Tuning the upper cut-off voltage for enabling Co3+ /Co2+ redox in a P2/P3/spinel composite cathode material for sodium-ion batteries: An in operando study

Recently, research developments on layered-spinel composite cathodes have surfaced as a promising approach to improving the electrochemical performance of cathode materials for sodium-ion batteries (SIBs). Here, a P2/P3 layered-spinel composite, (P2/P3-LS-Na1/2Mn2/3Ni1/6Co1/6O2 (LS-NMNC)), has been synthesized and evaluated as a promising cathode material for SIBs. The material exhibited distinct electrochemical characteristics across different voltage ranges of 1.50–4.00 V and 1.50–4.50 V. In operando X-ray diffraction and X-ray absorption spectroscopy were employed to investigate the remarkable charge capacity and rapid capacity degradation observed in broader voltage range. It was determined that material undergoes various electrochemical mechanisms when adjusting the upper cut-off voltage. A unique Co3+/Co2+ redox process was activated during the intercalation of sodium ions at a potential of 1.94 V within 1.50–4.00 V range, accompanied by a phase transition from P2/P3 to P′2/O′3. This phase transition, in conjunction with the Co3+/Co2+ redox process, is likely responsible for the enhanced structural stability and capacity exhibited by the material when upper cut-off voltage is restricted. Additionally, presence of an anionic redox couple and strain in the structure was noted with the increase in the upper cut-off to 4.50 V, which leads to instability and diminished electrochemical performance.