Secondary battery charging method and secondary battery charging device

Abstract

To provide a technique that can reduce the charging time of a secondary battery.SOLUTION: A charging method of a secondary battery comprises: a high-current charging step of supplying a charging current at a maximum current value during charging to the secondary battery and charging the secondary battery; and a current reducing step of reducing the current value of the charging current from the maximum current value in a stepwise manner with the lapse of a charging time. According to the charging method, the charging time can be reduced through high-efficiency charging by the high-current charging step. Also, with the current deterioration step, high-efficiency charging can be performed while suppressing the occurrence of a voltage overshoot.SELECTED DRAWING: Figure 5

Description

[Technical field]

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

[0002] As one example of a chargeable and dischargeable power storage device, a secondary battery such as a lithium ion secondary battery is known. The secondary battery may be charged by a constant current, a constant voltage, or a combination of these. For example, Patent Document 1 below discloses a charge control that switches from a constant current charge to a constant voltage charge. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2022-69459 A Summary of the Invention [Problem to be solved by the invention]

[0004] It has been desirable to shorten the charging time of secondary batteries, but conventional charging control has not been able to fully meet this demand.

[0005] Furthermore, in conventional charging control, repeated charging and discharging can result in a decrease in battery performance, such as a decrease in charging capacity. For example, according to research by the present inventors, in the case of a lithium-ion secondary battery in which carbon nanowalls are applied to the active material layer, depending on the charging method, the carbon nanowalls may fall off the current collector, resulting in a decrease in battery performance.

[0006] There is still room for improvement in various aspects of charging control of secondary batteries, including lithium-ion secondary batteries, including shortening the charging time. An object of the present application is to provide a charging technique for secondary batteries that can at least shorten the charging time. [Means for solving the problem]

[0007] The present invention can be realized, for example, in the following forms.

[0008] One aspect of the present invention is provided as a method for charging a secondary battery, comprising: a high-current charging step of supplying a charging current with a maximum current value during charging to the secondary battery, and a current reducing step of reducing the current value of the charging current in a stepwise manner from the maximum current value over charging time.

[0009] According to the charging method of this embodiment, rapid charging is possible by highly efficient charging at the maximum current value before the current reduction step. In addition, after rapid charging at the maximum current value, a current reduction step is executed to reduce the current value of the charging current in a stepwise manner, so that it is possible to prevent the voltage of the secondary battery from increasing significantly as the charge amount of the secondary battery increases, and to prevent the occurrence of voltage overshoot. Therefore, according to the charging method of this embodiment, smooth rapid charging at a high current value in accordance with the charge state of the secondary battery is possible, and the charging time of the secondary battery can be shortened.

[0010] The present invention can be realized in various forms other than the method for charging a secondary battery. For example, the present invention can be realized in the form of a charging device for charging a secondary battery, a control circuit / control device for controlling charging of a secondary battery, a vehicle or a mobile information terminal for executing the method for charging a secondary battery, or other appliances / devices / systems equipped with a secondary battery. The present invention can also be realized in the form of a control program for causing a computer to execute the method for charging a secondary battery, or a recording medium on which the program is recorded. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram showing the configuration of a charging device for a secondary battery. [Diagram 2] FIG. 1 is a schematic diagram showing a configuration of a secondary battery. [Diagram 3] FIG. 1 is a schematic diagram showing the configuration of a carbon nanostructure. [Figure 4] FIG. 4 is a process flow diagram of charging control in the first embodiment. [Diagram 5] FIG. 2 is an explanatory diagram for explaining charging control according to the first embodiment. [Figure 6] FIG. 11 is an explanatory diagram showing the change in voltage of a secondary battery over time as a reference experiment result. [Figure 7] FIG. 11 is a first explanatory diagram showing a photographed image of an electrode as a reference experimental result. [Figure 8] FIG. 2 is a second explanatory diagram showing a photographed image of an electrode as a reference experimental result. [Figure 9] FIG. 4 is an explanatory diagram showing changes over time in current and voltage of a secondary battery as an embodiment. [Figure 10] FIG. 11 is a first explanatory diagram for explaining charging control in a comparative example. [Figure 11] FIG. 2 is a second explanatory diagram for explaining charging control in the comparative example. [Figure 12] FIG. 11 is a third explanatory diagram for explaining charging control in the comparative example. [Figure 13] FIG. 4 is an explanatory diagram showing charging times in an example and a comparative example of the first embodiment. [Figure 14] FIG. 4 is an explanatory diagram showing changes in battery performance due to repeated charging and discharging in an example. [Figure 15] FIG. 4 is an explanatory diagram showing changes in battery performance due to repeated charging and discharging in a comparative example. [Figure 16] FIG. 11 is a process flow diagram of charging control in the second embodiment. [Figure 17] FIG. 11 is an explanatory diagram for explaining charging control according to a second embodiment. [Figure 18] FIG. 11 is an explanatory diagram for explaining an example of the second embodiment. [Figure 19] FIG. 11 is an explanatory diagram for explaining a comparative example of the second embodiment. [Figure 20]FIG. 11 is an explanatory diagram showing charging times in an example and a comparative example of the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, with reference to the drawings, an embodiment of a method for charging a secondary battery and a charging device for a secondary battery according to the present invention will be described.

[0013] 1. First embodiment: 1-1.Charging device configuration: 1 is a schematic diagram showing the configuration of a charging device 50 for a secondary battery 10 in the first embodiment. The charging device 50 supplies DC power to the secondary battery 10 to charge the secondary battery 10. The secondary battery 10 in this embodiment is a lithium ion battery in which lithium (Li) ions are involved in charging and discharging. The configuration of the secondary battery 10 will be described later.

[0014] The charging device 50 includes a charging circuit 59 electrically connected to the secondary battery 10. The charging circuit 59 may be capable of supplying power to the secondary battery 10 by contacting electrodes of the charging circuit 59 with the secondary battery 10 to establish electrical conduction, or may be capable of supplying power to the secondary battery 10 by a wireless power supply method.

[0015] In this embodiment, the charging circuit 59 is configured as an integrated circuit mounted on a microchip. The charging circuit 59 includes, as functional units, a power supply unit 51, a control unit 52, a current control unit 53, a switch unit 54, a current measurement unit 55, and a voltage measurement unit 56.

[0016] The power supply unit 51 is connected to an external power supply (not shown) and functions as a supply source of charging current for the secondary battery 10. When the external power supply is an AC power supply, the power supply unit 51 receives power converted to DC from the external power supply via an AC adapter. The power supply unit 51 may receive charging power from an electronic device such as a personal computer via a USB cable or the like.

[0017] The control unit 52 is configured by, for example, a microprocessor. The control unit 52 controls each component of the charging device 50 to execute charging control of the secondary battery 10. The charging control by the control unit 52 will be described later. The control unit 52 also has a function of detecting the state of charge (SOC) of the secondary battery 10 using the detection results of the current measurement unit 55 and the voltage measurement unit 56. The SOC is an index indicating the charge amount of the secondary battery 10, and for example, a fully charged state is indicated as 100%.

[0018] The current control unit 53 is configured with an element that realizes the function of a DC / DC converter, such as a switching regulator. The current control unit 53 is connected to the wiring between the power supply unit 51 and the secondary battery 10, and controls the magnitude of the charging current supplied from the power supply unit 51 to the secondary battery 10 according to the current value commanded by the control unit 52.

[0019] The switch unit 54 is configured, for example, by a switch element capable of controlling the opening and closing of a current. The switch unit 54 is provided in the wiring between the power supply unit 51 and the secondary battery 10, and opens and closes under the control of the control unit 52. The switch unit 54 is closed while the secondary battery 10 is being charged, and is opened when charging of the secondary battery 10 is completed.

[0020] Current measurement unit 55 is, for example, configured with an element that realizes the function of a current sensor, and detects the magnitude of the current charging current. Current measurement unit 55 outputs a signal proportional to the current charging current to control unit 52. Control unit 52 controls the driving of current control unit 53 based on the detection result of current measurement unit 55 during charging control.

[0021] The voltage measurement unit 56 is configured, for example, by an element that realizes the function of a voltage sensor, and detects the current voltage of the secondary battery 10. The voltage measurement unit 56 outputs a signal proportional to the voltage of the secondary battery 10 to the control unit 52. The control unit 52 executes charging control based on the detection result of the voltage measurement unit 56.

[0022] 1-2. Structure of secondary battery: Fig. 2 is a schematic diagram showing the configuration of a secondary battery 10 of the first embodiment. The secondary battery 10 includes a container 11, an electrolytic solution 12, a separator 15, a first electrode 20, and a second electrode 30. For convenience, in Fig. 1, the container 11 is illustrated by a dashed line, and the separator 15 is illustrated by a broken line.

[0023] The container 11 has an internal space filled with the electrolytic solution 12. The container 11 is liquid-tightly constructed from a material that is unlikely to react with the electrolytic solution 12. The electrolytic solution 12 has a property that metal ions involved in charging and discharging can be transferred between the first electrode 20 and the second electrode 30. In this embodiment, the electrolytic solution 12 is composed of a solution in which a lithium salt is dissolved in an organic solvent, and can transfer Li ions between the first electrode 20 and the second electrode 30. For example, lithium hexafluorophosphate (LiPF6) can be used as the lithium salt of the electrolytic solution 12. For example, dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc. can be used as the organic solvent.

[0024] The separator 15 divides the internal space of the container 11 into a first electrode chamber 16 in which the first electrode 20 is accommodated and a second electrode chamber 17 in which the second electrode 30 is accommodated. The separator 15 has electrical insulation and ion conductivity, and electrically insulates the first electrode 20 from the second electrode 30 while allowing metal ions (Li ions in this embodiment) transmitted via the electrolytic solution 12 to pass therethrough. The separator 15 is made of, for example, a resin film or nonwoven fabric having a porous structure.

[0025] The first electrode 20 constitutes a negative electrode. The first electrode 20 includes a first current collector 21 and a first active material layer 22. The first current collector 21 is made of a metal substrate. In this embodiment, the first current collector 21 is made of a metal foil of copper (Cu).

[0026] The first current collector 21 may be made of a Cu alloy or a metal other than Cu. The first current collector 21 may be made of, for example, aluminum (Al) or an Al alloy. The first current collector 21 may not be made of a metal foil, and may be made of, for example, a metal sheet or a metal thin film. The first current collector 21 may not be made of a flat plate, and may have, for example, a micron-order fine uneven structure formed on the surface, or may be bent into various shapes such as a cylindrical shape or a corrugated shape.

[0027] The first active material layer 22 is provided on both the first surface 21a and the second surface 21b of the first current collector 21. The first active material layer 22 contains carbon (C) as an active material. In this embodiment, the first active material layer 22 has a carbon nanostructure made of graphene. The carbon nanostructure will be described later.

[0028] The second electrode 30 constitutes the positive electrode of the secondary battery 10. The second electrode 30 has a second current collector 31 and a second active material layer 32. The second current collector 31 is made of, for example, a metal foil such as Al or titanium (Ti). The second current collector 31 may be made of other metals and may have a form other than a metal foil. The second current collector 31 does not have to be configured in a flat shape, and may be bent into various shapes such as a cylindrical shape or a corrugated shape.

[0029] The second active material layer 32 is formed on each of the first surface 31a and the second surface 31b of the second current collector 31. The second active material layer 32 contains an active material including metal ion atoms involved in charge and discharge (Li atoms in this embodiment), a conductive assistant, and a binder. The second active material layer 32 may contain a thickener.

[0030] As the active material of the second electrode 30, for example, a ternary material can be used, such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LMO), or lithium nickel oxide (NCA). As the conductive assistant, for example, acetylene black or carbon black can be used. As the binder, for example, polyvinylidene fluoride (PVDF) or styrene butadiene rubber (SBR) can be used. As the thickener, for example, carboxymethyl cellulose (CMC) can be used.

[0031] Fig. 3 is a schematic diagram showing the configuration of the carbon nanostructure 25 included in the first active material layer 22 of the first electrode 20. For convenience, in Fig. 3, graphene 26 constituting the carbon nanostructure 25 is illustrated as a substantially rectangular sheet.

[0032] Carbon nanostructures 25 are disposed over the entire surface of the first current collector 21 of the first electrode 20. The carbon nanostructure 25 has a configuration in which graphene 26 extends elongatedly with the first current collector 21 side as a base end. Graphene 26 is also called a "graphene sheet" and is a sheet-like substance having a thickness of one carbon atom and composed of a six-membered carbon ring structure, that is, a hexagonal lattice structure with carbon atoms as vertices. The carbon nanostructure 25 has a multilayer structure in which multiple graphenes 26 are stacked in the thickness direction. The carbon nanostructure 25 is conductive.

[0033] The carbon nanostructures 25 can be formed on the surface of the first current collector 21 by, for example, a chemical vapor deposition (CVD) method. Although not shown, the surface of the first current collector 21 is covered with an amorphous carbon layer. In the CVD method, after an amorphous carbon layer is formed on the surface of the first current collector 21, the carbon nanostructures 25 are formed so as to extend upward in an elongated manner, with the amorphous carbon layer serving as the starting point for growth.

[0034] The carbon nanostructure 25 is formed in a needle, plate, or pleat shape. The carbon nanostructure 25 can be interpreted as a structure of the same kind as, for example, a carbon nanowall or similar carbon nanoflakes, carbon nanoflowers, etc. The carbon nanostructure 25 is a graphite-like substance, and therefore has a higher electrical conductivity than carbon materials such as activated carbon.

[0035] The carbon nanostructure 25 does not have to be entirely composed of single crystals of carbon six-membered rings. The graphene 26 constituting the carbon nanostructure 25 may not have a complete graphene structure, but may be a thin film mainly composed of carbon with a six-membered ring structure. The graphene 26 may have a mosaic structure mainly composed of carbon with a six-membered ring structure. The mosaic structure means a structure in which a plurality of regions composed of carbon six-membered ring structures are discretely arranged.

[0036] 1-3. Battery reaction in secondary batteries: The chemical reactions occurring during charging and discharging in the secondary battery 10 can be expressed, for example, by the following reaction formula. When the positive electrode material is LiCoO2, the reaction formula at the second electrode 30, which is the positive electrode, is expressed by the following formula (1). x represents the proportion of reacting atoms and is a real number greater than 0 and less than 1. Li 1-x CoO2+ xLi + +xe - ⇔ LiCoO2…(1)

[0037] In contrast, the reaction formula at the first electrode 20, which is the negative electrode, is expressed by the following formula (2): As shown in formula (2), Li is precipitated on the surface of the first electrode 20 when the secondary battery 10 is charged. Li + + e - ⇔ Li …(2)

[0038] As shown in the above formula (2), according to the secondary battery 10 of this embodiment, theoretically, charging is possible as long as Li can be precipitated on the first electrode 20, and a high charge capacity can be obtained.

[0039] 1-4. How to charge secondary batteries: A charging method executed by a charging device 50 to which the charging method of the first embodiment is applied will be described with reference to Figures 4 and 5. Figure 4 shows a process flow diagram of charging control by the charging device 50. Figure 5 shows a graph showing an example of the current value of the charging current and the time change of the voltage of the secondary battery 10 while charging control of the charging device 50 is being executed.

[0040] In the charging control of the charging device 50, the secondary battery 10 is charged in multiple stages by a constant current at a current value commanded by the control unit 52 to the current control unit 53. In the charging control, the control unit 52 changes the current value of the charging current in a stepped manner for each period, as described below. In other words, in the charging control of the charging device 50, the current value of the charging current takes on discrete values ​​for each period.

[0041] As a preliminary preparation step, when starting charging of the secondary battery 10, if the switch unit 54 is open, the control unit 52 first closes the switch unit 54. If the switch unit 54 is closed, the control unit 52 keeps it closed.

[0042] In step 1, the control unit 52 executes low-current charging control. In the low-current charging control, charging is performed with a charging current of a predetermined low current value. The low-current charging control is based on a characteristic tendency immediately after the start of charging of the secondary battery 10 in which the carbon nanostructure 25 is applied to the first active material layer 22, which the inventor of the present invention found in his own research.

[0043] As described later, the inventors of the present invention found that in the secondary battery 10 of the first embodiment, the voltage tends to rise sharply in response to the current supplied immediately after the start of charging. Therefore, in the low-current charging control of step 1, the control unit 52 performs charging at a low current value that can suppress the occurrence of a voltage overshoot caused by a sharp rise in the voltage of the secondary battery 10 immediately after the start of charging.

[0044] The low current value, which is the current value of the charging current in the low current charging control, is preferably equal to or less than half the current value of the charging current in the high current charging control executed in step 2. The low current value is, for example, when the charging current in the high current charging control is 6.0 mA / cm 2 For current values ​​equivalent to a current density of 3.0 mA / cm 2 In this case, the low current value is preferably equal to or less than a current value corresponding to a current density of 2.0 mA / cm. 2 It is more preferable that the current value is equal to or less than the current density corresponding to the current density of 1000 kV.

[0045] The low current value of the low current charging control is preferably as large as possible within a range in which the occurrence of the above-mentioned overshoot can be suppressed, because this allows the charging speed of the secondary battery 10 to be increased.

[0046] Furthermore, as described later, in the secondary battery 10 of the first embodiment, if the flow value of the charging current immediately after the start of charging is too low, metal ions (Li ions in this embodiment) involved in charging and discharging are likely to precipitate as metal atoms between the first current collector 21 and the first active material layer 22 or in the first active material layer 22. If the amount of metal atoms precipitated between the first current collector 21 and the first active material layer 22 or in the first active material layer 22 becomes large, the carbon nanostructures 25 will be subject to external force from the metal atoms and will be likely to become detached from the first current collector 21, and will be likely to fall off from the first current collector 21.

[0047] On the other hand, if the current value of the charging current immediately after the start of charging is relatively high, the amount of metal ions involved in charging and discharging precipitated as metal atoms between the first current collector 21 and the first active material layer 22 or in the first active material layer 22 can be reduced. Also, a layer of precipitated metal atoms can be formed on the first active material layer 22. Therefore, the carbon nanostructures 25 are prevented from easily falling off the first current collector 21 due to the precipitation of the metal atoms between the first current collector 21 and the first active material layer 22 or in the first active material layer 22. Therefore, it is possible to prevent the battery performance of the secondary battery 10 from being deteriorated due to the falling off of the carbon nanostructures 25.

[0048] According to the experimental results of the inventors of the present invention described later, in order to suppress the falling off of the carbon nanostructures 25 from the first current collector 21, the low current value in the low current charging control is 0.4 mA / cm 2 The low current value in the low current charging control is preferably equal to or greater than the current value corresponding to the current density of 0.8 mA / cm. 2 More preferably, the current density is equal to or greater than 2.0 mA / cm 2 When the low current value in the low current charging control is specified based on the current value of the charging current in the high current charging control executed in step 2, the low current value in the low current charging control is preferably 1 / 4 or more, and more preferably 1 / 3 or more, of the current value of the charging current in the high current charging control.

[0049] It is more preferable that the low current value in the low current charging control is equal to or higher than the value at which metal growth nuclei, in which metal ions involved in charging and discharging are precipitated, are formed on the first active material layer 22. This promotes the precipitation of metal atoms on the first active material layer 22 to form a layer of metal atoms. If metal atoms can be precipitated on the first active material layer 22, the metal atoms can press the carbon nanostructures 25 from above, further suppressing the detachment of the carbon nanostructures 25 from the first current collector 21. This further suppresses the deterioration of battery performance when the secondary battery 10 is repeatedly charged and discharged.

[0050] As shown in Fig. 5, when low-current charging control at low current value I0 is started at time t0, the voltage of the secondary battery 10 rises steeply, instantaneously exceeds the first voltage threshold Va, then falls below the first voltage threshold Va, and then rises gradually as charging proceeds. The first voltage threshold Va may be a voltage value experimentally determined in advance to enable highly efficient charging. After detecting the instantaneous fluctuation in the voltage of the secondary battery 10 immediately after the start of charging, the control unit 52 ends the low-current charging control when the voltage of the secondary battery 10 reaches the first voltage threshold Va again, and starts the high-current charging control of step 2.

[0051] In the high current charging control of step 2, the control unit 52 supplies a charging current having a maximum current value during charging to charge the battery. The current value of the charging current in the high current charging control may be a value determined in advance based on experimental results or the like as the maximum current value at which the occurrence of a voltage overshoot of the secondary battery 10 can be suppressed. The current value of the charging current in the high current charging control is, for example, 1.0 mA / cm. 2 It is preferable that the current density is equal to or greater than 5.0 mA / cm 2 The current value of the charging current in the high current charging control is, for example, 6.0 mA / cm or more. 2 It is more preferable that the current value is equal to or greater than the current density corresponding to the current density of 1000 kV.

[0052] In high current charging control, the secondary battery 10 is charged at high speed with the maximum current value during charging. The higher the current value of the charging current in high current charging control, the shorter the charging time can be.

[0053] 5, when high current charging control is started at time t1 at current value I1, which is the maximum current value during charging, the voltage of the secondary battery 10 gradually increases as charging proceeds. When the voltage of the secondary battery 10 reaches a predetermined second voltage threshold Vb, the control unit 52 ends the high current charging control and starts the current reduction control of step 3. The second voltage threshold Vb is a value greater than the first voltage threshold Va, and may be a value determined in advance based on experimental results as a voltage at which the occurrence of overshoot can be suppressed.

[0054] In step 3, the control unit 52 uses current reduction control to reduce the current value of the charging current stepwise multiple times as charging time elapses from the high current value in the high current charging control. In this embodiment, the control unit 52 reduces the current value of the charging current to ⅔ or less of the previous current value. The control unit 52 reduces the current value of the charging current every time the voltage of the secondary battery 10 reaches the second voltage threshold Vb.

[0055] During current reduction control, the voltage of the secondary battery 10 drops once each time the charging current value is reduced. Therefore, by starting current reduction control after the charging of the secondary battery 10 has progressed to a certain extent by high current charging control, charging can be continued stably while preventing the voltage of the secondary battery 10 from becoming too high and causing an overshoot.

[0056] 5, at time t2, the current value of the charging current is reduced from current value I1 under high current charging control to current value I2. Also, the voltage of the secondary battery 10 gradually increases from time t2, and at time t3 when it reaches the second voltage threshold Vb, it is reduced from current value I2 to current value I3.

[0057] As described above, in this embodiment, the current value is reduced when the voltage of the secondary battery 10 reaches the second voltage threshold Vb, and the voltage of the secondary battery 10 is prevented from exceeding the critical second voltage threshold Vb at which the possibility of an overshoot occurring increases. Therefore, it is easy to continue highly efficient charging while maintaining the voltage at which the occurrence of a voltage overshoot is prevented.

[0058] The control unit 52 detects the SOC of the secondary battery 10 from time to time while the above-mentioned charging control is being executed, and when the SOC reaches a predetermined threshold, opens the switch unit 54 and terminates charging of the secondary battery 10.

[0059] As described above, according to the charging method of the secondary battery 10 of this embodiment, after performing highly efficient charging with the maximum current value, the current value of the charging current is stepped down, so that the secondary battery 10 can be charged in a short time while suppressing the occurrence of voltage overshoot. Furthermore, according to the method of the secondary battery 10 of this embodiment, low-current charging control is performed immediately after the start of charging, so that the occurrence of voltage overshoot immediately after the start of charging is suppressed. Furthermore, by performing low-current charging control immediately after the start of charging, it is possible to suppress the carbon nanostructure 25 from falling off from the first current collector 21 due to metal atoms precipitated on the first electrode 20 during charging. Therefore, it is possible to suppress the deterioration of battery performance when the secondary battery 10 is repeatedly charged and discharged. EXAMPLES

[0060] 1-5. Experimental example of voltage change of secondary battery immediately after charging starts: 6 shows graphs showing the time-dependent changes in voltage of two types of secondary batteries obtained through experiments by the inventors of the present invention as reference experimental results. The first graph Ga is a graph for the first type of secondary battery, and the second graph Gb is a graph for the second type of secondary battery. The first and second graphs Ga and Gb respectively show the changes in the measured voltage after the start of charging the first and second types of secondary batteries at the same constant current.

[0061] In the first type of secondary battery, carbon nanowalls were used as a carbon nanostructure in the active material layer of the negative electrode. In contrast, in the second type of secondary battery, the active material layer of the negative electrode was composed of graphite, not carbon nanostructures. Other than the above, the first and second type of secondary batteries had almost the same configuration.

[0062] As the second graph Gb shows, the voltage of the second type secondary battery in which graphite was applied to the negative electrode active material layer rose almost vertically immediately after the start of charging, then drew a gentle curve to reach approximately 3.5 V, and then gradually increased to about 3.8 V. This is presumably because in the second type secondary battery in which graphite was applied, the chemical reaction of the following formula (3), in which Li ions are inserted between the graphene layers in the graphite during charging and react with C atoms to produce a compound, takes time, resulting in a gradual rise in voltage. Li + + C ⇔ LiCx(x≦6) …(3)

[0063] In contrast, in the first type secondary battery in which carbon nanowalls were applied to the active material layer of the negative electrode, the voltage immediately after the start of charging exhibited a behavior that was clearly different from the change in voltage of the above-mentioned second type secondary battery. As shown in the first graph Ga, the voltage of the first type secondary battery rose almost vertically and steeply to over 4.0 V immediately after the start of charging, then dropped slightly and was maintained at around 3.9 V, which is slightly lower than 4.0 V. This is thought to be because, in the first type secondary battery, as shown in the above-mentioned reaction formula (2), Li ions are simply precipitated on the carbon nanostructure of the active material layer of the negative electrode during charging, and there is no chemical reaction to generate compounds, so the voltage tends to rise steeply.

[0064] From these experimental results, the inventors of the present invention found that while a secondary battery in which a carbon nanostructure is applied to the active material layer of the negative electrode has high battery performance, the voltage has a unique tendency to rise so steeply that it exceeds the maximum value immediately after the start of charging. From the consideration of this tendency, the inventors have come to the knowledge that when charging a secondary battery in which a carbon nanostructure is applied to the active material layer of the negative electrode, it is preferable to start charging with a low current in order to suppress the voltage overshoot immediately after the start of charging. Thus, the graph shown in FIG. 6 shows that, according to the charging control of the charging device 50 of this embodiment, the occurrence of a voltage overshoot immediately after the start of charging is suppressed by executing the low current charging control in step 1.

[0065] 1-6. Experimental example of change in state of secondary battery electrodes according to current value: 7 and 8 show, as reference experimental results, a table summarizing images of the negative electrode after constant current charging of a secondary battery of the same configuration in which carbon nanostructures are applied to the active material layer of the negative electrode as experimental examples S1 to S6. In Figs. 7 and 8, the "front image" is an image taken from the front direction corresponding to the thickness direction of the negative electrode current collector, and the "side image" is an image taken from the side direction corresponding to the direction along the surface of the negative electrode current collector. The carbon nanostructures of the secondary batteries in experimental examples S1 to S6 were carbon nanowalls.

[0066] The charging current values ​​I and current densities J of the charging currents in the experimental examples S1 to S6 are different from each other.

[0067] <Charging conditions for experimental examples S1 to S6> Experimental example S1: Current value I = 0.01 [mA], current density J = 0.008 [mA / cm 2 ] Experimental example S2: Current value I = 0.05 [mA], current density J = 0.040 [mA / cm 2 ] Experimental example S3: Current value I = 0.10 [mA], current density J = 0.080 [mA / cm 2 ] Experimental example S4: Current value I = 0.50 [mA], current density J = 0.400 [mA / cm 2 ] Experimental example S5: Current value I = 1.00 [mA], current density J = 0.800 [mA / cm 2 ] Experimental example S6: Current value I = 2.60 [mA], current density J = 2.000 [mA / cm 2 ]

[0068] The current value I is 0.05 mA or less, and the current density J is 0.040 mA / cm 2In the images taken in the following experimental examples S1 and S2, carbon nanowalls (CNWs) were present on top of the layer of Li that had been deposited on the negative electrode by charging. This shows that if the charging current is too small, Li will deposit between the current collector and the CNWs, and the CNWs will easily separate from the current collector. Once the negative electrode is in such a state, the CNWs will be prone to falling off when the battery is subsequently charged and discharged repeatedly.

[0069] The current value I is 0.10 mA and the current density J is 0.080 mA / cm 2 In the photographed image of the experimental example S3, a crack was generated in the CNW layer protruding from the current collector, and Li was protruding from the crack. If the crack in the CNW layer is large, as in the experimental example S3, the CNW is likely to fall off when the battery is subsequently charged and discharged repeatedly.

[0070] The current value I is 0.50 mA and the current density J is 0.400 mA / cm 2 In the photographed image of Experimental Example S3, the CNW layer was locally raised due to the precipitation of Li, but the cracks in the CNW layer were not large enough to expose the internal Li. If the cracks in the CNW layer were as large as those in Experimental Example S4, the detachment of the CNW would be more reliably suppressed than in Experimental Example S3, even if charging and discharging were repeated thereafter.

[0071] From the difference in the change in the electrode state in Experimental Examples S1 to S4, it can be seen that the larger the current value of the charging current, the more the deposition of Li between the current collector and the active material layer and in the active material layer is suppressed. 2 If this is the case, the occurrence of cracks in the CNW layer due to the precipitation of Li is suppressed, and therefore the CNW is less likely to fall off the current collector even if charge and discharge are repeated thereafter.

[0072] The current value I is 1.00 mA and the current density J is 0.800 mA / cm 2In the photographed image of experimental example S5, Li growth nuclei were formed on the CNW layer. This indicates that when the charging current is large to a certain extent, metal growth nuclei formed by the deposition of metal ions involved in charging and discharging can be formed on the carbon nanostructure layer that constitutes the active material layer of the negative electrode.

[0073] As in the experimental example S5, if a metal growth nucleus where metal ions involved in charging and discharging are precipitated can be formed on the carbon nanostructure layer, a layer of the metal atoms is likely to be formed on the carbon nanostructure layer as charging progresses. Therefore, the carbon nanostructure can be pressed from above by the layer of metal atoms, so that it is possible to more reliably prevent the carbon nanostructure from falling off the current collector even if charging and discharging are repeated thereafter.

[0074] The current value I is 2.60 mA and the current density J is 2.000 mA / cm 2 In the image of Experimental Example S6, it can be seen that the surface of the CNW is covered with Li. Thus, in Experimental Example S6, the CNW layer is covered with a layer of Li, and no cracks were generated in the CNW layer. This indicates that if the current value of the charging current is made larger than the value at which the metal growth nuclei formed by the deposition of metal ions involved in charging and discharging are formed on the carbon structure layer, the growth nuclei can be used as the starting point for growth to form a metal layer covering the carbon nanostructure.

[0075] If such a metal atomic layer can be formed once under low current charging control, the metal atomic layer can be easily grown with a uniform structure as charging proceeds. This can further increase the charging efficiency of the secondary battery. In addition, the carbon nanostructure is protected by the metal atomic layer, so that the carbon nanostructure can be further prevented from peeling off from the current collector due to repeated charging and discharging, which can cause a decrease in battery performance.

[0076] Thus, in a secondary battery in which carbon nanostructures are applied to the active material layer of the negative electrode, the detachment of carbon nanostructures from the electrode can be suppressed by increasing the current value of the charging current immediately after the start of charging within a range in which the occurrence of voltage overshoot is suppressed. From this, it can be seen that according to the charging control of the charging device 50 of this embodiment, the low current charging control of step 1 is executed, thereby suppressing the detachment of carbon nanostructures from the electrode and suppressing the deterioration of the battery performance of the secondary battery 10 due to repeated charging and discharging.

[0077] 1-7. Differences in charging time due to differences in secondary battery charging control: An example and a comparative example of the charging method for the secondary battery 10 in this embodiment will be described with reference to FIGS.

[0078] Figures 9 to 12 each show graphs showing the change over time in current I and voltage V when a secondary battery having a configuration equivalent to secondary battery 10 described in Figures 2 and 3 is charged using different charging controls until the secondary battery reaches a fully charged state with an SOC of 100%.

[0079] The graph in Fig. 9 is based on actual measurements taken when the charging control of Example E1 was performed. In Example E1 of Fig. 9, the secondary battery was charged by the charging control of the present embodiment described with reference to Figs. 4 and 5. In Example E1, the current value I of the charging current and the current density J were as follows:

[0080] <Charging conditions of Example E1> Low current charging control: I=2.6mA, J=2.0mA / cm 2 High current charging control: I=6.5mA, J=5.0mA / cm 2 Current reduction control: When the voltage reached 5.0 V, the current value I and current density J were reduced as follows: I: 6.5mA → 4.0mA → 2.6mA J: 5.0mA / cm 2 →3.0mA / cm 2 →2.0mA / cm 2

[0081] The graph in Fig. 10(a) shows a model of the change over time of current I and voltage V in charging control of Comparative Example C1, and the graph in Fig. 10(b) shows the change over time of current I and voltage V based on actual measured values. In Comparative Example C1 of Fig. 10, the secondary battery was charged by constant current control that maintained a constant current value without changing the current value of the charging current. The current value of the charging current in Comparative Example C1 was 0.5 mA, and the current density was 0.4 mA / cm 2 It was.

[0082] The graph in Fig. 11(a) shows a model of the change in current I and voltage V over time in the charging control of Comparative Example C2, and the graph in Fig. 11(b) shows the change in current I and voltage V over time based on actual measurements. In Comparative Example C2 of Fig. 11, the secondary battery was charged by constant voltage control that maintained the charging voltage at a constant voltage value. The charging voltage value in Comparative Example C2 was set to 4.5V.

[0083] The graph in Fig. 12(a) shows a model of the change over time of current I and voltage V in charging control of Comparative Example C3, and the graph in Fig. 12(b) shows the change over time of current I and voltage V based on actual measured values. In Comparative Example C3 of Fig. 12, charging control was performed in which charging of the secondary battery was started under constant current control, and then switched to constant voltage control during charging. The constant current control and constant voltage control were switched when the SOC of the secondary battery reached 80%. The charging current value under constant current control in Comparative Example C3 was 0.5 mA, and the current density was 0.4 mA / cm. 2 In Comparative Example C3, the charging voltage value in constant voltage control was set to 4.5V.

[0084] In Fig. 13, the results of the charging time by the charging control of each of Example E1 and Comparative Examples C1, C2, and C3 are shown in a bar graph for comparison. Also, Fig. 13 shows a bar graph of the specific capacity of the secondary battery obtained from the measured values ​​of Example E1 and Comparative Examples C1, C2, and C3.

[0085] According to the charging control of Example E1, charging was completed in approximately 1 / 10 or less of the charging time of Comparative Examples C1, C2, and C3. In addition, the specific capacity was almost unchanged due to the difference in charging control between Example E1 and Comparative Examples C1, C2, and C3. From this, it can be seen that according to the charging control in the charging device 50 of this embodiment, the charging time of the secondary battery 10 can be significantly shortened while maintaining the battery performance of the secondary battery 10.

[0086] 1-8. Changes in battery performance due to repeated charging and discharging of secondary batteries: 14 and 15, the change in the battery performance of the secondary battery when charging and discharging are repeated will be described for Example E2 and Comparative Example C4 of the charging method of this embodiment. In Example E2 and Comparative Example C4, the same secondary battery having a configuration corresponding to the secondary battery 10 described in Figs. 2 and 3 was used.

[0087] Fig. 14(a) is a graph showing the change over time in the current value of the charging current in the charging control of Example E2. In Example E2, the secondary battery was fully charged by the charging control at the current value shown in Fig. 14(a), and then the charging and discharging process in which the secondary battery was connected to a predetermined load and discharged was repeated 10 times.

[0088] 14(b) shows a graph showing the relationship between the charge capacity and voltage of the secondary battery obtained based on the actual measurements during charging and discharging. Graphs CG#1 to CG#10 show the relationship between the charge capacity and voltage during charging, and graphs DG#1 to DG#10 show the relationship between the charge capacity and voltage during discharging.

[0089] 15 is a graph showing the relationship between the charge capacity and voltage of the secondary battery obtained based on the actual measured values ​​during charging and discharging in Comparative Example C4. In Comparative Example C4, the secondary battery was charged by constant current control, and then connected to the same load as in Example E2 and discharged, and this charge / discharge process was repeated five times. In the constant current control of Comparative Example C4, the current value of the charging current was 0.5 mA, and the current density was 0.4 mA / cm. 2Graphs CG#11 to CG#15 show the relationship between the charge capacity and voltage of the secondary battery of Comparative Example C4 during charging, and graphs DG#11 to DG#15 show the relationship between the charge capacity and voltage of the secondary battery of Comparative Example C4 during discharging.

[0090] In Comparative Example C4, the maximum charge capacity decreased as the charge and discharge were repeated, whereas in Example E2, the variation in the maximum charge capacity when the charge and discharge were repeated was suppressed. This shows that the charge control of this embodiment suppresses the deterioration of the battery performance of the secondary battery when the charge and discharge are repeated.

[0091] As described above, the charging device and charging method for a secondary battery according to the first embodiment can significantly shorten the charging time of the secondary battery and suppress the deterioration of the battery performance of the secondary battery caused by repeated charging and discharging.

[0092] 2. Second embodiment: A method for charging a secondary battery in the second embodiment will be described with reference to Fig. 16 and Fig. 17. Fig. 16 shows a process flow diagram of the charging control executed by the charging device of the second embodiment. Fig. 17 shows a graph showing an example of the change over time of the charging current I and the voltage V of the secondary battery while the charging control of the second embodiment is being executed.

[0093] The configuration of the charging device of the second embodiment is almost the same as that of the charging device 50 shown in Fig. 1 described in the first embodiment. However, in the second embodiment, the secondary battery to be charged has graphite applied to the negative electrode active material layer instead of the carbon nanostructure described in the first embodiment. Other than the above-mentioned points, the secondary battery of the second embodiment has almost the same configuration as the secondary battery 10 described in the first embodiment.

[0094] As shown in Fig. 16, the charging control of the second embodiment is almost the same as that of the first embodiment, except that the constant current charging control of step 1 is omitted. As shown in Fig. 17, in the charging control of the second embodiment, when charging of the secondary battery starts at time t0, high current charging control is executed, and the secondary battery is charged at a constant current of the maximum current value I1.

[0095] When the voltage V of the secondary battery reaches a predetermined threshold Vth at time t1, current reduction control is started, and the current value of the charging current is reduced stepwise as the charging time passes. In the example of Fig. 17, the charging current is reduced to a current value I2 at time t1, to a current value I3 at time t2, and to a current value I4 at time t3.

[0096] According to the method for charging a secondary battery of the second embodiment, after highly efficient charging is performed at the maximum current value, the charging current value is stepped down multiple times, thereby making it possible to charge the secondary battery in a short time while suppressing the occurrence of voltage overshoot. Also, unlike the first embodiment, there is no period during which low current charging control is performed after charging starts, making it possible to shorten the charging time. EXAMPLES

[0097] An example and a comparative example of the charging method of the second embodiment will be described with reference to Fig. 18 to Fig. 20. Fig. 18 and Fig. 19 each show graphs showing the time variation of current I and voltage V when a secondary battery, which is the subject of charging in the above-mentioned second embodiment, is charged by different charging controls until the secondary battery reaches a fully charged state where the SOC of the secondary battery is 100%. Fig. 18 is a graph of Example E3, and Fig. 19 is a graph of Comparative Example E5.

[0098] In Example E3 of Fig. 18, the secondary battery was charged by the charge control of the second embodiment described with reference to Fig. 16 and Fig. 17. In Example E3, the current value I and current density J of the charging current were as follows.

[0099] <Charging conditions of Example E3> High current charging control: I=1.5mA, J=1.1mA / cm 2 Current reduction control: When the voltage reached 4.2 V, the current value I and the current density J were reduced as follows: I:1.5mA→1.0mA→0.5mA→0.3mA J: 1.1mA / cm 2 →0.8mA / cm 2 →0.4mA / cm 2 →0.2mA / cm 2

[0100] In Comparative Example C5 of Fig. 19, the secondary battery was charged by the same charging control as Comparative Example C3 described in the first embodiment with reference to Fig. 12. In Comparative Example C5, after the start of charging, charging was performed at a constant current of 1.0 mA, and when the SOC reached 80%, charging was switched to a constant voltage of 4.2 V.

[0101] In Fig. 20, the results of the charging time by the respective charging controls of the above-mentioned Example E3 and Comparative Example C5 are shown in a bar graph for comparison. Also, in Fig. 20, a bar graph of the specific capacity of the secondary battery obtained from the respective measured values ​​of Example E3 and Comparative Example C5 is shown.

[0102] According to the charge control of Example E3, charging was completed in about 70% of the charge time of Comparative Example C5. In addition, the specific capacity was almost unchanged between Example E3 and Comparative Example C5 due to the difference in charge control. From this, it can be seen that according to the charge control of the second embodiment, the charge time of the secondary battery can be significantly shortened while maintaining the battery performance of the secondary battery.

[0103] 3. Other embodiments: The present invention is not limited to the configurations of the above-mentioned embodiments and examples, and can be realized, for example, in the following forms. Any configurations described below as other embodiments are positioned as one example form for implementing the present invention, similar to the above-mentioned embodiment and the configurations and examples described as other embodiments within the above-mentioned embodiment.

[0104] 3-1. Other embodiment 1: In each of the above embodiments, the secondary battery to be charged may be a secondary battery other than a lithium ion secondary battery. For example, the secondary battery to be charged may be configured to involve metal ions other than Li ions in charging and discharging. The secondary battery may be configured to involve metal ions such as sodium (Na) ions, potassium (K) ions, magnesium (Mg) ions, etc. in charging and discharging. The secondary battery to be charged may be an all-solid-state battery in which metal ions involved in charging and discharging are conducted via a solid electrolyte.

[0105] 3-2. Other embodiment 2: In the secondary battery 10 described as the charging target of the above embodiment, the first current collector 21 of the first electrode 20 may be made of a metal other than Cu. The first current collector 21 may be made of, for example, a Cu alloy, Al, an Al alloy, or other metal. In the secondary battery 10, the first active material layer 22 of the first electrode 20 may be made of atoms other than C atoms. The first active material layer 22 may be omitted. In this case, it is preferable that the first current collector 21 of the first electrode 20 has a structure or configuration on the surface that serves as a growth nucleus that promotes the precipitation of metal ions involved in charge and discharge as metal atoms.

[0106] 3-3. Other embodiment 3: The charging device 50 may be configured as a charging system including devices having the functions of the power supply unit 51, the control unit 52, the current control unit 53, the switch unit 54, the current measurement unit 55, and the voltage measurement unit 56 described above, instead of an integrated circuit implemented on a microchip. The charging device 50 may be configured small so as to be used for charging portable information terminals and home appliances equipped with secondary batteries, or may be configured large so as to be used for charging secondary batteries installed in vehicles and large mobile objects.

[0107] 3-4. Other embodiment 4: The amount of reduction in the current value during the current reduction control is not limited to a value equal to or less than 2 / 3 of the previous value. It is preferable that the amount of reduction in the current value during the current reduction control be appropriately determined so as to suppress the occurrence of an overshoot in the voltage of the secondary battery during charging.

[0108] 3-5. Other embodiment 5: The timing at which the current value is reduced by the current reduction control is not limited to the timing at which the voltage of the secondary battery 10 reaches a predetermined threshold value. The timing at which the current value is reduced by the current reduction control may be, for example, the timing at which a predetermined time has elapsed after the start of charging or the start of high current charging control, or the timing at which the SOC of the secondary battery 10 reaches a predetermined threshold value.

[0109] 3-6. Other embodiment 6: In the above embodiment, in the low current charging control, charging does not have to be performed at a current value equal to or less than half the current value in the high current charging control. The current value in the low current charging control can be set to any value as appropriate so as to suppress the occurrence of a voltage overshoot immediately after the start of charging.

[0110] 4. Example of morphology: The present invention can be realized in the following forms.

[0111] [First aspect] The first aspect is provided as a method for charging a secondary battery. The charging method of the first aspect includes a high-current charging step of supplying a charging current with a maximum current value during charging to the secondary battery to charge it, and a current reducing step of reducing the current value of the charging current from the maximum current value in a stepwise manner with the passage of charging time. According to the charging method of the first aspect, rapid charging is possible by highly efficient charging at the maximum current value before the current reduction step. In addition, after rapid charging at the maximum current value, a current reduction step is executed to reduce the current value of the charging current in a stepwise manner, so that it is possible to prevent the voltage of the secondary battery from increasing significantly as the charge amount of the secondary battery increases, and to prevent the occurrence of voltage overshoot. Therefore, according to the charging method of the first aspect, smooth rapid charging at a high current value that matches the charge state of the secondary battery is possible, and the charging time of the secondary battery can be shortened.

[0112] [Second Mode] In the charging method of the first mode, the secondary battery comprises a first electrode having a current collector and an active material layer in which carbon nanostructures composed of graphene extending in elongated strips from the surface of the current collector are arranged all over the surface of the current collector, and a second electrode containing metal atoms that are ionized and migrate to the first electrode during charging, and the charging method may further comprise a low-current charging step of charging the secondary battery with the charging current having a low current value that is equal to or less than half of the maximum current value before the high-current charging step. According to the research of the inventor of the present invention, a secondary battery to which an active material layer to which carbon nanostructures are applied is applied has a tendency for the voltage to rise suddenly immediately after charging is started. According to the charging method of the second embodiment, charging at a low current value is performed before charging at a high current value is started, so that it is possible to suppress the occurrence of an overshoot caused by a sudden rise in voltage immediately after charging is started. In addition, by performing charging at a low current value before charging at a high current value, it is possible to suppress the precipitation of metal atoms between the current collector and the active material layer of the carbon nanostructure during charging. Therefore, it is possible to suppress the carbon nanostructure from falling off from the current collector due to the precipitation of such metal atoms. Therefore, it is possible to suppress the deterioration of the charging performance of the secondary battery due to repeated charging and discharging.

[0113] [Third Mode] In the charging method according to the second mode, the low current value may be equal to or greater than a current value at which growth nuclei of the metal atoms are formed on the active material layer. According to the third embodiment of the charging method, the formation of growth nuclei of metal atoms by charging at a low current promotes the deposition of metal atoms on the active material layer. The metal atoms deposited on the active material layer prevent the carbon nanostructures from peeling off from the current collector, so that the falling off of the carbon nanostructures from the current collector can be further prevented.

[0114] [Fourth feature] In the charging method according to any one of the first, second and third features, in the current reducing step, the current value may be reduced to 2 / 3 or less of the immediately preceding current value. According to the charging method of the fourth aspect, it is possible to further suppress the occurrence of voltage overshoot during charging.

[0115] [Fifth feature] In the charging method according to any one of the first, second, third and fourth features, in the current reducing step, the current value may be reduced when the voltage of the secondary battery reaches a predetermined threshold value. According to the charging method of the fifth aspect, highly efficient charging can be easily continued while maintaining a voltage at which the occurrence of voltage overshoot is suppressed.

[0116] [Sixth aspect] The sixth aspect is provided as a charging device for charging a secondary battery. The secondary battery of the sixth aspect includes a power supply unit that supplies a charging current to the secondary battery, a current control unit that controls the magnitude of the charging current, and a control unit that commands the current control unit to the current value of the charging current, and the control unit starts high current charging control for charging the secondary battery with the charging current having a maximum current value during charging, and then executes current reduction control for reducing the current value of the charging current from the maximum current value in a stepwise manner as charging time passes. According to the charging device of the sixth aspect, rapid charging is possible by performing highly efficient charging at the maximum current value in the high current charging control before performing the current reduction control. Furthermore, after rapid charging by the high current charging control, the current reduction control is performed to reduce the current value of the charging current in a stepped manner, so that the voltage of the secondary battery can be prevented from increasing significantly as the charge amount of the secondary battery increases, and the occurrence of voltage overshoot can be suppressed. Thus, according to the charging device of the sixth aspect, smooth rapid charging at a high current value in accordance with the charge state of the secondary battery is possible, and the charging time of the secondary battery can be shortened.

[0117] [Seventh Form] In the charging device of the sixth form, the secondary battery comprises a first electrode having a current collector and an active material layer in which a carbon nanostructure composed of graphene extending from the surface of the current collector is arranged over the entire surface of the current collector, and a second electrode containing metal atoms that ionize and migrate to the first electrode during charging, and the control unit may execute low current charging control to charge the secondary battery with the charging current at a low current value not greater than half of the maximum current value before starting the high current charging control. According to the charging device of the seventh embodiment, charging at a low current value is performed before charging at a high current value is started, so that it is possible to suppress the occurrence of a sudden rise in voltage and overshoot immediately after the start of charging. In addition, charging at a low current value suppresses the precipitation of metal atoms between the current collector and the active material layer of the carbon nanostructure, so that it is possible to suppress the carbon nanostructure from falling off the current collector due to the precipitation of such metal atoms. Therefore, the charging performance of the secondary battery is prevented from decreasing due to repeated charging and discharging.

[0118] [Eighth Mode] In the charging device of the seventh mode, the low current value may be equal to or greater than the current value at which the metal atoms precipitate on the active material layer. According to the charging device of the eighth embodiment, charging at a low current value promotes the deposition of metal atoms on the active material layer, and the deposited metal atoms prevent the carbon nanostructures from peeling off from the current collector, thereby further preventing the carbon nanostructures from falling off from the current collector.

[0119] [9th feature] In the charging device described in any one of the 6th, 7th, and 8th features, the control unit may reduce the current value of the charging current to 2 / 3 or less of the immediately preceding current value in the current reduction control. According to the charging device of the ninth aspect, it is possible to further suppress the occurrence of voltage overshoot during charging.

[0120] [Feature 10] The charging device described in any one of Features 6, 7, 8, and 9 may further include a voltage measurement unit that measures the voltage of the secondary battery, and in the current reduction control, the control unit may reduce the current value of the charging current when the voltage of the secondary battery reaches a predetermined threshold value. According to the charging device of the tenth aspect, highly efficient charging can be easily continued while maintaining a voltage at which the occurrence of voltage overshoot is suppressed. [Explanation of symbols]

[0121] Reference Signs List 10... secondary battery, 11... container, 12... electrolyte, 15... separator, 20... first electrode, 21... first current collector, 22... first active material layer, 25... carbon nanostructure, 26... graphene, 30... second electrode, 31... second current collector, 32... second active material layer, 50... charging device, 51... power supply unit, 52... control unit, 53... current control unit, 54... switch unit, 55... current measurement unit, 56... voltage measurement unit, 59... charging circuit

Claims

1. A method for charging a secondary battery, comprising the steps of: a high-current charging step of charging the secondary battery by supplying a charging current with a maximum current value during charging; a current decreasing step of decreasing the current value of the charging current from the maximum current value in a stepwise manner with the lapse of charging time; Equipped with The secondary battery is A first electrode including a current collector and an active material layer on a surface of the current collector, the active material layer having carbon nanostructures formed of graphene extending thinly from the surface of the current collector and disposed over the entire surface of the active material layer; a second electrode including metal atoms that ionize and migrate to the first electrode upon charging; Equipped with The charging method further comprises: a low-current charging step, prior to the high-current charging step, of charging the secondary battery with the charging current having a low current value equal to or less than half of the maximum current value; The charging method, wherein the low current value is equal to or greater than the current value at which growth nuclei of the metal atoms are formed on the active material layer.

2. 2. The charging method according to claim 1, The current reducing step reduces the current value to 2 / 3 or less of the immediately preceding current value.

3. The charging method according to claim 2, The charging method, wherein in the current reducing step, the current value is reduced when the voltage of the secondary battery reaches a predetermined threshold value.

4. A charging device for charging a secondary battery, a power supply unit for supplying a charging current to the secondary battery; A current control unit that controls the magnitude of the charging current; a control unit that commands the current control unit to control a current value of the charging current; Equipped with the control unit starts a high current charging control in which the secondary battery is charged with the charging current having a maximum current value during charging, and then executes a current reduction control in which the current value of the charging current is reduced in a stepwise manner from the maximum current value as charging time elapses; The secondary battery is A first electrode including a current collector and an active material layer on a surface of the current collector, the active material layer having carbon nanostructures formed of graphene extending thinly from the surface of the current collector and disposed over the entire surface of the active material layer; a second electrode including metal atoms that ionize and migrate to the first electrode upon charging; Equipped with the control unit executes a low current charging control to charge the secondary battery with the charging current having a low current value that is equal to or less than half of the maximum current value before starting the high current charging control; The low current value is equal to or greater than the current value at which the metal atoms precipitate on the active material layer.

5. 5. The charging device according to claim 4, The control unit, in the current reduction control, reduces a current value of the charging current to ⅔ or less of an immediately preceding current value.

6. The charging device according to claim 5 further comprises: a voltage measurement unit that measures a voltage of the secondary battery, The control unit, in the current reduction control, reduces a current value of the charging current when a voltage of the secondary battery reaches a predetermined threshold.

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