First-principles insight into the entanglements between superionic diffusion and Li/Al antisite in Al-doped Li1+xAlxGe2-x(PO4)3 (LAGP)

As an ion conductor, the Al-doped Li1+xAlxGe2-x(PO4)(3) (LAGP) demonstrates the superionic Li diffusion behavior, however, without the convinced verifications. In this context, the density functional theory (DFT) calculations are employed to clarify the structural origin of the fast Li ion migration kinetics in LAGP solid electrolytes. The calculated results show that doping of Al leads to an emerging high-energy 36f Li site, which plays an important role in promoting the Li diffusion and can largely lower the Li ion diffusion energy barrier. Moreover, the Li/Al antisite defect is investigated firstly, with which the Li ions are excited to occupy a relatively high energy site in LAGP. The obvious local structural distortion by Li/Al antisite results in the coordination change upon Li diffusion (lattice field distortion), which facilitates the Li diffusion significantly and is probably the main reason to account for the superionic diffusion phenomenon. Therefore, the occupation of Li at high-energy sites should be an effective method to establish the fast Li diffusion, which implies a rewarding avenue to build better Li-ion batteries.