The investigation of highly stable low strain orthorhombic Na2TiSiO5 as anode material for lithium ion batteries and the migration behavior of Na-Li ions

Abstract

As a member of silicate anode materials, orthorhombic Na2TiSiO5, synthesized by solid state method, has excellent electrochemical properties and is a promising Lithium-ion battery anode material with long cycle stability and low strain. At a current density of 100 mA g−1, Na2TiSiO5 has a reversible capacity of 395.2 mA h g−1 after 200 cycles with Coulombic efficiencies more than 97%. A reversible capacity of 251.3 mA h g−1 after 1000 cycles is retained with Coulombic efficiencies more than 98%. ex-situ experiments show the high structural stability and a less than 0.67% change of unit cell parameters in the charge/discharge process. The bond valence energy landscape (BVEL) calculation reveals 12 voids, namely, 4c-1, 4c-2, 2b and 2a, in a unit cell. That corresponds to a nominal composition Na2Li(4c-1)Li(4c-2)Li0.5(2b)Li0.5(2a)TiSiO5 and a theoretical capacity of 398.1 mAh g−1. The ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) calculations show that for both primitive orthorhombic Na2TiSiO5 and Li+ inserted Na2TiSiO5, Na+/Li+ are not active. However, with the insertion of 4C Li+, the occurrence of the synergic diffusion of Na+ and Ti4+ (4c Na+ -> 4c Ti4+ and 4c Ti4+ -> 2b vacancy) reliefs the extra strain, improves the structural stability during charge/discharge. The formation energy convex hull reveals 3 intermediate phases, that is Na2LiTiSiO5, Na2Li2TiSiO5 and Na2Li2.75TiSiO5, in the charge/discharge process.