The development of advanced cathode materials with high operating voltage and high reversible capacity is essential to promote the practical realization of sodium ion battery (SIB) technology. Here, O3 type Na_0.9Ca_0.035Cr_0.97Ti_0.03O₂ is designed by Ca and Ti co-substitution in O3 type NaCrO₂, and proposed as a new type of cathode material for high-energy density practical SIBs. On the basis of Chemical science, Alkalin ion successully incorporate NaO6 octahedron of Na_0.9Ca_0.035Cr_0.97Ti_0.03O₂.^1,2 In substitution process two Na+ per single Ca²⁺, while Cr³⁺ to Ti⁴⁺ ions.^3,4 As a result Na⁺ vacancies are formed Na⁺ layer. Through co-substitution, structural stability of O3 – Na_0.9Ca_0.035Cr_0.97Ti_0.03O₂ is increased, because stronger Ti – O bond and immobile Ca²⁺ ions in Na site synergistically make an effect the unit cell structures. This structural feature suppresses the irreversible phase transition and provide fast Na ion kinetics. In 1.5 – 3.8 voltage range, O3 – Na_0.9Ca_0.035Cr_0.97Ti_0.03O₂ cathode to deliver the high initial Coulombic efficiency of 95%. Also at a 10 C rate, cathode retains the 90% of its initial capacity after 1000 cycles. In situ X-ray diffraction confirmed the reversible structure properties during charging and discharging process. Moreover, the cathode guarantees the practical applicability with good cycling in a pouch- type full cell using a hard carbon anode, as well as with excellent water durability.

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