**4. Conclusions**

The effects of vinylene carbonate (VC) as an electrolyte additive on the surface chemistry and Na-ion storage mechanism was thoroughly investigated. TiO2 nanosheet anodes achieved enhanced long-term cycling stability and pseudocapacitance in VC-containing electrolyte solution compared to VC-free composition. FTIR and XPS surface chemical characterizations of TiO2 nanosheets cycled in VC-containing electrolyte solution confirmed the formation of polycarbonate rich surface film. Whereas, the VC-free electrolyte solution resulted in the formation of a sodium alkyl carbonate and sodium carbonate based surface film. The superior electrochemical performance of the TiO2 nanosheets in VC-containing electrolyte solution is credited to the faster pseudocapacitive Na-ion diffusion through the polycarbonate-based surface film. These results demonstrated that TiO2 nanosheet anodes in VC-containing alkyl carbonate electrolyte solutions can be utilized for advanced high-performance Na-ion batteries.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/2313-0 105/7/1/1/s1. Figure S1: High-resolution Raman spectra, Figure S2: Galvanostatic long cycling performance with different amount of VC additive, Figure S3: First cycle charge-discharge profiles of TiO2 nanosheet anodes, Figure S4: FTIR spectra of cycled TiO2 laminates without washing.

**Author Contributions:** R.R.M. and V.E., conceptualize the idea; R.R.M., performed materials synthesis and characterization; R.R.M. and V.E., together wrote the manuscript; V.E., also performed supervision of the work. All authors have read and agreed to the published version of the manuscript. **Funding:** We thank IMDEA Materials Institute for generous start-up funding. Vinodkumar Etacheri acknowledges Spanish Ministry of Economy, Industry and Competitiveness (MINECO), Spanish Ministry of Science and Innovation, and Comunidad de Madrid for Juan de la Cierva fellowship (IJCI-2015-25488), Retos investigacion project (MAT2017-84002-C2-2-R)/Ramon y Cajal fellowship (RYC-2018-025893-I), and Talent attraction fellowship (2016-T1/IND-1300) respectively.

**Data Availability Statement:** The data presented in this study are available within the article and/or supplementary material.

**Conflicts of Interest:** The authors declare no conflict of interest.
