Method for estimating state of electrode and method for manufacturing secondary battery

Abstract

To provide a novel method for estimating the state of an electrode that considers the deformation behavior of the electrode active material at the particle level.SOLUTION: The method for estimating the state of an electrode includes: a first step of preparing a three-dimensional model of an electrode including particles of an electrode active material; a second step of performing calculations shown in the following (1) to (3) by using the three-dimensional model; and a third step of calculating the expansion amount of the electrode on the basis of the results of the calculations: (1) the calculation of an electrochemical reaction of each particle; (2) the calculation of the potential distribution of each particle; and (3) the calculation of the stress of each particle.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to a method for estimating an electrode state and a method for manufacturing a secondary battery. [Background technology]

[0002] Various methods have been proposed for estimating the state of electrodes of secondary batteries containing electrode active materials using three-dimensional models. For example, Patent Document 1 describes a simulation method for analyzing the contact state between powder particles using a particle method in an electrode formed by compression molding multiple types of powder including an electrode active material powder and a solid electrolyte powder. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2017-59444 A Summary of the Invention [Problem to be solved by the invention]

[0004] In the electrodes of all-solid-state batteries, which have been studied for practical use in recent years, particulate electrode active material is in contact with particulate solid electrolyte. Therefore, if the electrode active material expands and contracts repeatedly, the contact interface between the electrode active material particles and the solid electrolyte particles may be destroyed, resulting in a decrease in battery performance. Furthermore, if the shape and size of the electrode active material particles contained in the electrode are not uniform, the expansion and contraction of each particle and the state of the contact interface may differ from one another. Therefore, from the viewpoint of optimizing the electrode structure, it is becoming increasingly important to estimate the change in the state of the electrode taking into account the deformation behavior of each particle of the electrode active material. In view of the above circumstances, an object of the present disclosure is to provide a novel method for estimating the state of an electrode that takes into account the deformation behavior of the electrode active material at the particle level. [Means for solving the problem]

[0005] Means for solving the above problems include the following embodiments. <1> A first step of preparing a three-dimensional model of an electrode comprising particles of an electrode active material; A second step of performing calculations shown in (1) to (3) below using the three-dimensional model; and a third step of calculating an amount of expansion of the electrode based on a result of the calculation. (1) Calculation of electrochemical reactions for each particle (2) Calculation of the electric potential distribution for each particle (3) Calculation of stress for each particle <2> The electrode is a negative electrode. <1> A method for estimating a state of an electrode according to claim 1. <3> The electrode active material includes silicon. <1> or <2> A method for estimating a state of an electrode according to claim 1. <4> The electrode further comprises particles of a solid electrolyte. <1> ~ <3> 13. A method for estimating an electrode state according to claim 12. <5> <1> ~ <4> determining a structure of the electrode based on a calculation result of the amount of electrode expansion obtained by the method for estimating an electrode state according to any one of the above; and forming an electrode having the above structure.

[0006] In the electrode state estimation method of the present disclosure, a plurality of pieces of learning data and teacher data relating to the deformation behavior of particles may be prepared, a trained model may be generated based on the plurality of teacher data, and the trained model may be used to estimate the deformation behavior of the electrode. The trained model may be generated, for example, by training a machine learning model using a deep learning algorithm. Effect of the Invention

[0007] According to one embodiment of the present disclosure, a novel method for estimating an electrode state is provided that takes into account the deformation behavior of each particle of an electrode active material. [Brief description of the drawings]

[0008] [Figure 1] FIG. 2 is a diagram showing an example of a three-dimensional model of an electrode. [Diagram 2] FIG. 2 is a diagram showing an example of a calculation flow performed by the method of the present disclosure. [Diagram 3] 1 is a graph comparing calculated and measured values ​​of the volume expansion rate of an electrode during charging. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] In the present disclosure, a numerical range indicated using "~" means a range that includes the numerical values ​​before and after "~" as the minimum and maximum values, respectively. In the numerical ranges described in the present disclosure, the upper or lower limit value described in a certain numerical range may be replaced with the upper or lower limit value of another numerical range described in the present disclosure. In the numerical ranges described in the present disclosure, the upper or lower limit value described in a certain numerical range may be replaced with a value shown in the examples.

[0010] <Electrode state estimation method> The electrode state estimation method of the present disclosure includes: A first step of preparing a three-dimensional model of an electrode comprising particles of an electrode active material; A second step of performing calculations shown in (1) to (3) below using the three-dimensional model; and a third step of calculating an amount of expansion of the electrode based on a result of the calculation. (1) Calculation of electrochemical reactions for each particle (2) Calculation of the electric potential distribution for each particle (3) Calculation of stress for each particle

[0011] According to the method of the present disclosure, it is possible to accurately estimate the deformation behavior of an electrode accompanying charging and discharging of a battery. Since an electrode contains a large number of particles (active material particles, solid electrolyte particles, etc.) inside, the deformation behavior of the electrode is linked to the deformation behavior of each of these particles. Conventional estimation methods do not take into account the state of individual particles contained in the electrode, whereas the method disclosed herein estimates the deformation behavior of the electrode based on calculation results regarding the deformation behavior of individual particles contained in the electrode.

[0012] The method of the present disclosure can be applied to electrodes of various secondary batteries (rechargeable batteries). The secondary battery may be a so-called all-solid-state battery using a particulate electrolyte, or a so-called liquid battery using a liquid electrolyte.

[0013] The method of the present disclosure is particularly effective when the active material particles contained in the electrode exhibit large deformation behavior. From this viewpoint, the electrode in the method of the present disclosure is preferably a negative electrode, and the electrode active material is preferably a negative electrode active material. Examples of the negative electrode active material include carbon materials such as graphite and hard carbon, silicon, metallic lithium, lithium alloys, and lithium titanate (LTO). Among the above negative electrode active materials, silicon and carbon materials are preferred, and silicon is more preferred. Silicon and carbon materials have a large degree of expansion and contraction during charging and discharging of a secondary battery, and silicon has a particularly significant expansion and contraction during charging and discharging of a secondary battery.

[0014] In the method of the present disclosure, the electrode may further include solid electrolyte particles, i.e., the electrode in the method of the present disclosure may be an electrode of an all-solid-state battery. Inside the electrode of an all-solid-state battery, particulate electrode active material is in contact with particulate solid electrolyte. In order to increase the battery capacity, it is desirable to increase the amount of electrode active material as much as possible. On the other hand, if the electrode active material expands and contracts repeatedly, the contact interface between the particles of the electrode active material and the particles of the solid electrolyte may be destroyed, resulting in a decrease in battery performance. Therefore, from the viewpoint of achieving both high capacity and high output of a battery, it is important to optimize the compounding ratio of the electrode active material and the solid electrolyte in the electrode. The method of the present disclosure is useful for determining the optimal compounding ratio of the electrode active material and the solid electrolyte in the electrode.

[0015] The solid electrolyte contained in the electrode is not particularly limited. Examples of the solid electrolyte include a sulfide solid electrolyte, an oxide solid electrolyte, and a halide solid electrolyte.

[0016] (1st step) In the first step, a three-dimensional model of an electrode containing particles of an electrode active material is prepared. When creating a three-dimensional model, information such as the number, blending ratio, particle size distribution, void ratio, and physical properties (elastic modulus, diffusion coefficient, electrical conductivity, reaction resistance, etc.) of the electrode active material particles contained in the electrode and, if necessary, the solid electrolyte particles contained therein, are set according to the results of experiments carried out separately. Information about particles included in the 3D model is set for each particle.

[0017] The number of particles of the electrode active material and, if necessary, the solid electrolyte contained in the three-dimensional model can each be selected, for example, from within the range of 100 to 100,000. The particle size of the electrode active material contained in the three-dimensional model and the solid electrolyte contained as necessary can each be selected, for example, from the range of 0.1 μm to 30 μm. The thickness of the electrodes in the three-dimensional model can be selected from the range of 10 μm to 200 μm, for example.

[0018] An example of a three-dimensional model of an electrode is shown in Figure 1. The three-dimensional model shown in Figure 1 is composed of silicon (Si, shown in white) as the electrode active material, a solid electrolyte (SE, shown in light gray), and voids (shown in black), and is an electrode structure in which rectangular parallelepiped Si particles and spherical SEs are randomly arranged. Spaces are provided above and below the electrode structure to allow for the expansion of the electrode. Figure 1 (a) shows the state before the electrode is charged, in which the Si particles have not expanded, and Figure 1 (b) shows the state after the electrode is charged to 674 mAh / g, in which the Si particles have expanded.

[0019] (2nd process) In the second step, the calculations shown in (1) to (3) are performed using the three-dimensional model of the electrode prepared in the first step.

[0020] In this disclosure, "particle-by-particle" means that the calculation is performed based on information set for each particle included in the three-dimensional model of the electrode. From the viewpoint of accuracy of the calculation results, the particles to be calculated in the second step preferably account for 80% or more of the total particles contained in the three-dimensional model of the electrode, more preferably 90% or more, and even more preferably 100%.

[0021] (1) Calculation of electrochemical reactions for each particle In calculating the electrochemical reaction of each particle, information regarding the absorption and release of metal ions in the electrode active material during charging and discharging of the electrode (for example, the amount of metal ions absorbed by each particle per unit charge amount) is calculated three-dimensionally.

[0022] (2) Calculation of the electric potential distribution for each particle In calculating the potential distribution for each particle, information on the potential distribution of the electrode active material during charging and discharging of the electrode (for example, the potential difference of each particle per unit charge amount) is calculated three-dimensionally.

[0023] (3) Calculation of stress for each particle In calculating the stress for each particle, information regarding the stress acting on the electrode active material during charging and discharging of the electrode (for example, internal stress caused by the occlusion of metal ions in each particle per unit charge amount) is calculated three-dimensionally. ε in equation (4) below is calculated here.

[0024] The second step may include carrying out the calculations shown in (4) to (6) below. (4) Calculation of metal ion diffusion coefficient for each particle (5) Calculation of metal ion concentration per particle (6) Calculation of the amount of expansion for each particle

[0025] (3rd step) In the third step, the amount of expansion of the electrodes is calculated based on the results of the calculation performed in the second step. In the third step, the phase field variables Si in the equations described below are output.

[0026] The expansion amount of the electrode is calculated by solving the following equations (1) to (4) using the Allen-Cahn equation (a reaction-diffusion equation named after Samuel M. Allen and John W. Cahn).

[0027]

Number

[0028] In the above equations, i, j, and k are symbols indicating specific particles, respectively. m indicates the total number of particles. n indicates the maximum allowable number of particles and is set to a value sufficiently larger than the actual number. S i is a phase field variable and is a value from 0 to 1 representing the occupancy ratio (particle size) of the i-th particle number. The type of particle is such that when the total number of particle numbers is n, S i (i < n / 4) is the active material, S i (n / 4 ≤ i < n / 2) is the solid electrolyte, S i (n / 2 ≤ i < n) is distinguished as a void. Ms is the change rate of the phase field variable. Since the calculation of the present invention is a convergence calculation to an equilibrium shape, it is set to a size such that the calculation does not diverge. G system means the virtual free energy of the system to be calculated (δ in Equation (2) means a functional derivative). S i 、S j 及びS k respectively mean the phase field variables of particle i, particle j, and particle k. G chem means the virtual chemical free energy of the system to be calculated. W ik means the interfacial energy barrier between particle i and particle k. a ik means the gradient coefficient between particle i and particle k. W ik とa ik は、粒子間の界面エネルギー(Siの表面エネルギーが凡そ1 J / m 2Considering that 2 Decided to reproduce the previous setting. ε c 11 , ε c 22 and ε c 33 means the restrained strain (actually achieved deformation amount), and can be obtained from the relational equation between the stress rate and the total strain rate defined for the elastic deformation region and the compositional deformation region, respectively. 11, 22, and 33 mean the crystal axis directions. Here, the plastic deformation region is regarded as a region (inclusion) with a different elastic coefficient from the elastic deformation region.

[0029] The first term on the right-hand side of equation (2) is a unique part of the method of the present disclosure and is calculated from equations (3) and (4). Equation (4) expresses the amount of deformation actually achieved as ε c ij The volume expansion rate (1+ε) of each particle is calculated by solving the stress balance equation. c 11 +ε c 22 +ε c 33 ) to the initial volume V0 i The value multiplied by this is the equilibrium volume (target volume), and the current volume V i This means that a penalty is introduced according to the difference between the equilibrium volume and the stress. This is the driving force that expands and contracts each particle to the actual expansion and contraction amount of the particle obtained by the stress calculation. f is the cohesive energy, and it should be set to a value that will obtain the target volume. For example, it can be set to 3000 J / mol in this calculation.

[0030] An example of the calculation flow performed by the method of the present disclosure is shown in FIG.

[0031] <Secondary battery manufacturing method> The method for manufacturing a secondary battery according to the present disclosure includes the steps of: determining a structure of an electrode based on a calculation result of an amount of electrode expansion obtained by the above-described method for estimating an electrode state; and forming an electrode having the structure.

[0032] In the manufacturing method of the secondary battery of the present disclosure, in the step of determining the structure of the electrode based on the calculation result of the amount of expansion of the electrode, one or more pieces of information related to the structure of the electrode are determined. Specific examples of the information related to the structure of the electrode include information related to the entire electrode such as the thickness, density, and porosity of the electrode, and information related to the components contained in the electrode such as the type, particle size, particle shape, and content of the electrode active material (and solid electrolyte contained as necessary).

[0033] The process for producing an electrode having a structure determined based on the calculation results of the amount of electrode expansion is not particularly limited, and can be carried out by a known method. There is no particular limitation on the method for producing a secondary battery using the electrode, and the battery can be produced by a known method.

[0034] The type of secondary battery produced by the production method of the present disclosure is not particularly limited. Specific examples of the secondary battery include alkali metal ion secondary batteries such as lithium ion secondary batteries, sodium ion secondary batteries, and potassium ion secondary batteries, nickel cadmium batteries, nickel hydrogen batteries, and lead storage batteries. The secondary battery may be either an all-solid-state battery or a liquid battery, and is preferably an all-solid-state battery. EXAMPLES

[0035] The present disclosure will be specifically described below based on examples, but the present disclosure is not limited to these examples.

[0036] We created a 3D model of an electrode consisting of particles of active material (silicon), particles of solid electrolyte, and voids, as shown in Figure 1. The number of active material particles included in this 3D model was approximately 2,000, and the number of solid electrolyte particles was approximately 20,000. Using the fabricated 3D model, the average volume expansion rate (%) of the Si particles was measured when constant current charging (C rate: 0.5) was performed. The results are shown in Figure 3. In Figure 3, the dashed line shows the expansion rate calculated from the charge amount, assuming that the experimental value of the volume expansion rate of the crystalline Si-Li system when the charge amount was 674 mAh / g is 150%.

[0037] As shown in Fig. 3, when the charge amount was 674 mAh / g, the calculated average volume expansion coefficient of the Si particles was 147%, and the experimental value of the volume expansion coefficient of the crystalline Si-Li system was 150%. Considering that the volume expansion coefficient of the Si particles in the actual electrode becomes negative by several percent due to elastic strain, it can be determined that the method disclosed herein can reproduce the volume expansion coefficient of the Si particles with high accuracy. Furthermore, the disclosed method can perform a process including three-dimensional calculations for 20,000 or more particles in a short period of time (e.g., a few days).This is believed to be because the disclosed method incorporates a mechanism that allows each particle to change to a set volume by setting a virtual energy distribution that takes a minimum value at a set particle size for each particle.

Claims

1. A first step of preparing a three-dimensional model of an electrode comprising particles of an electrode active material; A second step of performing calculations shown in (1) to (3) below using the three-dimensional model; and a third step of calculating an amount of expansion of the electrode based on a result of the calculation. (1) Calculation of electrochemical reactions for each particle (2) Calculation of electric potential distribution for each particle (3) Calculation of stress for each particle

2. The method for estimating a state of an electrode according to claim 1 , wherein the electrode is a negative electrode.

3. The method for estimating an electrode state according to claim 1 , wherein the electrode active material includes silicon.

4. The method for estimating a state of an electrode according to claim 1 , wherein the electrode further includes particles of a solid electrolyte.

5. determining a structure of the electrode based on a calculation result of the amount of electrode expansion obtained by the electrode state estimation method according to any one of claims 1 to 4; and forming an electrode having the above structure.

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