Cathode Materials for High Energy Density Lithium Batteries

Abstract

Two classes of cathode materials have been developed for high energy density applications. The Li-rich layered oxide material with the general formula Li1+xM1-xO2 (M = Ni, Mn, Co) and the lithium manganese silicate Li2MnSiO4. Both materials have theoretical capacities higher than commercialized ones, which may give rise to higher energy density batteries. Li-rich materials have been prepared by solid state and co-precipitation routes. Transmission Electron Microscopy (TEM) characterization showed an irreversible evolution of the structure through a spinel phase during the first charge. Electron Energy Loss Spectroscopy (EELS) also showed a continuous cation migration during cycling of the material leading to charge/discharge voltage decay. The redox process has been studied by X-Ray Diffraction (XRD) in synchrotron facilities (ESRF, Grenoble, France). Ni/Mn ratio has been identified to have a great role on capacity fading of the material. Finally, a Li-rich optimized composition has been prepared and stable reversible capacity of 250 mAh.g-1 has been obtained. Li2MnSiO4 has a large theoretical specific capacity (333 mAh/g) through exchange of 2 lithium ions per formula unit. The thermal stability due to strong Si-O bonds makes LiMnSiO a very promising material for future energy storage in space applications. Preparation in inert atmosphere showed beneficial improvements of LMSO’s electrochemical properties. Nano-sizing and carbon coating have been effective ways to improve electronic conductivity and therefore electrochemical performance. Up to 1.66 Li per formula unit can be re-inserted in the 1st cycle. XRD analysis showed complete amorphization of Li2MnSiO4 after the 1st charge at 4.8 V with complete modification of the charge/discharge curves in the next cycles. Increasing the carbon coating ratio limits capacity loss during cycling but did not avoid amorphization. Finally influence of voltage window on structure stability was investigated. Careful choice of upper limit voltage has been showed to stabilize Li2MnSiO4 structure but for now is still limited to low Li+ insertion/extraction from the host material.