Unusual Li-ion Intercalation Activation with Progressive Capacity Increase in Orthosilicate Nanocomposite Cathode

Lithium iron silicate Li₂FeSiO₄ (LFS) has received significant attention as a Li-ion intercalation cathode material. However, it exhibits relatively poor Li-ion intercalation kinetics. In this work, mechanochemically annealed low-temperature orthorhombic (β_II, Pmn2₁) LFS@C nanocomposite is shown to exhibit unusual Li-ion intercalation activation during cycling. In particular, we present evidence of an impressive 2-fold capacity increase over an initial dozen cycles from 90 to 180 mAh g^–1. Interestingly a clear shift in Li-storage mechanism was triggered by the in situ activation of the orthosilicate structure from a solid solution to biphasic type. This unusual electrochemical behavior was further analyzed by a systematic study on surface chemistry and crystal structure. Surface chemistry evaluation revealed the formation of a thick LiF-rich layer during cycling, which enabled stable cycling. More strikingly, a progressive irreversible in situ phase transition during the initial dozen cycles was revealed, which activated ion transport by two orders of magnitude. Such an unreported electrochemically induced structural activation provides unexplored opportunities for unlocking the full potential of high capacity nanostructured orthosilicate and other types of Li-ion battery cathodes.