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.

First-principles study of the structural stability and electrochemical properties of Na2MSiO4 (M = Mn, Fe, Co and Ni) polymorphs

“…16 Na-based orthosilicates are also known to exhibit a greater structural stability upon cycling with respect to their lithium counterparts, 15,18 due to the high Na/Fe exchange barriers observed for equilibrium structures exhibiting a diamond-like arrangement of AO 4 (A = Fe, Si) tetrahedral blocks. 12 DFT studies of these structures report also a very small volume change (below 3%) upon complete desodiation. 12 Experimental studies are overall in agreement with the predicted structural stability, 15,18,19 observed only when control over impurities is achieved 14 .…”

“…12 DFT studies of these structures report also a very small volume change (below 3%) upon complete desodiation. 12 Experimental studies are overall in agreement with the predicted structural stability, 15,18,19 observed only when control over impurities is achieved 14 . In 2011, Duncan and coworkers first successfully synthesised Na 2 MnSiO 4 (space group Pc) using a sol-gel method.…”

“…4 More recently, Na 2 MSiO 4 compounds are recognised as more promising candidates for novel cathode materials. [11][12][13][14][15] It has been inferred by means of ab initio calculations that full extraction of Na ions is easier to achieve with respect to the case of lithium due to a lower deintercalation voltage plateau. 16 Furthermore, the diffusion barrier for Na ions is found to be significantly lower.…”

Fe–Si networks and charge/discharge-induced phase transitions in Li2FeSiO4 cathode materials

“…The concept has been generalized to LiFePO4 systems by Lv et al where Fe-P networks are used to predict new low-energy structures with remarkable success 19 . Following previous work 21 , the Fe-Si networks in low-energy LiFeSiO4 structures are classified into 2D-ladder-like, 3D-diamond-like and 3D*-4-8-membered-rings types (For simplicity, we use name "2D", "3D" and "3D*" in the following discussion) according to the FeSi4 and SiFe4 polyhedra which are considered as building blocks of the Fe-Si networks ( fig.1).…”

“…A common diamond-like Fe-Si network was found for most of the lowenergy structures of Na2FeSiO4. The electrochemical properties are also related to the type of Fe-Si networks: crystal structures with common Fe-Si networks have similar electrochemical behaviour from first principles calculations 21 . The concept has been generalized to LiFePO4 systems by Lv et al where Fe-P networks are used to predict new low-energy structures with remarkable success 19 .…”