Nanoscale Mixed-Oxide Coatings for Improving Rate Performance of Li[Ni0.5Mn0.3Co0.2]O2 Cathode in a Rechargeable Li-Ion Battery
Authors: Laskar Masihhur R , Jackson David H. K. , Xu Shenzhen , Guan Yingxin , Fang Shuyu , Dreibelbis Mark , Mahanthappa Mahesh K. , Morgan Dane , Hamers Robert J , Kuech Thomas F. 
Published: February 2020

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-ZrO2 alloy 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 desirable thickness essential for the battery application.

Figure 1