Study On Preparation And Properties Of Li₂MnSiO₄/C Cathode Material Synthesized By Sol–gel Method

Silicate lithium battery cathode materials have attracted extensive attention of the researchers, because of the advantages of lower-cost, environmentally benign, good electrochemical performance, and the higher theoretical capacity especially. Lithium manganese silicate(Li2Mn Si O4) cathode material is one of the representatives of silicate battery cathode materials, which the theoretical capacity reached 333 m Ah/g. Li2 Mn Si O4 cathode material synthesized by sol-gel method, and through coating different carbon sources(glucose, sucrose or liquid polyacrylonitrile), adding polyethylene glycol(PEG) and Ni2+ doping method improved the conductivity, ion migration rate and cycle performance of Li2 Mn Si O4 cathode material. The synthesized materials were characterized by TG, XRD, SEM, galvanostatic charge-discharge test, electrochemical impedance spectroscopy and constant current charging and discharging test. The influence of presintering temperature, sintering temperature and holding time, different proportions of coating carbon source, different ratios of PEG and different proportions of doping Ni2+ on electrochemical properties of synthesized cathode materials were explored.(1) Li2 Mn Si O4 cathode material precursors were synthesized by sol-gel method, using CH3COOLi·2H2O,(CH3COO)2Mn·4H2O and ethylorthosilicate as raw materials, CH3 COOH as catalyst, and the cathode materials were obtained through sintering of precursors in argon atmosphere. The synthesized materials were characterized by XRD, the characteristic peaks were consistent with the standard spectrogram, but the electrochemical performance of materials was very poor, the first discharge capacity was 14.1 m Ah/g.(2) Li2 Mn Si O4 cathode material coating different carbon sources were synthesized by sol-gel method, and the presintering temperature, sintering temperature and holding time were studied: the best sintering temperature of Li2 Mn Si O4 cathode material did not change much using different carbon sources, the best presintering temperature was 400 oC, and the best sintering temperature was 700 oC, When using LPAN as the carbon source, the particle size of Li2 Mn Si O4/C cathode material was smaller, uniform, better crystallization property and high-purity was observed. The first discharge capacity was 225.6 m Ah/g, after 50 times circulation the discharge capacity was 86.1 m Ah/g, the retention rate was 38.2% and the ac impedance was about 180 ?.(3) Changing the amount of PEG during the coating of Li2 Mn Si O4 cathode material precursors with LPAN, a series of Li2 Mn Si O4/C cathode materials were synthesized. The results of the characterization of SEM, XRD, EIS and electrochemical performance test concluded that addition of PEG could effectively reduce the particle size of materials, and make the particle size distribution more homogenous. When every mol of Li2 Mn Si O4/C cathode material precursor was added 100 g PEG, the first discharge capacity could reach 235.6 m Ah/g, after 50 times circulation the discharge capacity still was 101.1m Ah/g, the retention rate was 42.9% and the ac impedance reduced to around 100 ?.(4) Using sol-gel method, we synthesized Ni2+ doping Li2 Mn Si O4 cathode material precursors. The ratio of Ni2+ with Mn2+was maintained at x:(1-x), x=0.04, 0.06, 0.08, 0.10 or 0.15. The obtained precursors were annealed at 400 oC for 2 h and 700 oC for 8 h in argon atmosphere, the Li2 Nix Mn(1-x)Si O4/C cathode materials were synthesized. Doping Ni2+ could improve the electrical conductivity of the material by the results of the characterization of SEM, XRD, EIS and electrochemical performance test. When the doping proportion was 0.08, its first discharge capacity could reach 300.1 m Ah/g, after 50 times circulation the discharge capacity was 92.5 m Ah/g, the retention rate was 30.8%.