Ultrathin atomic layer deposition Al₂O₃ coatings on graphite cathode materials for improving anti-self-discharging Al-ion battery

Objectives: Self-discharging is a natural behavior that occurs in all batteries. In this work, to reduce the self-discharging behavior of Al-ion battery, the graphite materials were coated with different thickness of Al2O3 coatings (3, 5, 7, and 10 nm) by using atomic layer deposition (ALD) technique. Because there is no solid electrolyte interface (SEI) formed in Al-ion battery with using ionic liquid electrolyte, ALD-Al2O3 coating could play a role in artificial SEI in Al-ion battery. Methods: The aluminum oxide (Al2O3) layer on the graphite electrode was deposited via ALD. During the deposition, the growth temperature for Al2O3 was kept at 80 degrees C. High-purity N2 was used to carry the gas precursors of trimethylaluminium Al-2(CH3)(6) (TMA) and H2O alternately into the reacting chamber of ALD. Each injection of gas precursor was followed by purging the reacting chamber with high-purity N-2. By controlling different reaction times, the different thickness of ALD-Al2O3 coatings on the surface of graphite materials can be obtained. Results: We found that SP1 graphite coated with 5 nm Al2O3 (SG-ALD-5 nm) as cathode materials in Al-ion battery exhibited a superior anti-self-discharging behavior (260 h) with long stability (similar to 900 cycles). The battery capacity can achieve up to 85 mAh/g at a current density of 300 mA/g (3C) with a coulombic efficiency (CE) of 98.5%. Furthermore, the ALD-Al2O3 coating method can also be applied to other graphite materials. The battery based on flexuous graphite with an ALD-Al2O3 coating (FG-ALD) also exhibited excellent discharging capacity of 140 mAh/g with good cycle stability over 3000 cycles at a current density of 3000 mA/g. Conclusion: Overall results indicated that ALD coatings on graphite materials demonstrated significantly promising on not only reducing the self-discharging problem but also remaining the superior battery performance.