Comparative Studies on the Electrochemical Properties of Li2MnSiO4/C and Li2Mn1-x-yFexMySiO4/C
Large scale lithium secondary batteries are required for the plug-in hybrid electric vehicles (PHEV) and electric vehicles (EV). However, currently used cathode materials cannot satisfy the energy density requirement for the PHEV or EV applications as well as cost and abuse tolerance requirements. The cathode materials superior to the currently used ones are required. The superior specifications include energy density, cycle life, lower cost, and abuse tolerance. Among the possible candidates for the cathode materials for large scale lithium secondary batteries, polyanion-type silicate is one of the materials that needs to be studied in depth due to its high theoretical capacity above 300 mAh/g and superior abuse tolerance. Li2MnSiO4 is one of the potentially promising candidates since it has high theoretical capacity (333 mAhg) and high operating voltage. However, it shows poor electrochemical performance after 1 charging and poor capacity retention. The practical capacity reported in the literature remains around 200 mAh/g at 60C. Many researches have tried to enhance the electrical conductivity and practical capacity of Li2MnSiO4 by Fe doping but the capacity retention has not been improved satisfactorily yet. Yang et al. demonstrated that the initial capacity as high as 214 mAhg could be achieved for Li2MnxFe1-xSiO4/C at x=0.5. They suggested that the poor electrochemical properties of Li2MnSiO4/C was due to the phase change such as the Jahn-Teller distortion occurring during 1 charging based on their ex-situ XRD and ATR-IR studies. Dominko claimed that the poor electrochemical performance was attributed to the low conductivity (5 x 10 Scm) of Li2MnSiO4. They also suggested the possibility that more than one electron per formula unit of Li2(Fe/Mn)SiO4 can be involved in the electrochemical reaction. In this study, electrochemical performances of Li2MnSiO4/C and Li2Mn1-x-yFexMySiO4/C have been compared. In order to overcome the low electrical conductivity of the lithium orthosilicate, carbon coating was incorporated into Li2Mn1-xFexSiO4 by sucrose precursor and metal ion with 3+, 4+, 5+ valences substituted Mn or Fe sites. The doping is expected to change the local electronic structure of the Li2Mn1xFexSiO4 by means of quasi free electron, electron hole, and cation vacancy. Li2Mn1-x-yFexMySiO4/C (0<x<1, 0<y<1, M=M) were prepared by ball milling Li-Mn-Fe oxide, doping agents, and sucrose precursor mixture followed by calcination at 600~800°C. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for structural analysis and observation of the lithium orthosilicate particles before and after the carbon coating and metal ion doping. The XRD studies have been done at beamline 8C2 and 10B at Pohang Accelerator Laboratory (PAL). The electrochemical performance of the samples was tested using coin cells (2032 type). It is observed that the simultaneous treatment of carbon coating and metal ion doping improved the electrochemical performance of Li2MSiO4. More details will be discussed at the meeting.