*3.2. Electrochemical Performance of the TiO2 Nanosheets*

The sodium-ion storage electrochemical performance of the TiO2 nanosheets was investigated in VC-containing (PC-VC/NaClO4) and VC-free (PC/NaClO4) electrolyte solutions. Second galvanostatic voltage profiles at different current densities (Figure 3a,b) indicated the marginally superior specific capacities of the TiO2 nanosheet anodes in PC-VC/NaClO4 electrolyte at various charge–discharge rates. First cycle coulombic efficiencies of 36% and 44.5% were observed in the PC/NaClO4 and PC-VC/NaClO4 electrolyte solutions, respectively (Figure S2). Irreversible capacity loss in the first cycle was attributed to unavoidable electrolyte decomposition, and irreversible Na2O formation associated with sodiation. Coulombic efficiency reached ~100% after the first charge–discharge in PC-VC/NaClO4 electrolyte, demonstrating the complete formation of high quality SEI during the first cycle. However, coulombic efficiency never reached ~100% on extended cycling in PC/NaClO4 electrolyte, which indicated the formation of an SEI with inferior passivation properties. This is also apparent from the consecutive cyclic voltammograms presented (Figure 3c,d). The TiO2 nanosheet anodes exhibited a similar electrochemical response, irrespective of the electrolyte composition. Three different electrochemical processes constitute the first cathodic response in both systems. The high-voltage region between 2.25 and 0.7 V represents diffusion-independent pseudocapacitive Na-ion intercalation [53]. SEI formation resulting from the electrolyte decomposition could be identified from the cathodic response between 0.7 and 0.5 V [9]. Lack of this signal in the second cathodic scan confirmed complete SEI formation in the first charge–discharge cycle [32,54]. A low voltage region between 0.2 and 0 V corresponded to the diffusion controlled Na-ion intercalation into the anatase and bronze crystal structure [36]. These electrochemical responses were in line with the plateau at low potential and sloping voltage profiles. Anodic response in the voltage range of 0.2 V, and a broad signal between 0.5 and 2.25 V represented Na-ion deintercalation through pseudocapacitive and diffusion dependent process, respectively.

**Figure 3.** Galvanostatic voltage profiles of TiO2 nanosheet anodes in Na-ion half-cells containing: (**a**) PC/NaClO4, and (**b**) PC-VC/NaClO4 electrolyte solutions at various current densities. Cyclic voltammograms of TiO2 nanosheet anodes at a scan rate of 1 mV/s in (**c**) PC/NaClO4, and (**d**) PC-VC/NaClO4 electrolyte solution.

Sodiation over a wide voltage range was observed in this case, which was noticeably different from the intercalation reaction at specific potentials for previously reported TiO2 anodes [27]. Distinct Na-ion storage was also evidenced by the marginally different shape of the voltammograms compared to earlier reports [37]. Although cathodic signals corresponding to SEI formation disappeared in the first cycle, there was a clear indication of irreversibility and incomplete SEI formation during the consecutive cycles of the VCfree battery, which was not observed in the case of the VC-containing Na-ion half-cells. This was a clear indication of efficient SEI formation on the TiO2 nanosheet anodes in the VC-containing electrolyte solution. Thus, disappearance of the SEI formation peak in the first cycle and stable consecutive cycles confirmed the highly reversible Na-ion intercalation reactions of the TiO2 nanosheet anodes in VC-containing electrolyte solution. The galvanostatic rate performance of the TiO2 nanosheet anodes in both VC-free and VCcontaining electrolyte solutions is presented in Figure 4a. Second cycle specific capacities were 247 and 219 mAh/g, respectively. Slightly higher capacities were identified in the case of VC-containing electrolyte solution during the following cycles at 25 mA/g. Capacity differences became more prominent at higher current densities, and also increased on reducing the current density to 25 mA/g. This was a clear indication of the capacity fading

of TiO2 anode in VC-free electrolyte, which was further verified during the extended galvanostatic cycling (Figure 4b).

**Figure 4.** (**a**) Galvanostatic rate performance, and (**b**) cycling performance of TiO2 nanosheet anodes in PC/NaClO4 and PC-VC/NaClO4 electrolyte solutions. Nyquist plots and equivalent circuit (inset) of TiO2 nanosheets in (**c**) PC/NaClO4 and (**d**) PC-VC/NaClO4 electrolyte solutions.

Both cells demonstrated ~100% coulombic efficiency after 750 cycles at a current density of 100 mA/g. Galvanostatic long-cycling performance of electrolyte solutions containing different amounts of VC confirmed 3% as the optimum concentration (Figure S3). For instance, at a current density of 100 mA/g, TiO2 nanosheet anode in Na-ion half-cell configuration retained 90% of the initial capacity, with ~100% coulombic efficiency after 750 galvanostatic charge–discharge cycles. On the other hand, Na-ion half-cells containing VC-free electrolyte solution retained only 16% of the initial capacity, with ~98% coulombic efficiency.

Electrochemical impedance spectroscopy (EIS) of pristine and cycled electrodes (Figure 4c,d) is performed to gain further insights into the effect of SEI films on the charge transfer, and Na-ion diffusion characteristics. High-to-medium frequency part of the EIS pattern can be assigned to the sum of SEI/contact resistance and charge-transfer resistance and represented as *R*Ω and *Rct* respectively in the Randles-like equivalent circuit (Figure 4d inset). Low frequency sloping line corresponds to solid-state diffusion kinetics of Naions shown as Warburg impedance (*Zw*) while constant phase element (CPE) is used to model the surface storage of Na-ions [7,49,55]. Charge transfer resistances of pristine TiO2 nanosheet anodes in VC-free and VC-containing electrolyte solutions, obtained by fitting

the Nyquist plot to the equivalent circuit, are 39 Ω and 171 Ω respectively. It is interesting to observe such impedance difference in pristine batteries as VC is supposed to influence SEI formation only during the first charge-discharge cycle. This could be related to the spontaneous SEI formation resulting from the increased reactivity of high surface area TiO2 nanosheets with the electrolyte solution even in the absence of applied potential. Such surface film formations were also observed previously in the case of pyrolytic graphite, carbon and silicon nanowire electrodes [41,56,57]. Charge transfer resistance in VC-free electrolyte solution increased to 517 Ω after 30 galvanostatic cycles. In contrast, resistance decreased to 105 Ω in the case of VC-containing half-cells. Na-ion diffusion coefficients of TiO2 nanosheet anodes after 30 galvanostatic cycles in VC-free and VC containing electrolyte solutions calculated from the Nyquist plot are 6.42 × <sup>10</sup>−<sup>16</sup> and 1.65 × <sup>10</sup>−<sup>14</sup> correspondingly. Hence it is clear that VC-addition significantly reduces the charge transfer resistance and enhances the Na-ion diffusion kinetics, facilitating improved Na-ion diffusion into the TiO2 nanosheets. These results are also in good agreement with the superior cycling performance of TiO2 nanosheet electrodes in VC-containing electrolyte solution.

The sodium-ion storage mechanism of TiO2 nanosheets in VC-free and VC-containing electrolyte solutions was further investigated by collecting cyclic voltammograms at various scan rates (Figure 5a,b). Considerable differences existed in the voltammograms in the two different electrolyte solutions. High peak current and area were identified for the TiO2 nanosheets in the VC-containing electrolyte, which is in line with their improved Na-ion storage performance observed from the galvanostatic rate performance and cycling studies. The shapes of the voltammograms were also different suggesting different Na-ion storage mechanisms. Anodic and cathodic current increased with an increase of scan rate in both electrolyte solutions, which is indicative of pseudocapacitive type Na-ion storage [21].

Sodium ion intercalation of TiO2 nanosheets, especially at higher scan rates, occurs over a wide voltage range in VC-containing electrolyte solution. Such broad signals indicate the increased diffusion independent behavior of Na-ion intercalation, which is typical in capacitive type electrode materials [58].

The sodium-ion storage of TiO2 nanosheet anodes consists of both diffusion limited and surface controlled process [59]. These components can be differentiated from the relationship between peak current density and scan rates, as presented in Equation (3) [21]

$$
\dot{a} = av^b \tag{3}
$$

where current shown as *i*, the scan rate as *v*, and *a* and *b* are adjustable variables. If b = 0.5, storage process is identified as diffusion-controlled, and surface controlled if b = 1.0 [60]. Moreover, as the Randles–Ševcik equation dictates, the increase of current is proportional to the square root of scan rate [55,61,62]. The slope of peak current density vs. scan rate plot (Figure 5c,d) represented dominant pseudocapacitive type Na-ion storage (increased slope) in TiO2 nanosheets, with a high degree of pseudocapacitance in the PC-VC/NaClO4 electrolyte solution.

The diffusion limited and pseudocapacitive contributions of specific capacities were quantified to further investigate the effect of electrolyte solutions on the Na-ion storage mechanism. Capacitive and diffusion controlled contributions of Na-ion half-cells containing PC/NaClO4 and PC-VC/NaClO4 electrolyte solutions at different scan rates are presented in Figure 6a,b. TiO2 nanosheet anode exhibited superior pseudocapacitive Na-ion storage in PC-VC/NaClO4 electrolyte solution (83% @ 1 mV/s) compared to PC/NaClO4 (63% @ 1 mV/s). Capacitance calculations were also performed after 30 galvanostatic cycles (Figure 6c,d) in order to investigate the capacity fading on extended cycling. The pseudocapacitance contribution of the TiO2 nanosheets reduced considerably (37% @ 1 mV/s), and slightly increased (87% @ 1 mV/s) in the PC/NaClO4 and PC-VC/NaClO4 electrolyte solutions, respectively (Figure 6e,f). The entirely different shape of the cyclic voltammograms also signified different Na-ion storage mechanisms in these TiO2 nanosheet anodes during extended cycling. It is worth noting that these results are in agreement with the EIS results presented earlier, where the impedance of the TiO2 anodes continually increased during

galvanostatic cycling in PC/NaClO4, and decreased in the PC-VC/NaClO4 electrolyte solutions. These observations led to the conclusion that the SEI formed in the VC-containing electrolyte solutions facilitated improved pseudocapacitive Na-ion diffusion into the TiO2 nanosheet anode. Hence, it is clear that the electrolyte composition played a crucial role in the cycling stability, impedance, and Na-ion storage mechanism of the TiO2 nanosheet anodes.

**Figure 5.** Cyclic voltammograms of TiO2 nanosheet anodes at various scan rates in (**a**) PC/NaClO4 and, (**b**) PC-VC/NaClO4 electrolyte solutions. Scan rate dependence of peak current density for TiO2 nanosheet anodes in (**c**) PC/NaClO4, and (**d**) PC-VC/NaClO4 electrolyte solutions.

**Figure 6.** Pseudocapacitance contributions of the TiO2 nanosheet anodes (**a**,**b**) before cycling, and (**c**,**d**) after cycling in PC/NaClO4 and PC-VC/NaClO4 electrolyte solutions. Pseudocapacitance contributions at a scan rate of 1 mV/s after 30 cycles in (**e**) PC/NaClO4, and (**f**) PC-VC/NaClO4 electrolyte solutions.
