Ti4+ substitution suppressing P2-O2 phase transition to construct stable P2-Na0.67Ni0.33Mn0.67O2 cathode for long-term durable sodium-ion batteries

Due to high safety and cost-effective resources, sodium-ion batteries show great prospect in large-scale energy storage systems. Owing to wide structural framework and high theoretical capacity, Na-based layered oxide with prismatic P2-type structure is an ideal cathode candidate for sodium-ion batteries. However, Na-based layered compounds always suffer from detrimental multiple phase transitions during cycle period, resulting in structure deterioration and rapid capacity decay. Herein, stable P2-Na0.67Ni0.33Mn0.67-xTixO2 cathodes are constructed through substituting Mn with Ti. In-situ XRD demonstrates that Ti substitution effectively successfully suppresses the undesired phase transition of P2-O2, and in turn appears a stable Z-symbiotic intermediate phase with minimal volume change. Hence, designed P2-Na0.67Ni0.33Mn0.37Ti0.3O2 cathode shows better structural stability and long-term cycling reliability, yielding a high reversible capacity of 123 mAh g−1 and a high capacity retention of 89 % after 100 cycles at 0.2C. More importantly, a high working voltage of 3.5 V and a boosted energy density of 400 Wh kg−1 is achieved in the NNMTO-3 || HC full cell, which further strengthens the competence of Ti-substituted Na-based layered oxide cathodes. This work provides a feasible design insight for long-life, high-voltage sodium-ion batteries, which can be applied to construct more stable layered compounds and further improves their electrochemical properties.