超临界溶剂热合成纳米磷酸铁锂晶体生长机制探究

Supercritical Solvothermal Synthesis and Growth Mechanism of LiFePO_(4)Nanocrystals

针对目前超临界溶剂热合成纳米磷酸铁锂(LiFePO_(4))的晶体生长机制及反应参数影响规律尚不明确的问题,采用超临界溶剂热合成方法在340~400℃反应温度下制备了平均粒径为68~85 nm的LiFePO_(4)粉体,表征并分析了产物的晶体结构及粒径等性能,评估了超临界乙醇中纳米LiFePO_(4)晶体生长过程表观活化能及生长机制,并建立了纳米LiFePO_(4)晶体生长动力学。结果表明,纳米LiFePO_(4)在超临界溶剂中的生长过程对其最终尺寸起主导作用,且生长过程受表面反应控制,符合多核控制的二阶生长动力学。纳米LiFePO_(4)产物的电化学性能测试表明,在400℃下获得的样品在0.1 C(C为电池充放电倍率)倍率下的初始放电比容量达到153.5 mA·h/g,放电倍率提高至5 C时容量保持率为68.3%。在340℃反应温度下:所得产物中的杂质(Li_(3)Fe_(2)(PO_(4))_(3)和Fe_(3)(PO_(4))_(2))导致电池出现了严重的极化效应;在高速充放电下,容量严重衰减,5 C倍率容量保持率仅为46.9%。本研究揭示了超临界溶剂热合成纳米LiFePO_(4)晶体的生长机制,可为后续工艺参数优化及工业化应用提供理论支撑。

At present,the growth mechanism of supercritical solvothermal synthetic lithium iron phosphate(LiFePO_(4))nanocrystals and the influence of reaction parameters are not clear.Given this circumstance,LiFePO_(4)powder with average particle size of 68-85 nm is prepared by supercritical solvothermal synthesis at reaction temperature of 340—400℃.The crystal structure and particle size of the product are characterized and analyzed,and the apparent activation energy and growth mechanism of LiFePO_(4)nanocrystals growth process are evaluated.On this basis,the growth kinetics of LiFePO_(4)nanocrystals is established.The results show that the growth process of nano LiFePO_(4)in supercritical solvent plays a dominant role in the final crystal size,and the growth process is controlled by the surface reaction,which conforms to the multinucleus-controlled second-order growth kinetics.In electrochemical performance tests,sample obtained at 400℃delivers an initial discharge specific capacity of 153.5 mA·h/g at 0.1 C(C referring to charge and discharge rates of the battery),and the capacity retention is 68.3%at 5 C.Impurities(Li_(3)Fe_(2)(PO_(4))_(3) and Fe_(3)(PO_(4))_(2))in the samples obtained at the reaction temperature of 340℃results in severe battery polarization.In high-speed charge-discharge,critical capacity decay is witnessed,ending up with a capacity retention of only 46.9%at 5 C.This study reveals the growth mechanism of LiFePO_(4)nanocrystals synthesized by supercritical solvothermal method,and lays a theoretical foundation for the subsequent process parameter optimization and even industrial application.