The high energy storage capacity and electrochemical stability of nanostructured lithium transition metal phosphates suggests that they are suitable positive-electrode materials for Li ion batteries.

The olivine LiFePO₄ has been singled out as particularly promising for large-capacity systems, such as plug-in hybrid electric vehicles, owing to its low cost, low environmental impact, and safety. However, it does show shortcomings in Li ion transport and has a two-phase redox reaction. Furthermore, it has been suggested that global Li reserves may limit the economic viability of large-scale Li ion energy storage systems.

Chemists from the University of Waterloo, Canada, have now looked at an alternative iron phosphate that could serve directly as a cathode in either Li ion or Na ion batteries [Ellis et al., Nat. Mater. (2007) 6, 749].

Brian L. Ellis and colleagues set out to synthesize an environmentally friendly iron-based alkali fluorophosphate that, like LiFePO₄, could be prepared in its reduced form to be used directly as a positive electrode. In the resulting material, Na₂FePO₄F, the Na cations are located between infinite FePO₄F layers, and possess facile two-dimensional migration pathways.

Electrodes prepared from a nanostructured polycrystalline form of Na₂FePO₄F were examined in coin-type cells using Li as a counter electrode. The reversible capacity was found to be 80% of the theoretical value, and this was sustained well on cycling. The average charge potential on cycling (3.6 V) was similar to that of LiFePO₄. The distinct two-phase behavior shown by LiFePO₄ was not observed in Na₂FePO₄F.

“This [new material] offers significant advantages with respect to cost and Li availability, and the possibility of developing viable Na ion cells,” the team concludes.