  The polyanionic compound Na2MnSiO4 is regarded as one of the promising
cathode materials for Na-ion batteries due to good specific capacity with its
attractive prospect of utilization of two electrons in the redox processes. So,
in this study, we have performed the thermodynamic and electronic structure
analysis of Na2MnSiO4 using first principles density functional theory
calculations. The intermediate ground state configurations for Na2MnSiO4of Na
de-intercalation were found using the cluster expansion method and are used to
obtain the 0 K voltage profile as a function of Na concentration. This material
shows an average voltage of 4.2 V and the finite temperature analysis at 300 K
using Monte Carlo simulations indicates that this material undergoes two phase
mixing when desodiate beyond 1.5 Na/f.u. The chemical bonding interactions
between the constituents are analyzed using the density of states, crystal
orbital Hamilton population, charge density, charge transfer and electron
localization function analyses. From these analyses we found that the
interaction between Mn and O is iono-covalent and that between Si-O is mainly
covalent in nature. The involvement of oxygen in the redox reaction apart from
the transition metal is identified using the Bader charge analysis. Relevant Na
diffusion pathways and their corresponding calculated energy barriers are
compared with the partially Fe substituted Na2MnSiO4 to understand the effect
of Mn-site substitution on the process of Na migration through this material.
