Method for restoring performance of positive electrode for lithium-ion secondary battery

Abstract

To provide a performance recovery method for a positive electrode for a lithium-ion secondary battery that can achieve an optimal recovered state of the positive electrode.SOLUTION: In a method for recovering performance of a positive electrode for a lithium-ion secondary battery by doping lithium ions into the positive electrode for the lithium-ion secondary battery whose capacity has decreased, the doping of lithium ions is performed by discharge using a lithium electrode as a counter electrode in an electrolyte, and the discharge is performed within a range of a predetermined cumulative discharge amount DG [Ah].SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for restoring the performance of a positive electrode for a lithium ion secondary battery. [Background technology]

[0002] In recent years, there has been growing interest in electric vehicles as a way to reduce CO2 emissions in light of climate-related disasters, and the use of lithium-ion secondary batteries for in-vehicle applications is also being considered. The performance of lithium ion secondary batteries generally deteriorates with repeated charging and discharging. Various methods have been proposed for restoring the performance of lithium ion secondary batteries. For example, Patent Document 1 discloses a capacity recovery device for a secondary battery, which includes a capacity estimation unit that calculates an estimated capacity, which is an estimated value of the capacity of the secondary battery, a capacity recovery processing unit that performs capacity recovery processing for the secondary battery by moving reactive species from the capacity recovery electrode to the positive electrode or the negative electrode, and an electrical quantity calculation unit that calculates the amount of electricity to be passed through the capacity recovery electrode, and the capacity recovery processing unit includes an electrical quantity monitoring unit that determines the amount of electricity passed from the capacity recovery electrode to the positive electrode or the negative electrode, or a voltage monitoring unit that monitors the voltage between the capacity recovery electrode and the positive electrode or the negative electrode. Patent Document 2 discloses a method for regenerating electrodes of a lithium ion secondary battery, which involves cleaning the electrodes of a used lithium ion secondary battery with a polar solvent, washing away degraded substances containing Li adhering to the surfaces of active material particles, which are the main component causing capacity degradation of the electrode, and thoroughly drying the electrodes to volatilize the cleaning solvent, followed by re-injecting the solvent into a battery having the dried electrodes. Patent Document 3 discloses a nonaqueous electrolyte secondary battery including a positive electrode, a negative electrode other than metallic lithium, and a nonaqueous electrolyte, characterized in that the nonaqueous electrolyte secondary battery further includes a third electrode that contains metallic lithium, is not in contact with the electrolytic solution, and is not connected to the positive electrode or the negative electrode. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2022 / 034717 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-022969 [Patent Document 3] Japanese Patent Application Laid-Open No. 2002-324585 Summary of the Invention [Problem to be solved by the invention]

[0004] None of Patent Documents 1 to 3 discloses a means for appropriately controlling the degree of recovery.

[0005] The present invention has been made in view of the above, and aims to provide a method for restoring the performance of a positive electrode for a lithium ion secondary battery, which is capable of achieving an optimal recovery state of the positive electrode for a lithium ion secondary battery. [Means for solving the problem]

[0006] In order to solve the above problems, the present invention proposes the following means. [1] A method for restoring the performance of a positive electrode for a lithium ion secondary battery by doping lithium ions into the positive electrode for the lithium ion secondary battery whose capacity has decreased, comprising: The doping of the lithium ions is carried out by discharging in an electrolyte solution using a lithium electrode as a counter electrode, The method for recovering the performance of a positive electrode for a lithium-ion secondary battery comprises discharging the battery within a range of an accumulated discharge amount DG [Ah] expressed by the following formula 1: 0.95×DG0≦DG≦1.05×DG0 Formula 1 (In the above formula 1, DG0 is a value calculated by the following formula 2. DB×[(DB-DA) / DB+X] Equation 2 In the above formula 2, DB [Ah] is the capacity of the new lithium ion secondary battery, DA [Ah] is the capacity of the lithium ion secondary battery whose capacity has decreased, and X is a correction coefficient selected from the following X1, X2, and X3. X1: Correction coefficient based on the lithium ions consumed in the formation of the film during the chemical formation process of the initial lithium ion secondary battery X2: Correction coefficient based on the deterioration of the negative electrode of the lithium-ion secondary battery X3: Correction coefficient based on the reaction resistance and migration resistance of lithium ions in lithium ion secondary batteries [2] A non-destructive method for restoring the performance of a positive electrode for a lithium ion secondary battery according to [1]. [3] The method for recovering the performance of a positive electrode for a lithium ion secondary battery according to [1] or [2], wherein the discharge is carried out at a constant current. [4] The method for recovering performance of a positive electrode for a lithium ion secondary battery according to any one of [1] to [3], wherein the (DB-DA) / DB is 0.7 or more. [5] The method for recovering performance of a positive electrode for a lithium ion secondary battery according to [1], wherein X1 is a numerical value in the range of 0 to 0.25, X2 is a numerical value in the range of 0 to 0.1, and X3 is a numerical value in the range of 0 to 0.18. [6] The method for recovering performance of a positive electrode for a lithium ion secondary battery according to any one of [1] to [5], wherein X3 is calculated by the following formula 3: X3=X3a+X3b+X3c+X3d Formula 3 (In the above formula 3, X3a is a correction coefficient based on the reaction resistance and migration resistance of lithium ions in the positive electrode when the performance-recovered lithium ion secondary battery is used, and is a numerical value within the range of 0 to 0.05; X3b is a correction coefficient based on the reaction resistance and migration resistance of lithium ions in the negative electrode when the performance-recovered lithium ion secondary battery is used, and is a numerical value within the range of 0 to 0.05; X3c is a correction coefficient based on the migration resistance of lithium ions in the electrolyte when the performance-recovered lithium ion secondary battery is used, and is a numerical value within the range of 0 to 0.04; and X3d is a correction coefficient based on the migration resistance of lithium ions in the separator when the performance-recovered lithium ion secondary battery is used, and is a numerical value within the range of 0 to 0.04.) [7] A method for recovering the performance of a positive electrode for a lithium ion secondary battery by doping lithium ions into the positive electrode for the lithium ion secondary battery whose capacity has decreased, comprising: The doping of the lithium ions is carried out by discharging in an electrolyte solution using a lithium electrode as a counter electrode, The doping of the lithium ions is controlled based on an integrated value of current during energization, a correction coefficient and a difference between the initial capacity of the lithium-ion secondary battery and the capacity of the lithium-ion secondary battery whose capacity has been reduced. [8] The performance recovery method for a positive electrode for a lithium ion secondary battery according to [7], wherein the correction coefficient includes at least one of a first correction coefficient based on the difference between the initial capacity of the lithium ion secondary battery and the capacity of the lithium ion secondary battery whose capacity has decreased, a second correction coefficient based on the amount of lithium ions consumed in a chemical formation process of the lithium ion secondary battery, and a third correction coefficient based on the discharge efficiency of the lithium ion secondary battery. [Effects of the Invention]

[0007] It is possible to provide a method for restoring the performance of a positive electrode for a lithium ion secondary battery, which is capable of appropriately restoring the performance of a positive electrode for a lithium ion secondary battery. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 2 is a diagram illustrating a configuration for performing discharge in one embodiment of the performance recovery method of the present invention. [Figure 2] 10 is a graph illustrating a method for determining the capacity deviation from a battery in an initial state (BOL) as the optimum value of the doping amount. [Figure 3] 1 is a graph illustrating the relationship between the degree of decrease in SOH of a battery and the capacity deviation due to negative electrode deterioration. [Figure 4] FIG. 2 is a schematic diagram illustrating deterioration of a negative electrode. [Figure 5]1 is a graph showing the relationship between the amount of Li in the negative electrode and the decrease in capacity, obtained by ICP emission spectroscopy. [Figure 6] FIG. 2 is a schematic diagram illustrating types of resistance in a battery. [Figure 7] 1 is a graph showing the change in capacity of a positive electrode active material due to pressing and washing. [Figure 8] 10 is a graph showing the results of performing a recovery treatment based on the optimum value of the doping amount without taking the correction coefficient X into consideration after performing pressing and cleaning. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, a method for recovering performance of a lithium ion secondary battery according to an embodiment of the present invention will be described with reference to the drawings. The method of the present embodiment is a method for recovering the performance of a positive electrode for a lithium ion secondary battery by doping lithium ions into the positive electrode for the lithium ion secondary battery whose capacity has decreased, and the doping of the lithium ions is carried out under predetermined conditions described below. Here, "initial state" means that the lithium ion secondary battery is in an unused state, or that the lithium ion secondary battery is in an undegraded state, i.e., that the capacity of the lithium ion secondary battery has not decreased due to charge / discharge cycles. Furthermore, the method of the present embodiment is preferably carried out non-destructively, without disassembling the positive electrode into its constituent elements.

[0010] (lithium-ion secondary battery) There are no particular limitations on the lithium ion secondary battery (hereinafter, sometimes simply referred to as "battery") whose performance can be restored by the method of this embodiment, and any known lithium ion secondary battery can be used. A lithium ion secondary battery is generally composed of a positive electrode, a negative electrode, and an electrolyte (electrolytic solution or solid electrolyte) disposed between the positive electrode and the negative electrode. A separation membrane (separator) may also be provided between the positive electrode and the negative electrode. The positive electrode and the negative electrode each include an active material, a binder, and a current collector. The configuration of the positive electrode and the negative electrode will be described below.

[0011] "Positive electrode" The positive electrode includes a positive electrode active material, a positive electrode conductive additive, a positive electrode binder, and a positive electrode current collector. A layer consisting of the positive electrode active material, the positive electrode conductive additive, and the positive electrode binder is referred to as a positive electrode mixture layer. The positive electrode mixture layer may be formed on one or both sides of the positive electrode current collector. Note that, as long as the positive electrode active material has sufficient conductivity, the positive electrode mixture layer does not need to contain a positive electrode conductive additive.

[0012] The positive electrode active material used in the positive electrode is not particularly limited as long as it can absorb and release Li ions. Examples of the positive electrode active material include lithium nickel oxide (e.g., LiNiO2), lithium cobalt oxide (e.g., LiCoO2), lithium nickel cobalt oxide, lithium nickel cobalt manganese oxide, LiFePO4, LiMn 1-x Fe x PO4, LiMnPO4, LiCoPO4, LiNiPO4, etc. The positive electrode active material preferably contains one or more elements selected from the group consisting of manganese, nickel, and cobalt.

[0013] The positive electrode conductive additive, which is a conductive additive used in the positive electrode, assists in the formation of a conductive path between the positive electrode active material and the positive electrode current collector. The positive electrode conductive additive is not particularly limited as long as it has conductivity, and examples thereof include carbon black such as acetylene black, carbon nanotubes, and graphite such as artificial graphite.

[0014] The positive electrode binder, which is a binder for the positive electrode active material, binds the positive electrode active material, the positive electrode conductive additive, and the positive electrode current collector. Examples of positive electrode binders include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyamide (PA), polyimide (PI), polyacrylic acid and its copolymers, polyamideimide (PAI), polybenzimidazole, polyethersulfone (PES), maleic anhydride-modified polypropylene, and mixtures thereof. The positive electrode binder preferably contains a crystalline polymer having a melting point. The positive electrode binder is preferably a polymer containing fluorine. Examples of fluorine-containing polymers include PVDF and PTFE.

[0015] Examples of the positive electrode current collector include metal foils such as aluminum foil, stainless steel foil, and nickel foil. The positive electrode current collector may have a carbon coating layer formed thereon. The positive electrode current collector may also be processed into a mesh shape.

[0016] "Negative electrode" The negative electrode includes a negative electrode active material, a negative electrode conductive additive, a negative electrode binder, and a negative electrode current collector. A layer consisting of the negative electrode active material, the negative electrode conductive additive, and the negative electrode binder is referred to as a negative electrode mixture layer. The negative electrode mixture layer may be formed on one or both sides of the negative electrode current collector. Note that, as long as the negative electrode active material has sufficient conductivity, the negative electrode mixture layer does not need to contain a negative electrode conductive additive.

[0017] The negative electrode active material is not particularly limited as long as it can absorb and release Li ions. Examples of the negative electrode active material include graphite (artificial graphite, natural graphite), amorphous carbon (hard carbon), mesocarbon microbeads, carbon fiber, and Si materials (silicon, Si alloys, Si oxides).

[0018] The negative electrode conductive additive, which is a conductive additive for the negative electrode, assists in the formation of a conductive path between the negative electrode active material and the negative electrode current collector. The negative electrode conductive additive is not particularly limited as long as it has conductivity, and examples thereof include carbon black such as acetylene black, carbon nanotubes, and graphite such as artificial graphite.

[0019] The negative electrode binder, which is a binder for the negative electrode, binds the negative electrode active material, the negative electrode conductive additive, and the negative electrode current collector. Examples of the negative electrode binder include carboxymethyl cellulose, polyvinylidene fluoride, polytetrafluoroethylene, polyacrylic acid, fluororubber, and diene rubber such as styrene-butadiene rubber. The negative electrode binder preferably contains a crystalline polymer having a melting point.

[0020] Examples of the negative electrode current collector include metal foils such as copper foil, stainless steel foil, and nickel foil. The negative electrode current collector may have a carbon coating layer formed thereon. The negative electrode current collector may also be processed into a mesh shape.

[0021] (Step of doping lithium ions into the positive electrode) In the method of this embodiment, the doping of lithium ions is performed by discharging in an electrolyte solution using a lithium electrode as a counter electrode. The configuration for discharging is shown in Figure 1. That is, as shown in Figure 1, a positive electrode and a lithium electrode as a counter electrode are immersed in the electrolyte solution, and a voltage is applied between the positive electrode and the lithium electrode to discharge from the lithium metal electrode. The discharge is carried out within the range of the accumulated discharge amount DG [Ah] shown in the following formula 1 (hereinafter, this process may be referred to as "recovery process"). 0.95×DG0≦DG≦1.05×DG0 Formula 1 (In the above formula 1, DG0 is a value calculated by the following formula 2. DB×[(DB-DA) / DB+X] Equation 2 In the above formula 2, DB [Ah] is the capacity of the lithium ion secondary battery in an initial state, DA [Ah] is the capacity of the lithium ion secondary battery after the capacity has decreased, and X is a correction coefficient selected from the following X1, X2, and X3. Here, in the formula 2, DB [Ah] may be the capacity of the positive electrode for the lithium ion secondary battery in an initial state, and DA [Ah] may be the capacity of the positive electrode for the lithium ion secondary battery whose capacity has decreased. X1: Correction factor based on the lithium consumed in film formation during the chemical formation process of a lithium-ion secondary battery in its initial state X2: Correction coefficient based on the deterioration of the negative electrode of the initial lithium-ion secondary battery X3: Correction coefficient based on the reaction resistance and migration resistance of lithium ions in lithium ion secondary batteries The (DB-DA) / DB ratio is preferably 0.7 or more. When the (DB-DA) / DB ratio is 0.7 or more, the performance of the positive electrode for a lithium ion secondary battery can be more reliably restored by the method of this embodiment. Furthermore, it is preferable to discharge at a constant current (CC).

[0022] Counter electrode The lithium electrode serving as the counter electrode is preferably made of lithium metal and can have the same structure as the negative electrode.

[0023] "Electrolyte" The electrolyte is not particularly limited, and any electrolyte generally used in lithium-ion secondary batteries can be used. For example, aprotic organic solvents such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate (MPC), and ethyl propyl carbonate (EPC) can be used.

[0024] In addition, the electrolyte solution may be one prepared by dissolving a lithium salt such as lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium iodide, lithium chloride, lithium bromide, LiB[OCOCF3]4, LiB[OCOCF2CF3]4, LiPF4(CF3)2, LiN(SO2CF3)2, or LiN(SO2CF2CF3)2, or a mixed lithium salt of two or more of these, in a solvent of a mixed organic compound of two or more of these aprotic organic solvents.

[0025] "Cumulative discharge amount DG [Ah]" The cumulative discharge amount DG [Ah] satisfies formula 1 and preferably satisfies the following formula 1a. 0.95×DG0≦DG≦1.05×DG0 Formula 1 0.98×DG0≦DG≦1.02×DG0 Formula 1a By discharging within the range of the cumulative discharge amount DG [Ah] that satisfies the above, it is possible to prevent the capacity after the recovery process from being insufficient or excessive compared to the initial capacity. If the capacity is excessively recovered and becomes larger than the initial capacity, the amount of Li in the positive electrode will be excessive, and when this positive electrode is reassembled into a battery, there is a risk of dendrites (also called dendritic crystals) being formed due to the precipitation of Li metal. From this perspective, the recovery rate, expressed as the ratio (%) (xd / x × 100) of the capacity of the lithium-ion secondary battery after doping (xd) to the capacity (x) of the lithium-ion secondary battery in the initial state, is preferably 90 to 110%, more preferably 95 to 105%, and most preferably substantially 100%. Alternatively, the recovery rate, which is expressed as the ratio (%) (xd / x × 100) of the capacity (x) of the positive electrode for the lithium ion secondary battery after completion of doping to the capacity (x) of the positive electrode for the lithium ion secondary battery in the initial state, is preferably 90 to 110%, more preferably 95 to 105%, and most preferably substantially 100%. Conventional methods were unable to control the recovery of only the capacity that had decreased due to use, and were unable to prevent the capacity from being insufficient or excessive after the recovery process.However, the present invention makes it possible to achieve an optimal recovery state by controlling discharge based on the accumulated discharge amount DG [Ah] that satisfies the above.

[0026] A method for determining the range of the cumulative discharge amount DG [Ah] will be described. First, the optimum value of the doping amount by discharge (a value that does not take into account the correction coefficient X) can be determined as follows: For example, charging and discharging are performed under the following conditions. Upper limit: 3V discharge (0.1C constant current (CC) discharge). Lower limit: 4.15V charging (0.1C constant current, constant voltage charging (CCCV: Constant Current, Constant Voltage)). The 0.1C (or 0.05C) capacity is measured in the cell state before disassembly, and a fitting analysis is performed on the capacity usage range of the positive and negative electrodes and the capacity deviation amount, and the capacity deviation amount relative to the cell in its initial state (BOL) is determined as the optimal value for the doping amount (see Figure 1). Alternatively, the open circuit voltage (OCV) of the cell when discharged to 3V and the open circuit potential (OCP) of the positive electrode after disassembling the cell are measured, and the capacity deviation from the cell in its initial state (BOL) is calculated as the optimal value for the doping amount from the SOC (State Of Charge)-OCV and SOC-OCP curves.

[0027] For example, if the SOH (State Of Health: capacity maintenance rate) has decreased to 90% from the initial state, and if the initial capacity is 8.0 Ah, the optimal value of the doping amount is 0.8 Ah, calculated by multiplying 8.0 Ah by (100% - 90%). Therefore, if the correction coefficient X is not taken into consideration, the counter electrode is discharged, the discharge amount from the start of discharge is accumulated, and the discharge ends when the accumulated value reaches 0.8 Ah. If the SOH has decreased to 80% from the initial state, and the initial capacity is 8.0 Ah, the optimal doping amount is 1.6 Ah, calculated by multiplying 8.0 Ah by (100% - 80%). Therefore, if the correction coefficient X is not taken into consideration, the counter electrode is discharged, the discharge amount is accumulated from the start of discharge, and the discharge ends when the accumulated value reaches 1.6 Ah.

[0028] There are several advantages to controlling discharge based on the accumulated discharge amount DG [Ah]. First, regardless of the specific potential, control is possible by focusing only on the accumulated current value. Also, since the battery capacity itself is evaluated in Ah, direct control based on the accumulated current is possible. Furthermore, control is possible using only the current without using a voltage sensor. However, if discharge is controlled based on the optimum value assuming an ideal state as described above, it is inevitable that the capacity after the recovery process will be insufficient or excessive compared to the capacity in the initial state, so it is necessary to take the correction coefficient X into consideration.

[0029] Each of the correction coefficients X (X1, X2, X3) will be explained below. <x1> X1 is a correction coefficient based on the lithium ions consumed in forming a film in the chemical formation process of a lithium ion secondary battery in an initial state, and is preferably a value within the range of 0 to 0.25. That is, X1 is a correction coefficient that takes into account the influence of chemical conversion, and is a value obtained by dividing the percentage of Li consumed in forming a film by 1 / 100. This value usually decreases as the number of chemical conversions increases. The percentage of Li consumed in forming a film by chemical conversion changes, for example, as follows: First chemical treatment: 20.3% Second chemical treatment: 1.4% 3rd chemical treatment: 0.8% 4th chemical treatment: 0.4%. This correction coefficient X1 is determined from the average value of the experimental results shown in Table 1.

[0030] [Table 1]

[0031] The correction coefficient X1 may be a value measured after the performance recovery process.

[0032] <x2> X2 is a correction coefficient based on the deterioration of the negative electrode of the lithium ion secondary battery, and is preferably a numerical value within the range of 0 to 0.1. That is, X2 is a correction coefficient that takes into account the influence of the deterioration of the negative electrode, and is a value that divides the capacity deviation (%) due to the deterioration of the negative electrode by 1 / 100. The degree of decrease in the battery's SOH and the capacity deviation due to anode degradation are proportional (see Figure 3). The main causes of the decrease in SOH are a decrease in the positive electrode capacity and anode capacity deviation, but in both cases, Li accumulates on the anode side. In other words, the capacity decrease progresses due to the deactivation of Li caused by the formation of a solid electrolyte interface (SEI) in the anode (see Figure 4). By analyzing the battery using ICP optical emission spectroscopy, the relationship between the amount of Li in the negative electrode and the decrease in capacity can be confirmed (see Figure 5). When calculating the correction coefficient X2 based on the SOH, it can be estimated from the number of charge / discharge cycles or calculated based on the dQ / dV curve. For example, if the SOH decreases by 10% from the initial state due to use, the negative electrode capacity deviation is 1.2% (X2 = 0.012), and if the SOH decreases by 10%, the negative electrode capacity deviation is 2.4% (X2 = 0.024), so X2 can be calculated.

[0033] <x3> X3 is a correction coefficient based on the reaction resistance and migration resistance of lithium ions in the lithium ion secondary battery, and is preferably a value within the range of 0 to 0.18. More specifically, X3 is preferably calculated by the following formula 3. X3=X3a+X3b+X3c+X3d Formula 3 (In the above formula 3, X3a is a correction coefficient based on the reaction resistance and migration resistance of lithium ions in the positive electrode when the performance-recovered lithium ion secondary battery is used, and is a numerical value within the range of 0 to 0.05; X3b is a correction coefficient based on the reaction resistance and migration resistance of lithium ions in the negative electrode when the performance-recovered lithium ion secondary battery is used, and is a numerical value within the range of 0 to 0.05; X3c is a correction coefficient based on the migration resistance of lithium ions in the electrolyte when the performance-recovered lithium ion secondary battery is used, and is a numerical value within the range of 0 to 0.04; and X3d is a correction coefficient based on the migration resistance of lithium ions in the separator when the performance-recovered lithium ion secondary battery is used, and is a numerical value within the range of 0 to 0.04.) The types of resistance within a battery are shown in Figure 6. That is, X3 is a correction coefficient that takes into account the resistance inside the battery, and is the sum (%) of various resistances divided by 100. Each resistance component can be measured using an AC impedance method, a multi-point probe method, or the like.

[0034] <Other correction factors> When a capacity recovery process other than the doping of lithium ions (hereinafter also referred to as "other recovery processes") is also performed, it is preferable to consider the recovered capacity due to the other recovery processes as the correction coefficient XN. It is also preferable to consider the amount of collapse of the crystalline structure of the positive electrode predicted by impedance measurement as the correction coefficient XN. The correction coefficient XN is preferably 0 to 0.2. In this case, it is preferable that DG0 in the above formula 1 is a value calculated by the following formula 2a. DB×[(DB-DA) / DB+X-XN] Equation 2a Other recovery processes include pressing, washing, coating removal, etc. For example, if the volume recovered by pressing, washing, and coating removal are 1%, 6%, and 1%, respectively, the volume recovered due to other recovery processes is 8%, and this is divided by 100 to obtain 0.08, which is the correction coefficient XN.

[0035] The pressing is performed by compressing the positive electrode in the thickness direction, and can be performed by a known means such as a roll press. When the positive electrode active material was pressed at temperatures of 80°C, 110°C, and 130°C, the change in capacity was observed, and the results are shown in FIG. For cleaning, known methods can be used. For example, the positive electrode is preferably cleaned using a solvent such as dimethyl carbonate (DMC), acetone, or propylene carbonate (PC). For removing the coating, a known method can be used. For a battery whose capacity had decreased by 12% from the initial state (BOL) due to use, the positive electrode was washed with dimethyl carbonate (DMC), acetone, and propylene carbonate (PC), and the results are shown in Figure 7. Here, for example, in the case of DMC linear, XN is 0.07. The results of the recovery treatment performed by combining pressing and cleaning and based on the optimal value of the doping amount without considering the correction coefficient X are shown in Figure 8. As is clear from Figure 8, the recovery rate was about 96%, which was not sufficient.

[0036] In another embodiment of the present invention, there is provided a method for recovering the performance of a positive electrode for a lithium ion secondary battery by doping lithium ions into a positive electrode for a lithium ion secondary battery whose capacity has decreased, the method comprising: The doping of the lithium ions is carried out by discharging in an electrolyte solution using a lithium electrode as a counter electrode, The doping of the lithium ions is controlled based on an integrated value of current during energization, The present invention provides a method for recovering performance of a positive electrode for a lithium ion secondary battery, in which an integrated value of current when the energization is completed is set based on a difference between the capacity of the lithium ion secondary battery in an initial state and the capacity of the lithium ion secondary battery whose capacity has decreased, and a correction coefficient. Here, it is preferable that the correction coefficients include at least one of a first correction coefficient based on the difference between the initial capacity of the lithium ion secondary battery and the capacity of the lithium ion secondary battery after the capacity has decreased, a second correction coefficient based on the amount of lithium ions consumed in a chemical formation process of the lithium ion secondary battery, and a third correction coefficient based on the discharge efficiency of the lithium ion secondary battery.

[0037] Although the embodiments of the present invention have been described above, the technical scope of the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention. In addition, the components in the above embodiments can be replaced with well-known components as appropriate without departing from the spirit of the present invention.

Claims

1. A method for recovering the performance of a positive electrode for a lithium ion secondary battery by doping lithium ions into the positive electrode for the lithium ion secondary battery whose capacity has decreased, comprising: The doping of the lithium ions is carried out by discharging in an electrolyte solution using a lithium electrode as a counter electrode, The method for recovering performance of a positive electrode for a lithium ion secondary battery comprises discharging the battery within a range of an accumulated discharge amount DG [Ah] expressed by the following formula 1: 0.95×DG0≦DG≦1.05×DG0 Formula 1 (In the above formula 1, DG0 is a value calculated by the following formula 2. DB×[(DB-DA) / DB+X] Formula 2 In the above formula 2, DB [Ah] is the capacity of the lithium ion secondary battery in an initial state, DA [Ah] is the capacity of the lithium ion secondary battery after the capacity has decreased, and X is a correction coefficient that is set based on at least one of X1, X2, and X3 below. X1: Correction coefficient based on lithium ions consumed in film formation in the chemical formation process of a lithium ion secondary battery in its initial state X2: Correction coefficient based on deterioration of the negative electrode of the lithium ion secondary battery X3: Correction coefficient based on the reaction resistance and migration resistance of lithium ions in a lithium ion secondary battery)

2. The method for recovering the performance of a positive electrode for a lithium ion secondary battery according to claim 1, which is carried out non-destructively.

3. 2. The method for recovering performance of a positive electrode for a lithium ion secondary battery according to claim 1, wherein the discharge is carried out at a constant current.

4. The method for recovering performance of a positive electrode for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the (DB-DA) / DB is 0.7 or more.

5. 2. The method for recovering performance of a positive electrode for a lithium ion secondary battery according to claim 1, wherein X1 is a numerical value in the range of 0 to 0.25, X2 is a numerical value in the range of 0 to 0.1, and X3 is a numerical value in the range of 0 to 0.

18.

6. The method for recovering performance of a positive electrode for a lithium ion secondary battery according to claim 1 , wherein X3 is calculated by the following formula 3: X3=X3a+X3b+X3c+X3d Formula 3 (In the above formula 3, X3a is a correction coefficient based on the reaction resistance and migration resistance of lithium ions in the positive electrode when the performance-recovered lithium ion secondary battery is used, and is a numerical value within the range of 0 to 0.05; X3b is a correction coefficient based on the reaction resistance and migration resistance of lithium ions in the negative electrode when the performance-recovered lithium ion secondary battery is used, and is a numerical value within the range of 0 to 0.05; X3c is a correction coefficient based on the migration resistance of lithium ions in the electrolyte when the performance-recovered lithium ion secondary battery is used, and is a numerical value within the range of 0 to 0.04; and X3d is a correction coefficient based on the migration resistance of lithium ions in the separator when the performance-recovered lithium ion secondary battery is used, and is a numerical value within the range of 0 to 0.04.)

7. A method for recovering the performance of a positive electrode for a lithium ion secondary battery by doping lithium ions into the positive electrode for the lithium ion secondary battery whose capacity has decreased, comprising: The doping of the lithium ions is carried out by discharging in an electrolyte solution using a lithium electrode as a counter electrode, The doping of the lithium ions is controlled based on an integrated value of current during energization, an integrated value of current when the energization is completed is set based on a difference between a capacity of the lithium ion secondary battery in an initial state and a capacity of the lithium ion secondary battery whose capacity has decreased, and a correction coefficient; the correction coefficient is set based on at least one of a first correction coefficient based on the difference between the capacity of the lithium-ion secondary battery in an initial state and the capacity of the lithium-ion secondary battery whose capacity has been reduced, a second correction coefficient based on the amount of lithium ions consumed in a chemical formation process of the lithium-ion secondary battery, and a third correction coefficient based on the discharge efficiency of the lithium-ion secondary battery.

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