The use of renewable but intermittent energy sources, such as solar and wind energy, means that it is necessary to store this energy for later use. Hence, a shift towards renewable sources of energy requires the development and use of efficient and inexpensive large-scale energy storage devices. Whereas portable electronics conventionally utilize lithium ion batteries (LIBs), this is not an option for large-scale energy storage, since the increased demand for lithium would result in a significant price increase. Therefore, alternative technologies, such as sodium ion batteries (SIBs) should be considered¹. Since sodium is widely available across the globe, and is the lightest alkali metal after lithium, SIBs can be a valuable alternative to LIB technologies, despite the lesser reducing nature of sodium. Because of dendrite formation, the use of sodium metal as anode results in a significant safety hazard. Therefore, it is a better option to use a technology where sodium ions are not reduced to their metallic form. Potential alternative SIB anodes are mainly carbonaceous materials, alloys and phosphorus/phosphide, as well as oxides and polyanionic compounds². However, many reported SIB anode materials suffer from a variety of disadvantages, such as a low electrochemical stability and energy density. Therefore, one of the challenges that the commercialization of SIBs faces is the development of a suitable anode material.

Recently, a reduced sodium titanate was reported as SIB anode. However, its low electrochemical stability, together with its cumbersome synthesis method and the lack of understanding of the mechanism of sodium ion intercalation, makes its use in a commercial SIB unlikely. Our density functional theory (DFT) calculations show that the material is an insulator due to its large band gap. Therefore, a new, convenient synthesis method was developed, which results in a composite of reduced sodium titanate with conductive amorphous carbon (NTO/C). This composite material was characterized by powder X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR). The formation mechanism was studied using thermogravimetric analysis (TGA) and electron paramagnetic resonance (EPR). The electrochemical characteristics were studied using galvanostatic cycling and the galvanostatic intermittent titration technique (GITT). Operando XRD was used to investigate the mechanism of sodium insertion. The NTO/C, with an average operation voltage of ~0.9 V vs. Na/Na⁺, shows a high electrochemical stability (89% capacity retention after 250 cycles at 1C), and high capacity (135 mAh g^-1 at 0.1C, 105 mAh g^-1 at 1C and 61 mAh g^-1 at 10C). Consequently, this NTO/C is currently the most energy dense material of all stable lithium-free SIB anode materials with an appropriate operation potential, even slightly surpassing O3-NaTiO₂³. As it also requires only abundant and inexpensive elements, we believe that NTO/C is an excellent material for commercial SIB anodes.

1. Slater, M. D., Kim, D., Lee, E. & Johnson, C. S. Sodium-ion batteries. Adv. Funct. Mater. 23, 947–958 (2013).
2. Yabuuchi, N., Kubota, K., Dahbi, M. & Komaba, S. Research Development on Sodium-Ion Batteries. Chem. Rev. 114, 11636–11682 (2014).
3. Wu, D. et al. NaTiO₂: a layered anode material for sodium-ion batteries. Energy Environ. Sci. 8, 195–202 (2015).