Mechanically durable and electrochemically stable NbTiO2 cathode for flexible thin-film batteries

With the rapid advancement of flexible and portable electronics, energy storage systems require both excellent mechanical flexibility and stable electrochemical performance. Thin film structures offer promising advantages for satisfying these requirements, including uniform thickness control and strong adhesion to flexible substrates. Accordingly, in this study, rutile-phase NbTiO₂ (NTO) thin films were fabricated on Cu substrates via radio frequency magnetron sputtering at room temperature. By precisely controlling the NTO thickness and O₂ flow rate, optimal electrochemical properties were achieved under the conditions of a 100-nm film thickness and 0.4-sccm O₂ flow rate. The optimized NTO electrode exhibited excellent electrochemical performance, including a high initial charge capacity of 647.1 mAh g⁻¹, discharge capacity of 407.2 mAh g⁻¹, capacity retention of 72.2 % after 100 cycles, and Coulombic efficiency of >99 %. In addition, the mechanical flexibility of NTO was investigated through repeated bending tests of >10,000 cycles, and no significant change in electrical resistance was observed. These results demonstrate the potential of sputtered NTO thin films as reliable cathode materials for flexible thin-film batteries and next-generation wearable electronics.