Ion Exchange of Layer-Structured Titanate Cs_xTi₂-_x/₂Mg_x/₂O₄ (x = 0.70) and Applications as Cathode Materials for Both Lithium- and Sodium-Ion Batteries

Cathode materials for rechargeable batteries have been extensively investigated. Sodium-ion batteries are emerging as alternatives to lithium-ion batteries. In this study, a novel cathode material for both lithium- and sodium-ion batteries has been derived from a layered crystal. Layer-structured titanate Cs_xTi_2-x/₂Mg_x/₂O₄ (x = 0.70) with lepidocrocite (γ-FeOOH)-type structure has been prepared in a solid-state reaction from Cs₂CO₃, anatase-type TiO₂, and MgO at 800°C. Ion-exchange reactions of Cs⁺ in the interlayer space were studied in aqueous solutions. The single phases of Li⁺, Na⁺, and H⁺ exchange products were obtained, and these were found to contain interlayer water. The interlayer water in the lithium ion-exchange product was removed by heating at 180°C in vacuum. The resulting titanate Li_0.53H_0.13Cs_0.14Ti_1.65Mg_0.30O₄ was evaluated for use as cathodes in both rechargeable lithium and sodium batteries. The Li⁺ intercalation-deintercalation capacities were found to be 151 mAh/g and 114 mAh/g, respectively, for the first cycle in the voltage range 1.0 - 3.5 V. The amounts of Li⁺ corresponded to 0.98 and 0.74 of the formula unit, respectively. The Na⁺ intercalation-deintercalation capacities were 91 mAh/g and 77 mAh/g, respectively, for the first cycle in the voltage range 0.70 - 3.5 V. The amounts of Na⁺ corresponded to 0.59 and 0.50 of the formula unit, respectively. The new cathode material derived from the layer-structured titanate is non-toxic, inexpensive, and environmentally benign.