【摘要】 为了研究碳包覆对LiFePO4结构的影响,以柠檬酸为碳源,采用机械活化-高温固相法,合成了不同碳包覆量的LiFePO4/C复合正极材料.通过XRD、SEM、BET、HRTEM、选区电子衍射(SAED)、交流阻抗谱(ACI)和恒电流充放电等现代分析方法,全面研究了碳包覆量不同时,LiFePO4/C复合正极材料的结构、形貌和电化学性能,并对C包覆对结构影响的成因进行了分析.结果表明,柠檬酸高温分解后生成无定形碳非晶物质,在LiFePO4颗粒表面包覆形成一种网络结构,抑制了颗粒的生长;C包覆影响了晶体的生长方向和微观结构,LiFePO4/C的优势生长为[121]方向;交流阻抗分析表明包覆后锂离子扩散系数比未改性的LiFePO4提高了两个数量级,且各项阻抗值均降低,从而提高了材料的离子及电子电导性、放电性能和循环性能.更多还原

【Abstract】 LiFePO4/C composite cathode materials containing different amounts of carbon were fabricated by mechanical activation-high temperature solid state reaction using citric acid as a carbon source,with the aim to study the effect of carbon-coating on LiFePO4 internal structure.The crystal structure,surface morphology and electrochemical properties of the obtained composites coated with various carbon contents were investigated by the techniques of XRD,SEM,BET,HRTEM,SAED,AC Impedance spectra,and constant current charge-discharge testing.And the structural changes originating from the carbon-coating were also analyzed.Under high temperature,citric acid was decomposed into network-like structure amorphous carbon,and coated on the surface of LiFePO4 particles,which could inhibit the grain growth and cause the smaller sizes.The crystal growth direction and micro-structure of LiFePO4/C were significantly influenced by carbon-coating that resulted in growing along direction preferentially.According to AC Impedance spectra measurements,all the impedances of carbon coated LiFePO4 decreased evidently,and the diffusion coefficient of Li ions increased by up to two orders of magnitude compared with uncoated LiFePO4,indicating the ion and electric conductivity,discharge capacity and cycling performance were greatly improved.更多还原