Evaluation method for positive electrode slurry for all-solid-state batteries

Abstract

To precisely evaluate a coating state of a solid electrolyte in a positive active material in a positive electrode slurry for an all-solid-state battery.SOLUTION: A method for evaluating a positive electrode slurry for an all-solid-state battery includes: a measurement step of sealing a positive electrode slurry for an all-solid-state battery in which a positive electrode active material, a solid electrode, a conductive auxiliary agent, and a binder material are mixed together into a measurement container with an electrode provided in both ends and measuring an AC impedance; a frequency region identification step of identifying a frequency region from the AC impedance data; and an evaluation step of evaluating quality of a coating state of the solid-state electrolyte in the positive active material on the basis of the parameter of an AC impedance imaginary number axis of the identified frequency region and on the basis of the parameter of an AC impedance real number axis of the identified frequency region.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for accurately evaluating the quality of a slurry for a positive electrode containing an active material, a solid electrolyte, a conductive additive, a binder material, etc. for an all-solid-state battery in a slurry state. [Background technology]

[0002] In the production of electrode slurries for use in liquid electrolyte lithium-ion batteries and all-solid-state lithium-ion batteries, the quality of the electrode slurries is generally controlled by rheological evaluation such as viscosity. A known method for controlling the quality of electrode slurries is, for example, to sample electrode slurries immediately after production and measure the AC impedance of the electrode slurries to determine whether the electrode slurries are good or bad. Furthermore, a method for evaluating the quality of positive electrode slurries for all-solid-state lithium-ion batteries includes, for example, evaluating the coating state of a coating material on an electrode active material.

[0003] Patent Document 1 describes a method for producing a paste containing an active material, a solid electrolyte, and a conductive additive, which includes a measurement step of measuring the AC impedance of the paste corresponding to a predetermined measurement frequency band, a composition ratio determination step of determining whether or not the composition ratio of the paste is outside a predetermined range based on the width in the real part direction of an arc portion corresponding to the predetermined frequency band in a locus drawn by the measured AC impedance on a complex impedance plane, and a removal step of removing paste whose composition ratio is determined to be outside the predetermined range.

[0004] Patent Document 2 describes a paste evaluation method for evaluating a paste to be applied to the electrode surface of a battery, which uses a container having a rotation mechanism and a measurement unit that measures the AC impedance of the paste, and describes that the paste contained in the container is rotated by the rotation mechanism while the measurement unit measures the AC impedance of the paste, and that the measurement unit has a pair of application electrode plates arranged in parallel to apply an AC voltage to the paste, and that the measured values ​​of the AC impedance for one or more rotations by the rotation mechanism are averaged to correct measurement errors arising from errors in the parallelism of the pair of application electrode plates. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-222651 [Patent Document 2] Patent No. 5505318 Summary of the Invention [Problem to be solved by the invention]

[0006] Compared to liquid electrolyte lithium-ion batteries, cathode slurries for all-solid-state batteries use more materials and contain a composite element between the active material and the solid electrolyte, and it is believed that the composite (coating state) of the solid electrolyte on the cathode active material contributes greatly to battery performance. However, no specific method has been found to evaluate the coating state of the solid electrolyte on the cathode active material in a slurry state, making it difficult to accurately evaluate the coating state.

[0007] In order to solve the above problems, the present application aims to accurately evaluate the coating state of a solid electrolyte on a positive electrode active material in a positive electrode slurry for an all-solid-state battery, thereby contributing to stabilization of battery performance, improvement of quality control in the manufacturing process, and ultimately to energy efficiency. [Means for solving the problem]

[0008] [1] A measurement step of sealing a positive electrode slurry for an all-solid-state battery, in which a positive electrode active material, a solid electrolyte, a conductive additive, and a binder material are kneaded and dispersed, in a measurement container provided with electrodes on both ends, and measuring AC impedance; a frequency domain identification step of identifying a frequency domain from the AC impedance data; and an evaluation step of evaluating whether a coating state of the solid electrolyte on the positive electrode active material is good or bad, based on a parameter on an imaginary axis of AC impedance in the specified frequency region and a parameter on a real axis of AC impedance in the specified frequency region.

[0009] The method for evaluating a positive electrode slurry for an all-solid-state battery of the present invention can accurately evaluate the coating state of a solid electrolyte on a positive electrode active material from the measured AC impedance of the positive electrode slurry for an all-solid-state battery, based on a parameter on the imaginary axis of the AC impedance in a specific frequency range and a parameter on the real axis of the AC impedance in a specific frequency range.

[0010] [2] The method for evaluating a positive electrode slurry for an all-solid-state battery according to [1], wherein the parameter on the imaginary axis of AC impedance is an imaginary component (Zim) of AC impedance of the positive electrode slurry for an all-solid-state battery, or a polarization charge (Cp) derived from the imaginary component (Zim) of AC impedance.

[0011] The evaluation method for a positive electrode slurry for an all-solid-state battery of the present invention uses the imaginary component of AC impedance (Zim) or the polarization charge (Cp) derived from the imaginary component of AC impedance (Zim) as a parameter of the imaginary axis of AC impedance, and therefore can accurately evaluate the coating state of the solid electrolyte on the positive electrode active material.

[0012] [3] The method for evaluating a positive electrode slurry for an all-solid-state battery according to [1] or [2], wherein the parameter of the real axis of the AC impedance is a real component (Zre) of the AC impedance of the positive electrode slurry for an all-solid-state battery.

[0013] The method for evaluating a positive electrode slurry for an all-solid-state battery of the present invention uses the real component (Zre) of the AC impedance of the positive electrode slurry for an all-solid-state battery as a parameter of the real axis of the AC impedance, and therefore can accurately evaluate the dispersion state of the positive electrode slurry.

[0014] [4] The method for evaluating a positive electrode slurry for an all-solid-state battery according to any one of [1] to [3], wherein in the frequency region specifying step, the AC impedance of the positive electrode slurry for an all-solid-state battery is evaluated in two or more frequency regions.

[0015] The method for evaluating a positive electrode slurry for an all-solid-state battery of the present invention evaluates the AC impedance of the positive electrode slurry for an all-solid-state battery in two or more frequency ranges, and therefore can accurately evaluate the coating state of the solid electrolyte on the positive electrode active material.

[0016] [5] The method for evaluating a positive electrode slurry for an all-solid-state battery according to any one of [1] to [4], wherein in the frequency region specifying step, the polarization charge (Cp) derived from the imaginary component (Zim) of the AC impedance and the real component (Zre) of the AC impedance obtained in different frequency regions are evaluated for quality either alone or in combination.

[0017] The evaluation method for a positive electrode slurry for an all-solid-state battery of the present invention evaluates the AC impedance of the positive electrode slurry for an all-solid-state battery by combining a parameter on the imaginary axis of the AC impedance and a parameter on the real axis of the AC impedance obtained by dividing the AC impedance of the positive electrode slurry for an all-solid-state battery into two or more frequency ranges, and therefore can more accurately evaluate the coating state of the solid electrolyte on the positive electrode active material. [Effects of the Invention]

[0018] According to the present invention, in a positive electrode slurry for an all-solid-state battery, the coating state of a positive electrode active material with a solid electrolyte can be evaluated with high accuracy. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a flowchart showing a method for evaluating a positive electrode slurry for an all-solid-state battery according to the present embodiment. [Figure 2] FIG. 1 is a diagram showing an example of a Nyquist plot showing the measurement results of AC impedance of a positive electrode slurry for an all-solid-state battery. [Figure 3] 1 is a graph showing the relationship between the load strength and the polarization charge (Cp) in the solid electrolyte coating step of the all-solid-state battery active material in Example 1. FIG. [Figure 4] FIG. 2 is a diagram showing the relationship between load strength and real component (Zre) in the solid electrolyte coating step of the active material for an all-solid-state battery in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, embodiments of the present invention will be described in detail.

[0021] [Evaluation method for positive electrode slurry for all-solid-state batteries] A method for evaluating a positive electrode slurry for an all-solid-state battery according to an embodiment of the present invention includes a measuring step of enclosing the positive electrode slurry for an all-solid-state battery, in which a positive electrode active material, a solid electrolyte, a conductive additive, and a binder material are kneaded and dispersed, in a measuring container provided with electrodes on both ends, and measuring AC impedance; a frequency domain specifying step of specifying a frequency domain from the AC impedance data; and an evaluation step of evaluating the quality of the coating state of the solid electrolyte on the positive electrode active material, based on a parameter on the AC impedance imaginary axis of the specified frequency domain and a parameter on the AC impedance real axis of the specified frequency domain.

[0022] The method for evaluating a positive electrode slurry for an all-solid-state battery of this embodiment may include a step of preparing a positive electrode slurry for an all-solid-state battery to be evaluated by the method for evaluating a positive electrode slurry for an all-solid-state battery of this embodiment.

[0023] FIG. 1 is a flowchart showing a method for evaluating a positive electrode slurry for an all-solid-state battery according to this embodiment.

[0024] "Preparation process" In the method for evaluating a positive electrode slurry for an all-solid-state battery according to this embodiment, the positive electrode slurry for an all-solid-state battery prepared in the preparation step (S1 shown in FIG. 1) is evaluated. The all-solid-state battery positive electrode slurry that is the target of quality control by the all-solid-state battery positive electrode slurry evaluation method of this embodiment contains a positive electrode active material, a solid electrolyte, a conductive additive, and a binder material.

[0025] The positive electrode active material is not particularly limited as long as it can reversibly absorb and release lithium ions and transport electrons, and known positive electrode active materials applicable to the positive electrode of all-solid-state lithium ion batteries can be used. For example, lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4), solid solution oxide (Li2MnO3-LiMO2 (M = Co, Ni, etc.)), lithium manganese-nickel-cobalt oxide (LiNi x Mn y Co z O2, x+y+z=1), composite oxides such as olivine-type lithium phosphate (LiFePO4); conductive polymers such as polyaniline and polypyrrole; sulfides such as Li2S, CuS, Li-Cu-S compounds, TiS2, FeS, MoS2, and Li-Mo-S compounds; mixtures of sulfur and carbon; etc. The positive electrode active material may be composed of one kind of the above materials alone, or may be composed of two or more kinds.

[0026] The solid electrolyte is not particularly limited as long as it has lithium ion conductivity and insulating properties, and materials generally used in all-solid-state lithium ion batteries can be used. Examples include inorganic solid electrolytes such as sulfide solid electrolyte materials, oxide solid electrolyte materials, halide solid electrolytes, and lithium-containing salts, polymer-based solid electrolytes such as polyethylene oxide, and gel-based solid electrolytes containing lithium-containing salts and lithium-ion conductive ionic liquids. Among these, sulfide solid electrolyte materials are preferred from the viewpoints of high lithium ion conductivity, structural formability by pressing, and interfacial bonding. The form of the solid electrolyte material is not particularly limited, but may be, for example, in the form of particles.

[0027] The positive electrode slurry for an all-solid-state battery may contain a conductive additive to improve the conductivity of the positive electrode. The conductive additive may be any conductive additive that is generally usable in all-solid-state lithium-ion batteries. Examples of the conductive additive include carbon black such as acetylene black and Ketjen black; carbon fiber; vapor-grown carbon fiber; graphite powder; and carbon materials such as carbon nanotubes. The conductive additive may be composed of one or more of the above materials.

[0028] The positive electrode slurry for an all-solid-state battery may contain a solvent from the viewpoint of adjusting the viscosity.

[0029] The positive electrode slurry for an all-solid-state battery can be prepared by blending a positive electrode active material, a solid electrolyte, a conductive additive, and a binder material at a predetermined blending ratio, and kneading the mixture containing the positive electrode active material, the solid electrolyte, the conductive additive, and the binder material in a kneader.

[0030] "Measurement process" In the measurement step, the positive electrode slurry for an all-solid-state battery is placed between two electrodes, and an AC voltage or an AC current is applied between the two electrodes to measure the AC impedance of the positive electrode slurry for an all-solid-state battery (S2 in Figure 1). In the method for evaluating a positive electrode slurry for an all-solid-state battery of this embodiment, the frequency of the AC voltage or AC current used to measure the AC impedance of the positive electrode slurry for an all-solid-state battery is continuously changed, for example, from 5 MHz to 1 Hz.

[0031] "Frequency domain identification process" In the frequency range specifying step, an AC frequency range used for evaluating the coating state of the solid electrolyte on the positive electrode active material is specified from the measurement data of the AC impedance acquired in the measuring step (S3 shown in FIG. 1). In the method for evaluating the coating state of a solid electrolyte in a positive electrode active material of the present invention, the coating state can be evaluated with high accuracy by using parameters on the imaginary axis of AC impedance in a specific frequency range and parameters on the real axis of AC impedance in a specific frequency range from among the AC impedance measurement data obtained in the measurement step. The AC frequency range used for evaluating the coating state is selected, for example, from 5 kHz to 50 kHz, 100 Hz to 1 kHz, or 5 Hz to 20 Hz.

[0032] The parameter of the imaginary axis of the AC impedance of the positive electrode slurry for an all-solid-state battery is preferably the imaginary component of the AC impedance (Zim) or the polarization charge (Cp) derived from the imaginary component of the AC impedance (Zim). A method for specifying the frequency domain of the imaginary component (Zim) or the polarization charge (Cp) used to evaluate the coating state will be described. The imaginary component (Zim) of AC impedance or the polarization charge (Cp) derived from the imaginary component (Zim) of AC impedance in the AC frequency range of 5 Hz to 20 Hz changes most significantly depending on the type of all-solid-state battery positive electrode slurry being measured. The imaginary component (Zim) or the polarization charge (Cp) at the frequency at which this imaginary component (Zim) or the polarization charge (Cp) changes most significantly is used as the imaginary axis parameter used to evaluate the coating state. The frequency at which this imaginary axis parameter changes most significantly varies depending on the material of the active material, etc. For example, the frequency at which the imaginary component (Zim) or the polarization charge (Cp) changes most significantly can be identified by evaluating samples in which the material of the all-solid-state battery active material is subjected to different load intensities in the solid electrolyte coating process. In the case of the all-solid-state battery positive electrode slurry in the examples, the frequency at which the imaginary component (Zim) or the polarization charge (Cp) changes most significantly is, for example, 10 Hz.

[0033] The parameter of the real axis of the AC impedance of the positive electrode slurry for an all-solid-state battery is preferably the real component (Zre) of the AC impedance of the positive electrode slurry for an all-solid-state battery. A method for specifying the frequency domain of the real component (Zre) used to evaluate the coating state will be described. The AC impedance data obtained in the measurement step is displayed on a complex plane (Nyquist plot) with the real component (Zre) on the horizontal axis and the imaginary component (Zim) on the vertical axis. FIG. 2 shows an example of a Nyquist plot of the AC impedance measurement of the positive electrode slurry for an all-solid-state battery obtained in the measurement step. As shown in FIG. 2, the Nyquist plot of the positive electrode slurry for an all-solid-state battery theoretically has three arcs. However, it is difficult to obtain the first arc in the real part direction using currently commonly used measuring equipment. In FIG. 2, the frequency at the boundary between the second arc and the third arc in the real part direction from the low impedance side is the frequency of the real component (Zre) used to evaluate the coating state. The second arc portion corresponds to a frequency range from 1 kHz to 1 MHz, and the third arc portion corresponds to a frequency range from 5 Hz to 1 kHz.

[0034] "Process for evaluating the coating state of solid electrolyte on positive electrode active material" In the step of evaluating the coating state of the solid electrolyte on the positive electrode active material (hereinafter referred to as the evaluation step), the coating state of the solid electrolyte on the positive electrode active material is evaluated based on the parameters of the AC impedance imaginary axis and the parameters of the AC impedance real axis at the frequency identified in the frequency range identification step, from the AC impedance measurement results of the all-solid-state battery positive electrode slurry obtained in the measurement step (S4 in FIG. 1).

[0035] If the parameter of the imaginary axis of the AC impedance in the frequency range specified in the frequency range specification step satisfies a predetermined reference value based on the AC impedance measurement results of an all-solid-state battery positive electrode slurry (hereinafter, "good slurry") having a good solid electrolyte coating state on the positive electrode active material, and if the parameter of the real axis of the AC impedance in the frequency range specified in the frequency range specification step satisfies a predetermined reference value based on the AC impedance measurement results of a good slurry, the positive electrode slurry is determined to have a good solid electrolyte coating state on the positive electrode active material. The reference value is determined from the AC impedance measurement results of the slurry determined to be good based on, for example, battery performance, the dispersion state of the slurry determined using an analytical method such as a particle size distribution analyzer, and the solid electrolyte coating thickness calculated using an analytical method such as an electron microscope. The upper and lower limits of the reference value are determined depending on the type of all-solid-state battery positive electrode slurry.

[0036] If the parameter of the imaginary axis of the AC impedance in the frequency region specified in the frequency region specifying step does not satisfy a predetermined reference value based on the AC impedance measurement results of a good-quality slurry, or if the parameter of the real axis of the AC impedance in the frequency region specified in the frequency region specifying step does not satisfy a predetermined reference value based on the AC impedance measurement results of a good-quality slurry, the positive electrode slurry for an all-solid-state battery is determined to have a poor coating state of the solid electrolyte in the positive electrode active material.

[0037] According to the evaluation method for a positive electrode slurry for an all-solid-state battery of this embodiment, the AC impedance of the positive electrode slurry for an all-solid-state battery is measured in a specific frequency range, and the coating state of the solid electrolyte on the positive electrode active material can be evaluated with high accuracy based on the obtained parameters on the imaginary axis of the AC impedance and the parameters on the real axis of the AC impedance. [Example]

[0038] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0039] [Example 1] "Preparation of cathode slurry for all-solid-state batteries" In advance, in order to change the coating state of the positive electrode active material, composite materials were prepared by changing the processing conditions in four patterns in the solid electrolyte coating process. The four patterns were prepared under conditions with low to high processing loads. Thereafter, a positive electrode slurry for an all-solid-state battery was prepared as follows. The binder solution and the conductive additive dispersion were mixed by stirring at 2000 rpm for 1 minute using a planetary centrifugal mixer (kneading device), and mixture 1 was obtained. To the obtained mixture 1, zirconia balls with a diameter of 2 mm, a positive electrode active material (coated with a solid electrolyte, for example, positive electrode active material:solid electrolyte=75:25 to 90:10 (mass ratio)), and a solvent (butyl butyrate) were added, and the mixture was stirred and mixed at 2000 rpm for 1 minute to obtain mixture 2. To the obtained mixture 2, a solid electrolyte and a solvent (butyl butyrate) were added, and the mixture was kneaded for 1 minute at 2000 rpm using a kneading device, to obtain mixture 3. To the obtained mixture 3, a dilution solvent (butyl butyrate) was added, and the mixture was stirred and mixed at 2000 rpm for 2 minutes, to obtain mixture 4. Thereafter, a step of kneading was carried out while repeatedly diluting.

[0040] "Impedance Measurement: Imaginary Component (Cp)" The AC impedance of the positive electrode slurries for all-solid-state batteries obtained by changing each processing condition was measured, and the polarization charge (Cp) derived from the parameter (Zim) on the imaginary axis of the AC impedance was calculated. The relationship between the load intensity conditions in the coating process and the polarization charge (Cp) was investigated. The results are shown in Figure 3. The results shown in Figure 3 indicate that the load intensity and the polarization charge (Cp) are proportional, and the greater the load intensity, the smaller the polarization charge (Cp). This indicates that as the load intensity increases, the coating of the active material with solid electrolyte progresses, reducing the exposed surface area of ​​the positive electrode active material, and the particle state is better coated with solid electrolyte.

[0041] "Impedance Measurement: Real Component (Zre)" The real component (Zre) of the AC impedance of the positive electrode slurry for all-solid-state batteries was calculated as a parameter on the real axis of the obtained AC impedance, and the relationship between the processing conditions of the positive electrode slurry for all-solid-state batteries and the real component (Zre) was investigated. The results are shown in Figure 4. The results shown in Figure 4 indicate that the load intensity and the real component (Zre) are proportional, and the greater the load intensity, the greater the real component (Zre). Based on other verification results, this is inferred to have no direct causal relationship with the coating state of the solid electrolyte. However, the dispersion state differs in all-solid-state slurries with different processing conditions, and it is inferred that this indicates a difference in the dispersion state. If the real axis parameter falls within a predetermined reference value, it can be determined that the dispersion state is good.

[0042] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications and variations are possible within the scope of the gist of the present invention as set forth in the claims.

Claims

[Claim 1] a measuring step of enclosing an all-solid-state battery positive electrode slurry in which a positive electrode active material, a solid electrolyte, a conductive additive, and a binder material are kneaded and dispersed in a measuring container provided with electrodes on both ends, and measuring the AC impedance of the all-solid-state battery positive electrode slurry to obtain AC impedance data of the all-solid-state battery positive electrode slurry; a frequency range identifying step of identifying an AC frequency range used for evaluating the coating state of the solid electrolyte on the positive electrode active material from the AC impedance data; an evaluation step of evaluating whether a coating state of the solid electrolyte on the positive electrode active material is good or bad, based on a parameter on an AC impedance imaginary axis of the specified frequency region and a parameter on an AC impedance real axis of the specified frequency region, In the frequency domain specifying step, the AC impedance data is displayed on a complex plane by dividing the data into a real component on the horizontal axis and an imaginary component on the vertical axis, and a frequency at a boundary between a second arc portion in the real component direction from the low impedance side and a third arc portion from the low impedance side among three arc portions of the complex plane display is determined as a frequency of a real component of the AC impedance of the all-solid-state battery positive electrode slurry used for evaluating a coating state of the solid electrolyte in the positive electrode active material; In the evaluation step, if the parameter on the imaginary axis of the AC impedance in the specified frequency range satisfies a reference value previously set based on the AC impedance measurement result of a good-quality all-solid-state battery positive electrode slurry in which the positive electrode active material is well coated with the solid electrolyte, and if the parameter on the real axis of the AC impedance in the specified frequency range satisfies a reference value previously set based on the AC impedance measurement result of a good-quality all-solid-state battery positive electrode slurry, the positive electrode active material is judged to be a positive electrode slurry in which the solid electrolyte is well coated with the solid electrolyte; if the parameter on the imaginary axis of the AC impedance in the specified frequency range does not satisfy the reference value previously set based on the AC impedance measurement result of a good-quality all-solid-state battery positive electrode slurry, or if the parameter on the real axis of the AC impedance in the specified frequency range does not satisfy the reference value previously set based on the AC impedance measurement result of a good-quality all-solid-state battery positive electrode slurry, the positive electrode active material is judged to be a positive electrode slurry in which the solid electrolyte is poorly coated with the solid electrolyte; the parameter of the AC impedance imaginary axis is an imaginary component (Zim) of the AC impedance of the all-solid-state battery positive electrode slurry or a polarization charge (Cp) derived from the imaginary component (Zim); the parameter on the real axis of AC impedance is a real component (Zre) of AC impedance of the positive electrode slurry for the all-solid-state battery.

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