Cationic and transition metal co-substitution strategy of O3-type NaCrO2 cathode for high-energy sodium-ion batteries

The development of advanced cathode materials with high operational voltage and high reversible capacity is crucial for facilitating the practical realization of sodium-ion battery (SIB) technology. Herein, O3-type Na0.9Ca0.035Cr0.97Ti0.03O2 is designed by co-substitution of Ca and Ti into O3-type NaCrO2, and proposed as a new cathode material for high-energy and practical SIBs. On the basis of the stoichiometry, alkali earth metal ions successfully incorporate into the NaO6 octahedron of NaCrO2 by substituting a single Ca2+ per two Na+, while Ti4+ ions are substituted with Cr3+ ions into the CrO6 octahedral site, resulting in formation of Na+ vacancies in the Na+ layer for the charge compensation. This co-substitution strategy reinforces the structural stability of the O3-type Na0.9Ca0.035Cr0.97Ti0.03O2 cathode, induced by the stronger Ti–O bond than Cr–O bond and presence of immobile Ca2+ ions between the CrO6 slabs. These structural features suppress the irreversible phase transition and provide excellent Na+ ion-diffusion kinetics in a wide operation voltage window of 1.5–3.8 V, allowing the cathode to deliver the high initial Coulombic efficiency of 95% and retain the 90% of its initial capacity after 1000 cycles at a 10 C rate. Moreover, the cathode guarantees the practical applicability with long-term cycling in a pouch-type full cell using a hard carbon anode, as well as with durability against water.