Impact of Carbon Coating on the Electrochemical Property of Li2MnSiO4/C Cathode Material By Sol-Gel Method

Abstract

Lithium ion batteries have been widely used on the portable electronic devices due to its high energy density and long cycle life. In order to meets the market requests, various cathode materials such as lithium transition metal oxide (LiMO2, M=Mn、Co) and polyanion cathodes ( LiMPO4,Li2MSiO4, M=Mn、Co、Fe ) have been investigated. The lithium transition metal oxide is easily to be collapsed during the charge-discharge process because of the layer structure. The advantages of polyanion cathodes are the better stability than lithium transition metal oxides due to strong covalent bonds ( ex：P─O、Si─O) and environment more friendly, lower safety risk as well as lower cost. On the other hand, the lithium transition metal orthosilicates, Li2MSiO4 ( M=Fe, Co or Mn), which offers the possibility of two insertion/extraction lithium ions during charge-discharge, have higher theoretical capacity than the phosphate cathode materials and had been considering as promising cathode materials. Moreover, both Li2FeSiO4 and Li2MnSiO4 are more attractive cathode materials among the orthosilicates but Li2MnSiO4 can offer higher power density than Li2FeSiO4 because the Mn2+/Mn3+, Mn3+/Mn4+ redox potential is higher than Fe2+/Fe3+, Fe3+/Fe4+ redox potential.Nevertheless, Li2MnSiO4 also has some drawbacks that include poor electronic conductivity and Jahn-Teller distortion, resulting in collapse of the structure and formation of the amorphous Li2MnSiO4. To solve this problem, following methods have been suggested: 1) decreasing the particle size 2) coating a conductive carbon layer 3) doping transition metal. However, it is found that the characteristics, particle size, morphology etc., of the products are highly dependent on the synthesis method applied. In this study, the sol-gel method is used with polyvinylpyrrolidone (PVP) as the in-situ carbon coating material to obtain the nanoscale primary particles with lower calcination temperature and without pressure that can lower the cost. Various amounts of PVP are used in Li2MnSiO4 solution for the sol-gel process to adjust the carbon content of the Li2MnSiO4/C cathode materials in the range of 3~6 wt%. The XRD spectra show that all the Li2MnSiO4/C materials synthesized are in Pmn21 structure. The TEM images indicate that a conducting amorphous carbon layer is deposited on the particle surface that improves the electronic conductivity of the Li2MnSiO4 cathode materials. In addition, it is found that the as –prepared LMS/C exhibits high capacity at 0.05C in the voltage of 1.5–4.8V and strong redox peaks in the cyclic-voltammetry curves. This is ascribed to the homogeneous carbon coating and nano-size primary particle that enhances the electronic conductivity and shortens the Li+ ion diffusion path, respectively. Acknowledgements : The authors gratefully acknowledge the Ministry of Science and Technology, Taiwan, R. O. C. and Chung Yuan Christian University for supporting this research work.