Investigations of Thermal Stability and Solid Electrolyte Interphase on Na₂Ti₃O₇/C as a Non-carbonaceous Anode Material for Sodium Storage Using Non-flammable Ether-based Electrolyte

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.