两步固相法合成LiFe1-xNbxPO4/C复合正极材料及其电化学性能

以LiH₂PO₄和FeC₂O₄·2H₂O为原料, 采用分步添加聚乙烯醇和葡萄糖两种碳源的方式, 通过两步固相法合成了碳包覆的LiFePO₄材料. 700℃下处理的产物结晶良好, 颗粒分布均匀, 具有良好的电化学性能, 0.1C和1C倍率下放电比容量分别为157.3 和138.3 mAh·g^-1. 在碳包覆的基础上, 选择高价Nb⁵⁺进行铁位取代获得了复合改性的LiFe_1-xNb_xPO₄/C (x=0.005, 0.01, 0.015, 0.02)材料. 优化的LiFe_0.99Nb_0.01PO₄/C 材料显示了良好的倍率充放电能力和循环稳定性, 0.1C和5C倍率下放电比容量分别为160.5 和136.0 mAh·g^-1, 5C倍率下循环50 次后比容量保持在134.8 mAh·g^-1, 容量保持率为99.1%. 循环伏安测试结果表明, Nb⁵⁺离子掺杂减少了锂离子扩散阻力, 降低了充放电过程中的动力学限制, 提高了电极的可逆性.

Synthesis and Electrochemical Properties of LiFe1-xNbxPO4/C Composite Cathode Material by Two-Step Synthesis Route

LiFePO₄/C composite was prepared by two-step synthesis route, with LiH₂PO₄ and FeC₂O₄· 2H₂O as starting materials. In the process, the carbon sources polyvinyl alcohol (PVA) and glucose were added in a stepwise fashion. The well-crystallized LiFePO₄/C composite with homogeneous small particles was obtained after reacting at 700℃ for 4 h. This composite had a discharge specific capacity of 157.3 mAh·g^-1 at 0.1C rate and 138.4 mAh·g^-1 at 1C rate. On the basis of carbon coating modification, Nb-ion-doped LiFe_1-xNb_xPO₄/C (x=0.005, 0.01, 0.015, 0.02) composites were prepared. The optimized LiFe_0.99Nb_0.01PO₄/C cathode displayed a discharge specific capacity 160.5 mAh·g^-1 at 0.1C discharge rate, 136.0 mAh·g^-1 at 5C and maintained 134.8 mAh·g^-1 after 50 cycles, showing good rate properties and cycling stability. Cyclic voltammetry (CV) measurements indicated that aliovalent dopant substituting on the Fe sites can reduce the resistance of Li ion diffusion in the electrode process, increase phase transformation kinetics during cycling, and enhance the reversibility of LiFePO₄ electrodes.