Low Cost, High Energy Density Na-Ion Batteries
Of the many types of Na-based active materials available, layered oxides such as NaxMO2 (M = transition metal) have recently shown promise in terms of both performance and cost.3,4 These layered oxides can conform to several structural types, the most common of which are O3, P2 and P3, as designated by Delmas et al.5In these descriptions, the O and P refer to an octahedral (O), or prismatic (P), coordination of the Na-ions, and the number refers to the number of layers in the unit cell.
The data presented here show the electrochemical performance of a material from the NaaNi(1-x-y-z)MnxMgyTizO2 range, which is covered by a recent Faradion patent application. The material was synthesised via a conventional solid state technique, targeting a stoichiometry to maximise capacity whilst minimising cost and eliminating impurities. In the as-made material, nickel and magnesium are present as M2+, whereas manganese and titanium are present as M4+.
The material was electrochemically evaluated in small scale Na-ion pouch cells, using a CC/CV testing regime. Electrodes of ~ 3 mAh/cm2 were used and these were cycled against a commercially available hard carbon, at 30 °C, using an NaPF6-based electrolyte.
Figure 1 shows the low rate (~ C/10) voltage profiles of this material vs. hard carbon. A reversible capacity of 132 mAh/g (cathode) is achieved, at an average discharge voltage of 3.44 V, giving a cathode specific energy of 453 Wh/kg, which is comparable to commercially available Li-ion cathode materials.
Figure 2 shows the typical life cycle behaviour of this material. The data were collected at a rate of ~ C/10, using voltage limits of 4.3 V and 1.0 V. A reversible capacity of 117 mAh/g (cathode) is retained after 100 cycles, indicating a discharge capacity retention of 97 %. Most of this loss in capacity is recoverable when lower discharge rates are used, suggesting that this is due to cell impedance changes, rather than an irreversible loss of Na.
Figures 3 and 4 show the rate capability behaviour of a typical example of this material. This shows good rate capability up to rates of 2C and moderate rate capability at a rate of 4C. This is a notable result, as this material has not yet been optimised for power applications. Subsequent low rate cycling allows higher capacities to be re-accessed, signifying that the loss of capacity is recoverable.
 V. Palomares, P. Serras, I. Villaluenga, K.B. Hueso, J. Carretero-Gonz, T. Rojo, Energy Environ. Sci. 5, 2012, 5884.
 J. Barker, M.Y. Saidi, J. Swoyer, Electrochem. Solid St. 6, 2003, A1.
 M. Slater, D. Kim, E. Lee, C.S. Johnson, Adv. Func. Mater. 23, 2013, 947.
 S.W. Kim, D.H. Seo, X. Ma, G. Ceder, K. Kang, Adv. Energy Mater. 2, 2012, 710.