In situ Raman spectroscopy reveals the mechanism of titanium substitution in P2–Na2/3Ni1/3Mn2/3O2: Cathode materials for sodium batteries

Layered P2–Na2/3Ni1/3Mn2/3O2 is a promising cathode material. It exhibits a high capacity and suitable operating voltage and undergoes a phase transition from P2 to O2 during charge/discharge. Researchers have used Ti substitution to improve the cathode, yet the chemical principles that underpin elemental substitution and functional improvement remain unclear. To clarify these principles, we used in situ Raman spectroscopy to monitor chemical changes in P2–Na2/3Ni1/3Mn1/3Ti1/3O2 and P2–Na2/3Ni1/3Mn2/3O2 during charge/discharge. Based on the change in the A1g and Eg peaks during charge/discharge, we concluded that Ti substitution compressed the transition metal layer and expanded the planar oxygen layer in the unit cell. Titanium stabilized the P2 phase structure, which improved the cycling stability of P2–NaNMT. Our results provide clear theoretical support for future research on modifying electrodes by elemental substitution.