Method for evaluating substituted silicon clathrate

Abstract

To provide a method for evaluating the expansion size and synthesizability of substituted silicon clathrate through calculation.SOLUTION: A method of the present disclosure for evaluating substituted silicon clathrate causes a computer to perform operations including the steps of: (a) acquiring structural information of silicon clathrate from a structural database; (b) specifying a plurality of positions of silicon forming a six-membered ring in the structure acquired in step (a); (c) generating structural information of a substituted structure in which the silicon is substituted for another element at one or more of the positions specified in step (b); (d) evaluating structural stability of the substituted structure generated in step (c); (e) generating structural information of a lithium ion insertion structure in which lithium ions are inserted into the substituted structure; and (f) evaluating a volume expansion rate due to insertion of lithium ions in step (e).SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to methods for evaluating substituted silicon clathrates. [Background technology]

[0002] In recent years, the development of batteries has been actively carried out. For example, in the automobile industry, development of batteries for use in electric vehicles or hybrid vehicles is underway. Silicon is also known as an electrode active material for batteries, particularly lithium ion batteries.

[0003] Silicon electrode active materials have a large theoretical capacity and are effective in increasing the energy density of batteries. On the other hand, silicon electrode active materials have a problem of large expansion during charging. It is known that the expansion during charging can be suppressed by using a silicon clathrate electrode active material as the silicon electrode active material.

[0004] Meanwhile, a technique for evaluating a battery by using a predetermined calculation has been developed.

[0005] For example, Patent Document 1 discloses a method for estimating the state of a battery using regression analysis. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2001-223033 A Summary of the Invention [Problem to be solved by the invention]

[0007] Although silicon clathrate electrode active materials can suppress expansion during charging compared to ordinary silicon electrode active materials, there is a demand for further suppression of the expansion of silicon clathrate electrode active materials during charging. In response to this demand, studies have been conducted to suppress the expansion during charging by replacing silicon in silicon clathrate with specific elements.

[0008] However, it is not easy to synthesize silicon clathrates substituted with specific elements (substituted silicon clathrates). Therefore, it is necessary to evaluate the degree of expansion and ease of synthesis (synthesis possibility) of substituted silicon clathrates by calculation, optionally in combination with machine learning.

[0009] The present disclosure aims to provide a method for evaluating the degree of expansion and the synthetic feasibility of substituted silicon clathrates through calculations. [Means for solving the problem]

[0010] The present inventors have found that the above problems can be solved by the following means. <Aspect 1> A method for evaluating a substituted silicon clathrate, comprising causing a computer to carry out a process comprising the steps of: (a) obtaining structural information of a silicon clathrate from a structural database; (b) identifying multiple positions of silicon atoms constituting a six-membered ring in the structure obtained in step (a); (c) generating structural information of a substituted structure in which the silicon is substituted with another element at one or more of the positions identified in step (b); (d) evaluating the structural stability of the substituted structure produced in step (c); (e) generating structural information of a lithium ion insertion structure obtained by inserting a lithium ion into the substituted structure; and (f) Evaluating the volume expansion rate due to the insertion of lithium ions in step (e). Effect of the Invention

[0011] According to the present disclosure, a method for evaluating the battery characteristics of a substituted silicon clathrate by calculation can be provided. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram showing the lithium ion intercalation structure of an aluminum-substituted silicon clathrate. [Diagram 2] FIG. 2 is a diagram showing the relationship between the aluminum substitution position and the decomposition energy in aluminum-substituted silicon clathrate. [Diagram 3] FIG. 3 is a diagram showing the relationship between the aluminum substitution site and the volume expansion coefficient in the lithium ion insertion structure of aluminum-substituted silicon clathrate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, embodiments of the present disclosure will be described in detail. Note that the present disclosure is not limited to the following embodiments, and various modifications can be made within the scope of the present disclosure.

[0014] <Evaluation method for substituted silicon clathrate> The disclosed method for evaluating a substituted silicon clathrate includes having a computer execute processes including: (a) acquiring structural information of the silicon clathrate from a structural database; (b) identifying multiple positions of silicon constituting a hexagonal ring in the structure acquired in step (a); (c) generating structural information of a substituted structure in which silicon is substituted with another element at one or more of the multiple positions identified in step (b); (d) evaluating the structural stability of the substituted structure generated in step (c); (e) generating structural information of a lithium ion insertion structure in which lithium ions are inserted into the substituted structure; and (f) evaluating the volume expansion coefficient due to the insertion of lithium ions in step (e).

[0015] Silicon clathrate has a cage structure consisting of five- and six-membered rings as shown in Figure 1, and lithium ions enter and exit this cage during charging and discharging.

[0016] As described above, conventionally, the silicon in the silicon clathrate has been replaced with a specific element to suppress the expansion during charging, but it is not easy to synthesize a silicon clathrate substituted with a specific element. The present inventors have found that the element replacement position in the clathrate structure has a large effect on the volume expansion rate and the possibility of synthesis. Furthermore, they have found that the most effective expansion suppression is achieved in the replacement structure with the highest possibility of synthesis.

[0017] <Process (a)> The disclosed method includes having a computer execute a process to obtain structural information of a silicon clathrate from a structural database.

[0018] An example of the process of acquiring information is the process of saving information stored on a network or in the cloud to a terminal such as a personal computer.

[0019] <Step (b)> The method of the present disclosure includes having a computer execute a process of identifying multiple positions of silicon that constitutes a hexagonal ring in the structure obtained in step (a). It is preferable to have a computer execute this process when replacing silicon that constitutes a hexagonal ring, because the expansion suppression effect of the silicon clathrate is particularly enhanced. <Process (c)> The method of the present disclosure includes having a computer execute a process to generate structural information of a substituted structure in which silicon is substituted with another element at one or more of the positions identified in step (b).

[0020] An example of the "other elements" is aluminum.

[0021] <Step (d)> The method of the present disclosure includes having a computer execute a process for evaluating the structural stability of the substituted structure generated in step (c).

[0022] The process of evaluating the structural stability can be, for example, a process of evaluating the decomposition energy. The evaluation of the structural stability can be used as an index of the synthetic feasibility of the substituted structure.

[0023] <Step (e)> The method of the present disclosure includes causing a computer to execute a process for generating structural information of a lithium ion insertion structure in which lithium ions are inserted into a substituted structure.

[0024] The lithium ions may be inserted into the spaces in the crystal structure one atom at a time, or multiple atoms may be inserted simultaneously, and may be inserted regularly or randomly starting from a predetermined space in the crystal structure.

[0025] 〈Process (f)〉 The method of the present disclosure includes having a computer execute a process for evaluating the volume expansion rate due to the insertion of lithium ions in step (e).

[0026] In the present disclosure, the term "volume expansion rate" refers to the rate of increase in volume due to the insertion of lithium ions, with the volume before the insertion of lithium ions being taken as 100%.

[0027] The method of the present disclosure can be combined with machine learning as desired. For example, such a combination can include, after step (f) in the method of the present disclosure, having a computer learn the relationship between a lithium ion insertion structure in which a specific element is substituted and the volume expansion coefficient of the structure, thereby predicting a structure with a small volume expansion coefficient. EXAMPLES

[0028] <Evaluation of substituted silicon clathrate> 〈Process (a)~Process (c)〉 Structural information of silicon clathrate was obtained from the open materials database THE Materials Project, and a computer was used to identify the multiple positions of silicon that compose the six-membered ring from the obtained structure. Structural information of an Al-substituted structure was generated in which one of the multiple positions of silicon was randomly replaced with aluminum (Al) (structural formula: Si 33 Al1, Al is about 3 atomic %. In addition, a computer was used to evaluate the volume of the Al-substituted structures. The generated structures were classified into three patterns, taking into account the symmetry of the structures, out of a total of 34 patterns.

[0029] <Step (d)> Using the Python library pymatgen, we calculated the decomposition energy of the Al-substituted structure by performing convex hull calculations based on the structural data contained in the open materials database THE Materials Project, and evaluated the structural stability, i.e., the feasibility of synthesis. The results are shown in Figure 2. As shown in Figure 2, of the three structures, the structure in which Al is substituted at the vertex containing the six-membered ring had the smallest decomposition energy, i.e., the highest structural stability. This means that this structure is the easiest to synthesize.

[0030] <Step (e)> Structural information on the lithium ion insertion structure was generated by inserting lithium ions into the Al-substituted structure and running a structural optimization process on a computer. The first-principles calculation package VASP was used for the structural optimization. A program written in Python was used for the regression calculation, and the analysis was performed on Jupyter Notebook. In addition, the computer was run to evaluate the volume of the lithium ion insertion structure.

[0031] 〈Process (f)〉 The volume expansion rate of the Al-substituted structure due to the insertion of lithium ions was evaluated. The volume expansion rate was calculated as the rate of increase in volume due to the insertion of lithium ions, with the volume before lithium ion insertion being taken as 100%. For reference, the volume expansion rate of unsubstituted silicon clathrate was also evaluated in the same way. At a capacity of around 450 mAh / g, where the difference in the volume expansion rate between the Al-substituted structure and the unsubstituted silicon clathrate was at its maximum, the Al-substituted structure suppressed volume expansion by up to approximately 15% compared to the unsubstituted silicon clathrate. On the other hand, some Al-substituted structures had almost no effect in suppressing volume expansion.

[0032] Focusing on data for a capacity of around 450 mAh / g, the results of an analysis of the Al substitution position are shown in Figure 3. As shown in Figure 3, the structure with the smallest volume expansion coefficient at this capacity was one in which Al was substituted at the vertex including the six-membered ring.

[0033] From the above results, it was confirmed that there is a correlation between the substitution position, the possibility of synthesizing the substitution structure, and the volume expansion rate of the lithium ion insertion structure, that is, the structure in which Al is substituted at the apex including the six-membered ring, which is the easiest to synthesize, has the smallest volume expansion after lithium ion insertion. As shown in Figure 3, the structure in which Al is substituted at the apex including the six-membered ring has the largest volume, which provides a large space for lithium ions to be inserted, and therefore it is believed that the volume expansion rate when lithium ions are inserted into this structure was the smallest.

Claims

[Claim 1] A method for evaluating a substituted silicon clathrate, comprising causing a computer to execute a process including the steps of: (a) obtaining structural information of a silicon clathrate from a structural database; (b) identifying multiple positions of silicon atoms constituting a six-membered ring in the structure obtained in step (a); (c) generating structural information of a substituted structure in which the silicon is substituted with another element at one or more of the positions identified in step (b); (d) assessing the structural stability of the substituted structure produced in step (c); (e) generating structural information of a lithium ion insertion structure in which lithium ions are inserted into the substituted structure; and (f) Evaluating the volume expansion rate due to the insertion of lithium ions in step (e).

Images