Structure And Electrochemical Performance Of Dual Ions Co-doped LiMnPO₄

Olivine structured manganese phosphate (LiMnP04) has become a potential applications of cathode material for lithium-ion battery due to its low cost, environmentally benife, high operating voltage (4.1 V), high energy density (697Wh · kg-1) and good compatibility with most liquid electrolytes presently used. However, LiMnP04 has a very poor electronic conductivity, which limits its electrochemical properties at rapid charge/discharge and hinders its commercialization.Now, carbon coating and dual ions co-doping were applied to improve the performance of LiMnPO4/C. The doped samples were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, galvanostatic charge/discharge, cyclic voltammetry and electrochemical impedance spectroscopy.The results indicate that Fe2+-Ti4+-Fe2+-V3+, Fe2+-Nb5+and Fe2+-Al3+ co-doping are in the form of solid solution anf into the LiMnPO4 lattice, leading to the shrink or expand of the lattice cell. Among those dual ions doped samples, Li(Mn0.85Fe0.15)0.92Ti0.08PO4//C, Li(Mn0.85Fe0.15)0.95 V0.05PO4/C, Li0.995Nb0.0055Mn0.85Fe0.15PO4/C and Lio.995Alo.oo5Mno.85Fe0.15PO4/C exhibit a discharge capacity of 139.8,146.8,143.1 and 141.4 mAh·g-1 with the capacity retention ration of 102.3%、99.5%、100.9% and 91.2% after 50 cycles at 1 C and 25 ℃, and they also give a capacity of 166.0,165.5,161.3 and 154.4 mAh·g-1 at 1C and 60 ℃, respectively. Even at 5 C, Li(Mno.85Feo.i5)0.92Ti0.08PO4/C, Li(Mn0.8sFe0.15)0.95 V0.05PO4/C, Lio.995Nbo.oosMno.85Feo.15PO4C and Lio.995Alo.oo5Mno.85Feo.i5PO4/C still delivere 100,71,97 and 61 mAh·g-1, respectively. The charge and discharge results show that dual ions doping can significantly improve the electrochemical performance of LiMnPO4/C. Electrochemical impedance spectroscopy and cyclic voltammetry plots demonstrate that dual ion doping can reduce the resistance of lithium ion insertion-extraction process and the degree of polarization, increase the lithium-ion diffusion rate and make it easier to overcome the dynamics limit, resulting the improvement of the performance of LiMnPO4/C.