**1. Introduction**

Lithium-ion batteries (LIBs) are widely used in the current generation of portable electronics, electric vehicles, and smart grids coupled with renewable energy sources [1–4]. Despite their high energy density, moderate power density, and good cycle life, there are concerns regarding the future large-scale implementation of LIBs due to the limited availability of expensive (\$17,000 per metric ton) lithium resources, where market demand will be up to two to six times extraction capacity in the next two decades [5,6]. Rechargeable sodium-ion batteries (SIBs) are very attractive in this regard, due to the abundance of inexpensive sodium resources [3,5,7,8]. Moreover, the similar electrochemistry and redox potentials of lithium and sodium (−3.02 and −2.71 V vs. SHE, respectively), make it a suitable candidate for efficient electrochemical energy storage [9–11]. However, the larger size of Na-ions compared to Li-ions (1.02 and 0.76 Å radius, respectively) hinders their intercalation in the most commonly used Li-ion battery anode material, graphite (interlayer d-spacing of 3.4 Å) [1,12,13]. Shuttling of solvated Na-ions between individual electrodes is also sluggish, leading to the poor rate performance and cycling stability of Na-ion batteries [14]. Hence, numerous studies have focused on the development of alternative high-performance anode materials [15–20].

Carbonaceous materials with large interlayer spacing, such as hard carbon, graphene, and amorphous carbon have been widely investigated as anode materials [16,17]. However, these anodes exhibited low specific capacities, poor rate performances, and mediocre cycling stability. Despite the high specific capacity of conversion (Co3O4, SnO2, Fe3O4,

**Citation:** Maça, R.R.; Etacheri, V. Effect of Vinylene Carbonate Electrolyte Additive on the Surface Chemistry and Pseudocapacitive Sodium-Ion Storage of TiO2 Nanosheet Anodes. *Batteries* **2020**, *7*, 1. https://dx.doi.org/10.3390/ batteries7010001

Received: 20 November 2020 Accepted: 21 December 2020 Published: 24 December 2020

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NiO, CuO, MnO2, etc.) and alloying type (Sn, Ge, Sb, etc.) anodes, rapid capacity fading associated with huge volume changes make them less attractive for Na-ion battery applications [18–20]. Insertion type metal oxides, such as Nb2O5, Na2Ti3O7, TiO2, and Li4Ti5O12 are another class of Na-ion battery anodes [21,22]. Titanium dioxide (TiO2) has received much attention among anode materials due to its high chemical stability, non-toxicity, abundance, low volume change during Na-ion intercalation, and inexpensive nature [23–26]. Amorphous and different crystalline (anatase, bronze, rutile, etc.) polymorphs of TiO2 have been investigated as promising Na-ion battery anodes [7,27–30]. The existence of 2D intercalation channels in crystalline polymorphs make them superior to amorphous TiO2 for Na-ion storage [4,9,31]. Hence, anatase TiO2 composed of TiO6 octahedra and zigzag edges of the 3-D network have become the most studied polymorph [4,32]. Bronze polymorph of TiO2 has recently attracted a lot of attention as a Na-ion battery anode due to the presence of more open channels and layered crystal structure [29,33]. The Na-ion intercalation kinetics' dependence on crystal structure and orientation has also been proved in recent studies [22,33]. Nevertheless, TiO2 anodes suffer from low electronic conductivity, mediocre specific capacity, poor rate-performance, and low cycling stability [25,34].

Numerous strategies such as carbon coating, doping with other transition metal ions, and the synthesis of different morphologies in the nanoscale have been established for improving the Na-ion storage performance of TiO2 anodes [23,26,34]. Nevertheless, these methods have only resulted in a trivial improvement of specific capacities and cycling stabilities. Another advanced approach to improve the electrochemical performance is to increase the diffusion independent pseudocapacitive-type Na-ion storage [14,21]. This surface/near-surface ion storage is independent of the electronic and ionic conductivity of the electrode material [21]. In addition to the excellent rate performance and cycling stabilities, due to negligible structural changes during the charge–discharge process, high specific capacities can also be achieved due to the synergy with diffusion limited Na-ion storage [30,35]. Improved pseudocapacitance can usually be achieved by either precise nanostructuring, or the formation of hybrids with carbonaceous materials. These approaches were recently demonstrated for enhancing the intrinsic (~4%) pseudocapacitive Na-ion storage of TiO2 anodes [22]. For instance Chen et al. demonstrated the excellent electrochemical performance of TiO2-nanosheets grown on RGO [36]. Our recent study demonstrated the significantly improved pseudocapacitance of nanointerface engineered anatase-bronze hybrid TiO2 nanosheets [9]. The formation of additional Na-ion diffusion pathways, and efficient charge separation were responsible for the improved pseudocapacitance of these electrodes [37].

Solid electrolyte interface (SEI) characteristics are crucial in deciding the electrochemical performance, in addition to the physiochemical properties of an electrode material [38,39]. Carefully engineered electrode–electrolyte interfaces can enable producing batteries with superior energy/power densities and cycling stabilities [38,40]. Since SEIs are formed as a result of electrolyte decomposition, the most appropriate approach to engineer SEI properties is to tune the electrolyte composition [39,40]. Carbonate based electrolyte compositions usually contain solvents such as ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), etc., and a suitable sodium salt (NaPF6, NaTFSI, NaClO4, etc.). Electrochemical decomposition of these electrolytes results in the formation of an electrode–electrolyte interface composed of sodium alkyl carbonates, sodium alkoxides, polyenes, polycarbonates, and inorganic salts [5,38,41,42]. Excess electrolyte decomposition, due to the extreme reactivity of high surface area nanostructured electrodes, can often lead to increased SEI formation, which deteriorates the Na-ion storage performance [24,39]. Only a thin SEI layer capable of protecting the electrode from continuous reaction with the electrolyte solution is necessary to maintain optimal Na-ion diffusion, and a high degree of reversibility [43]. The chemical stability of SEI components is also critical to prevent dissolution in the electrolyte solution, which can lead to continuous SEI formation at the expense of electrolyte decomposition, and cell failure [38,41,44].

Using electrolyte additives is one widely employed strategy to endow the SEIs with superior electrochemical properties [40,44]. Vinylene carbonate (VC) is one of the most commonly used electrolyte additives in the case of Li-ion batteries [41,44,45]. This has been mainly aimed at flexible SEI formation in large volume expansion conversion/alloying type anodes, and at preventing metal-ion leaching from high voltage cathodes [38,42,46]. The high electrochemical reactivity of VC results in the preferential formation of polymeric SEIs, which prevent further reaction of the electrode material with the electrolyte solution during extreme operating environments, including high temperature, and large volume change [40,44,47]. Use of VC in the case of Na-ion batteries is also mainly aimed at improving the cycling stability of high capacity conversion/alloying type anodes [46]. However, none of these studies investigated SEI composition tuning to enhance the pseudocapacitive Na-ion storage of TiO2 anodes.

Herein, we report significantly improved pseudocapacitive Na-ion storage and surface chemistry of TiO2 nanosheet anodes in VC-containing electrolyte solution. Vinylene carbonate, a widely used Li-ion battery additive was chosen in this case due to its ability to form SEIs with superior passivation and Na-ion diffusion properties. The presence of a double bond is mainly responsible for the preferential decomposition/polymerization of VC associated with the generation of polycarbonate based SEIs. Ultrathin, mesoporous, and high surface area TiO2 nanosheets were selected as the preferred anode material in this case, to improve the contact with the electrolyte solution and thereby amplify the effect of the electrolyte additive on the surface chemistry and electrochemical performance. The excellent electrochemical performance of TiO2 nanosheet anode in VC-containing electrolyte solution is credited to the superior pseudocapacitance, resulting from the faster Na-ion diffusion through the polycarbonate based surface film.

Figure 1 shows the different solid electrolyte interface (SEI) composition on TiO2 nanosheet anodes formed in vinylene carbonate (VC)-containing and VC-free electrolyte solutions.

**Figure 1.** Schematic of the different solid electrolyte interface (SEI) composition on TiO2 nanosheet anodes formed in vinylene carbonate (VC)-containing and VC-free electrolyte solutions.
