Method for restoring performance of lithium-ion secondary batteries

Abstract

To provide a performance recovery method for a lithium-ion secondary battery that can achieve an optimal recovered state of the lithium-ion secondary battery.SOLUTION: In a method for recovering performance of a lithium-ion secondary battery by doping lithium ions into a positive electrode included in the lithium-ion secondary battery whose capacity has been reduced, 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 to a predetermined potential V_E (V).SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to a method for recovering the performance of 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 batteries in vehicles is also being considered. The performance of lithium-ion batteries generally deteriorates with repeated charging and discharging. Various methods have been proposed to restore the performance of lithium-ion 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 battery, which involves cleaning the electrodes of a used lithium-ion 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 secondary battery device that can prevent the loss of the potential measurement function of the third electrode in a secondary battery in which the third electrode for measuring the potential of the positive electrode and the negative electrode also serves as a source of lithium ions to the positive electrode and the negative electrode. Patent Document 4 discloses a capacity recovery method for a lithium-ion secondary battery using a third electrode, in which the potential difference (V) between the positive electrode and the third electrode is measured and the capacity recovery process is stopped when the measured potential difference reaches a predetermined stop reference value. Here, the stop reference value is set by first establishing conduction between the positive electrode and the third electrode of a reference lithium-ion secondary battery having the same configuration as the lithium-ion secondary battery to be capacity recovered, monitoring the potential difference (V) between the positive electrode and the third electrode as it decreases over time from the start of conduction, determining the fluctuation of the potential difference over time (hr) from the monitored potential difference, determining a potential difference decrease fluctuation period, a potential difference fluctuation transient period, and a potential difference decrease stable period from the potential difference fluctuation, and adopting the potential difference corresponding to the potential difference fluctuation transient period. [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 Publication No. 2021-151169 [Patent Document 4] Japanese Patent Application Publication No. 2017-091923 Summary of the Invention [Problem to be solved by the invention]

[0004] Furthermore, none of Patent Documents 1 to 4 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 recovering the performance of a lithium ion secondary battery that can appropriately recover the performance of the 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 recovering the performance of a lithium ion secondary battery by doping lithium ions into a positive electrode included in 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, and the discharging is carried out up to a potential VE (V) expressed by the following formula 1. 0.90×VB≦VE≦1.10×VB Formula 1 (In the above formula 1, VB is y when x=0 in the following function shown in the following formula 2. y=f(x) Equation 2 In the formula 2, x is the capacity of the positive electrode included in the lithium ion secondary battery in the initial state, and y is the potential of the positive electrode included in the lithium ion secondary battery in the initial state. [2] The method for recovering performance of a lithium ion secondary battery according to [1], which is carried out without destroying the positive electrode. [3] The method for recovering performance of a lithium ion secondary battery according to [1] or [2], wherein the discharge is carried out under constant current and constant voltage conditions. [4] determining a mode of capacity decline by comparing a differential capacity curve of the lithium ion secondary battery whose capacity has declined with a differential capacity curve of the lithium ion secondary battery in an initial state; The method for recovering performance of a lithium ion secondary battery according to any one of [1] to [3], wherein it is determined whether or not to perform the discharging based on the determined mode of capacity decrease. [5] By comparing a differential capacity curve 1 of the lithium ion secondary battery whose capacity has decreased with a differential capacity curve 2 of the lithium ion secondary battery in an initial state, it is determined whether the mode of the capacity decrease is due to (1) structural deterioration of a positive electrode active material contained in a positive electrode, (2) a decrease in negative electrode capacity, or (3) a deviation between a positive electrode potential and a negative electrode potential, and it is determined whether or not to perform the discharge based on the determination result; The performance recovery method for a lithium ion secondary battery according to [4], wherein the differential capacity curve 1 is a differential capacity curve obtained by differentiating the following formula 3 with respect to x1, and the differential capacity curve 2 is a differential capacity curve obtained by differentiating the following formula 4 with respect to x2: y1=f(x1) Equation 3 (In the above formula 3, x1 is the capacity of the lithium ion secondary battery whose capacity has decreased, and y1 is the potential of the lithium ion secondary battery whose capacity has decreased.) y2=f(x2) Equation 4 (In the above formula 4, x2 is the capacity of the lithium ion secondary battery in the initial state, and y2 is the potential of the lithium ion secondary battery in the initial state.) [6] The method for recovering performance of a lithium ion secondary battery according to [5], wherein the classification into modes (1), (2), and (3) is performed based on the following criteria: Mode (1): The width of the peak derived from the positive electrode differs between the differential capacity curve 1 and the differential capacity curve 2. Mode (2): The width of the peak derived from the negative electrode differs between the differential capacity curve 1 and the differential capacity curve 2. Mode (3): The position of the peak derived from the positive electrode or the position of the peak derived from the negative electrode differs between differential capacity curve 1 and differential capacity curve 2. [7] The method for recovering performance of a lithium ion secondary battery according to [4], wherein the discharge is performed when it is confirmed that there is no capacity reduction in the mode (1) but that there is a capacity reduction in the mode (2) or the mode (3). [8] A method for recovering the performance of a lithium ion secondary battery by doping lithium ions into a positive electrode included in a 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 the potential of the positive electrode during current application, the potential of the positive electrode when the energization is completed is set based on the potential of the positive electrode in an initial state of the lithium ion secondary battery. A method for restoring the performance of a lithium-ion secondary battery. [9] The potential of the positive electrode when the energization is completed is set based on the potential of the positive electrode in the initial state of the lithium ion secondary battery when the charging rate is equal to or lower than a predetermined value. [8] The method for recovering performance of a lithium-ion secondary battery according to [8].

[10] The potential of the positive electrode when the energization is completed is set based on the potential of the positive electrode when the charging rate of the lithium ion secondary battery is 0% in an initial state. [9] The method for recovering performance of a lithium-ion secondary battery according to [9].

[11] The performance recovery method for a lithium ion secondary battery according to any one of [8] to

[10] , wherein the potential of the positive electrode when the current flow is completed is set based on the open-end potential of the positive electrode of the lithium ion secondary battery in an initial state. [Effects of the Invention]

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

[0008] [Figure 1] 1 is a graph illustrating factors that cause a decrease in capacity of a lithium ion secondary battery. [Figure 2] FIG. 2 is a diagram illustrating a configuration for performing discharge in one embodiment of the performance recovery method of the present invention. [Figure 3] FIG. 2 is a diagram for explaining an example of a dV / dQ curve at each of a positive electrode and a negative electrode of a lithium ion secondary battery in an initial state. [Figure 4] FIG. 1 is a diagram for explaining an example of a curve obtained by fitting an actually measured dV / dQ curve of a lithium ion secondary battery. [Figure 5] FIG. 10 is a diagram for explaining a comparison between a dV / dQ curve in an initial state and a dV / dQ curve in a deteriorated state. [Figure 6] 1 is a graph showing the relationship between the capacity retention rate (%) after recovery treatment and the capacity retention rate (%) in a deteriorated state of a lithium ion secondary battery in Comparative Example 1. [Figure 7] 1 is a graph illustrating the recovery state of a lithium ion secondary battery in Comparative Example 1. [Figure 8] FIG. 2 is a diagram showing the flow of a performance recovery method of the present invention. 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 lithium ion secondary battery whose capacity has decreased by doping lithium ions into a positive electrode included in the lithium ion secondary battery. The doping of the lithium ions is carried out by discharging in an electrolyte solution using a lithium electrode as a counter electrode, and the discharging is carried out up to a potential VE (V) expressed by the following formula 1: 0.90×VB≦VE≦1.10×VB Formula 1 (In the above formula 1, VB is y when x=0 in the following function shown in the following formula 2. y=f(x) Equation 2 In the formula 2, x is the capacity of the positive electrode included in the lithium ion secondary battery in the initial state, and y is the potential of the positive electrode included in the lithium ion secondary battery in the initial state. Here, the term "initial state" means that the lithium ion secondary battery is unused or that the lithium ion secondary battery is not deteriorated, i.e., the capacity of the lithium ion secondary battery is not reduced by charge / discharge cycles. More specifically, the initial state is preferably the state at the time when formation is completed. 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, 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 2. That is, as shown in Figure 2, 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 up to a potential VE (V) expressed by the following formula 1 (hereinafter, this step may be referred to as a "recovery process"). 0.90×VB≦VE≦1.10×VB Formula 1 (In the above formula 1, VB is y when x=0 in the following function shown in the following formula 2. y=f(x) Equation 2 In the formula 2, x is the capacity of the positive electrode included in the lithium ion secondary battery in the initial state, and y is the potential of the positive electrode included in the lithium ion secondary battery in the initial state.

[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] "Potential VE(V)" The potential VE (V) satisfies the above formula 1, preferably satisfies the following formula 1a, and more preferably satisfies the following formula 1b. 0.95×VB≦VE≦1.05×VB Equation 1a 0.98×VB≦VE≦1.02×VB Equation 1b (In the above formula, VB is as defined in Formula 1.) By discharging to a potential VE (V) 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 greater than the initial capacity, the amount of Li in the positive electrode becomes excessive, and Li precipitation may occur when this positive electrode is reassembled into a battery. From this perspective, the recovery rate, expressed as the ratio (%) (xd / x × 100) of the capacity of the positive electrode after doping to the capacity (x) of the positive electrode contained in the initial state of the lithium-ion secondary battery, is preferably 90 to 110%, more preferably 95 to 102%, and most preferably substantially 100%. Conventional methods have not been able to control the recovery of only the capacity that has decreased due to use, and have not been able to prevent the capacity from becoming 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 potential VE (V) that satisfies the above.

[0026] It is preferable that x (the capacity of the positive electrode included in the lithium ion secondary battery in an initial state) and y (the potential of the positive electrode included in the lithium ion secondary battery in an initial state) in the above formula 2 are calculated based on the SOC (State Of Charge)-OCP (Open Circuit Potential) obtained in advance for the positive electrode included in a lithium ion secondary battery having the same specifications as the lithium ion secondary battery to be subjected to the recovery process.

[0027] "Discharge conditions" The discharge is preferably carried out under constant current, constant voltage (CCCV) conditions. More specifically, it is preferably carried out under the following conditions. After discharging at a constant current value (I0) until a predetermined voltage (V0) is reached, the control switches to maintaining the predetermined voltage (V0), and discharge ends when the current value decays to a predetermined value (I1). V0: Voltage at the intercept point of the capacitance-voltage curve I0: A current rate of 0.3 C or less, preferably 0.1 C or less. (The lower the current rate I0, the more accurate the recovery amount becomes, but the longer the discharge process time becomes. From this perspective, the current rate I0 is preferably 0.01 C or more.) I1: A current rate equal to or less than half of I0, preferably equal to or less than 0.05 C. (The lower the current rate I1, the more accurate the recovery amount, but the longer the discharge process time. From this perspective, the current rate I1 is preferably equal to or greater than 0.001 C.) Here, the current value at which the rated capacity of a new battery can be discharged in one hour is 1C.

[0028] "Identification and classification of capacity degradation modes (factors)" In the performance recovery method for a lithium ion secondary battery of this embodiment, it is preferable to compare a differential capacity curve 1 of the lithium ion secondary battery whose capacity has decreased with a differential capacity curve 2 of the lithium ion secondary battery in an initial state, classify the modes (causes) of the capacity decrease into three modes (causes): (1) structural deterioration of the positive electrode active material contained in the positive electrode, (2) a decrease in the negative electrode capacity, and (3) a difference between the positive electrode potential and the negative electrode potential, and determine whether or not to perform the discharge based on the classification results. Here, the differential capacity curve 1 is a differential capacity curve obtained by differentiating the following formula 3 with x1, and the differential capacity curve 2 is a differential capacity curve obtained by differentiating the following formula 4 with x2. y1=f(x1) Equation 3 (In the above formula 3, x1 is the capacity of the lithium ion secondary battery whose capacity has been reduced, and y1 is the voltage or single-electrode potential of the lithium ion secondary battery whose capacity has been reduced.) y2=f(x2) Equation 4 (In the above formula 4, x2 is the capacity of the lithium ion secondary battery in the initial state, and y2 is the voltage or single-electrode potential of the lithium ion secondary battery in the initial state.) Figures 1(a), (b), and (c) show the image of the capacity reduction due to the above factors (1), (2), and (3).

[0029] The method for obtaining the differential capacity curve 1 and the differential capacity curve 2 and the method for classifying the modes will be described below. <Advance preparation> As a preliminary step, for example, a battery with the same specifications as the target lithium-ion secondary battery is first disassembled, and single-electrode data for each coin cell is obtained. This involves obtaining the dV / dQ curves for the initial positive and negative electrodes, as shown in Figure 3. These single-electrode dV / dQ curves are then added together and fitted to the measured curve for the lithium-ion secondary battery (the dotted curve in Figure 4), as shown in Figure 4. The fitted curve is shown as a solid line in Figure 4. This allows the extreme values ​​of the measured curve to be assigned to the positive and negative electrodes. In this way, the dV / dQ curve (differential capacity curve 2) for the initial state of a used lithium-ion secondary battery with reduced capacity is obtained in advance. The peak positions of the dV / dQ curve for the positive and negative electrodes are then determined, along with the peak widths for the initial state of the positive and negative electrodes.

[0030] 3 and 4, the horizontal axis indicates the cell capacity (Ah), and the vertical axis indicates the amount of change in voltage relative to a change in reference capacity (dV / dQ).

[0031] Here, assuming a fully charged state as 100%, if a peak position can be detected between 5% and 95% of the charge rate or state of charge (SOC: State Of Charge), differential capacity analysis becomes possible. Specifically, in the dV / dQ curve (differential capacity curve 2) in the initial state, two peaks based on the positive electrode are identified, and the width between these peaks is determined. In Figures 3 and 4, the two peaks based on the positive electrode are located at the intersections with the dotted line. Furthermore, in the dV / dQ curve (differential capacity curve 2) in the initial state, two peaks based on the negative electrode are identified, and the width between these peaks is determined. In Figures 3 and 4, the two peaks based on the negative electrode are located at the intersections with the dotted line.

[0032] For example, as a guide, the two peaks between 0 and 30% on the low SOC side may be determined as the positive electrode peaks, the two peaks between 30 and 60% as the negative electrode (graphite in this embodiment) peaks (intersections with the dotted line Gr), and the two peaks between 60 and 100% on the high SOC side as the silicon oxide (SiO) peaks in the negative electrode (intersections with the dotted line SiO). The positive electrode peaks can also be determined by being convex on the negative side of dV / dQ, and the negative electrode peaks by being convex on the positive side of dV / dQ. Note that although peaks originating from the materials constituting the electrodes will appear, for example, if the negative electrode does not contain SiO, no peaks originating from SiO will appear.

[0033] <After battery capacity decreases> For the lithium-ion secondary battery whose capacity has decreased due to use, the voltage and current are continuously measured by charging at a low current. It is important that the current be relatively low. The capacity is obtained by integrating the current value with respect to time, and a curve of the voltage value versus the capacity is obtained from this. Furthermore, a differential capacity curve 1 (dV / dQ curve) versus the capacity can be obtained by differentiating the voltage value with respect to the capacity.

[0034] Analysis is possible even with a typical charging rate of 0.2 to 0.5 C. However, a lower rate of, for example, 0.02 to 0.07 C allows for more accurate analysis. Note that once charging has progressed to a certain extent and the voltage has stabilized, charging can be continued by gradually reducing the current.

[0035] As described above, the dV / dQ curve (differential capacity curve 1) is obtained by calculating the differential value of the voltage on the charge / discharge curve of a lithium-ion secondary battery at a reference capacity. This method makes it possible to accurately recognize the fluctuation characteristics of the voltage with respect to the reference capacity of the battery whose deterioration is to be determined. Therefore, by comparing the generated dV / dQ curve (differential capacity curve 1) for determining deterioration with the dV / dQ curve (differential capacity curve 2) obtained when the battery is in its initial state, it becomes possible to evaluate and determine the degree to which the battery has deteriorated from its initial state at that time.

[0036] Next, based on the obtained dV / dQ curve after capacity reduction (differential capacity curve 1) and the dV / dQ curve in the initial state (differential capacity curve 2), the first capacity reduction rate due to deterioration of the positive electrode, which is mode (1), the second capacity reduction rate due to deterioration of the negative electrode, which is mode (2), and the capacity reduction amount due to the difference between the positive electrode potential and the negative electrode potential, which is mode (3), are evaluated.

[0037] Specifically, for a used battery with reduced capacity, two peaks originating from each electrode are identified, and the change in the width between these two peaks (peak width) between the initial state and the state after the battery has started to be used is evaluated. That is, the peak widths of the positive and negative electrodes after the battery has started to be used are determined, and the change in peak width due to battery use is evaluated by comparing them with the peak widths in the initial state described above.

[0038] Determining Mode (1) and Mode (2) The dV / dQ curve in the initial state (differential capacity curve 2) is compared with the dV / dQ curve after capacity reduction (differential capacity curve 1), and the rate of change in the width between the two peaks based on the positive and negative electrodes is calculated. The rate of change in the width between the two peaks based on the positive electrode of the dV / dQ curve after capacity reduction (differential capacity curve 1) relative to the dV / dQ curve in the initial state (differential capacity curve 2) is defined as the first capacity reduction rate, and the rate of change in the width between the two peaks based on the negative electrode of the dV / dQ curve after capacity reduction (differential capacity curve 1) relative to the dV / dQ curve in the initial state (differential capacity curve 2) is defined as the second capacity reduction rate. Specifically, the first and second capacity reduction rates are calculated as follows: First capacity decrease rate (%)=(peak width originating from positive electrode in differential capacity curve 2−peak width originating from positive electrode in differential capacity curve 1) / peak width originating from positive electrode in differential capacity curve 2×100 Second capacity decrease rate (%)=(negative electrode peak width in differential capacity curve 2−negative electrode peak width in differential capacity curve 1) / negative electrode peak width in differential capacity curve 2×100

[0039] FIG. 5 shows a comparison between the dV / dQ curve (differential capacity curve 2) of the lithium-ion secondary battery in the initial state and the dV / dQ curve (differential capacity curve 1) of the lithium-ion secondary battery after capacity reduction. In FIG. 5, the solid line represents the differential capacity curve 2 in the initial state, and the dotted line represents the differential capacity curve 1 after capacity reduction. The differential capacity curve 1 after capacity reduction is shifted along the vertical axis to avoid overlap. As shown in FIG. 5, comparing the differential capacity curve 2 in the initial state with the differential capacity curve 1 after capacity reduction reveals that the width between the two peaks based on the positive and negative electrodes has changed.

[0040] The presence or absence of capacity degradation in mode (1) and mode (2) is determined based on the first capacity degradation rate and the second capacity degradation rate. Specifically, it is preferable to make the determination based on the following criteria. Mode (1): The width of the peak derived from the positive electrode differs between the differential capacity curve 1 and the differential capacity curve 2. Specifically, when the first capacity decrease rate is 10% or more, preferably 5% or more, it is determined that the peak widths are different, and that a capacity decrease due to mode (1) has occurred. Mode (2): The width of the peak derived from the negative electrode differs between the differential capacity curve 1 and the differential capacity curve 2. Specifically, when the second capacity decrease rate is 10% or more, preferably 5% or more, it is determined that the peak widths are different, and that a capacity decrease due to mode (2) has occurred. In the case of a battery in which a capacity decrease due to mode (1) is observed, it is preferable not to treat the battery by the method of this embodiment because a significant capacity recovery effect cannot be expected by the method of this embodiment. Furthermore, in the case where a deterioration analysis of the positive electrode active material (e.g., analysis by SEM) shows that a capacity decrease is caused by cracks in the positive electrode active material particles, it is also preferable not to treat the battery by the method of this embodiment for the same reason.

[0041] 《Determining Mode (3)》 Mode (3) is based on the shift of the dV / dQ curve (differential capacity curve 1) after capacity reduction from the dV / dQ curve (differential capacity curve 2) in the initial state. The shift of the dV / dQ curve (differential capacity curve 1) after capacity reduction from the dV / dQ curve (differential capacity curve 2) in the initial state is determined to be primarily due to a decrease in the amount of Li involved in charge and discharge due to the deposition of a Li-containing negative electrode film caused by a side reaction on the active material surface. For example, the percentage of shift in the midpoint of the two peaks based on the negative electrode is taken as the capacity reduction rate (%) due to the difference between the positive electrode potential and the negative electrode potential. The presence or absence of capacity reduction due to factor (3) is judged based on the rate of capacity reduction. Specifically, it is preferable to make the judgment based on the following criteria. Mode (3): The position of the peak derived from the negative electrode differs between differential capacity curve 1 and differential capacity curve 2. Specifically, when the rate of capacity decrease is 10% or more, preferably 5% or more, it is determined that a capacity decrease due to mode (3) has occurred.

[0042] 8 shows an example of a flow for carrying out the regenerative performance recovery method of the present invention after measuring the battery capacity and diagnosing the degradation mode (identifying and classifying the capacity degradation mode). This flow will be specifically described below. First, the battery capacity is measured, and then the degradation mode is diagnosed. If the degradation mode is mode (2) or (3), the degraded positive electrode is combined with a lithium electrode as a counter electrode to make a battery, and lithium ions are doped into the positive electrode by the discharge treatment. Then, the recovered positive electrode is combined with the negative electrode to make a battery. In the deterioration mode diagnosis, if the deterioration mode is mode (1), the battery is either reused by a recovery process other than battery regeneration, or discarded. Furthermore, if the deterioration mode diagnosis does not reveal any decrease in capacity, the battery continues to be used.

[0043] In another embodiment of the present invention, there is provided a method for recovering the performance of a lithium ion secondary battery by doping lithium ions into a positive electrode included in 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 the potential of the positive electrode during current application, the potential of the positive electrode when the energization is completed is set based on the potential of the positive electrode in an initial state of the lithium ion secondary battery. A method for restoring the performance of a lithium-ion secondary battery is provided. Here, the potential of the positive electrode when the current flow is completed is preferably set based on the potential of the positive electrode in the initial state of the lithium ion secondary battery when the charging rate is equal to or lower than a predetermined value, more preferably based on the potential of the positive electrode in the initial state of the lithium ion secondary battery when the charging rate is 0%, and even more preferably based on the open end potential of the positive electrode in the initial state of the lithium ion secondary battery.

[0044] (Action and effect) The method for recovering performance of a lithium ion secondary battery according to the present embodiment described above has the following advantages. Regardless of the state of the battery's internal resistance or the state of charge at the time of disassembly, the amount of capacity recovery can be uniquely determined by the positive electrode potential, making it possible to appropriately recover performance. By acquiring the negative electrode potential data at the time of completion of anodization and adjusting the positive electrode potential range accordingly, any voltage range can be used. No special equipment is required for capacity recovery. The internal structure of the battery is the same as that of a regular battery, so there is no trade-off with the initial battery performance. By using the charging data when the battery return device (so-called battery exchanger: BEX) is connected, it is possible to arbitrarily determine the timing for collecting the battery and subjecting it to recovery processing. By predicting or detecting the causes of capacity decline (degradation) and the degree of degradation, it is possible to determine in real time the secondary uses for the battery and the recovery methods to be applied when recycling the positive electrode.

[0045] The technical scope of the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. In addition, the components in the above-described embodiments can be replaced with well-known components as appropriate without departing from the spirit of the present invention. [Example]

[0046] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0047] Example 1 (Lithium-ion secondary battery used) A lithium ion secondary battery made of a ternary positive electrode active material was used. The potential (y) of the positive electrode of this battery was approximately 3.6 V. Therefore, the potential VE according to the above formula 1 is 3.24 to 3.96 V.

[0048] (Creating a degraded sample with reduced capacity) The lithium ion secondary battery was subjected to the following charge / discharge test. The lithium ion secondary battery was placed in a thermostatic chamber at 45°C, and 1000 charge / discharge cycles were repeated at a current value of 0.3 C relative to the rated capacity. This resulted in a sample with reduced capacity (deteriorated). The SOH (State Of Health: capacity retention rate) of the obtained deteriorated sample 1 was 89%. Furthermore, when a differential capacity curve 1 was obtained for this deteriorated sample 1 and compared with a differential capacity curve 2 of the lithium ion secondary battery in the initial state, it was found that a deviation in the negative electrode potential had occurred, and the mode (cause) of the capacity decrease was identified as the mode (3) (capacity decrease due to a potential deviation).

[0049] (electrolyte) The electrolyte was composed of LiPF6 as a salt and an aprotic solvent containing DMC.

[0050] (Counter electrode) Metallic lithium was used as the counter electrode.

[0051] (Discharge test) The deteriorated sample 1 was discharged (Li doped) at 0.1 C and 3.6 V using a constant current constant voltage (CCCV) method until the positive electrode potential reached 3.6 V, based on the potential VE calculated from Equation 1. The positive electrode potential was monitored using a charge / discharge device, and the temperature was set to 25°C. As a result, the SOH recovered to 97%.

[0052] <Comparative Example 1> The preparation conditions for Degraded Sample 1 were changed to prepare Degraded Sample 2 with an SOH of 80% and Degraded Sample 3 with an SOH of 93%, and the same operations as in Example 1 were carried out except that both samples were discharged (Li doping treatment) until the positive electrode potential reached 3 V. As a result, the SOH of degraded sample 2 recovered to 93%, and that of degraded sample 3 recovered to 106%. Figure 6 shows the relationship between the SOH of these degraded samples and the SOH after recovery treatment. Figure 7 also shows the change in capacity over time from the initial state (BOL). (In Figure 7, n=2 means that there were two samples, the same test was performed twice, and the average values ​​were graphed.) These results show that the capacity after recovery treatment may be excessive if the positive electrode potential does not satisfy the formula 1. If the capacity is excessively recovered and becomes larger than the initial state, the amount of Li in the positive electrode becomes excessive, and there is a risk of Li precipitation occurring when this positive electrode is reassembled into a battery.

Claims

1. A method for recovering the performance of a lithium ion secondary battery by doping lithium ions into a positive electrode included in the lithium ion secondary battery whose capacity has decreased, comprising: The doping of lithium ions is performed by discharging in an electrolyte solution using a lithium electrode as a counter electrode, and the discharging is performed by immersing a positive electrode and a lithium electrode as a counter electrode in the electrolyte solution until a potential V (V) between the positive electrode and the lithium electrode satisfies the following formula 1: 0.90×VB≦VE≦1.10×VB Formula 1 (In the above formula 1, VB is y when x=0 in the following function shown in the following formula 2. y=f(x) Formula 2 In the formula 2, x is the capacity of the positive electrode included in the lithium ion secondary battery in the initial state, and y is the potential of the positive electrode included in the lithium ion secondary battery in the initial state.

2. The method for recovering performance of a lithium ion secondary battery according to claim 1, wherein the method is carried out without destroying the positive electrode.

3. 3. The method for recovering performance of a lithium ion secondary battery according to claim 1, wherein the discharging is carried out under constant current and constant voltage conditions.

4. determining a mode of capacity reduction by comparing a differential capacity curve of the lithium ion secondary battery whose capacity has been reduced with a differential capacity curve of the lithium ion secondary battery in an initial state; 3. The method for recovering performance of a lithium ion secondary battery according to claim 1, wherein whether or not to perform the discharging is determined based on the determined mode of capacity decrease.

5. a differential capacity curve 1 of the lithium ion secondary battery whose capacity has decreased and a differential capacity curve 2 of the lithium ion secondary battery in an initial state are compared to determine whether the mode of the capacity decrease is due to (1) structural deterioration of a positive electrode active material contained in a positive electrode, (2) a decrease in negative electrode capacity, or (3) a deviation between a positive electrode potential and a negative electrode potential, and determine whether or not to perform the discharge based on the determination result; 5. The method for recovering performance of a lithium ion secondary battery according to claim 4, wherein the differential capacity curve 1 is a differential capacity curve obtained by differentiating the following formula 3 with respect to x1, and the differential capacity curve 2 is a differential capacity curve obtained by differentiating the following formula 4 with respect to x2: y1=f(x1) Equation 3 (In the above formula 3, x1 is the capacity of the lithium ion secondary battery whose capacity has decreased, and y1 is the potential of the lithium ion secondary battery whose capacity has decreased.) y2=f(x2) Equation 4 (In the above formula 4, x2 is the capacity of the lithium ion secondary battery in the initial state, and y2 is the potential of the lithium ion secondary battery in the initial state.)

6. 6. The method for recovering performance of a lithium ion secondary battery according to claim 5, wherein the modes (1), (2), and (3) are classified based on the following criteria: Mode (1): The width of the peak derived from the positive electrode differs between the differential capacity curve 1 and the differential capacity curve 2. Mode (2): The width of the peak derived from the negative electrode differs between the differential capacity curve 1 and the differential capacity curve 2. Mode (3): The position of the peak originating from the negative electrode differs between differential capacity curve 1 and differential capacity curve 2.

7. 6. The performance recovery method for a lithium ion secondary battery according to claim 5, wherein the discharging is performed when it is confirmed that there is no capacity reduction due to the mode (1) and that there is a capacity reduction due to the mode (2) or the mode (3).

8. A method for recovering the performance of a lithium ion secondary battery by doping lithium ions into a positive electrode included in 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 the potential of the positive electrode during current application, the potential of the positive electrode when the energization is completed is set based on the potential of the positive electrode in an initial state of the lithium ion secondary battery. A method for restoring the performance of a lithium-ion secondary battery.

9. 9. The performance recovery method for a lithium ion secondary battery according to claim 8, wherein the potential of the positive electrode at the time of completing the energization is set based on the potential of the positive electrode in an initial state of the lithium ion secondary battery when the charging rate is equal to or lower than a predetermined value.

10. the potential of the positive electrode when the energization is completed is set based on the potential of the positive electrode when the lithium ion secondary battery is in an initial state and the charging rate is 0%. The method for recovering performance of a lithium ion secondary battery according to claim 9.

11. The performance recovery method for a lithium ion secondary battery according to any one of claims 8 to 10, wherein the potential of the positive electrode when the energization is completed is set based on the open end potential of the positive electrode of the lithium ion secondary battery in an initial state.

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