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 8a_init ↔ 16c ↔ 8a_final or a 8a_init ↔ 16c ↔ 48f ↔ 16d_final 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 8a_init ↔ 16c ↔ 8a_final. Moreover, the results indicate that the 16d Wyckoff position is a trapping site for Li vacancies. The dominant 8a_init ↔ 16c ↔ 8a_final path appears to compete with its back diffusion, which can be identified by the lifetime t₁₆ of the 16c site. Our studies show that for t₁₆ < 100 fs the back diffusion dominates, whereas for 100 fs ≤ t₁₆ < 200 fs the 8a_init ↔ 16c ↔ 8a_final 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 E_A 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 E_A = 0.33 eV.