Molecular Analysis of Transport Characteristics of Li Ion in Solid State Electrolyte
Authors: Nakajima Koki , Mabuchi Takuya , Tokumasu Takashi 
Published: February 2020

These days, the demands for a new technology for producing energy without consuming fossil fuels are increasing. One reason is, for example, 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 21 century. So, one of the solutions for the problems in the 21 century is inventing new battery without emitting CO2 and NO. Above all, lithium ion battery is given attention as the 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. In our study, as the material for the electrolyte Li7La3Zr2O12 (LLZ) is chosen. LLZ can treat easily due to high chemical stability of Li ions and high ion conductivity at high temperatures. In previous research, it was revealed that LLZ has phase transition from tetragonal phase to cubic phase at about 900 K. It was also found that there are no sites in tetragonal LLZ but some sites in cubic LLZ. That is, it means LLZ has no sites below 900K and some sites above 900K. Above all, the transport mechanism of Li ion in both tetragonal LLZ and cubic LLZ at 300K was revealed by Ab-initio MD simulation. However, how the structure property of LLZ influences the transport property of Li ion at various temperatures was not revealed.

The objective of this research is to reveal how the structure property of LLZ influences the transport property of Li ion in LLZ at various temperatures by molecular dynamics (MD) simulation. In this research, we simulated the self-diffusivity of Li ion from 300K to 1100K with 100K decrements. As the analysis of transport property, we calculated self-diffusion coefficients and root-mean square deviation (RMSD) of each Li ion. As the analysis of structure property, we calculated the lattice parameters and Li-Li’s radius distribution function (RDF).

As the simulation system, we constructed the initial structure at first, performed NVT or NPT simulation for 225 ps to reach equilibrium state and after that started sampling for 1ns.

Firstly we found a peak at 2.5Å from the results of RDF. The result is consistent with the fact that the distance between Li site and the nearest other Li site is about 2.5Å from other research. From the result of self diffusion coefficient of Li ion against temperature, we found the slope from 500K to 900K is different from the slope from 900K to 1100K. It can be considered that the difference is due to the difference of the transport mechanisms of Li ion in each LLZ phases. From the results of RMSD, we found that the temperature dependence on RMSD is large. From 600K to 800K we found some Li ions jumped to the neighboring sites and change the places. It is called “a synchronous collective motion”. From 900K to 1100K the synchronous collective motion was not seen in the graphs.

From these results and the fact that there is some sites in LLZ from 900K but no sites below 900K, it can be said that Li ions move as synchronous collective motion below 900 while they do not move as synchronous collective motion but as the motion through the vacant sites from 900K to 1100K.