Layered Sodium Cobalt Oxides Anchored on Graphene for Improvement of Capacity Retention in Sodium Ion Batteries

Na_xCoO₂ is a well-known material for a promising cathode in sodium ion batteries. The crystal structures of Na_xCoO₂ have been reported to have four different phases. Each phase is built up by sheets of edge-sharing CoO₆ octahedra, allowing Na⁺ ions to be intercalated with octahedral and/or trigonal prismatic surroundings. According to the stacking sequence of the oxygen layers, two or three repeating CoO₂ sheets per unit cell can be present. Two-layer structure has γ phase in 0.6≤x≤0.75 and three-layer structure has β phase in 0.55≤x≤0.6, α phase in 1≤x≤0.9, and α’ phase at x=0.75. In this study, stoichiometric NaCoO₂ (NCO) was anchored on graphene oxide (GO) and used as a cathode for sodium ion batteries. Co₃O₄ was first prepared on GO and thermally converted to NCO in an inert atmosphere (to minimize the loss of GO). During this process, GO was simultaneously reduced to graphene to provide NCO with a conducting network and possibly structural stability. Since the stoichiometric NCO, which has an O3-type layered structure, is in an electronically semiconducting state in contrast to non-stoichiometric NCO (metallic), the improvement of electrochemical property of NCO could be expected by incorporating graphene. Preliminary study showed that the capacity retention of NCO anchored on graphene was substantially better than the pristine NCO. Multi-step phase transition was not altered, but the capacity fading was negligible during 50 cycles. We are currently focusing on the synthesis of phase-pure NCO in an inert atmosphere.