Co-substitution Strategy for Boosting Rate-Capability of Lithium-Superionic-Conductor (LISICON)-Type Anode Materials in γ-Li3VO4–Li4GeO4–Li3PO4 Quasi-Ternary-System

Abstract

Using simple solid-state calcination, γ-Li3+x V1–x–y Ge x P y O4 (LVGePO) anode materials with lithium superionic conductor (LISICON)-related crystal structures have been successfully synthesized for next-generation energy storage applications with high-energy and high-power densities. The correlation among their chemical compositions, crystal-phase formations, and rate performances has been elucidated and mapped in the quasi-ternary phase diagram of the Li3VO4–Li4GeO4–Li3PO4 system. The crystal phase formation and surface stability can be controlled by the Ge4+- and/or P5+- substitution ratio; 5 at% or more Ge4+-substitution resulted in a pure γ-phase structure with high Li+ conductivity, while the presence of P5+ suppressed the SEI formation. Fine-tuning of the chemical composition brings about the highest charge (delithiation) capacity retention of ca. 62% of the theoretical capacity at 10 A g–1 (ca. 40C-rate) obtained in the typical chemical composition range of Li3.05–3.1V0.7–0.8Ge0.05–0.1P0.1–0.25O4 with the γ-phase crystal structure. Such co-substituted LVGePO anodes exhibited superior rate performances compared to any binary solid solutions of Li3+x V1–x Ge x O4 and Li3V1–y P y O4. The improvement in the electrochemical performances are induced by the distinct roles of co-substituted cations, viz., P5+ suppresses the reductive decomposition of electrolytes on the LVGePO crystal surfaces, while Ge4+ stabilizes the high Li+ conductive γ-phase structure.