Molybdenum Substituted Vanadyl Phosphate ε-VOPO4 with Enhanced Two-Electron Transfer Reversibility and Kinetics for Lithium-Ion Batteries

详细信息    查看全文

• 作者：Bohua Wen ; Qi Wang ; Yuhchieh Lin ; Natasha A. Chernova ; Khim Karki ; Youngmin Chung ; Fredrick Omenya ; Shawn Sallis ; Louis F. J. Piper ; Shyue Ping Ong ; M. S. Whittingham
• 刊名：Chemistry of Materials
• 出版年：2016
• 出版时间：May 10, 2016
• 年：2016
• 卷：28
• 期：9
• 页码：3159-3170
• 全文大小：796K
• 年卷期：0
• ISSN：1520-5002

文摘

We have investigated the possibility of molybdenum substitution into ε-VOPO₄ structure and its effects on the electrochemical performance of this material as a cathode in Li-ion battery. We have found that up to 5% of Mo can substitute V upon hydrothermal synthesis at 180 °C with further annealing at 550 °C. The substitution is confirmed by the increase of the unit cell volume with Mo content. A combination of X-ray absorption and photoelectron spectroscopy, magnetic studies, and density functional theory calculations indicates an Mo⁶⁺ oxidation state which is charge compensated by reduction of the same amount of V to 4+. Mo-substituted samples show much smaller particle size as compared to unsubstituted ε-VOPO₄ and significantly improved electrochemical behavior. ε-V_0.95Mo_0.05OPO₄ shows the initial reversible capacity ∼250 mAh/g (∼1.6 Li) and ∼80% retention for up to 20 cycles at C/25. Sloping voltage profile, faster kinetics, and lower voltage hysteresis of Mo substituted VOPO₄ are demonstrated by the galvanostatic intermittent titration technique. This enhanced electrochemical performance is attributed to the smaller particles and possible existence of partial Li_xMo_yV_1–yOPO₄ solid solution supported by X-ray diffraction, which leads to less abrupt and completely reversible structure changes upon Li cycling evidenced by X-ray absorption spectroscopy.