Na(2+x)Ti6O13 as potential negative electrode material for Na-ion batteries

Area of Science:

• Materials Science
• Electrochemistry
• Energy Storage

Background:

• Sodium-ion (Na-ion) batteries are a promising alternative to lithium-ion batteries due to sodium's abundance and low cost.
• Na-ion batteries offer comparable cell potentials to Li-ion batteries.

Purpose of the Study:

• To investigate the effect of lowering the cutoff voltage on the capacity of Na2Ti6O13 negative electrode material.
• To understand the structural evolution and intercalation mechanism of Na-ions in Na2Ti6O13.

Main Methods:

• In situ X-ray diffraction (XRD) was used to monitor structural changes during cycling.
• Density functional theory (DFT) calculations were employed to complement experimental observations.
• Electrochemical performance was evaluated by cycling the Na2Ti6O13 electrode at different cutoff voltages.

Main Results:

• Lowering the cutoff voltage from 0.3 V to 0 V vs Na/Na(+) increased the capacity of Na2Ti6O13 from 49.5 mAh/g to 196 mAh/g for at least 10 cycles.
• Na-ion intercalation is limited to Na(2+2)Ti6O13 and proceeds via a reversible solid solution reaction with minimal lattice parameter changes.
• Decreased crystallinity and solid electrolyte interphase (SEI) formation at higher Na compositions (below 0.3 V) contribute to capacity fade.

Conclusions:

• Na2Ti6O13 exhibits significantly enhanced capacity at lower cutoff voltages, demonstrating its potential as a negative electrode material for Na-ion batteries.
• The intercalation mechanism is primarily a reversible solid solution process, but capacity fade occurs due to structural degradation and SEI formation at deeper discharge levels.
• Further optimization of electrode materials and electrolytes is needed to improve the long-term cycling stability of Na-ion batteries operating at low voltages.