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J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al Figure 11. Average voltage and energy density vs. gravimetric capacity for various Ti-based anodes for Na-ion batteries (NIBs). Energy density is calculated based on the weight of active materials in positive and negative electrodes using their reversible discharge capacity. as NaBF4 or NaFSI and binders, such as sodium alginate or sodium carboxymethylcellulose have shown superior cycling stability, although their performance still lags behind those of their Li analogues [114]. Therefore, the development of next-generation electrolytes and binders is crucial to further improve battery efficiency. Another issue is the structural relaxation of the fully sodiated phase (Na4Ti3O7), attributed to the large electrostatic repulsion in the crystal structure [115]. This phenomenon may lead to self-discharge in full cells containing NTO, compromising cycling stability. Advances in science and technology to meet challenges To address the poor electronic conductivity and high Na+ ion migration barriers in NTO, different approaches, including bulk and surface (micro)structure design and carbon composite fabrication have been considered. Elemental doping using small concentrations (<5 at.%) of Nb [116] and elements from the lanthanide series (e.g. Yb) [117] has been shown to decrease the bandgap energy, enabling superior high-rate performance in NTO. For example, Nb-doped NTO demonstrated a 0.3 eV reduction in bandgap energy with respect to pristine NTO [116]. The enhanced electrochemical performance was attributed to doping-induced crystal structure distortions, together with the formation of Ti3+ ions. Furthermore, oxygen vacancies, which act as n-type defects, were claimed to contribute to the higher electrical conductivity observed in Yb-doped NTO [117]. Similarly, surface treatments involving the direct or indirect use of hydrogen have been used to create oxygen vacancies, conferring high capacities at fast rates [118]. Besides changes in the crystal structure, nanostructuring strategies have been extensively developed, showing an improved electrochemical performance through more efficient Na+ ion (de)intercalation, exploiting larger surface areas which can be further enhanced by creating continuous networks of material to improve electronic conductivity. For instance, Anwer et al used solvothermal synthesis to create 3D networks of 2D NTO nanosheets [119], generating microflowers (figure 12(a)) with a stable capacity of 108 mAh g−1 (200 mA g−1) over 1000 cycles, while NTO nanotubes 10–20 nm in diameter (figure 12(b)) have shown a high stable capacity of 100 mAh g−1 [120] (100 mA g−1) over 2000 cycles. Moreover, the use of carbon coatings [115] and carbon composite mixtures [121] is a common strategy to improve the electronic conductivity and pseudocapacitive contribution during Na+ ion storage. For instance, nanosheet-coated carbon shell particles delivered a capacity of 110 mAh g−1 (885 mA g−1) over 1000 cycles [122]. Finally, understanding the impact of electrolyte composition on interfacial electrochemistry is crucial for developing approaches to further prolong the cycle life of NIBs. Although this area is still in its infancy, preliminary work has been developed to mitigate SEI film instability by improving electrolyte formulations (e.g., by the incorporation of fluoroethylene carbonate (FEC) additives) and through surface engineering, among others. 26PDF Image | 2021 roadmap for sodium-ion batteries
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