JP2015187949A - lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide means for measuring the concentration lithium ions in an electrolyte appropriately.SOLUTION: A lithium ion secondary battery has an electrode group, a third electrode and a fourth electrode, the electrode group has a positive electrode and a negative electrode, potentials of the positive electrode and negative electrode are detected by the third electrode and fourth electrode, the third electrode is arranged in the electrode group, and the fourth electrode is arranged on the outside of the electrode group. For example, the third electrode and fourth electrode are composed of any one kind or more of phosphoric acid transition metal lithium or lithium titanate, and the fourth electrode is double junction type.

Description

  The present invention relates to a lithium ion secondary battery.

  In recent years, development of lithium ion secondary batteries has been actively promoted. The performance of a lithium ion secondary battery is degraded by repeated charge and discharge. As a means for knowing battery performance deterioration, there is a method of detecting the internal state of the battery. As one means for detecting the internal state of the battery, there is a method of detecting the potential of the positive electrode and the negative electrode by installing a reference electrode inside the battery.

  In Patent Literature 1, a reference electrode is provided in addition to a positive electrode and a negative electrode inside a lithium ion secondary battery. The positive electrode, the negative electrode, and the reference electrode are ion-conductive with each other through the electrolytic solution with a separator interposed therebetween. With this configuration, it is possible to detect the potential of the positive electrode and the negative electrode based on the reference electrode.

Japanese Patent Laid-Open No. 2002-50407

  It is known that the performance deterioration of the lithium ion secondary battery depends on the deterioration of the electrode and the decrease in the electrolyte concentration. Although the technology of Patent Document 1 can detect the potential of the positive electrode and the negative electrode and know the deterioration of the electrode, both the positive electrode and the negative electrode are located in the reaction part between the positive electrode and the negative electrode, Since the liquid concentration fluctuates from the beginning, it is difficult to appropriately calculate the concentration of lithium ions in the electrolytic solution when measuring the voltage value between the reference electrodes.

  An object of this invention is to measure the density | concentration of the lithium ion in electrolyte solution appropriately.

  The features of the present invention for solving the above problems are as follows, for example.

  The electrode group has a third electrode and a fourth electrode, the electrode group has a positive electrode and a negative electrode, the potentials of the positive electrode and the negative electrode are detected by the third electrode and the fourth electrode, and the third electrode The fourth electrode is a lithium ion secondary battery that is disposed outside the electrode group.

  By this invention, the density | concentration of the lithium ion in electrolyte solution can be measured appropriately. Problems, configurations, and effects other than those described above will be clarified by the following description of embodiments.

It is the schematic which shows the structure of the lithium ion secondary battery in one Embodiment of this invention. It is the schematic which shows the arrangement | positioning location of the 3rd electrode in one Embodiment of this invention. It is the schematic which shows the arrangement | positioning location of the 4th electrode in one Embodiment of this invention. It is the schematic which shows the arrangement | positioning location of the 4th electrode in one Embodiment of this invention. It is the schematic which shows one form of a 4th electrode. It is a calibration curve of the estimated electrolyte concentration and detection potential in one embodiment of the present invention. It is the schematic which shows the structure of the lithium ion secondary battery in one Example of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The following description shows specific examples of the contents of the present invention, and the present invention is not limited to these descriptions. Various modifications by those skilled in the art are within the scope of the technical idea disclosed in this specification. Changes and modifications are possible. In all the drawings for explaining the present invention, components having the same function are denoted by the same reference numerals, and repeated description thereof may be omitted.

<Lithium ion secondary battery 130>
FIG. 1 is a schematic diagram showing the configuration of a lithium ion secondary battery in one embodiment of the present invention. As an example, an example using a laminated lithium ion secondary battery is given. FIG. 1B is a side view of the stacked portion of the electrode group 100 in which the lithium ion secondary battery 130 is stacked in FIG. 1A viewed from above the electrode stacked surface.

  The lithium ion secondary battery 130 includes an electrode group 100, a third electrode 103, a fourth electrode 104, and an insulating sheet 110 installed in a battery case 160. In the electrode group 100, positive electrodes 101, separators 105, and negative electrodes 102 are alternately stacked. The shape of the lithium ion secondary battery is a square type or a laminate type when the electrode group is laminated, and a cylindrical type, a flat oval type, a square type, etc. when the electrode group is wound. Yes, any shape may be selected.

  The third electrode 103 is disposed inside the electrode group 100 while the positive electrode 101 and the negative electrode 102 face each other. Further, the fourth electrode 104 is disposed outside the electrode group 100 at a location away from the portion where the positive electrode 101 and the negative electrode 102 face each other.

  The positive electrode 101, the negative electrode 102, the third electrode 103, and the fourth electrode 104 are ionic conductive with each other through the electrolytic solution. In order to avoid conduction with the positive electrode 101 and the negative electrode 102, the third electrode 103 is covered with a separator 105. The fourth electrode 104 is covered with an insulating sheet 110.

  The positive electrode terminal 121 and the negative electrode terminal 122 are energized with the positive electrode 101 and the negative electrode 102, respectively, and the lithium ion secondary battery 130 is charged and discharged by an external circuit via the positive electrode terminal 121 and the negative electrode terminal 122. The third electrode 103 and the fourth electrode 104 are electrically connected to the third electrode terminal 123 and the fourth electrode terminal 124, respectively.

<Positive electrode 101>
The positive electrode 101 includes a positive electrode active material made of a lithium-containing oxide that can reversibly insert and desorb lithium ions. Examples of the positive electrode active material include layered transition metal oxides with or without substitution elements, lithium transition metal phosphates, and spinel type transition metal oxides. For example, as the layered transition metal oxide, lithium nickelate LiNiO ₂ or lithium cobaltate LiCoO ₂ , as the transition metal lithium phosphate lithium iron phosphate LiFePO ₄ , manganese phosphate lithium LiMnPO ₄ , spinel type transition metal oxide Examples thereof include lithium manganate LiMn ₂ O ₄ . One kind or two or more kinds of the above materials may be contained as the positive electrode active material. In the positive electrode active material in the positive electrode 101, lithium ions are desorbed in the charging process, and lithium ions desorbed from the negative electrode active material in the negative electrode 102 are inserted in the discharging process.

<Negative electrode 102>
The negative electrode 102 is, for example, a carbon material that can reversibly insert and desorb lithium ions, silicon-based material Si, SiO, lithium titanate with or without a substitution element, lithium vanadium composite oxide, lithium and metal, for example, , A negative electrode active material made of an alloy with tin, aluminum, antimony or the like. As a carbon material, as a raw material, natural graphite, a composite carbonaceous material obtained by forming a film on natural graphite by a dry CVD method or a wet spray method, a resin material such as epoxy or phenol, or a pitch-based material obtained from petroleum or coal Examples thereof include artificial graphite and non-graphitizable carbon material produced by firing. The above materials may be contained singly or in combination of two or more as the negative electrode active material. The negative electrode active material in the negative electrode 102 undergoes insertion / extraction reaction or conversion reaction of lithium ions in the charge / discharge process.

<Separator 105>
For the separator 105, for example, a polypropylene separator is used. In addition to polypropylene, a microporous film or non-woven fabric made of polyolefin such as polyethylene can be used.

<Third electrode 103, fourth electrode 104>
The third electrode 103 and the fourth electrode 104 are made of a material containing an active material used for the lithium ion battery or the positive and negative electrodes of the lithium battery described above. The third electrode 103 and the fourth electrode 104 are used to detect the potentials VP and VN of the positive electrode 101 and the negative electrode 102 or the potential difference VR between the third electrode 103 and the fourth electrode 104. Therefore, it is desirable that the potentials of the third electrode 103 and the fourth electrode 104 do not change. Since a minute current flows through the third electrode 103 and the fourth electrode 104 in order to measure the potentials of VP, VN, and VR, the potential of the third electrode 103 and the fourth electrode 104 may not change even if a minute current flows. desirable.

Therefore, lithium transition metal lithium with or without a substitution element (LiMPO ₄ (M is a transition metal and one or more of iron, manganese, and vanadium)), lithium manganese phosphate LiMnPO _{4 and the} like It has a region where Li desorption occurs as a two-phase coexistence reaction, such as lithium transition metal lithium, spinel type transition metal oxides such as lithium manganate LiMn ₂ O ₄ , lithium titanate, lithium vanadium composite oxide, graphite, etc. It is desirable to select at least one material group having a flat and stable charge / discharge potential. Among these, it is desirable from the viewpoint of potential stability that it is composed of at least one of lithium transition metal lithium and lithium titanate having a wide, flat and stable charge / discharge potential region. In order not to change the potential, it is desirable that Li is desorbed from the third electrode 103 and the fourth electrode 104 containing the above materials to the region where the two-phase coexistence reaction occurs.

  The third electrode 103 and the fourth electrode 104 may contain one or more of the above materials. However, when two or more of the above materials are contained, a plurality of potential stable regions exist, Each region becomes narrower with respect to 1 mol of the substance. Therefore, in consideration of potential stability, it is desirable to use one kind alone.

<Electrolyte>
As the electrolytic solution, for example, a non-aqueous solution in which 1 mol / l of lithium hexafluorophosphate as a lithium salt is dissolved in a mixed solvent of ethylene carbonate and diethyl carbonate having a volume ratio of 1: 1 is injected.

The lithium salt is not particularly limited, but for inorganic lithium salts, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiI, LiCl, LiBr, etc., and for organic lithium salts, LiB [OCOCF ₃ ] ₄ , LiB [OCOCF ₂ CF ₃ ] ₄ , LiPF ₄ (CF ₃ ) ₂ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ CF ₂ CF ₃ ) ₂ or the like can be used.

  As a solvent, ethylene carbonate (EC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate An aprotic organic solvent such as (MPC) or ethylpropyl carbonate (EPC), or a solvent of two or more of these mixed organic compounds is used.

<Insulation sheet 110>
For the insulating sheet 110, a film made of polypropylene or a polyolefin such as polyethylene used in the separator 105, a nonwoven fabric, or the like is used, but a material having a small pore diameter and a high density is preferable. The electrolyte solution concentration of the electrode group 100 is calculated using the potential difference VR between the third electrode 103 disposed inside the electrode group 100 and the fourth electrode 104 disposed outside the electrode group 100. In order to accurately calculate the electrolyte concentration of the electrode group 100, it is necessary that the electrolyte concentration of the fourth electrode 104 be kept uniformly at a certain specified amount. It is known that the performance of the lithium ion secondary battery is deteriorated by repeatedly charging and discharging, and the electrolyte concentration in the electrode group 100 is also reduced. When distribution occurs in the electrolyte concentration, lithium ions move from the high concentration side to the low concentration side, and the concentration tends to be uniform. Therefore, it is desirable that the material of the insulating sheet 110 has a small hole diameter, a high density, a low lithium ion diffusion rate, and a small change in the electrolyte concentration. Further, in order to reduce the change in the electrolyte concentration, it is desirable that the insulating sheet 110 is thick, such as multiple layers.

  A specific location of the third electrode 103 is shown in FIG. The third electrode 103 is disposed inside the electrode group 100. For example, the third electrode 103 is disposed in the middle where the positive electrode 101 and the negative electrode 102 face each other.

  As for the arrangement location, as shown in FIG. 2 (a), the electrode group 100 may be arranged on the outside of a plurality of stacked electrodes, in other words, on the electrode group 100 near the battery case 160, It may be arranged on the inner side, in other words, on the side far from the battery case 160 in the electrode group 100. In FIG. 2A, it can be said that the third electrode 103 is disposed at the edge portion in the direction that becomes the vertical surface in the direction in which the positive electrode 101 and the negative electrode 102 are laminated in the electrode group 100.

  Like FIG.2 (b), you may arrange | position to the center part of an electrode facing part, and may arrange | position to an edge part. In FIG. 2B, it can be said that the third electrode 103 is disposed in the center of the electrode group 100 in the direction in which the positive electrode 101 and the negative electrode 102 are laminated. However, even in the case of a plurality of stacked electrodes, there is a difference in the electrolyte solution concentration, so that it is desirable to arrange the electrodes on the inner side, which is considered to exhibit more average characteristics. In addition, since it is considered that the influence of inhibiting lithium ion diffusion between the positive electrode 101 and the negative electrode 102 is large when arranged in the center in the electrode facing portion, it is desirable that the electrode is disposed in the edge portion rather than the central portion. Therefore, as shown in FIG. 2C, the inside of the plurality of layers, particularly the central portion of the plurality of layers, the edge portion of the electrode facing portion, in other words, in the direction in which the positive electrode 101 and the negative electrode 102 are stacked in the electrode group 100. It is desirable to arrange at the edge portion in the direction of the vertical surface.

  Although FIG. 2 illustrates a stacked lithium ion battery, the same applies to a wound lithium ion secondary battery. In FIG. 2, there is one third electrode 103 disposed inside the electrode group 100, but a plurality of third electrodes 103 may be disposed in order to confirm the electrolyte solution distribution in the electrode.

  A specific location of the fourth electrode 104 is shown in FIG. The fourth electrode 104 is disposed outside the electrode group 100. For example, the fourth electrode 104 is disposed outside the electrode group 100 at a location away from the portion where the positive electrode 101 and the negative electrode 102 face each other.

  First, a stacked lithium ion secondary battery will be described. As for the arrangement location, as shown in FIG. 3A, the fourth electrode 104 is located outside the electrode group 100 and above the battery case 160, in other words, outside the electrode group 100 close to the positive electrode terminal 121 and the negative electrode terminal 122. May be arranged. Further, as shown in FIG. 3B, the fourth electrode 104 is disposed outside the electrode group 100, below the battery case 160, in other words, outside the electrode group 100 far from the positive electrode terminal 121 and the negative electrode terminal 122. You may do it. In order to accurately calculate the electrolyte concentration of the electrode group 100, it is necessary that the electrolyte concentration of the fourth electrode 104 is kept uniform at a certain specified amount. Therefore, when the electrolyte concentration of the electrode group 100 decreases, the fourth electrode 104 faces the positive electrode 101 and the negative electrode 102 in order to inhibit the outflow of lithium ions from the insulating sheet 110 enclosing the fourth electrode 104. It is desirable to place it at a location farther from the part where it is located. Therefore, as shown in FIG. 3C, the central portion of the outer periphery of the negative electrode 102 not related to the charge / discharge reaction, in other words, the vertical surface of the electrode group 100 in the direction in which the positive electrode 101 and the negative electrode 102 are stacked. It is desirable to arrange in the center.

  Next, a wound lithium ion secondary battery will be described. 4A, the fourth electrode 104 may be disposed on the battery case 160 outside the electrode group 100 wound around the winding shaft 140, as shown in FIG. As shown in (b), the fourth electrode 104 may be disposed outside the electrode group 100 and below the battery case 160. For the same reason, the fourth electrode 104 is related to the charge / discharge reaction as shown in FIG. It is desirable to dispose the fourth electrode 104 in the central part of the outer periphery of the negative electrode 102 without any electrode. By disposing the fourth electrode 104 as shown in FIG. 4C, it is possible to suppress the outflow of lithium ions from the insulating sheet 110 that surrounds the fourth electrode 104. Further, as shown in FIG. 4D, the winding shaft 140 may be a cavity, and the fourth electrode 104 may be disposed in the hollow portion, particularly the central portion of the winding shaft 140.

  Since it is desirable that the lithium ion concentration in the electrolyte solution around the fourth electrode 104 be kept uniform, as shown in FIG. 5, the fourth electrode 104 may be a double junction type reference electrode that is double-isolated. In FIG. 5, the fourth electrode 104 is disposed in a region formed by the connection site 150 and the container 170. The connection part 150 is in contact with the container 170. In the double junction type reference electrode, the fourth electrode 104 and the electrode group 100 have ionic conductivity due to the connection portion 150. By isolating the fourth electrode 104 by the double junction type reference electrode container 170, it is possible to keep the concentration of the surrounding electrolyte more uniform. When the fourth electrode 104 is a double junction type reference electrode, the insulating sheet 110 may or may not be provided. When the insulating sheet 110 is provided, the distance from the reaction portion between the positive electrode and the negative electrode from the fourth electrode 104 is longer, and the lithium ion concentration in the electrolyte solution around the fourth electrode can be maintained.

The connection part 150 is constituted by a salt bridge using a gel having glass-type or ceramic-type liquid junction or ionic conductivity. From the viewpoint of leakage of the electrolytic solution, it is desirable to use a material other than ceramics as the connection portion 150 in the above. Among these, it is desirable to use a salt bridge of an ionic liquid gel with a small change in the concentration of the internal solution and a small amount of impurity mixing into the electrolyte. A hydrophobic material is used as the material of the ionic liquid gel, and the cation of the hydrophobic ionic liquid is at least one of a quaternary ammonium cation, a quaternary phosphonium cation, and a quaternary arzonium cation, The anion is [R ₁ SO ₂ NSO ₂ R ₂ ] ⁻ (R _{1 and} R ₂ are each a C 1-5 perfluoroalkyl group), a borate ion containing fluorine and tetravalent boron, bis (2-ethylhexyl) Sulfosuccinate, AlCl ₄ ⁻ , Al ₃ Cl ₇ ⁻ , NO ₃ ⁻ , BF ₄ ⁻ , PF ₆ ⁻ , CH ₃ COO ⁻ , CF ₃ COO ⁻ , CF ₃ SO ₃ ⁻ , (CF ₃ SO ₂ ) ₂ N ⁻ , (CF ₃ SO ₂ ) ₃ C ⁻ , AsF ₆ ⁻ , SbF ₆ ⁻ , F (HF) _n ⁻ , CF ₃ CF ₂ CF ₂ CF ₂ SO ₃ ⁻ , (CF ₃ CF ₂ SO ₂ ) ₂ N ⁻ , or CF ₃ CF ₂ CF ₂ CO ^- it includes the at least one or more. The double junction type as shown in FIG. 5 is a desirable configuration for the fourth electrode 104 from the viewpoint of uniform electrolyte concentration. Moreover, although the double junction type which isolate | separates twice was demonstrated, the structure more than double may be sufficient.

  The container 170 of the double junction type reference electrode is preferably made of a material that does not hinder the voltage measurement of the reference electrode 104 and has a good sealing property. Specifically, glass, an insulating sheet, etc. are mentioned. Insulating sheets are desirable because they are compact and hard to break. When glass is used for the container 170, for example, the connection part 150 is a cavity, and the periphery of the cavity in the connection part 150 is made of glass. Various materials are packed into the cavity of the connection portion 150. From the viewpoint of joining by rubbing, it is desirable to use Vycor glass for the glass of the container 170. When an insulating sheet is used for the container 170, for example, the insulating sheet can be a metal film whose surface is insulated like a sealing material of a laminate cell. The metal film and the connection part 150 are welded by pressurization and heating using an adhesive. For example, a urethane resin adhesive is used as the adhesive. In the case where a metal film is used for the container 170, the manufacturing process is simplified by using an ion conductive gel for the connection portion 150.

  The principle of measuring the lithium ion concentration in the electrolytic solution of the present invention will be described. A case where lithium titanate is used for the third electrode 103 and the fourth electrode 104 will be described as an example.

  In lithium titanate, a charge / discharge reaction as shown in the following formula proceeds. (Formula 1) indicates a charging reaction, and (Formula 2) indicates a discharge reaction.

Li ₇ Ti ₅ O ₁₂ → Li ₄ Ti ₅ O ₁₂ + 3Li ⁺ + 3e ⁻ (Formula 1)
Li ₄ Ti ₅ O ₁₂ + 3Li ⁺ + 3e ⁻ → Li ₇ Ti ₅ O ₁₂ (Formula 2)
The potential of the lithium ion insertion reaction into lithium titanate is represented by the reduction potential of (Formula 2) calculated from the Nernst formula of (Formula 3).

E = E ⁰ + RT / 3F · ln (a (Li ₄ Ti ₅ O ₁₂ ) · a (Li ⁺ ) ³ / a (Li ₇ Ti ₅ O ₁₂ )) (Formula 3)
Here, E is the reduction potential, E ⁰ is the standard reduction potential, R is the gas constant, T is the temperature [K], and a is the activity. Since the activity depends on the concentration and the solid activity of Li ₄ Ti ₅ O ₁₂ and Li ₇ Ti ₅ O ₁₂ can be regarded as 1, (Equation 3) can be simplified as (Equation 4).

E = E ⁰ + RT / 3F · ln (a (Li ⁺ ) ³ )
= E ⁰ + RT / F · ln (a (Li ⁺ )) (Formula 4)
From (Equation 4), the reduction potential of the lithium insertion reaction into lithium titanate is proportional to the activity of the lithium ion concentration. That is, it is considered that it depends on the lithium ion concentration. Therefore, a calibration curve can be created by examining the lithium ion concentration and the potential difference using two lithium titanate electrodes having different lithium ion concentrations.

  For example, when a non-aqueous solution in which lithium hexafluorophosphate is dissolved as a lithium salt in a mixed solvent of ethylene carbonate and diethyl carbonate having a volume ratio of 1: 1 is used as the electrolyte, FIG. 6 shows a prediction of the potential difference detected when the other lithium titanate is inserted into the electrolyte solution having a different concentration and inserted into the electrolyte solution concentration of 1. In FIG. 6, it is considered that the potential difference is detected depending on the electrolyte concentration according to the Nernst equation of (Equation 4). It is considered that the calibration curve as shown in FIG. 6 differs depending on the reference electrode and the electrolyte material to be used. Although FIG. 6 assumes a 25 ° C. environment, it is possible to cope with various environmental temperatures by substituting the environmental temperature into T in (Expression 4). As an example, the case where lithium titanate, which is the same material, is used for the third electrode 103 and the fourth electrode 104, but even when different materials are used, by obtaining a calibration curve, the electrolyte concentration Can be calculated.

  FIG. 7 is a schematic diagram in which the lithium ion battery of the present invention is simplified and wiring is added. By outputting the potential difference between V1 of the positive electrode terminal 121 and V4 of the fourth electrode 104 to the outside, the potential of the positive electrode 101 can be detected. Further, the potential of the negative electrode 102 can be detected by outputting the potential difference between V2 of the negative electrode terminal 122 and V4 of the fourth electrode 104 to the outside. Then, the potential difference between V3 of the third electrode 103 and V4 of the fourth electrode 104 is output to the outside, and the concentration difference is calculated from the potential difference using a calibration curve as shown in FIG. It is possible to measure the electrolyte concentration. When detecting the potential of the positive electrode 101 and the negative electrode 102, the potential difference between V3, V1, and V2 of the third electrode 103 may be output to the outside, but the electrolyte around the third electrode 103 inside the electrode group 100 The concentration varies not only from a long-term point of view, but also changes temporarily because the charge / discharge reaction rate differs between the positive electrode side and the negative electrode side. For this reason, when the third electrode 103 is used as a reference electrode, the reference potential is unstable. Therefore, the fourth electrode 104 is preferably used to detect the potentials of the positive electrode 101 and the negative electrode 102.

  By using the above configuration, in addition to detection of the positive electrode potential and the negative electrode potential, it is possible to appropriately measure the lithium ion concentration in the electrolytic solution. In addition, by adopting the configuration of this example, the concentration of lithium ions in the electrolyte can be measured nondestructively.

DESCRIPTION OF SYMBOLS 100 Electrode group 101 Positive electrode 102 Negative electrode 103 3rd electrode 104 4th electrode 105 Separator 110 Insulation sheet 121 Positive electrode terminal 122 Negative electrode terminal 123 3rd electrode terminal 124 4th electrode terminal 130 Lithium ion secondary battery 140 Winding axis 150 Connection site 160 Battery case 170 container

Claims (7)

An electrode group, a third electrode and a fourth electrode;
The electrode group has a positive electrode and a negative electrode,
The potentials of the positive electrode and the negative electrode are detected by the third electrode and the fourth electrode,
The third electrode is disposed inside the electrode group,
The fourth electrode is a lithium ion secondary battery disposed outside the electrode group. In claim 1,
The third electrode and the fourth electrode are lithium ion secondary batteries composed of at least one of lithium transition metal lithium and lithium titanate. In any one of Claims 1 thru | or 2.
The lithium ion secondary battery is a stacked type,
The third electrode is a lithium ion secondary battery disposed in a central portion in a direction in which the positive electrode and the negative electrode are stacked in the electrode group. In any one of Claims 1 thru | or 3,
The lithium ion secondary battery is a stacked type,
The third electrode is a lithium ion secondary battery disposed at an edge portion in a direction that becomes a vertical surface in a direction in which the positive electrode and the negative electrode are stacked in the electrode group. In any one of Claims 1 thru | or 4,
The fourth electrode is a lithium ion secondary battery disposed in a central portion in a direction that is a vertical surface in a direction in which the positive electrode and the negative electrode are stacked in the electrode group. In claim 1,
The lithium ion secondary battery is a wound type,
The lithium ion secondary battery has a winding axis,
The electrode group is wound around the winding shaft,
The winding shaft is provided with a cavity,
The fourth electrode is a lithium ion secondary battery disposed in a cavity of the winding shaft. In any one of Claims 1 thru | or 6.
The lithium ion secondary battery has a connection site and a container,
The fourth electrode is disposed in a region formed by the connection site and the container;
The connecting portion is in contact with the container;
A lithium ion secondary battery in which the fourth electrode and the electrode group have ionic conductivity depending on the connection site.