Abstract :
Two iso-structural end members of the family of orthosilicates, i.e. Li2MSiO4 (M = Mn and Fe) and their solid solutions, were prepared and electrochemically characterized for potential use in Li-ion batteries. Due to the low specific conductivity (~5 × 10−16 S cm−1 for Li2MnSiO4 and~ 6 × 10−14 S cm−1 for Li2FeSiO4 at room temperature), small particles in an intimate contact with a conducting phase (i.e. carbon) are needed. Li2MSiO4/C composites (M = Mn and/or Fe) prepared by the Pechini synthesis generally leads to 30–50 nm large particles embedded in a carbon matrix. The amount of carbon in the composite is close to 10 wt.% for the Li2FeSiO4/C composite and slightly more than 5 wt.% for the Li2MnSiO4/C composite. In situ XRD experiment confirms a structural collapse of Li2MnSiO4 and the observed structural stability is completely different for Li2FeSiO4, which undergoes a fully reversible two-phase transition. Solid solutions between Li2MnSiO4 and Li2FeSiO4 in principle lead to higher capacities (>1e− per transition metal is feasible). For a long-term operation the cut-off voltage should be properly chosen. Electrochemical characterisation and in situ XRD experiments suggest the use of cut-off voltage close to 4.2 V.
