Fast ion conducting solids (FICs) are an integral component of portable energy devices, such as solid-state batteries and solid-oxide fuel cells that are economic, sustainable and environmentfriendly. Improving the efficiency and cost-effectiveness of these devices for large-scale practical applications relies upon better understanding of microscopic factors that control fast ion transport in solids. However, the experimental limitations to probe matter at the atomic level, rather constricts the search for improved FICs largely guided by intuition. Alternatively, molecular dynamics (MD) simulations, being a powerful tool to probe microscopic structure and dynamics of materials, can bridge this gap and complement the experimental efforts in this pursuit.
Recent MD simulations on NASICONs (Na3Zr2Si2PO12), an experimentally well studied Na+ fast ion conductor, predict that ordering of framework ions in aliovalently substituted materials can enhance their ionic conductivity by more than an order of magnitude. The correlation between the mobile ions themselves forms another important governing factor for fast ion transport. MD simulations on Na2Ni2TeO6, a recently synthesized class of Na+ conducting oxides, suggest that strong ion-ion interactions in the system results in the Na+ ions preferring energetically less favorable sites available in larger numbers to energetically more favorable sites of fewer numbers. This introduces higher degree of disorder for the Na+ sublattice, and the system gains in terms of free-energy through tradeoff between energy and entropy. These simulations studies will be discussed in detail.