Synthesis And Modification Of Lithium Manganum Phosphate Cathode Material For Lithium-ion Batteries

LiMnPO₄, one of the most important member of LiMPO₄（M=Mn, Fe, Co, Ni）, attractsmore and more attention. It has many advantages such as low cost, environmentalfriendliness and stable electrochemical performance. The theoretical capacity and theoperation potential （vs. Li/Li+） of LiMnPO₄is170mAh/g and4.1V, respectively. In thispaper, both solid state method and sol-gel method are employed to prepare LiMnPO₄.Moreover, cation doping on both Mn site and Li site are employed to modify LiMnPO₄.The amount of oxalate was adjusted to improve the morphological and electrochemicalcharacteristic of LiMnPO₄when the solid state method was used to synthesize LiMnPO₄.The results show that the best morphological characteristic was obtained when theoxalate/Mn ratio is2:1. Several carbon sources were employed to coat carbon on LiMnPO₄,and the carbon contents were optimized. The result confirmed that the electrochemicalperformance of LiMnPO₄prepared by solid state method can barely be improved by carboncoating.LiMnPO₄/C nanoparticles were prepared by a citric acid assisted sol-gel method. Citricacid acts as both a chelating agent in the precursor preparation and a carbon source in thefollowing calcination. The effects of synthetic conditions such as citric acid content andcalcination temperature on the structure, morphology and electrochemical performance ofLiMnPO₄/C were investigated. Higher citric acid content facilitates the formation ofuniform nanoparticles, which is benefit to the electrochemical property of LiMnPO₄.However, higher calcinations temperature results in better crystallinity and larger particlesize. Therefore, the capacity first increases and then decreases with increasing calcinationtemperature. The best electrochemical performance were obtained when the ratio of citricacid to manganese is2.0and the calcinations temperature is650℃.Cation doping was applied on both Mn site abd Li site. The electrochemicalperformance of LiMnPO₄can be greatly improved by cation doping. When Mn ispartiallysubstituted by Fe, LiMn_0.5Fe_0.5PO₄/C has the best electrochemical performance at lowcurrent density. The first charge capacity and the first discharge capacity are169.5mAh/gand150.9mAh/g respectively, thus the coulombic efficiency of the first cycle is89%. Thecapacity retention after50charge-discharge cycling is95.7%. When Li is partiallysubstituted by Na (Li_1-xNa_xMnPO₄/C, x=0,_0.02,0.04,0.06,0.08), the specific capacity of Li_0.98Na_0.02MnPO₄/C is the highest. When the current density is low, the first chargecapacity and the first discharge capacity of Li_0.98Na_0.02MnPO₄/C is159.3mAh/g and154.2mAh/g, respectively, thus the coulombic efficiency of the first cycle is96.7%.