Structural Investigation of Na2MnSiO4, a Novel Cathode Material for Rechargeable Sodium Ion Batteries

Abstract

Lithium ion batteries dominate the world’s requirements of portable power storage due to their superior energy and power capability. However, as the focus of battery applications shifts towards larger scale energy storage, placing pressure on the declining amount of usable lithium, research interests shift towards the development of sodium ion batteries. Recently, various sodium based cathode materials have been based on structures that allow lithium insertion. Mixed-polyanion lithium compounds have been shown to be suitable precursors as they have the structural stability required for the complete exchange of lithium with sodium.1 The silicates Li2 MSiO4, (M = Mn and Fe), fulfil today’s requirements of safe, cheap and sustainable cathode materials especially due to the abundance of iron, manganese and silicon. The relatively higher theoretical capacities for these materials arise due to the possible extraction of two lithium ions accessing the M 2+/M 3+ and M 3+/M 4+ redox couples.2 The clear advantages of Li2 MSiO4, (M = Mn and Fe), over other lithium insertion compounds have made these materials attractive as possible analogues for cathode materials in Na-ion batteries.While the structure of Na2MnSiO4 (monoclinic, space group Pn) has been reported previously employing conventional X-ray diffraction technique,3 the possibility of this compound to be utilised as a cathode material in sodium ion batteries has not been explored. To date, we have been able to synthesise the Na2MnSiO4 and investigated the structural parameters of the compounds through the combination of synchrotron X-ray and neutron powder diffraction studies. This contribution will also address the electrochemical behaviour of this compound, including structural changes which occur in Na2MnSiO4during electrochemical cycling and the connection of these structural changes to the observed properties.