Evaluation of electrochemical performance and redox activity of Fe in Ti doped layered P2-Na_0.67Mn_0.5Fe_0.5O₂ cathode for sodium ion batteries

Here, we report a wet synthesis-based titanium doping strategy to improve the structural stability and electrochemical performances, such as cycling stability and rate capability, of P2-type Na_0.67Fe_0.5Mn_0.5O₂ layered oxide cathodes. Through Ti⁴⁺ doping aimed at replacing some of the Mn and Fe atoms in the crystal structure, effective mitigation of the Jahn Teller distortion caused by active Mn³⁺ before charging and Fe⁴⁺ after charging was achieved. X-ray diffraction (XRD), Raman spectroscopy, Electrochemical Impedance Spectroscopy (EIS) and Mössbauer spectroscopy were used to investigate the effects of the Ti⁴⁺ dopant before and after cycling. It was observed that Ti⁴⁺ doping increased the Na layer thickness, minimized the lattice volume strain, showed better structural stability, minimally decreased Fe migration to the Na layer, and lowered charge transfer resistance in these P2-type cathodes. Overall, our reported synthesis methodology and electrochemical characterizations highlight the feasibility of Ti doping in sodium layered oxide P2-type cathodes.