Oxygen defects boost the lithium-ion storage kinetics of γ-phase Li3.1V0.9Ge0.1O4−x for high-rate lithium-ion capacitors

Abstract

The kinetics of electrode materials are fundamentally governed by ion and electron transport efficiency within their crystal structures, which critically determines their rate performance. Current γ-Li₃PO₄-type lithium superionic conductors suffer from compromised kinetics that restrict their power density and rate capability due to suboptimal ionic/electronic conductivity. Here, we develop a strategy to stabilize the gamma-phase lithium vanadate under ambient conditions through germanium doping, achieving the target composition γ-Li_3.1Ge_0.1V_0.9O₄. Subsequent introduction of oxygen vacancies further optimizes the material's kinetics, generating the defective phase γ-Li_3.1Ge_0.1V_0.9O_4−x. These oxygen defects induce band structure modification, manifested as a narrowed bandgap that promotes electron excitation between valence and conduction bands. Concurrently, the defect strategy lowers the activation energy barrier for lithium-ion diffusion, synergistically enhancing both ionic and electronic transport pathways. The enhanced γ-Li_3.1Ge_0.1V_0.9O_4−x exhibits excellent lithium storage rate performance in a half cell, delivering a remarkable specific capacity of 262.3 mAh g⁻¹ at a high rate of 30C, while maintaining stable cycling performance, outperforming similar materials. When configured into γ-Li_3.1Ge_0.1V_0.9O_4−x//AC lithium-ion capacitors, the hybrid device achieves outstanding energy/power density combinations up to 57.0 Wh k⁻¹ and 9166.6 W kg⁻¹, respectively, surpassing most reported counterparts in this material category.