Li2MnSiO4 obtained by microwave assisted solvothermal method: electrochemical and surface studies

Abstract

Nanostructured nominally pure Li₂MnSiO₄ (LMS) is successfully synthesized by a microwave assisted solvothermal method followed by successful coating with conductive carbon using carbonizing agents such as D-(+)-glucono-1,5-lactone (GA) and citric acid (CA). CA is found to be a better carbonizing agent than GA as it shows two times higher capacity than GA. Reversible capacity of 250 mA h g⁻¹ is obtained for carbon coated LMS at C/10 with flat charge/discharge plateau in the narrow potential window 2.0 to 4.5 V vs. Li/Li⁺. Capacity fades drastically after the first few cycles with a sloppy charge/discharge profile. The possible mechanism for the observed discharge plateau is discussed on the basis of XPS method and electrochemical testing. Ex situ XPS analysis confirms that such a large capacity (exchange of more than one mol of Li per compound reaction) after the first charging of the Li₂MnSiO₄ material is connected with the complete oxidation of Mn²⁺ to Mn⁴⁺ on the surface. To the best of our knowledge, the Mn⁴⁺ oxidation state is confirmed experimentally for the first time in the case of the manganese orthosilicate cathode material that exhibits best theoretical capacity of 330 mA h g⁻¹.