From tunnel NMO to layered polymorphs oxides for sodium ion batteries

The search for highly performing cathode materials for sodium batteries is a fascinating topic. Unfortunately, Na 0.44 MnO 2 (NMO), the well-known cathode material with good electrochemical performances, suffers from structural degradation due to reduction of Mn 4+ to the Jahn–Teller Mn 3+ ion, limiting the long-term cyclability. The cation substitution can be a useful way to mitigate the problem, thanks to the possible stabilization of mixtures of different polymorphs. In this paper, NMO was first substituted with Fe ions, obtaining Na 0.44 Mn 0.5 Fe 0.5 O 2 , with layered structure, then Al, Si and Cu (10% atom) were substituted on both Mn and Fe ions. Mixtures of P3 type phases, in different amount depending on dopant, were obtained and quantified by Rietveld refinements, and relationships between chemical composition, polymorph type and morphology were proposed. Cyclic voltammetry showed broad peaks, due to the complex structural transitions consequent to the intercalation/deintercalation of sodium. Charge discharge cycles disclosed the superior performances of Cu doped sample, which also benefits from improved air stability, a well-known issue of layered compounds. Discharge capacity values of about 63 mAh/g were detected at 1C, and after 50 cycles at C/2, capacities of about 80 mAh/g are obtained, with a capacity retention of 86%.