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

Abstract

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.

A mini review on cathode materials for sodium‐ion batteries

“…Full coin cells with Na 0.9 Cu 0.12 Ni 0.10 Fe 0.30 Mn 0.43 Ti 0.05 O 2 versus hard carbon demonstrated close to 200 Wh/kg (cathode+anode) using a nonflammable glyme-based electrolyte. 21 18650 Cells with the same configuration demonstrated an energy density of 62 Wh/kg cell and a stable storage performance for 100 cycles. Based on various electrochemical and material characterization experiments, considerable improvements in the structure and storage performance were observed by Ti 4+ and Ni 2+ substitution.…”

Reviewing the Safe Shipping of Lithium-Ion and Sodium-Ion Cells: A Materials Chemistry Perspective

“…(2) In conducting the above studies, particular emphasis should be put on the effect of the electrolyte as it is known that there is an intimate relationship between the safety of a Li-ion cell and the electrolyte it contains [6,12]. Some recent results on large-scale Na-ion cells indicate that Na-ion electrolytes utilising high weight fractions of thermally stable solvents such as PC or Tetraglyme can indeed result in lower rates of heat generation or higher thresholds for onset of exothermic self-heating reactions [65][66][67][68] (3) Recently, some articles have analysed the economic potential of Na-ion batteries from a materials perspective [19][20][21]. Detailed life-cycle cost estimations for Na-ion batteries, focussing on not just material costs or resource availability, but other factors such as costs of shipping/storing Na-ion cells at 0 V, relative to Li-ion cells at 30% SOC, and also their recycling, would certainly be timely and illuminating for the alkali-ion battery community…”

Enabling both ultrahigh initial coulombic efficiency and superior stability of Na2Ti3O7anodes by optimizing binders

“…Later, Du et al found that changing the electrolyte (1 M NaBF 4 in tetraglyme instead of 1 M NaClO 4 in ethylene carbonate (EC) and propylene carbonate (PC) (v/v = 1 : 1)) could increase ICE of Na 2 Ti 3 O 7 /C from 33% to 73%, however, the compromise behind this breakthrough is to increase the cut-off voltage, which greatly limits the release of capacity. 26…”

“…Later, Du et al found that changing the electrolyte (1 M NaBF 4 in tetraglyme instead of 1 M NaClO 4 in ethylene carbonate (EC) and propylene carbonate (PC) (v/v = 1 : 1)) could increase ICE of Na 2 Ti 3 O 7 /C from 33% to 73%, however, the compromise behind this breakthrough is to increase the cut-off voltage, which greatly limits the release of capacity. 26 To nd a more general approach, we turn our attention to the binders suitable for NTO. From the previously reported but rare literature, it has been demonstrated that PVdF undergoes dehydrouorination during the electrode preparation of NTO, 27 and the catalytic activity of titanate materials may affect the binder and electrolyte.…”