Performance recovery method of lithium ion secondary battery

Abstract

Provided is a method for recovering the performance of a lithium ion secondary battery with which it is possible to achieve an optimal recovery state of the lithium ion secondary battery. A method for recovering the performance of a lithium ion secondary battery by doping a positive electrode included in a lithium ion secondary battery having reduced capacity with lithium ions, in which the doping of the lithium ions is performed by using a lithium electrode as a discharge of a counter electrode in an electrolyte solution, the discharge being performed to a predetermined potential (VE (V)).

Description

[0001] Cross-references between related applications

[0002] This application claims priority based on Japanese Patent Application No. 2024-057915 filed on March 29, 2024, the contents of which are incorporated herein by reference. Technical Field

[0003] The invention relates to a performance recovery method for a lithium ion secondary battery. Background Art

[0004] In recent years, interest in electric vehicles has increased for CO2 reduction from the perspective of climate-related disasters, and the use of lithium-ion batteries for automotive applications has been studied.

[0005] Lithium-ion batteries generally experience performance degradation due to repeated charge and discharge cycles, and various methods have been proposed to restore the performance of lithium-ion batteries.

[0006] For example, International Publication No. 2022 / 034717 discloses a device that is a capacity recovery device for a secondary battery and includes a capacity estimating unit that calculates an estimated capacity as an estimated value of the capacity of the secondary battery, a capacity recovery processing unit that performs capacity recovery processing on the secondary battery by moving a reactant species from a capacity recovery electrode to a positive electrode or a negative electrode, and an electric quantity calculating unit that calculates an electric quantity that should be supplied to the capacity recovery electrode. The capacity recovery processing unit includes an electric quantity monitoring unit that determines the electric quantity flowing 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.

[0007] Japanese Patent Publication No. 2012-022969 discloses a method for regenerating lithium-ion battery electrodes, wherein a used lithium-ion battery electrode is cleaned with a polar solvent to flush out degraded products containing lithium attached to the surface of active material particles, which are the main component of capacity degradation of the electrode. The battery is then dried thoroughly to volatilize the cleaning solvent, and an electrolyte is reinjected into the battery having the dried electrode.

[0008] Japanese Patent Application Laid-Open No. 2021-151169 discloses a secondary battery device in which a third electrode for measuring the potential of a positive electrode and a negative electrode also serves as a lithium ion supply source to the positive and negative electrodes, wherein loss of the potential measurement function by the third electrode can be prevented.

[0009] Japanese Patent Application Publication No. 2017-091923 discloses a method for restoring the capacity of a lithium-ion secondary battery using a third electrode. The method measures the potential difference (V) between the positive electrode and the third electrode, and stops the capacity restoration process when the measured potential difference reaches a predetermined stop reference value. This stop reference value is used by pre-establishing electrical conduction between the positive electrode and the third electrode of a reference lithium-ion secondary battery having the same structure as the lithium-ion secondary battery to be restored. The potential difference (V) between the positive electrode and the third electrode, which decreases over time from the start of electrical conduction, is monitored. Based on the monitored potential difference, fluctuations in the potential difference over time (hr) are determined. Based on this potential difference fluctuation, a potential difference decrease period, a potential difference change transition period, and a potential difference decrease stabilization period are determined, and the potential difference corresponding to the potential difference change transition period is used. Summary of the Invention

[0010] Furthermore, Patent Documents 1 to 4 do not disclose a means for appropriately controlling the degree of recovery.

[0011] The present invention has been made in view of the above, and an object of the present invention is 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.

[0012] Solutions to Problems

[0013] In order to solve the above-mentioned problems, the present invention proposes the following solutions.

[0014] [1] A method for recovering the performance of a lithium-ion secondary battery, which is performed by doping lithium ions into a positive electrode of a lithium-ion secondary battery whose capacity has been reduced, wherein:

[0015] The doping with lithium ions is performed by discharging in an electrolyte using a lithium electrode as a counter electrode, and the discharge is performed to a potential VE (V) represented by the following formula 1.

[0016] 0.90×VB≤VE≤1.10×VB Formula 1

[0017] (In the above formula 1, VB is y when x=0 in the following function represented by the following formula 2.

[0018] y=f(x) Formula 2

[0019] In the above 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.

[0020] [2] The method for recovering the performance of a lithium-ion secondary battery according to [1], wherein:

[0021] The performance recovery method of the lithium-ion secondary battery is performed in a non-destructive manner on the positive electrode.

[0022] [3] The method for recovering the performance of a lithium-ion secondary battery according to [1] or [2], wherein:

[0023] Discharging is carried out under constant current and constant voltage conditions.

[0024] [4] The method for recovering the performance of a lithium-ion secondary battery according to any one of [1] to [3], wherein:

[0025] By comparing the differential capacity curve of the lithium ion secondary battery after capacity reduction with the differential capacity curve of the lithium ion secondary battery in the initial state, a capacity reduction pattern is determined.

[0026] Based on the determined pattern of capacity reduction, it is determined whether to perform the discharge.

[0027] [5] The method for recovering the performance of a lithium-ion secondary battery according to [4], wherein:

[0028] The performance recovery method of the lithium ion secondary battery includes the following processing: by comparing the differential capacity curve 1 of the lithium ion secondary battery after capacity reduction with the differential capacity curve 2 of the lithium ion secondary battery in the initial state, determining whether the capacity reduction mode is (1) structural degradation of the positive electrode active material included in the positive electrode, (2) reduction in negative electrode capacity, and (3) deviation of the positive electrode potential and the negative electrode potential, and determining whether to perform the discharge based on the determination result,

[0029] The differential capacity curve 1 is a differential capacity curve obtained by differentiating the following equation 3 using x1, and the differential capacity curve 2 is a differential capacity curve obtained by differentiating the following equation 4 using x2.

[0030] y1=f(x1) Formula 3

[0031] (In the above formula 3, x1 is the capacity of the lithium-ion secondary battery after the capacity is reduced, and y1 is the potential of the lithium-ion secondary battery after the capacity is reduced.)

[0032] y²=f(x²) Formula 4

[0033] (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.)

[0034] [6] The method for recovering the performance of a lithium-ion secondary battery according to [5], wherein:

[0035] The classification of the above modes (1), (2), and (3) is based on the following criteria.

[0036] Mode (1): The distance between the peaks from the positive electrode is different between the differential capacity curve 1 and the differential capacity curve 2.

[0037] Mode (2): The distance between the peaks from the negative electrode is different between the differential capacity curve 1 and the differential capacity curve 2.

[0038] Mode (3): The position of the peak from the positive electrode or the position of the peak from the negative electrode is different between the differential capacity curve 1 and the differential capacity curve 2.

[0039] [7] The method for recovering the performance of a lithium-ion secondary battery according to [4], wherein:

[0040] The discharge is performed when it is confirmed that there is no capacity reduction due to the mode (1) and that a capacity reduction occurs due to the mode (2) or the mode (3).

[0041] [8] A method for recovering the performance of a lithium-ion secondary battery, which is performed by doping lithium ions into a positive electrode of a lithium-ion secondary battery whose capacity has been reduced, wherein:

[0042] The lithium ion doping is performed by discharging the electrolyte using a lithium electrode as a counter electrode.

[0043] The lithium ion doping is controlled based on the potential of the positive electrode when power is applied.

[0044] The potential of the positive electrode when energization is completed is set based on the potential of the positive electrode in the initial state of the lithium ion secondary battery.

[0045] [9] The method for recovering the performance of a lithium-ion secondary battery according to [8], wherein:

[0046] The potential of the positive electrode when 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 charge rate is equal to or lower than a predetermined value.

[0047]

[10] The method for recovering the performance of a lithium-ion secondary battery according to [9], wherein:

[0048] The potential of the positive electrode when energization is completed is set based on the potential of the positive electrode in the initial state of the lithium ion secondary battery at a charge rate of 0%.

[0049]

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

[10] , wherein:

[0050] The potential of the positive electrode when 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.

[0051] A method for recovering the performance of a lithium-ion secondary battery capable of appropriately recovering the performance of the lithium-ion secondary battery can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 This is a graph explaining the cause of capacity reduction in lithium-ion secondary batteries.

[0053] Figure 2 This is a diagram for explaining a configuration when discharging is performed in one embodiment of the performance recovery method of the present invention.

[0054] Figure 3 This is a diagram for explaining an example of a dV / dQ curve for each of the positive electrode and the negative electrode in the initial state of a lithium-ion secondary battery.

[0055] Figure 4 This is a diagram for explaining an example of a curve fitted to an actually measured curve of a dV / dQ curve of a lithium-ion secondary battery.

[0056] Figure 5 This is a diagram for explaining the comparison between the dV / dQ curve in the initial state and the dV / dQ curve in the degraded state.

[0057] Figure 6 This is a graph showing the relationship between the capacity retention rate (%) after the recovery process and the capacity retention rate (%) in the degraded state of the lithium ion secondary battery in Comparative Example 1.

[0058] Figure 7 This is a graph illustrating the recovery state of the lithium ion secondary battery in Comparative Example 1.

[0059] Figure 8 It is a diagram showing the flow of the performance recovery method of the present invention. DETAILED DESCRIPTION

[0060] Hereinafter, a method for recovering the performance of a lithium-ion secondary battery according to an embodiment of the present invention will be described with reference to the accompanying drawings.

[0061] The method of this embodiment is a method for recovering the performance of a lithium-ion secondary battery, which is performed by doping a positive electrode included in a lithium-ion secondary battery whose capacity has decreased with lithium ions. The lithium ion doping is performed by discharging in an electrolyte using a lithium electrode as a counter electrode, and the discharge is performed until the potential VE (V) represented by the following formula 1 is reached.

[0062] 0.90×VB≤VE≤1.10×VB Formula 1

[0063] (In the above formula 1, VB is y when x=0 in the following function represented by the following formula 2.

[0064] y=f(x) Formula 2

[0065] In the above 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.

[0066] It should be noted that the "initial state" herein refers to a lithium-ion secondary battery that is unused or undegraded, that is, a state in which the capacity of the lithium-ion secondary battery has not decreased due to charge and discharge cycles. More specifically, the initial state is preferably the state at the time when chemical generation is complete.

[0067] Furthermore, the method of this embodiment is preferably carried out without decomposing the positive electrode into its constituent elements.

[0068] (Lithium-ion secondary battery)

[0069] There are no particular limitations on the lithium-ion secondary battery (hereinafter sometimes simply referred to as a "battery") whose performance is restored by the method of this embodiment, and any known lithium-ion secondary battery can be used as the subject. A lithium-ion secondary battery is generally composed of a positive electrode, a negative electrode, and an electrolyte (electrolyte or solid electrolyte) disposed between the positive and negative electrodes. In addition, a separator (diaphragm) may be provided between the positive and negative electrodes. The positive electrode and the negative electrode each contain an active material, a binder, and a current collector. The structures of the positive and negative electrodes are described below.

[0070] "positive electrode"

[0071] The positive electrode comprises a positive electrode active material, a positive electrode conductive additive, a positive electrode binder, and a positive electrode current collector. The layer composed of the positive electrode active material, the positive electrode conductive additive, and the positive electrode binder is referred to as the positive electrode composite material layer. The positive electrode composite material layer may also be formed on one or both sides of the positive electrode current collector. It should be noted that if the positive electrode active material has sufficient conductivity, the positive electrode conductive additive may not be included in the positive electrode composite material layer.

[0072] The positive electrode active material used as the active material 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 xPO4, LiMnPO4, LiCoPO4, LiNiPO4, etc. The positive electrode active material preferably contains one or more selected from the group consisting of manganese, nickel, and cobalt.

[0073] A positive electrode conductive agent, used as a conductive aid in the positive electrode, assists in forming a conductive path between the positive electrode active material and the positive electrode current collector. The positive electrode conductive agent is not particularly limited as long as it has conductivity. Examples include carbon black such as acetylene black, carbon nanotubes, and graphite such as artificial graphite.

[0074] The positive electrode binder, which serves as 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 copolymers thereof, polyamideimide (PAI), polybenzimidazole, polyethersulfone (PES), maleic anhydride-modified polypropylene, and mixtures thereof. It is preferred that the positive electrode binder contain a crystalline polymer having a melting point. The positive electrode binder is preferably a polymer containing fluorine. Examples of polymers containing fluorine include PVDF and PTFE.

[0075] Examples of the positive electrode current collector include metal foils such as aluminum foil, stainless steel foil, and nickel foil. A carbon coating may also be formed on the positive electrode current collector. Furthermore, the positive electrode current collector may be processed into a mesh shape.

[0076] "negative electrode"

[0077] The negative electrode comprises a negative electrode active material, a negative electrode conductive aid, a negative electrode binder, and a negative electrode current collector. The layer composed of the negative electrode active material, the negative electrode conductive aid, and the negative electrode binder is referred to as the negative electrode composite material layer. The negative electrode composite material layer may be formed on one or both sides of the negative electrode current collector. It should be noted that, as long as the negative electrode active material has sufficient conductivity, the negative electrode conductive aid may not be included in the negative electrode composite material layer.

[0078] The negative electrode active material is not particularly limited as long as it can occlude and release lithium ions. Examples of the negative electrode active material include graphite (artificial graphite, natural graphite), amorphous carbon (hard carbon), mesocarbon microbeads, carbon fibers, and Si materials (silicon, Si alloys, Si oxides).

[0079] The negative electrode conductive aid, serving as a conductive additive for the negative electrode, assists in forming a conductive path between the negative electrode active material and the negative electrode current collector. The negative electrode conductive aid is not particularly limited as long as it is conductive. Examples include carbon black such as acetylene black, carbon nanotubes, and graphite such as artificial graphite.

[0080] The negative electrode binder, which serves as 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 negative electrode binders include carboxymethyl cellulose, polyvinylidene fluoride, polytetrafluoroethylene, polyacrylic acid, fluororubber, styrene-butadiene rubber, and other diene rubbers. The negative electrode binder preferably contains a crystalline polymer with a melting point.

[0081] 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 also be coated with a carbon coating. Furthermore, the negative electrode current collector may be processed into a mesh shape.

[0082] (Process of doping the positive electrode with lithium ions)

[0083] In the method of this embodiment, lithium ion doping is performed by discharging in an electrolyte using a lithium electrode as a counter electrode. Figure 2 The structure for discharging is shown in FIG. Figure 2 As shown, a positive electrode and a lithium electrode serving as a counter electrode are immersed in an electrolyte solution, and a voltage is applied between the positive electrode and the lithium electrode to cause discharge from the lithium metal electrode.

[0084] The discharge is continued until the potential VE (V) is expressed by the following formula 1 (hereinafter, this step may be referred to as “recovery processing”).

[0085] 0.90×VB≤VE≤1.10×VB Formula 1

[0086] (In the above formula 1, VB is y when x=0 in the following function represented by the following formula 2.

[0087] y=f(x) Formula 2

[0088] In the above 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.

[0089] "Counter electrode"

[0090] The counter electrode is a lithium electrode, preferably lithium metal, and can have the same structure as the negative electrode.

[0091] "electrolyte"

[0092] The electrolyte is not particularly limited, and any electrolyte commonly used for lithium-ion secondary batteries can be used. For example, aprotic organic solvents such as vinylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methylpropyl carbonate (MPC), and ethylpropyl carbonate (EPC) can be used.

[0093] In addition, as an electrolyte, an electrolyte in which lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium iodide, lithium chloride, lithium bromide, LiB[OCOCF3]4, LiB[OCOCF2CF3]4, LiPF4(CF3)2, LiN(SO2CF3)2, LiN(SO2CF2CF3)2 or a mixed lithium salt of two or more thereof is dissolved in a solvent of a mixed organic compound of two or more of these aprotic organic solvents can be used.

[0094] "Electric potential VE (V)"

[0095] The potential VE (V) satisfies the above-mentioned formula 1, preferably satisfies the following formula 1a, and more preferably satisfies the following formula 1b.

[0096] 0.95×VB≤VE≤1.05×VB Formula 1a

[0097] 0.98×VB≤VE≤1.02×VB Formula 1b

[0098] (In the formula, VB is as defined by Formula 1.)

[0099] By discharging until the potential VE (V) described above is met, it is possible to prevent the capacity after recovery from being insufficient or excessive compared to the initial capacity. It should be noted that when the capacity is excessively recovered and becomes greater than the initial capacity, the amount of Li in the positive electrode becomes excessive, and when the positive electrode is reassembled into a battery, Li precipitation may occur. From this perspective, the recovery rate, expressed as the ratio (%) of the capacity (xd) of the positive electrode after doping to the capacity (x) of the positive electrode included in the initial state of the lithium-ion secondary battery (x) (xd / x×100), is preferably set to 90-110%, more preferably 95-102%, and most preferably substantially 100%.

[0100] Conventional methods have failed to control the amount of capacity lost due to use, preventing insufficient or excessive capacity after recovery. However, the present invention achieves an optimal recovery state by controlling discharge based on the aforementioned potential VE (V).

[0101] In the above formula 2, x (the capacity of the positive electrode included in the lithium-ion secondary battery in the initial state) and y (the potential of the positive electrode included in the lithium-ion secondary battery in the initial state) are preferably obtained based on the SOC (State Of Charge) - OCP (Open Circuit Potential) of the positive electrode included in a lithium-ion secondary battery of the same specifications as the lithium-ion secondary battery to be recovered.

[0102] Discharge conditions

[0103] The discharge is preferably performed under constant current constant voltage (CCCV) conditions. More specifically, it is preferably performed under the following conditions.

[0104] After discharging at a constant current value (I0) until reaching a predetermined voltage (V0), control is switched to maintain the predetermined voltage (V0), and discharging is terminated when the current value decays to a predetermined value (I1).

[0105] V0: The voltage at the intercept of the capacity-voltage curve

[0106] I0: A current rate of 0.3C or less, preferably 0.1C or less. (The lower the current rate I0, the higher the accuracy of the recovery amount, but the longer the discharge process time. From this point of view, the current rate I0 is preferably 0.01C or more.)

[0107] I1: A current rate of 1 / 2 or less of I0, preferably 0.05C or less. (Lower current rate I1 improves the accuracy of the recovery amount, but the discharge process takes longer. From this perspective, current rate I1 is preferably 0.001C or more.)

[0108] It should be noted that the current value that can discharge the rated capacity of a new battery in one hour is 1C.

[0109] "Identification and Classification of Capacity Degradation Patterns (Causes)"

[0110] In the performance recovery method of the lithium ion secondary battery of the present embodiment, it is preferred that a differential capacity curve 1 of the lithium ion secondary battery after capacity reduction is compared with a differential capacity curve 2 of the lithium ion secondary battery in an initial state, the mode (cause) of capacity reduction is classified into three modes (cause) of (1) structural deterioration of the positive electrode active material included in the positive electrode, (2) reduction in negative electrode capacity, and (3) deviation of the positive electrode potential and the negative electrode potential, and based on the classification results, it is determined whether to perform the discharge.

[0111] Here, the differential capacity curve 1 is a differential capacity curve obtained by differentiating the following equation 3 with respect to x1, and the differential capacity curve 2 is a differential capacity curve obtained by differentiating the following equation 4 with respect to x2.

[0112] y1=f(x1) Formula 3

[0113] (In the above formula 3, x1 is the capacity of the lithium-ion secondary battery after the capacity is reduced, and y1 is the voltage or single-pole potential of the lithium-ion secondary battery after the capacity is reduced.)

[0114] y²=f(x²) Formula 4

[0115] (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.)

[0116] It should be noted that in Figure 1 (a), (b), and (c) show images of capacity reduction caused by the above-mentioned reasons (1), (2), and (3).

[0117] The following describes a method for obtaining the differential capacity curve 1 and the differential capacity curve 2 and a method for classifying the patterns.

[0118] <Preparation>

[0119] As a preliminary preparation, for example, a battery of the same specification as the target lithium-ion secondary battery is first disassembled to obtain single-electrode data in a button cell unit, that is, Figure 3 The dV / dQ curves of the positive electrode and the negative electrode in the initial state are shown in FIG. Then, the dV / dQ curves of these electrodes are added together, as shown in FIG. Figure 4 As shown in the figure, the measured curve of lithium-ion secondary battery (by Figure 4 The dotted line depicts the curve). Figure 4 In the figure, the fitted curve is represented by a solid line. This allows the extreme values ​​of the measured curve to be attributed to the positive and negative electrodes. In this way, the dV / dQ curve (differential capacity curve 2) of the lithium-ion secondary battery in its initial state after use and reduced capacity is obtained in advance. Furthermore, the peak positions of the dV / dQ curve based on the positive and negative electrodes are determined, and the distance between the peaks in the initial state of the positive and negative electrodes is determined.

[0120] It should be noted that in Figure 3 as well as Figure 4 In FIG. 1 , the horizontal axis represents the capacity (Ah) of the single cell, and the vertical axis represents the amount of change in voltage (dV / dQ) relative to a change in a reference capacity.

[0121] Here, the fully charged state is set to 100%. If the peak position can be detected between 5% and 95% of the charge rate or state of charge (SOC: State Of Charge), differential capacity analysis can be performed. 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 distance between these peaks is determined. Figure 3 as well as Figure 4 In the graph, the two peaks (downward peaks) based on the positive electrode are located at the intersection with the dotted line. In addition, in the dV / dQ curve in the initial state (differential capacity curve 2), the two peaks based on the negative electrode are determined, and the distance between these peaks is determined. Figure 3 as well as Figure 4 In the figure, the two peaks based on the negative electrode are located at the intersection with the dotted line.

[0122] For example, as a benchmark, on the lower SOC side, two peaks between 0 and 30% are identified as peaks of the positive electrode, two peaks between 30 and 60% are identified as peaks of the negative electrode (graphite in this embodiment) (intersections with the dashed line Gr), and on the higher SOC side, two peaks between 60 and 100% are identified as peaks of silicon oxide (SiO) in the negative electrode (intersections with the dashed line SiO). Alternatively, identification can be made by convexing the positive electrode peak toward the negative side of dV / dQ and the negative electrode peak toward the positive side of dV / dQ. It should be noted that while peaks originating from the materials that make up the electrodes appear, if, for example, the negative electrode does not contain SiO, the SiO-derived peak will not appear.

[0123] <After battery capacity decreases>

[0124] The lithium-ion secondary battery, after its capacity has decreased due to use, is charged at a low current to obtain voltage and current in a time series. A relatively low current is important. The current value is integrated over time to obtain the capacity, which then yields a voltage-to-capacity curve. Furthermore, by differentiating the voltage value by the capacity, a differential capacity curve 1 (dV / dQ curve) is obtained relative to the capacity.

[0125] Analysis can be performed even at a typical charging rate of 0.2 to 0.5 C. However, a lower rate, such as 0.02 to 0.07 C, allows for more accurate analysis. It should be noted that charging can also be performed by gradually reducing the current when the voltage remains constant after charging has progressed to a certain extent.

[0126] As described above, the dV / dQ curve (differential capacity curve 1) is generated by calculating the differential value of the voltage in the charge / discharge curve of the lithium-ion secondary battery with respect to the reference capacity. This method accurately identifies the voltage fluctuation characteristics of the battery subject to degradation determination relative to the reference capacity. Therefore, by comparing the generated dV / dQ curve for degradation determination (differential capacity curve 1) with the dV / dQ curve (differential capacity curve 2) obtained in the initial state of the battery, the extent of battery degradation from its initial state at that point in time can be evaluated and determined.

[0127] 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 caused by degradation of the positive electrode as the above-mentioned mode (1), the second capacity reduction rate caused by degradation of the negative electrode as the above-mentioned mode (2), and the capacity reduction amount caused by the shift between the positive electrode potential and the negative electrode potential as the above-mentioned mode (3) were evaluated respectively.

[0128] Specifically, for batteries with reduced capacity after use, two peaks originating from each electrode are identified, and changes in the distance between these two peaks (inter-peak distance) between the initial state and the state after the battery begins use are evaluated. Specifically, the inter-peak distances for each of the positive and negative electrodes after the battery begins use are determined and compared with the inter-peak distances in the initial state to evaluate changes in the inter-peak distances due to battery use.

[0129] 《Determination of Mode (1) and Mode (2)》

[0130] 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) to calculate the rate of change in the distance between the two peaks at the positive and negative electrodes. Specifically, the rate of change in the distance between the two peaks at the positive electrode in 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 distance between the two peaks at the negative electrode in 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.

[0131] First capacity reduction rate (%) = (distance between peaks from the positive electrode in differential capacity curve 2 - distance between peaks from the positive electrode in differential capacity curve 1) / distance between peaks from the positive electrode in differential capacity curve 2 × 100

[0132] Second capacity reduction rate (%) = (distance between peaks from the negative electrode in differential capacity curve 2 - distance between peaks from the negative electrode in differential capacity curve 1) / distance between peaks from the negative electrode in differential capacity curve 2 × 100

[0133] exist Figure 5 ] shows a comparison between the dV / dQ curve of the lithium ion secondary battery in the initial state (differential capacity curve 2) and the dV / dQ curve of the lithium ion secondary battery after capacity reduction (differential capacity curve 1). Figure 5 In FIG, the solid line shows the differential capacity curve 2 in the initial state, and the dotted line shows the differential capacity curve 1 after the capacity is reduced. In addition, the differential capacity curve 1 after the capacity is reduced is offset along the vertical axis to avoid overlap. Figure 5 As shown, when the differential capacity curve 2 in the initial state is compared with the differential capacity curve 1 after the capacity decreases, it can be seen that the distance between the two peaks based on the positive electrode and the negative electrode changes.

[0134] The presence or absence of capacity reduction due to mode (1) or mode (2) is determined based on the first capacity reduction rate and the second capacity reduction rate. Specifically, it is preferable to make the determination based on the following criteria.

[0135] Mode (1): The distance between the peaks from the positive electrode is different between the differential capacity curve 1 and the differential capacity curve 2. Specifically, when the first capacity reduction rate is 10% or more, preferably 5% or more, it is determined that the distance between the peaks is different, and it is determined that capacity reduction caused by mode (1) has occurred.

[0136] Mode (2): The distance between the peaks from the negative electrode is different between the differential capacity curve 1 and the differential capacity curve 2. Specifically, when the second capacity reduction rate is 10% or more, preferably 5% or more, it is determined that the distance between the peaks is different, and it is determined that capacity reduction caused by mode (2) has occurred.

[0137] In the case of a battery whose capacity reduction is believed to be caused by mode (1), the significant capacity recovery effect of the method according to this embodiment is not significant, and therefore it is preferable not to treat it as a target of treatment according to the method according to this embodiment. In addition, in the case where the capacity reduction is caused by cracks in the positive electrode active material particles through degradation analysis of the positive electrode active material (for example, analysis based on SEM), it is also preferable not to treat it as a target of treatment according to the method according to this embodiment for the same reason.

[0138] 《Determination of Mode (3)》

[0139] The above-mentioned pattern (3) is based on the shift of the dV / dQ curve (differential capacity curve 1) after capacity reduction relative to 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 relative to the dV / dQ curve (differential capacity curve 2) in the initial state is determined to be mainly due to the accumulation of negative electrode films containing Li on the surface of the active material due to side reactions, resulting in a decrease in the amount of Li involved in charge and discharge. For example, the ratio of the midpoint shift of the two peaks based on the negative electrode is used as the capacity reduction rate (%) caused by the shift of the positive electrode potential and the negative electrode potential.

[0140] The presence or absence of capacity reduction due to the cause (3) is determined based on the capacity reduction rate. Specifically, it is preferable to make the determination based on the following criteria.

[0141] Mode (3): The position of the peak from the negative electrode differs between differential capacity curve 1 and differential capacity curve 2. Specifically, when the capacity reduction rate is 10% or more, preferably 5% or more, it is determined that capacity reduction due to mode (3) has occurred.

[0142] exist Figure 8 1 shows an example of a flow of a case where the regenerative performance recovery method of the present invention is implemented or not implemented after battery capacity measurement and degradation pattern diagnosis (capacity degradation pattern identification and classification). This flow is described in detail below.

[0143] First, the battery capacity is measured, and then the degradation mode diagnosis is performed. If the degradation mode is mode (2) or (3), the deteriorated positive electrode is combined with a lithium electrode as a counter electrode to form a battery, and the positive electrode is doped with lithium ions through the discharge treatment. Furthermore, the positive electrode that has undergone the recovery treatment is combined with the negative electrode to form a battery.

[0144] In the above-mentioned degradation pattern diagnosis, when the degradation pattern is pattern (1), the battery is submitted to reuse or disposal other than battery regeneration by recovery processing.

[0145] Furthermore, in the degradation mode diagnosis, if no capacity reduction occurs, the battery continues to be used.

[0146] In another embodiment of the present invention, a method for recovering the performance of a lithium-ion secondary battery is provided, which is performed by doping lithium ions into a positive electrode included in a lithium-ion secondary battery with reduced capacity, wherein:

[0147] The lithium ion doping is performed by discharging the electrolyte using a lithium electrode as a counter electrode.

[0148] The lithium ion doping is controlled based on the potential of the positive electrode when power is applied.

[0149] The potential of the positive electrode when energization is completed is set based on the potential of the positive electrode in the initial state of the lithium ion secondary battery.

[0150] Here, the potential of the positive electrode when the power-on is completed is preferably set based on the potential of the positive electrode in the initial state of the lithium-ion secondary battery with a charging rate below a specified value, more preferably set based on the potential of the positive electrode in the initial state of the lithium-ion secondary battery with a charging rate of 0%, and further preferably set based on the open end potential of the positive electrode in the initial state of the lithium-ion secondary battery.

[0151] (Effect)

[0152] The performance recovery method of a lithium ion secondary battery according to the present embodiment described above produces the following effects.

[0153] Independent of the state of the internal resistance of the battery or the state of charge at the time of disassembly, the capacity recovery amount can be uniquely defined by the positive electrode potential, thereby appropriately restoring the performance.

[0154] By acquiring the negative electrode potential data at the time point when the chemical generation is completed and adjusting the positive electrode potential range accordingly, an arbitrary voltage range can be used.

[0155] No special equipment is required for capacity recovery.

[0156] The internal structure of the battery does not change from that of a normal battery, so the battery performance in the initial state is not affected.

[0157] By using the charge data when a battery return device (so-called battery exchanger: BEX) is connected, the timing of collecting the battery and submitting it to the recovery process can be arbitrarily determined.

[0158] By predicting or detecting the cause of capacity reduction (degradation) and the degree of degradation, it is possible to make real-time decisions on the battery's secondary use destination and the recovery method for recycling the positive electrode.

[0159] It should be noted that the technical scope of the present invention is not limited to the embodiment, and various changes can be applied without departing from the scope of the present invention. In addition, without departing from the scope of the present invention, the constituent elements in the embodiment can be appropriately replaced with well-known constituent elements.

[0160] [Example]

[0161] The present invention is further described in detail below with reference to examples, but the present invention is not limited to the following examples.

[0162] <Example 1>

[0163] (Lithium-ion secondary battery used)

[0164] A lithium-ion secondary battery composed 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 obtained from the above formula 1 was 3.24 to 3.96 V.

[0165] (Preparation of Degraded Samples After Capacity Reduction)

[0166] The lithium ion secondary battery was subjected to the following charge and discharge test.

[0167] The lithium-ion secondary battery was placed in a thermostatic chamber at 45° C., and charge and discharge were repeated for 1000 cycles at a current value of 0.3 C relative to the rated capacity.

[0168] Thus, a sample with reduced (degraded) capacity was obtained.

[0169] The obtained SOH (State of Health: capacity retention rate) of the degraded sample 1 was 89%.

[0170] Furthermore, a differential capacity curve 1 was obtained for the deteriorated sample 1 and compared with a differential capacity curve 2 of the lithium-ion secondary battery in the initial state. It was determined that a shift in the negative electrode potential had occurred, and the mode (cause) of capacity reduction was the mode (3) (capacity reduction caused by potential shift).

[0171] (electrolyte)

[0172] The electrolyte solution was set to a structure using LiPF6 as a salt and an aprotic solvent containing DMC.

[0173] (Counter electrode)

[0174] As the counter electrode, metallic lithium was used.

[0175] (Discharge test)

[0176] Degraded sample 1 was discharged (Li doping treatment) at 0.1C and 3.6V using a constant current constant voltage (CCCV) method, based on the potential VE obtained from equation 1, until the positive electrode potential reached 3.6V. The positive electrode potential was monitored on a charge-discharge device at a temperature of 25°C. As a result, the SOH level recovered to 97%.

[0177] <Comparative Example 1>

[0178] The production conditions of degraded sample 1 were changed to produce degraded sample 2 with an SOH of 80% and degraded sample 3 with an SOH of 93%. Both samples were subjected to the same operation as in Example 1 except that discharge (Li doping treatment) was performed until the positive electrode potential reached 3V.

[0179] As a result, the SOH of degraded sample 2 recovered to 93%, and the SOH of degraded sample 3 recovered to 106%. Figure 6 The relationship between the SOH of these degraded samples and the SOH after recovery is shown in FIG. Figure 7 The transition of capacity change from the initial state (BOL) is shown in Figure 7 In the example, n=2 means that the number of samples is 2, and the same experiment is carried out twice, and the average value is plotted in a graph.

[0180] These results show that the capacity after recovery is excessive when the positive electrode potential does not satisfy the above formula 1. If the capacity is excessively recovered and becomes larger than the initial state, the amount of Li in the positive electrode is excessive, and when the positive electrode is assembled into a battery again, Li precipitation may occur.

Claims

1. A method for recovering the performance of a lithium-ion secondary battery, comprising doping a positive electrode of a lithium-ion secondary battery with reduced capacity with lithium ions, wherein: The lithium ion doping is performed by discharging the electrolyte using a lithium electrode as a counter electrode, and the discharge is performed until a potential VE represented by the following formula 1 is reached, where the unit of VE is V: 0.90×VB≤VE≤1.10×VB Formula 1 In the above formula 1, VB is y when x=0 in the following function represented by the following formula 2, y=f(x) Formula 2 In the above 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 the performance of a lithium-ion secondary battery according to claim 1, wherein: The performance recovery method of the lithium-ion secondary battery is performed in a non-destructive manner on the positive electrode.

3. The method for recovering the performance of a lithium-ion secondary battery according to claim 1 or 2, wherein: Discharging is carried out under constant current and constant voltage conditions.

4. The method for recovering the performance of a lithium-ion secondary battery according to claim 1 or 2, wherein: By comparing the differential capacity curve of the lithium ion secondary battery after capacity reduction with the differential capacity curve of the lithium ion secondary battery in the initial state, a capacity reduction pattern is determined. Based on the determined pattern of capacity reduction, it is determined whether to perform the discharge.

5. The method for recovering the performance of a lithium-ion secondary battery according to claim 4, wherein: The performance recovery method of the lithium ion secondary battery includes the following processing: by comparing a differential capacity curve 1 of the lithium ion secondary battery after capacity reduction with a differential capacity curve 2 of the lithium ion secondary battery in an initial state, determining whether the capacity reduction mode is (1) structural degradation of a positive electrode active material included in a positive electrode, (2) reduction in negative electrode capacity, and (3) deviation of positive electrode potential and negative electrode potential, and determining whether to perform the discharge based on the determination result, The differential capacity curve 1 is obtained by differentiating the following equation 3 using x1, and the differential capacity curve 2 is obtained by differentiating the following equation 4 using x2. y1=f(x1) Formula 3 In the above formula 3, x1 is the capacity of the lithium ion secondary battery after the capacity is reduced, y1 is the potential of the lithium ion secondary battery after the capacity is reduced, y²=f(x²) Formula 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 the performance of a lithium-ion secondary battery according to claim 5, wherein: The classification of the above modes (1), (2) and (3) is based on the following criteria: Mode (1): The distance between the peaks from the positive electrode is different between the differential capacity curve 1 and the differential capacity curve 2, Mode (2): The distance between the peaks from the negative electrode is different between the differential capacity curve 1 and the differential capacity curve 2, Mode (3): The position of the peak from the positive electrode or the position of the peak from the negative electrode is different between the differential capacity curve 1 and the differential capacity curve 2.

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

8. A method for restoring the performance of a lithium-ion secondary battery, comprising doping a positive electrode of a lithium-ion secondary battery with reduced capacity with lithium ions, wherein: The lithium ion doping is performed by discharging the electrolyte using a lithium electrode as a counter electrode. The lithium ion doping is controlled based on the potential of the positive electrode when power is applied. The potential of the positive electrode when energization is completed is set based on the potential of the positive electrode in the initial state of the lithium ion secondary battery.

9. The method for recovering the performance of a lithium-ion secondary battery according to claim 8, wherein: The potential of the positive electrode when 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 charge rate is equal to or lower than a predetermined value.

10. The method for recovering the performance of a lithium-ion secondary battery according to claim 9, wherein: The potential of the positive electrode when energization is completed is set based on the potential of the positive electrode in the initial state of the lithium ion secondary battery at a charge rate of 0%.

11. The method for recovering the performance of a lithium-ion secondary battery according to any one of claims 8 to 10, wherein: The potential of the positive electrode when 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.

Images