Synthesis And Modification Of The Composite Cathode Material LiMnPO₄/C For Lithium-ion Batteries

Olivine structured LiMnPO4becomes one of the most promising cathode materials of lithium ion battery, owing to abundant source of raw material, high discharge plateau, good thermostability, and friendliness to environment etc. However, The utilization of LiMnPO4is difficult because of its poor rate performance and the metastable nature of the delithiated phase.In this paper, to improving the performance of LiMnPO4, tow efficient synthesis methods of carbon-coated LiMnPO4/C powders has been investigated in detail.A fast precipitation method was adopted for synthesis of nano-MnP04·H20, with MnSO4·H2O, H3PO4, NH4NO3and NaOH as raw materials. MnPO4·H2O precipitate was characterized by XRD (X-ray diffraction) and SEM (scanning electron microscope). Fine-sized, well-crystallized, carbon-coated LiMnPO4/C nano-composites were obtained by using mechanochemical activation assisted carbothermal reduction route from as-prepared MnPO4·H2O, Li2CO3and PVA (Polyvinyl alcohol). The effect of calcination temperature on the structure and properties of obtained materials was investigated by XRD, SEM, TEM (transmission electron microscopy) and electrochemical measurements. The in situ6.8wt%carbon coated LiMnPO4/C displayed discharge specific capacity of124mAhg-1at0.05C rate and108mAhg-1at1C rate. The capacity retention was nearly100%after20cycles at1C rate. This mechanochemical activation assisted precipitation technique was a facile approach for the fabrication of LiMnPO4cathode materials.With LiH2PO4and Mn powder as raw material, PVA as carbon source, uniform distribution and high reactivity precursor of [Mn3(PO4)2·xH2O+Li3PO4] was prepared by mechanical liquid activation precipitation method. Then carbon coating LiMnPO4material was obtained after roasting of as-prepared precursor. The impact of roasting temperature on the structure, microstructure and electrochemical performance of LiMnPO4/C has been investigated. The result of optimizing demonstrates that, uniform fine particles and well-crystallized LiMnPO4/C can be produced at650℃for4h, with initial discharge rate capacity of113.1mAhg-1at0.05C, but with poor rate performance and cycle performance. Therefore, Mn-site substitution LiMn0.95M0.05PO4/C (M＝Fe2+,V3+,Ti4+) was conducted by doping different valency ions. Among these alternatives, LiMn0.95Fe0.05PO4/C with Fe doping exhibits the highest discharge rate capacity of123.5mAhg-1(0.05C). Under the optimization conditions, LiMn0.95Fe0.05PO4/C displays discharge rate capacity of120.3and99.9mAhg-1, at0.1C and1C, respectively.At1C,the cycle discharge rate capacity is maintained well, with99.2mAhg-1after40times cycling, which demonstrates the good rate performance and cycle stability.Li1+xMn1-x/2PO4/C (x=-0.05-0.2) materials were synthesized by mechanical liquid activation precipitation method. The results show that Li1.05Mn0.975PO4/C displaying the best electrochemical capacity and rate capability. It delivers discharge capacity of123and more than110mAhg-1at0.05C and1C rate. Furthermore, it expresses excellent plateau at4.03V when charge and discharge at0.05C. The cycle discharge rate capacity is maintained very well, with nerly100mAhg-1after20times cycling. Furthermore, Li1.05Mn0.925Fe0.05PO4/C material displays a very good discharge capacity of132、109and90mAhg-1at0.05C、1C and2C rate, respectively, and also displays excellent plateau apound4.1V, The capacity retention is nearly100%after100cycles at1C rate.