*3.3. Surface Chemical Characterization of TiO2 Nanosheets*

It is crucial to understand the surface chemistry of TiO2 nanosheet electrodes cycled in VC-free and VC-containing electrolyte solutions due to the significant role of surface films on electrochemical performance. Surface chemical characterizations of these electrodes cycled in PC/NaClO4 and PC-VC/NaClO4 electrolyte solutions were performed using ex-situ XPS and FTIR techniques. Electrodes were thoroughly washed with acetonitrile and dried under vacuum to make sure that there was no residual NaClO4 salt and solvent left to interfere with these spectroscopic measurements. The quantification of elements present in the surface films formed in VC-free and VC-containing electrolyte solutions (Table 1) indicated the contribution of both solvent and Na-salt towards SEI formation. Higher oxygen and carbon concentrations found in the surface film confirmed solvent

reduction as the major reaction responsible for SEI formation. Surface films formed in VC-containing electrolyte had a higher carbon and oxygen content, signifying the increased formation of numerous oxygen containing species, such as Na-carbonate, Na-alkyl carbonate, Na-alkoxides, and Na-polycarbonates, due to PC and VC reduction [63]. Lower Na and Cl content in this case was a clear indication of reduced Na-salt decomposition, which is vital to maintain good ionic conductivity during extended cycling. Increased Ti-content signifies the formation of surface films that allow superior X-ray/ electron penetration and detection of Ti-atoms underneath.

**Table 1.** Quantification of elements present in the TiO2 nanosheet electrode surface film after 30 cycles in VC-containing and VC-free electrolyte solutions (based on XPS measurements).


XPS spectra of the elements of interest present in the TiO2 anode surface films formed in both electrolyte solutions (PC/NaClO4 and PC-VC/NaClO4) are presented in Figures 7 and 8. These spectra demonstrated a significant difference in the chemical composition of surface films. Alkyl carbonate solvents (PC and VC in this case) can be easily reduced to organic and inorganic Na-salts under an applied potential [64].

**Figure 7.** High-resolution XPS spectra of TiO2 nanosheet electrodes after 30 cycles in PC/NaClO4 electrolyte solution for (**a**) survey, (**b**) C 1s, (**c**) O 1s, (**d**) Cl 2p, (**e**) Na 1s, (**f**) Ti 2p.

**Figure 8.** High-resolution XPS spectra of TiO2 nanosheet electrodes after 30 cycles in PC-VC/NaClO4 electrolyte solution for (**a**) survey, (**b**) C 1s, (**c**) O 1s, (**d**) Cl 2p, (**e**) Na 1s, (**f**) Ti 2p.

As presented in Scheme 1, two electron PC reduction in the presence of Na-ions leads to sodium carbonate (Na2CO3) and propylene (CH2=CH-CH3), whereas one electron reduction results in sodium alkyl carbonate (CH2OCO2Na)2 and ethylene (CH2=CH2) formation. On the other hand, PC reaction with perchlorate anion (ClO4 *−*) forms hydrogen perchlorate (HClO4), carbondioxide, and acetaldehyde radical [65]. The sodium alkyl carbonate produced interacting with the trace amounts of water present in the electrolyte could produce more sodium carbonate (where R = -CH3, or CH3-CH2), carbondioxide, sodium hydroxide, and/or ROH species [66]. Protons generated by the dissociation of HClO4 could react with NaClO4 to form HCl, and sodium hydroxide (NaOH). Previously formed NaOH species reacted with the PC solvent to produce sodium alkyl carbonates [67]. Initially formed sodium alkyl carbonates react with the sodium carbonate and hydrochloric acid (HCl), and in the following steps generating sodium chloride (NaCl), desodiated alkyl carbonate (ROCO2H), and carbonic acid (H2CO3). This step can also form sodium bicarbonate (NaHCO3) instead of carbonic acid [66]. Most importantly, the increased reactivity of VC due to the presence of a double bond could form higher complexity sodiated polymers, such as sodium ethylene dicarbonate (NaO2CO-C2H4-OCO2Na), sodium butylene carbonate (CH2CH2OCO2Na)2, and poly-vinylene carbonate [41,64,66].

Hence, the effect of solvents on the salt reduction can be investigated by comparing the Cl-concentration in the surface film. Among the carbonate solvents used in this case, VC is more reactive than PC towards electrochemical reduction, which leads to the difference in the SEI composition and properties [47]. In the case of PC/NaClO4 electrolyte solution, high-resolution C 1s spectrum (Figure 7b) consisted of three distinct peaks located at 284.8 eV, 286.4 eV, and 289.6 eV, and characteristic of elemental carbon (C-C bonds), C-O-C/ C-O groups of RONa, and C=O groups of ROCO2Na/ Na2CO3 species, respectively [28,68]. Signals corresponding to alkoxy groups were absent in the case of the VC-containing electrolyte solution (Figure 8b). The relative intensity of the ROCO2Na peaks was also less, which clearly indicated difference in the surface film composition. The O 1s spectra of the TiO2 anodes cycled in VC-free electrolyte solution (Figure 7c) could be deconvoluted into individual peaks located at 531.0 eV, 532.6 eV, and 533.3 eV, corresponding to alkoxides, ether, and carbonate groups present in the surface film [49,68,69].

Due to the presence of similar functional groups, it was difficult to distinguish ROCO2Na present in VC-free and polycarbonate groups formed in VC-containing electrolyte solutions. However, slightly increased O 1s binding energies (531.2, 532.8, 533.5 eV, respectively) and increased width in the case of PC-VC/NaClO4 indicated (Figure 8c) the presence of higher carbonate concentration in the SEI, possibly due to polycarbonate formation resulting from VC-decomposition. Increased oxygen and carbon content quantified from the high-resolution spectra (Table 1) also indicated the presence of higher carbonate content in the case of the VC-containing electrolyte solution. Hence, it can be concluded

that the surface films formed in both VC-free electrolyte solutions consisted of alkoxides, carbonates, and ether species. Whereas alkoxides were absent in the polycarbonate rich surface film formed in the VC-containing electrolyte solution.

High-resolution Cl 2p spectra related to both the VC-free and VC-containing electrolyte solutions exhibited similar features (Figures 7d and 8d). Individual peaks at 198.1 and 199.7 eV correspond to the Cl 2p3/2 and Cl 2p1/2 components, respectively, of inorganic chlorides (mainly NaCl) present in the surface film [70]. Signals corresponding to organic chlorides were not identified in the case of both electrolyte solutions. Increased salt (NaClO4) decomposition in the case of the VC-free electrolyte solution is also evidenced by the lower relative amount (Table 1) of inorganic chlorides in the surface film. This can be credited to the more pronounced ClO4 − reduction in the absence of highly reactive VC electrolyte additive. High-resolution Na 1s spectra in the case of both electrolyte solutions (Figures 7e and 8e) were identical, with a prominent peak at 1071.4 eV indicating the presence of NaCl in the surface film [70]. High-resolution Ti 2p spectra in the case of the surface films formed in both electrolyte solutions (Figures 7f and 8f) exhibited signals at 458.1 eV and 463.9 eV, which are characteristic of the Ti 2p3/2 and Ti 2p1/2 components of Ti4+ ions [53].

This is in good agreement with the intercalation-type pseudocapacitive Na-ion storage mechanism we reported earlier for these anatase–bronze hybrid nanosheets [9]. Identical Na 1s and Ti 2p signals also verified a similar Na-ion storage mechanism of TiO2 nanosheets in both VC-free and VC-containing electrolyte solutions. Increased Na and Ti concentration in the case of PC/NaClO4 and PC-VC/NaClO4 can be attributed to the enhanced NaClO4 decomposition in the absence of VC, and superior transparency of polycarbonate based surface film for X-rays/electron beam, correspondingly.

Further surface chemical studies of the TiO2 were performed by ATR-FTIR spectral measurements. FTIR spectra of non-cycled TiO2 nanosheet electrodes, after cycling in VC-free and VC-containing electrolyte solutions (Figure 9a), exhibited clear differences in the surface chemical composition. Presence of ROCO2Na and Na2CO3 as the major components resulting from PC reduction was evident from these spectra [64,67,70]. Peaks indicative of the symmetric and asymmetric stretch mode of C-O bond (v C-O) present in ROCO2Na appeared in the 1450–1360 cm−<sup>1</sup> and 1650–1540 cm−<sup>1</sup> regions, respectively [67,71]. Scissoring vibrations for OCO2 <sup>−</sup> (δ OCO2 −) from the same species appeared at 871/877 cm−<sup>1</sup> for both electrolyte compositions [67,72]. C-H stretching (v C-H) bands appear at higher frequencies between 2999 and 2930, which were identical to the previous reports for electrodes cycled in VC-free electrolytes [57,67]. Signals in the 2340–2350 cm−<sup>1</sup> range indicated the presence of atmospheric CO2 [41]. Peaks at 1295 cm−1, 1075 cm−1, and 1067 cm−<sup>1</sup> belong to CO stretching/ CH3 deformation coming from the organic species containing –ONa and/or –OCO2Na functional groups and double bonds [57,71]. Peaks originating from the carbonate group of Na2CO3 (v CO3 <sup>−</sup>2) were clearly visible at 1398 cm−1/1400 cm−<sup>1</sup> in the spectra [72]. These signals were absent in the spectra of the pristine TiO2 electrode, confirming surface film formation only during the electrochemical charge–discharge process. Interestingly, additional peaks corresponding to PC/VC solvents and NaClO4 salts are not visible in the case of the non-washed electrodes (Figure S4). This could be due to the dominant FTIR intensities of the SEI components compared to trace amounts of solvent and Na-salt. Similar FTIR spectra before and after washing confirmed that the SEI components were not damaged by acetonitrile solvent.

**Figure 9.** (**a**) ATR-FTIR spectra of non-cycled TiO2 nanosheet electrodes, and after cycling in PC/NaClO4 and PC-VC/NaClO4 electrolyte solutions. Post-cycling SEM images of TiO2 nanosheet electrodes in (**b**,**c**) PC/NaClO4, and (**d**,**e**) PC-VC/NaClO4 electrolyte solution. (**f**) SEM image and (**g**–**j**) corresponding SEM-EDX elemental mapping of TiO2 nanosheet electrodes after 30 cycles in PC-VC/NaClO4 electrolyte solution.

Thus, the FTIR signals clearly demonstrated the formation of alkoxides and carbonates formed as a result of electrolyte decomposition (Scheme 1). The spectra related to PC/NaClO4 and PC-VC/NaClO4 electrolyte solutions are somewhat similar. The major difference between these spectra is the presence of polycarbonate peaks at around 1780 cm−<sup>1</sup> in the case of the VC-containing electrolyte [57]. It is indeed clear that VC-addition to PC/NaClO4 electrolyte solution resulted in the formation of a polycarbonate rich surface

film, along with Na2CO3 and ROCO2Na species. Post-cycling SEM images (Figure 9b–e) of the TiO2 nanosheet anode confirmed their structural stability during the sodiation– desodiation process. Individual petals of the agglomerated TiO2 nanosheets were clearly visible through the surface films formed in both electrolyte solutions. EDX mapping of the TiO2 nanosheet anodes cycled in PC-VC/NaClO4 electrolyte solution (Figure 9f–j) also verified homogeneous SEI formation. Hence, it is clear that the difference in pseudocapacitance and cycling stabilities was a consequence of the different chemical compositions of the surface films rather than their thickness. This should be expected, considering the minimal volume changes of the TiO2 anode during the charge–discharge process, which is unlikely to form thicker SEI due to limited electrolyte consumption only in the initial cycles.

Surface chemical analysis results obtained from the ATR-FTIR measurements were in line with the XPS results discussed above. These spectral studies enabled us to differentiate the surface chemical aspects responsible for the increased pseudocapacitive Na-ion storage in VC-containing electrolyte solution. Surface film formation in the two different electrolyte solutions can be described as follows: In PC/NaClO4 electrolyte, PC reduction into Na2CO3 and ROCO2Na are the primary processes contributing to the surface film. There exists a competition between the PC and VC reduction in the case of PC-VC/NaClO4 electrolyte solution [47]. VC is the most reactive component, and surface films formed in this case constitute both VC polymerization and PC reduction products. Polycarbonates formed during the initial charge–discharge cycles, as a result of VC-decomposition, protect the highly reactive TiO2 anode from further reaction with the electrolyte solution. Improved passivation also resulted in the reduced decomposition of the NaClO4 salt, which is crucial for maintaining good ionic conductivity during extended charge–discharge cycles. Significantly improved pseudocapacitance resulted from the ultrafast Na-ion diffusion through the polycarbonate based surface film. It should be noted that improved electrochemical performance of graphite and silicon nanowire anodes in VC/ FEC containing electrolyte solutions has been observed previously [41]. Nevertheless, such drastic difference in the pseudocapacitive Na-ion storage mechanism has never been identified. In conclusion VC electrolyte additive resulted in excellent long term cycling stability and pseudocapacitive Na-ion storage, a consequence of the superior passivation and Na-ion transport properties of the polycarbonate-rich surface film.
