Solvothermal synthesis of Mg-doped Li$_{\mathrm{2}}$FeSiO$_{\mathrm{4}}$/C nanocomposite cathode materials for lithium-ion batteries

Lithium transition metal orthosilicates, such as Li$_{\mathrm{2}}$FeSiO$_{\mathrm{4\thinspace }}$and Li$_{\mathrm{2}}$MnSiO$_{\mathrm{4}}$, as cathode material have attracted much attention lately due to their high theoretical capacity (\textasciitilde 330 mAh/g), low cost, and environmental friendliness. However, they suffer from poor electronic conductivity and slow lithium ion diffusion in the solid phase. Several cation-doped orthosilicates have been studied to improve their electrochemical performance. We have synthesized partially Mg-substituted Li$_{\mathrm{2}}$Mg$_{x}$Fe$_{\mathrm{1-}}_{x}$SiO$_{\mathrm{4}}$-C, ( $x =$ 0.0, 0.01, 0.02, and 0.04) nano-composites by solvothermal method followed by annealing at 600$^{\mathrm{o}}$C in argon flow. The structure and morphology of the composites were characterized by XRD, SEM and TEM. The surface area and pore size distribution were measured by using N$_{\mathrm{2}}$ adsorption/desorption curves. The electrochemical performance of the Li$_{\mathrm{2}}$Mg$_{x}$Fe$_{\mathrm{1-}}_{x}$SiO$_{\mathrm{4}}$-C composites was evaluated by Galvanostatic cycling against metallic lithium anode, electrochemical impedance spectroscopy, and cyclic voltammetry. Li$_{\mathrm{2}}$Mg$_{\mathrm{0.01}}$Fe$_{\mathrm{0.99}}$SiO$_{\mathrm{4}}$-C sample shows a capacity of \textasciitilde 278 mAh/g (at C/30 rate in the 1.5-4.6 V voltage window) with an excellent rate capability and stability, compared to the other samples. We attribute this observation to its higher surface area, enhanced electronic conductivity and higher lithium ion diffusion coefficient.