碳包覆LiFe0.5Co0.5PO4固溶体正极材料的制备及其电化学性能

分别以四水磷酸铁（FePO₄·4H₂O）和二水草酸亚铁（FeC₂O₄·2H₂O）为铁源，采用简单便捷的流变相法制备了碳包覆LiFe_0.5Co_0.5PO₄固溶体材料（LiFe_0.5Co_0.5PO₄/C，简称为LFCP/C）。采用X射线衍射（XRD）、扫描电镜（SEM）、透射电镜（TEM）、恒流充放电等测试手段对复合材料的物相、形貌结构和电化学性能进行了表征和测试。结果表明，2种铁源得到的材料均为橄榄石晶型结构且结晶度良好，二者在颗粒尺寸分布、碳包覆效果和电化学性能方面具有显著的差别。用作锂离子电池正极材料时，以FeC₂O₄·2H₂O为原料得到的LFCP/C具有更优异的电性能：在2.5~5.0 V电压范围内，0.1C倍率下（1C=150 mA·g^-1），放电比容量为137.5 mAh·g^-1，在10C仍具有57.6 mAh·g^-1的放电比容量；0.5C循环100次后容量仍保持78.1%。该样品更佳的电化学性能主要得益于其更小的平均颗粒尺寸，更高的比表面积和理想的碳包覆效果。

Synthesis and Electrochemical Performances of Carbon Coated LiFe0.5Co0.5PO4 Solid Solution as Cathode Materials

Cabon coated LiFe_0.5Co_0.5PO₄ solid solution (LiFe_0.5Co_0.5PO₄/C, LFCP/C) were synthesized via a facile rheological phase method using iron phosphate tetrahydrate (FePO₄·4H₂O) and iron oxalate dihydrate (FeC₂O₄·2H₂O) as iron source, respectively. The phase composition, particle morphology and electrochemical performance for the as-prepared materials were characterized by methods including X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and galvanostatic charge-discharge measurements. Results demonstrate that both LFCP/C samples possess olivine structure with high crystallinity, while there are significant difference in particle size distribution, carbon coating effect and electrochemical properties. As cathode for lithium ion batteries (LIBs), the LFCP/C obtained by FeC₂O₄·2H₂O as reactant exhibits better electrochemical performance than that by FePO₄·4H₂O, delivering a specific discharge capacity of 137.5 mAh·g^-1 at 0.1C (1C=150 mA·g^-1) rate in the voltage range of 2.5~5.0 V, and even at 10C rate, a specific capacity of 57.6 mAh·g^-1 is still maintained. Meanwhile, it shows excellent cyclability with a capacity retention rate of 78.1% after 100 cycles at 0.5C. The better electrochemical performance of LFCP/C obtained by FeC₂O₄·2H₂O can be mainly ascribed to its smaller average particle size, higher BET specific surface area, and more appealing carbon coating effect.