Density Functional Theory Analysis of Na4Mn4Ti5O18 for Sodium Ion Battery Electrode

Density functional theory study has been performed on Na4Mn4Ti5O18 to evaluate its application feasibility as an electrode in Na ion battery. Two types of void in the structure of Na4Mn4Ti5O18, viz., Z shaped tunnel and polyhedral pentagonal void, has been observed that act as guest sites for reversible migration of Na+ ions. It is also noted that electrochemically active sites are placed in Z shaped tunnel that act as host for Na+ during anodic response while it acts as the Na+ donor during cathodic response. The Mn atom, located in the Z-shape tunnel, acts as redox active site for both cathodic and anodic response via formation of redox couple Mn+3/Mn+4 and Mn+3/Mn+2 respectively. On the other hand, Mn atom present at square pyramidal site remains inert during charge-discharge process. The redox action in Na4Mn4Ti5O18, occurring due to intercalation and de-intercalation of 2Na(+) ions per formula unit, has been noted to exhibit voltage -2.63 V and 3.82 V respectively. The theoretical capacity -128 mAhg(-1) has been estimated corresponding to migration of 2Na(+) in Na4Mn4Ti5O18 structure. The present simulation study indicates the possibility of electrode action in Na4Mn4Ti5O18 for sodium ion battery application.