Fe–Si networks in Na2FeSiO4 cathode materials

Abstract

Using a combination of adaptive genetic algorithm search, motif-network search scheme and first-principles calculations, we have systematically studied the low-energy crystal structures of Na₂FeSiO₄. We show that the low-energy crystal structures with different space group symmetries can be classified into several families based on the topologies of their Fe–Si networks. In addition to the diamond-like network which is shared by most of the low-energy structures, another three robust Fe–Si networks are also found to be stable during the charge/discharge process. The electrochemical properties of representative structures from these four different Fe–Si networks in Na₂FeSiO₄ and Li₂FeSiO₄ are investigated and found to be strongly correlated with the Fe–Si network topologies. Our studies provide a new route to characterize the crystal structures of Na₂FeSiO₄ and Li₂FeSiO₄ and offer useful guidance for the design of promising cathodes for Na/Li ion batteries.