Metal oxide coatings on cathodes can improve electrochemical stability and longer life cycle of rechargeable Li-ion batteries. Such coatings protects the cathode surface from the direct contact with electrolyte, helps to prevent the formation of SEI (solid electrolyte interphase) layer, reduce ionic dissolution and that results in improved capacity retention, columbic efficiency and longer cycle performance ^[1,2]. In literature, several oxides and fluorides coatings such as - Al₂O₃, MgO, TiO₂, ZnO, SnO₂, ZrO₂ and LaF₃,  AlF₃, MgF₂, have been suggested which have demonstrated improved capacity retention ^[3].

Among those coatings, amorphous Al₂O₃ is probably the most effective coating for capacity retention and stability of the cell ^[4]. But Al₂O₃ is a high bandgap insulating material which impose a large resistance on cathode surface resulting smaller charge-discharge capacity at higher C-rates i.e. poor rate performance ^[5]. Therefore, we propose mixed oxides coatings: Al₂O₃-Ga₂O₃, Al₂O₃-MgO and Al₂O₃-ZrO₂ to improve the conductivity of the coatings. The key reason for selectively choosing Ga, Mg and Zr to introduce into Al₂O₃ system is following. As Ga₂O₃ is known for higher electron conductivity, the mixing of Ga should increase the electron conductivity and that can be tuned by varying Ga-content into film. Here Mg, Al, and Zr has valance 2, 3 and 4 respectively, therefore alloying MgO or ZrO₂ with Al₂O₃ will create broken atomic bonds (i.e. increase porosity) in amorphous atomic network and we believe such broken atomic bonds or nano-voids will generate additional path for transporting Li-ion through the coatings resulting less overpotential on cathode surface. Our experiments with such mixed oxides coatings indeed demonstrated improved rate performance and better capacity retention compared to uncoated and Al₂O₃-coated NMC cathodes, tested in 2032 coin-type cell assembly. Amongst the three different mixed oxide coatings Al₂O₃-ZrO₂alloy found to be best as shown in Fig.1. An optimum mixing of Zr/(Al+Zr)=0.5 and 5-ALD cycles thickness (~0.5nm) coating helps to retain ~90% of the initial capacity of the NMC-cathode after 100 charge-discharge cycles and also it gives ~300% more improvement in capacity at 10C-rate in contrast with uncoated NMC.

To obtain nanoscale mixed-oxides coatings via ALD (as shown in Fig.2), we have demonstrated a new method ‘co-pulsing ALD’ where the metal-precursors are pulsed into reactor at the same time, instead in a sequence. That allows the metal precursors to mix into their gas-phase before reacting on cathode surface providing an intimate mixing into deposited film. The content of mixing can be controlled by varying the partial pressures of the precursors and the total thickness can be controlled by pulse number. In contrast to separate pulsing ^[6], co-pulsing allows independent control on content of mixing into ternary alloys and total thickness of the coatings at atomic-scale. That enables to tune the cathode surface properties with desired thickness essential for the battery application.

References

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2.             X. Li, J. Lu, X. Meng, Y. tang, M. N. Banis, J. Yang, Y. Hu, R. Li, M. Cai, X. Sun, J. Power Sources, 247, 57 (2014)

3.             M. Akyol, S. Kirklin, C. Wolverton. Adv. Energy Mater. 4, 1400690 (2014).

4.             J. Cho, Y. J. Kim, B. Park Chem. Mater. 4, 3788 (2000).

5.             X. Xiao, P. Lu, D. Ahn Adv. Mater. 23, 3911 (2011).  

6.             S. M. George, Chemical Reviews, 2010, 110, 111-131.