Improved electrochemical performance of Li2FeSiO4/CNF/rGO nanocomposites for lithium ion batteries

We have investigated electrochemical performance of Li2FeSiO4 (LFS) nanocomposites prepared with carbon nanofibers (CNF) and reduced graphene oxide (rGO) as conducting fillers. LFS/CNF(10 wt%) and LFS/CNF(5 wt%)/rGO(5 wt%) nanocomposites were synthesized using solvothermal method followed by annealing at 600 °C for 4 h under continuous flow of argon. Rietveld refinement of X-ray diffraction data confirmed the formation of monoclinic P21/n LFS without any impurity phase. Although both samples exhibit very similar electrical conductivity of 1.2 ± 0.2 S·cm−1, LFS/CNF/rGO shows a higher surface area of 88 m2/g compared to 62 m2/g for LFS/CNF. Correspondingly, the glavanostatic charge-discharge curves show an improved discharge capacity of 260 mAh·g−1 and 135 mAh·g−1 at C/20 and 8 C rates for LFS/CNF/rGO, compared to 230 mAh·g−1 and 116 mAh·g−1 at the same rates for LFS/CNF. This is further corroborated by the analysis of electrochemical impedance spectroscopy (EIS) data using a modified Randles circuit showing a factor of two increase in Li-ion diffusion coefficient (4.2 × 10−13 cm2/s) and higher exchange current density in LFS/CNF/rGO nanocomposite. Increased porosity caused by conducting fillers of CNF/rGO seems to provide a large contact area between the LFS nanoparticles and electrolyte for efficient insertion/extraction of Li-ions.