The effect of polymorphism on the lithium storage performance of Li2MnSiO4

Abstract

We report the synthesis of low and high temperature polymorphs of Li2MnSiO4 and dependence of lithium storage performance on such polymorphs. Two polymorphs namely, Pmn21 (low temperature/orthorhombic polymorph) and P21/n (high temperature/monoclinic polymorph) are isolated by controlling the calcination temperature. Among them the electrochemical performance of Pmn21 is found to be better than P21/n. Orthorhombic polymorph (Pmn21) of carbon coated Li2MnSiO4 exhibits an impressive discharge capacity of 262 mAh g−1 at 0.1C and rate performance up to 5C; in contrast P21/n delivers only a discharge capacity of 164 mAh g−1 at 0.1C at room temperature. Notably, the capacity of Pmn21 phase is almost 1.5–2 times higher than P21/n phase at all current rates. Capacity retention of 90% is reported for orthorhombic polymorph until 30 cycles at 0.1C. Further the voltage profiles and polarization of orthorhombic phase are much better than the monoclinic phase. Such perceivable differences in the rate performances and voltage profiles is argued to be due to variations in the activation energy barrier and hopping distance of lithium-ion in the crystal structures. Besides the role of polymorphism, we also show here that the structural stability during cycling is critical in retaining high storage performance.