Overcoming transport and electrochemical limitations in the high-voltage Na0.67Ni0.33Mn0.67-yTiyO2 (0 ≤ y ≤ 0.33) cathode materials by Ti-doping

This paper shows a Ti-doping strategy for improvement of transport and electrochemical properties of P2-type Ni- and Mn-based layered oxides, candidate cathode materials for the high-voltage Na-ion batteries. Pure and Ti-substituted Na0.67Ni0.33Mn0.67-yTiyO2 (0 ≤ y ≤ 0.33) oxides are synthesized using a standard high temperature solid state method. While Ti does not affect structural parameters of the materials considerably, it induces change of occupancy of both Na-sites, which is found to be beneficial. For higher concentrations of Ti (y ≥ 0.2) both, electronic and ionic components of the electrical conductivity are enhanced, with sodium conductivity remaining about three orders of magnitude higher than the electronic part. Improved conductivity is associated unexpectedly with higher activation energy, and such peculiar behavior seems to be interrelated with disruption of the ordering at Na-sites. As documented, moderate level of Ti-doping enables to design functional layered transition metal oxide cathode materials for Na-ion batteries, exhibiting high voltage and high reversible capacity.