Secondary battery charging method and charging system

Abstract

This method for charging a secondary battery provided with a positive electrode for storing lithium ions during discharging and releasing the lithium ions during charging, a negative electrode onto which lithium metal is deposited during charging and from which the lithium metal is dissolved during discharging, and a non-aqueous electrolyte having lithium ion conductivity, includes a process for charging the secondary battery in accordance with a first charging profile. In the secondary battery, a surface of the negative electrode is covered by a protective layer. In the first charging profile, charging begins from a first charging step in which charging is performed using a constant current having a first current density I1, and a second charging step in which charging is performed using a constant current having a second current density I2 greater than the first current density I1 is executed following the first charging step.

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 [Problem to be solved by 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. [Means for solving the problem]

[0008] In view of the above, one aspect of the present invention relates to a method for charging a secondary battery including a positive electrode that absorbs lithium ions during discharge and releases the lithium ions during charge, a negative electrode where lithium metal precipitates during charge and dissolves during discharge, and a non-aqueous electrolyte having lithium ion conductivity, wherein the surface of the negative electrode of the secondary battery is covered with a protective layer, and the method includes a step of charging the secondary battery according to a first charging profile, wherein the first charging profile starts with a first charging step in which charging is performed at a constant current of a first current density I1, and following the first charging step, a second charging step is performed in which charging is performed at a constant current of a second current density I2 that is higher than the first current density I1.

[0009] In view of the above, another aspect of the present invention relates to a charging system for a secondary battery, comprising: a positive electrode that absorbs lithium ions during discharge and releases the lithium ions during charge; a negative electrode into which lithium metal precipitates during charge and from which the lithium metal dissolves during discharge; a non-aqueous electrolyte having lithium ion conductivity; and a protective layer covering the surface of the negative electrode; and a charge control unit that controls charging of the secondary battery, wherein the charge control unit selects one from one or more charge profiles including at least a first charge profile and controls charging of the secondary battery, and in the first charge profile, charging is started from a first charge step in which charging is performed at a constant current of a first current density I1, and following the first charge step, a second charge step is performed in which charging is performed at a constant current of a second current density I2 that is greater than the first current density I1. [Effects of the Invention]

[0010] According to the present disclosure, dendritic precipitation of lithium metal in a secondary battery is suppressed, and deterioration of cycle characteristics is suppressed. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings. [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] 1 is a schematic configuration diagram of a charging system for a secondary battery according to an embodiment of the present disclosure. [Figure 3] 1 is a vertical cross-sectional view schematically illustrating a lithium secondary battery used in a charging system for a secondary battery according to an embodiment of the present disclosure. [Figure 4] 4 is an enlarged view of a main part showing an example of the electrode group of FIG. 3. FIG. [Figure 5] 4 is an enlarged view of a main part showing another example of the electrode group of FIG. 3. FIG. 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 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 0.98 × C or more (SOC = 98% or more) 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] A secondary battery charged by the charging method according to the present disclosure includes a positive electrode that absorbs lithium ions during discharge and releases lithium ions during charge, a negative electrode where lithium metal precipitates during charge and dissolves during discharge, and a non-aqueous electrolyte having lithium ion conductivity. 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] In a secondary battery charged by the charging method according to the present disclosure, the surface of the negative electrode is covered with a protective layer. This covers the lithium metal deposited on the negative electrode, suppressing contact between the lithium metal and the non-aqueous electrolyte. The protective layer also acts to press the lithium metal. The protective layer can suppress the growth of lithium metal dendrites. This suppresses side reactions caused by contact between the lithium metal and the non-aqueous electrolyte, thereby suppressing deterioration in cycle characteristics associated with the side reactions. Furthermore, the charge control unit controls the charge as described below according to the depth of discharge, thereby synergistically suppressing dendrite growth.

[0018] (Secondary battery charging method) A charging method according to the present disclosure includes charging a secondary battery according to a first charging profile. The first charging profile is selected, for example, when the depth of discharge (DOD) of the secondary battery is equal to or greater than a first threshold. In the first charging profile, charging begins with a first charging step in which charging is performed at a constant current of a first current density I1. Following the first charging step, a second charging step is performed in which charging is performed at a constant current of a second current density I2 that is greater than the first current density I1. The first threshold may be set to 0% or a small value such as 10% or less. However, from the perspective of shortening the charging time, it is desirable to set the threshold to a relatively large value. Note that when the first threshold is set to 0%, the secondary battery is always charged according to the first charging profile regardless of the DOD.

[0019] 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.

[0020] 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.

[0021] The smaller the charging 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. However, when the DOD is shallow, there is a sufficient lithium metal underlayer on the negative electrode, making it less likely for lithium metal to deposit in a dendritic form than when the DOD is deep. In other words, when the DOD is deep, it is necessary to discharge with a small charging current to suppress the deposition of dendritic lithium metal. On the other hand, when the DOD is shallow, it is not necessary to reduce the charging current as much as when the DOD is deep; rather, increasing the charging current value shortens the charging time and is more practical.

[0022] Therefore, in a charging method according to an embodiment of the present disclosure, when the DOD is deep and equal to or greater than a first threshold, charging is initiated at a small constant current of a first current density I1 based on a first charging profile, and then charging is continued at a constant current of a second current density I2 greater than the first current density I1. This makes it possible to suppress the deposition of dendritic lithium metal and to prevent deterioration of cycle characteristics. Charging at the constant current of the first current density I1 (first charging step) is continued, for example, until the depth of discharge falls below a predetermined second threshold, and once the depth of discharge falls below the second threshold, charging is switched to charging at a constant current of the second current density I2 (second charging step).

[0023] On the other hand, when the DOD is shallow and is less than the first threshold, there is little need to charge at the small first current density I1 in order to suppress the deposition of dendritic lithium metal. Rather, priority may be given to charging at a current density greater than the first current density I1 to shorten the charge time. Therefore, when the DOD is less than the first threshold, charging may be performed based on a second charge profile different from the first charge profile. In the second charge profile, charging begins with a third charge step in which charging is performed at a constant current of a third current density I3 greater than the first current density I1. This makes it possible to suppress the deposition of dendritic lithium metal and shorten the charge time at the same time.

[0024] The first current density I1 is, for example, 3.0 mA / cm 2 less than 1mA / cm2 Considering the balance between the charging time and the cycle characteristics, the first current density I1 may be 0.1 mA / cm or less. 2 More than 0.5mA / cm is desirable. 2 It may be more than that.

[0025] The second current density I2 is greater than the first current density I1, for example, 4.0 mA / cm 2 or more. I1 / I2 may be, for example, 0.1 or more and 0.5 or less, or 0.2 or more and 0.4 or less. I1 / I2 may be, for example, 0.2 or more and 0.9 or less, or 0.3 or more and 0.7 or less.

[0026] The third current density I3 in the second charging profile may be equal to the second current density I2. In this case, in the first charging step when the first charging profile is selected, charging may be performed until the DOD becomes less than the first threshold, and then the second charging step may be started. That is, the second threshold may be the same as the first threshold. In this case, the second charging profile corresponds to the first charging profile in which the first charging step is skipped when the DOD is less than the first threshold. In this case, the secondary battery is charged at the current density I1 until the DOD becomes less than the first threshold when the DOD is equal to or greater than the first threshold, and is charged at the second current density I2 when the DOD is less than the first threshold. In other words, charging is essentially controlled based on a single charging profile having multiple charging steps in which the charging start and end conditions are set by the DOD. This allows for a simple and easy control circuit structure to be adopted when implementing the charging method.

[0027] The DOD threshold (first threshold) that determines whether to select the first or second charge profile may be, for example, 50% or more and 90% or less, or 70% or more and 90% 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, priority is given to selecting the second charge profile and completing charging as quickly as possible. On the other hand, at a deep depth of discharge (DOD) of 90% or more (i.e., SOC of less than 10%), 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, priority is given to selecting the first charge profile and proceeding with charging carefully.

[0028] The first charging profile and the second charging profile may each be configured with multiple charging steps. The first charging profile may further include one or more charging steps performed after the second charging step. Similarly, the second charging profile may further include one or more charging steps performed after the third charging step.

[0029] The number of charging steps n included in the first charging profile may be greater than the number of charging steps m included in the second charging profile. When the DOD is deep, the first charging profile including many charging steps is selected, and the charging current is increased as the charging period approaches the end, thereby prioritizing shortening the charging time. On the other hand, when the DOD is shallow, the second charging profile does not require the charging current to be increased in such small increments. This allows for simpler control (or a simpler control circuit structure) to be adopted.

[0030] The number m of charging steps included in the second charging profile is, for example, 2 to 5, and may be 2 to 4 or 2 to 3, but m = 2 is efficient. nm may be 1 or greater, but nm = 1 or 2 is efficient. Of the charging steps included in the first charging profile and / or the second charging profile, the last charging step may be a constant voltage charging step.

[0031] In both the first and second charging profiles, the charging current value is small at first and gradually increases. In the first charging profile including n charging steps, the charging current increases in smaller increments than in the second charging profile including m charging steps. 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 charging profile until it reaches a fully charged state (SOC 100%).

[0032] 1 is a flow diagram showing an example of charge control by a charge control unit in a secondary battery system of the present disclosure. The illustrated charge control method includes a step (S1) of selecting either a first charge profile or a second charge profile before the start of charging. Such selection is made based on the DOD detected by the DOD detection unit before the start of the process of charging the secondary battery (hereinafter also referred to as time T). This allows the charge 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.

[0033] Specifically, the second charging profile includes at least two charging steps, and the first charging profile includes more charging steps than the second charging profile, i.e., if the second charging profile includes m (≧2) charging steps, the first charging profile includes n (≧3) charging steps.

[0034] 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 first threshold, the second charging profile is selected, and if it is equal to or greater than the first threshold, the first 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 first charging profile, the charging current may be increased in smaller increments than in the second charging profile. That is, in the first charging profile, charging may start at a charging current smaller than in the second charging profile. Furthermore, in the charging steps executed in the latter half of the first charging profile, charging may be performed at a charging current larger than in the second charging profile.

[0035] The first charging profile may be, for example, a current density I 11 (=I1) and the subsequent charging step S11 (first charging step) at a current density I 11 current density I 12 (=I2) and a charging step S12 (second charging step) (I 11 12 (I1 <I2))。

[0036] The second charging profile may be, for example, a current density I 21 (=I3) and the subsequent charging step S21 (third charging step) at a current density I 21 current density I 22 and charging step S12 at (I 21 22 ). current density I 21 is the current density I 11 greater than (I 21 >I 11 (I3>I1)).

[0037] The first charging profile has more charging steps than the second charging profile, and the current density I following charging step S12 13 A charging step S13 is performed at a current density I 13 is the current density I 12 ​​It is desirable that it is larger than (I 12 13 ).

[0038] 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.

[0039] Charging step S11 is set as the first charging step in the first profile, for example. Charging step S21 is set as the first charging step in the second profile, for example. Charging steps S11, S12, and S13 in the first profile may all be constant current charging steps. Charging steps S21 and S22 in the second profile may all be constant current charging steps.

[0040] As mentioned above, I 11 (=I1) 21 (=I3). I 11 / I 21 (=I1 / I3) is not particularly limited, but may be, for example, 0.2 to 1, or 0.5 to 1.

[0041] I 21 I 22 The closer to (I 21 / I 22 The closer I12 is to 1, the shorter the charging time can be when charging the secondary battery with the second profile. 11 ​​ / I 12 The smaller the value, the less likely it is that lithium metal will grow in a dendrite shape. 21 / I 22 >I 11 / I 12 It is desirable to satisfy the following.

[0042] I 22 is 6.0mA / cm 2 However, I 22 If I is too large, the possibility of lithium metal becoming isolated during charging increases gradually. 22 is 8.0mA / cm 2 The following is desirable: I 21 / I 22 may be, for example, 0.1 or more and 0.8 or less, or 0.4 or more and 0.7 or less.

[0043] I 12 is 1.0mA / cm 2 More than 2.0mA / cm is desirable. 2 or more than 4.0mA / cm 2 However, I 12 If I is too large, the possibility of lithium metal becoming isolated during charging increases gradually. 12 is 6.0mA / cm 2 The following is desirable: I 13 is 6.0mA / cm 2 More than 8.0mA / cm is desirable. 2 However, I 13 If I is too large, the possibility of lithium metal becoming isolated during charging increases gradually. 13 is 10.0mA / cm 2 The following is desirable:

[0044] I 11 / 12 (=I1 / I2) may be, for example, 0.1 or more and 0.5 or less, or 0.2 or more and 0.4 or less. 12 / I 13 may be, for example, 0.2 or more and 0.9 or less, or 0.3 or more and 0.7 or less.

[0045] Small first current density I 11 In the charging step S11 (initial stage of charging) at a current density I (=I1), lithium metal precipitates in a lump (granular) form on the negative electrode current collector, and a good lithium metal underlayer is easily formed. 12 I 11 If the current density is set to be larger than 1000 kJ / s, the lithium metal dendrites do not grow easily. In addition, the lithium metal underlayer grows further in the charging step S12, so the current density I 13 I 12 Even if it is made even larger than (I 12 13 ), the growth of lithium metal dendrites is suppressed, which significantly reduces charging time.

[0046] The charge quantity of electricity (Q2) in the charging step S21 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. Hereinafter, the "total charge quantity of electricity" charged according to the second profile will also be referred to as the total charge quantity of electricity P2. When Q2 is 5% or more of the total charge quantity of electricity P2, the effect of suppressing the growth of dendritic lithium metal is increased. Furthermore, when Q2 is 15% or less of the total charge quantity of electricity P2, the charging time can be shortened while a sufficient effect of suppressing the growth of dendritic lithium metal can be obtained.

[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, DOD, or charging rate. The SOC or DOD 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 along the first profile, the current density I 11 When the battery voltage reaches the first voltage by the charging step S11 at the current density I 11 ​Charging step S11 is terminated at current density I 12 Then, the charging step S12 is started at a current density I 12 When the battery voltage reaches the second voltage by charging at a current density I 12 Charging step S12 is completed at a current density I 13 Then, the charging step S13 is started at a current density I 13 When the battery voltage reaches the third voltage due to the charging step at 13 The first voltage is, for example, the voltage when a charging amount of electricity equivalent to 15% or less of the total charging amount of electricity P1 is charged, the second voltage is, for example, the voltage when a charging amount of electricity equivalent to 50% or less of the total charging amount of electricity P1 is charged in total, and the third voltage is, for example, the voltage when a charging amount of electricity equivalent to 90% or more of the total charging amount of electricity P1 is charged in total.

[0049] Also, for example, in charging along the second profile, the current density I 21 When the battery voltage reaches the fourth voltage by the charging step S21 at the current density I 21 Charging is terminated at a current density I 22 Then, a charging step S22 is started at a current density I 22 When the battery voltage reaches the fifth voltage by the charging step S22 at the current density I 22 The fourth voltage is, for example, the voltage when a charging amount of electricity equivalent to 15% or less of the total charging amount of electricity P2 is charged, and the fifth voltage is, for example, the voltage when a charging amount of electricity equivalent to 90% or more of the total charging amount of electricity P2 is charged in total.

[0050] When the charge end voltage in each charging step is set based on the SOC or DOD, the second voltage may be equal to the fourth voltage, and the third voltage may be equal to the fifth voltage. That is, the charge end condition in charging step S12 may be the same as the charge end condition in charging step S21, and the charge end condition in charging step S13 may be the same as the charge end condition in charging step S22. In this case, I 12 =I 21(I2=I3) and I 13 =I 22 That is, the charging conditions in charging step S12 and charging step S21 may be the same, and the charging conditions in charging step S13 and charging step S22 may be the same.

[0051] Additionally, the first voltage may be a voltage corresponding to the threshold (first threshold) for determining DOD in step S1. If these conditions are met, charging is essentially controlled based on a single charging profile having multiple charging steps (three in the example of FIG. 1) in which charging start and end conditions are defined based on DOD. This simplifies control and allows the use of a simple control circuit structure. That is, charging of the secondary battery may be controlled based on a charging profile that includes at least a first charging step in which, when the DOD is below a threshold, charging is performed at a current density I1 until the DOD reaches the threshold, and a second charging step in which charging is performed at a current density I2 when the DOD is equal to or greater than the threshold.

[0052] 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.

[0053] (protective layer) The protective layer covers the surface of the negative electrode, thereby suppressing contact between lithium metal deposited on the negative electrode and the non-aqueous electrolyte and inhibiting the growth of lithium metal dendrites. As a result, side reactions due to contact between lithium metal and the non-aqueous electrolyte are suppressed, and deterioration of cycle characteristics associated with the side reactions can be suppressed. The protective layer may be, for example, a resin layer containing a resin. The resin layer may be a mixed layer of a resin and inorganic particles. This makes it easier to obtain good lithium ion conductivity in the protective layer, and lithium ions can move smoothly between the negative electrode and the non-aqueous electrolyte via the protective layer during charge and discharge.

[0054] Examples of resins include fluorine-containing polymers (fluorine-based resins), polyolefin resins, acrylic resins, silicone resins, epoxy resins, polyimides, polyamideimides, polyvinyl alcohols, polyacrylic acids, polymethacrylic acids, polyethylene oxides, and polystyrenes. One type of resin may be used alone, or two or more types may be used in combination. From the viewpoints of thermal stability and chemical stability, it is preferable that the resin contains a fluorine-based resin. It is preferable that the fluorine-based resin contains at least one selected from the group consisting of polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), a copolymer of vinylidene fluoride (VdF) and hexafluoropropylene (HFP), and a copolymer of VdF and tetrafluoroethylene (TFE). Of these, PVdF is more preferable.

[0055] The inorganic particles are particles containing an inorganic material (for example, metal oxide, metal hydroxide, metal composite oxide, metal nitride, metal carbide, metal fluoride, etc.). Specific examples of inorganic materials include copper oxide, bismuth oxide, tungsten oxide, indium oxide, silver oxide, etc. One type of inorganic particle (inorganic material) may be used alone, or two or more types may be used in combination. Among these, it is preferable that the inorganic particles contain at least one type selected from the group consisting of copper oxide particles and bismuth oxide particles. In order to increase the strength of the protective layer, the density of the inorganic particles is 6 g / cm. 3 It may be more than that.

[0056] When the protective layer is a layer containing a resin and inorganic particles, the density ratio of the inorganic particles to the resin may be 3.5 or more. When the density ratio is 3.5 or more, it is believed that the inorganic particles are deposited thinly and densely during the formation of the protective layer, forming a film of uniform thickness. Therefore, uneven distribution of the inorganic particles due to aggregation in the surface direction of the protective layer is suppressed, and the variation in the strength of the protective layer in the surface direction is suppressed, thereby improving the reliability of the strength of the protective layer.

[0057] PVdF is advantageous in that it is easy to adjust the density ratio of inorganic particles to resin material to 3.5 or more, which makes it easy to increase the strength of the protective layer, and it has an appropriate degree of swelling in the nonaqueous electrolyte solvent and is easy to process. The appropriate degree of swelling makes it easy to obtain good lithium ion conductivity in the protective layer.

[0058] The protective layer preferably contains one or more lithium salts. When the protective layer contains a lithium salt, the lithium ion conductivity can be increased. The lithium salt contained in the protective layer may be lithium bis(fluorosulfonyl)imide (LFSI). Among lithium salts, LFSI is preferred because it can increase the plasticity of the protective layer and improve film stability. The content of the lithium salt in the entire protective layer is, for example, 0.1 to 20 mass %.

[0059] The lithium salt contained in the protective layer is incorporated when the protective layer is formed. The lithium salt contained in the non-aqueous electrolyte may penetrate into the protective layer. Furthermore, a portion of the lithium salt contained in the protective layer may dissolve in the non-aqueous electrolyte. However, even if the lithium salt contained in the non-aqueous electrolyte penetrates into the protective layer, the lithium salt derived from the non-aqueous electrolyte remains on the surface of the protective layer, while the lithium salt incorporated during film formation is present in the depths of the protective layer. Therefore, the lithium salt contained in the protective layer can be identified by disassembling the secondary battery after charge and discharge, removing the protective layer, and analyzing its composition.

[0060] The molecular weight (weight average molecular weight) of the resin may be, for example, 10,000 or more and 2,000,000 or less. When the weight average molecular weight is within the above range, a protective layer with good flexibility is easily obtained. When the negative electrode is wound during the battery manufacturing process, the resin easily conforms to the negative electrode, and the coverage of the negative electrode surface by the protective layer is easily maintained.

[0061] From the viewpoint of preventing contact between lithium metal and the non-aqueous electrolyte and preventing dendrites from penetrating the protective layer, the thickness of the protective layer may be 0.1 μm or more and 5 μm or less, or 0.5 μm or more and 2 μm or less. The thickness of the protective layer is determined by obtaining a cross-sectional image of the protective layer on the surface of the negative electrode using a scanning electron microscope (SEM), measuring the thickness of any 10 points on the protective layer using the cross-sectional image, and calculating the average value of the thicknesses.

[0062] The protective layer must have high lithium-ion conductivity. However, high lithium-ion conductivity and film stability of the protective layer are generally inversely related. Protective layers with high lithium-ion conductivity tend to have low film strength, which can lead to uneven deposition of lithium metal with repeated charge-discharge cycles, making the protective layer susceptible to destruction. This can lead to lithium metal dendrites penetrating the protective layer or tearing the protective layer due to tension caused by expansion and contraction of the wound body during charge and discharge. As a result, the film stability of the protective layer decreases with increasing cycle count. Therefore, simply providing a protective layer on the negative electrode may not be effective in suppressing dendrite growth, and thus may not be able to sufficiently suppress deterioration in cycle performance.

[0063] However, according to the charging system for a secondary battery disclosed herein, by combining it with control of the charging current density according to the depth of discharge, the protective layer continues to function while suppressing uneven precipitation of lithium metal, thereby fully demonstrating the effect of suppressing dendrite growth and achieving significantly high cycle characteristics.

[0064] The protective layer can be formed, for example, by applying a protective layer-forming ink to the surface of the negative electrode current collector and drying it. The application can be performed using, for example, a bar coater, an applicator, a gravure coater, or the like. The protective layer-forming ink can be prepared, for example, by adding and mixing a resin material, a liquid component, and, if necessary, a lithium salt and inorganic particles. The liquid component can be a component capable of dispersing inorganic particles and dissolving the resin material, such as N-methyl-2-pyrrolidone (NMP), dimethyl ether (DME), or tetrahydrofuran (THF).

[0065] A space for depositing lithium metal is preferably provided between the negative electrode and the positive electrode. The space can be formed by providing a spacer between the negative electrode and the positive electrode. From the viewpoint of improving productivity, the spacer may be made of the same material as the protective layer. The thickness of the protective layer may be partially increased during the formation of the protective layer, and the thicker portion may be used as the spacer. That is, the spacer may be disposed integrally with the protective layer. For example, a coater (e.g., a gravure coater) may be used to apply a protective layer-forming ink to the surface of the negative electrode current collector to form a protective layer, and then a dispenser may be used to apply the same protective layer-forming ink in a line shape on the protective layer to form linear convex portions (spacers). From the viewpoint of improving productivity, the protective layer and the convex portions may be dried simultaneously after the convex portions are formed.

[0066] (Charging system) FIG. 2 is a configuration diagram showing an example of a charging system for a secondary battery according to an embodiment of the present disclosure. The charging system includes a secondary battery 10 and a charging device 101. An external power supply 105 that supplies power to the charging device 101 is connected to the charging device 101. The charging device 101 includes a charging control unit 102 that includes a charging circuit. The charging control unit 102 selects one charging profile from one or more charging profiles including at least a first charging profile, and controls charging of the secondary battery according to the selected charging profile. The charging profile may be selected based on, for example, the depth of discharge (DOD) of the secondary battery before charging begins.

[0067] For example, when the DOD before the start of charging is equal to or greater than a predetermined threshold, the charge control unit selects a first charge profile and controls charging of the secondary battery in accordance with the first charge profile. In the first charge profile, charging begins with a first charge step in which charging is performed at a constant current of a first current density I1, and following the first charge step, a second charge step is performed in which charging is performed at a constant current of a second current density I2 that is greater than the first current density I1. On the other hand, when the DOD before the start of charging is less than the predetermined threshold, the charge control unit selects, for example, a second charge profile and controls charging of the secondary battery in accordance with the second charge profile. In the second charge profile, charging begins with a third charge step in which charging is performed at a constant current of a third current density I3 that is greater than the first current density I1.

[0068] The charging device 101 includes a voltage detection unit 103 that detects the voltage of the secondary battery 10 as a DOD detection unit that detects the DOD of the secondary battery. The voltage detection unit 103 detects the voltage of the secondary battery 10 before starting charging of the secondary battery and includes a calculation unit that calculates the DOD based on the detected voltage. Based on the DOD calculated by the calculation unit, the charging control unit 102 selects either a first charging profile or a second charging profile. Then, the charging of the secondary battery is controlled according to the selected charging profile. Based on the DOD calculated by the calculation unit, the charging control unit 102 may start a second charging step after completing execution of a first step in the first charging profile.

[0069] The charging device 101 also includes a current detection unit 104 that detects the current output from the secondary battery. The charging control unit 102 controls the charging current so that the current value detected by the current detection unit 104 does not deviate significantly from a predetermined value.

[0070] 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 103, 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.

[0071] Next, the secondary battery will be described in more detail. (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.

[0072] 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 develops 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). If 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 develops capacity due to the deposition and dissolution of lithium metal. The protective layer covers the surface of the lithium ion absorption layer when lithium metal is not deposited on the surface of the lithium ion absorption layer, and covers the surface of the lithium metal when lithium metal is deposited on the surface of the lithium absorption layer.

[0073] 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.

[0074] 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.

[0075] 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).

[0076] The conductive material is, for example, a carbon material, such as carbon black, acetylene black, ketjen black, carbon nanotubes, and graphite.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] (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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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, mM, in the positive electrode may be set to, for example, 1.1 or less.

[0085] 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-b The 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.

[0086] 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.

[0087] 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).

[0088] (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.

[0089] 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.

[0090] (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.

[0091] The liquid non-aqueous electrolyte is prepared by dissolving a lithium salt in a non-aqueous solvent, which generates lithium ions and anions.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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.

[0097] 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.

[0098] The configuration of a lithium secondary battery will be described below with reference to the drawings, taking as an example a cylindrical battery equipped with a wound-type electrode group. Figure 3 is a vertical cross-sectional view of a lithium secondary battery 10 as an example of this embodiment.

[0099] The lithium secondary battery 10 is a cylindrical battery including a cylindrical battery case, a wound electrode group 14 housed in the battery case, and a nonaqueous electrolyte (not shown). The battery case is composed of a case body 15, which is a cylindrical metal container with a bottom, and a sealing body 16 that seals the opening of the case body 15. The case body 15 has an annular step 21 formed by pressing a portion of the side wall from the outside near the opening. The sealing body 16 is supported by the surface of the step 21 on the opening side. A gasket 27 is disposed between the case body 15 and the sealing body 16, thereby ensuring the hermeticity of the battery case. Within the case body 15, insulating plates 17 and 18 are disposed at both ends of the electrode group 14 in the winding axis direction.

[0100] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26. The cap 26 is disposed outside the case body 15, and the filter 22 is disposed inside the case body 15. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective centers, with an insulating member 24 interposed between their respective peripheral edges. The filter 22 and the lower valve body 23 are connected to each other at their respective peripheral edges. The upper valve body 25 and the cap 26 are connected to each other at their respective peripheral edges. The lower valve body 23 has an air vent. If the internal pressure of the battery case increases due to abnormal heat generation or the like, the upper valve body 25 bulges toward the cap 26 and separates from the lower valve body 23. This breaks the electrical connection between the lower valve body 23 and the upper valve body 25. If the internal pressure increases further, the upper valve body 25 breaks, and gas is discharged through an opening formed in the cap 26.

[0101] The electrode group 14 is composed of a positive electrode 11, a negative electrode (negative electrode current collector) 12, and a separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 interposed therebetween are all strip-shaped and are spirally wound so that their width directions are parallel to the winding axis. Insulating plates 17 and 18 are disposed at both axial ends of the electrode group 14, respectively.

[0102] Here, Fig. 4 is an enlarged view of a main part showing an example of the electrode group in Fig. 3. Fig. 4 is an enlarged view schematically showing the region X surrounded by the dashed line in Fig. 3, and shows a state in which lithium metal is not deposited on the surface of the negative electrode current collector.

[0103] As shown in Fig. 4, the positive electrode 11 includes a positive electrode current collector and a positive electrode composite layer. The positive electrode 11 is electrically connected to a cap 26, which also serves as a positive electrode terminal, via a positive electrode lead 19. One end of the positive electrode lead 19 is connected, for example, near the center of the positive electrode 11 in the longitudinal direction (the exposed portion of the positive electrode current collector). The other end of the positive electrode lead 19 extending from the positive electrode 11 is welded to the inner surface of the filter 22 through a through-hole formed in the insulating plate 17.

[0104] The negative electrode 12 includes a negative electrode current collector 32, and the surface of the negative electrode current collector 32 is covered with a protective layer 40. The protective layer 40 is a layer containing the block polymer described above. The negative electrode 12 is electrically connected to the case body 15, which also serves as a negative electrode terminal, via a negative electrode lead 20. One end of the negative electrode lead 20 is connected, for example, to an end of the negative electrode 12 in the longitudinal direction (the exposed portion of the negative electrode current collector 32), and the other end is welded to the inner bottom surface of the case body 15. During charging, lithium metal is deposited on the surface of the negative electrode current collector 32, and the surface of the lithium metal is covered with the protective layer 40.

[0105] Here, Fig. 5 is an enlarged view of a main part showing another example of the electrode group of Fig. 3. Fig. 5 is an enlarged view schematically showing the region X surrounded by the dashed line in Fig. 3, and shows a state in which lithium metal is not deposited on the surface of the negative electrode current collector. The same components as those in Fig. 4 are assigned the same reference numerals, and their explanations will be omitted.

[0106] As shown in FIG. 5, a spacer 50 is provided between a negative electrode 12 having a protective layer 40 on its surface and a separator 13. The spacer 50 is formed of linear protrusions provided along the longitudinal direction of the separator 13. The height of the spacer 50 (linear protrusions) based on the protective layer 40 is, for example, 10 μm or more and 100 μm or less. The width of the spacer 50 (linear protrusions) is 200 μm or more and 2000 μm or less. The spacer 50 may be made of the same material as the protective layer 40 and may be integrated with the protective layer 40. Multiple linear protrusions may be provided in parallel at a predetermined interval. As shown in FIG. 5, when lithium metal is not deposited on the surface of the negative electrode current collector 32, a space 51 is formed between the negative electrode 12 and the separator 13. The lithium metal deposited on the surface of the negative electrode current collector 32 during charging is received in the space 51 between the negative electrode 12 and the separator 13 while being subjected to the pressing force of the separator 13 .

[0107] Because the lithium metal is accommodated in the space 51 between the negative electrode 12 and the separator 13, the apparent volume change of the electrode assembly due to the deposition of the lithium metal during charge-discharge cycles is reduced. This also reduces the stress applied to the negative electrode current collector 32. Furthermore, because pressure is applied from the separator 13 to the lithium metal accommodated between the negative electrode 12 and the separator 13, the deposition state of the lithium metal is controlled, the lithium metal is less likely to become isolated, and a decrease in charge-discharge efficiency is suppressed.

[0108] In the illustrated example, the cross-sectional shape of the spacer 50 is rectangular. However, embodiments of the present disclosure are not limited to this, and the cross-sectional shape may be, for example, a trapezoid, a rectangle with at least one curved corner, an ellipse, or a partial ellipse. In the illustrated example, the spacer 50 is provided between the negative electrode 12 and the separator 13. However, embodiments of the present disclosure are not limited to this, and the spacer may be provided between the positive electrode and the separator, or between the positive electrode and the negative electrode and the separator, respectively.

[0109] In the illustrated example, a cylindrical lithium secondary battery having a wound-type electrode group has been described, but the shape of the lithium secondary battery is not limited to this and can be appropriately selected from various shapes such as cylindrical, coin, prismatic, sheet, and flat depending on the application. The shape of the electrode group is also not particularly limited and may be a laminated type. In addition, known components other than the electrode group and non-aqueous electrolyte of the lithium secondary battery can be used without any particular limitation.

[0110] [Example] The present invention will be specifically described below based on examples, but the present invention is not limited to the following examples.

[0111] Example 1 (Preparation of positive electrode) Lithium nickel composite oxide (LiNi 0.9 Co 0.05 Al 0.05O2), 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.

[0112] (Preparation of negative electrode) A strip of electrolytic copper foil (15 μm thick) was prepared as the negative electrode current collector, and a 25 μm Li foil was attached to the negative electrode current collector. PVdF, LFSI, and NMP (dispersion medium) were mixed in a mass ratio of PVdF:LFSI = 90:10 to prepare the protective layer ink. The protective layer ink was applied to both sides of the negative electrode and dried to form a protective layer (2 μm thick).

[0113] (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%.

[0114] (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.

[0115] The voltage at SOC 100% is 4.1V, and the voltage at DOD 100% is 3.0V.

[0116] (Pre-charge / discharge) The resulting battery A1 (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 corresponding 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 corresponding to the rated capacity C is charged or discharged at a constant current for 10 hours.

[0117] (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. (Pre-discharge) After a 10-minute rest, the voltage was increased to 3.0 V (SOC 0%) at 0.6 C (current density 6.0 mA / cm 2 ) constant current discharge was performed.

[0118] (Charge-discharge cycle test) Using the battery after preliminary charging and discharging, a charge-discharge cycle test was performed under the following charge-discharge conditions in an environment of 25° C. In the charge cycle, charging was performed based on a charge profile including two constant current charging steps in accordance with the first charge profile described above.

[0119] (1) DOD at the start of charging: 100% (voltage 3.0V) (2) First charging step (first charging step) First current density I1:3mA / cm 2 Charging quantity of electricity Q1: 11% of total charging quantity of electricity P1 (until DOD 89%) (3) Subsequent charging step (second charging step) 2nd current density I2: 4mA / cm 2 Charged electricity: Remaining part of total charged electricity P1 (4) Constant voltage charging step The current is 0.02C (current density 0.2mA / cm 2 ) Constant voltage charging at 4.1V (5) Discharge step After a 10-minute rest, the voltage was increased to 3.0 V at 0.6 C (current density 6.0 mA / cm 2 ) constant current discharge

[0120] The above charge / discharge cycle was repeated 250 times, and the discharge capacity after 250 cycles was C 250 C 250 The ratio C of the discharge capacity after pre-discharge to C0 250 / C0×100(%) was evaluated as the capacity retention rate.

[0121] Comparative Example 1 In the example, a negative electrode without a protective layer was prepared to obtain a battery B1. After preliminary charging and discharging, B1 was subjected to a charge-discharge cycle test under the following charge-discharge conditions in an environment of 25°C, and the discharge capacity C after 250 cycles was 250 The ratio C to the discharge capacity C0 after preliminary discharge 250 The capacity retention rate was evaluated as / C0 × 100 (%). As shown below, in the charging cycle, charging was performed based on a charging profile including one constant current charging step.

[0122] (1) DOD at the start of charging: 100% (voltage 3.0V) (2) First charging step 2nd current density I2: 4mA / cm 2Then, charge the full amount of electricity (the amount of electricity that will result in SOC 100%). (3) Constant voltage charging 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 3.0 V at 0.6 C (current density 6.0 mA / cm 2 ) constant current discharge

[0123] Comparative Example 2 The battery A1 of Example 1 was used. After the preliminary charge and discharge, the battery A1 was subjected to a charge and discharge cycle test at 25° C., including charging with the same charge profile as in Comparative Example 1, and the capacity retention rate was determined in the same manner as in Comparative Example 1.

[0124] Comparative Example 3 Battery B1 of Comparative Example 1 was used. After the preliminary charge and discharge, the battery B1 was subjected to a charge and discharge cycle test at 25° C., including charging with the same charge profile as in Example 1, and the capacity retention rate was determined in the same manner as in Example 1.

[0125] The evaluation results of the capacity retention rate of each battery are shown in Table 1. Table 1 also shows the configuration of the protective layer and the charging conditions (number of constant current charging steps) of the batteries used in the examples and comparative examples.

[0126] [Table 1]

[0127] In Comparative Examples 1 and 2, when the number of constant-current charging steps was one and charging to 100% SOC was performed at a second current density I2 higher than the first current density I1, the capacity retention rate was low. Furthermore, there was no clear difference in the capacity retention rate depending on whether or not a protective layer was present.

[0128] In contrast, in Example 1 and Comparative Example 3, the constant current charging step was divided into two steps, and charging was performed up to SOC 100% according to the constant current charging step at the first current density I1 and the subsequent constant current charging step at the second current density I2 (I1 < I2), thereby improving the capacity retention rate.

[0129] When the protective layer is not provided, from the difference between Comparative Example 1 and Comparative Example 3, the improvement width of the capacity retention rate by dividing the charging step into two steps is about 2.7%. In contrast, when the protective layer is provided, from the difference between Example 1 and Comparative Example 2, the improvement width of the capacity retention rate by dividing the charging step into two steps is about 6.2%, which is significantly larger.

Industrial Applicability

[0130] The charging method and charging system according to the present invention are preferably used for charging a lithium secondary battery of a type in which lithium metal is deposited on the negative electrode current collector during charging and the lithium metal is dissolved during discharging.

[0131] Although the present invention has been described with respect to the preferred embodiments at the present time, such disclosure should not be construed in a limiting sense. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the technical field to which the present invention pertains upon reading the above disclosure. Therefore, the appended claims should be construed to encompass all modifications and alterations without departing from the true spirit and scope of the present invention.

Explanation of Signs

[0132] 10 Lithium secondary battery <00006​​​​​​​​​​​​​​​​​​​​​​​ 24 Insulating material 25 Upper valve body 26 Cap 27 Gasket 32 Negative electrode current collector 40 protective layer 50 spacer 51 Space 101 Charging device 102 Charging control unit 103 Voltage detection unit 104 Current detection unit 105 External power supply

Claims

1. A method for charging a secondary battery comprising: a positive electrode that absorbs lithium ions during discharge and releases the lithium ions during charge; a negative electrode in which lithium metal precipitates during charge and in which the lithium metal dissolves during discharge; and a non-aqueous electrolyte having lithium ion conductivity, In the secondary battery, the surface of the negative electrode is covered with a protective layer, charging the secondary battery according to a first charging profile; In the first charging profile, a first current density I １ Charging is started from a first charging step in which charging is performed at a constant current of the first current density I １ a second current density I greater than ２ a second charging step is performed in which charging is performed at a constant current of the first current density I 1 is 3.0 mA / cm 2 or less; The method for charging a secondary battery, wherein the second current density I 2 is 4.0 mA / cm 2 or more.

2. The method further comprises the step of selecting one of a plurality of charging profiles including at least the first charging profile and a second charging profile; In the second charging profile, charging is performed at a constant current of a third current density I 3 that is greater than the first current density I 1 ; When the depth of discharge is equal to or greater than a first threshold, charging is performed according to the first charging profile; The method for charging a secondary battery according to claim 1 , wherein when the depth of discharge is less than the first threshold, charging is performed according to the second charging profile.

3. 3. The method for charging a secondary battery according to claim 1, wherein the first charging step is performed until a depth of discharge becomes less than a predetermined second threshold.

4. 4. The method for charging a secondary battery according to claim 1, wherein the amount of electricity charged in the first charging step is 5% or more and 15% or less of the total amount of electricity charged in the step of charging the secondary battery.

5. 5. The method for charging a secondary battery according to claim 1, wherein the protective layer contains one or more lithium salts.

6. the protective layer contains a resin, 6. The method for charging a secondary battery according to claim 1, wherein the molecular weight of the resin is 10,000 or more and 2,000,000 or less.

7. The method for charging a secondary battery according to claim 6 , wherein the resin includes a fluorine-based resin.

8. 8. The method for charging a secondary battery according to claim 1, wherein the protective layer has a thickness of 0.1 μm or more and 5 μm or less.

9. the non-aqueous electrolyte contains lithium ions and anions, 9. The method for charging a secondary battery according to claim 1, wherein the anion includes an anion of an oxalate complex.

10. The method for charging a secondary battery according to claim 9 , wherein the anion of the oxalate complex includes a difluorooxalate borate anion.

11. The method for charging a secondary battery according to any one of claims 1 to 10, wherein a space in which the lithium metal is precipitated is provided between the negative electrode and the positive electrode.

12. a positive electrode that absorbs lithium ions during discharge and releases the lithium ions during charge; a negative electrode in which lithium metal is deposited during charging and in which the lithium metal dissolves during discharging; a non-aqueous electrolyte having lithium ion conductivity; a protective layer covering the surface of the negative electrode; a secondary battery comprising: a charge control unit that controls charging of the secondary battery, the charge control unit selects one from one or a plurality of charge profiles including at least a first charge profile and controls charging of the secondary battery; In the first charging profile, a first current density I １ Charging is started from a first charging step in which charging is performed at a constant current of the first current density I １ a second current density I greater than ２ a second charging step is performed in which charging is performed at a constant current of the first current density I 1 is 3.0 mA / cm 2 or less; The charging system for a secondary battery, wherein the second current density I 2 is 4.0 mA / cm 2 or more.

13. The plurality of charging profiles further includes a second charging profile in which charging is performed at a constant current of a third current density I 3 that is greater than the first current density I 1 ; The charging control unit selecting the first charging profile when the depth of discharge is equal to or greater than a first threshold; The charging system for a secondary battery according to claim 12 , wherein the second charging profile is selected when the depth of discharge is less than the first threshold.

14. a DOD detection unit for detecting a depth of discharge of the secondary battery; 14. The charging system for a secondary battery according to claim 12, wherein in the first charging profile, the first charging step is performed until the depth of discharge becomes less than a predetermined second threshold.

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