Enhancing the Phase Stability of γ-Phase Li<sub>3</sub>VO<sub>4</sub> for High-Performance Hybrid Supercapacitors: Investigating Influential Factors and Mechanistic Insights
Tatsuya Kondo, Keisuke Matsumura, Patrick Rozier, Patrice Simon, Kenji Machida, Sekihiro Takeda, Shuichi Ishimoto, Kenji Tamamitsu, Etsuro Iwama, Wako Naoi, Katsuhiko Naoi
Abstract
This study explores the potential of titanium ion (Ti 4+ ) substitution in γ-Li 3 VO 4 (γ-LVO) as an anode material for high-energy supercapacitors. A series of Li 3+ x V 1– x Ti x O 4 ( x = 0, 0.05, 0.10, 0.15, and 0.20) are systematically synthesized to vary the Ti-substitution ratio within LVO, and their crystal phases were analyzed using X-ray diffraction (XRD). Additionally, the reactivity of Ti during charge–discharge cycles is assessed by monitoring in situ X-ray absorption fine structure (XAFS) spectral changes. Quenching methods and XRD measurements quantitatively reveal that substituting 20 atom % of V 5+ with Ti 4+ achieves a single-phase γ-LVO, distinct from the nonsubstituted LVO (β-phase LVO). The Ti-substituted γ-phase LVO electrode displays a supercapacitor-like voltage curve and exceptional high-power performance during charge–discharge tests, benefiting from its high ionic conductivity stemming from the LISICON (Lithium Super Ionic CONductor) crystal structure. Furthermore, the Ti-substituted γ-phase LVO electrode exhibits an impressive rate capability, retaining 50% of its capacity at a very high current density of 2 A g –1 (10C-rate), while the nonsubstituted LVO retained only 13% under the same conditions. GITT analysis confirms a 100-fold higher Li + diffusion coefficient for the Ti-substituted γ-phase LVO electrode. A novel approach is employed to examine the kinetic effects of Ti substitution on γ-phase stabilization: halting or quenching the γ → β phase transition during cooling using liquid nitrogen, coupled with XRD measurements, facilitates a quantitative evaluation of the phase transition rate. The primary goal of this study is to conduct a comprehensive assessment of the crystal structure and its stability by taking advantage of the excellent traceability of the Ti element through X-ray measurements to achieve this aim.