*Article* **Effect of Vinylene Carbonate Electrolyte Additive on the Surface Chemistry and Pseudocapacitive Sodium-Ion Storage of TiO2 Nanosheet Anodes**

**Rudi Ruben Maça 1,2 and Vinodkumar Etacheri 1,\***


**\*** Correspondence: vinodkumar.etacheri@imdea.org

**Abstract:** Although titanium dioxide has gained much attention as a sodium-ion battery anode material, obtaining high specific capacity and cycling stability remains a challenge. Herein, we report significantly improved surface chemistry and pseudocapacitive Na-ion storage performance of TiO2 nanosheet anode in vinylene carbonate (VC)-containing electrolyte solution. In addition to the excellent pseudocapacitance (~87%), the TiO2 anodes also exhibited increased high-specific capacity (219 mAh/g), rate performance (40 mAh/g @ 1 A/g), coulombic efficiency (~100%), and cycling stability (~90% after 750 cycles). Spectroscopic and microscopic studies confirmed polycarbonate based solid electrolyte interface (SEI) formation in VC-containing electrolyte solution. The superior electrochemical performance of the TiO2 nanosheet anode in VC-containing electrolyte solution is credited to the improved pseudocapacitive Na-ion diffusion through the polycarbonate based SEI (coefficients of 1.65 <sup>×</sup> <sup>10</sup>−<sup>14</sup> for PC-VC vs. 6.42 <sup>×</sup> <sup>10</sup>−<sup>16</sup> for PC). This study emphasizes the crucial role of the electrolyte solution and electrode–electrolyte interfaces in the improved pseudocapacitive Na-ion storage performance of TiO2 anodes.

**Keywords:** electrolyte; additive; interface; pseudocapacitance; intercalation; energy storage; secondary battery; sodium-ion
