Positive electrode and power storage device

Abstract

This invention provides a positive electrode having a positive electrode current collector and a positive electrode composite material layer. The aforementioned positive electrode composite material layer comprises a positive electrode active material and dielectric particles, and the peak pore diameter of the positive electrode is less than or equal to the median particle size of the aforementioned dielectric particles. Additionally, this invention provides an energy storage device having the aforementioned positive electrode, negative electrode, and electrolyte.

Description

Technical Field

[0001] This invention relates to a positive electrode and an energy storage device. Background Technology

[0002] Previously, lithium-ion rechargeable batteries, as energy storage devices with high energy density, have been widely used. A lithium-ion rechargeable battery, for example, has a structure in which a separator exists between the positive and negative electrodes and is filled with an electrolyte. Furthermore, the positive electrode, for example, has a structure in which a positive electrode composite material layer is formed on the positive electrode current collector.

[0003] Patent Document 1 describes the following: a dispersion of dielectric particles with a relative permittivity of 500 or more and a particle size of 200 nm or less is impregnated into a positive electrode current collector having a positive electrode composite material layer formed thereon, thereby disposing the dielectric particles inside the positive electrode composite material layer.

[0004] [Previous Technical Documents]

[0005] (Patent Documents)

[0006] Patent Document 1: Japanese Patent Application Publication No. 2016-119180 Summary of the Invention

[0007] [The problem the invention aims to solve]

[0008] However, dielectric particles with a diameter of less than 200 nm tend to aggregate. Therefore, if a large number of such dielectric particles are used, the proportion of electrolyte in contact with the dielectric particles due to aggregation will decrease. Consequently, the following problems arise: even if the electric field generated inside the lithium-ion battery acts on the dielectric particles, the dielectric polarization of the dielectric particles is insufficient to increase the dissociation of the supporting salts in the electrolyte, thus increasing the cell resistance. Furthermore, because the dielectric particles' ability to capture trace amounts of acid in the electrolyte and their stabilizing effect through interaction with the electrolyte is insufficient, corrosion of the positive electrode active material and decomposition of the electrolyte cannot be suppressed, resulting in decreased durability of the lithium-ion battery.

[0009] The purpose of this invention is to provide a positive electrode that can reduce the individual resistance of an energy storage device and improve its durability.

[0010] [Technical means to solve the problem]

[0011] One embodiment of the present invention is a positive electrode having a positive electrode current collector and a positive electrode composite material layer, wherein the aforementioned positive electrode composite material layer comprises a positive electrode active material and dielectric particles, and the peak pore diameter of the positive electrode is less than or equal to the median particle size of the aforementioned dielectric particles.

[0012] The content of the aforementioned dielectric particles in the aforementioned positive electrode composite material layer can also be 0.1% by mass or more and 2% by mass or less.

[0013] The relative permittivity of the aforementioned dielectric particles can also be 20 or higher.

[0014] The peak pore diameter of the aforementioned positive electrode can also be above 0.1 μm and below 0.6 μm.

[0015] The density of the aforementioned positive electrode can also be above 3.4 g / cc.

[0016] The aforementioned positive electrode active material can also have a bimodal particle size distribution.

[0017] Another aspect of the present invention is an energy storage device having the above-mentioned positive electrode, negative electrode and electrolyte.

[0018] (The effect of the invention)

[0019] According to the present invention, a positive electrode can be provided that can reduce the individual resistance of the energy storage device and improve its durability. Attached Figure Description

[0020] Figure 1 It is a graph showing the measurement results of the pore diameter distribution of the positive electrodes of Examples 1-5 and Comparative Examples 1-3.

[0021] Figure 2 This is a scanning electron microscope (SEM) image of the cross-section of the positive electrode of Example 1.

[0022] Figure 3 This is a SEM image of the cross-section of the positive electrode of Comparative Example 1. Detailed Implementation

[0023] The embodiments of the present invention will be described below.

[0024] <Positive electrode>

[0025] The positive electrode of this embodiment has a positive electrode current collector and a positive electrode composite material layer, the positive electrode composite material layer comprising a positive electrode active material and dielectric particles. The peak pore diameter of the positive electrode of this embodiment is less than or equal to the median particle size of the dielectric particles.

[0026] Furthermore, in this embodiment, the positive electrode can be formed on one side of the positive electrode current collector or on both sides of the positive electrode current collector.

[0027] In this embodiment, since the peak pore diameter of the positive electrode is less than or equal to the median particle size of the dielectric particles, the number of pores with diameters larger than the median particle size of the dielectric particles in the positive electrode composite layer is reduced, while the proportion of electrolyte in contact with the dielectric particles increases. Therefore, when the electric field generated inside the energy storage device acts on the dielectric particles, the dielectric polarization of the dielectric particles increases the dissociation degree of the supporting salt in the electrolyte, thereby reducing the individual resistance of the energy storage device. In addition, by utilizing the effect of the dielectric particles to capture trace amounts of acid present in the electrolyte and the effect of the dielectric particles stabilizing the electrolyte through interaction with it, the corrosion of the positive electrode active material and the decomposition of the electrolyte can be suppressed, resulting in improved durability of the energy storage device.

[0028] To reduce the individual resistance of the energy storage device and improve its durability, it is preferable to increase the relative permittivity of the dielectric particles.

[0029] In this embodiment, the peak pore diameter of the positive electrode is preferably 0.1 μm or more and 0.6 μm or less, and more preferably 0.3 μm or more and 0.6 μm or less. When the peak pore diameter of the positive electrode in this embodiment is 0.1 μm or more and 0.6 μm or less, the individual resistance of the energy storage device decreases and its durability improves.

[0030] Furthermore, the peak pore diameter of the positive electrode in this embodiment can be controlled by adjusting the median particle size of the positive electrode active material or by pressing it during the fabrication of the positive electrode.

[0031] In this embodiment, the density of the positive electrode is preferably 3.4 g / cc or higher, and more preferably 3.5 g / cc or higher. When the density of the positive electrode in this embodiment is 3.4 g / cc or higher, the energy density of the single cell of the energy storage device is increased. In addition, the effect of the dielectric particles on the electrolyte is improved, resulting in improved durability of the energy storage device.

[0032] [Dielectric particles]

[0033] Dielectric particles can also be oxide particles.

[0034] Examples of oxides that constitute oxide particles include BaTi. 1-x Zr x O3(0≦X≦0.5), SrBi2Ta2O9, (K 1-x Na x )NbO3(0≦X≦1), BiFeO3, CaCu3Ti4O 12 Li 0.33 La 0.55 TiO3, Li 1.3 Al 0.3 Ti 1.7 P3O 12LiNbO3, etc.

[0035] The relative permittivity of the dielectric particles is preferably 20 or higher, and more preferably 30 or higher. When the relative permittivity of the dielectric particles is 20 or higher, the individual resistance of the energy storage device decreases and its durability improves.

[0036] The median particle size of the dielectric particles is preferably 0.3 μm or more and 1.0 μm or less, and more preferably 0.4 μm or more and 0.6 μm or less. When the median particle size of the dielectric particles is 0.3 μm or more and 1.0 μm or less, the dielectric particles are less likely to aggregate, and the contact area with the electrolyte is increased.

[0037] The content of dielectric particles in the positive electrode composite material layer is preferably 0.1% by mass or more and 2% by mass or less, and more preferably 0.5% by mass or more and 1.0% by mass or less. When the content of dielectric particles in the positive electrode composite material layer is 0.1% by mass or more, the individual resistance of the energy storage device decreases and the durability improves; when it is 2% by mass or less, the energy density of the energy storage device increases.

[0038] [Positive electrode composite material layer]

[0039] The positive electrode composite material layer contains positive electrode active material and dielectric particles, and may also contain other components.

[0040] Other components include, for example, solid electrolytes, conductive additives, and binders.

[0041] As a positive electrode active material, there are no particular limitations as long as it can adsorb and release lithium ions. For example, LiCoO2 and Li(Ni) can be listed. 5 / 10 Co 2 / 10 Mn 3 / 10 O2, Li(Ni) 6 / 10 Co 2 / 10 Mn 2 / 10 O2, Li(Ni) 8 / 10 Co 1 / 10 Mn 1 / 10 O2, Li(Ni) 0.8 Co 0.15 Al 0.05 O2, Li(Ni) 1 / 6 Co 4 / 6 Mn 1 / 6 O2, Li(Ni) 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiCoO4, LiMn2O4, LiNiO2, LiFePO4, lithium sulfide, sulfur, etc.

[0042] The positive electrode active material can also have a bimodal particle size distribution. As a result, the filling capacity of the positive electrode active material is improved, and the pore diameter distribution of the positive electrode in this embodiment is reduced.

[0043] Positive electrode active materials with bimodal particle size distributions are obtained, for example, by mixing positive electrode active materials with different peak values ​​in their particle size distributions.

[0044] [Positive Current Collector]

[0045] As a positive current collector, there are no particular limitations; examples include metal foil, etc.

[0046] Examples of metals that make up metal foil include aluminum.

[0047] <Method for manufacturing the positive electrode>

[0048] The manufacturing method of the positive electrode in this embodiment is not particularly limited. Conventional methods in this technical field can be applied. Examples include coating a positive electrode composite material paste containing positive electrode active material and dielectric particles onto a positive electrode current collector and then drying it.

[0049] After forming a positive electrode composite material layer on the positive electrode current collector, conventional methods in this technical field can be applied. For example, the positive electrode current collector with the positive electrode composite material layer formed is pressed to obtain the positive electrode. At this time, the density of the positive electrode can be adjusted by pressing.

[0050] <Electric Storage Devices>

[0051] The energy storage device of this embodiment has a positive electrode, a negative electrode, and an electrolyte as described in this embodiment.

[0052] Examples of energy storage devices include secondary batteries such as lithium-ion batteries and capacitors.

[0053] There are no particular limitations on the negative electrode; any known negative electrode that can be used in energy storage devices can be used.

[0054] There are no particular limitations on the electrolyte; any known electrolyte that can be used in energy storage devices can be used.

[0055] [Lithium-ion rechargeable battery]

[0056] The lithium-ion secondary battery of this embodiment includes a positive electrode, a negative electrode, an electrolyte, and a separator located between the positive electrode and the negative electrode.

[0057] In the lithium-ion secondary battery of this embodiment, two materials can be selected from those that can form electrodes. The charge and discharge potentials of the two materials are compared, and the material with the higher potential is applied to the positive electrode, while the material with the lower potential is applied to the negative electrode, so as to form any battery.

[0058] There are no particular limitations on the separator; any known separator that can be used in lithium-ion secondary batteries can be used.

[0059] [Example]

[0060] The following describes embodiments of the present invention, but the present invention is not limited to these embodiments.

[0061] [Dielectric particles]

[0062] After obtaining dielectric particles from Toshima Manufacturing Co., Ltd., for dielectric particles whose median particle size needs to be adjusted, the dielectric particles are pulverized using a ball mill with isopropyl alcohol (IPA).

[0063] The properties of dielectric particles are shown in Table 1.

[0064] [Table 1]

[0065] Abbreviation Relative permittivity / - <![CDATA[D 50 / μm]]> <![CDATA[KNbO3]]> KNO 40 0.6 <![CDATA[Li 0.33 The 0.55 TiO3]]> LLT 23 0.4 <![CDATA[LiNbO3]]> LNO 38 0.5 <![CDATA[BaZr 0.2 Ti 0.8 O3]]> BZTO 42 0.6

[0066] [Methods for determining the relative permittivity of powders]

[0067] After the powder is introduced into a tablet forming machine with a diameter (R) of 38 mm for measurement, the powder is compressed using a hydraulic press to a thickness (d) of 1–2 mm, forming compressed powder. At this time, the relative density (D) of the powder is adjusted. powder The powder is formed by pressing it in a manner where the mass density of the pressed powder (ratio of the powder's mass density to its true specific gravity × 100) is 40% or higher. Next, the electrostatic capacitance C of the pressed powder at 25°C and 1 kHz is measured using an inductance-capacitance-resistance (LCR) meter and an automatic balancing bridge method. total The relative permittivity ε of the pressed powder was calculated. total Next, the dielectric constant ε0 of vacuum is set to 8.854 × 10⁻⁶. -12 The relative permittivity ε of air air Let ε be 1, and use the following equations (1) to (3) to calculate the relative permittivity ε of the powder (actual volume portion). power .

[0068] The contact area between the pressed powder and the electrode is A = (R / 2). 2 ×π···(1)

[0069] C total =ε total ×ε0×(A / d)···(2)

[0070] εtotal =ε powder ×D powder +ε air ×(1-D powder )···(3)

[0071] Median particle size (D) 50 [Determination method]

[0072] The particle size distribution of the powder was determined using a particle size distribution measuring device MT3000II (manufactured by Microtrac). Water was used as the solvent, the refractive index was set to 1.81, and the particle size at a cumulative percentage of 50% was taken as the median particle size.

[0073] <Examples 1-5, Comparative Examples 1-3>

[0074] [The production of the positive electrode]

[0075] Dielectric particles, acetylene black (AB) as a conductive additive, polyvinylidene fluoride (PVDF) as a binder, and polyvinyl pyrrolidone (PVP) as a dispersant, along with N-methyl-2-pyrrolidinone (NMP) as a dispersion medium, were premixed. Then, wet mixing was performed using a rotary mixer to obtain a premixed slurry. Next, LiNi, as the positive electrode active material, was... 0.8 Co 0.1 Mn 0.1 O2 (NCM811) was mixed with the premixed slurry, and then dispersed using a planetary mixer to obtain the cathode composite paste. The mass ratios of the components in the NCM811 cathode composite paste are shown in Table 2. Here, NCM811 exhibits peak values ​​at 4 μm and 14 μm when it has a bimodal particle size distribution, and at 12 μm when it does not have a bimodal particle size distribution.

[0076] After coating the aluminum foil, which serves as the positive electrode current collector, with a positive electrode composite material layer paste, it is dried. Next, the dried positive electrode current collector is pressed by rolling and then dried in a vacuum at 120°C to form the positive electrode composite material layer, thus obtaining the positive electrode plate. The obtained positive electrode plate is then stamped to a size of 30mm × 40mm to form the positive electrode.

[0077] [Making the negative electrode]

[0078] An aqueous solution of carboxymethyl cellulose (CMC) as a binder was premixed with acetylene black (AB) as a conductive additive using a planetary mixer. Next, natural graphite (NG) as the negative electrode active material was mixed and premixed using a planetary mixer. Then, water as a dispersion medium and styrene butadiene rubber (SBR) as a binder were added, and dispersion was performed using a planetary mixer to obtain the negative electrode composite paste. The mass ratio of the components in the negative electrode composite paste was set as NG:AB:CMC:SBR = 97.5:0.5:1.0:1.0. The median particle size of NG was 12 μm.

[0079] Next, a negative electrode composite material layer paste is coated onto the copper foil, which serves as the negative electrode current collector, and then dried. The dried negative electrode current collector is then pressed using a roller press and dried under vacuum at 130°C to form the negative electrode composite material layer, thus obtaining the negative electrode plate. The obtained negative electrode plate is then stamped to a size of 32mm × 42mm to form the negative electrode.

[0080] [The fabrication of lithium-ion secondary batteries]

[0081] The secondary battery was heat-sealed into a bag shape using an aluminum laminate (manufactured by Dai Nippon Printing) to obtain a container. Next, a laminate containing a separator between the positive and negative electrodes was introduced into the container, and electrolyte was injected into the interfaces of each electrode. Then, the pressure was reduced to -95 kPa, and the container was sealed to fabricate a lithium-ion secondary battery. As the separator, a microporous membrane made of polyethylene with a single-sided coating of alumina particles approximately 5 μm thick was used. As the electrolyte, a solution of LiPF6 as the electrolyte salt was used, dissolved at a concentration of 1.2 mol / L in a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 30:30:40.

[0082] <Pore diameter distribution of the positive electrode>

[0083] After pretreatment by vacuum drying the positive electrode at 120°C for 12 hours, the pore diameter distribution of the positive electrode was measured using an AutoPore V9605 (manufactured by Micromeritics) via mercury porosimetry in the range of approximately 0.0036–200 μm.

[0084] Here, the diameter of the pores in the positive electrode is calculated using the Washburn formula.

[0085] Washburn's formula: P×D=-4×σ×cosθ

[0086] (P: pressure, σ: surface tension of mercury, D: pore diameter, θ: contact angle of the positive electrode relative to mercury)

[0087] Here, the surface tension of mercury is 480 dynes / cm, and the contact angle between the positive electrode and mercury is 140°.

[0088] exist Figure 1 The results of the pore diameter distribution measurements of the positive electrodes of Examples 1-5 and Comparative Examples 1-3 are shown below. Figure 1 The values ​​recorded are the peak pore diameters of each positive electrode.

[0089] <Cross-section observation of the positive electrode>

[0090] The cross-sections of the positive electrodes of Example 1 and Comparative Example 1 were observed using SEM.

[0091] exist Figure 2 The image shows a cross-sectional SEM photograph of the positive electrode of Example 1. Additionally, in... Figure 3 The image shown is a SEM photograph of the cross-section of the positive electrode of Comparative Example 1.

[0092] from Figure 2 and Figure 3 It can be seen that the positive electrode of Example 1 has less space (the part surrounded by the dotted line) compared to the positive electrode of Comparative Example 1. Therefore, if the positive electrode of Example 1 is used, the proportion of electrolyte in contact with the dielectric particles will be increased.

[0093] Evaluation of the initial performance of lithium-ion secondary batteries

[0094] The following initial performance evaluations were performed on the lithium-ion secondary batteries of Examples 1-5 and Comparative Examples 1-3.

[0095] [Discharge Capacity]

[0096] The prepared lithium-ion secondary battery was placed at a measured temperature (25°C) for 1 hour, then charged with a constant current of 12.4 mA to 4.2V, followed by a constant voltage charge at 4.2V for 1 hour. Next, the lithium-ion battery was placed for 30 minutes, then discharged with a constant current of 12.4 mA to 2.5V. This process was repeated 5 times, and the discharge capacity at the 5th discharge was defined as the discharge capacity (mAh). Furthermore, the current value that could complete the discharge in 1 hour based on the obtained discharge capacity was defined as 1C.

[0097] [Cell resistance]

[0098] After measuring the discharge capacity, the lithium-ion secondary battery was placed at the measurement temperature (25°C) for 1 hour, then charged with a constant current at a charging rate of 0.2C. After adjusting to 50% State of Charge (SOC), it was placed for 10 minutes. Next, a 10-second pulse discharge was performed at a discharge rate of 0.5C, and the voltage at 10 seconds was measured. Then, the voltage at 10 seconds of discharge was plotted with the horizontal axis representing current and the vertical axis representing voltage at a discharge rate of 0.5C. Next, the lithium-ion secondary battery was placed for 10 minutes, then supplemented with charging to restore the SOC to 50%, and placed for another 10 minutes. Then, the above operation was performed at discharge rates of 1.0C, 1.5C, 2.0C, 2.5C, and 3.0C, and the voltage at 10 seconds of discharge was plotted for each discharge rate. The slope of the approximate straight line obtained from each plot, based on the least squares method, was then used as the single-cell resistance (mΩ).

[0099] <Evaluation of the Post-Durability Performance of Lithium-ion Secondary Batteries>

[0100] The following post-durability performance evaluations were performed on the lithium-ion secondary batteries of Examples 1-5 and Comparative Examples 1-3.

[0101] [Discharge Capacity]

[0102] A cycle was performed in a 45°C constant-temperature bath, charging the lithium-ion secondary battery to 4.2V at a charging rate of 1C and discharging it to 2.5V at a discharging rate of 2C. This cycle was repeated 500 times. Next, the temperature of the constant-temperature bath was changed to 25°C, and the lithium-ion secondary battery was left to stand for 24 hours. Then, it was charged to 4.2V at a charging rate of 0.2C, followed by constant-voltage charging at 4.2V for 1 hour. Afterward, the lithium-ion secondary battery was left to stand for 30 minutes, and then discharged to 2.5V at a discharging rate of 0.2C. The discharge capacity (mAh) was measured.

[0103] [Single-unit resistor]

[0104] Similar to the initial performance test, the lithium-ion secondary battery after the discharge capacity test was adjusted to 50% of the State of Charge (SOC), and the single-cell resistance (mΩ) was calculated.

[0105] [Capacity Maintenance Rate]

[0106] The ratio of the discharge capacity after durability to the discharge capacity of the initial performance is calculated as the capacity retention rate (%).

[0107] [Rate of change of resistance]

[0108] The ratio of the resistance of a single unit after durability testing to the resistance of a single unit after initial performance is calculated as the resistance change rate (%).

[0109] Table 2 shows the evaluation results of the initial performance and post-durability performance of the lithium-ion secondary batteries.

[0110] [Table 2]

[0111]

[0112]

[0113] As shown in Table 2, the lithium-ion secondary batteries of Examples 1 to 5 have low single-cell resistance and high durability.

[0114] In contrast, the lithium-ion secondary battery of Comparative Example 1 has high single-cell resistance and low durability because the positive electrode composite material layer does not contain dielectric particles. Furthermore, the lithium-ion secondary batteries of Comparative Examples 2 and 3 have high single-cell resistance and low durability because the peak pore diameter of the positive electrode is larger than the median particle size of the dielectric particles.

Claims

1. A positive electrode, comprising a positive electrode current collector and a positive electrode composite material layer, The aforementioned positive electrode composite material layer includes positive electrode active material and dielectric particles. The peak pore diameter of the positive electrode is below the median particle size of the aforementioned dielectric particles. The aforementioned positive electrode active material has a bimodal particle size distribution. The aforementioned peak pore diameter is greater than 0.1 μm and less than 0.6 μm. The aforementioned dielectric particles contain BaZr 0.2 Ti 0.8 O3 or KNbO3, wherein the relative permittivity of the aforementioned dielectric particles is 20 or higher, and the median particle size of the aforementioned dielectric particles is 0.3 μm or higher and 1.0 μm or lower.

2. The positive electrode according to claim 1, wherein, The content of the aforementioned dielectric particles in the aforementioned positive electrode composite material layer is more than 0.1% by mass and less than 2% by mass.

3. The positive electrode according to claim 1, wherein, The density of the aforementioned positive electrode is above 3.4 g / cc.

4. An energy storage device comprising a positive electrode, a negative electrode, and an electrolyte as described in claim 1.

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