Novel strategies to improve the structural and electrochemical stability of Li2CoSiO4 during cycling

Li2MSiO4 (M = Co, Fe, Mn) is an interesting group of cathode materials for lithium-ion batteries (LIB) because of its high theoretical capacity (about 300 mAh g(-1) Li2CoSIO4 (LCS) is the most interesting of the three, owing to its high redox potential and hence the higher operating voltage of the cell compared to Li2FeSiO4 and Li2MnSiO4. However, LCS suffers from severe structural stability problems during electrochemical cycling. After the first charging, an irreversible structural rearrangement in the material leads to a highly irreversible capacity loss, followed by drastic capacity fading, as has been reported in several papers. In this work, we show that the structure of LCS could be preserved by partially substituting lithium with sodium. The difference in the atomic size of these two alkali elements stabilizes the structure during electrochemical cycling. LCS, 10% Na-substituted LCS (LNCS), 20% Na-substituted LCS (LN2CS) and a composite of LNCS and multi-walled carbon nanotubes (LNCS-CNT) were prepared by the sol-gel method. The LNCS cathode shows an improved electrochemical performance (first discharge capacity of 135 mAh g(-1)) when compared to pristine LCS (40 mAh g(-1) first discharge capacity). To improve the electronic conductivity of LNCS, functionalized MWCNTs were added. Among the four samples, LNCS-CNT showed enhanced electrochemical performance with a charge and discharge capacity of 218 and 186 mAh g(-1), respectively, with the lithium diffusion co-efficient in the order of 10(-13) cm2 s(-1).