Unravelling Anomalous Pseudocapacitive Sodium-Ion Storage of TiO ₂ Nanosheets in the Diglyme-Based Electrolyte: An Interfacial Analysis

Although TiO₂ offers multiple advantages as an anode material for Na-ion batteries, achieving both elevated specific capacity and durable cycling stability remains a major hurdle for practical applications. This work demonstrates the outstanding Na-ion storage capability of TiO₂ nanosheet anodes when employed in an ether-based electrolyte system (diglyme/NaPF₆). The TiO₂ anode achieved a superior specific capacity (214 mAh g^-1 @ 25 mA g^-1), demonstrated excellent rate capability (135 mAh g^-1 @ 1 A g^-1), retained 67% of its capacity after 6000 cycles, maintained nearly 100% Coulombic efficiency, and a pronounced pseudocapacitive contribution (∼59%), substantially outperforming the conventional carbonate-based electrolyte (EC-EMC/NaPF₆), which showed only 18% pseudocapacitance. Interfacial analysis using ATR-FTIR and XPS verified the development of polyether-derived solid electrolyte interphase (SEI) upon charge-discharge cycling in a diglyme/NaPF₆ solution. The superior Na-ion storing behavior is credited to superfast and pseudocapacitive (1.29 × 10^-12 cm²/s) Na-ion diffusion through a SEI composed of polyether, in contrast to the slower diffusion observed with the polycarbonate-based counterpart (7.54 × 10^-14 cm²/s). Furthermore, a full sodium-ion cell comprising a NaV₃O₈ cathode, nanosheet TiO₂ anode, and 1 M diglyme/NaPF₆ solution electrolyte demonstrated outstanding performance, delivering 67 mAh g^-1 @ 1 A g^-1 after 400 cycles, with 161 Wh kg^-1 energy density. These findings underscore the critical role of the electrode-electrolyte interface in enhancing the pseudocapacitive Na-ion storage in TiO₂-based anodes.