Unusual Li-Ion Intercalation Activation With Progressive Capacity Increase in Orthosilicate Nanocomposite Cathode

Lithium iron silicate Li2FeSiO4 (LFS) has received significant attention as Li-ion intercalation cathode material. However, it exhibits relatively poor Li-ion intercalation kinetics. In this work, mechanochemically annealed low-temperature orthorhombic (βII, Pmn21) LFS@C nanocomposite is shown to exhibit unusual Li-ion intercalation activation during cycling. In particular, we present evidence of impressive two-fold capacity increase over the initial dozen cycles from 90 mAh g-1 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 bi-phasic type. This unusual electrochemical behavior was further analysed by a systematic study on surface chemistry and crystal structure. Surface chemistry evaluation revealed a thick LiF-rich layer formation during cycling, which enabled stable cycling. More strikingly, a progressive irreversible in-situ phase transition during the initial dozen cycles was revealed,...