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

Although TiO 2 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 TiO 2 nanosheet anodes when employed in an ether-based electrolyte system (diglyme/NaPF 6 ). The TiO 2 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/NaPF 6 ), 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/NaPF 6 solution. The superior Na-ion storing behavior is credited to superfast and pseudocapacitive (1.29 × 10 –12 cm 2 /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 cm 2 /s). Furthermore, a full sodium-ion cell comprising a NaV 3 O 8 cathode, nanosheet TiO 2 anode, and 1 M diglyme/NaPF 6 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 TiO 2 -based anodes.