Temperature-dependent Li vacancy diffusion in Li4Ti5O12 by means of first principles molecular dynamic simulations

Lithium-ion batteries (LIBs) are a key electrochemical energy storage technology for mobile applications. In this context lithium titanate (LTO) is an attractive anode material for fast-charging LIBs and solid-state batteries (SSBs). The Li ion transport within LTO has a major impact on the performance of the anode in LIBs or SSBs. The Li vacancy diffusion in lithium-poor Li4Ti5O12 can take place either via 8ainit ↔ 16c ↔ 8afinal or a 8ainit ↔ 16c ↔ 48f ↔ 16dfinal diffusion path. To gain a more detailed understanding of the Li vacancy transport in LTO, we performed first principles molecular dynamics (FPMD) simulations in the temperature range from 800 K to 1000 K. To track the Li vacancies through the FPMD simulations, we introduce a method to distinguish the positions of multiple (Li) vacancies at each time. This method is used to characterize the diffusion path and the number of different diffusion steps. As a result, the majority of Li vacancy diffusion steps occur along the 8ainit ↔ 16c ↔ 8afinal. Moreover, the results indicate that the 16d Wyckoff position is a trapping site for Li vacancies. The dominant 8ainit ↔ 16c ↔ 8afinal path appears to compete with its back diffusion, which can be identified by the lifetime t16c of the 16c site. Our studies show that for t16c < 100 fs the back diffusion dominates, whereas for 100 fs ≤ t16c < 200 fs the 8ainit ↔ 16c ↔ 8afinal path dominates. In addition, the temperature-independent pre-factor D0 of the diffusion coefficient, as well as the attempt frequency Γ0 and the activation energy EA in lithium-poor LTO have been determined to be D0 = 1.5 × 10−3 cm2 s−1, as well as Γ0 = 6.6 THz and EA = 0.33 eV.