Insight into cations substitution on structural and electrochemical properties of nanostructured Li2FeSiO4/C cathodes

Structural instability is the major obstacle in the Li2FeSiO4/C cathode during charge and discharge process, which can be improved by the substitution of cations in the host cage. In this study, the transition metal ions with different valence (Ag1+, Zn2+, Cr3+, and Ti4+) have been substituted in Li2FeSiO4/C via modified sol-gel method and the impact on the structural, electrical, and electrochemical performances has been systematically explored. The Rietveld-refined XRD pattern and HR-TEM (SAED) result reveal that all the prepared samples maintain orthorhombic structure (S.G- Pmn2(1)). The FE-SEM and TEM micrographs of bare and doped Li2FeSiO4/C display nanoparticle formation with 20-40 nm size. Among different cation-substituted silicates, Li2Fe0.9Ti0.1SiO4/C sample exhibits an excellent total conductivity of 1.20 x 10(-4) S cm(-1) which is one order of magnitude higher than the bare Li2FeSiO4/C sample. The galvanostatic charge-discharge curves and cyclic voltammetric analysis reveal that the Li2Fe0.9Ti0.1SiO4/C material provides an excellent initial specific capacity of 242 mAh g(-1) and maintains a capacity of 226 mAh g(-1) after 50 cycles with capacity retention of 93.38%. The Ti doping is a promising strategy to overcome the capacity fading issues, by preventing the structural collapse during Li-ion intercalation/de-intercalation processes in the Li2FeSiO4/C electrode through the strong hybridization between the 3d and 4s orbitals in titanium and 2p orbital in oxygen.