We request 3 days on POLARIS to study the detailed crystal structure of a sodium battery cathode material that is currently under consideration for use in commercial sodium batteries. This material will be studied in conjunction with two related prototypical layered sodium metal oxides. Na-ion batteries are becoming of increasing importance because they offer a more sustainable, cost-effective and safer energy storage solution than Li-ion batteries. Previously considered to be substantially inferior in performance to Li-ion batteries, Na-ion batteries have now reached higher energy densities than commercial lithium iron phosphate batteries at operating temperatures between -20C and +60C. This proposal will provide definitive information about the stoichiometry, and, in particular, the oxygen deficiency in this promising sodium cathode system.