Charging method and charging system for secondary battery

Abstract

The charging method for a lithium secondary battery includes a process of charging the lithium secondary battery according to any one of a first charging curve and a second charging curve. The first charging curve includes at least two charging steps, and the second charging curve includes more charging steps than the first charging curve. At the start time of the charging process, the first charging curve is selected when the depth of discharge of the secondary battery is less than a predetermined threshold, and the second charging curve is selected when the depth of discharge of the secondary battery is above the threshold.

Description

Technical Field

[0001] This disclosure relates to a charging method and charging system for a secondary battery. Background Technology

[0002] Non-aqueous electrolyte secondary batteries, represented by lithium-ion secondary batteries, have high energy density and high output, and are considered promising for use as power sources for mobile devices such as smartphones, power sources for vehicles such as electric vehicles, and storage devices for natural energy such as sunlight.

[0003] In Patent Document 1, a non-aqueous electrolyte secondary battery of the following type is proposed with the aim of increasing battery capacity: lithium metal is deposited on the negative electrode current collector during charging and the lithium metal dissolves during discharging.

[0004] Patent document 2 proposes a charging method for a secondary battery with a multi-stage charging process. The secondary battery sequentially comprises a positive electrode current collector foil, a positive electrode active material layer, a solid electrolyte layer, and a negative electrode current collector foil. The reaction at the negative electrode utilizes the precipitation-dissolution reaction of metallic lithium. The charging method for this secondary battery is characterized by at least the following: a first charging step, wherein the secondary battery is charged at a first current density I1 (mA / cm²). 2 The first charging step involves charging the secondary battery with a second current density I2 that is greater than the first current density I1, after the first charging step, to increase the thickness of the rough coating layer. In the first charging step, when the roughness height of the solid electrolyte layer on the side surface of the negative electrode current collector foil is set to Y (μm) and the thickness of the rough coating layer is set to X (μm), the secondary battery is charged with the first current density I1 until X / Y becomes 0.5 or more.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2001-243957

[0008] Patent Document 2: Japanese Patent Application Publication No. 2020-9724 Summary of the Invention

[0009] In secondary batteries that deposit lithium metal at the negative electrode during charging and dissolves in the non-aqueous electrolyte during discharging, it is generally difficult to control the deposition morphology of lithium metal, and a technology to suppress dendrite formation is desired. Lithium metal deposited as dendrites during charging is easily partially isolated from the conductive network of the negative electrode during discharging. Therefore, with repeated charge-discharge cycles, cycle characteristics deteriorate.

[0010] While reducing the charging current can suppress the precipitation of dendritic lithium metal, it also tends to increase the side reactions between lithium metal and non-aqueous electrolytes and lengthens the charging time, thus reducing the practicality of the battery.

[0011] In view of the above, one aspect of this disclosure relates to a charging method for a secondary battery, the secondary battery comprising: a positive electrode, a negative electrode having a negative current collector, and a non-aqueous electrolyte, wherein lithium metal is deposited on the negative electrode during charging and dissolves in the non-aqueous electrolyte during discharging, the charging method comprising a step of charging the secondary battery according to any one of a first charging curve and a second charging curve, wherein the first charging curve comprises at least two charging steps and the second charging curve comprises more charging steps than the first charging curve, wherein at the start time of the charging step of the secondary battery, the first charging curve is selected when the depth of discharge of the secondary battery is less than a predetermined threshold, and the second charging curve is selected when the depth of discharge of the secondary battery is greater than or equal to the threshold.

[0012] Other aspects of this disclosure relate to a charging system for a secondary battery, the charging system comprising a secondary battery, a depth of discharge (DOD) detection device for detecting the DOD of the secondary battery, and a charging control unit for controlling the charging of the secondary battery, wherein the secondary battery comprises: a positive electrode, a negative electrode having a negative current collector, and a non-aqueous electrolyte, wherein lithium metal is deposited at the negative electrode during charging and dissolves in the non-aqueous electrolyte during discharging, the DOD detection device determines the DOD of the secondary battery before charging begins, and the charging control unit controls the charging of the secondary battery according to any one of a first charging curve and a second charging curve, wherein the first charging curve includes at least two charging steps, the second charging curve includes more charging steps than the first charging curve, the first charging curve is selected when the DOD is less than a predetermined threshold, and the second charging curve is selected when the DOD is greater than or equal to the threshold.

[0013] According to this disclosure, a good balance is achieved between the charging time and cycle characteristics of the secondary battery. Attached Figure Description

[0014] Figure 1 This is a flowchart of a charging method for a secondary battery according to one embodiment of the present disclosure.

[0015] Figure 2 This is a schematic diagram of a charging system for a secondary battery according to one embodiment of the present disclosure.

[0016] Figure 3 This is a schematic perspective view showing a portion of the secondary battery used in the charging method and charging system according to one embodiment of this disclosure, after it has been cut off. Detailed Implementation

[0017] The following describes examples of embodiments of the charging method and charging system disclosed herein. However, this disclosure is not limited to the examples described below. Specific numerical values ​​and materials are sometimes shown in the following description, but other numerical values ​​and materials can be applied as long as the effects of this disclosure are achieved. Furthermore, examples of the charging method and charging system disclosed herein will be described below with appropriate reference to the accompanying drawings.

[0018] Hereinafter, Depth of Discharge (DOD) refers to the proportion of discharged charge relative to the charge remaining in a fully charged battery. Conversely, State of Charge (SOC) refers to the proportion of remaining charge in a battery relative to the charge remaining in a fully charged battery. Furthermore, the amount of charge (i.e., full charge) obtained by charging a fully discharged battery (SOC = 0%, DOD = 100%) to a fully charged state (SOC = 100%, DOD = 0%) is equivalent to its rated capacity. The voltage of a fully charged battery corresponds to the charging termination voltage. The voltage of a fully discharged battery corresponds to the discharging termination voltage. However, when the rated capacity of the battery is set as C, for example, the state of charging the battery to a state of charge of 0.98 × C or higher (SOC = 98%) can also be considered a fully charged state.

[0019] Additionally, current density (mA / cm) 2 () refers to the area per unit of facing area between the positive and negative electrodes (1 cm²). 2 The charging density is obtained by dividing the current applied to the battery by the total area of ​​the positive electrode composite material layer (or positive electrode active material layer) facing the negative electrode (hereinafter also referred to as the effective total area of ​​the positive electrode). Regarding the effective total area of ​​the positive electrode, for example, when the positive electrode has positive electrode composite material layers on both sides of the positive electrode current collector, the effective total area of ​​the positive electrode refers to the total area of ​​the positive electrode composite material layers on both sides (that is, the sum of the projected areas of the positive electrode composite material layers on one and the other surfaces of the positive electrode current collector on each of the two sides).

[0020] Furthermore, the secondary battery charged by the charging method disclosed herein is a secondary battery having a positive electrode, a negative electrode having a negative current collector, and a non-aqueous electrolyte. During charging, lithium metal is deposited at the negative electrode, and during discharging, the lithium metal dissolves in the non-aqueous electrolyte. That is, while not limited to a single type of secondary battery, it primarily refers to lithium (metal) secondary batteries.

[0021] In lithium-ion batteries, over 70% of the rated capacity is achieved through the deposition and dissolution of lithium metal. During charging and discharging, the movement of electrons in the negative electrode primarily occurs through the deposition and dissolution of lithium metal. Specifically, 70-100% (e.g., 80-100% or 90-100%) of the movement of electrons (or, in other words, current) in the negative electrode during charging and discharging occurs through the deposition and dissolution of lithium metal. That is, the negative electrode involved in this embodiment differs from negative electrodes where the movement of electrons during charging and discharging is primarily achieved through the adsorption and release of lithium ions by the negative electrode active material (graphite, etc.).

[0022] [How to charge a secondary battery]

[0023] In lithium-ion rechargeable batteries, controlling the deposition morphology of lithium metal is crucial for achieving good cycle characteristics. Reducing the charging current can suppress the deposition of dendritic lithium metal, but this increases charging time. To improve the practicality of rechargeable batteries, it is necessary to find a way to suppress the deposition of dendritic lithium metal while shortening the charging time.

[0024] Therefore, the charging method disclosed herein includes the step of charging the secondary battery according to any one of the first charging curve and the second charging curve. Figure 1 This is a flowchart illustrating an example of the charging method disclosed herein. The illustrated charging method includes a step (S1) of selecting any one of a first charging curve and a second charging curve before charging begins. Such selection is based on the DOD prior to the start time point (hereinafter also referred to as time point T) of the step of charging the secondary battery. Therefore, during the use of the repeatedly charged and discharged secondary battery, the charging current can be selectively reduced within the necessary range, and the overall charging time can be shortened.

[0025] Specifically, the first charging curve includes at least two charging steps, and the second charging curve includes more charging steps than the first charging curve. That is, if the first charging curve includes n (≥2) charging steps, the second charging curve includes (n+m) charging steps. n can be, for example, 2–5, 2–4, or 2–3, but n=2 is efficient. m can be 1 or more, but m=1 or m=2 is efficient.

[0026] Here, the charging profile refers to the process that determines the charging method, voltage, current, and other conditions when charging a secondary battery. In other words, the charging profile specifies the timeline for each step of the process until the secondary battery is fully charged. Furthermore, each charging step involves charging the secondary battery under different charging conditions. These charging conditions are defined by the charging current and / or charging voltage. Each charging step can be a constant current charging step or a constant voltage charging step.

[0027] In either the first or second charging curve, the charging current initially appears small and gradually increases. In the second charging curve, which includes (n+m) charging steps, the charging current increases more incrementally than in the first charging curve. However, it is also possible that the charging current decreases after a period of gradual increase from the start of charging. Typically, in the charging process of a secondary battery, the battery is charged according to either the first or second curve until it reaches a fully charged state (SOC 100%).

[0028] At the start time (time point T) of the charging process for the secondary battery, if the depth of discharge (DOD) of the secondary battery is less than a predetermined threshold, a first charging curve is selected; if the DOD of the secondary battery is above the threshold, a second charging curve (S2) is selected. That is, at time point T, when the DOD is shallow (when the battery's SOC is high), the number of charging steps is less; when the DOD is deep (when the battery's SOC is low), the number of charging steps becomes more. In the second charging curve, the charging current increases more gradually. In other words, in the second charging curve, charging can begin with a smaller charging current than in the first charging curve. Furthermore, in the second charging curve, charging can be performed with a larger charging current than in the first charging curve.

[0029] The smaller the charging current, the less likely lithium metal is to precipitate as dendrites, and the more likely it is to precipitate planarly. The larger the charging current, the shorter the charging time. In other words, the second charging curve contributes to improved cycle characteristics and / or shorter charging time. On the other hand, when the DOD is shallow, a sufficient lithium metal substrate layer exists at the negative electrode, so lithium metal is less likely to precipitate as dendrites compared to when the DOD is deep. That is, the first charging curve does not need to make the charging current rise as piecemeal. Therefore, simpler control (or a simpler control circuit construction) can be used. Furthermore, when the first curve is selected, the charging current at the start of charging can be larger than the charging current at the start of charging in the second curve.

[0030] The threshold of DOD for the selection of the first and second curves on the left and right can be, for example, 50% or more and 70% or less, or can be 55% or more and 70% or less. In the shallow depth of discharge where DOD is less than 50% (i.e., SOC is 50% or more), there is a sufficient substrate layer for lithium metal precipitation during charging, so lithium metal is not easily isolated. In this case, the first curve is selected, and priority is given to simple control or completing charging as quickly as possible. Additionally, from the perspective of capacity retention rate, there is no benefit in further increasing the number of charging steps and charging with a large current at the end of charging in the case of a shallow DOD. On the other hand, in the deep depth of discharge where DOD exceeds 80% (i.e., SOC is less than 20%), the presence of the substrate layer for lithium metal precipitation during charging is insufficient, and the possibility of lithium metal growing into dendrites is high. In this case, the second curve is selected, and priority is given to carefully advancing the charging. Or, select the second curve with more charging steps, and charge with a larger current as it gets closer to the end of charging, giving priority to shortening the charging time.

[0031] The first charging curve includes, for example, a charging step S11 of charging at a first current density I1, and a charging step S12 of charging at a second current density I2 greater than the first current density I1 (I1 < I2) following the charging step S11.

[0032] The second charging curve includes, for example, a charging step S21 of charging at a third current density I3, and a charging step S22 of charging at a fourth current density I4 greater than the third current density I3 (I3 < I4) following the charging step S21.

[0033] The number of charging steps of the second charging curve is more than that of the first charging curve, and at least further includes a charging step S23 of charging at a fifth current density I5 following the charging step S22. Preferably, the fifth current density I5 of the charging step S23 following the charging step S22 is greater than the fourth current density I4 (I4 < I5).

[0034] The charging step S11 is, for example, set as the initial charging step in the first curve. Additionally, the charging step S21 is, for example, set as the initial charging step in the second curve. Both the charging step S11 and the charging step S12 in the first curve can be constant current charging steps. All of the charging step S21, the charging step S22, and the charging step S23 in the second curve can be constant current charging steps.

[0035] The third current density I3 can be smaller than the first current density I1 (I3 < I1). When the second charging curve is selected at a deep DOD, it is preferred to start charging with a smaller current. There is no particular limitation on I3 / I1. For example, it can be 0.2 to 1, or can be 0.5 to 1.

[0036] The closer I1 is to I2 (the closer I1 / I2 is to 1), the more the charging time can be shortened when charging the secondary battery with the first curve. On the other hand, the larger I4 is relative to I3 (the smaller I3 / I4 is), the less likely the lithium metal is to grow into dendrites. Therefore, it is preferable to satisfy I1 / I2 > I3 / I4.

[0037] I1 / I2 can be, for example, 0.6 or more, or 0.7 or more, or 0.75 or more (it can also be 0.8 or more). However, if I1 / I2 is too close to 1, the possibility of lithium metal isolation during charging in charging step S11 gradually increases. Therefore, it is preferable that I1 / I2 is 0.9 or less.

[0038] The charging amount Q1 in charging step S11, the charging amount Q2 in charging step S12, the charging amount Q3 in charging step S21, and the charging amount Q4 in charging step S22 can satisfy Q1 / Q2 < Q3 / Q4. The larger Q2 is relative to Q1 (the smaller Q1 / Q2 is), the shorter the charging time when charging the secondary battery with the first curve. In addition, the larger Q4 is relative to Q3 (the smaller Q3 / Q4 is), the shorter the charging time when charging the secondary battery with the second curve. However, in terms of shortening the charging time, it is preferable that Q2 < Q4, and it is preferable that Q1 / Q2 < Q3 / Q4.

[0039] In the first charging curve, the first current density I1 is, for example, 3.0 mA / cm 2 or less, and the second current density I2 can be 4.0 mA / cm 2 or more.

[0040] If considering the balance between charging time and cycle characteristics, the first current density I1 is preferably 1.0 mA / cm 2 or more, and can also be 2.0 mA / cm 2 or more.

[0041] The second current density I2 is preferably 4.0 mA / cm [[ID=​​​​​​​​​The charge amount (Q1) obtained through charging step S11 can be 5% or more and 15% or less of the total charge amount charged in the process of charging the secondary battery. Here, "total charge amount charged in the process of charging the secondary battery" refers to the charge amount from the start of charging until the secondary battery is fully charged, and varies depending on the DOD or SOC of the secondary battery at the start of charging. Hereinafter, the "total charge amount" charged according to the first curve will also be referred to as the total charge amount P1. When Q1 is 5% or more of the total charge amount P1, the effect of suppressing the growth of dendritic lithium metal is greater. In addition, when Q1 is 15% or less of the total charge amount, a sufficient effect of suppressing the growth of dendritic lithium metal is obtained while shortening the charging time.

[0044] In the second charging curve, the third current density I3 is, for example, 1 mA / cm. 2 Below, the fourth current density I4 is greater than the third current density, and is 4 mA / cm². 2 Below, the fifth current density I5 is greater than the second current density, and can be 4 mA / cm². 2 above.

[0045] Considering the balance between charging time and cycle characteristics, the third current density I3 is preferably 0.1 mA / cm². 2 The above can also be 0.5 mA / cm. 2 above.

[0046] The fourth current density I4 is preferably 1.0 mA / cm². 2 The above can also be 2.0 mA / cm 2 That's all. However, if the fourth current density I4 is too high, the possibility of lithium metal isolation during charging gradually increases; therefore, I4 is preferably 4.0 mA / cm². 2 the following.

[0047] The fifth current density I5 is preferably 6.0 mA / cm². 2 The above can also be 8.0 mA / cm 2 That's all. However, if the fifth current density I5 is too high, the possibility of lithium metal isolation during charging gradually increases. Therefore, I5 is preferably 10.0 mA / cm². 2 the following.

[0048] For example, I3 / I4 can be above 0.1 and below 0.5, or above 0.2 and below 0.4.

[0049] For example, I4 / I5 can be above 0.2 and below 0.9, or above 0.3 and below 0.7.

[0050] In the charging step S21 (initial charging stage) where charging is performed at a small third current density I3, lithium metal is deposited on the negative electrode current collector in a块状 (granular) form, and it is easy to form a good base layer of lithium metal. Therefore, even if the fourth current density I4 in the subsequent charging step S22 is larger than the third current density I3, dendritic lithium metal is not likely to grow. Additionally, in the charging step S22, the base layer of lithium metal further grows, so even when the current density I5 in the subsequent charging step S23 is larger than I4 (I4 < I5), the growth of dendritic lithium metal is suppressed. Thus, a significant shortening of the charging time can be achieved.

[0051] The charging charge (Q3) charged through the charging step S21 can be 5% or more and 15% or less of the total charging charge charged in the process of charging the secondary battery. Hereinafter, the "total charging charge" charged in accordance with the second curve is also referred to as the total charging charge P2. When Q3 is 5% or more of the total charging charge P2, the effect of suppressing the growth of dendritic lithium metal becomes greater. Additionally, when Q3 is 15% or less of the total charging charge P2, a sufficient effect of suppressing the growth of dendritic lithium metal is obtained while shortening the charging time.

[0052] The total charging charge charged through the charging step S21 and the charging step S22 can be 50% or less of the total charging charge P2, or can be 40% or less. In this case, it is possible to significantly shorten the charging time while sufficiently suppressing the growth of dendritic lithium metal. More than 50% of the remaining part of the total charging charge P2 is charged through the charging step S23 where charging is performed at a higher current density I5.

[0053] The timing for ending each charging step can be controlled based on, for example, the charging time, the charging charge, the voltage, etc., can also be controlled based on the ratio of the charged charge in each charging step to the total charging charges P1, P2, and can also be controlled based on the SOC or the charging rate. The SOC can be estimated based on the voltage. For example, the SOC can be estimated based on the voltage, and the charging termination voltage for charging through each charging step can be set.

[0054] For example, in the charging performed in accordance with the first curve, when the battery voltage reaches the first voltage through the charging step S11 where charging is performed at the first current density I1, the charging at the first current density Ii is ended, and the charging step S12 where charging is performed at the second current density I2 is started. Then, when the battery voltage reaches the second voltage through the charging step S12 where charging is performed at the second current density I2, the charging at the second current density I? is ended. The first voltage is, for example, the voltage when a charging charge equivalent to 15% or less of the total charging charge P1 has been charged, and the second voltage is, for example, the voltage when a total charging charge equivalent to 90% or more of the total charging charge P1 has been charged.

[0055] Additionally, for example, in charging according to the second curve, when the battery voltage reaches the third voltage through charging step S21 at the third current density I3, charging step S21 at the third current density I3 ends, and charging step S22 at the fourth current density I4 begins. Then, when the battery voltage reaches the fourth voltage through charging at the fourth current density I4, charging step S22 at the fourth current density I4 ends, and charging step S23 at the fifth current density I5 begins. Then, when the battery voltage reaches the fifth voltage through charging step S23 at the fifth current density I5, charging at the fifth current density I5 ends. The third voltage is, for example, the voltage when less than 15% of the total charge P2 has been charged; the fourth voltage is, for example, the voltage when less than 50% of the total charge P1 has been charged; and the fifth voltage is, for example, the voltage when more than 90% of the total charge P1 has been charged.

[0056] Alternatively, a constant-voltage charging step S3 can be performed after the constant-current charging step for more reliable charging. This charging step, for example, continues until the current reaches a predetermined value. For instance, after a final constant-current charging step to a predetermined charging termination voltage, a constant-voltage charging step can be performed at that voltage. Afterward, discharging is carried out until a predetermined discharge termination voltage is reached.

[0057] [Rechargeable battery charging system]

[0058] The charging system disclosed herein includes a secondary battery, a depth of discharge (DOD) detection device for detecting the depth of discharge of the secondary battery, and a charging control unit for controlling the charging of the secondary battery. The secondary battery is a battery having a positive electrode, a negative electrode having a negative current collector, and a non-aqueous electrolyte, wherein lithium metal is deposited at the negative electrode during charging and dissolves in the non-aqueous electrolyte during discharging. The depth of discharge detection device measures the depth of discharge of the secondary battery before charging begins. The charging control unit controls the charging of the secondary battery according to any one of the first and second charging curves described above. When the depth of discharge (DOD) of the secondary battery detected by the depth of discharge detection device before charging begins is less than a predetermined threshold, the first charging curve is selected; when the DOD is above the threshold, the second charging curve is selected.

[0059] Figure 2This illustrates an example of a charging system according to one embodiment. The charging system includes a secondary battery 11 and a charging device 12. An external power source 13 is connected to the charging device 12 to supply power to it. The charging device 12 includes a charging control unit 14 with a charging circuit. The charging control unit 14 controls the charging of the secondary battery according to a selected charging curve.

[0060] The charging device 12 includes a voltage detection unit 15 that detects the voltage of the secondary battery 11, serving as a DOD detection device for detecting the DOD of the secondary battery. The voltage detection unit 15 detects the voltage of the secondary battery 11 before charging begins, and includes an arithmetic unit that calculates the DOD based on the detected voltage. The charging control unit 14 selects any one of a first curve and a second curve based on the DOD calculated by the arithmetic unit. Then, it controls the charging of the secondary battery according to the selected charging curve.

[0061] In addition, the charging device 12 includes a current detection unit 16 for detecting the current output from the secondary battery. The charging control unit 14 controls the charging current so that the current value detected by the current detection unit 16 does not deviate significantly from a predetermined value.

[0062] exist Figure 2 In this system, the switching timing and ending timing of the charging steps are controlled based on the voltage (or DOD (or SOC)) detected by the voltage detection unit 15, but the control method is not limited. For example, at least some of the control can be based on charging time, charging capacity, etc.

[0063] Next, the details of the secondary battery will be explained in more detail.

[0064] [negative electrode]

[0065] The negative electrode has a negative current collector. In a lithium-ion secondary battery, charging causes lithium metal to deposit on the surface of the negative electrode. More specifically, charging causes lithium ions in the non-aqueous electrolyte to gain electrons at the negative electrode and become lithium metal, which is then deposited on the surface of the negative electrode. Discharging causes the lithium metal deposited on the surface of the negative electrode to dissolve into the non-aqueous electrolyte as lithium ions. Furthermore, the lithium ions in the non-aqueous electrolyte can originate from lithium salts added to the non-aqueous electrolyte, or they can be supplied from the positive electrode active material due to charging, or both.

[0066] The negative electrode may include a negative electrode current collector and a thin sheet of lithium metal adhering to the surface of the current collector. It may also include a lithium-ion storage layer supported on the current collector (a layer that exhibits capacity through the storage and release of lithium ions by a negative electrode active material (such as graphite)). In this case, the open-circuit potential of the negative electrode at full charge can be 70 mV or less relative to the lithium metal (the dissolution potential of lithium). When the open-circuit potential of the negative electrode at full charge is 70 mV or less relative to the lithium metal, lithium metal is present on the surface of the lithium-ion storage layer at full charge. That is, the negative electrode exhibits capacity through the deposition and dissolution of lithium metal.

[0067] Full charge refers to the state of charge (SOC) of a battery when its rated capacity is set to C, reaching, for example, 0.98 × C or higher. The open-circuit potential of the negative electrode at full charge can be measured as follows: The battery at full charge is disassembled under an argon atmosphere, the negative electrode is removed, and a single cell is reassembled using lithium metal as the counter electrode. The composition of the non-aqueous electrolyte in the single cell can be the same as that in the disassembled battery.

[0068] The lithium-ion storage layer is obtained by forming a layered structure of a negative electrode composite material containing negative electrode active material. In addition to the negative electrode active material, the negative electrode composite material may also contain binders, thickeners, conductive agents, etc.

[0069] Examples of anode active materials include carbonaceous materials, Si-containing materials, and Sn-containing materials. An anode may contain one type of anode active material or a combination of two or more types. Examples of carbonaceous materials include graphite, easily graphitized carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon).

[0070] Conductive materials include, for example, carbon materials. Examples of carbon materials include carbon black, acetylene black, Ketjen black, carbon nanotubes, and graphite.

[0071] Examples of adhesive materials include fluoropolymers, polyacrylonitrile, polyimide resins, acrylic resins, polyolefin resins, and rubber-like polymers. Examples of fluoropolymers include polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF).

[0072] The negative electrode current collector can be a conductive thin sheet. Foil, thin film, etc., can be used as the conductive sheet. There are no particular restrictions on the thickness of the negative electrode current collector; for example, it can be 5 μm or more but less than 300 μm.

[0073] The negative electrode current collector (conductive sheet) can be made of any conductive material other than lithium metal and lithium alloys. The conductive material can be a metal, alloy, or other metallic material. Preferably, the conductive material is a material that does not react with lithium. More specifically, it is preferably a material that does not form any alloys or intermetallic compounds with lithium. Examples of such conductive materials include copper (Cu), nickel (Ni), iron (Fe), and alloys containing these metallic elements, or graphite with a preferentially exposed substrate. Examples of alloys include copper alloys and stainless steel (SUS). Copper and / or copper alloys with high conductivity are particularly preferred. The negative electrode current collector can be copper foil or copper alloy foil.

[0074] [positive electrode]

[0075] The positive electrode, for example, includes a positive electrode current collector and a positive electrode composite material layer supported on the positive electrode current collector. The positive electrode composite material layer, for example, comprises a positive electrode active material, a conductive material, and a binder material. The positive electrode composite material layer may be formed on only one side of the positive electrode current collector or on both sides of the positive electrode current collector. The positive electrode is obtained, for example, by coating both sides of the positive electrode current collector with a positive electrode composite material slurry comprising a positive electrode active material, a conductive material, and a binder material, and then rolling the coating after it has dried.

[0076] The positive electrode active material is a material that absorbs and releases lithium ions. Examples of positive electrode active materials include, for instance, composite oxides containing lithium and a metal Me other than lithium (e.g., lithium-containing transition metal oxides containing at least a transition metal as the metal Me), transition metal fluorides, polyanions, fluorinated polyanions, and transition metal sulfides. Lithium-containing transition metal oxides are particularly preferred due to their low manufacturing cost and high average discharge voltage. Lithium-containing transition metal oxides having a layered rock salt-type crystal structure are especially preferred.

[0077] Lithium contained in lithium-containing transition metal oxides is released from the positive electrode as lithium ions during charging and deposited as lithium metal at the negative electrode or in the negative electrode current collector. During discharge, lithium metal dissolves from the negative electrode, releasing lithium ions, which are then absorbed into the composite oxide at the positive electrode. In other words, the lithium ions involved in charging and discharging originate primarily from the solute in the non-aqueous electrolyte and the positive electrode active material. In this case, the molar ratio of the total stoichiometry of Li (mLi) in the positive and negative electrodes to the amount of metallic Me (mMe) in the lithium-containing transition metal oxide (mMe) is, for example, 1.2 or less.

[0078] Examples of transition metal elements that can be included in lithium-containing transition metal oxides include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, and W. Lithium-containing transition metal oxides can contain one or more transition metal elements. The transition metal element can be Co, Ni, and / or Mn. Depending on the requirements, lithium-containing transition metal oxides can contain more than one main group element. Examples of main group elements include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, and Bi. Al can be a main group element.

[0079] In lithium-containing transition metal oxides, composite oxides with a rock-salt-type crystal structure exhibiting a layered structure are preferred for achieving high capacity. These composite oxides contain Co, Ni, and / or Mn as transition metal elements, and sometimes Al as an arbitrary component. Furthermore, lithium-containing transition metal oxides containing at least Ni as a transition metal are preferred for achieving particularly high capacity. In this case, in the lithium secondary battery, the molar ratio of the total stoichiometry of lithium (mLi) in the positive and negative electrodes to the amount of metal M (excluding lithium) in the positive electrode (mM): mLi / mM can, for example, be set to 1.1 or less.

[0080] Lithium-containing transition metal oxides, for example, those of general formula (1): Li a Ni b M 1-b O2 represents. General formula (1) satisfies, for example, 0.9 ≤ a ≤ 1.2 and 0.65 ≤ b ≤ 1. M can be, for example, at least one element selected from the group consisting of Co, Mn, Al, Ti, Fe, Nb, B, Mg, Ca, Sr, Zr and W.

[0081] As a binder, conductive agent, etc., the material exemplified in the negative electrode can be used. The shape and thickness of the positive electrode current collector can be selected from the shape and range of the positive electrode current collector.

[0082] Materials used as positive electrode current collectors (conductive sheets) include, for example, metallic materials such as Al, Ti, and Fe. These metallic materials can be Al, Al alloys, Ti, Ti alloys, Fe alloys, etc. Fe alloys can be stainless steel (SUS).

[0083] [Septum]

[0084] The diaphragm uses a porous sheet with ion permeability and insulation properties. Examples of porous sheets include microporous films, woven fabrics, and non-woven fabrics. There are no particular limitations on the material of the diaphragm; it can be a polymer material. Examples of polymer materials include olefin resins, polyamide resins, and cellulose. Examples of olefin resins include polyethylene, polypropylene, and copolymers of ethylene and propylene. The diaphragm may contain additives as needed. Examples of additives include inorganic fillers.

[0085] There is no particular limitation on the thickness of the diaphragm; for example, it can be 5 μm or more and 20 μm or less, preferably 10 μm or more and 20 μm or less.

[0086] [Non-aqueous electrolytes]

[0087] Non-aqueous electrolytes with lithium-ion conductivity include, for example, a non-aqueous solvent, lithium ions, and anions dissolved in the non-aqueous solvent. Non-aqueous electrolytes can be in liquid or gel form.

[0088] Liquid non-aqueous electrolytes are prepared by dissolving lithium salts in a non-aqueous solvent. Lithium ions and anions are generated by dissolving the lithium salt in the non-aqueous solvent.

[0089] Gel-like non-aqueous electrolytes include lithium salts and matrix polymers, or lithium salts, non-aqueous solvents, and matrix polymers. As a matrix polymer, for example, a polymer material that gels by absorbing a non-aqueous solvent is used. Examples of polymer materials include fluoropolymers, acrylic resins, and polyether resins.

[0090] As an anion, known substances used as non-aqueous electrolytes in lithium secondary batteries can be used. Specifically, BF4 can be cited as an example. - ClO4 - PF6 - CF3SO3 - CF3CO2 - Anions of imides, anions of oxalate complexes, etc. Examples of anions of imides include N(SO₂CF₃)₂. - 、N(C m F 2m+1 SO2) x (C n F 2n+1 SO2) y - (m and n are independent integers greater than or equal to 0 or 1, and x and y are independent integers of 0, 1, or 2, respectively, satisfying x + y = 2.) etc. The anion of the oxalate complex may contain boron and / or phosphorus. Examples of anions for oxalate complexes include bis(oxalateborate) anion and difluoro(oxalateborate) anion (BF2(C2O4)). -), PF4(C2O4) - PF2(C2O4)2 - Non-aqueous electrolytes can contain these anions individually or in combination with two or more anions.

[0091] From the viewpoint of suppressing the dendritic precipitation of lithium metal, the non-aqueous electrolyte preferably contains at least an oxalate complex anion, and particularly preferably contains a fluorinated oxalate complex anion (especially a difluorooxalate-borate anion). Through the interaction between the fluorinated oxalate complex anion and lithium, lithium metal readily and uniformly precipitates in fine particles. Therefore, localized precipitation of lithium metal is easily suppressed. The fluorinated oxalate complex anion can also be combined with other anions. Other anions can be PF6. - And / or imide anions.

[0092] Examples of non-aqueous solvents include esters, ethers, nitriles, amides, or their halogenated derivatives. Non-aqueous electrolytes may contain only one of these non-aqueous solvents or two or more of them. Examples of halogenated derivatives include fluorides.

[0093] The concentration of lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 3.5 mol / L or less. The concentration of anion in the non-aqueous electrolyte can be set to 0.5 mol / L or more and 3.5 mol / L or less. In addition, the concentration of anion of oxalate complex in the non-aqueous electrolyte can be set to 0.05 mol / L or more and 1 mol / L or less.

[0094] As an example of the structure of a lithium secondary battery, the following structure can be cited: the electrode assembly and electrolyte are housed together in a casing, and the electrode assembly is formed by winding the positive and negative electrodes together with a separator in between. However, it is not limited to this, and other forms of electrode assemblies can also be used. For example, it can also be a stacked electrode assembly in which the positive and negative electrodes are stacked together with a separator in between. There are no limitations on the shape of the lithium secondary battery, for example, it can be cylindrical, square, coin-shaped, button-shaped, laminated, etc.

[0095] Figure 3This is a schematic perspective view of a square lithium secondary battery according to one embodiment of the present disclosure, after a portion has been cut away. The battery includes a square-bottomed battery casing 4, and an electrode assembly 1 and an electrolyte contained within the battery casing 4. The electrode assembly 1 has a strip-shaped negative electrode, a strip-shaped positive electrode, and a separator between them. The negative electrode current collector of the negative electrode is electrically connected to a negative terminal 6 disposed on a sealing plate 5 via a negative electrode lead 3. The negative terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector of the positive electrode is electrically connected to the back side of the sealing plate 5 via a positive electrode lead 2. That is, the positive electrode is electrically connected to the battery casing 4, which also serves as the positive terminal. The periphery of the sealing plate 5 is fitted into the open end of the battery casing 4, and the fitting portion is laser welded. The sealing plate 5 has an electrolyte injection hole, which is plugged with a sealing plug 8 after electrolyte injection.

[0096] [Example]

[0097] The present disclosure will now be specifically described based on embodiments, but the present invention is not limited to the following embodiments.

[0098] Example 1

[0099] [The production of the positive electrode]

[0100] Lithium-nickel composite oxide (LiNi) 0.9 Co 0.05 Al 0.05 O2, acetylene black, and polyvinylidene fluoride (PVdF) were mixed in a mass ratio of 95:2.5:2.5, and N-methyl-2-pyrrolidone (NMP) was added before stirring to prepare a positive electrode slurry. Next, the positive electrode slurry was coated onto the surface of an Al foil, which would serve as the positive electrode current collector. After the coating was dried, it was rolled to form a positive electrode composite material layer (density 3.6 g / cm³) on both sides of the Al foil. 3 The positive electrode of ).

[0101] [Making the negative electrode]

[0102] Electrolytic copper foil (10 μm thick) is cut into specified electrode sizes to obtain a negative electrode current collector.

[0103] [Preparation of non-aqueous electrolytes]

[0104] A non-aqueous electrolyte was prepared by dissolving lithium salts in a mixed solvent. The mixed solvent was a mixture of fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of FEC:EMC:DMC = 20:5:75. The lithium salts used were LiPF6, LiN(FSO2)2 (hereinafter referred to as LiFSI), and LiBF2(C2O4) (hereinafter referred to as LiFOB). The concentrations of LiPF6 and LiFSI in the non-aqueous electrolyte were set to 0.5 mol / L and 1% by mass, respectively.

[0105] [Battery assembly]

[0106] An Al-made positive electrode lead is attached to the positive electrode obtained above, and a Ni-made negative electrode lead is attached to the negative electrode obtained above. In an inactive gas atmosphere, the positive and negative electrodes are wound into a vortex shape with a polyethylene film (separator) in between to create a wound electrode assembly. The electrode assembly is housed in a bag-shaped outer casing formed of a laminated sheet having an Al layer. After injecting the aforementioned non-aqueous electrolyte, the casing is sealed to create a non-aqueous electrolyte secondary battery. Furthermore, when housing the electrode assembly in the casing, a portion of the positive and negative electrode leads are exposed to the outside of the casing.

[0107] Since all lithium originates from the non-aqueous electrolyte and the positive electrode, the molar ratio of the total amount of lithium (mLi) in the positive and negative electrodes to the amount of metal Me (Ni, Co, and Al) (mMe) in the positive electrode is 1.0. The voltage at 100% SOC is 4.1V, and the voltage at 100% DOD is 3.0V.

[0108] [Preparing for charging / discharging]

[0109] Using the obtained battery (rated capacity 100mAh), the following pre-charge and discharge tests were performed at 25°C. Furthermore, the current value (1 / X)C represents the current value when a charge equivalent to the rated capacity C is charged or discharged at a constant current for X hours. For example, 0.1C represents the current value when a charge equivalent to the rated capacity C is charged or discharged at a constant current for 10 hours.

[0110] (Preparing to charge)

[0111] At 0.05C (current density 0.5mA / cm²) 2 The current I0 is used to charge the battery at a constant current until the voltage reaches 4.1V (SOC 100%).

[0112] (Preparing to discharge)

[0113] After a 10-minute rest, at 0.6C (current density 6.0 mA / cm²) 2 Perform constant current discharge until the voltage reaches 3.75V.

[0114] [Charge-discharge cycle test]

[0115] Using the battery after pre-charge and discharge, a charge-discharge cycle test was conducted at 25°C, including the first and second curves shown below.

[0116] <First Curve>

[0117] (1) DOD at the start of charging: 52% (voltage 3.75V)

[0118] (2) Initial charging step S11

[0119] First current density I1: 1 mA / cm 2 or 3mA / cm 2

[0120] Charging capacity Q1: 15% of the total charging capacity P1

[0121] (3) Charging step S12 following S11

[0122] Second current density I2: 4 mA / cm 2

[0123] Charging capacity Q2: The remaining portion (85%) of the total charging capacity P1.

[0124] Charging termination voltage: 4.1V

[0125] (4) Constant voltage charging following S21

[0126] Constant voltage charging at 4.1V until the current reaches 0.02C (current density 0.2mA / cm²). 2 )until

[0127] (5) After a 10-minute rest, apply a current of 0.6C (current density 6.0 mA / cm²). 2 Perform constant current discharge until the voltage reaches 3.75V.

[0128] Based on the above, I1 / I2 = 1 / 4, Q1 / Q2 = 15 / 85.

[0129] <Second Curve>

[0130] (1) DOD at the start of charging: 80% (voltage 3.55V)

[0131] (2) Initial charging step S21

[0132] Third current density I3: 1 mA / cm 2

[0133] Charging capacity Q3: 15% of the total charging capacity P2

[0134] (3) Charging step S22 following S21

[0135] Fourth current density I4: 4 mA / cm 2

[0136] Charging capacity Q4: 35% of the total charging capacity P1 (Q3 + Q4 = 50%)

[0137] (4) Charging step S23 following S22

[0138] Fifth current density I5: 8 mA / cm 2

[0139] Charging capacity Q5: The remaining portion (50%) of the total charging capacity P1 (Q3 + Q4 + Q5 = 100%)

[0140] (5) Constant voltage charging following S23

[0141] Constant voltage charging at 4.1V until the current reaches 0.02C (current density 0.2mA / cm²). 2 )until

[0142] (6) Discharge

[0143] After a 10-minute rest, apply a current of 0.6C (current density 6.0 mA / cm²). 2 Perform constant current discharge until the voltage reaches 3.55V.

[0144] Based on the above, I3 / I4 = 1 / 4, I4 / I5 = 4 / 8, Q3 / Q4 = 15 / 35.

[0145] Table 1 shows the time required to charge to full charge according to the first and second curves. Table 2 shows the capacity retention rate after 60 cycles.

[0146] [Table 1]

[0147]

[0148] Comparative Example 1

[0149] Next, in contrast to Example 1, using the battery after pre-charge and discharge, a charge-discharge cycle test was conducted at 25°C with the DOD at the start of charging set to 80% according to the first curve and the DOD at the start of charging set to 52% according to the second curve.

[0150] <First Curve>

[0151] (1) DOD at the start of charging: 80% (voltage 3.55V)

[0152] (2) Initial charging step S21

[0153] First current density I1: 1 mA / cm 2

[0154] Charging capacity Q1: 15% of the total charging capacity P1

[0155] (3) Charging step S12 following S11

[0156] Second current density I2: 4 mA / cm 2

[0157] Charging capacity Q2: The remaining portion (85%) of the total charging capacity P1.

[0158] Charging termination voltage: 4.1V

[0159] (4) Constant voltage charging following S12

[0160] Constant voltage charging at 4.1V until the current reaches 0.02C (current density 0.2mA / cm²). 2 )until

[0161] (5) Discharge

[0162] After a 10-minute rest, apply a current of 0.6C (current density 6.0 mA / cm²). 2 Perform constant current discharge until the voltage reaches 3.75V.

[0163] <Second Curve>

[0164] (1) DOD at the start of charging: 52% (voltage 3.75V)

[0165] (2) Initial charging step S21

[0166] Third current density I3: 1 mA / cm 2

[0167] Charging capacity Q3: 15% of the total charging capacity P2

[0168] (3) Charging step S22 following S21

[0169] Fourth current density I4: 4 mA / cm 2

[0170] Charging capacity Q4: 35% of the total charging capacity P2 (Q3 + Q4 = 50%)

[0171] (4) Charging step S23 following S22

[0172] Fifth current density I5: 8 mA / cm 2

[0173] Charging capacity Q5: The remaining portion (50%) of the total charging capacity P2 (Q3+Q4+Q5=100%)

[0174] (5) Constant voltage charging following S23

[0175] Constant voltage charging at 4.1V until the current reaches 0.02C (current density 0.2mA / cm²). 2 )until

[0176] (6) Discharge

[0177] After a 10-minute rest, apply a current of 0.6C (current density 6.0 mA / cm²). 2 Perform constant current discharge until the voltage reaches 3.55V.

[0178] Table 2 shows the time required to charge to full charge according to the first and second curves. Additionally, Table 2 shows the capacity retention rate after 60 cycles.

[0179] [Table 2]

[0180]

[0181] Comparing Tables 1 and 2 reveals that when the initial DOD is shallow (DOD = 52%), there is almost no benefit to using the second charging curve from the perspective of shortening charging time. In other words, even with the second curve and the addition of a charging step S23 at a higher fifth current density I5, the charging time is only reduced by 0.02 hours from 1.82 hours to 1.80 hours compared to using the first curve. In this case, the simpler control of the first charging curve is sufficient. On the other hand, it is evident that when the initial DOD is shallow (DOD = 52%), using the first curve and increasing the first current density I1 not only significantly reduces the charging time from 1.82 hours to 1.46 hours but also maintains a high level of capacity retention.

[0182] Furthermore, when the initial DOD is shallow (DOD = 52%) and the second curve is selected, the capacity retention rate tends to decrease somewhat (96.0% → 93.8%). This is believed to be because metallic lithium tends to precipitate in small amounts as dendrites during charging at the fifth current density I5. In other words, when the DOD is shallow, the charging time does not change significantly regardless of whether the first or second curve is selected; therefore, the first curve, which is more favorable for capacity retention, is preferred.

[0183] On the other hand, when the DOD is high at the start of charging (DOD = 80%), the charging time is significantly shortened by using the second curve. That is, if the second curve is used, the charging time is reduced from 3.04 hours to 2.72 hours compared to the case of using the first curve, a reduction of 0.32 hours.

[0184] When the DOD is 52%, the benefit of reduced charging time is negligible; therefore, it can be said that the threshold of DOD, which serves as the boundary between the first and second charging curves, is preferably set to at least 50%. Conversely, when the DOD is 80%, the benefit of reduced charging time is significant; therefore, it can be said that the threshold of DOD is preferably set, for example, to 70% or less.

[0185] Comparative Example 2

[0186] Next, using the battery after pre-charge and discharge, a charge-discharge cycle test was conducted at 25°C using the following comparison curve with the DOD at the start of charging set to 52% to omit charging step S11 from the first curve.

[0187] <Comparison Curve>

[0188] (1) DOD at the start of charging: 52% (voltage 3.75V)

[0189] (2) Charging step S12

[0190] Current density: 4 mA / cm 2

[0191] Charging capacity: 100% of the total charging capacity P1

[0192] Charging termination voltage: 4.1V

[0193] (3) Constant voltage charging following S12

[0194] Constant voltage charging at 4.1V until the current reaches 0.02C (current density 0.2mA / cm²). 2 )until

[0195] (4) Discharge

[0196] After a 10-minute rest, apply a current of 0.6C (current density 6.0 mA / cm²). 2 Perform constant current discharge until the voltage reaches 3.75V.

[0197] In contrast to the case where charging was performed according to the first curve, the capacity retention rate after 60 cycles is shown in Table 3.

[0198] [Table 3]

[0199]

[0200] The results in Table 3 show that the capacity retention rate decreases even when charging is not performed according to the first charging curve which includes at least two charging steps, even when the DOD is shallow at the beginning of charging.

[0201] Industrial availability

[0202] The charging method and charging system disclosed herein are suitable for charging lithium secondary batteries of the type in which lithium metal is deposited on the negative electrode current collector during charging and dissolves during discharging.

[0203] Explanation of reference numerals in the attached figures

[0204] 1: Electrode assembly; 2: Positive lead; 3: Negative lead; 4: Battery casing; 5: Sealing plate; 6: Negative terminal; 7: Gasket; 8: Seal; 11: Non-aqueous electrolyte secondary battery; 12: Charging device; 13: External power supply; 14: Charging control unit; 15: Voltage detection unit; 16: Current detection unit.

Claims

1. A method for charging a secondary battery, the secondary battery comprising: a positive electrode, a negative electrode having a negative current collector, and a non-aqueous electrolyte, wherein lithium metal is deposited on the negative electrode during charging, and the lithium metal dissolves in the non-aqueous electrolyte during discharging. The charging method includes the step of charging the secondary battery according to any one of the first charging curve and the second charging curve. in, The first charging curve includes at least two charging steps. The second charging curve includes more charging steps than the first charging curve. At the start time of the charging process for the secondary battery, the first charging curve is selected when the depth of discharge of the secondary battery is less than a predetermined threshold, and the second charging curve is selected when the depth of discharge of the secondary battery is greater than or equal to the threshold. The first charging curve includes a charging step S11 of charging at a first current density I1, and a charging step S12 of charging at a second current density I2, which is greater than the first current density I1, following the charging step S11. The second charging curve includes a charging step S21 of charging at a third current density I3, and a charging step S22 of charging at a fourth current density I4, which is larger than the third current density I3, following the charging step S21. The second charging curve also includes a charging step S23, which follows the charging step S22 and charges at a fifth current density I5 that is larger than the fourth current density I4.

2. The charging method for a secondary battery according to claim 1, wherein, The threshold for the depth of discharge is above 50% and below 70%.

3. The charging method for a secondary battery according to claim 1, wherein, The third current density I3 is smaller than the first current density I1.

4. The charging method for a secondary battery according to any one of claims 1 to 3, wherein, The condition I1 / I2 > I3 / I4 is satisfied.

5. The charging method for a secondary battery according to claim 4, wherein, The charging amount Q1 in charging step S11, the charging amount Q2 in charging step S12, the charging amount Q3 in charging step S21, and the charging amount Q4 in charging step S22 satisfy Q1 / Q2. <Q3 / Q4。 6. The charging method for a secondary battery according to any one of claims 1 to 3, wherein, The first current density I1 is 3.0 mA / cm². 2 the following, The second current density I2 is 4.0 mA / cm². 2 above.

7. The charging method for a secondary battery according to any one of claims 1 to 3, wherein, The amount of charge generated by the charging step S11 is more than 5% and less than 15% of the total amount of charge generated during the process of charging the secondary battery.

8. The charging method for a secondary battery according to any one of claims 1 to 3, wherein, The third current density is 1 mA / cm². 2 the following, The fourth current density is greater than the third current density and is 4.0 mA / cm². 2 the following, The fifth current density is greater than the fourth current density and is 4.0 mA / cm². 2 above.

9. The charging method for a secondary battery according to any one of claims 1 to 3, wherein, The amount of charge generated by the charging step S21 is more than 5% and less than 15% of the total amount of charge generated during the process of charging the secondary battery.

10. The charging method for a secondary battery according to claim 9, wherein, The total charging capacity through charging steps S21 and S22 is less than 50% of the total charging capacity.

11. The charging method for a secondary battery according to any one of claims 1 to 3, wherein, The negative electrode current collector is a copper foil or a copper alloy foil.

12. The charging method for a secondary battery according to any one of claims 1 to 3, wherein, The negative electrode includes the negative electrode current collector and a sheet of lithium metal adhering to the surface of the negative electrode current collector.

13. The charging method for a secondary battery according to any one of claims 1 to 3, wherein, The positive electrode comprises a composite oxide containing lithium and a metal Me other than lithium. The metal Me includes at least transition metals.

14. The charging method for a secondary battery according to claim 13, wherein, The molar ratio of the total amount of Li (mLi) in the positive electrode and the negative electrode to the amount of metal Me (mMe) in the composite oxide is mLi / mMe or less.

15. The charging method for a secondary battery according to claim 13, wherein, The composite oxide has a layered rock salt-type crystal structure. The metal Me includes at least Ni as the transition metal.

16. The charging method for a secondary battery according to claim 15, wherein, The composite oxide is of general formula (1): Li a Ni b M 1-b O2 indicates that In the general formula (1), 0.9 ≤ a ≤ 1.2 and 0.65 ≤ b ≤ 1 are satisfied. M is at least one element selected from the group consisting of Co, Mn, Al, Ti, Fe, Nb, B, Mg, Ca, Sr, Zr and W.

17. The charging method for a secondary battery according to any one of claims 1 to 3, wherein, The non-aqueous electrolyte contains lithium ions and anions. The anions include the anions of oxalate complexes.

18. The charging method for a secondary battery according to claim 17, wherein, The anions of the oxalate complex include difluorooxaloborate anions.

19. A charging system for a secondary battery, comprising a secondary battery, a depth of discharge (DOD) detection device for detecting the DOD of the secondary battery, and a charging control unit for controlling the charging of the secondary battery. in, The secondary battery comprises: a positive electrode, a negative electrode with a negative current collector, and a non-aqueous electrolyte. During charging, lithium metal is deposited on the negative electrode, and during discharging, the lithium metal dissolves in the non-aqueous electrolyte. The DOD detection device measures the depth of discharge of the secondary battery before charging begins. The charging control unit controls the charging of the secondary battery according to any one of the first charging curve and the second charging curve. The first charging curve includes at least two charging steps. The second charging curve includes more charging steps than the first charging curve. The first charging curve is selected when the depth of discharge is less than a predetermined threshold, and the second charging curve is selected when the depth of discharge is greater than or equal to the threshold. The first charging curve includes a charging step S11 of charging at a first current density I1, and a charging step S12 of charging at a second current density I2, which is greater than the first current density I1, following the charging step S11. The second charging curve includes a charging step S21 of charging at a third current density I3, and a charging step S22 of charging at a fourth current density I4, which is larger than the third current density I3, following the charging step S21. The second charging curve also includes a charging step S23, which follows the charging step S22 and charges at a fifth current density I5 that is larger than the fourth current density I4.

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