Bipolar electrode and power storage device

Abstract

To provide a bipolar electrode and a power storage device, in which both increase in resistance and deterioration in durability are suppressed.SOLUTION: There is provided a bipolar electrode in which a cathode active material layer, an electrode current collector, and an anode active material layer are provided in this order. The cathode active material layer includes a first cathode active material and a second cathode active material having a layered crystal structure, the first positive electrode active material has a cation mixing ratio of 5.0% or less and an excess lithium ratio of less than 1.0% by mass, and the second positive electrode active material has a cation mixing ratio of 3.3% or less and an excess lithium ratio of less than 0.8% by mass.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to bipolar electrodes and energy storage devices.

Background Art

[0002] Japanese Patent Application Laid-Open No. 2023-91566 (Patent Document 1) discloses using two types of positive electrode active materials with different average particle diameters for the purpose of increasing the volume capacity density of the positive electrode of a non-aqueous electrolyte secondary battery.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In Patent Document 1, although the volume capacity density of the positive electrode of the non-aqueous electrolyte secondary battery is improved, there is room for improvement from the viewpoint of resistance.

[0005] Here, a bipolar electrode (bipolar battery) having a positive electrode active material layer on one surface of a current collector and a negative electrode active material layer on the other surface is known. Bipolar batteries have attracted attention from the viewpoint of improving energy density compared to conventional non-aqueous electrolyte secondary batteries.

[0006] However, since the bipolar electrode presses the positive electrode and the negative electrode simultaneously, the positive electrode is not sufficiently pressed, and the contact of the positive electrode active material deteriorates. Further, in a bipolar battery, current flows in the thickness direction of the electrode, but if the contact between the positive electrode active materials or between the positive electrode active material and the conductive material is insufficient, the current flow path becomes complicated or interrupted, resulting in problems such as an increase in resistance and deterioration of durability.

[0007] An object of the present disclosure is to provide a bipolar electrode and a power storage device in which suppression of an increase in resistance and suppression of deterioration of durability are compatible.

Means for Solving the Problems

[0008] [1] A bipolar electrode including a positive electrode active material layer, an electrode current collector, and a negative electrode active material layer in this order, The positive electrode active material layer includes a first positive electrode active material and a second positive electrode active material having a layered crystal structure, The first positive electrode active material has a cation mixing ratio of 5.0% or less and an excess lithium ratio of less than 1.0% by mass, The second positive electrode active material has a cation mixing ratio of 3.3% or less and an excess lithium ratio of less than 0.8% by mass, the bipolar electrode.

[0009] The positive electrode active material of the present technology includes a first positive electrode active material and a second positive electrode active material. The first positive electrode active material and the second positive electrode active material have a cation mixing ratio and an excess lithium ratio within a specific range. By mixing these positive electrode active materials, it is expected to achieve both suppression of an increase in resistance and suppression of deterioration of durability.

[0010] [2] The first positive electrode active material is polycrystalline particles, The second positive electrode active material is single crystal particles, the bipolar electrode according to [1].

[0011] [3] The first positive electrode active material and the second positive electrode active material are included in a mass ratio of 30:70 to 90:10, the bipolar electrode according to [1] or [2].

[0012] [4] A power storage device including the bipolar electrode according to any one of [1] to [3].

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Mode for Carrying Out the Invention

[0014] Hereinafter, embodiments of the present disclosure (hereinafter may be abbreviated as "the present embodiment"), and examples of the present disclosure (hereinafter may be abbreviated as "the present examples") will be described. However, the present embodiment and the present examples do not limit the technical scope of the present disclosure.

[0015] In this specification, when a compound is represented by a stoichiometric composition formula such as "LiCoO2", the stoichiometric composition formula is merely a representative example. For example, when lithium cobaltate is represented as "LiCoO2", unless otherwise specified, lithium cobaltate is not limited to a composition ratio of "Li / Co / O = 1 / 1 / 2", and may contain Li, Co, and O in any composition ratio. The composition ratio may be non-stoichiometric.

[0016] "D50" in this specification indicates the particle diameter at which the integration becomes 50% in the volume-based particle size distribution (cumulative distribution). The particle size distribution can be measured by the laser diffraction method.

[0017] In this specification, "at least one of the first positive electrode active material and the second positive electrode active material" may be collectively referred to as "the positive electrode active material".

[0018] <Bipolar electrode> The bipolar electrode of this embodiment is used as an electrode of an energy storage device. The energy storage device is, for example, a non-aqueous electrolyte secondary battery such as a lithium-ion secondary battery. In this embodiment, the bipolar electrode used in a lithium-ion secondary battery will be described.

[0019] FIG. 1 is a schematic diagram showing an example of the bipolar electrode of this embodiment. The bipolar electrode 20 includes a positive electrode active material layer 21, an electrode current collector 10, and a negative electrode active material layer 22 in this order. That is, the positive electrode active material layer 21 is adhered to one main surface of the electrode current collector 10, and the negative electrode active material layer 22 is adhered to the other main surface, respectively.

[0020] 《Positive Electrode Active Material Layer》 FIG. 2 is a schematic diagram showing an example of the positive electrode active material layer of this embodiment. The positive electrode active material layer 21 contains a positive electrode active material. The positive electrode active material includes a first positive electrode active material 1 and a second positive electrode active material 2. The first positive electrode active material 1 and the second positive electrode active material 2 have a layered crystal structure. The crystal structure of the positive electrode active material can be identified, for example, by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

[0021] (Positive Electrode Active Material) The positive electrode active material of this embodiment can reversibly intercalate and release lithium ions. The positive electrode active material is a positive electrode active material containing a lithium metal-containing composite oxide. The positive electrode active material of this embodiment may be a positive electrode active material composed of a lithium metal-containing composite oxide. As the lithium metal-containing composite oxide, for example, at least one selected from the group consisting of LiCoO2, LiNiO2, LiMnO2, Li(NiCoMn)O2, and Li(NiCoAl)O2 may be used. Among them, Li(NiCoMn)O2 is preferable because of its particularly excellent resistance characteristics. Such a lithium metal-containing composite oxide preferably has a composition represented by the following formula (i). Note that the first positive electrode active material 1 and the second positive electrode active material 2 may have the same composition or different compositions. Also, the chemical composition of the positive electrode active material can be measured by, for example, ICP-AES or the like. Li z Ni 1-x-y Co x Mn y O2(i) In the above formula (1), x, y, and z satisfy the relationships of 0.1 ≦ x ≦ 0.4, 0.1 ≦ y ≦ 0.5, and 0.95 ≦ z ≦ 1.2.

[0022] (First positive electrode active material) The first positive electrode active material 1 is polycrystalline particles. In this embodiment, "polycrystalline particles" means that in a SEM image (scanning electron microscope) at 5000 times magnification, particles (secondary particles) formed by aggregation of 11 or more primary particles are contained in an amount of 90% or more. That is, in this embodiment, the proportion of polycrystalline particles contained in the first positive electrode active material 1 is 90% or more. The polycrystalline particles may be formed, for example, by aggregation of 50 or more primary particles. In the polycrystalline particles, there is no upper limit to the number of primary particles. The polycrystalline particles may be formed, for example, by aggregation of 10,000 or fewer primary particles, or may be formed by aggregation of 1,000 or fewer primary particles.

[0023] The primary particles can have any shape. The primary particles may be, for example, spherical, columnar, massive, or the like.

[0024] The polycrystalline particles can have any shape. The polycrystalline particles may be, for example, spherical, columnar, massive, etc. The polycrystalline particles may have a D50 larger than that of the single crystal particles described later, for example. Thereby, for example, reduction of resistivity is expected. The D50 of the polycrystalline particles may be, for example, 5 to 20 μm, or may be 15 to 19 μm.

[0025] The crystallite size of the first positive electrode active material 1 may be 300 Å or more and 1100 Å or less. In the present embodiment, "crystallite" refers to a region (aggregate) that can be regarded as a single crystal in the crystal structure within one particle constituting the positive electrode active material, and "crystallite size" refers to the size of the crystallite. The crystallite size of the first positive electrode active material 1 is preferably 500 Å or more and 1000 Å or less.

[0026] In the present embodiment, for the crystallite size of the positive electrode active material, for example, a value calculated by Scherrer's formula based on an X-ray Diffraction (XRD) pattern can be adopted. The crystallite size of the positive electrode active material can be obtained, for example, by calculating the half-value width of the (003) plane found in the range of 2θ = 19.1 to 20.1 in the spectrum obtained by XRD measurement using Scherrer's formula.

[0027] The first positive electrode active material 1 has a cation mixing ratio of 5.0% or less. In the present embodiment, "cation mixing" refers to a state in which lithium ions and transition metal cations are substituted for each other in the crystal structure of the lithium metal-containing composite oxide, or a state in which the lithium ion sites are occupied by transition metal cations.

[0028] When the proportion of cation mixing is high, the cations of transition metals substituted for lithium ions cause deterioration of the crystal structure stability of the lithium metal-containing composite oxide, such as hindering the passage of lithium ions, resulting in an increase in resistance and deterioration of durability. In the present embodiment, when the proportion of cation mixing in the first positive electrode active material 1 is 5.0% or less, improvement in resistance increase and durability deterioration is expected. The proportion of cation mixing in the first positive electrode active material 1 is preferably 4.0% or less. The proportion of cation mixing in the first positive electrode active material 1 may be, for example, 0.5% or more, or may be 1.0% or more.

[0029] The proportion of cation mixing in the present embodiment is calculated, for example, by performing Rietveld analysis. Rietveld analysis is a method of refining various parameters (such as lattice constants) of the crystal structure so that the diffraction intensity calculated assuming a crystal structure model matches the XRD pattern (diffraction intensity) actually measured by powder XRD or the like. The diffraction peak can be obtained by performing powder XRD measurement using an XRD apparatus.

[0030] The proportion of excess lithium in the first positive electrode active material 1 is less than 1.0% by mass. In the present embodiment, "excess lithium" means lithium other than the lithium metal-containing composite oxide that is the positive electrode active material and exists on the surface of the positive electrode active material. Specifically, the positive electrode active material contained in the positive electrode active material layer 21 is generated by reacting a lithium compound (precursor) such as lithium carbonate or lithium hydroxide with other materials as described later. When such a positive electrode active material is generated, in order to appropriately react the precursor with other materials, the precursor may be input in excess, and the unreacted precursor may be contained in the generated positive electrode active material. Such a lithium compound contained in an unreacted state is referred to as "excess lithium".

[0031] When the proportion of excess lithium is high, it is considered that the excess lithium itself becomes a resistance layer and the excess lithium decomposes, resulting in an increase in resistance and deterioration of durability. In the present embodiment, when the proportion of excess lithium in the first positive electrode active material 1 is less than 1.0% by mass, improvement in increase in resistance and deterioration of durability is expected. The proportion of excess lithium in the first positive electrode active material 1 is preferably 0.9% or less.

[0032] In the present embodiment, the proportion of excess lithium can be measured, for example, by adding a predetermined amount of the positive electrode active material to pure water, stirring for a certain period of time, and then neutralizing and titrating the mass of lithium eluted in the pure water with hydrochloric acid.

[0033] (Second positive electrode active material) The second positive electrode active material 2 is composed of single crystal particles. In the present embodiment, the "single crystal particles" mean that in an SEM image at 5000 times magnification, the total of particles existing alone and aggregates formed by aggregation of 2 or more and 10 or less particles is 80% or more. That is, in the present embodiment, the proportion of single crystal particles contained in the second positive electrode active material 2 is 80% or more.

[0034] The single crystal particles in the present embodiment are particles in which grain boundaries cannot be confirmed visually in a cross-sectional SEM image. The single crystal particles can have any shape. The single crystal particles may be, for example, spherical, columnar, massive, or the like.

[0035] The crystallite size of the second positive electrode active material 2 may be 800 Å or more and 1500 Å or less. In the present embodiment, the crystallite size of the second positive electrode active material 2 is preferably larger than the crystallite size of the first positive electrode active material 1. The crystallite size of the second positive electrode active material 2 is preferably 1000 Å or more and 1400 Å or less.

[0036] The second positive electrode active material 2 has a cation mixing ratio of 3.3% or less. When the cation mixing ratio of the second positive electrode active material 2 is 3.3% or less, improvement in the increase of resistance and the deterioration of durability is expected. The cation mixing ratio of the second positive electrode active material 2 is preferably 2.9% or less. The cation mixing ratio of the second positive electrode active material 2 may be, for example, 0.2% or more, and may also be 0.5% or more.

[0037] The second positive electrode active material 2 has a surplus lithium ratio of less than 0.8% by mass. When the surplus lithium ratio of the second positive electrode active material 2 is less than 0.8% by mass, improvement in the increase of resistance and the deterioration of durability is expected. The surplus lithium ratio of the first positive electrode active material 1 is preferably 0.6% or less.

[0038] (Content ratio) The content ratio of the first positive electrode active material 1 and the second positive electrode active material 2 is, in terms of their mass ratio (first positive electrode active material 1: second positive electrode active material 2), for example, 30:70 to 90:10. When the mass ratio of the first positive electrode active material 1 and the second positive electrode active material 2 is within the above range, suppression of the increase in resistance and suppression of the deterioration of durability are expected. The mass ratio of the first positive electrode active material 1 and the second positive electrode active material 2 may also be 40:60 to 80:20.

[0039] (Content rate) The content of the positive electrode active material in the positive electrode active material layer 21, that is, the total content of the first positive electrode active material 1 and the second positive electrode active material 2 with respect to the total mass of the positive electrode active material layer 21, may be, for example, 50% by mass or more, or may be 80% by mass or more, or may be 90% by mass or more, or may be 95% by mass or more, or may be substantially 100% by mass.

[0040] (Other components) The positive electrode active material layer 21 may further contain, for example, a conductive material, a binder, etc. The conductive material may contain, for example, acetylene black (AB), etc. The binder may contain, for example, PVDF, etc. The conductive material and the binder may be, for example, 0.1% by mass or more and 10% by mass or less with respect to the positive electrode active material layer 21.

[0041] (Preparation of the positive electrode active material) The preparation of the positive electrode active material is carried out, for example, according to the following procedure. Sulfates such as Ni are prepared. An acidic aqueous solution is formed by dissolving the sulfate in water. For example, an acidic aqueous solution can be formed by dissolving nickel sulfate, cobalt sulfate, and manganese sulfate in water.

[0042] A reaction vessel is prepared. An ammonium ion donor such as aqueous ammonia is added to the reaction vessel and stirred. While stirring, a pH adjuster such as sodium hydroxide is added to the reaction vessel to prepare an alkaline aqueous solution. While controlling the pH of the reaction solution to be constant, the acidic aqueous solution is dropped into the alkaline aqueous solution. Thereby, a precipitate can be formed. The precipitate is considered to contain a composite hydroxide (precursor). The precipitate is washed, filtered, and dried to form a dried product.

[0043] A mixture is formed by mixing the dried product and a lithium compound. The lithium compound may contain, for example, lithium carbonate, lithium hydroxide, etc. The mixture is heat-treated (fired) to synthesize a lithium metal-containing composite oxide powder. The synthesized lithium metal-containing composite oxide powder is pulverized to prepare the positive electrode active material.

[0044] In this embodiment, by appropriately adjusting the firing temperature, firing time, etc., a positive electrode active material having a desired ratio of cation mixing and a ratio of excess lithium can be prepared.

[0045] 《Current collector for electrode》 The current collector 10 for the electrode may contain at least one selected from the group consisting of, for example, aluminum (Al), stainless steel, nickel (Ni), chromium (Cr), platinum (Pt), niobium (Nb), iron (Fe), titanium (Ti), copper (Cu), and zinc (Zn). Further, the current collector 10 for the electrode may be obtained by plating the surface of a metal foil.

[0046] 《Negative electrode active material layer》 The negative electrode active material layer 22 contains a negative electrode active material. The negative electrode active material may include, for example, at least one selected from the group consisting of graphite, soft carbon, and hard carbon. The negative electrode active material layer may further contain, for example, a binder, a thickener, and the like. Examples of the binder include styrene butadiene rubber (SBR) and the like. Examples of the thickener include carboxymethyl cellulose (CMC) and the like.

[0047] <Power storage device> FIG. 3 is a schematic diagram showing an example of the power storage device of the present embodiment. In the present embodiment, the case where the power storage device 100 is a lithium ion secondary battery (hereinafter may be abbreviated as "battery") will be described.

[0048] The battery 100 may include an exterior body (not shown). The exterior body may house the power generation element 50 and an electrolytic solution (not shown). The exterior body may have any form. The exterior body may be, for example, a metal case, or may be a pouch made of a metal foil laminate film or the like. The exterior body may contain, for example, Al or the like.

[0049] The battery 100 includes a power generation element 50. The power generation element 50 may also be referred to as an electrode body or an electrode group. The power generation element 50 includes a bipolar electrode 20 and a separator 30. The power generation element 50 may be formed by alternately laminating the bipolar electrode 20 and the separator 30.

[0050] <Separator> The separator 30 is porous. The separator 30 can permeate the electrolytic solution. The separator 30 separates the positive electrode active material layer 21 and the negative electrode active material layer 22. The separator 30 is electrically insulating. The separator 30 may contain, for example, a polyolefin resin such as polyethylene (PE) or polypropylene (PP). The separator 30 may have, for example, a single-layer structure or a multilayer structure. The separator 30 may consist essentially of a PE layer, or may be formed by laminating a PP layer, a PE layer, and a PP layer in this order.

[0051] "Electrolyte" The electrolyte contains a solvent and a Li salt. The solvent is aprotic. The solvent may contain any component. The solvent may contain, for example, at least one selected from the group consisting of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC).

[0052] The Li salt is a supporting electrolyte. The Li salt is dissolved in the solvent. The Li salt may contain, for example, at least one selected from the group consisting of LiPF6 and LiBF4. The Li salt may have, for example, a molar concentration of 0.5 mol / L or more and 2.0 mol / L or less.

[0053] The electrolyte may further contain any additive. The electrolyte may contain, for example, 0.01% by mass or more and 5% by mass or less of the additive. The additive may contain, for example, at least one selected from the group consisting of vinylene carbonate (VC) and vinyl ethylene carbonate (VEC).

Example

[0054] <Manufacture of Cathode Active Material> 《No.A》 An acidic aqueous solution was obtained by dissolving nickel sulfate, cobalt sulfate, and manganese sulfate in ion-exchanged water. The molar ratio of Ni, Co, and Mn in the acidic aqueous solution was 8:1:1, and the concentration of the acidic aqueous solution was 30% by mass.

[0055] Ammonia water was supplied to the reaction vessel and stirred with a stirrer. Next, an aqueous sodium hydroxide solution was supplied to the reaction vessel to prepare an alkaline aqueous solution. While the alkaline aqueous solution in the reaction vessel was stirred by the stirrer, an acidic aqueous solution was dropped into the alkaline aqueous solution. During the dropping of the acidic aqueous solution, ammonia water and the aqueous sodium hydroxide solution were appropriately added so that the ammonia concentration and pH of the reaction solution became constant. The precipitate after the reaction was washed with water and filtered to obtain a composite hydroxide. The obtained composite hydroxide was dried at 120 °C for 16 hours to obtain a dried product. In the above reaction, the atmosphere of the reaction vessel, the stirring speed, the pH of the reaction solution, the mixing volume ratio of ammonia water to the acidic solution (NH3 / acidic solution), the dropping speed and the dropping time of the acidic solution are as shown in Figure 4.

[0056] The dried product and lithium carbonate were mixed in a mortar to obtain a mixture. The mixture was fired in a muffle furnace under an oxygen atmosphere to obtain a lithium metal-containing composite oxide powder. The lithium metal-containing composite oxide powder was pulverized using a jet mill to obtain a positive electrode active material of No. A. The mixing molar ratio of Li to Ni, Co and Mn in the dried product (Li / Ni+Co+Mn), the firing temperature and the firing time are as shown in Figure 4.

[0057] 《No.B~C, No.a~c》 Except for the points where the manufacturing conditions were changed as shown in Figure 4, the positive electrode active materials of No. B~C and No. a~c were obtained by the same process as No. A. It was confirmed by SEM images (magnification: 5000 times) that each positive electrode active material of No. A~C is a first positive electrode active material (polycrystalline particles), and each positive electrode active material of No. a~c is a second positive electrode active material (single crystal particles).

[0058] 《Composition》 The composition of the positive electrode active material of each No. was confirmed by an ICP emission spectroscopic analyzer (manufactured by Hitachi High-Technologies Corporation, PS3520UVDD). The results are shown in Figure 5. Also, as a result of confirming the crystal structure of each No., it was confirmed that all the positive electrode active materials have a layered crystal structure.

[0059] "Cation Mixing" Using an XRD instrument ((manufactured by Rigaku Corporation, Smartlab)), X-rays were irradiated onto the cathode active materials of each No. under the following measurement conditions to obtain an XRD pattern. From the obtained XRD pattern, crystal peak data was subjected to Rietveld analysis to calculate the ratio of cation mixing in the cathode active materials of each No. The results are shown in Figure 5. [Measurement Conditions] X-ray output: 45 kV, 200 mA X-ray source: CuKα ray (wavelength: 1.54051 Å), single crystal monochromator Diffraction angle: 10 - 120° Measurement temperature: room temperature (25 °C) Scanning speed: 1 second / step

[0060] "Excess Lithium" 1 g of each No. of the cathode active material was added to pure water and stirred for 30 minutes. Then, the mass of lithium eluted into the pure water was neutralized and titrated with hydrochloric acid to calculate the ratio of excess lithium. The results are shown in Figure 5.

[0061] "Crystallite Size" Among the spectra obtained under the above measurement conditions, the crystallite size of the cathode active materials of each No. was calculated by calculating the half-width of the (003) plane observed in the range of 2θ = 19.1 - 20.1 using Scherrer's equation. The results are shown in Figure 5.

[0062] <Manufacture of Lithium Ion Secondary Battery> As materials for the cathode active material layer, each of the cathode active materials of No. A - C and No. a - c, AB (manufactured by Denka Co., Ltd.) as the conductive material, and PVdF (manufactured by Kuraray Co., Ltd.) as the binder were prepared. Using the above materials, a cathode active material layer was fabricated. The mass ratio of each material in the cathode active material layer is cathode active material (first cathode active material and second cathode active material): conductive material: binder = 90:5:5. Also, the mass ratio of the first cathode active material and the second cathode active material is shown in Figure 6.

[0063] As materials for the negative electrode active material layer, natural graphite (Hitachi Chemical Co., Ltd.) as the negative electrode active material, SBR (JSR Corporation) as the binder, and CMC (Nippon Paper Industries Co., Ltd.) as the thickener were prepared. Using the above materials, a negative electrode active material layer was fabricated. The mass ratio of each material in the negative electrode active material layer is negative electrode active material: binder: thickener = 95:2.5:2.5.

[0064] Al foil (thickness: 15 μm) was prepared as the electrode current collector. A bipolar electrode was fabricated by laminating the positive electrode active material layer, the electrode current collector, and the negative electrode active material layer in this order.

[0065] As the separator, a porous resin (PP / PE / PP) (thickness: 24 μm) with PP layers laminated on both sides of the PE layer was prepared. The bipolar electrode, the separator, and the bipolar electrode were laminated so that the separator separated the positive electrode active material layer and the negative electrode active material layer. Thereby, an electrode body was formed.

[0066] As the exterior body, a pouch made of a laminate film was prepared. The electrode body was housed in the exterior body. As the electrolytic solution, a mixture obtained by dissolving a supporting salt (LiPF6) at a concentration of 1 mol / L in a mixed solvent containing EC, DMC, and EMC was prepared. The electrolytic solution was injected into the exterior body. After injecting the electrolytic solution, the exterior body was sealed. Thus, test cells for evaluation Nos. 1 to 7 were manufactured. The first positive electrode active material and the second positive electrode active material used in each No. are shown in FIG. 6.

[0067] <Evaluation> 《Normalized IV Resistance》 Under a temperature environment of 25°C, the SOC (State of Charge) of the test battery was adjusted to 50% by constant current - constant voltage (CC - CV) charging. The current during constant current (CC) charging was 1It. "1It" is defined as the current that discharges the rated capacity of the battery in 1 hour. At 50% SOC, the voltage of the battery was 3.7V. After the adjustment of SOC, with a 30 - minute pause in between, the battery was discharged for 10 seconds at a current of 10It. The initial discharge resistance (normalized IV resistance) was obtained by the following formula (ii). The results are shown in Figure 6. The initial resistance in Figure 6 is a relative value. The initial resistance of No.3 is defined as 1.

[0068] r=(V0 - V 10 ) / current (ii) In the above formula (ii), r represents the discharge resistance. V0 represents the voltage at the start of discharge. V 10 represents the voltage 10 seconds after the start of discharge.

[0069] 《Rate of increase in resistance》 After the measurement of the initial resistance, under a temperature environment of 25°C, the SOC of the battery was adjusted to 80% by CC - CV charging. The current during CC charging was 1It. After the adjustment of SOC, a cycle test was carried out under a temperature environment of 25°C. That is, the following discharge and charge were alternately repeated over 15 days.

[0070] Discharge: Current = 1It, Discharge capacity = Capacity corresponding to 20% SOC Charge: Current = 1It, Charge capacity = Capacity corresponding to 20% SOC

[0071] After the cycle test, similar to the initial resistance, the discharge resistance after the cycle (post - cycle resistance) was measured. By dividing the post - cycle resistance by the initial resistance, the rate of increase in resistance (percentage) was obtained. The results are shown in Figure 6. The rate of increase in resistance in Figure 6 is a relative value. The rate of increase in resistance of No.3 is defined as 1.

[0072] <Results> As shown in Fig. 6, in Nos. 4 to 7, it can be seen that the normalized IV resistance and the resistance increase rate are lower compared to Nos. 1 to 3. That is, in Nos. 4 to 7, it can be seen that both the suppression of the increase in resistance and the suppression of the deterioration of durability are achieved.

[0073] This embodiment and these examples are illustrative in all respects. This embodiment and these examples are not restrictive. The technical scope of the present disclosure includes all modifications within the meaning and scope equivalent to the description of the claims. For example, any configurations are extracted from this embodiment and these examples, and their arbitrary combinations are also contemplated from the beginning.

Description of Reference Numerals

[0074] 1 First positive electrode active material, 2 Second positive electrode active material, 10 Electrode current collector, 20 Bipolar electrode, 21 Positive electrode active material layer, 22 Negative electrode active material layer, 30 Separator, 50 Power generation element, 100 Power storage device (lithium ion secondary battery).

Claims

1. A bipolar electrode comprising a positive electrode active material layer, an electrode current collector, and a negative electrode active material layer in this order, wherein the positive electrode active material layer contains a first positive electrode active material and a second positive electrode active material having a layered crystal structure, the first positive electrode active material has a cation mixing ratio of 5.0% or less and an excess lithium ratio of less than 1.0% by mass, and the second positive electrode active material has a cation mixing ratio of 3.3% or less and an excess lithium ratio of less than 0.8% by mass. The bipolar electrode.

2. The first positive electrode active material is polycrystalline particles, The second positive electrode active material is single crystal particles. The bipolar electrode according to claim 1.

3. The bipolar electrode according to claim 1, wherein the first positive electrode active material and the second positive electrode active material are contained in a mass ratio of 30:70 to 90:

10.

4. A power storage device comprising the bipolar electrode according to any one of claims 1 to 3.

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