Investigation of Ti-substitution and MnPO4 coating effects on structural and electrochemical properties of P2-Na0.67TixNi0.33Mn0.67-xO2 cathode materials

Here, we successfully designed and fabricated a titanium doped cobalt-free P2-Na0.67Ni0.33Mn0.37O2 cathode material synthesized by solid state method. The introduction of Ti effectively inhibited the P2-O2 phase transition and stabilized the structure of the cathode material. By exploring the effect of different doping amount on the cycle performance and rate performance of cathode materials, it is found that when the doping amount x = 0.3, the structural stability of the material can be effectively improved. Ti doping increases the thickness of Na layer, reduces the volume strain of lattice and exhibits better structural stability. In addition, a thin layer of MnPO4 was coated on the surface of the doped sample, which further improved the structural stability of the material, inhibited the reaction of the active material/electrolyte interface and the fragmentation and spalling of the particles after cycling. Although the initial discharge capacity decreased slightly after coating, the capacity retention of the sample with x = 1 wt% coating increased from 66.4% to 87.2% after 200 cycles. Overall, the synthesis methods and electrochemical characterizations reported highlight the feasibility of titanium doping in P2 type sodium ion batteries, and the stability of cathode materials guaranteed.