  Natural abundance of sodium and its similar behavior to lithium triggered
recent extensive studies of cost-effective sodium-ion batteries (SIBs) for
large-scale energy storage systems. A challenge is to develop electrode
materials with a high electrode potential, specific capacity and a good rate
capability. In this work we propose mixed eldfellite compounds
\ce{Na_x(Fe_{1/2}M_{1/2})(SO4)2} (M = Mn, Co, Ni) as a new family of high
electrode potential cathodes of SIBs and present their material properties
predicted by first-principles calculations. The structural optimizations show
that these materials have significantly small volume expansion rates below 5\%
upon Na insertion/desertion with negative Na binding energies. Through the
electronic structure calculations, we find band insulating properties and hole
(and/or electron) polaron hoping as a possible mechanism for the charge
transfer. Especially we confirm the high electrode voltages over 4 V with
reasonably high specific capacities. We also investigate the sodium ion
mobility by estimating plausible diffusion pathways and calculating the
corresponding activation barriers, demonstrating the reasonably fast migrations
of sodium ions during the operation. Our calculation results indicate that
these mixed eldfellite compounds can be suitable materials for high performance
SIB cathodes.
