Defect properties and solution energies of dopants in NASICON-type LiGe2(PO4)3 solid electrolyte: a first-principles study

Abstract

NASICON-type solid electrolytes are suitable choices for solid state batteries considering safer and more stable electrochemical performance compared to other potential solid electrolytes. The present study investigates intrinsic defects and dopant incorporation energetics in the LiGe₂(PO₄)₃ (LGP) electrode material using density functional theory-based calculations. The formation energies of intrinsic defects (Frenkel, Schottky and anti-sites) indicate that Li Frenkel pair formation is the most energetically feasible process. With an aim to improve the lithium ion conductivity and chemical stability by suitable doping, solution energies are calculated for various trivalent (M³⁺ = B³⁺, Al³⁺, Ga³⁺, Sc³⁺, In³⁺, Y³⁺, Gd³⁺, La³⁺) and tetravalent (M⁴⁺ = Si⁴⁺, Ti⁴⁺, Sn⁴⁺ and Zr⁴⁺) ions substituted at the Ge⁴⁺ site. The most favourable trivalent and tetravalent dopants are Al³⁺ and Ti⁴⁺, respectively. The changes in lattice parameters with doping are correlated with channel/bottleneck size for Li⁺ migration. Alkali atom doping at the Li⁺ site is energetically favourable whereas alkali-earth doping at the Li⁺ site is not. Analysis based on Bader charges and density of states delineates changes in chemical interactions between the dopant atoms and the host LGP.