4V级锂离子电池用橄榄石型LiMnPO_4的电化学性能

Electrochemical performance of 4 V olivine LiMnPO_4 for the cathode material of lithium ion batteries

对反应物与中间产物进行球磨,采用固相反应法在600℃合成了掺碳的橄榄石型LiMnPO4。通过XRD表征样品的晶体结构,采用SEM观察样品的微观形貌,利用电化学手段测试样品的充放电性能,并对样品进行交流阻抗和扩散系数的测定。研究结果表明,得到的样品物相较纯,粒径小(100～200nm)且分布均匀,首次放电容量接近100mA.h.g-1,但样品循环容量衰减快,大电流放电性能较差。通过对样品的交流阻抗测试发现,电化学反应阻抗随放电的进行而不断增大,说明材料的荷电量越高,界面电化学反应速度越快。扩散系数的测量结果表明,充电态和放电态的扩散系数分别1.2×10-12和5×10-13cm2.s-1,表明晶格中锂离子的浓度越高,越容易脱出。

Olivine LiMnPO4 was synthesized at 600 ℃ by the method of solid-state reaction combining with the addition of carbon black and ball-milling of reagents and precursors. Structure, surface morphology and charge/discharge performance of LiMnPO4 were characterized by x-ray diffraction （XRD）, scanning electron microscope （SEM）, and electrochemical measurement respectively. AC impedance and diffusion coefficient were also measured by electrochemical method. The results indicate that the synthesized sample has pure olivine structure with the small particle size of about 100 - 200 nm. The initial discharge capacity of the sample is approximately 100 mA · h · g^-1. However, the capacity decreases rapidly with the cycling of charge and discharge, and the rate capability is also poor. The electrochemical reaction resistance increases with the deintercalation of lithium ions, which indicates that electrochemical reaction occurring at the interface is enhanced with more lithium ions occupying in the materials. Diffusion coefficients in the charged state and fully discharged state are 1.2× 10-12 cm^2 · s^-1 and 5×10^-13 cm^2 · s^-1 respectively. It is suggested that the higher the concentration of lithium ions in the lattice is, the more easily lithium ions are deintercalated out of materials.