". Prior to the first step, a preparation step may be performed in which the lithium ion battery is discharged to an end-of-discharge voltage.

[0029] SOC (State Of Charge) is the charging rate (%) of a lithium ion battery. In the recovery processing method of this embodiment, a first step in which the SOC is set to an upper limit value S1 and a second step in which the SOC is set to a lower limit value S2 are repeated, so that the SOC repeatedly increases and decreases. In the recovery processing method of this embodiment, the SOC increases linearly in the first step. In the second step, the SOC decreases linearly. In the example shown in FIG. 6, the first step is the first step, but either the first step or the second step may be performed first.

[0030] FIG. 7 is an example of an "SOC-voltage curve" showing the relationship between SOC and voltage. The horizontal axis of FIG. 7 indicates SOC (%). The vertical axis of FIG. 7 indicates voltage (V). The SOC-voltage curve can be obtained as follows.

[0031] The capacity of a lithium ion battery is calculated, for example, as follows. Take as an example a lithium-ion battery that uses a ternary lithium composite oxide containing cobalt, nickel, and manganese as the positive electrode active material. The rated voltage is 3.6V. The capacity is 3Ah. The upper limit voltage is 4.2V. The lower limit voltage is 2.5V.

[0032] The lithium ion battery is placed in a thermostatic chamber at 25°C and left there for 4 hours, after which the following operations are carried out in the thermostatic chamber at a temperature of 25°C. (1) Discharge the lithium-ion battery to 2.5V at a current of 3A (equivalent to 1C at rated capacity) and leave it for 10 seconds. (2) Charge the lithium-ion battery at a constant current of 3A up to 4.2V. (3) Charge the lithium-ion battery at a constant voltage of 4.2 V and a current of 0.6 A (equivalent to 0.2 C at rated capacity). (4) Discharge the lithium-ion battery at a constant current of 3 A down to 2.5 V. Measure the capacity during this discharge.

[0033] During discharge (operation (4)), the voltage is measured every second. The SOC is calculated by (capacity - current - time) / capacity x 100(%). The obtained SOC and voltage can be used to generate the SOC-voltage curve shown in FIG.

[0034] The slope of the SOC-voltage curve is the slope of a straight line obtained by linearly approximating the range of SOC corresponding to 30 seconds before to 30 seconds after the point in time using the least squares method for SOC and voltage. The slope of the SOC-voltage curve is the ratio (V / %) of the change in voltage (V) to the change in SOC (%).

[0035] The upper limit S1 of the SOC (see FIG. 6) is set to be equal to or less than the SOC value at point M1 in FIG. 7, where the slope of the SOC-voltage curve is at a minimum. In the example shown in FIG. 7, the slope of the SOC-voltage curve is at a minimum value of 0.0047 at point M1. The SOC at this time is 36%. Therefore, the upper limit S1 of the SOC is set to be equal to or less than 36%. The upper limit S1 is preferably equal to or less than 15%, and more preferably equal to or less than 12%. By setting the upper limit S1 of the SOC to be equal to or less than the value of the SOC at point M1, the amount of lithium ions absorbed in the negative electrode can be reduced, making it easier to decompose lithium (eg, dendrite) in the negative electrode.

[0036] The upper limit S1 of the SOC (see FIG. 6) is preferably equal to or less than the SOC value at point M2 in FIG. 7, where the slope of the SOC-voltage curve is twice the minimum value. The SOC at point M2 is smaller than the SOC at point M1. In the example shown in FIG. 7, the slope of the SOC-voltage curve is twice the minimum value 0.0047 (0.0095) at point M2. The SOC at this time is 12.9%. Therefore, the upper limit S1 of the SOC is preferably equal to or less than 12.9%. By setting the upper limit S1 of the SOC to be equal to or less than the value of the SOC at point M2, it is possible to facilitate decomposition of lithium (eg, dendrite) in the negative electrode.

[0037] The upper limit S1 of the SOC (see FIG. 6) is greater than 0%. The upper limit S1 may be 0.2% or more (for example, 1.1% or more).

[0038] The lower limit value S2 of the SOC (see FIG. 6) is equal to or greater than 0% and is smaller than the upper limit value S1. The lower limit value S2 may be 0%. The lower limit value S2 is, for example, 0% to 7%.

[0039] FIG. 8 is an SEM image of the surface of the negative electrode of a lithium-ion battery after the recovery treatment. As shown in FIG. 8, the amount of lithium (eg, dendrites) on the surface of the negative electrode is reduced compared to before the recovery treatment (see FIG. 5).

[0040] In the recovery processing method of this embodiment, prior to the recovery processing including the above-mentioned first and second steps, it may be determined whether or not the performance of the lithium ion battery has deteriorated, and the recovery processing may be executed only if the deterioration in performance is confirmed. The deterioration in performance may be determined, for example, based on the capacity recovery rate. By determining the deterioration in performance, the non-operational period of the lithium ion battery may be shortened.

[0041] [Effects of the recovery processing method according to the embodiment] According to the recovery method of this embodiment, a cycle including a first step of setting the SOC to an upper limit value S1 by charging and a second step of setting the SOC to a lower limit value S2 by discharging is repeated multiple times. The upper limit value S1 is set to be equal to or lower than the SOC value at point M1 where the slope of the SOC-voltage curve is at a minimum value. This reduces the amount of lithium ions absorbed in the negative electrode, making it easier to decompose lithium (e.g., dendrites) in the negative electrode. This allows the performance of the lithium ion battery to be restored.

[0042] By restoring the performance of lithium-ion batteries, it is possible to extend the lifespan of the lithium-ion batteries and thereby improve their energy efficiency.

[0043] The above describes the form for carrying out the present invention using an embodiment, but the present invention is not limited to such an embodiment, and various modifications and substitutions can be made within the scope that does not deviate from the gist of the present invention.

[0044] The present invention will be described in detail below based on specific examples, but the present invention is not limited to the following examples.

[0045] <Example 1> A lithium-ion battery was prepared using a ternary lithium composite oxide containing cobalt, nickel, and manganese as the positive electrode active material. The rated voltage is 3.6V. The capacity is 3Ah. The upper limit voltage is 4.2V. The lower limit voltage is 2.5V.

[0046] (Preparation of samples with reduced performance) The lithium ion battery was subjected to the following charge / discharge test. The lithium ion battery was placed in a thermostatic chamber at −10° C. and left there for 4 hours, after which the following operations (A) and (B) were repeated 300 cycles in the thermostatic chamber at a temperature of −10° C. (A) Discharge the lithium-ion battery to 2.5V at a current of 9A and leave it for 10 seconds. (B) Charge the lithium-ion battery to 4.2 V at a current of 9 A and leave it for 10 seconds. This resulted in a sample with reduced (degraded) performance.

[0047] (Recovery process) As shown in Fig. 6, a cycle including the following two steps was repeated several times for the above-mentioned sample using a charge / discharge device 10 (see Fig. 2). The treatment time was 72 hours. The SOC-voltage curve is shown in Fig. 7. First step: The SOC of the lithium-ion battery is set to the upper limit value S1 by charging. The charging time is t. The charging current is 9A. Second step: The SOC of the lithium ion battery is set to the lower limit S2 by discharging. The discharge time is t. The current during discharge is 9A.

[0048] Using the above-mentioned capacity measurement method, the initial capacity of the lithium ion battery, the capacity after performance degradation (after deterioration), and the capacity after recovery were measured.

[0049] The recovery rate was calculated by the following formula: The results are shown in Table 1 and Figure 9. Recovery rate = (Capacity after recovery - Capacity after degradation) / (Initial capacity - Capacity after degradation)

[0050] In Examples 1-1 to 1-3, t was set to 10 seconds, 100 seconds, and 300 seconds, respectively. In Comparative Example 1-2, t was set to 730 seconds. For comparison, Table 1 and FIG. 9 also show Comparative Example 1-1 in which no recovery process was performed.

[0051] [Table 1]

[0052] As shown in Table 1 and FIG. 9, in Examples 1-1 to 1-3 in which the recovery treatment was performed, a high recovery rate was obtained. In contrast, the recovery rate was low in Comparative Example 1-1 where no recovery treatment was performed, and the recovery rate was also low in Comparative Example 1-2 where the upper limit of SOC was set to 75.1%.

[0053] <Example 2> A recovery treatment test was carried out in the same manner as in Example 1 except for the discharge voltage. The results are shown in Table 2 and Figure 10. "Discharge voltage" is the voltage at the end of discharge in the second step. Table 2 and FIG. 10 also show the results of Example 1-1 for comparison.

[0054] [Table 2]

[0055] As shown in Table 2 and FIG. 10, the recovery rate was higher in the Examples having a low SOC than in the Comparative Examples having a high SOC.

[0056] <Example 3> A recovery treatment test was carried out in the same manner as in Example 1, except for the current during charging and discharging. The results are shown in Table 3 and FIG. For comparison, Table 3 and FIG. 11 also show the results of Example 1-1 and Comparative Example 1-1.

[0057] [Table 3]

[0058] As shown in Table 3 and FIG. 11, in the examples, a high recovery rate was obtained even when the current during charging and discharging were different.

[0059] <Example 4> Except for the treatment time, the recovery treatment test was carried out in the same manner as in Example 1. The results are shown in Table 3 and FIG. For comparison, Table 4 and FIG. 12 also show the results of Example 1-1 and Comparative Example 1-1.

[0060] [Table 4]

[0061] As shown in Table 4 and FIG. 12, a higher recovery rate was obtained with a longer treatment time. [Explanation of symbols]

[0062] 1 Lithium-ion battery S1 Upper limit (first value) S2 Lower limit (second value)

Claims

1. A method for recovering the capacity of a lithium ion battery, comprising: a first step of charging the lithium ion battery to a first value that is equal to or lower than an SOC value at which a slope of an SOC-voltage curve becomes a minimum; a second step of discharging the lithium ion battery to a second value that is smaller than the first value; A recovery treatment method for a lithium-ion battery, comprising repeating a cycle including the steps of:

2. The first value is greater than 0% and is equal to or less than an SOC value at which the slope of the SOC-voltage curve is twice the minimum value. The recovery method for a lithium ion battery according to claim 1.

3. The first value is less than or equal to 15%. The recovery treatment method for a lithium ion battery according to claim 1 or 2.

4. Prior to the first step and the second step, the presence or absence of a decrease in capacity of the lithium ion battery is determined, and the first step and the second step are carried out only when the decrease in capacity is confirmed. The method for recovering a lithium ion battery according to any one of claims 1 to 3.

5. A charging / discharging device electrically connected to a lithium ion battery to restore capacity of the lithium ion battery, A control unit that charges and discharges the lithium ion battery, The control unit is a first step of charging the lithium ion battery to a first value that is equal to or lower than an SOC value at which a slope of an SOC-voltage curve becomes a minimum; a second step of discharging the lithium ion battery to a second value that is smaller than the first value; A charging and discharging device that repeats a cycle including the above multiple times.

6. A program for recovering the capacity of a lithium-ion battery, comprising: A charging / discharging device electrically connected to the lithium ion battery, a first step of charging the lithium ion battery to a first value that is equal to or lower than an SOC value at which a slope of an SOC-voltage curve becomes a minimum; a second step of discharging the lithium ion battery to a second value that is smaller than the first value; A program that repeats a cycle including multiple times.

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