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Here, we present a comprehensive study of choice of electrolyte, anode and cathode to develop commercially viable non-flammable sodium-ion battery. We report hard carbon (HC) vs. Na using ether-based non-flammable electrolyte (1 ​M NaBF4 in tetraglyme) and compare storage performance, thermal stability and SEI formation with those obtained using carbonate-based electrolyte (1 ​M NaClO4 in EC:PC = 1:1 v/v). The results shows that 1 ​M NaBF4 in tetraglyme works as a better electrolyte than carbonate-based electrolyte for HC anode. We present and compare storage performances of pristine and aliovalent-doped Na3V2(PO4)3 (NVP) vs. Na. Doped-NVP outperforms pristine cathode in terms of specific capacity and rate capability. 18650-type non-flammable sodium-ion cells fabricated using modified NVP vs. HC exhibits energy density of 60 ​Wh kg−1. When discharged at a high rate close to 5C, the cell successfully retains 83% of its storage capacity obtained at low rate. When cycled at C/5, doped NVP vs. HC 18650 ​cell retains 90% of its initial capacity after 200 cycles.

In this manuscript, the impact of operating conditions such as voltage window, and operating temperature on electrochemical performance and cycle life of Zn-substituted Na3.2V1.8Zn0.2(PO4)3 (NVZP) vs hard carbon (HC) coin cells filled with 1 mol dm−3 NaBF4 in tetraglyme is presented. Initially, the cells are cycled for 500 times at C/2 charge and 1 C discharge in three different voltage windows (4.20–1.00 V, 4.05–1.00 V and 4.05–1.50 V) and at two temperatures (28 °C and 40 °C) and are subjected to periodic internal resistance and impedance measurements. The elemental composition of the electrodes harvested after cycling reveals that vanadium dissolution with accompanying deposition on the HC electrode and irreversible loss of sodium causes increased cell impedance. The identified degradation mechanisms, which causes severe capacity fade, are found to be accelerated in the cells cycled over wider voltage windows, particularly at elevated temperature. The best cycling performance and lowest impedance are recorded for the cells cycled within 4.05–1.50 V at 28 °C owing to negligible vanadium dissolution. Under these optimized testing conditions, a prototype 18650 cell, shows impressive capacity retention of 77% after 1000 cycles.

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.