Ca-substituted Na3ZnGaS4 with enhanced ionic conductivity and its applicability for all-solid-state sodium-ion batteries

Abstract

Na3ZnGaS4 is a new class of Na + -conducting materials with a sturdy framework consisting of corner-shared super-tetrahedra. The ionic conductivity (σion) of pristine Na3ZnGaS4 is quite low, but it can be improved by introducing defects. Herein, we describe the incorporation of vacancies into Na2 sites of Na3ZnGaS4 via Ca-doping (Na3-xCaxZnGaS4; x = 0–0.2), which leads to a significant increase in σion, reaching 0.28 mS cm−1 (hot-pressed Na2.7Ca0.15ZnGaS4 pellet). The preferential residence of vacancies and Ca2+ in Na + -conducting Na2 sites is theoretically and experimentally verified. Because of simultaneous location of vacancies and Ca2+ in Na2 sites, the increase in σion with 'x' in Na3-xCaxZnGaS4 is not related to a decrease in the migration energy barrier (EA) but rather to an increase in effective hopping frequencies, compensating for the increase in EA. Na2.7Ca0.15ZnGaS4 also exhibits excellent compatibility with Na2Sn anode and TiS2 cathode. A full cell (Na2Sn│Na2.7Ca0.15ZnGaS4│TiS2) delivers a discharge capacity of over 146 mAh g−1TiS2 during 100 charge/discharge cycles at 0.12 mA cm−2 (120 mA g−1TiS2). In comparison to Na3SbS4, Na2.7Ca0.15ZnGaS4 also demonstrates inertness in various organic solvents, which is important for layer-by-layer stacking processes in the fabrication of all-solid-state batteries.