硫掺杂Na_(3)(VOPO_(4))_(2)F正极材料的制备及储钠性能

聚阴离子型Na_(3)(VOPO_(4))_(2)F材料具有结构稳定、安全性高及工作电压高等特性,其开放的三维框架结构可以为钠离子的快速迁移提供路径,是目前最具发展潜力的钠离子电池正极材料之一。然而,Na_(3)(VOPO_(4))_(2)F的本征电子导电性较差,导致倍率性能不理想。离子掺杂是一种提升材料导电性和电化学性能的有效策略。通过水热法成功制备了S^(2-)掺杂的Na_(3)(VOPO_(4))_(2)S_(x)F材料。XRD和电化学阻抗结果表明,S^(2-)掺杂可以扩大离子扩散通道并降低电荷传输电阻;恒电流间歇滴定测试结果显示S^(2-)掺杂可以加快离子迁移速率。因此,Na_(3)(VOPO_(4))_(2)S_(x)F正极材料在钠离子半电池中表现出优异的电化学性能,在30C倍率下可逆比容量为66.8mA·h/g,在10C倍率下循环500次后容量保持率可达96%。Na_(3)(VOPO_(4))_(2)S_(x)F正极与硬碳负极组成的钠离子全电池可获得121.7mA·h/g的高比容量,在1C倍率下循环60次后,其容量衰减可以忽略不计。

Improved rate and cycling performances of Na_(3)V_(2)(PO_(4))_(2)F_(2)O by Ti^(3+/4+)doping with two oxidation states for sodium cathodes

Ti at the oxidation states of Ti^(3+)and Ti^(4+),was used to enhance the performance of Na_(3)V_(2)(PO_(4))_(2)F_(2)O by partially substituting vanadium.After doping Ti,the crystallographic volume is decreased due to the less radii of Ti^(3+/4+),and the valence of Ti is demonstrated identical to V.During sodium insertion in Ti-doped Na_(3)V_(2)(PO_(4))_(2)F_(2)O,the two discharge plateaus split into three because of the rearrangement of local redox environment.Consequently,the optimized Na_(3)V_(0.96)Ti_(0.04)(PO_(4))_(2)F_(2)O shows a specific capacity of 123 and 63 mA·h/g at 0.1C and 20C,respectively.After 350 cycles at 0.5C,the capacity is gradually reduced corresponding to a retention of 71.05%.The significantly improved performance is attributed to the rapid electrochemical kinetics,and showcases the strategy of replacing V^(3+/4+)with Ti^(3+/4+)for high-performance vanadium-based oxyfluorophosphates.