On controlling the P2-O2 phase transition by optimal Ti-substitution on Ni-site in P2-type Na0.67Ni0.33Mn0.67O2 (NNMO) cathode for Na-ion batteries

Novel electrode materials of P2-type layered oxides, Na-0.67 Ni0.33-xTixMn0.67O2 (x = 0, 0.08 and 0.16) are synthesized via solid state synthesis methods and electrochemically characterized as cathode materials for rechargeable non-aqueous sodium ion batteries. Na0.67Ni0.17Ti0.16Mn0.67O2 electrodes can deliver initial discharge capacity of 167 mAhg(-1 )at 0.1C rate with an average voltage of 3.7 V. Na0.67Ni0.25Ti0.08Mn0.67O2 electrodes lose only 12% of its initial capacity after 50 cycles when cycled to an upper cut-off voltage of 4.3 V. Controlling the P2-O2 transition at higher voltages through either optimized cut-off voltages or substitution is critical for improving the capacity retention. A complete reversible P2-OP4 transition occur only in optimally substituted sample, Na0.67Ni0.25Ti0.08Mn0.67O2 where as P2-O2 transition reemerges with higher amount of substitution in Na0.67Ni0.17Ti0.16Mn0.67O2.