Cathode material brings possibility of Na ion batteries Energy storage materials
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.