Na-Fe-S-O System As Cathode Materials for Na-Ion Batteries: An Ab Initio Study

Sodium-ion rechargeable batteries have been attracting increasing interest as a cost effective alternative to lithium-ion batteries, particular for larger energy storage systems. However, sodium system has the intrinsic drawback of having lower voltage than lithium one. Exploration of cathode materials containing polyanions can led to high voltages and therefore is a meaningful remedy. Experiences gained in the studies of lithium battery show that sulphates (Li₂Fe(SO₄)₂) deliver the highest voltage (3.83 V vs Li/Li⁺) among polyanions such as phosphates and silicates due to the high formal charge of S and inductive effects[1]. The Na-Fe-S-O system also has the advantage of composing of elements high in natural abundances and low in costs.

In exploring the Na-Fe-S-O system, our group found some new phases[2]. One of these can extract nearly all the Na atoms in the formula unit at an average voltage of 3.6 V vs Na/Na⁺. This is a significant improvement over the known Fe₂(SO₄)₃ in the Nasicon phase[3], where only one Na atom can be inserted and at a lower voltage of 3.4 V vs Na/Na⁺. The rate capability and cycling performance are also very satisfactory. The new structure consists of peculiar edge sharing Fe₂O₁₀subunits forming tunnel spaces partially occupied by Na ions.

In order to understand the structural origin of the excellent performance of this material we carried out ab initio study employing density functional theory. Migration activation energy (E_a) calculations found that Na ions are very mobile in the structure with E_aas low as 0.14 eV (Figure). This accounts for the high rate performance and hints at the application as solid-state electrolytes. On the voltage aspect, emphasis will be put on the origin of the higher voltage of the present system in comparison to the Nasicon phase. Theory for the trend of voltage in cathode material employing different polyanions is well known, however, it is highly controversial for polymorphs of compounds with the same chemical compositions. Phase behavior at different state of charge will also be discussed.

　　　　　　　　　　　　

References

[1] J.B. Goodenough in H Reiss (Ed.) Progress in Solid State Chemistry, Vol 5. Pergamon Press, 1971, pp 145-387

[2] A. Yamada, P. Barpanda, S. Nishimura, G. Oyama, Patent JP-2013-187914 (2013).

[3] A.K. Padhi, V. Manivannan, J.B. Goodenough, J. Electrochem. Soc. , 145(5), 1518 (1998).