Secondary battery charging method and charging system

Abstract

This lithium secondary battery charging method comprises a step for charging a lithium secondary battery in accordance with a first charging profile or a second charging profile. The first charging profile includes at least two charging steps. The second charging profile includes more charging steps than the first charging profile. At the start of the step for charging the secondary battery, if the depth of discharge of the secondary battery is less than a predetermined threshold value, the first charging profile is selected, and if the depth of discharge of the secondary battery is greater than or equal to the threshold value, the second charging profile is selected.

Description

[Technical Field]

[0001] The present disclosure relates to a charging method and a charging system for a secondary battery. [Background technology]

[0002] BACKGROUND ART Non-aqueous electrolyte secondary batteries, typified by lithium-ion secondary batteries, have high energy density and high output, and are considered promising 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 solar power.

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

[0004] Patent Document 2 describes a method for charging a secondary battery having a positive electrode current collector foil, a positive electrode active material layer, a solid electrolyte layer, and a negative electrode current collector foil in this order, and utilizing a deposition-dissolution reaction of metallic lithium as a negative electrode reaction, in which the secondary battery is charged at a first current density I1 (mA / cm 2 ), thereby depositing metallic lithium on the surface of the solid electrolyte layer facing the negative electrode current collector foil, thereby forming a roughness coating layer that is part of the negative electrode active material layer and is made of the metallic lithium; and a second charging step, after the first charging step, charging the secondary battery at a second current density I2 that is greater than the first current density I1, thereby increasing the thickness of the roughness coating layer, wherein the first charging step involves charging the secondary battery at the first current density I1 until X / Y becomes 0.5 or greater, where Y (μm) is a roughness height on the surface of the solid electrolyte layer facing the negative electrode current collector foil, and X (μm) is a thickness of the roughness coating layer. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-243957 [Patent Document 2] Japanese Patent Publication No. 2020-9724 Summary of the Invention

[0006] In secondary batteries in which lithium metal precipitates on the negative electrode during charging and dissolves in the non-aqueous electrolyte during discharging, it is generally difficult to control the morphology of lithium metal precipitation, and a technology to suppress dendrite formation is desired. The lithium metal precipitated in a dendritic form during charging tends to become partially isolated from the conductive network of the negative electrode during discharging. This results in a deterioration in cycle performance with repeated charge and discharge.

[0007] Although it is possible to suppress the deposition of dendritic lithium metal by reducing the current during charging, this tends to increase side reactions between the lithium metal and the non-aqueous electrolyte, and also lengthens the charging time, reducing the practical utility of the battery.

[0008] In view of the above, one aspect of the present disclosure relates to a charging method for a secondary battery including a positive electrode, a negative electrode including a negative electrode current collector, and a non-aqueous electrolyte, wherein lithium metal precipitates on the negative electrode during charging and the lithium metal dissolves in the non-aqueous electrolyte during discharging, the charging method including a step of charging the secondary battery according to either a first charging profile or a second charging profile, wherein the first charging profile includes at least two charging steps and the second charging profile includes more charging steps than the first charging profile, and wherein, at the start of the step of charging the secondary battery, when a depth of discharge of the secondary battery is less than a predetermined threshold, the first charging profile is selected, and when a depth of discharge of the secondary battery is equal to or greater than the threshold, the second charging profile is selected.

[0009] Another aspect of the present disclosure relates to a charging system for a secondary battery, the charging system including: a secondary battery; a DOD detection device that detects a depth of discharge of the secondary battery; and a charge control unit that controls charging of the secondary battery, wherein the secondary battery includes a positive electrode, a negative electrode having a negative electrode current collector, and a non-aqueous electrolyte, wherein lithium metal precipitates on the negative electrode during charging and the lithium metal dissolves in the non-aqueous electrolyte during discharging, the DOD detection device measures the depth of discharge of the secondary battery before starting charging of the secondary battery, and the charge control unit controls charging of the secondary battery according to either a first charge profile or a second charge profile, wherein the first charge profile includes at least two charge steps and the second charge profile includes more charge steps than the first charge profile, and wherein the first charge profile is selected when the depth of discharge is less than a predetermined threshold, and the second charge profile is selected when the depth of discharge is equal to or greater than the threshold.

[0010] According to the present disclosure, a good balance between the charging time and cycle characteristics of a secondary battery is achieved. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a flow diagram of a method for charging a secondary battery according to an embodiment of the present disclosure. [Figure 2] FIG. 2 is a schematic configuration diagram of a charging system for a secondary battery according to an embodiment of the present disclosure. [Figure 3] FIG. 3 is a schematic perspective view, with a portion cut away, of a secondary battery used in a charging method and charging system according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0012] Embodiments of a charging method and a charging system according to the present disclosure will be described below using examples. However, the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be used as examples, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In addition, examples of a charging method and a charging system according to the present disclosure will be described below with reference to the drawings as appropriate.

[0013] Hereinafter, the depth of discharge (DOD) is the ratio of the amount of discharged electricity to the amount of electricity contained in a battery in a fully charged state. Conversely, the state of charge (SOC) is the ratio of the amount of electricity remaining in a battery to the amount of electricity contained in a battery in a fully charged state. Note that the amount of electricity charged (i.e., the fully charged amount) when a battery in a fully discharged state (SOC = 0%, DOD = 100%) is charged until it reaches a fully charged state (SOC = 100%, DOD = 0%) corresponds to the rated capacity. The voltage of a battery in a fully charged state corresponds to the end-of-charge voltage. The voltage of a battery in a fully discharged state corresponds to the end-of-charge voltage. However, if the rated capacity of a battery is C, a state in which the battery is charged to a state of charge of, for example, 0.98 × C or more (SOC = 98%) may also be considered to be a fully charged state.

[0014] Also, the current density (mA / cm 2 ) is the unit opposing area (1 cm) between the positive and negative electrodes 2 ) and is obtained by dividing the current value applied to the battery by the total area of ​​the positive electrode composite layer (or positive electrode active material layer) facing the negative electrode (hereinafter also referred to as the total effective area of ​​the positive electrode). For example, when the positive electrode has a positive electrode composite layer on both sides of the positive electrode current collector, the total effective area of ​​the positive electrode is the total area of ​​the positive electrode composite layer on both sides (i.e., the sum of the projected areas of the positive electrode composite layers on both sides onto one and the other surface of the positive electrode current collector).

[0015] Furthermore, the secondary battery charged by the charging method according to the present disclosure is a secondary battery that includes a positive electrode, a negative electrode including a negative electrode current collector, and a non-aqueous electrolyte, in which lithium metal precipitates on the negative electrode during charging and dissolves in the non-aqueous electrolyte during discharging. That is, the secondary battery mainly refers to, but is not limited to, a lithium (metal) secondary battery.

[0016] In a lithium secondary battery, for example, 70% or more of the rated capacity is achieved by the deposition and dissolution of lithium metal. The movement of electrons in the negative electrode during charging and discharging is mainly due to the deposition and dissolution of lithium metal in the negative electrode. Specifically, 70 to 100% (e.g., 80 to 100% or 90 to 100%) of the movement of electrons (current from another perspective) in the negative electrode during charging and discharging is due to the deposition and dissolution of lithium metal. In other words, the negative electrode according to this embodiment differs from a negative electrode in which the movement of electrons in the negative electrode during charging and discharging is mainly due to the absorption and release of lithium ions by the negative electrode active material (such as graphite).

[0017] [How to charge a secondary battery] In lithium secondary batteries, controlling the morphology of lithium metal deposition is important for achieving good cycle characteristics. Although it is possible to suppress the deposition of dendritic lithium metal by reducing the current during charging, this increases the charging time. To improve the practicality of secondary batteries, it is necessary to suppress the deposition of dendritic lithium metal while shortening the charging time.

[0018] Therefore, a charging method according to the present disclosure includes a step of charging a secondary battery according to either a first charging profile or a second charging profile. FIG. 1 is a flow diagram showing an example of the charging method according to the present disclosure. The illustrated charging method includes a step (S1) of selecting either the first charging profile or the second charging profile before the start of charging. Such a selection is made based on the DOD before the start of the step of charging the secondary battery (hereinafter also referred to as time T). This allows the charging current to be selectively reduced within a necessary range during the use period of the secondary battery, which is repeatedly charged and discharged, and the overall time required for charging can be shortened.

[0019] Specifically, the first charging profile includes at least two charging steps, and the second charging profile includes more charging steps than the first charging profile. That is, if the first charging profile includes n (≧2) charging steps, the second charging profile includes (n+m) charging steps. n is, for example, 2 to 5, and may be 2 to 4 or 2 to 3, but n=2 is effective. m may be 1 or greater, but m=1 or m=2 is effective.

[0020] Here, a charging profile is a recipe that defines conditions such as the charging method, voltage, and current when charging a secondary battery. In other words, a charging profile specifies a schedule for each step of charging a secondary battery to a fully charged state. Each charging step is a step in which the secondary battery is charged under different charging conditions. The charging conditions are regulated by the charging current and / or charging voltage. Each charging step may be a constant current charging step or a constant voltage charging step.

[0021] In both the first and second charging profiles, the charging current value is small at first and gradually increases. In the second charging profile, which includes (n+m) charging steps, the charging current increases in smaller increments than in the first charging profile. However, the charging current may begin to decrease after a period in which it gradually increases from the start of charging. Typically, in the process of charging a secondary battery, the battery is charged according to the first or second profile until it reaches a fully charged state (SOC 100%).

[0022] At the start of the process of charging the secondary battery (time T), if the depth of discharge of the secondary battery is less than a predetermined threshold, the first charging profile is selected, and if it is equal to or greater than the threshold, the second charging profile is selected (S2). That is, at time T, if the DOD is shallow (if the battery SOC is high), the number of charging steps is small, and if the DOD is deep (if the battery SOC is low), the number of charging steps is large. In the second charging profile, the charging current increases in smaller increments. That is, in the second charging profile, charging can be started at a smaller charging current than in the first charging profile. Also, in the second charging profile, charging can be performed at a larger charging current than in the first charging profile.

[0023] The smaller the charge current, the less likely lithium metal is to deposit in a dendritic form, and the more likely it is to deposit in a planar form. The larger the charge current, the shorter the charge time. In other words, the second charge profile contributes to improving cycle characteristics and / or shortening charge time. On the other hand, when the DOD is shallow, a sufficient lithium metal underlayer is present on the negative electrode, making it less likely that lithium metal will deposit in a dendritic form than when the DOD is deep. In other words, the first charge profile does not require such small increases in charge current. Therefore, simpler control (or a simpler control circuit structure) can be adopted. Furthermore, when the first profile is selected, the charge current at the start of charge may be higher than the charge current at the start of charge in the second profile.

[0024] The DOD threshold that determines whether to select the first or second profile may be, for example, 50% or more and 70% or less, or 55% or more and 70% or less. At a shallow depth of discharge (DOD) of less than 50% (i.e., SOC of 50% or more), there is a sufficient base layer on which lithium metal precipitates during charging, making it difficult for lithium metal to become isolated. In this case, selecting the first profile and completing charging with simple control or as quickly as possible is preferred. Furthermore, when the DOD is shallow, there is no benefit in increasing the number of charging steps and charging at a high current toward the end of charging, even from the perspective of capacity retention. On the other hand, at a deep depth of discharge (DOD) of more than 80% (i.e., SOC of less than 20%), there is an insufficient base layer on which lithium metal precipitates during charging, making it highly likely that lithium metal will grow into a dendrite-like structure. In this case, selecting the second profile and proceeding with careful charging is preferred. Alternatively, selecting the second profile with more charging steps and charging at a higher current toward the end of charging is preferred, thereby shortening the charging time.

[0025] The first charging profile includes, for example, a charging step S11 at a first current density I1, followed by a charging step S12 at a second current density I2 that is greater than the first current density I1 (I1 <I2)。

[0026] The second charging profile includes, for example, a charging step S21 at a third current density I3 followed by a charging step S22 at a fourth current density I4 greater than the third current density I3 (I3 <I4)。

[0027] The second charging profile has more charging steps than the first charging profile, and at least includes a charging step S23 at a fifth current density I5 following the charging step S22. The fifth current density I5 of the charging step S23 following the charging step S22 is preferably greater than the fourth current density I4 (I4 <I5)。

[0028] The charging step S11 is set, for example, as the first charging step in the first profile. Also, the charging step S21 is set, for example, as the first charging step in the second profile. Both the charging step S11 and the charging step S12 in the first profile may be constant current charging steps. All of the charging step S21, the charging step S22, and the charging step S23 in the second profile may be constant current charging steps.

[0029] The third current density I3 may be smaller than the first current density I1 (I3 < I1). When the second charging profile is selected, since the DOD is deep, it is desirable to start charging with a smaller current. I3 / I1 is not particularly limited, but for example, it may be 0.2 to 1, or may be 0.5 to 1.

[0030] The closer I1 is to I2 (the closer I1 / I2 is to 1), the shorter the charging time when charging the secondary battery with the first profile can be. On the other hand, the larger I4 is with respect to I3 (the smaller I3 / I4 is), the less likely the lithium metal is to grow dendritically. Therefore, it is desirable to satisfy I1 / I2 > I3 / I4.

[0031] I1 / I2 may be, for example, 0.6 or more, may be 0.7 or more, may be 0.75 or more (further 0.8 or more). However, if I1 / I2 gets too close to 1, the possibility of the lithium metal becoming isolated during charging in the charging step S11 gradually increases, so it is desirable that I1 / I2 is 0.9 or less.

[0032] The charge amount Q1 in the charging step S11, the charge amount Q2 in the charging step S12, the charge amount Q3 in the charging step S21, and the charge amount Q4 in the charging step S22 may satisfy Q1 / Q2 < Q3 / Q4. The larger Q2 is with respect to Q1 (the smaller Q1 / Q2 is), the shorter the charging time when charging the secondary battery with the first profile. Also, the larger Q4 is with respect to Q3 (the smaller Q3 / Q4 is), the shorter the charging time when charging the secondary battery with the second profile. However, in terms of shortening the charging time, it is desirable that Q2 < Q4, and it is desirable that Q1 / Q2 < Q3 / Q4.

[0033] In the first charging profile, the first current density I1 is, for example, 3.0 mA / cm 2 or less, and the second current density I2 may be 4.0 mA / cm 2 or more.

[0034] Considering the balance between the charging time and the cycle characteristics, the first current density I1 is desirably 1.0 mA / cm 2 or more, and may be 2.0 mA / cm 2 or more.

[0035] The second current density I2 is desirably 4.0 mA / cm 2 or more, and may be 6.0 mA / cm 2 or more. However, if the second current density I2 is too high, the possibility of lithium metal isolation during charging gradually increases, so I2 is desirably 8.0 mA / cm 2 or less.

[0036] I1 / I2 may be, for example, 0.1 or more and 0.8 or less, or may be 0.4 or more and 0.7 or less.

[0037] The charge quantity of electricity (Q1) in the charging step S11 may be 5% or more and 15% or less of the total charge quantity of electricity charged in the process of charging the secondary battery. Here, the "total charge quantity of electricity charged in the process of charging the secondary battery" refers to the charge quantity of electricity 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 quantity of electricity" charged according to the first profile will also be referred to as the total charge quantity of electricity P1. When Q1 is 5% or more of the total charge quantity of electricity P1, the effect of suppressing the growth of dendritic lithium metal is enhanced. Furthermore, when Q1 is 15% or less of the total charge quantity of electricity, the charge time can be shortened while a sufficient effect of suppressing the growth of dendritic lithium metal can be obtained.

[0038] In the second charging profile, the third current density I3 is, for example, 1 mA / cm 2 the fourth current density I4 is greater than the third current density and is equal to or less than 4 mA / cm 2 the fifth current density I5 is greater than the second current density and is equal to or less than 4 mA / cm 2 It may be more than that.

[0039] The third current density I3 is set to 0.1 mA / cm, taking into consideration the balance between charging time and cycle characteristics. 2 More than 0.5mA / cm is desirable. 2 It may be more than that.

[0040] The fourth current density I4 is 1.0 mA / cm 2 More than 2.0mA / cm is desirable. 2 However, if the fourth current density I4 is too high, the possibility of lithium metal becoming isolated during charging gradually increases, so I4 should be 4.0 mA / cm 2 The following is desirable:

[0041] The fifth current density I5 is 6.0 mA / cm 2 More than 8.0mA / cm is desirable. 2The above may be applicable. However, if the fifth current density I5 is too high, the possibility of lithium metal becoming isolated during charging gradually increases. Therefore, I5 is preferably 10.0 mA / cm 2 or less.

[0042] I3 / I4 may be, for example, 0.1 or more and 0.5 or less, or may be 0.2 or more and 0.4 or less.

[0043] I4 / I5 may be, for example, 0.2 or more and 0.9 or less, or may be 0.3 or more and 0.7 or less.

[0044] In the charging step S21 (initial charging) at a small third current density I3, lithium metal is deposited in a块状 (granular) form on the negative electrode current collector, and a good underlying layer of lithium metal is likely to be formed. Therefore, even if the fourth current density I4 in the subsequent charging step S22 is made larger than the third current density I3, dendritic lithium metal is unlikely to grow. Also, since the underlying layer of lithium metal further grows in the charging step S22, even if the current density I5 in the subsequent charging step S23 is made even larger than I4 (I4 < I5), the growth of dendritic lithium metal is suppressed. As a result, it is possible to significantly shorten the charging time.

[0045] The charge amount (Q3) in the charging step S21 may be 5% or more and 15% or less of the total charge amount charged in the process of charging the secondary battery. Hereinafter, the "total charge amount" charged along the second profile is also referred to as the total charge amount P2. When Q3 is 5% or more of the total charge amount P2, the effect of suppressing the growth of dendritic lithium metal becomes greater. Also, when Q3 is 15% or less of the total charge amount P2, a sufficient effect of suppressing the growth of dendritic lithium metal while shortening the charging time can be obtained.

[0046] The total amount of charge electricity in charging step S21 and charging step S22 may be 50% or less, or even 40% or less, of the total amount of charge electricity P2. In this case, it is possible to significantly shorten the charging time while sufficiently suppressing the growth of dendritic lithium metal. The remaining 50% or more of the total amount of charge electricity P2 is charged in charging step S23 at a higher current density I5.

[0047] The timing to end each charging step may be controlled, for example, by the charging time, the amount of charged electricity, or the voltage, or by the ratio of the amount of electricity charged to the total amount of charged electricity P1 or P2 in each charging step, or by the SOC or charging rate. The SOC may be estimated from the voltage. For example, the SOC may be estimated from the voltage, and the end-of-charge voltage for each charging step may be set.

[0048] For example, in charging according to the first profile, when the battery voltage reaches a first voltage in charging step S11 at a first current density I1, charging at the first current density I1 is terminated and charging step S12 at a second current density I2 is initiated. Then, when the battery voltage reaches a second voltage in charging step S12 at the second current density I2, charging at the second current density I2 is terminated. The first voltage is, for example, the voltage when a quantity of charge electricity equivalent to 15% or less of the total charging quantity of electricity P1 has been charged, and the second voltage is, for example, the voltage when a total quantity of charge electricity equivalent to 90% or more of the total charging quantity of electricity P1 has been charged.

[0049] Furthermore, for example, in charging according to the second profile, 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 at the fifth current density I5, charging at the fifth current density I5 ends. The third voltage is, for example, the voltage when a charging quantity of electricity equivalent to 15% or less of the total charging quantity of electricity P2 has been charged. The fourth voltage is, for example, the voltage when a total charging quantity of electricity equivalent to 50% or less of the total charging quantity of electricity P1 has been charged. The fifth voltage is, for example, the voltage when a total charging quantity of electricity equivalent to 90% or more of the total charging quantity of electricity P1 has been charged.

[0050] To ensure reliable charging, a constant voltage charging step S3 may be performed following the constant current charging step. This charging step may be performed, for example, until the current reaches a predetermined value. For example, a final charging step may be performed at a constant current up to a predetermined charge cut-off voltage, followed by a constant voltage charging step at that voltage. Discharging is then performed, with the discharge limit set to the predetermined discharge cut-off voltage.

[0051] [Secondary battery charging system] A charging system according to the present disclosure includes a secondary battery, a DOD detection device that detects the depth of discharge of the secondary battery, and a charge control unit that controls charging of the secondary battery. The secondary battery includes a positive electrode, a negative electrode with a negative electrode current collector, and a non-aqueous electrolyte, in which lithium metal precipitates on the negative electrode during charging and dissolves in the non-aqueous electrolyte during discharge. The depth of discharge detection device measures the depth of discharge of the secondary battery before charging of the secondary battery begins. The charge control unit controls charging of the secondary battery according to either the first charge profile or the second charge profile described above. If the depth of discharge (DOD) of the secondary battery detected by the depth of discharge detection device before charging of the secondary battery begins is less than a predetermined threshold, the first charge profile is selected, and if the DOD is equal to or greater than the threshold, the second charge profile is selected.

[0052] 2 shows an example of a charging system according to an embodiment. The charging system includes a secondary battery 11 and a charging device 12. An external power supply 13 that supplies power to the charging device 12 is connected to the charging device 12. The charging device 12 includes a charging control unit 14 that includes a charging circuit. The charging control unit 14 controls charging of the secondary battery according to a selected charging profile.

[0053] The charging device 12 includes a voltage detection unit 15 that detects the voltage of the secondary battery 11 as a DOD detection device that detects the DOD of the secondary battery. The voltage detection unit 15 detects the voltage of the secondary battery 11 before charging of the secondary battery begins and includes a calculation unit that calculates the DOD based on the detected voltage. Based on the DOD determined by the calculation unit, the charging control unit 14 selects either the first profile or the second profile. Then, charging of the secondary battery is controlled according to the selected charging profile.

[0054] The charging device 12 also includes a current detection unit 16 that detects 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.

[0055] In FIG. 2, the timing of switching and terminating the charging step is controlled by the voltage (or DOD (or SOC)) detected by the voltage detection unit 15, but the control method is not limited thereto. For example, at least a part of the control may be performed based on the charging time, the amount of charged electricity, etc.

[0056] Next, the secondary battery will be described in more detail.

[0057] [Negative electrode] The negative electrode includes a negative electrode current collector. In a lithium secondary battery, lithium metal is deposited on the surface of the negative electrode upon charging. More specifically, lithium ions contained in the non-aqueous electrolyte receive electrons on the negative electrode upon charging, becoming lithium metal, which is then deposited on the surface of the negative electrode. The lithium metal deposited on the surface of the negative electrode dissolves as lithium ions in the non-aqueous electrolyte upon discharging. The lithium ions contained in the non-aqueous electrolyte may be derived from a lithium salt added to the non-aqueous electrolyte, may be supplied from the positive electrode active material upon charging, or may be both.

[0058] The negative electrode may include a negative electrode current collector and a sheet-like lithium metal in close contact with the surface of the negative electrode current collector, and may further include a lithium ion absorption layer (a layer that generates capacity by absorption and desorption of lithium ions by a negative electrode active material (such as graphite)) supported on the negative electrode current collector. In this case, the open circuit potential of the negative electrode when fully charged may be 70 mV or less relative to lithium metal (lithium dissolution and deposition potential). When the open circuit potential of the negative electrode when fully charged is 70 mV or less relative to lithium metal, lithium metal is present on the surface of the lithium ion absorption layer when fully charged. In other words, the negative electrode generates capacity by deposition and dissolution of lithium metal.

[0059] When the rated capacity of the battery is C, the fully charged state refers to a state in which the battery is charged to a state of charge of, for example, 0.98×C or more (SOC=98% or more). The open circuit potential of the negative electrode when fully charged can be measured by disassembling a fully charged battery under an argon atmosphere, removing the negative electrode, and assembling a cell with lithium metal as the counter electrode. The nonaqueous electrolyte in the cell may have the same composition as the nonaqueous electrolyte in the disassembled battery.

[0060] The lithium ion occlusion layer is a layer of a negative electrode mixture containing a negative electrode active material. The negative electrode mixture may contain a binder, a thickener, a conductive agent, etc. in addition to the negative electrode active material.

[0061] Examples of the negative electrode active material include a carbonaceous material, a Si-containing material, and a Sn-containing material. The negative electrode may contain one type of negative electrode active material or a combination of two or more types. Examples of the carbonaceous material include graphite, easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon).

[0062] The conductive material is, for example, a carbon material, such as carbon black, acetylene black, ketjen black, carbon nanotubes, and graphite.

[0063] Examples of the binder include fluororesin, polyacrylonitrile, polyimide resin, acrylic resin, polyolefin resin, rubber polymer, etc. Examples of the fluororesin include polytetrafluoroethylene, polyvinylidene fluoride, etc.

[0064] The negative electrode current collector may be a conductive sheet, such as a foil or film. The thickness of the negative electrode current collector is not particularly limited and is, for example, 5 μm or more and 300 μm or less.

[0065] The material of the negative electrode current collector (conductive sheet) may be any conductive material other than lithium metal and lithium alloys. The conductive material may be a metallic material such as a metal or alloy. The conductive material is preferably a material that does not react with lithium. More specifically, a material that does not form an alloy or an intermetallic compound with lithium is preferred. Examples of such conductive materials include copper (Cu), nickel (Ni), iron (Fe), alloys containing these metal elements, and graphite with a preferentially exposed basal plane. Examples of alloys include copper alloys and stainless steel (SUS). Among these, copper and / or copper alloys, which have high conductivity, are preferred. The negative electrode current collector may be copper foil or copper alloy foil.

[0066] [Positive electrode] The positive electrode includes, for example, a positive electrode current collector and a positive electrode composite layer supported on the positive electrode current collector. The positive electrode composite layer includes, for example, a positive electrode active material, a conductive material, and a binder. The positive electrode composite layer may be formed on only one side of the positive electrode current collector, or may be formed on both sides. The positive electrode is obtained, for example, by applying a positive electrode composite slurry including the positive electrode active material, the conductive material, and the binder to both sides of the positive electrode current collector, drying the coating, and then rolling.

[0067] The positive electrode active material is a material that absorbs and releases lithium ions. Examples of the positive electrode active material include composite oxides containing lithium and a metal other than lithium, Me (for example, 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. Among these, lithium-containing transition metal oxides are preferred because of their low production cost and high average discharge voltage. Among these, those having a layered rock salt crystal structure are preferred.

[0068] During charging, the lithium contained in the lithium-containing transition metal oxide is released from the positive electrode as lithium ions and precipitates as lithium metal on the negative electrode or negative electrode current collector. During discharging, the lithium metal dissolves from the negative electrode, releasing lithium ions that are then absorbed into the composite oxide of the positive electrode. That is, the lithium ions involved in charging and discharging are generally derived from the solute in the non-aqueous electrolyte and the positive electrode active material. In this case, the molar ratio of the total amount of Li, mLi, contained in the positive and negative electrodes to the amount of metal Me, mMe, contained in the lithium-containing transition metal oxide, mLi / mMe, is, for example, 1.2 or less.

[0069] Examples of transition metal elements contained in the lithium-containing transition metal oxide include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, and W. The lithium-containing transition metal oxide may contain one or more transition metal elements. The transition metal element may be Co, Ni, and / or Mn. The lithium-containing transition metal oxide may contain one or more typical elements as needed. Examples of typical elements include Mg, Al, Ca, Zn, Ga, Ge, Sn, Sb, Pb, and Bi. The typical element may be Al, etc.

[0070] Among lithium-containing transition metal oxides, composite oxides containing Co, Ni, and / or Mn as transition metal elements, and optionally containing Al, and having a layered rock-salt crystal structure are preferred in terms of achieving high capacity. Furthermore, lithium-containing transition metal oxides containing at least Ni as a transition metal are particularly preferred in terms of achieving high capacity. In this case, in a lithium secondary battery, the molar ratio mLi / mM of the total amount of lithium in the positive electrode and negative electrode, mLi, to the amount of metal M other than lithium in the positive electrode, mM, may be set to, for example, 1.1 or less.

[0071] The lithium-containing transition metal oxide is, for example, a lithium-containing transition metal oxide represented by the general formula (1): Li a Ni b M 1-bThe general formula (1) satisfies, for example, m0.9≦a≦1.2 and 0.65≦b≦1. M may 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.

[0072] As the binder, conductive agent, etc., for example, those exemplified for 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.

[0073] Examples of materials for the positive electrode current collector (conductive sheet) include metal materials containing Al, Ti, Fe, etc. The metal material may be Al, an Al alloy, Ti, a Ti alloy, an Fe alloy, etc. The Fe alloy may be stainless steel (SUS).

[0074] [Separator] The separator is made of a porous sheet having ion permeability and insulating properties. Examples of porous sheets include thin films, woven fabrics, and nonwoven fabrics having micropores. The material of the separator is not particularly limited, but may be a polymeric material. Examples of polymeric materials include olefin resins, polyamide resins, and cellulose. Examples of olefin resins include polyethylene, polypropylene, and copolymers of ethylene and propylene. The separator may contain additives as needed. Examples of additives include inorganic fillers.

[0075] The thickness of the separator is not particularly limited, but is, for example, 5 μm or more and 20 μm or less, and more preferably 10 μm or more and 20 μm or less.

[0076] [Non-aqueous electrolyte] The non-aqueous electrolyte having lithium ion conductivity contains, for example, a non-aqueous solvent and lithium ions and anions dissolved in the non-aqueous solvent. The non-aqueous electrolyte may be in a liquid state or a gel state.

[0077] The liquid non-aqueous electrolyte is prepared by dissolving a lithium salt in a non-aqueous solvent, which generates lithium ions and anions.

[0078] The gel-like non-aqueous electrolyte contains a lithium salt and a matrix polymer, or a lithium salt, a non-aqueous solvent, and a matrix polymer. The matrix polymer is, for example, a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, and polyether resin.

[0079] As the anion, a known anion used in the non-aqueous electrolyte of a lithium secondary battery can be used. Specifically, BF4 - , ClO4 - , PF6 - , CF3SO3 - , CF3CO2 - , anions of imides, anions of oxalate complexes, etc. Examples of imide anions include N(SO2CF3)2 - , N(C m F 2m+1 SO2) x (C n F 2n+1 SO2)y - (m and n are each independently an integer of 0 or 1 or greater, and x and y are each independently 0, 1, or 2, satisfying the relationship x+y=2.) The anion of the oxalate complex may contain boron and / or phosphorus. Examples of the anion of the oxalate complex include bisoxalate borate anion and difluorooxalate borate anion (BF2(C2O4) - ), PF4(C2O4) - , PF2(C2O4)2 - The non-aqueous electrolyte may contain one of these anions alone or two or more of them.

[0080] From the viewpoint of suppressing the deposition of lithium metal in a dendritic form, the nonaqueous electrolyte preferably contains at least an anion of an oxalate complex, and more preferably contains an oxalate complex anion having fluorine (particularly a difluorooxalate borate anion). The interaction between the oxalate complex anion having fluorine and lithium facilitates the uniform deposition of lithium metal in the form of fine particles. This makes it easier to suppress localized deposition of lithium metal. The oxalate complex anion having fluorine may be combined with other anions. The other anions may be PF6 - and / or an anion of an imide.

[0081] Examples of non-aqueous solvents include esters, ethers, nitriles, amides, and halogen-substituted derivatives thereof. The non-aqueous electrolyte may contain one or more of these non-aqueous solvents. Examples of halogen-substituted derivatives include fluorides.

[0082] The concentration of the 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 the anion in the non-aqueous electrolyte may be 0.5 mol / L or more and 3.5 mol / L or less. Furthermore, the concentration of the anion of the oxalate complex in the non-aqueous electrolyte may be 0.05 mol / L or more and 1 mol / L or less.

[0083] An example of the structure of a lithium secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween is housed in an exterior body together with an electrolyte solution. However, this is not limited thereto, and other forms of electrode groups may also be applied. For example, a stacked electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween may also be used. The shape of the lithium secondary battery is also not limited, and may be, for example, a cylindrical type, a prismatic type, a coin type, a button type, a laminate type, or the like.

[0084] FIG. 3 is a schematic perspective view, partially cut away, of a prismatic lithium secondary battery according to an embodiment of the present disclosure. The battery includes a bottomed prismatic battery case 4, an electrode group 1, and an electrolyte housed within the battery case 4. The electrode group 1 includes a long, strip-shaped negative electrode, a long, strip-shaped positive electrode, and a separator interposed therebetween. The negative electrode current collector is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. In other words, the positive electrode is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The periphery of the sealing plate 5 fits into the open end of the battery case 4, and the fitting is laser-welded. The sealing plate 5 has an electrolyte injection hole, which is closed with a seal 8 after injection.

[0085] [Example] The present disclosure will be specifically described below based on examples, but the present invention is not limited to the following examples.

[0086] Example 1 [Preparation of positive electrode] 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 and stirred to prepare a positive electrode slurry. Next, the positive electrode slurry was applied to the surface of an Al foil, which serves as a positive electrode current collector. The coating was dried and then rolled to form a positive electrode mixture layer (density 3.6 g / cm) on both sides of the Al foil. 3 ) was formed on the positive electrode.

[0087] [Preparation of negative electrode] An electrolytic copper foil (thickness: 10 μm) was cut into a predetermined electrode size to obtain a negative electrode current collector.

[0088] [Preparation of non-aqueous electrolyte] A non-aqueous electrolyte was prepared by dissolving a lithium salt 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 concentration of LiPF6 in the non-aqueous electrolyte was 0.5 mol / L, and the concentration of LiFSI was 0.5 mol / L. The content of LiFOB in the non-aqueous electrolyte was 1 mass%.

[0089] [Battery assembly] An Al positive electrode lead was attached to the positive electrode obtained above, and a Ni negative electrode lead was attached to the negative electrode obtained above. The positive electrode and negative electrode were spirally wound with a polyethylene thin film (separator) interposed therebetween in an inert gas atmosphere to produce a wound electrode assembly. The electrode assembly was housed in a bag-shaped exterior body formed of a laminate sheet having an Al layer, and the nonaqueous electrolyte was injected thereinto. The exterior body was then sealed to produce a nonaqueous electrolyte secondary battery. When the electrode assembly was housed in the exterior body, a portion of the positive electrode lead and the negative electrode lead were each exposed to the outside from the exterior body.

[0090] Since all the lithium comes from the nonaqueous 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 (here, Ni, Co, and Al) in the positive electrode (mMe) (mLi / mMe) was 1.0. The voltage at 100% SOC was 4.1 V, and the voltage at 100% DOD was 3.0 V.

[0091] [Pre-charge / discharge] The resulting battery (rated capacity 100 mAh) was subjected to the following preliminary charge / discharge tests at 25°C. The current value (1 / X)C represents the current value when a quantity of electricity equivalent to the rated capacity C is charged or discharged at a constant current for X hours. For example, 0.1C is the current value when a quantity of electricity equivalent to the rated capacity C is charged or discharged at a constant current for 10 hours.

[0092] (Pre-charging) The current density was 0.05 C (0.5 mA / cm) until the voltage reached 4.1 V (SOC 100%). 2 ) was charged at a constant current I0.

[0093] (Pre-discharge) After a 10-minute rest, the voltage was increased to 3.75 V at 0.6 C (current density 6.0 mA / cm 2 ) constant current discharge was performed.

[0094] [Charge / discharge cycle test] Using the battery after preliminary charging and discharging, a charge-discharge cycle test including charging according to the following first and second profiles was carried out in an environment of 25°C.

[0095] <First Profile> (1) DOD at the start of charging: 52% (voltage 3.75V) (2) First charging step S11 First current density I1: 1mA / cm 2 or 3mA / cm 2 Charged electricity quantity Q1: 15% of total charged electricity quantity P1 (3) Charging step S12 following S11 2nd current density I2: 4mA / cm 2 Charged electricity quantity Q2: Remaining part (85%) of total charged electricity quantity P1 Charge end voltage: 4.1V (4) Constant voltage charging following S21 The current is 0.02C (current density 0.2mA / cm 2 ) Constant voltage charging at 4.1V (5) After a 10-minute rest, the voltage was increased to 3.75 V at 0.6 C (current density 6.0 mA / cm 2 ) constant current discharge From the above, I1 / I2=1 / 4, Q1 / Q2=15 / 85.

[0096] <Second Profile> (1) DOD at the start of charging: 80% (voltage 3.55V) (2) First charging step S21 Third current density I3: 1mA / cm 2 Charging electricity quantity Q3: 15% of total charging electricity quantity P2 (3) Charging step S22 following S21 4th current density I4: 4mA / cm 2 Charging electricity quantity Q4: 35% of total charging electricity quantity P1 (Q3 + Q4 = 50%) (4) Charging step S23 following S22 5th current density I5:8mA / cm 2 Charging quantity of electricity Q5: Remaining portion (50%) of total charging quantity of electricity P1 (Q3 + Q4 + Q5 = 100%) (5) Constant voltage charging following S23 The current is 0.02C (current density 0.2mA / cm 2 ) Constant voltage charging at 4.1V (6)Discharge After a 10-minute rest, the voltage was increased to 3.55 V at 0.6 C (current density 6.0 mA / cm 2 ) constant current discharge From the above, I3 / I4=1 / 4, I4 / I5=4 / 8, and Q3 / Q4=15 / 35.

[0097] The time required for full charge according to the first and second profiles is shown in Table 1. The capacity retention rate after 60 cycles is shown in Table 2.

[0098] [Table 1]

[0099] Comparative Example 1 Next, contrary to Example 1, a charge-discharge cycle test was conducted using the battery after preliminary charging and discharging in an environment of 25°C, in which the battery was charged according to the first profile with a DOD of 80% at the start of charging, and then according to the second profile with a DOD of 52% at the start of charging.

[0100] <First Profile> (1) DOD at the start of charging: 80% (voltage 3.55V) (2) First charging step S21 First current density I1: 1mA / cm 2 Charged electricity quantity Q1: 15% of total charged electricity quantity P1 (3) Charging step S12 following S11 2nd current density I2: 4mA / cm 2 Charged electricity quantity Q2: Remaining part (85%) of total charged electricity quantity P1 Charge end voltage: 4.1V (4) Constant voltage charging following S12 The current is 0.02C (current density 0.2mA / cm 2 ) Constant voltage charging at 4.1V (5)Discharge After a 10-minute rest, the voltage was increased to 3.75 V at 0.6 C (current density 6.0 mA / cm 2 ) constant current discharge <Second Profile> (1) DOD at the start of charging: 52% (voltage 3.75V) (2) First charging step S21 Third current density I3: 1mA / cm 2 Charging electricity quantity Q3: 15% of total charging electricity quantity P2 (3) Charging step S22 following S21 4th current density I4: 4mA / cm 2 Charging electricity quantity Q4: 35% of total charging electricity quantity P2 (Q3 + Q4 = 50%) (4) Charging step S23 following S22 5th current density I5:8mA / cm 2 Charging quantity of electricity Q5: Remaining portion (50%) of total charging quantity of electricity P2 (Q3 + Q4 + Q5 = 100%) (5) Constant voltage charging following S23 The current is 0.02C (current density 0.2mA / cm 2 ) Constant voltage charging at 4.1V (6)Discharge After a 10-minute rest, the voltage was increased to 3.55 V at 0.6 C (current density 6.0 mA / cm 2 ) constant current discharge The time required for full charge according to the first and second profiles is shown in Table 2. Table 2 also shows the capacity retention rate after 60 cycles.

[0101] [Table 2]

[0102] Comparing Tables 1 and 2, it can be seen that when the DOD at the start of charging is shallow (DOD = 52%), there is little benefit to adopting the second profile in terms of shortening the charge time. In other words, even when the second profile is adopted and charging step S23 is incorporated at a higher fifth current density I5, the charge time is reduced by only 0.02 hours, from 1.82 hours to 1.80 hours, compared to when the first profile is adopted. In this case, it is sufficient to adopt the first charge profile, which is simpler to control. On the other hand, when the DOD at the start of charging is shallow (DOD = 52%), adopting the first profile and increasing the first current density I1 not only significantly reduces the charge time from 1.82 hours to 1.46 hours, but also maintains a high level of capacity retention.

[0103] Furthermore, when the second profile is selected when the DOD at the start of charging is shallow (DOD = 52%), the capacity retention tends to decrease slightly (96.0% → 93.8%). This is thought to be because metallic lithium is more likely to precipitate in a dendrite-like form 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 profile is selected, so it is preferable to select the first profile, which is more advantageous for capacity retention.

[0104] On the other hand, when the DOD at the start of charging is deep (DOD = 80%), the charging time is significantly reduced by using the second profile. That is, the charging time is reduced by 0.32 hours, from 3.04 hours to 2.72 hours, when using the second profile, compared to when using the first profile.

[0105] When the DOD is 52%, the benefit of shortening the charging time is minimal, so it is desirable to set the DOD threshold, which is the boundary between the first and second charging profiles, to at least 50% or more. Also, when the DOD is 80%, the benefit of shortening the charging time is significant, so it is desirable to set the DOD threshold to, for example, 70% or less.

[0106] Comparative Example 2 Next, using the battery after preliminary charging and discharging, a charge-discharge cycle test was conducted in an environment of 25°C, in which the DOD at the start of charging was set to 52%, and charging was performed using the following comparative profile, which omitted charging step S11 from the first profile.

[0107] <Comparison Profile> (1) DOD at the start of charging: 52% (voltage 3.75V) (2) Charging step S12 Current density: 4mA / cm 2 Charged electricity amount: 100% of total charged electricity amount P1 Charge end voltage: 4.1V (3) Constant voltage charging following S12 The current is 0.02C (current density 0.2mA / cm 2 ) Constant voltage charging at 4.1V (4)Discharge After a 10-minute rest, the voltage was increased to 3.75 V at 0.6 C (current density 6.0 mA / cm 2 ) constant current discharge Table 3 shows the capacity retention rate after 60 cycles compared with when charging according to the first profile.

[0108] [Table 3]

[0109] The results in Table 3 show that even when the DOD at the start of charging is shallow, the capacity retention rate decreases if charging is not performed according to the first charging profile, which includes at least two charging steps. [Industrial Applicability]

[0110] The charging method and charging system according to the present disclosure are suitable for use in charging a type of lithium secondary battery in which lithium metal is deposited on a negative electrode current collector during charging and the lithium metal dissolves during discharging. [Explanation of symbols]

[0111] 1 electrode group 2 positive leads 3 Negative lead 4 Battery case 5 Sealing plate 6 Negative terminal 7 Gasket 8. Seal 11 Nonaqueous electrolyte secondary battery 12 Charging device 13 External power supply 14 Charging control unit 15 Voltage detection section 16 Current detection section

Claims

1. A method for charging a secondary battery comprising: a positive electrode; a negative electrode including a negative electrode 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, charging the secondary battery according to either a first charging profile or a second charging profile; the first charging profile includes at least two charging steps; the second charging profile includes more charging steps than the first charging profile; At the start of the step of charging the secondary battery, When the depth of discharge of the secondary battery is less than a predetermined threshold, the first charging profile is selected; When the depth of discharge of the secondary battery is equal to or greater than the threshold, the second charging profile is selected; The first charging profile comprises: A charging step S11 at a first current density I1, followed by a charging step S12 at a second current density I2 greater than the first current density I1, The second charging profile is a charging step S21 at a third current density I3, followed by a charging step S22 at a fourth current density I4 greater than the third current density I3; The method for charging a secondary battery, wherein the second charging profile includes a charging step S23 following the charging step S22 at a fifth current density I5 that is greater than the fourth current density I4.

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

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

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

5. 5. The charging method for a secondary battery according to claim 4, wherein the charging quantity Q1 of the charging step S11, the charging quantity Q2 of the charging step S12, the charging quantity Q3 of the charging step S21, and the charging quantity Q4 of the charging step S22 satisfy Q1 / Q2<Q3 / Q4.

6. The first current density I1 is 3.0 mA / cm 2 is as follows: The second current density I2 is 4.0 mA / cm 2 The method for charging a secondary battery according to any one of claims 1 to 5.

7. The method for charging a secondary battery according to any one of claims 1 to 6, wherein the amount of electricity charged in the charging step S11 is 5% or more and 15% or less of the total amount of electricity charged in the process of charging the secondary battery.

8. The third current density is 1 mA / cm 2 is as follows: The fourth current density is greater than the third current density and is 4 mA / cm 2 is as follows: The fifth current density is greater than the fourth current density and is 4 mA / cm 2 The method for charging a secondary battery according to any one of claims 1 to 7.

9. The method for charging a secondary battery according to any one of claims 1 to 8, wherein the amount of electricity charged in the charging step S21 is 5% or more and 15% or less of the total amount of electricity charged in the process of charging the secondary battery.

10. 10. The method for charging a secondary battery according to claim 9, wherein a total amount of charged electricity in the charging step S21 and the charging step S22 is 50% or less of the total amount of charged electricity.

11. The method for charging a secondary battery according to any one of claims 1 to 10, wherein the negative electrode current collector is a copper foil or a copper alloy foil.

12. The method for charging a secondary battery according to any one of claims 1 to 11, wherein the negative electrode comprises the negative electrode current collector and a sheet-like lithium metal in close contact with a surface of the negative electrode current collector.

13. the positive electrode includes a composite oxide including lithium and the metal other than lithium (Me), The method for charging a secondary battery according to any one of claims 1 to 12, wherein the metal Me includes at least a transition metal.

14. 14. The method for charging a secondary battery according to claim 13, wherein the molar ratio mLi / mMe of the total amount mLi of Li contained in the positive electrode and the negative electrode to the amount mMe of the metal Me contained in the composite oxide is 1.2 or less.

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

16. The composite oxide has the general formula (1): Li ａ Ni ｂ M １－ｂ O ２ is expressed as In the general formula (1), 0.9≦a≦1.2 and 0.65≦b≦1 are satisfied, 16. The method for charging a secondary battery according to claim 15, wherein 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 non-aqueous electrolyte contains lithium ions and anions, The method for charging a secondary battery according to any one of claims 1 to 16, wherein the anion includes an anion of an oxalate complex.

18. 18. The method for charging a secondary battery according to claim 17, wherein the anion of the oxalate complex includes a difluorooxalate borate anion.

19. a secondary battery; a DOD detection device that detects the depth of discharge of the secondary battery; and a charge control unit that controls charging of the secondary battery; The secondary battery comprises a positive electrode, a negative electrode including a negative electrode current collector, and a non-aqueous electrolyte, wherein lithium metal is deposited on the negative electrode during charging, and the lithium metal is dissolved in the non-aqueous electrolyte during discharging; the DOD detection device measures the depth of discharge of the secondary battery before starting charging of the secondary battery; the charge control unit controls charging of the secondary battery according to either a first charge profile or a second charge profile; the first charging profile includes at least two charging steps; the second charging profile includes more charging steps than the first charging profile; When the depth of discharge is less than a predetermined threshold, the first charging profile is selected, and when the depth of discharge is equal to or greater than the threshold, the second charging profile is selected; The first charging profile comprises: A charging step S11 at a first current density I1, followed by a charging step S12 at a second current density I2 greater than the first current density I1, The second charging profile is a charging step S21 at a third current density I3, followed by a charging step S22 at a fourth current density I4 greater than the third current density I3; The second charging profile includes a charging step S23 at a fifth current density I5 greater than the fourth current density I4, following the charging step S22.

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