Recovery processing method for lithium ion battery, charging and discharging device, and storage medium

Abstract

This invention provides a lithium-ion battery recovery treatment method, a charging and discharging device, and a storage medium that effectively restores the performance of lithium-ion batteries. A lithium-ion battery has a positive electrode and a negative electrode, and its performance is reduced due to residual lithium ions at the negative electrode. The lithium-ion battery recovery treatment method repeatedly performs multiple cycles including a first step and a second step. In the first step, the state of charge (SOC) of the lithium-ion battery is brought to a first value (S1) by charging. This first value (S1) is below the SOC value when the slope of the SOC-voltage curve reaches its minimum, and above the SOC value when the slope of the SOC-voltage curve reaches twice the minimum. In the second step, the SOC of the lithium-ion battery is brought to a second value (S2) that is smaller than the first value by discharging.

Description

[0001] This application claims priority based on Japanese Patent Application No. 2021-214861, filed on December 28, 2021, the contents of which are incorporated herein by reference. Technical Field

[0002] This invention relates to a method for recovering lithium-ion batteries, a charging and discharging device, and a storage medium. Background Technology

[0003] In recent years, from the perspective of climate-related disasters and CO2 reduction, there has been a surge of interest in electric vehicles, and the use of lithium-ion batteries for vehicle applications is being studied.

[0004] Lithium-ion batteries can experience performance degradation due to repeated charge-discharge cycles. As a method to restore the performance of lithium-ion batteries, a method of placing the lithium-ion battery under specified conditions has been proposed (for example, see Japanese Patent Application Publication No. 2000-277164 and Japanese Patent Application Publication No. 2014-127283). Summary of the Invention

[0005] The aforementioned technologies have not been able to fully restore the performance of lithium-ion batteries.

[0006] One of the objectives of this invention is to provide a lithium-ion battery recovery method, charging and discharging device, and storage medium that effectively restores the performance of lithium-ion batteries.

[0007] The lithium-ion battery recovery processing method, charging and discharging device, and storage medium of the present invention adopt the following structure.

[0008] (1): One aspect of the present invention relates to a method for recovering a lithium-ion battery, the lithium-ion battery having a positive electrode and a negative electrode, and the performance of the lithium-ion battery being reduced due to lithium ion residue at the negative electrode, wherein the method for recovering the lithium-ion battery repeatedly performs multiple cycles including a first step and a second step, in the first step, the SOC of the lithium-ion battery is made to a first value by charging, the first value being below the SOC value when the slope of the SOC-voltage curve becomes the minimum value, and above the SOC value when the slope of the SOC-voltage curve becomes twice the minimum value, in the second step, the SOC of the lithium-ion battery is made to a second value smaller than the first value by discharging.

[0009] (2): Based on the above (1) scheme, the first value is less than 30%.

[0010] (3): Based on the above scheme (1) or (2), the second value is less than 10%.

[0011] (4): Based on any of the above (1) to (3), before the first step and the second step, determine whether the lithium-ion battery has a performance degradation, and only if the performance degradation is confirmed, implement the first step and the second step.

[0012] (5): One aspect of the present invention relates to a charging and discharging device electrically connected to a lithium-ion battery having a positive electrode and a negative electrode, and the performance of the lithium-ion battery being reduced due to lithium ion residue at the negative electrode. The charging and discharging device includes a control unit for charging and discharging the lithium-ion battery. The control unit repeatedly performs multiple cycles including a first process and a second process. In the first process, the SOC of the lithium-ion battery is made to a first value by charging. The first value is below the SOC value when the slope of the SOC-voltage curve becomes the minimum value and above the SOC value when the slope of the SOC-voltage curve becomes twice the minimum value. In the second process, the SOC of the lithium-ion battery is made to a second value smaller than the first value by discharging.

[0013] (6): One aspect of the present invention relates to a storage medium, which is a non-temporary storage medium storing a program that can be read by a computer, wherein the program causes a charging and discharging device to repeatedly perform multiple cycles including a first step and a second step, the charging and discharging device being electrically connected to a lithium-ion battery having a positive electrode and a negative electrode, and the performance of the lithium-ion battery being reduced due to lithium ion residue at the negative electrode, in the first step, the SOC of the lithium-ion battery is made to a first value by charging, the first value being below the SOC value when the slope of the SOC-voltage curve becomes the minimum value and above the SOC value when the slope of the SOC-voltage curve becomes twice the minimum value, and in the second step, the SOC of the lithium-ion battery is made to a second value smaller than the first value by discharging.

[0014] Based on the above schemes (1) to (6), a method for restoring lithium-ion batteries, a charging and discharging device, and a storage medium are provided that have excellent performance restoration effects on lithium-ion batteries. Attached Figure Description

[0015] Figure 1 This is a 3D diagram of an example of a lithium-ion battery.

[0016] Figure 2 This is a structural diagram of the charging and discharging device.

[0017] Figure 3A It is a diagram that illustrates the movement pattern of lithium ions at the negative electrode, and it is a diagram that shows the charging process.

[0018] Figure 3BIt is a diagram that shows the movement pattern of lithium ions at the negative electrode, and it is a diagram that shows the discharge process.

[0019] Figure 4 This is a chart illustrating an example of the change in discharge capacity during a charge-discharge test.

[0020] Figure 5 This is an explanatory diagram illustrating an example of a lithium-ion battery recovery process according to an embodiment.

[0021] Figure 6 This is an example of a SOC-voltage curve.

[0022] Figure 7 It is a chart representing the results of the experiment.

[0023] Figure 8 It is a chart representing the results of the experiment.

[0024] Figure 9 It is a chart representing the results of the experiment.

[0025] Figure 10 It is a chart representing the results of the experiment.

[0026] Figure 11 It is a chart representing the results of the experiment.

[0027] Figure 12 It is a chart representing the results of the experiment. Detailed Implementation

[0028] Hereinafter, embodiments of the lithium-ion battery recovery processing method, charging and discharging device, and storage medium of the present invention will be described with reference to the accompanying drawings.

[0029] [Lithium-ion batteries]

[0030] Figure 1 This is a 3D diagram of an example of a lithium-ion battery.

[0031] like Figure 1 As shown, the lithium-ion battery 1 includes: a laminate 2 comprising electrodes; an outer casing 4 housing the laminate 2; and a cover 5 sealing the outer casing 4. The outer casing 4 is, for example, a metal housing. A positive terminal 6 and a negative terminal 7 (see reference) are provided on the outer casing 4 or the cover 5. Figure 2 ).

[0032] The laminate 2 includes a positive electrode 21, a negative electrode 22, and a separator 23. The separator 23 is sandwiched between the positive electrode 21 and the negative electrode 22. An electrolyte is impregnated in the positive electrode 21, the negative electrode 22, and the separator 23.

[0033] The positive electrode 21 has a positive current collector and a positive active material layer. The positive active material is, for example, a lithium composite oxide containing nickel, cobalt, etc. Examples of lithium composite oxides include lithium-nickel composite oxide, lithium-cobalt composite oxide, lithium-manganese composite oxide, lithium-nickel-cobalt composite oxide, lithium-nickel-manganese composite oxide, and lithium-nickel-cobalt-manganese composite oxide.

[0034] The negative electrode 22 has a negative current collector and a negative active material layer. The negative active material is, for example, a carbon material such as graphite.

[0035] The diaphragm 23 is formed of resins such as polyethylene (PE) and polypropylene (PP).

[0036] Electrolytes include, for example, non-aqueous solvents and lithium salts (electrolytes). Examples of non-aqueous solvents include vinylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC). Examples of electrolytes include lithium hexafluoride phosphate (LiPF6) and lithium tetrafluoride borate (LiBF4).

[0037] Lithium-ion batteries 1 are used, for example, in vehicles.

[0038] [Charging and discharging device]

[0039] Figure 2 This is a structural diagram of the charging and discharging device 10 according to the embodiment.

[0040] like Figure 2 As shown, the charging / discharging device 10 is electrically connected to the positive terminal 6 and the 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 the recovery processing method described later. The charging / discharging device 10 may also include a power source for charging the lithium-ion battery 1. In the absence of a power source, an external power source is used. The charging / discharging device 10 is, for example, mounted in a vehicle. The charging / discharging device 10 may also be mounted in a battery replacement device.

[0041] The control unit 11 is implemented by executing programs (software) through hardware processors such as CPUs (Central Processing Units). Some or all of these components can be implemented by hardware (including circuitry) such as LSIs (Large Scale Integration), ASICs (Application Specific Integrated Circuits), FPGAs (Field-Programmable Gate Arrays), and GPUs (Graphics Processing Units), or through the coordinated use of software and hardware. The program can be pre-stored in a storage device such as an HDD or flash memory (a storage device with a non-transitory storage medium) of the control unit 11, or it can 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 (a non-transitory storage medium) onto a drive device.

[0042] [Performance degradation of lithium-ion batteries due to repeated charge and discharge cycles]

[0043] Lithium-ion batteries can experience performance degradation, such as capacity shift, due to repeated charging and discharging.

[0044] According to "Kentaro Kaji and four others, 'Research related to the modeling of lithium-ion battery degradation', Proceedings of the Motor Vehicle Technical Association, Motor Vehicle Technical Association, Vol. 44 (2013), No. 2, pp. 429-434", three factors can be cited as the degradation mechanism of lithium-ion batteries.

[0045] (1) Because lithium ions remain in the state of entering the negative electrode material and do not come out, the total number of active lithium is reduced.

[0046] (2) Because lithium ions are reduced (i.e. gain electrons) on the surface of the negative electrode and react with the electrolyte, the total amount of active lithium ions is reduced.

[0047] (3) Because of the resin coating formed by the reaction of the electrolyte on the surface of the negative electrode, lithium ions are difficult to pass through, thus increasing the internal resistance of the battery.

[0048] (1) is called “residual lithium ions deep in the negative electrode” (residual lithium ions in the negative electrode). (2) is called “lithium deposition”. (3) is called “SEI formation”. The precipitation of needle-like crystals in lithium deposition is called dendrite precipitation.

[0049] Lithium deposition is more likely to occur at low temperatures (e.g., below 0°C). In contrast, the retention of lithium ions deep within the negative electrode is more likely to occur at temperatures above room temperature (e.g., above 0°C; specifically, 10°C to 65°C).

[0050] Detailed explanation of the residual lithium ions deep within the negative electrode.

[0051] Figure 3A and Figure 3B It is a diagram that illustrates the movement pattern of lithium ions at the negative electrode. Figure 3A Indicates charging. Figure 3B This indicates the state during discharge. 34 represents the electrolyte.

[0052] like Figure 3A As shown, during charging, the negative electrode 32 is filled with lithium ions 30. Figure 3B As shown, during discharge, the number of lithium ions 30 in the negative electrode 32 decreases. With repeated charge-discharge cycles, even during discharge, a significant amount of lithium ions 30 remain deep within the negative electrode 32. This situation is referred to as residual lithium ions deep within the negative electrode. If the residual lithium ions deep within the negative electrode increase, the number of active lithium ions decreases, leading to a reduction in capacity.

[0053] Figure 4 This is an example graph showing the change in discharge capacity during a charge-discharge test. In Figure 4 In the figure, the change in discharge capacity is shown as a baseline based on the capacity maintenance rate. The capacity maintenance rate is expressed as "current capacity / initial capacity × 100 (%)".

[0054] like Figure 4 As shown, the discharge capacity of the lithium-ion battery in this example gradually decreases as the number of charge-discharge cycles (repetitions) increases.

[0055] [Recovery methods for lithium-ion batteries]

[0056] Lithium-ion batteries that have experienced performance degradation can have their performance restored using the method described below. The restoration process shown below can be performed using the charge / discharge device 10 (see reference 10). Figure 2 To implement this.

[0057] Figure 5 This is an explanatory diagram illustrating an example of the recovery processing method of the embodiment. It should be noted that the recovery processing method for lithium-ion batteries is also referred to simply as a "recovery processing method".

[0058] like Figure 5 As shown, the recovery process of this embodiment is repeatedly performed in a cycle including the following two steps.

[0059] The first step: charging the lithium-ion battery to its maximum SOC (State of Charge) value, S1. The maximum SOC value, S1, is an example of the "first value".

[0060] The second step: discharging the lithium-ion battery to bring its state of charge (SOC) to the lower limit value S2. The lower limit value S2 is an example of the "second value".

[0061] Alternatively, a preparatory step can be performed before the first step, in which the lithium-ion battery is discharged to the discharge termination voltage.

[0062] SOC (State of Charge) is the charge rate (%) of a lithium-ion battery. In the recovery process of this embodiment, a first step that brings the SOC to the upper limit S1 and a second step that brings the SOC to the lower limit S2 are repeatedly performed, thus the SOC repeatedly increases and decreases. In the recovery process of this embodiment, the SOC increases linearly in the first step and decreases linearly in the second step.

[0063] exist Figure 5 In the example shown, the initial step is the first step, but either the first step or the second step can come first.

[0064] Figure 6 This is an example of a "SOC-voltage curve" that represents the relationship between SOC and voltage. Figure 6 The horizontal axis represents SOC (%). Figure 6 The vertical axis represents voltage (V). The SOC-voltage curve can be obtained as follows.

[0065] For example, calculate the capacity of a lithium-ion battery as follows.

[0066] An example of a lithium-ion battery using a ternary lithium composite oxide containing cobalt, nickel, and manganese as the positive electrode active material is given. The rated voltage is 3.6V. The capacity is 3Ah. The upper limit voltage is 4.2V. The lower limit voltage is 2.5V.

[0067] After placing the lithium-ion battery in a constant temperature bath at 25°C for 4 hours, the following operations were performed in the same constant temperature bath at 25°C.

[0068] (1) Discharge the lithium-ion battery to 2.5V at a current of 3A (equivalent to 1C according to the rated capacity) and leave it for 10 seconds.

[0069] (2) Charge the lithium-ion battery with a constant current of 3A until it reaches 4.2V.

[0070] (3) Charge the lithium-ion battery at a constant voltage of 4.2V until the current becomes 0.6A (equivalent to 0.2C according to the rated capacity).

[0071] (4) Discharge the lithium-ion battery at a constant current of 3A until it reaches 2.5V. Measure the capacity during this discharge.

[0072] During discharge (operation (4)), the voltage is measured every 1 second.

[0073] SOC is calculated using the formula: (Capacity - Current - Time) / Capacity × 100 (%).

[0074] Based on the obtained SOC and voltage, it is possible to manufacture... Figure 6 The SOC-voltage curve is shown.

[0075] The slope of the SOC-voltage curve is the slope of the following straight line, which is obtained by approximating the range of SOC from 30 seconds before to 30 seconds after a given time point using the least squares method. The slope of the SOC-voltage curve is the ratio (V / %) of the change in voltage (V) to the change in SOC (%).

[0076] The upper limit of SOC, S1 (refer to) Figure 5 ) set to in Figure 6 The SOC value at point M1, where the slope of the SOC-voltage curve becomes minimum, is below the value of the SOC. Figure 6 In the example shown, the slope of the SOC-voltage curve reaches a minimum of 0.0031 at point M1. At this point, the SOC is 34%. Therefore, the upper limit of SOC, S1, is set to be below 34%. Preferably, the upper limit S1 is below 50% (e.g., below 30%).

[0077] By setting the upper limit of SOC S1 below the SOC value at point M1, the amount of lithium ions remaining at the negative electrode can be reduced, and performance degradation such as capacity shift can be suppressed.

[0078] The upper limit of SOC, S1 (refer to) Figure 5 Preferred to be in Figure 6 The SOC at point M2 is greater than the minimum SOC value when the slope of the SOC-voltage curve is twice that of the minimum value. The SOC at point M2 is smaller than the SOC at point M1. Figure 6 In the example shown, the slope of the SOC-voltage curve is twice the minimum value of 0.0031 (0.0062) at point M2. At this point, the SOC is 10%. Therefore, the upper limit of SOC S1 is preferably 10% or higher.

[0079] By setting the upper limit of SOC S1 to be higher than the SOC value at point M2, the amount of lithium ions remaining at the negative electrode can be reduced, and performance degradation such as capacity shift can be suppressed.

[0080] The lower limit of SOC, S2 (refer to...) Figure 5The lower limit S2 can be 0% or higher and less than the upper limit S1. For example, the lower limit S2 can be less than 10%.

[0081] The lower limit value S2 can also be in Figure 6 The SOC value at point M2, where the slope of the SOC-voltage curve is twice the minimum value, is above the SOC value at point M1, where the slope of the SOC-voltage curve is below the minimum value.

[0082] In the recovery process of this embodiment, it is also possible to determine whether the lithium-ion battery has experienced performance degradation before the recovery process including the aforementioned first and second steps, and to perform the recovery process only if performance degradation is confirmed. The presence or absence of performance degradation can be determined, for example, based on the capacity recovery rate. By determining whether performance degradation exists, the non-operation period of the lithium-ion battery can be shortened.

[0083] [The effect of the recovery processing method in the implementation method]

[0084] According to the recovery processing method of this embodiment, multiple cycles including a first step and a second step are repeatedly performed. In the first step, the State of Charge (SOC) is brought to an upper limit value S1 by charging, and in the second step, the SOC is brought to a lower limit value S2 by discharging. The upper limit value S1 is set below the SOC value at point M1 where the slope of the SOC-voltage curve reaches its minimum. The upper limit value S1 is set above the SOC value at point M2. This reduces the amount of lithium-ion residue at the negative electrode and suppresses performance degradation such as capacity shift. Therefore, the performance of the lithium-ion battery can be restored.

[0085] By restoring the performance of lithium-ion batteries, their lifespan can be extended, thereby improving energy efficiency.

[0086] The above description illustrates specific embodiments of the present invention, but the present invention is not limited to such embodiments in any way, and various modifications and substitutions can be made without departing from the spirit of the present invention.

[0087] The present invention will now be described in detail with reference to specific examples. It should be noted that the present invention is not limited to the following examples.

[0088] <Example 1>

[0089] A lithium-ion battery using a ternary lithium composite oxide comprising cobalt, nickel, and manganese as the positive electrode active material was prepared. The rated voltage is 3.6V. The capacity is 3Ah. The upper limit voltage is 4.2V. The lower limit voltage is 2.5V.

[0090] (Creation of samples with reduced performance)

[0091] The lithium-ion battery was subjected to the charge-discharge test shown below.

[0092] After placing the lithium-ion battery in a constant temperature bath at 25°C for 4 hours, the following operations (A) and (B) were repeated 269 times in the same constant temperature bath at 25°C.

[0093] (A) Discharge the lithium-ion battery to 2.5V at a current of 9A and leave it for 10 seconds.

[0094] (B) Charge the lithium-ion battery to 4.2V with a current of 9A and leave it for 10 seconds.

[0095] Thus, a sample with degraded (degraded) performance was obtained.

[0096] (Recovery Processing)

[0097] like Figure 5 As shown, the charging and discharging device 10 (see reference) is used. Figure 2 Regarding the aforementioned samples, the first and second processes were repeated multiple times (120 times). The processing time was 20 hours. The temperature was 25°C. The sample size was 5. The SOC-voltage curve is shown below. Figure 6 middle.

[0098] Preparation process: Discharge the lithium-ion battery at a current of 9A to 2.5V (lower limit S2: SOC 0%) (discharge termination voltage).

[0099] First step: Charge the lithium-ion battery to 0.75Ah (upper limit S1: SOC 25%) at a current of 9A. Set the charging time to 5 minutes.

[0100] The second step: Discharge the lithium-ion battery at a current of 9A to 2.5V (lower limit S2: SOC 0%) (discharge termination voltage). The discharge time is set to 5 minutes.

[0101] Using the aforementioned capacity measurement method, the initial capacity, the capacity after performance degradation (deterioration), and the capacity after recovery of the lithium-ion battery were measured.

[0102] The recovery rate was calculated using the following formula. The results are shown in Table 1.

[0103] Recovery rate = (Recovered capacity - Degraded capacity) / (Initial capacity - Degraded capacity)

[0104] Table 1

[0105]

[0106] As shown in Table 1, a high recovery rate was obtained through the aforementioned recovery process.

[0107] <Example 2>

[0108] Similar to Example 1, except for the upper limit value S1 of SOC, recovery treatment tests were conducted. The results are shown in Table 2 and... Figure 7 middle.

[0109] For comparison, the results are also shown when the upper limit S1 and lower limit S2 are both 0% (i.e., no charging or discharging is performed).

[0110] Table 2

[0111]

[0112] As shown in Table 2 and Figure 7 As shown, the recovery rate is high when the upper limit S1 is above 10% and below 30%.

[0113] <Example 3>

[0114] Similar to Example 1, except for the upper limit value S1 and the lower limit value S2, a recovery treatment test was conducted. The results are shown in Table 3 and... Figure 8 middle.

[0115] Table 3

[0116]

[0117] As shown in Table 3 and Figure 8 As shown, the recovery rate is high when the lower limit S2 is less than 10%.

[0118] <Example 4>

[0119] Similar to Example 1, except for the number of repetitions (cycle number) of the cycle consisting of the first and second processes, a recovery treatment test was conducted. The results are shown in Table 4 and... Figure 9 middle. Figure 9 The horizontal axis represents processing time. Processing time is roughly proportional to the number of loops.

[0120] In Table 4 and Figure 9 In order to make comparisons, the results for the case where the loop count is zero are also shown.

[0121] Table 4

[0122]

[0123] As shown in Table 4 and Figure 9 As shown, the more cycles, the higher the recovery rate.

[0124] <Example 5>

[0125] Similar to Example 2, except that the current was set to 3A, a recovery treatment test was performed. With a current of 3A, the charging and discharging times were three times longer than in Example 2. The treatment time was 20 hours, the same as in Example 2, therefore the number of cycles was approximately one-third of the number of cycles (120) in Example 2. The results are shown in Table 5 and... Figure 10 middle.

[0126] Table 5

[0127]

[0128] As shown in Table 5 and Figure 10 As shown, compared to Example 2, the tendency regarding the recovery rate is that it does not change much even with changes in current (see reference). Figure 7 ).

[0129] <Example 6>

[0130] Except that the temperature condition for making the performance-degraded sample was changed from 25°C to 50°C, the performance-degraded sample was obtained in the same manner as in Example 2.

[0131] For this sample, a recovery treatment test was performed in the same manner as in Example 2. The results are shown in Table 6 and... Figure 11 middle.

[0132] Table 6

[0133]

[0134] As shown in Table 6 and Figure 11 As shown, the tendency of the recovery rate is not significantly different compared to the case at a temperature of 25°C (see reference). Figure 7 ).

[0135] <Comparative Example 1>

[0136] Except that the temperature conditions for making the performance-degraded sample were changed from 25°C to -10°C, the same performance-degraded sample was obtained as in Example 1.

[0137] For this sample, the recovery treatment test was performed in the same manner as in Example 1. The results are shown in Table 7 and... Figure 12 .

[0138] Table 7

[0139]

[0140] As shown in Table 7 and Figure 12 As shown, the recovery rate of samples that experienced performance degradation at low temperatures was lower than that of samples that experienced performance degradation at room temperature.

[0141] Therefore, the recovery method of this embodiment is effective for lithium-ion batteries whose performance has degraded at room temperature.

[0142] <Comparative Example 2>

[0143] Similar to Example 1, a performance-degraded sample was obtained (the temperature at which the performance was degraded was 25°C) (the performance-degraded sample was obtained under the condition of room temperature).

[0144] Except for the case where the temperature condition for making the performance-degraded sample was -10°C, the same performance-degraded sample as in Example 1 (the performance-degraded sample under low temperature conditions) was obtained.

[0145] These samples underwent multiple cycles, including the following two procedures. The processing time was 72 hours.

[0146] First step: Charge the lithium-ion battery to a SOC of 1.1%. The charging time is set to 10 seconds.

[0147] The second step: Discharge the lithium-ion battery to 0% SOC. The discharge time is set to 10 seconds.

[0148] Table 8

[0149]

[0150] As shown in Table 8, in Comparative Example 2, where the SOC (upper limit S1) of the first process was reduced compared to Comparative Example 1, the recovery rate was higher when performance degradation occurred at low temperature compared to when performance degradation occurred at room temperature.

[0151] Therefore, the effectiveness of recovery processing varies depending on the conditions under which performance is degraded.

[0152] <Example 7>

[0153] Except for the current and processing time during charging and discharging, the recovery treatment test was conducted in the same manner as in Example 1. The results are shown in Table 9.

[0154] Table 9

[0155]

[0156] As shown in Table 9, the number of cycles and the current did not significantly affect the recovery rate.

[0157] The range of SOC is defined as the SOC value at point M2, where the slope of the SOC-voltage curve is twice the minimum, being above the SOC value at point M1, where the slope of the SOC-voltage curve is the minimum. (Refer to...) Figure 6This is referred to as a "specific range".

[0158] Samples that exhibit performance degradation at room temperature (e.g., 25°C) are referred to as "samples with degraded performance at room temperature". Samples that exhibit performance degradation at low temperature (e.g., -10°C) are referred to as "samples with degraded performance at low temperature".

[0159] In the recovery process where the upper limit value S1 of SOC is within a specific range, a high recovery rate was obtained in the performance-degraded samples at room temperature (Examples 1 and 2) compared with the performance-degraded sample at low temperature (Comparative Example 1).

[0160] In contrast, the recovery process for upper limit S1 below the lower limit of a certain range is not very effective for performance degradation samples at room temperature (see Comparative Example 2).

[0161] The recovery processing method of this embodiment is useful when the following condition [1] is met, and when both condition [1] and condition [2] are met.

[0162] [1] When performing recovery processing (as described above) on the performance degradation samples at room temperature and at low temperature to make the upper limit value S1 of SOC within a specific range, the recovery rate of the performance degradation at room temperature is higher than that of the performance degradation at low temperature (see Table 7).

[0163] [2] When the upper limit of SOC S1 of the sample with reduced performance at room temperature and the sample with reduced performance at low temperature were restored (as described above), the recovery rate of the sample with reduced performance at room temperature was lower than that of the sample with reduced performance at low temperature (see Table 8).

[0164] Under the condition of satisfying [1], and under the condition of satisfying both [1] and [2], the reason for the performance reduction can be inferred to be the large amount of lithium ion residue deep in the negative electrode (lithium ion residue at the negative electrode).

[0165] The recovery processing method of this embodiment can be implemented when condition [1] is met. The recovery processing method of this embodiment can also be implemented when both condition [1] and condition [2] are met.

Claims

1. A method for recovering a lithium-ion battery, wherein the lithium-ion battery has a positive electrode and a negative electrode, and the performance of the lithium-ion battery is reduced due to residual lithium ions at the negative electrode, wherein... The lithium-ion battery recovery process involves repeated cycles, including the first and second steps. In the first step, the state of charge (SOC) of the lithium-ion battery is brought to a first value through charging. This first value is below the SOC value when the slope of the SOC-voltage curve reaches its minimum, and above the SOC value when the slope of the SOC-voltage curve is twice the minimum value. In the second process, the SOC of the lithium-ion battery is reduced to a second value that is smaller than the first value by discharging.

2. The method for recovering lithium-ion batteries according to claim 1, wherein, The first value is below 30%.

3. The method for recovering lithium-ion batteries according to claim 1, wherein, The second value is less than 10%.

4. The method for recovering lithium-ion batteries according to any one of claims 1 to 3, wherein, Before the first and second steps, it is determined whether the lithium-ion battery has experienced performance degradation. The first and second steps are only performed if the performance degradation is confirmed.

5. A charging and discharging device electrically connected to a lithium-ion battery, the lithium-ion battery having a positive electrode and a negative electrode, wherein the performance of the lithium-ion battery is reduced due to lithium-ion residue at the negative electrode, wherein... The charging and discharging device includes a control unit for charging and discharging the lithium-ion battery. The control unit repeatedly performs multiple cycles, including the first and second processes. In the first step, the state of charge (SOC) of the lithium-ion battery is brought to a first value through charging. This first value is below the SOC value when the slope of the SOC-voltage curve reaches its minimum, and above the SOC value when the slope of the SOC-voltage curve is twice the minimum value. In the second process, the SOC of the lithium-ion battery is reduced to a second value that is smaller than the first value by discharging.

6. A storage medium that stores a non-transitory program that can be read by a computer, wherein, The program causes the charging and discharging device to repeatedly perform multiple cycles, including the first and second processes. The charging and discharging device is electrically connected to a lithium-ion battery, which has a positive electrode and a negative electrode. The performance of the lithium-ion battery is reduced due to lithium ion residue at the negative electrode. In the first step, the state of charge (SOC) of the lithium-ion battery is brought to a first value through charging. This first value is below the SOC value when the slope of the SOC-voltage curve reaches its minimum, and above the SOC value when the slope of the SOC-voltage curve is twice the minimum value. In the second process, the SOC of the lithium-ion battery is reduced to a second value that is smaller than the first value by discharging.

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