Method for manufacturing lithium ion capacitor

Abstract

A method for manufacturing a lithium ion capacitor (1) includes a step (S11) in which doping is performed at a first doping current value, and a step (S12) in which doping is performed at a second doping current value. In the step (S11) in which doping is performed at a first doping current value, doping is performed at a first doping current value of 0.05 C or more and 0.2 C or less from the beginning of doping. In the step (S12) of doping at a second doping current value, doping is performed at the second doping current value having a C-rate higher than that of the first current value.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a lithium ion capacitor.

[0002] This application claims priority to Japanese Patent Application No. 2022-143169 filed in Japan on September 8, 2022, the contents of which are incorporated herein by reference. Background Art

[0003] Patent document 1 discloses the structure of a lithium ion electrochemical device, which comprises: an electrode group in which a positive electrode and a negative electrode are wound with a separator interposed therebetween; a case for housing the electrode group; and an organic electrolyte that permeates or impregnates the electrode group in the case.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2014-116237 Summary of the invention

[0007] Problems to be solved by the invention

[0008] When the structure disclosed in Patent Document 1 is applied to, for example, a lithium ion capacitor, if a durability test of the product is performed, the deactivation rate of lithium may increase. In order to improve the durability of the product, it is preferred to suppress the deactivation rate of lithium. To this end, the inventors of the present invention conducted in-depth research and found that when the lithium ion capacitor is doped, the film thickness (SEI (Solid Electrolyte Interphase: solid electrolyte interface) film) formed on the surface of the negative electrode is uneven, which may have an adverse effect on the deactivation rate of lithium.

[0009] The present invention has been made in view of such problems, and an object of the present invention is to provide a method for manufacturing a lithium ion capacitor capable of reducing the deactivation rate of lithium and improving the durability of the product by improving the uniformity of a coating formed on the surface of a negative electrode during doping.

[0010] Means for solving problems

[0011] A method for manufacturing a lithium ion capacitor according to one aspect of the present invention includes: performing doping at a first doping current value of 0.05C to 0.2C from the start of doping; and performing doping at a second doping current value higher than the first doping current value.

[0012] Effects of the Invention

[0013] According to the present invention, by improving the uniformity of the coating formed on the surface of the negative electrode during doping, the deactivation rate of lithium can be reduced, thereby improving the durability of the product. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a cross-sectional view showing a schematic structure of a lithium ion capacitor doped by the method for manufacturing a lithium ion capacitor according to an embodiment of the present disclosure.

[0015] Figure 2 It is a plan view of a cylindrical battery cell according to an embodiment of the present disclosure in a developed state.

[0016] Figure 3 Yes Figure 2 A cross-sectional view of a cylindrical battery cell in an unfolded state.

[0017] Figure 4 It is shown Figure 2 Side view of a cylindrical battery cell.

[0018] Figure 5 This is a flowchart showing the flow of a method for manufacturing a lithium ion capacitor according to one embodiment of the present disclosure.

[0019] Figure 6 It is schematically shown for the purpose of Figure 1 A diagram showing a state in which a lithium ion capacitor is doped and a power supply device is connected.

[0020] Figure 7 : is a graph showing the change in voltage of a lithium ion capacitor doped by the method for manufacturing a lithium ion capacitor according to one embodiment of the present disclosure.

[0021] Figure 8 This is a diagram showing changes in the voltage of a lithium ion capacitor in a comparative example of a method for manufacturing a lithium ion capacitor according to an embodiment of the present disclosure.

[0022] Fig. 9 This is a graph showing changes in the deactivation rate of lithium in a cycle endurance test in an example of a method for producing a lithium ion capacitor.

[0023] Fig.10 This is a SEM observation image of the negative electrode surface in the example of the method for producing a lithium ion capacitor.

[0024] Fig.11 This is a SEM observation image of the negative electrode surface in a comparative example of the method for producing a lithium ion capacitor.

[0025] Fig.12It is a diagram showing the analysis results by X-ray photoelectron spectroscopy in an example of a method for manufacturing a lithium ion capacitor.

[0026] Fig.13 It is a diagram showing the analysis results by X-ray photoelectron spectroscopy in a comparative example of a method for producing a lithium ion capacitor. DETAILED DESCRIPTION

[0027] <Implementation Method>

[0028] 《Structure of lithium-ion capacitors》

[0029] Hereinafter, embodiments will be described in detail with reference to the drawings.

[0030] like Figure 1 As shown, a lithium ion capacitor (LIC) 1 manufactured by the method for manufacturing a lithium ion capacitor according to the present embodiment has a structure of an electric double layer capacitor at the positive electrode and a structure of a lithium ion battery at the negative electrode.

[0031] The lithium ion capacitor 1 includes a casing 2 , a cylindrical battery cell 3 , a current collector plate 4 , a terminal plate 5 , and an electrolyte 6 .

[0032] The housing 2 is formed of a metal such as an aluminum alloy and has a bottomed cylindrical shape. The housing 2 is formed with a housing space 7 for housing a cylindrical battery cell 3, a collector plate 4, and an electrolyte 6. A terminal plate 5 is mounted on an opening 8 of the housing 2 in this embodiment by drawing or the like. The terminal plate 5 closes the opening 8.

[0033] The cylindrical battery cell 3 is formed into a cylindrical shape that can be accommodated in the accommodation space 7 of the housing 2. The cylindrical battery cell 3 formed into this cylindrical shape is accommodated in the accommodation space 7 together with the electrolyte 6. The central axis a of the cylindrical battery cell 3 extends along the central axis of the accommodation space 7 of the housing 2 in a state of being accommodated in the accommodation space 7 of the housing 2. In addition, in the following description, the direction in which the central axis a of the cylindrical battery cell 3 extends is referred to as the central axis direction Da, and the side on which the opening 8 of the housing 2 is arranged in the central axis direction Da is referred to as the central axis direction first side Da1, and the opposite side is referred to as the central axis direction second side Da2.

[0034] like Figure 1 to Figure 4 As shown in FIG. 1 , the cylindrical battery cell 3 includes a plurality of electrode foils 9, a plurality of separators 10, and a plurality of protrusions 11. The cylindrical battery cell 3 is formed by winding the electrode foils 9 and the separators 10 in a state of being alternately stacked in a cylindrical shape. Figure 2As shown, the cylindrical battery cell 3 includes a positive electrode foil 9P containing lithium and a negative electrode foil 9N capable of absorbing lithium as electrode foils 9. The cylindrical battery cell 3 of this embodiment includes a positive electrode protrusion 11P and a negative electrode protrusion 11N as protrusions 11.

[0035] like Figure 3 As shown, the positive electrode foil 9P of the present embodiment comprises an aluminum layer 12 composed of an aluminum alloy, and a positive electrode carbon material layer 13 composed of a carbon material applied to the surface and back of the aluminum layer 12. The positive electrode carbon material layer 13 of the present embodiment is formed of a material containing lithium carbonate (Li2CO3). The negative electrode foil 9N of the present embodiment comprises a copper layer 14 composed of copper as a metal having a melting point of 1000°C or more, and a negative electrode carbon material layer 15 composed of a carbon material capable of absorbing lithium ions applied to the surface and back of the copper layer 14. These aluminum layer 12 and copper layer 14 have a thickness of, for example, 6 to 20 μm.

[0036] The separator 10 is composed of at least an electrical insulating material that maintains electrical insulation between the electrodes of the lithium ion capacitor 1. The separator 10 is in a sheet shape when the cylindrical battery cell 3 is unfolded. The separator 10 is arranged between the positive electrode foil 9P and the negative electrode foil 9N. The separator 10 is arranged to sandwich the negative electrode foil 9N. Thus, the positive electrode foil 9P (electrode foil 9), the separator 10, the negative electrode foil 9N (electrode foil 9) and the separator 10 are alternately stacked. The separator 10 has a thickness of, for example, 18 to 22 μm.

[0037] like Figure 2 As shown, the protrusion 11 is formed integrally with the electrode foil 9. Figure 2 as well as Figure 4 As shown, the cylindrical battery cell 3 of this embodiment includes a negative electrode protrusion 11N formed on the first side Da1 in the central axis direction, and a positive electrode protrusion 11P formed on the second side Da2 in the central axis direction as a plurality of protrusions 11. The positive electrode protrusion 11P is formed to protrude from the positive electrode foil 9P to the second side Da2 in the central axis direction. The positive electrode protrusion 11P protrudes further to the second side Da2 in the central axis direction than the positive electrode foil 9P, the negative electrode foil 9N, and the separator 10. The negative electrode protrusion 11N is formed to protrude from the negative electrode foil 9N to the first side Da1 in the central axis direction. The negative electrode protrusion 11N protrudes further to the first side Da1 in the central axis direction than the positive electrode foil 9P, the negative electrode foil 9N, and the separator 10.

[0038] like Figure 4 As shown, the cylindrical battery cell 3 is formed by winding the positive electrode foil 9P (electrode foil 9), the separator 10, the negative electrode foil 9N (electrode foil 9) and the separator 10, which are respectively extended in strips, into a cylindrical shape in a stacked state. Thus, the positive electrode foil 9P, the negative electrode foil 9N and the separator 10 constituting the cylindrical battery cell 3 are respectively spirally shaped when viewed from the central axis direction Da.

[0039] The cylindrical battery cell 3 thus formed has a separator 10 disposed on its outer peripheral surface. In the present embodiment, adhesive tapes 50 and the like are respectively wound around the edge of the first side Da1 in the central axis direction and the edge of the second side Da2 in the central axis direction of the outer peripheral surface of the cylindrical battery cell 3. By means of these adhesive tapes 50 and the like, the end 25 of the separator 10 does not expand radially outwardly with respect to the central axis a as the center.

[0040] When the cylindrical battery cell 3 is in a single state, that is, when the cylindrical battery cell 3 is not housed in the housing 2, the electrode foil 9 (positive electrode foil 9P, negative electrode foil 9N) and the separator 10 are provided with a direction De in which the electrode foil 9 and the separator 10 extend in a strip shape (see Figure 2 , Figure 3 ) tension. This tension applies a pressure within a predetermined range between the electrode foils 9 facing each other across the separator 10 in the cylindrical battery cell 3. A pressure within a predetermined range, for example, 0.5 to 0.7 MPa, is applied between the positive electrode foil 9P and the negative electrode foil 9N facing each other across the separator 10.

[0041] In this way, by applying tension to the electrode foil 9 and the separator 10, uniform pressure is applied between the positive electrode foil 9P and the negative electrode foil 9N constituting the cylindrical battery cell 3 throughout the entire winding direction of the cylindrical battery cell 3. As a result, the gap between the positive electrode foil 9P and the negative electrode foil 9N is made uniform throughout the entire cylindrical battery cell 3.

[0042] Here, if the gap between positive electrode foil 9P and negative electrode foil 9N is uneven, a portion where the resistance between positive electrode foil 9P and negative electrode foil 9N is locally increased may occur. In the portion where the resistance is locally increased, it is considered that the overvoltage between positive electrode foil 9P and negative electrode foil 9N increases, leading to the precipitation of lithium (Li) into negative electrode foil 9N.

[0043] In contrast, the contact pressure between the positive electrode foil 9P and the negative electrode foil 9N is appropriately managed throughout the entire cylindrical battery cell 3, thereby achieving uniformity in the gap between the positive electrode foil 9P and the negative electrode foil 9N. This can suppress the variation in resistance between the positive electrode foil 9P and the negative electrode foil 9N, and suppress the precipitation of lithium.

[0044] In the cylindrical battery cell 3, if the pressure applied between the electrode foils 9 facing each other through the diaphragm 10 is less than, for example, 0.5 MPa, the effect of suppressing the precipitation of lithium by achieving the uniformity of the gap between the positive electrode foil 9P and the negative electrode foil 9N is reduced. In addition, in the cylindrical battery cell 3, if the pressure applied between the electrode foils 9 facing each other through the diaphragm 10 is less than, for example, 0.5 MPa, the tension acting on the electrode foils 9 and the diaphragm 10 during the manufacture of the cylindrical battery cell 3 becomes low, and it is difficult to stably exert the pressure applied between the electrode foils 9. In addition, in the cylindrical battery cell 3, if the pressure applied between the electrode foils 9 facing each other through the diaphragm 10 is greater than, for example, 0.7 MPa, the diaphragm 10 between the positive electrode foil 9P and the negative electrode foil 9N is plastically deformed and squeezed, and the gap between the positive electrode foil 9P and the negative electrode foil 9N may be excessively narrowed. In addition, in the cylindrical battery cell 3, since the pressure applied between the electrode foils 9 facing each other with the separator 10 between them is greater than, for example, 0.7 MPa, the tension acting on the electrode foils 9 and the separator 10 becomes excessively high during the manufacture of the cylindrical battery cell 3, especially exceeding the mechanical strength of the separator 10, sometimes causing adverse effects such as breakage of the separator 10.

[0045] In addition, in the area of ​​the outer peripheral surface of the cylindrical battery cell 3 where the ends of the electrode foil 9 and the separator 10 are fixed by the adhesive tape 50, the tension applied to the electrode foil 9 and the separator 10 may be lower than that in the inner peripheral portion of the cylindrical battery cell 3. Therefore, the area where the pressure within a certain range is applied between the positive electrode foil 9P and the negative electrode foil 9N facing each other with the separator 10 interposed therebetween as described above is the portion excluding the area facing the outer peripheral surface of the cylindrical battery cell 3.

[0046] When forming the cylindrical battery cell 3, it is preferable that the appropriate numerical ranges of the tension applied to the electrode foil 9 and the separator 10 and the pressure applied between the electrode foils 9 facing each other through the separator 10 are set, for example, at the trial production stage of the lithium ion capacitor 1 before the lithium ion capacitor 1 is actually manufactured as a product. For this purpose, for example, when forming the cylindrical battery cell 3 while applying tension to the electrode foil 9 and the separator 10, a pressure-sensitive paper, a pressure sensor, etc. are sandwiched between the electrode foils 9 facing each other through the separator 10 to grasp the pressure value applied between the electrode foils 9 facing each other through the separator 10. Using the cylindrical battery cell 3 whose pressure value applied between the electrode foils 9 is grasped, a long-term storage test, a cycle test in which a load is repeatedly applied, etc. are carried out under specified conditions, and the cylindrical battery cell 3 after the implementation is evaluated to confirm the presence or absence of lithium precipitation. As a result, appropriate numerical ranges of the tension applied to electrode foil 9 and separator 10 and the pressure applied between electrode foils 9 facing each other with separator 10 interposed therebetween can be set based on conditions under which lithium deposition is not confirmed.

[0047] In this embodiment, if Figure 1 , Figure 4 As shown, a positive electrode collector plate 4P and a negative electrode collector plate 4N are provided as the collector plate 4. The negative electrode collector plate 4N is fixed to the negative electrode protrusion 11N by welding or the like. The positive electrode collector plate 4P is fixed to the positive electrode protrusion 11P by welding or the like. These positive electrode collector plate 4P and negative electrode collector plate 4N are formed into a substantially flat plate shape having a circular outer edge centered on the central axis a, and have an inner side surface 27 facing the protrusion 11 side in the central axis direction Da, and an outer side surface 28 facing the opposite side of the inner side surface 27 in the central axis direction Da and forming a back-to-back relationship.

[0048] The positive electrode collector plate 4P is formed of a metal including the same metal as the positive electrode protrusion 11P. That is, the positive electrode collector plate 4P of the present embodiment is formed of an aluminum alloy. The negative electrode collector plate 4N is formed of a metal including the same metal as the negative electrode protrusion 11N. The negative electrode collector plate 4N is formed of a material having a melting point of 1000°C or more. The negative electrode collector plate 4N of the present embodiment is formed of copper. In addition, as Figure 1 As shown, in the present embodiment, a convex portion 29 is formed at the central portion of the current collector plate 4, which protrudes toward the protruding portion 11 side in the central axis direction Da. Furthermore, a through hole 30 is formed in the convex portion 29 of the current collector plate 4. The convex portion 29 is inserted into a hollow portion 31 with a circular cross section formed in the central portion of the cylindrical battery cell 3 and extending in the central axis direction Da.

[0049] like Figure 1 As shown, the terminal plate 5 closes the opening 8 of the shell 2. The terminal plate 5 of the present embodiment comprises at least a terminal plate body 35, a pressure regulating valve 36 and a sealing rubber 37. The terminal plate body 35 is circular when viewed from the central axis direction Da, and has a hole 35h in its central portion. The pressure regulating valve 36 is arranged in the central portion of the terminal plate body 35, and adjusts the pressure of the receiving space 7 via the hole 35h. The sealing rubber 37 seals the gap between the terminal plate body 35 and the inner peripheral surface of the opening 8 of the shell 2. The pressure regulating valve 36 is installed in such a manner as to block the hole 35h after the electrolyte 6 is injected into the receiving space 7 from the hole 35h of the terminal plate body 35.

[0050] 《Manufacturing method of lithium ion capacitor》

[0051] A method for manufacturing the above-mentioned lithium ion capacitor will be described.

[0052] like Figure 5As shown, the manufacturing method S10 of the lithium ion capacitor includes: step S11, doping with a first doping current value; step S12, doping with a second doping current value; step S13, discharging; step S14, repeatedly charging and discharging; and step S15, charging to the factory voltage. The manufacturing method S10 of the lithium ion capacitor is performed after the lithium ion capacitor 1 is manufactured and when doping is performed. In the manufacturing method S10 of the lithium ion capacitor, as shown in FIG. Figure 6 As shown, by connecting the power supply device 200 to the terminal plate 5, a voltage is applied between the aluminum layer 12 of the positive electrode foil 9P and the copper layer 14 of the negative electrode foil 9N. As a result, the lithium carbonate (Li2CO3) contained in (carried on) the positive electrode carbon material layer 13 is electrochemically decomposed to generate lithium ions (Li + ) and supplied to the negative electrode carbon material layer 15. As a result, the negative electrode carbon material layer 15 reversibly absorbs and carries lithium ions, and so-called lithium ion doping is performed.

[0053] like Figure 7 As shown, in the step S11 of doping with a first doping current value, a current is passed between the positive electrode foil 9P and the negative electrode foil 9N at the first doping current value from the start of doping by the power supply device 200. In the step S11, the first doping current value, for example, the C rate of the doping current is set within a range of 0.05C or more and 0.2C or less. Here, the so-called C rate generally indicates the speed of charging and discharging. Here, the magnitude of the current that fully charges (or discharges) the capacity of the negative electrode (in the case of this embodiment, the negative electrode of the lithium ion capacitor 1) that is the object within 1 hour is defined as 1C. If the first doping current value is greater than 0.2C, the uniform effect of SEI thin filming brought about by suppressing the doping current to a low level immediately after the doping starts is reduced. In addition, if the first doping current value is less than 0.05C, the time required for doping becomes longer, resulting in a decrease in production efficiency.

[0054] In the present embodiment, 1C of the lithium ion capacitor 1 is, for example, 5A (ampere). In this case, in step S11, doping is performed at a first doping current value set within a range of 0.05C or more and 0.2C or less, that is, 0.25A or more and 1.0A. In the present embodiment, doping is performed at a first doping current value set to 0.1C=0.5A in step S11. In addition, in step S11 of doping at the first doping current value, so-called CC (Constant Current) charging is performed at a constant current.

[0055] In step S11 of doping with a first doping current value, doping based on the first doping current value is continued until the voltage between the positive electrode and the negative electrode (hereinafter referred to as the inter-electrode voltage) reaches a preset target voltage Vt. Here, the target voltage is set to be lower than the rated voltage of the lithium ion capacitor. If doping is started in step S11, the film-forming components such as additives and solvents contained in the electrolyte 6 are decomposed, thereby forming a SEI (Solid Electrolyte Interphase: solid electrolyte interface) film on the surface of the negative electrode carbon material layer 15 (the interface with the electrolyte 6). In step S11, for example, the inter-electrode voltage at which the SEI film is formed on the entire surface of the negative electrode carbon material layer 15 is set to the target voltage Vt. In this embodiment, the voltage at which the SEI film is formed on the entire surface of the negative electrode carbon material layer 15, which has been confirmed by preliminary experiments, is set to the target voltage Vt. As the target voltage Vt, for example, 4.2V can be listed.

[0056] In the power supply device 200 , the inter-electrode voltage in the lithium ion capacitor 1 to be doped is monitored, and when the inter-electrode voltage reaches the target voltage Vt, the process proceeds to step S12 to switch to doping with the second doping current value.

[0057] In step S12 of doping with a second doping current value, doping is performed with a second doping current value by the power supply device 200. In step S12, the second doping current value is set within a range where the C rate is higher than that of the first doping current value in step S11 and is less than 1C. In the present embodiment, in step S12, for example, doping is performed with a second doping current value set within a range of greater than 0.25C and less than 1C. In the present embodiment, in step S12, doping is performed with a second doping current value set to 0.32C=1.6A. In addition, in step S12 of doping with a second doping current value, so-called CCCV (Constant Current Constant Voltage) charging is performed, which is charging with a constant current followed by a constant voltage.

[0058] In step S12 of doping with a second doping current value, doping is continued with the second doping current value until the inter-electrode voltage reaches the rated voltage of the lithium ion capacitor 1, or is set to a second target voltage Vr below the rated voltage. In the present embodiment, the second target voltage Vr is set to 4.5V, for example. In step S12, after the SEI film is formed on the entire surface of the negative electrode carbon material layer 15 in step S11, doping can be efficiently performed with a second doping current value higher than that in step S11. Thus, the doping operation of the lithium ion capacitor 1 is completed. If the inter-electrode voltage reaches the second target voltage Vr, after constant voltage charging is performed to maintain the second target voltage Vr for a specified time, it is transferred to step S13.

[0059] In step S13 of performing discharge, the lithium ion capacitor 1 is discharged. In step S13, discharge is performed until the inter-electrode voltage of the lithium ion capacitor 1 becomes a preset lower limit target voltage Vd from the second target voltage Vr (e.g., 4.5V). In the present embodiment, the power supply device 200 performs discharge at a current value (0.32C=1.6A) of the same C rate as step S12 until the inter-electrode voltage becomes a lower limit target voltage Vd set to, for example, 2.0V. When the inter-electrode voltage reaches the lower limit target voltage Vd, the process proceeds to step S14.

[0060] In step S14 of repeatedly performing charge and discharge, the charging and discharging of the lithium ion capacitor 1 are repeatedly performed a predetermined number of times by the power supply device 200. Thus, the fixation of the SEI film formed on the negative electrode carbon material layer 15 is achieved. In the present embodiment, in step S14, charging and discharging are repeatedly performed between the second target voltage Vr (e.g., 4.5V) and the lower limit target voltage Vd (e.g., 2.0V) with a current value (1.0C=5A) at which the C rate is set to, for example, 1.0C. After charging and discharging are repeatedly performed a predetermined number of times, transfer to step S15. In addition, during the period of repeated charging and discharging in step S14, the second target voltage Vr and the lower limit target voltage Vd may be changed, or a time period during which neither charging nor discharging is performed may be set.

[0061] In the charging step S15 to the factory voltage, the battery is charged with a current value of, for example, 1.0C (1.0C=5A) until the preset factory voltage Vs (for example, 3.8V) is reached. When the voltage value reaches the factory voltage Vs, the charging is stopped and the power supply device 200 is disconnected.

[0062] In addition, the above-mentioned step S13 of discharging, step S14 of repeatedly charging and discharging, and step S15 of charging to the factory voltage can be omitted.

[0063] <Effects>

[0064] In the manufacturing method S10 of the above-mentioned lithium ion capacitor 1, after doping is performed at a first doping current value with a C rate set to be greater than 0.05C and less than 0.2C from the start of doping, doping is performed at a second doping current value with a C rate higher than the first doping current value. In this way, in the initial stage just after the doping starts, the doping current is suppressed to be low, thereby the potential on the surface of the negative electrode carbon material layer 15 is uniformized, and the formation rate of the SEI coating on the surface of the negative electrode carbon material layer 15 is uniformized. As a result, the film thickness of the SEI coating formed on the surface of the negative electrode carbon material layer 15 is uniformized, and the local generation of high resistance areas can be suppressed. As a result, during charging and discharging when the lithium ion capacitor 1 is actually used, the absorption property of lithium ions in the negative electrode carbon material layer 15 is improved, and the amount of lithium ions that are not absorbed by the negative electrode carbon material layer 15 and precipitated can be suppressed. As a result, the deactivation rate of lithium can be reduced, and the durability of the lithium ion capacitor 1 can be improved.

[0065] Furthermore, in the step S11 of doping with the first doping current value, doping is performed until the inter-electrode voltage reaches a target voltage Vt set lower than the rated voltage Vr. By setting the target voltage Vt so that the SEI film is formed on the entire surface of the negative electrode carbon material layer 15 , a uniform SEI film can be stably formed.

[0066] Furthermore, in the step S11 of doping with the first doping current value, doping is performed with a constant current, and in the step S12 of doping with the second doping current value, doping is performed with a constant current and then with a constant voltage. Thus, in the step S11, the SEI film can be stably formed, and in the step S12, the lithium ion capacitor 1 can be doped efficiently.

[0067] In addition, the lithium ion capacitor 1 described above applies tension to the electrode foil 9 and the separator 10 of the cylindrical battery cell 3, and applies uniform pressure between the positive electrode foil 9P and the negative electrode foil 9N constituting the cylindrical battery cell 3, throughout the entire winding direction of the cylindrical battery cell 3. Thus, the gap between the positive electrode foil 9P and the negative electrode foil 9N is made uniform throughout the entire cylindrical battery cell 3. Therefore, the contact pressure between the positive electrode foil 9P and the negative electrode foil 9N is properly managed, and the deviation of the resistance between the positive electrode foil 9P and the negative electrode foil 9N can be suppressed. As a result, the precipitation of lithium in the electrode foil 9 (negative electrode foil 9N) can be suppressed. For such a lithium ion capacitor 1, the manufacturing method S10 of the lithium ion capacitor 1 described above is used to perform doping, thereby reducing the deactivation rate of lithium and improving the durability of the lithium ion capacitor 1. Such an effect becomes more significant.

[0068] [Example]

[0069] The doping of the lithium ion capacitor 1 was performed by the method S10 for manufacturing the lithium ion capacitor 1 as described above, and the evaluation was performed. The results are shown below.

[0070] The cylindrical battery cell 3 was produced so that a pressure of 0.5 MPa was applied between the positive electrode foil 9P and the negative electrode foil 9N. The lithium ion capacitor 1 was produced using the cylindrical battery cell 3 .

[0071] (Example 1)

[0072] The manufactured lithium ion capacitor 1 was doped by the manufacturing method S10 of the lithium ion capacitor 1 as described above. In step S11 immediately after the doping was started, doping was performed at a constant current with a doping current of 0.1C=0.5A until the inter-electrode voltage reached the target voltage Vt=4.2V. After the inter-electrode voltage reached 4.2V, the process was transferred to step S12, and doping was performed at a constant voltage and a constant current with a doping current of 0.32C=1.6A until the inter-electrode voltage reached the second target voltage Vr=4.5V. Thereafter, in step S13, discharge was performed at a current value of 0.32C=1.6A until the inter-electrode voltage reached 2.0V. Furthermore, in step S14, 5 cycles of charging and discharging were performed between the inter-electrode voltages of 2.0V and 4.5V with a current value of 0.32C=1.6A. In step S15 , charging was performed at a current value of 0.32C=1.6A until the factory voltage Vs=3.8V was reached.

[0073] (Comparative Example)

[0074] For comparison, Figure 8 As shown, for the manufactured lithium ion capacitor 1, in step S21 immediately after the start of doping, doping was performed at a constant voltage and constant current with a doping current of 0.32C = 1.6 A until the inter-electrode voltage reached the rated voltage Vr = 4.5 V. Thereafter, doping was performed under the same conditions as steps S13, S14, and S15 of Example 1.

[0075] For each of Example 1 and Comparative Example, the charged lithium ion capacitor 1 was subjected to a cycle durability test in which the process of discharging until the inter-electrode voltage reached 2.8 V and the process of charging until the inter-electrode voltage reached 3.95 V were repeated for 600 to 1000 hours, and the deactivation rate of lithium was measured. The results are shown in Fig. 9 .

[0076] like Fig. 9As shown, it was confirmed that the lithium deactivation rate was suppressed lower in Example 1 in which doping was performed by the manufacturing method S10 of the lithium ion capacitor 1 than in the comparative example.

[0077] In addition, for each of Example 1 and Comparative Example, the surface state of the negative electrode carbon material layer 15 of the negative electrode foil 9N after disassembly of the battery cell at a battery cell voltage of 4.0 V after doping, storage aging, exhaust, and initial input and output measurement was observed by scanning electron microscope (SEM). The results are shown in Fig.10 , Fig.11 .

[0078] Confirm that Fig.11 As shown, the negative electrode surface of the comparative example has large concave and convex portions distributed thereon. Fig.10 As shown, the concave and convex portions on the surface of the negative electrode of Example 1 doped by the manufacturing method S10 of the lithium ion capacitor 1 are finely distributed. Here, the convex portion is a deposit derived from metal Li, so in Example 1, the generation of initial coarse deposits is suppressed, thereby improving the life.

[0079] In addition, for each of Example 1 and Comparative Example, the surface analysis of the binding state with fluorine (F) was performed by X-ray photoelectron spectroscopy (X-ray photoelectron spectroscopy, XPS) for each of the negative electrode carbon material layer 15 just after doping and after the cycle durability test. The results are shown in Fig.12 , Fig.13 .

[0080] like Fig.12 , Fig.13 As shown in FIG. 1 , both Example 1 and Comparative Example confirmed a peak PL indicating the combination of lithium (Li) and fluorine (F) at around 688 eV. Fig.13 In the comparative example shown, a peak PC indicating the combination of carbon (C) and fluorine (F) was confirmed at around 690 eV. In this comparative example, it was confirmed that the peak PC indicating the combination of carbon (C) and fluorine (F) became significantly higher after the cycle endurance test compared to just after doping. In the process of lithium precipitation, a compound composed of carbon (C) and fluorine (F) is generated. That is, the high peak PC indicating the combination of carbon (C) and fluorine (F) indicates that lithium has been precipitated.

[0081] In contrast, in Example 1, the peak PC indicating the combination of carbon (C) and fluorine (F) was not confirmed immediately after doping, and the peak PC indicating the combination of carbon (C) and fluorine (F) was slightly confirmed after the cycle endurance test, but it was confirmed to be significantly lower than that of the comparative example. Therefore, it was confirmed that the precipitation of lithium was suppressed by doping in the manufacturing method S10 of the lithium ion capacitor 1.

[0082] <Other embodiments>

[0083] As mentioned above, although embodiment of this invention was described, this invention is not limited to this, It can change suitably within the range which does not deviate from the technical idea of ​​this invention.

[0084] In the above-mentioned embodiment, the structure of the lithium ion capacitor 1 is described, but the lithium ion capacitor 1 may be set to another structure as appropriate.

[0085] Industrial Applicability

[0086] According to the present invention, by improving the uniformity of the coating formed on the surface of the negative electrode during doping, the deactivation rate of lithium can be reduced, thereby improving the durability of the product.

[0087] Description of the mark

[0088] 1: Lithium ion capacitor, 2: Housing, 3: Cylindrical battery cell, 9: Electrode foil, 10: Separator.

Claims

1. A method for manufacturing a lithium ion capacitor, comprising: A step of performing doping at a first doping current value of 0.05C or more and 0.2C or less; and A step of performing doping at a second doping current value higher than the first doping current value.

2. The method for manufacturing a lithium ion capacitor according to claim 1, wherein: The step of performing doping at the first doping current value is performed until the inter-electrode voltage reaches a target voltage set lower than a rated voltage.

3. The method for manufacturing a lithium ion capacitor according to claim 2, wherein: When the inter-electrode voltage reaches the target voltage, the process switches to the step of performing doping at the second doping current value.

4. The method for manufacturing a lithium ion capacitor according to claim 1 or 3, wherein: In the step of performing doping at the second doping current value, doping is continued at the second doping current value until the inter-electrode voltage reaches the rated voltage or a second target voltage set to be lower than the rated voltage.

5. The method for manufacturing a lithium ion capacitor according to claim 1 or 2, wherein: In the step of performing doping at the first doping current value, doping is performed at a constant current.

6. The method for manufacturing a lithium ion capacitor according to claim 5, wherein: In the step of performing doping at the second doping current value, doping is performed at a constant current and then at a constant voltage.

7. The method for manufacturing a lithium ion capacitor according to claim 1, wherein: The second doping current value is greater than or equal to 0.25C and less than 1C.

8. The method for manufacturing a lithium ion capacitor according to claim 1 or 2, wherein: The lithium ion capacitor has: A cylindrical battery cell is formed by winding an electrode foil and a separator each extending in a strip shape into a cylindrical shape in a stacked state; and A cylindrical housing for accommodating the cylindrical battery cell. The cylindrical battery cell applies tension to the electrode foil and the separator in the directions in which the electrode foil and the separator extend, thereby applying a pressure of 0.5 MPa or more between the electrode foils that face each other with the separator interposed therebetween.

9. The method for manufacturing a lithium ion capacitor according to claim 8, wherein: The pressure applied between the electrode foils facing each other via the separator is 0.7 MPa or less.

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