Study On The Doped LiMnPO₄Used As Cathode Materials For Lithium Ion Battery

Olivine-structured LiMnPO4is considered as the most promising candidate forcathode materials of Li-ion batteries due to its high theoretical specific capacity, goodthermal stability, excellent safety, wide material sources, low cost and benignity to theenvironment. However, LiMnPO4synthesized via different methods shows differentelectrochemical performances, and LiMnPO4shows low reversible capacity and poorcharge-discharge property at high discharging rate due to its poor conductivity. Inorder to overcome these shortcomings, some measures such as different syntheticmethods and cation-doping were taken to improve the electrochemical performancesof LiMnPO4in this thesis.In this thesis, LiMnPO4/C, LiMnPO4/C/Ag, and Li(1-2x)MxMnPO4/C（M=Mg、Ni、Cu）were synthesized by coprecipitation method and sol-gel method. The crystalstructures and morphologies of these materials were investigated by IR, XRD andESEM. The electrochemical performances were measured by galvanostaticcharge-discharge and AC Impedance tests. The effects of sintering temperature,sintering time, carbon content, doping ratio, etc. on the crystal structures andelectrochemical performances were studied.The results of the structural analysis indicated that all the samples synthesized bycoprecipitation method and sol-gel method sintered at different temperatures in airwere of olivine structure with space group of Pmnb. ESEM analysis indicated thatcarbon content could have great impact on the morphologies of the samples, and thatthe particle size decreased with the elevation of carbon content.The results of electrochemical measurements showed that the performances ofsamples synthesized by sol-gel method were better than those of prepared bycoprecipitation method. For the samples prepared by sol-gel method, those with20wt%conductive carbon added to the precursors possessed the best performance, butthe optimal sintering temperatures varied slightly with various compositons. Theundoped LiMnPO4/C sintered at400℃possessed the best performance, when testedat1/50C rate, its discharge capacity reached156mAh/g, which was about92%of thetheoretical capacity, and maintained at51mAh/g after60cycles. For the systems of Li(1-2x)MxMnPO4/C （M=Mg、 Ni、 Cu）, the sample Li0.94Mg0.03MnPO4/C andLi0.84Ni0.08MnPO4/C sintered at400℃possessed the best electrochemical properties.When tested at1/50C rates, both initial discharge capacity reached146mAh/g, andremained116mAh/g and80mAh/g respectively after60cycles. The sampleLi0.98Cu0.01MnPO4/C sintered at500℃possessed the best performance, when tested at1/50C rate, its discharge capacity reached98mAh/g, and maintained at91mAh/gafter43cycles with93%capacity retained. The above results confirmed that thereversible capacity and cycle performance of LiMnPO4had been greatly improved bycarbon-coating and cation-doping.