Temperature-dependent Li vacancy diffusion in Li 4 Ti 5 O 12 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 Li 4 Ti 5 O 12 can take place either via 8 a init ↔ 16 c ↔ 8 a final or a 8 a init ↔ 16 c ↔ 48 f ↔ 16 d 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 8 a init ↔ 16 c ↔ 8 a final . Moreover, the results indicate that the 16 d Wyckoff position is a trapping site for Li vacancies. The dominant 8 a init ↔ 16 c ↔ 8 a final path appears to compete with its back diffusion, which can be identified by the lifetime t 16 c of the 16 c site. Our studies show that for t 16 c < 100 fs the back diffusion dominates, whereas for 100 fs ≤ t 16 c < 200 fs the 8 a init ↔ 16 c ↔ 8 a final path dominates. In addition, the temperature-independent pre-factor D 0 of the diffusion coefficient, as well as the attempt frequency Γ 0 and the activation energy E A in lithium-poor LTO have been determined to be D 0 = 1.5 × 10 -3 cm 2 s -1 , as well as Γ 0 = 6.6 THz and E A = 0.33 eV.