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

Abstract

Sodium orthosilicates NaMSiO (M = Mn, Fe, Co and Ni) have attracted much attention due to the possibility of exchanging two electrons per formula unit. They are also found to exhibit great structural stability due to a diamond-like arrangement of tetrahedral groups. In this work, we have systematically studied the possible polymorphism of these compounds by means of density functional theory, optimising the structure of a number of systems with different group symmetries. The ground state is found to be Pc-symmetric for all the considered M = Mn, Fe, Co, Ni, and several similar structures exhibiting different symmetries coexist within a 0.3 eV energy window from this structural minimum. The intercalation/deintercalation potential is calculated for varying transition metal atoms M. Iron sodium orthosilicates, attractive due to the natural abundance of both materials, exhibit a low voltage, which can be enhanced by doping with nickel. The diffusion pathways for Na atoms are discussed, and the relevant barriers are calculated using the nudged elastic band method on top of DFT calculations. Also in this case, nickel impurities would improve the material performances by lowering the barrier heights. Notably, the ionic conductivity is found to be systematically larger with respect to the case of lithium orthosilicates, due to a larger spacing between atomic layers and to the non-directional bonding between Na and the neighbouring atoms. Overall, the great structural stability of the material together with the low barriers for Na diffusion indicates this class of materials as good candidates for modern battery technologies.

High‐Abundance and Low‐Cost Metal‐Based Cathode Materials for Sodium‐Ion Batteries: Problems, Progress, and Key Technologies

“…[202] The nature of the diamond-like FeSi bonds is found to be extremely robust, so that it can offer only tiny volume changes during cycling. [203,204] This is also verified via the as-prepared carbon-coated Na 2 FeSiO 4 reported by Li et al [205] ( Figure 16e). The crystallization of highly pure Na 2 FeSiO 4 material is troublesome, however, since impurity phases such as Na 2 SiO 3 seem inevitable and Fe 2+ is easily oxidized to Fe 3+ .…”

Research Progress in Multielectron Reactions in Polyanionic Materials for Sodium‐Ion Batteries

“…Galvanostatic cycling demonstrated a reversible capacity of about 100 and 125 mAh g −1 for monoclinic and orthorhombic phases, respectively. It is clear that nearly one Na + ion can be removed from Na 2 CoSiO 4 at an average voltage of 3.3 V, consistent with the theoretical predictions . Rangasamy et al have also prepared the Pn ‐Na 2 CoSiO 4 through a solvothermal method .…”

“…By summarizing the reported Na 2 MnSiO 4 with >1‐e reactions, we can tentatively conclude that the nanosized particles and highly efficient carbon conductive network enable multielectron reactions of Na 2 MnSiO 4 due to the enhancement in intrinsic low electronic/ionic conductivities . Doping strategies that are widely used in the Li counterparts might be an effective and necessary way to improve the electronic/ionic conductivities substantially in the future works to obtain a decent cycling rate performance .…”

“…More specifically, the complicated sodium extraction/insertion mechanism requires in‐depth investigation by means of in situ XRD, XAFS, pair distribution function (PDF), and NMR. In addition, the low ionic conductivity needs to be substantially improved by, e.g., doping methods on the basis of the theoretical prediction . Finally, the operating voltage of Na 2 FeSiO 4 is somewhat low, which might be enhanced by introducing redox couples with higher potential (e.g., Co and Ni) …”

“…They obtained much more complicated features of charge/discharge curves than that obtained by Treacher et al, as compared in Figure 19e,f (Supporting Information). However, whether the second Na can be extracted is still unknown from both of the two reported papers because the upper cut‐off voltages were set below the predicted potential of ≈4.0 V for the second Na deinsertion …”