Sodium ion batteries attracted a large research community because of earth-abundance, low cost and possibility of high energy density batteries. Despite the cost motivation of the research a considerable fraction of this activity still focuses on electrode materials analogous to those in Li-ion batteries often containing costly transition metals such as Co, which appears problematic for various reasons: differences in reaction mechanisms of the Na analogues to the Li transition metal oxides, different structural requirements for Na⁺ mobility, the fact that a cost advantage of Na over Li will only translate into low cost batteries if the concept of earth-abundance is applied consistently. Understanding of transport in solids both electrode and electrolyte is pivotal in designing such batteries. The analysis and prediction of ion transport in solids from static and dynamic structure models has become an interesting application for the bond valence (BV) approach [1]. Here we present a rational and quantitative characterization of rate performances of insertion-type cathode materials based on the diffusion relaxation model.
Bond-valence site energy modelling and DFT simulations were employed to clarify Na⁺ ion migration barriers in more than 20 sodium-ion battery cathode materials based on earth-abundant elements. Migration energy barriers calculated from our bond valence site energy models closely agree with the DFT models and with the help of the diffusion relaxation model allow for a direct semi-quantitative prediction of the rate performance of half-cells based on local structure models. The demonstrated method is computationally cheap enough to be applied for the fast screening of candidate structure databases and to allow for the testing of a series of local structure models (e.g. to explore the role of antisite dopants or stoichiometry deviations) for each structure type. Rate performance predictions are tested with molecular dynamics and Kinetic Monte Carlo simulations and yield specific guidelines for the design of novel sodium-ion battery cathode materials in terms of pathway dimensionality, migration barriers the effect of low-lying unoccupied sites, dopants or antisite defects as well as particle sizes.
As an example of the materials identified by the computational study as promising for high rate performance the alluaudite-type non-stoichiometric Na_2+xFe_2-x/2(SO₄)₃, (wherein Fe vacancies and Na_Fe^/ antisite defects cross-link the otherwise one-dimensional Na⁺ pathways [2]) was synthesized by mechanical milling of Na₂SO₄ and FeSO₄ followed by annealing at 400ºC under argon environment and its favourable rate performance is demonstrated in a room-temperature all-solid state sodium-ion battery using Na₃PS₄ as the solid electrolyte.  

 References

[1]     Adams S., Prasada Rao R., “Understanding ionic conduction and energy storage materials with bond valence-based methods”, Bond Valences (Edited by I. D. Brown et al.), Structure and Bonding 158, pp. 129-160, Springer (2014).

[2]     Wong L.L., Chen H.M., Adams S., “Sodium-ion diffusion mechanisms in the low cost high voltage cathode material Na_2+dFe_2-d/2(SO₄)₃^” Phys. Chem. Chem. Phys. 17 (2015), 9186 - 9193.