The Sodium Ion Battery

Sodium-ion batteries (NIBs) have regained interest in the scientific community since first discovered in the 1980s due to the increasing worldwide energy demand for large-scale and low-cost batteries. Na-ion technology stands up as the perfect candidate to satisfy these requirements; it is highly abundant (6th most abundant element in earth), accessible and widely distributed worldwide (and hence cheap).

The sodium chemistry per se is an exciting new field of research for materials scientists. Sodium exhibits a larger ionic radius than lithium which translates into robust materials that may not suffer as many transformations upon electrochemical cycling and therefore offer potentially improved stability. Furthermore, sodium research may broaden the materials design aspect. For example, novel lithium insertion materials with unusual Li coordination geometry can be synthesised by prior synthesis of the sodiated analogue and subsequent Na+/Li+ ion exchange.

Synthesis of novel transition metal sodium layered oxide cathodes

Currently, an active search for earth-abundant and low-cost sodium materials is being undertaken to develop Na-ion technology. Our main interests are focussed primarily on the synthesis of layered structure oxide materials, able to perform as cathodes in sodium batteries. Layered LiMO2 materials, where M is usually a first row transition metal, are well studied materials in Li-ion batteries due to their high capacity, materials cost and safety. By analogy, our studies focus in their transition metal sodium analogues (i.e. NaMO2). Specifically, P2-type NaMO2 materials (where P indicates Na in prismatic sites and 2 indicates the number of MO6 octahedral layers within the unit cell) have arisen as the most promising candidates of this type of layered materials due to their high capacity and reversibility versus other polymorphs (e.g. O3 (Na in octahedral sites) and P3-type). Nevertheless, the P2-type polymorph still suffers from volume changes at high voltages due to a P2-(O2 or a more complex OP4) phase transition occurring, causing a detrimental effect in the reversibility of these compounds. Consequently, our current efforts are directed to minimising this effect by means of doping.

Current studies in our group show that a slight replacement of Mn atoms with Mg in the P2-type Na0.67Mn1-xMgxO2 structure (with x up to 0.2) enhances the electrochemical properties vs. the pristine material in terms of reduction in polarisation, smoother voltage curves (reduction in phase transitions) and improved capacity retention over extended cycling. We explain the improved properties in these materials by the dilution of Mn3+ centres (Jahn-Teller distorted) with Mg doping and we supported this information with powder X-ray diffraction and galvanostatic measurements.

 

J. Billaud, G. Singh, A. R. Armstrong, E. Gonzalo, V. Roddatis, M. Armand, T. Rojo and P. G. Bruce, "Na0.67Mn1−xMgxO2 (0 ≤ x ≤ 0.2): a high capacity cathode for sodium-ion batteries", Energy Environ. Sci. 2014, 7, 1387.

We are also studying other types of layered materials such as β-NaMnO2, which vary from the typical NaMO2 layered oxides in terms of structural and electrochemical behaviour. In β-NaMnO2, Na atoms are located in Oh sites and sandwiched by zig-zag layers of edge-sharing MnO6 octahedra. Moreover, we have shown in recent reports using in-situ XRD, 23Na NMR and HRTEM that a reversible capacity of 100 mA h g-1 after 100 cycles can be achieved despite the extensive volume changes during cycling. This result is remarkable, considering that the poor cycling performance of layered materials is commonly attributed to phase transitions upon charge and discharge.

Billaud, Juliette, et al. "β-NaMnO2: A High-Performance Cathode for Sodium-Ion Batteries." Journal of the American Chemical Society 136.49 (2014): 17243-17248.