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Sodium-ion batteries (NIBs) have been emerging as one of the most promising candidates for stationary storage applications such as telecommunication towers, micro-grids etc., mainly because Na is one of the most abundant elements on the Earth’s crust.1,2 NIB operating at ambient temperature is expected to be durable, safe and inexpensive. Regardless of the relatively lower energy density of NIBs, they can be effectively employed for stationary applications, where the weight and footprint requirements are not severe.3 However, identifying appropriate anode, cathode, electrolyte, as well as the combination of these three components have always been challenging to develop robust NIB.4,5 In this talk, we will present investigation of the storage performance, thermal stability6 and SEI layers of four notable anodes, viz., hard carbon, graphite, TiO2 and Na2Ti3O7 7 using ether-based non-flammable electrolyte: 1M NaBF4 in tetraglyme and compare with the results obtained with commonly used carbonate-based electrolyte: 1M NaClO4 in EC:PC. We report better storage performance with higher first cycle coulombic efficiency of above anodes tested against metallic Na using ether-based electrolyte compared to carbonate-based electrolytes. Thermal studies, ATR-FTIR and impedance spectroscopy recorded at fully sodiated and fully desodiated states of these four anodes further confirm that a more stabilized SEI is formed by ether-based electrolyte. Above studies further suggests that the ether-based electrolyte is much safer for NIBs compared to carbonate-based electrolytes such as 1M NaClO4 in EC:PC. For the cathode, Na3V2(PO4)3 (NVP) was chosen due to a high redox potential of 3.37 V vs. Na/Na+. By employing a highly scalable synthesis procedure8 two types of NVP are prepared: pristine NVP and modified NVP by aliovalent doping. Sodium storage performances (specific capacity, rate performance and cycle life) of modified NVP outperforms the pristine NVP. The observed superior storage performance in modified NVP is attributed to enhanced activity of vanadium (V3+ to V4+ and V4+ to V5+)9 as confirmed by XPS studies and higher chemical diffusion coefficient. We also present storage performance, XPS studies, measurement of heat loss and internal resistance of 18650-type non-flammable NIB cells made using NVP (pristine- and modified- NVP) vs. HC with 1M NaBF4 in tetraglyme as electrolyte. The 18650 cell of pristine NVP vs. HC shows low energy density (47 Wh.kg− 1), moderate rate performance and poor cyclability. On the other hand, the 18650 cell of modified NVP vs. HC exhibits improved energy density (60 Wh.kg− 1) and enhanced rate and cyclic performances. Further, we report lesser heat generation in modified NVP vs. HC cell compared to pristine NVP vs. HC cell. Corresponding internal resistance of these 18650 cells measured at different depths of discharge (DoD) and temperature intervals reveal improved chemical diffusion coefficient, and substantial reduction in charge transfer resistance of the modified NVP vs. HC cell caused by aliovalent doping of NVP. The work presented here for introducing a safe NIB technology for stationary storage application is an illustration of R&D with a long value chain: scale-up production of cathode materials, commercial type cell fabrication, investigation of storage performance, estimation of heat generation, quantification of heat loss in terms of internal resistance. This translational R&D at NUS thus bridges academics and industries. References: B. Dunn, H. Kamath and J.-M. Tarascon, Science, 2011, 334, 928-935. N. Yabuuchi, K. Kubota, M. Dahbi and S. Komaba, Chemical Reviews, 2014, 114, 11636-11682. M. Armand and J.-M. Tarascon, Nature, 2008, 451, 652-657. J. Wang, Y. Yamada, K. Sodeyama, E. Watanabe, K. Takada, Y. Tateyama and A. Yamada, Nature Energy, 2018, 3(1), 22–29. C. Delmas, Advanced Energy Materials, 2018, 8(17), 1–9. A.Ponrouch, E. Marchante, M. Courty, J. M. Tarascon and M. R. Palacin, Energy and Environmental Science, 2012, 5(9), 8572–8583. J. Xu, C. Ma, M. Balasubramanian and Y. S. Meng, Chemical Communications, 2014, 3, 1–4. 8. Saravanan, C. W. Mason, A. Rudola, K. H. Wong, P. Balaya, Advanced Energy Materials, 2013, 3, 444-450. F. Lalère, V. Seznec, M. Courty, R. David, J. N. Chotard and C. Masquelier, Journal of Materials Chemistry A, 2015, 3, 16198-16205.

The accelerating consumption of the limited lithium resources may affect future prices of lithium-ion batteries. In contrast, sodium-ion battery is considered to be a cheaper alternative to lithium-ion battery on account of the globally abundant sodium resources1. In order to become commercially viable, the sodium-ion battery needs to deliver long cycling life with good capacity and energy density while still ensuring safety. In this study, the interactions between selected sodium-ion anodes and electrolytes were carefully investigated. A newly designed ether-based non-flammable electrolyte, 1M NaBF4 in tetraglyme2 was tested with three types of anodes (Na2Ti3O7/C, Graphite and Hard Carbon), and the results were compared with those obtained with the popularly used carbonate-based electrolyte, 1M NaClO4 in EC:PC (v:v=1:1). With 1M NaBF4 in tetraglyme electrolyte, stable half-cell cycling performance was achieved for all three anodes. Being glyme-based, this electrolyte also enabled excellent cycling of graphite anode through solvent co-intercalation reaction mechanism (with conventional carbonate-based electrolytes, graphite cannot store sodium)2,3. Compared with 1M NaClO4 in EC:PC, 1M NaBF4 in tetraglyme showed much higher first cycle coloumbic efficiencies for Na2Ti3O7/C and Hard Carbon half-cells, indicating less solid -electrolyte interphase (SEI) formation on the surface of the electrodes. Furthermore, the cycling stability and capacity of Na2Ti3O7/C were similar with both of the two electrolytes while better performance was achieved with 1M NaBF4 in tetraglyme for Hard Carbon. Thermal stability studies were conducted for all the electrodes at the states of pristine, fully sodiated and fully desodiated using Differential Scanning Calorimetry (DSC). Furthermore, investigations on the SEI formation on the various anodes were also performed using the techniques of Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR), Field Emission Scanning Electron Microscope (FESEM), Energy-Dispersive X-ray Spectroscopy (EDX), Electrochemical Impedance Spectroscopy (EIS) and DSC. The DSC analysis showed that the anodes cycled with 1M NaBF4 in tetraglyme were more thermally stable than their counterparts cycled with 1M NaClO4 in EC:PC. The SEI investigations corroborated the cycling and DSC results, indicating that the SEI formed on the various anodes using 1M NaBF4 in tetraglyme was not only thinner, but also thermally more stable than the SEI formed using 1M NaClO4 in EC:PC. Such details from the various studies, which will be revealed in the presentation, unambiguously proved that 1M NaBF4 in tetraglyme was a much safer choice as an electrolyte for sodium-ion batteries compared with 1M NaClO4 in EC:PC especially for low voltage anodes. [1] Kubota, K., & Komaba, S. (2015). Review—Practical Issues and Future Perspective for Na-ion Batteries. Journal of The Electrochemical Society, 162(14), A2538–A2550. [2] Rudola, A., Du, K., & Balaya, P. (2017). Monoclinic Sodium Iron Hexacyanoferrate Cathode and Non-Flammable Glyme-Based Electrolyte for Inexpensive Sodium-ion Batteries, Journal of The Electrochemical Society, 164 (6), A1098-A1109. [3] Jache, B.; Adelhelm, P. (2014) Use of Graphite as a Highly Reversible Electrode with Superior Cycle Life for Sodium-Ion Batteries by Making Use of Co-Intercalation Phenomena. Angew. Chemie - Int. Ed., 53 (38), 10169–10173.

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