One of the key requirements of large-scale grid-storage systems is development of inexpensive and safe batteries. Sodium-ion batteries using earth-abundant Fe or Ti based cathodes and anodes would be ideal candidates for such storage systems. Herein, a new phase of Na-rich and all Fe Prussian Blue Analogue, monoclinic Na2Fe2(CN)6.2H2O, is reported as a potential cathode for such grid-storage sodium-ion batteries. This water-insoluble and air-stable cathode can deliver 85 mAh g−1 at an average discharge voltage of 3 V vs Na/Na+ with excellent cycle life (3,000 cycles). Many facets about its sodium storage characteristics are discussed with particular emphasis on the role of interstitial water on the sodium storage performance and its conversion to the dehydrated rhombohedral phase. Its compatibility with a newly developed non-flammable glyme-based liquid electrolyte, 1M NaBF4 in tetraglyme, is also disclosed along with general electrochemical and thermal characterization of this electrolyte for sodium-ion battery application. Finally, three different types of full cells are revealed with either monoclinic or rhombohedral phase as cathode and graphite or the recently reported Na2Ti3O7 ⇋ Na3-xTi3O7 pathway of Na2Ti3O7 as anode. Full cell energy densities of 70–90 Wh kg−1 (using cumulative cathode and anode weights) could be obtained without any pre-cycling steps. This new cathode and safe electrolyte may hold great promise toward development of inexpensive, non-flammable and highly stable grid-storage sodium-ion batteries.

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We introduce a fire-retarding phosphate-based electrolyte, 1 M NaBF4 in triethyl phosphate with 3% vinylene carbonate as an SEI-forming additive for sodium-ion battery. With this electrolyte formulation, we achieved a...

In order to become commercially viable, sodium-ion batteries need to deliver long cycle life with good capacity and energy density while still ensuring safety. Electrolyte plays a key role forming solid electrolyte interphase (SEI) layers at low potential, which affects the thermal stability and cycle life of the anode materials under consideration. In this study, an ether-based non-flammable electrolyte, 1 M NaBF4 in tetraglyme, is tested for sodium storage using a non-carbonaceous anode material Na2Ti3O7/C, and the results are compared with those obtained with the popularly used carbonate-based electrolyte, 1 M NaClO4 in ethylene carbonate (EC) and propylene carbonate (PC) (v/v = 1:1). The Na2Ti3O7/C versus Na cells using 1 M NaBF4 in tetraglyme show a much higher first cycle Coulombic efficiency (73%) than those using 1 M NaClO4 in EC/PC (33%). Thermal stability studies using differential scanning calorimetry (DSC) conclusively show that Na2Ti3O7/C electrodes cycled with 1 M NaBF4 in tetraglyme are more thermally stable than the one cycled with 1 M NaClO4 in EC/PC. Further investigations on the formation of SEI layers were performed using attenuated total reflection–Fourier transform infrared spectroscopy, field-emission scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, electrochemical impedance spectroscopy, and DSC studies. These studies unambiguously demonstrate that the SEI formed on Na2Ti3O7/C using 1 M NaBF4 in tetraglyme is not only less resistive but also more stable than the SEI formed using 1 M NaClO4 in EC/PC.

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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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