Facile Electrochemical Activity of Monoclinic Li₂MnSiO₄ as Potential Cathode for Li-Ion Batteries

Synthesis of pure single-phase Li₂MnSiO₄ is challenging because of its rich polymorphism. Here, we demonstrate our success in preparing crystalline pure, battery-grade monoclinic phase Li₂MnSiO₄ (LMS) employing the temperature-programmed reaction technique. Systematic analysis of the electrochemical behavior of Li₂MnSiO₄ reveals its excellent battery activity in the monoclinic phase, with an initial discharge capacity of ∼250 mAh g^-1 associated with the reversible intercalation of more than one Li⁺. The extraction of Li⁺ ions from Li₂MnSiO₄ corresponding to the oxidation of Mn²⁺ to Mn³⁺ then to Mn⁴⁺ appears as single oxidation/reduction peaks at 4.3/3.9 V in the first charge/discharge sweep of cyclic voltammogram within the potential window of 3.0-4.4 V. However, an extension of cathodic sweep to 2.5 V results in the appearance of an additional redox peak at 2.7/3.1 V vs Li⁺/Li^o due to the reversible phase transition of monoclinic phase into battery-active orthorhombic phase induced by Jahn-Teller-active Mn³⁺ as evident from ex situ X-ray diffractograms. Indeed, the reversible intercalation of Li⁺ into the newly formed phase accounts for the high specific capacity of LMS within the potential window of 2.5-4.4 V. The capacity loss in the repeated cycles of monoclinic Li₂MnSiO₄ is explained by the formation of Mn₂O₃ owing to the dissolution of Mn³⁺.