Molecular Analysis of Li Ion Transport Characteristics in Polymer Electrolytes of All Solid-State Lithium Ion Battery
Authors: Nakajima Koki , Tokumasu Takashi , Mabuchi Takuya 
Published: December 2020

These days, the demands for a new technology for producing energy without consuming fossil fuels are increasing. One of the reasons is strict regulations of emitting CO2 and NO on cars like Euro 6 in European countries. It is predicted these regulations will be much stricter in many countries in the future. Also, the demands for a new battery that is smaller and has higher energy outputs for smartphones and computers are increasing in the twenty first century. So, one of the solutions for the problems in the twenty first century is inventing new battery without emitting CO2 and NO. Above all, lithium ion battery is given attention as one of the new batteries because it has high energy density and can be stored for a long time.

Lithium ion battery has some merits such as rechargeable battery, high energy density and large range of operating temperatures. With these benefits, lithium ion battery is used for electronic vehicles (EV), cellphones and computers. However, lithium ion battery has some demerits. Because its electrolyte is liquid, liquid leak and automatic firing occurs and it has low degree of design freedom. One of the solutions for these problems is all solid-state lithium ion battery whose electrolyte is solid. Now, electrolyte materials that have high Li ion transport property are required.

In this study, we focus on polymer electrolyte for its high brittleness. In polymer electrolyte, it is known that Li ion transports by four diffusion mechanisms, inter-segmental hopping, intra-segmental hopping, inter-chain hopping and co-diffusion. We focus on polyethylene oxiside (PEO) and P(2EO-MO) as the electrolyte materials for this study. PEO is known for the most common polymer electrolyte material that has high Li ion transport property. P(2EO-MO) was reported in 2018 [1] that it has higher transference number than PEO but lower ionic conductivity than PEO. However, the reason why P(2EO-MO) has different Li ion transport property from PEO have not been investigated. To reveal the reason in molecular level is required because the knowledge will help to produce a new polymer that has better Li ion transport property in the future. For that, Li ion transport mechanisms in PEO and P(2EO-MO) need to be revealed.

The objective of this study is to reveal the Li ion transport mechanism in PEO and P(2EO-MO). In particular, the ratio of each four types of Li ion transport mechanisms was investigated using molecular dynamics (MD) simulation because the Li ion diffusion is nano scale dynamics. The simulation systems are constructed with 30 LiTFSI molecules as Lithium salts for r = 0.01 and either 30 PEO chains or 30 P(2EO-MO) chains. The salt concentration ratio r is set 0.01, 0.04, 0.08, 1.2 and 1.6. The r is defined by the equation r = [Li]/[O]. Here, [O] means all O atoms concentration in the polymer. TraPPE-UA forcefield is used for all calculations with LAMMPS and the motion equations are integrated with timestep of 1 fs by Velocity Verlet method. Pressure and temperature are controlled by Nose-Hoover barostat and thermostat and 3-dimensional boundary conditions are applied. After equilibrating the system, NVT simulation was applied 300 ns for production. The temperature and pressure were set as 400 K and 1 bar, respectively.

As the analysis of structural property, radial distribution function (RDF) was calculated. As the analysis of transport property, self-diffusion coefficient of Li ion was calculated by mean square displacement (MSD) and each Li ion displacements were also calculated. Additionally, with calculating oxygen index, Li ion transport mechanism was identified. With LiTFSI cluster analysis, the relationship between the Li or TFSI diffusion and the salt concentration ratio was explained.

Reference

[1] Zheng, Q. et al. Optimizing Ion Transport in Polyether-Based Electrolytes for Lithium Batteries. Macromolecules
51, 2847–2858 (2018)