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A detailed electrochemical study of Li insertion in the α-form of VOPO4 and the optimization of the cycling performance are presented. The redox process occurs in one step close to 3.76 V, along with a phase transition. In order to improve the intercalation kinetics, the electronic conductivity was optimized by introducing a mixed valency, and the ionic conductivity was favored by introducing ‘pillaring’ molecules or ions in the interlayer space. In this way, the electrochemical behaviors of α-VOPO4·2H2O, α-NaxVOPO4, α-KxVOPO4 and α-MgxVOPO4 (0≤x≤1) have been studied: the hydrate compound shows a good specific capacity (100 mA h/g at a C/5 regime), but a poor cyclability was mainly because of water oxidation. The inserted large alkaline ions improved the cyclability up to 80 cycles (Na+) or over 100 cycles (K+). Enhancements of the VOPO4 specific capacity have been obtained as well by mechanical ball-millings. |
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