Method for recovering performance of positive electrode for lithium ion secondary battery

Abstract

Provided is a method for recovering the performance of a positive electrode for a lithium ion secondary battery, with which it is possible to achieve an optimal recovery state of the positive electrode for a lithium ion secondary battery. A method for recovering the performance of a positive electrode for a lithium ion secondary battery by doping a positive electrode for a lithium ion secondary battery, the capacity of which has been reduced, with lithium ions by discharging using a lithium electrode as a counter electrode in an electrolyte solution, the discharge is performed within a prescribed cumulative discharge amount (DG [Ah]) range.

Description

[0001] Cross-references between related applications

[0002] This application claims priority based on Japanese Patent Application No. 2024-057912 filed on March 29, 2024, the contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to a method for recovering the performance of a positive electrode used in 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 secondary batteries for automotive applications has been studied.

[0005] Lithium-ion secondary batteries generally suffer performance degradation due to repeated charge and discharge. Various schemes have been proposed as methods for restoring the performance of lithium-ion secondary batteries. For example, International Publication No. 2022 / 034717 discloses a device that is a capacity recovery device for a secondary battery, and is provided with 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 of the secondary battery by moving the reactants from the capacity recovery electrode to the positive electrode or the negative electrode, and an electric quantity calculating unit that calculates the electric quantity that should be energized to the capacity recovery electrode, the capacity recovery processing unit being provided with 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.

[0006] Japanese Patent Publication No. 2012-022969 discloses a method for regenerating electrodes of lithium-ion secondary batteries, wherein the electrodes of a used lithium-ion secondary battery are 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 the battery having the dried electrodes is refilled with liquid.

[0007] Japanese Patent Publication No. 2002-324585 discloses a non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode other than metallic lithium, and a non-aqueous electrolyte. The non-aqueous electrolyte secondary battery is characterized in that it comprises a third electrode containing metallic lithium, which is not in contact with the electrolyte solution and is not connected to the positive electrode and the negative electrode. Summary of the Invention

[0008] Patent Documents 1 to 3 do not disclose a means for appropriately controlling the degree of recovery.

[0009] 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 positive electrode for a lithium ion secondary battery, which can achieve an optimal recovery state of the positive electrode for the lithium ion secondary battery.

[0010] The solution of the present invention proposes the following structure.

[0011] [1] A method for recovering the performance of a positive electrode for a lithium-ion secondary battery, which is performed by doping a positive electrode for a lithium-ion secondary battery whose capacity has been reduced with lithium ions, wherein:

[0012] The lithium ion doping is performed by discharging the electrolyte using a lithium electrode as a counter electrode.

[0013] The discharge is performed within the range of the cumulative discharge amount DG [Ah] expressed by the following formula 1.

[0014] 0.95×DG0≤DG≤1.05×DG0 Formula 1

[0015] (In the above formula 1, DG0 is the value calculated by the following formula 2.

[0016] DB×[(DB-DA) / DB+X] Formula 2

[0017] In the above formula 2, DB [Ah] is the capacity of the new lithium ion secondary battery, DA [Ah] is the capacity of the lithium ion secondary battery after the capacity is reduced, and X is a correction coefficient selected from the following X1, X2, and X3.

[0018] X1: Correction coefficient based on lithium ions consumed in film formation during the chemical conversion process of the lithium ion secondary battery in the initial state

[0019] X2: Correction factor based on degradation of the negative electrode of the lithium-ion secondary battery

[0020] X3: Correction factor based on the reaction resistance and transfer resistance of lithium ions in lithium-ion secondary batteries)

[0021] [2] The method for recovering the performance of a positive electrode for a lithium-ion secondary battery according to [1], wherein:

[0022] The method for recovering the performance of a positive electrode for a lithium-ion secondary battery is performed non-destructively.

[0023] [3] The method for recovering the performance of a positive electrode for a lithium-ion secondary battery according to [1] or [2], wherein:

[0024] Discharge is carried out at a constant current.

[0025] [4] The method for recovering the performance of a positive electrode for a lithium-ion secondary battery according to any one of [1] to [3], wherein:

[0026] The (DB-DA) / DB is greater than 0.7.

[0027] [5] The method for recovering the performance of a positive electrode for a lithium-ion secondary battery according to [1], wherein:

[0028] The X1 is a value within the range of 0 to 0.25, the X2 is a value within the range of 0 to 0.1, and the X3 is a value within the range of 0 to 0.18.

[0029] [6] The method for recovering the performance of a positive electrode for a lithium-ion secondary battery according to any one of [1] to [5], wherein:

[0030] The X3 is calculated by the following formula 3.

[0031] X3=X3a+X3b+X3c+X3d Formula 3

[0032] (In Formula 3, X3a is a correction coefficient obtained based on the reaction resistance and transfer resistance of lithium ions in the positive electrode when the performance-recovered lithium ion secondary battery is used, and is a value within the range of 0 to 0.05; X3b is a correction coefficient obtained based on the reaction resistance and transfer resistance of lithium ions in the negative electrode when the performance-recovered lithium ion secondary battery is used, and is a value within the range of 0 to 0.05; X3c is a correction coefficient obtained based on the transfer resistance of lithium ions in the electrolyte when the performance-recovered lithium ion secondary battery is used, and is a value within the range of 0 to 0.04; and X3d is a correction coefficient obtained based on the transfer resistance of lithium ions in the separator when the performance-recovered lithium ion secondary battery is used, and is a value within the range of 0 to 0.04.)

[0033] [7] A method for recovering the performance of a positive electrode for a lithium-ion secondary battery, which is performed by doping a positive electrode for a lithium-ion secondary battery whose capacity has been reduced with lithium ions, wherein:

[0034] The lithium ion doping is performed by discharging the electrolyte using a lithium electrode as a counter electrode.

[0035] The lithium ion doping is controlled based on the integrated value of the current during power-on.

[0036] The integrated value of the current at the completion of energization is set based on a difference between the capacity of the lithium ion secondary battery in the initial state and the capacity of the lithium ion secondary battery after the capacity reduction, and a correction factor.

[0037] [8] The method for recovering the performance of a positive electrode for a lithium-ion secondary battery according to [7], wherein:

[0038] The correction coefficient includes at least one of a first correction coefficient obtained based on the difference between the capacity of the lithium ion secondary battery in the initial state and the capacity of the lithium ion secondary battery after the capacity is reduced, a second correction coefficient obtained based on the amount of lithium ions consumed by the chemical generation process of the lithium ion secondary battery, and a third correction coefficient obtained based on the discharge efficiency of the lithium ion secondary battery.

[0039] A method for recovering the performance of a positive electrode for a lithium ion secondary battery can be provided, which can appropriately recover the performance of the positive electrode for the lithium ion secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 This is a diagram for explaining a configuration when discharging is performed in one embodiment of the performance recovery method of the present invention.

[0041] Figure 2 This chart explains how to determine the optimal value of the doping amount by using the capacity deviation of the battery relative to the initial state (BOL) (Vm.cell: measured full cell capacity, Vcal: expected full cell capacity, Vp: calculated positive electrode capacity, Vc: measured positive electrode capacity, Vn: calculated negative electrode capacity, Vn': measured negative electrode capacity).

[0042] Figure 3 This is a graph illustrating the relationship between the degree of reduction in battery SOH and capacity deviation caused by negative electrode degradation.

[0043] Figure 4 It is a schematic diagram explaining the deterioration of the negative electrode.

[0044] Figure 5 This is a graph showing the relationship between the negative electrode Li amount and capacity reduction based on ICP emission spectroscopy.

[0045] Figure 6 A schematic diagram illustrating the types of resistance within a battery.

[0046] Figure 7 This is a graph showing changes in the capacity of the positive electrode active material due to pressing and washing.

[0047] Figure 8 Graph showing the results of performing pressing and cleaning and performing a restoration treatment based on the optimal value of the doping amount when the correction factor X is not taken into consideration. DETAILED DESCRIPTION

[0048] Hereinafter, a method for recovering the performance of a positive electrode for a lithium ion secondary battery according to an embodiment of the present invention will be described with reference to the drawings.

[0049] The method of this embodiment is a method for recovering the performance of a positive electrode for a lithium ion secondary battery by doping a positive electrode for a lithium ion secondary battery whose capacity has decreased with lithium ions, and the lithium ion doping is performed under predetermined conditions described below.

[0050] It should be noted that the “initial state” here refers to a state where the lithium ion secondary battery is unused or has not deteriorated, that is, a state where the capacity of the lithium ion secondary battery has not decreased due to charge and discharge cycles.

[0051] Furthermore, the method of this embodiment is preferably carried out without decomposing the positive electrode into its constituent elements.

[0052] (Lithium-ion secondary battery)

[0053] 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.

[0054] "positive electrode"

[0055] 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.

[0056] 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 x PO4, LiMnPO4, LiCoPO4, LiNiPO4, etc. The positive electrode active material preferably contains one or more selected from the group consisting of manganese, nickel, and cobalt.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] "negative electrode"

[0061] 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.

[0062] 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).

[0063] 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.

[0064] 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.

[0065] 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.

[0066] (Process of doping the positive electrode with lithium ions)

[0067] In the method of this embodiment, the lithium ion doping is performed by discharging in an electrolyte using a lithium electrode as a counter electrode. Figure 1 The structure for discharging is shown in FIG. Figure 1 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.

[0068] The discharge is performed within the range of the cumulative discharge amount DG [Ah] expressed by the following formula 1 (hereinafter, this step may also be referred to as “recovery processing”).

[0069] 0.95×DG0≤DG≤1.05×DG0 Formula 1

[0070] (In the above formula 1, DG0 is the value calculated by the following formula 2.

[0071] DB×[(DB-DA) / DB+X] Formula 2

[0072] In the above formula 2, DB [Ah] is the capacity of the lithium ion secondary battery in the initial state, DA [Ah] is the capacity of the lithium ion secondary battery after the capacity is reduced, and X is a correction coefficient selected from the following X1, X2, and X3.

[0073] Here, in the above formula 2, DB [Ah] may be the capacity of the positive electrode for the lithium ion secondary battery in the initial state, and DA [Ah] may be the capacity of the positive electrode for the lithium ion secondary battery after the capacity is reduced.

[0074] X1: Correction coefficient based on lithium ions consumed in film formation during the chemical conversion process of the lithium ion secondary battery in the initial state

[0075] X2: Correction factor based on degradation of the negative electrode of the lithium-ion secondary battery

[0076] X3: Correction factor based on the reaction resistance and transfer resistance of lithium ions in lithium-ion secondary batteries)

[0077] The (DB-DA) / DB is preferably 0.7 or greater. If the (DB-DA) / DB is 0.7 or greater, the performance of the positive electrode for a lithium ion secondary battery can be more reliably restored by the method of this embodiment.

[0078] In addition, the discharge is preferably performed at a constant current (CC).

[0079] "Counter electrode"

[0080] The counter electrode is a lithium electrode, preferably lithium metal, and can have the same structure as the negative electrode.

[0081] "electrolyte"

[0082] 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.

[0083] 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.

[0084] "Cumulative discharge capacity DG [Ah]"

[0085] The cumulative discharge amount DG [Ah] satisfies Formula 1, and preferably satisfies the following Formula 1a.

[0086] 0.95×DG0≤DG≤1.05×DG0 Formula 1

[0087] 0.98×DG0≤DG≤1.02×DG0 Formula 1a

[0088] By discharging within the range of the cumulative discharge capacity DG [Ah] described above, 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. When the positive electrode is reassembled into a battery, dendrites (also called dendrites) caused by the precipitation of Li metal may form. From this perspective, the recovery rate, expressed as the ratio (%) of the capacity (xd) of the lithium-ion secondary battery after doping to the capacity (x) of the lithium-ion secondary battery in its initial state (x) (xd / x×100), is preferably set to 90-110%, more preferably 95-105%, and most preferably substantially 100%. Alternatively, the recovery rate, which is expressed as the ratio (%) of the capacity (xd) of the positive electrode for the lithium ion secondary battery after doping to the capacity (x) of the positive electrode for the lithium ion secondary battery in the initial state (x) (xd / x×100), is preferably set to 90 to 110%, more preferably 95 to 105%, and most preferably substantially 100%.

[0089] 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 requirement for the cumulative discharge capacity DG [Ah].

[0090] A method for determining the range of the cumulative discharge amount DG [Ah] will be described.

[0091] First, the optimal value of the doping amount due to discharge (without considering the value of the correction coefficient X) can be determined as follows: For example, charge and discharge are performed under the following conditions.

[0092] Upper limit: 3V discharge (0.1C constant current (CC) discharge).

[0093] Lower limit: 4.15V charging (0.1C constant current constant voltage (CCCV) charging).

[0094] The 0.1C (or 0.05C) capacity of the single cell before disassembly is measured in advance, and the capacity usage range and capacity deviation of the positive and negative electrodes are fitted and analyzed. The capacity deviation of the single cell relative to the initial state (BOL) is determined as the optimal value of the doping amount (refer to Figure 2 ).

[0095] Alternatively, the open circuit voltage (OCV) of the cell during 3V discharge and the open circuit potential (OCP) of the positive electrode after disassembly are measured. Based on the SOC (State of Charge)-OCV and SOC-OCP curves, the capacity shift of the cell relative to the initial state (BOL) is determined as the optimal value of the doping amount.

[0096] For example, if the SOH (State of Health) decreases to 90% from the initial state, and the initial capacity is 8.0 Ah, then the optimal doping amount becomes 0.8 Ah by multiplying 8.0 Ah by (100% - 90%). Therefore, ignoring the correction factor X, discharge is performed from the counter electrode, and the discharge amount from the start of discharge is accumulated, ending when the accumulated value reaches 0.8 Ah.

[0097] If the SOH decreases to 80% from the initial state, and the initial capacity is 8.0 Ah, then the optimal doping amount becomes 1.6 Ah by multiplying 8.0 Ah by (100% - 80%). Therefore, ignoring the correction factor X, discharging from the counter electrode and accumulating the discharge amount from the start of discharge will terminate when the accumulated value reaches 1.6 Ah.

[0098] Controlling discharge based on the accumulated discharge capacity DG [Ah] offers several advantages. Specifically, regardless of the potential, control can be performed solely based on the accumulated current value. Furthermore, since the battery capacity itself (Ah) is used for evaluation, direct control based on the accumulated current is possible. Furthermore, control based solely on the current can be performed without using a voltage sensor.

[0099] However, when discharging is controlled based on the optimal value obtained by assuming an ideal state as described above, it is inevitable that the capacity after the recovery process will be insufficient or excessive compared to the capacity in the initial state. Therefore, the correction coefficient X needs to be taken into consideration.

[0100] Hereinafter, each correction coefficient X (X1, X2, X3) will be described.

[0101] <X1>

[0102] X1 is a correction coefficient obtained based on lithium ions consumed in film formation in the chemical conversion process of the lithium ion secondary battery in the initial state, and is preferably a value within the range of 0 to 0.25.

[0103] Specifically, X1 is a correction factor that takes into account the effects of chemical generation. It is calculated by dividing the percentage (%) of Li consumed in film formation due to chemical generation by 1 / 100. Generally, the value of X1 decreases as the number of chemical generation events increases. The percentage (%) of Li consumed in film formation due to chemical generation varies, for example, as follows.

[0104] Chemical generation for the first time: 20.3%

[0105] Chemical generation second time: 1.4%

[0106] Chemical generation for the third time: 0.8%

[0107] Chemical generation fourth time: 0.4%.

[0108] The correction coefficient X1 is obtained based on the average value of the experimental results shown in Table 1.

[0109]

Table 1

[0110]

[0111] Note: “1st efficiency”, “2nd efficiency” and “3rd efficiency” refer to the ratio (%) of Li not consumed in the film formation during the first, second and third chemical generation steps, respectively.

[0112] Note that the correction coefficient X1 may be a value measured after the performance recovery process.

[0113] <X2>

[0114] X2 is a correction factor based on the degradation of the negative electrode of the lithium-ion secondary battery, and is preferably a value within the range of 0 to 0.1. Specifically, X2 is a correction factor that takes into account the effects of negative electrode degradation and is calculated by dividing the capacity deviation (%) due to negative electrode degradation by 1 / 100.

[0115] The degree of reduction in battery SOH is directly proportional to the capacity deviation caused by negative electrode degradation (refer to Figure 3 The main reasons for the decrease in SOH are the decrease in positive electrode capacity and the shift in negative electrode capacity. However, in any case, Li is accumulated on the negative electrode side. That is, the capacity decrease progresses due to the deactivation of Li caused by the formation of SEI (Solid Electrolyte Interface) in the negative electrode (refer to Figure 4 ).

[0116] By analyzing the battery using ICP emission spectroscopy, the relationship between the negative electrode Li content and capacity reduction can be confirmed (refer to Figure 5 ).

[0117] When the correction coefficient X2 is determined based on the SOH, it can be estimated based on the number of charge and discharge cycles or determined based on the dQ / dV curve.

[0118] For example, when the SOH decreases by 10% due to use from the initial state, the negative electrode capacity deviation is 1.2% (X2=0.012), and X2 can be obtained. When the SOH decreases by 20%, the negative electrode capacity deviation is 2.4% (X2=0.024), and X2 can be obtained.

[0119] <X3>

[0120] X3 is a correction coefficient obtained based on the reaction resistance and transfer resistance of lithium ions in the lithium ion secondary battery, and is preferably a value within the range of 0 to 0.18.

[0121] More specifically, X3 is preferably calculated by the following formula 3.

[0122] X3=X3a+X3b+X3c+X3d Formula 3

[0123] (In Formula 3, X3a is a correction coefficient obtained based on the reaction resistance and transfer resistance of lithium ions in the positive electrode when the performance-recovered lithium ion secondary battery is used, and is a value within the range of 0 to 0.05; X3b is a correction coefficient obtained based on the reaction resistance and transfer resistance of lithium ions in the negative electrode when the performance-recovered lithium ion secondary battery is used, and is a value within the range of 0 to 0.05; X3c is a correction coefficient obtained based on the transfer resistance of lithium ions in the electrolyte when the performance-recovered lithium ion secondary battery is used, and is a value within the range of 0 to 0.04; and X3d is a correction coefficient obtained based on the transfer resistance of lithium ions in the separator when the performance-recovered lithium ion secondary battery is used, and is a value within the range of 0 to 0.04.)

[0124] exist Figure 6 The types of resistors in the battery are shown in FIG.

[0125] That is, X3 is a correction coefficient that takes into account the resistance in the battery, and is a value obtained by dividing the total (%) of various resistances by 1 / 100.

[0126] Each resistance component can be measured using an AC impedance method, a multipoint probing method, or the like.

[0127] <Other correction factors>

[0128] When a capacity recovery treatment other than lithium ion doping (hereinafter sometimes referred to as "other recovery treatment") is performed, the amount of capacity recovered due to the other recovery treatment is preferably also considered as the correction factor XN. Furthermore, the amount of crystalline structure destruction of the positive electrode estimated by impedance measurement is preferably also considered as the correction factor XN. The correction factor XN is preferably 0 to 0.2.

[0129] In this case, in the above-mentioned formula 1, DG0 is preferably a value calculated by the following formula 2a.

[0130] DB×[(DB-DA) / DB+X-XN] Formula 2a

[0131] Other recovery processes include pressing, cleaning, and film removal. For example, if the capacity recovered by pressing, cleaning, and film removal is 1%, 6%, and 1%, respectively, the capacity recovered by other recovery processes is 8%. This value, calculated as 1 / 100, is 0.08, which is used as the correction factor XN.

[0132] The pressing is performed by compressing the positive electrode in the thickness direction. Compression can be performed by a known means such as roll pressing.

[0133] After observing the change in the capacity of the positive electrode active material when pressed at 80°C, 110°C, and 130°C, it became Figure 7 The results shown.

[0134] Furthermore, a known method can be used for cleaning. For example, it is preferable to clean the positive electrode using a solvent such as dimethyl carbonate (DMC), acetone, or propylene carbonate (PC).

[0135] Regarding the film removal, a known method can also be employed.

[0136] Regarding the battery whose capacity decreased by 12% from the initial state (BOL) due to use, after cleaning the positive electrode with dimethyl carbonate (DMC), acetone, and propylene carbonate (PC), it became Figure 7 Here, for example, in the case of DMC cleaning, it becomes XN0.07.

[0137] It should be noted that in Figure 8 The results of the restoration process performed by combining pressing and cleaning and based on the optimal value of the doping amount when the correction factor X is not considered are shown in FIG. Figure 8 It is clear that the recovery rate is about 96%, which cannot be said to be a full recovery.

[0138] In another embodiment of the present invention, a method for recovering the performance of a positive electrode for a lithium ion secondary battery is provided, wherein the method is performed by doping lithium ions into a positive electrode for a lithium ion secondary battery after the capacity of the positive electrode has been reduced.

[0139] The lithium ion doping is performed by discharging the electrolyte using a lithium electrode as a counter electrode.

[0140] The lithium ion doping is controlled based on the integrated value of the current during power-on.

[0141] The integrated value of the current at the completion of energization is set based on a difference between the capacity of the lithium ion secondary battery in the initial state and the capacity of the lithium ion secondary battery after the capacity reduction, and a correction factor.

[0142] Here, preferably, the correction coefficient includes at least one of a first correction coefficient obtained based on the difference between the capacity of the lithium-ion secondary battery in the initial state and the capacity of the lithium-ion secondary battery after the capacity is reduced, a second correction coefficient obtained based on the amount of lithium ions consumed by the chemical generation process of the lithium-ion secondary battery, and a third correction coefficient obtained based on the discharge efficiency of the lithium-ion secondary battery.

[0143] While the embodiments of the present invention have been described above, the technical scope of the present invention is not limited to the embodiments, and various modifications can be made without departing from the gist of the present invention. In addition, the components in the embodiments can be appropriately replaced with well-known components without departing from the gist of the present invention.

Claims

1. A method for recovering the performance of a positive electrode for a lithium ion secondary battery, which is performed by doping a positive electrode for a lithium ion secondary battery whose capacity has been reduced with lithium ions, wherein: The lithium ion doping is performed by discharging the electrolyte using a lithium electrode as a counter electrode. The discharge is performed within the range of the cumulative discharge amount DG represented by the following formula 1, 0.95×DG0≤DG≤1.05×DG0 Formula 1 In the above formula 1, DG0 is the value calculated by the following formula 2, DB×[(DB-DA) / DB+X] Formula 2 In the above formula 2, DB is the capacity of the lithium-ion secondary battery in the initial state, DA is the capacity of the lithium-ion secondary battery after the capacity is reduced, the unit of DG, DB and DA is [Ah], and X is a correction coefficient set based on at least one of the following X1, X2 and X3. X1: Correction coefficient based on lithium ions consumed in film formation during the chemical conversion process of the lithium ion secondary battery in the initial state X2: Correction factor based on degradation of the negative electrode of the lithium-ion secondary battery X3: A correction factor obtained based on the reaction resistance and transfer resistance of lithium ions in the lithium ion secondary battery.

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

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

4. The method for recovering the performance of a positive electrode for a lithium ion secondary battery according to any one of claims 1 to 3, wherein: The (DB-DA) / DB is greater than 0.

7.

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

18.

6. The method for recovering the performance of a positive electrode for a lithium-ion secondary battery according to claim 1, wherein: The X3 is calculated by the following formula 3: X3=X3a+X3b+X3c+X3d Formula 3 In Formula 3, X3a is a correction coefficient obtained based on the reaction resistance and movement resistance of lithium ions in the positive electrode when using the lithium ion secondary battery after performance recovery, and is a numerical value in the range of 0 to 0.

05. X3b is a correction coefficient obtained based on the reaction resistance and movement resistance of lithium ions in the negative electrode when using the lithium ion secondary battery after performance recovery, and is a numerical value in the range of 0 to 0.

05. X3c is a correction coefficient obtained based on the movement resistance of lithium ions in the electrolyte when using the lithium ion secondary battery after performance recovery, and is a numerical value in the range of 0 to 0.

04. X3d is a correction coefficient obtained based on the movement resistance of lithium ions in the diaphragm when using the lithium ion secondary battery after performance recovery, and is a numerical value in the range of 0 to 0.

04.

7. A method for recovering the performance of a positive electrode for a lithium ion secondary battery, which is performed by doping a positive electrode for a lithium ion secondary battery whose capacity has been reduced 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 integrated value of the current during power-on. The integrated value of the current at the completion of energization is set based on a difference between the capacity of the lithium ion secondary battery in an initial state and the capacity of the lithium ion secondary battery after the capacity reduction, and a correction factor.

8. The method for recovering the performance of a positive electrode for a lithium-ion secondary battery according to claim 7, wherein: The correction coefficient is set based on at least one of a first correction coefficient, a second correction coefficient, and a third correction coefficient, the first correction coefficient being obtained based on the difference between the capacity of the lithium-ion secondary battery in the initial state and the capacity of the lithium-ion secondary battery after the capacity is reduced, the second correction coefficient being obtained based on the amount of lithium ions consumed by the chemical generation process of the lithium-ion secondary battery, and the third correction coefficient being obtained based on the discharge efficiency of the lithium-ion secondary battery.

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