Manganese pyrosilicates as novel positive electrode materials for Na-ion batteries

Abstract

A carbon-coated pyrosilicate, Na₂Mn₂Si₂O₇/C, was synthesized and characterized for use as a new positive-electrode material for sodium ion batteries. The material consists of 20–80 nm primary particles embedded in a ≈10 nm-thick conductive carbon matrix. Reversible insertion of Na⁺ ions is clearly demonstrated with ≈25% of its theoretical capacity (165 mA h g⁻¹) being accessible at room temperature at a low cycling rate. The material yields an average potential of 3.3 V vs. Na⁺/Na on charge and 2.2 V on discharge. DFT calculations predict an equilibrium potential for Na₂Mn₂Si₂O₇ in the range of 2.8–3.0 V vs. Na⁺/Na, with a possibility of a complete flip in the connectivity of neighboring Mn-polyhedra – from edge-sharing to disconnected and vice versa. This significant rearrangement in Mn coordination (≈2 Å) and large volume contraction (>10%) could explain our inability to fully desodiate the material, and illustrates well the need for a new electrode design strategy beyond the conventional “down-sizing/coating” procedure.