D. H. K. Jackson, M. Laskar, S. Fang, S. Xu (University of Wisconsin - Madison), R. G. Ellis (Purdue, University of Wisconsin - Madison), X. Li (University of Wisconsin - Madison), M. Dreibelbis (The Dow Chemical Company), S. E. Babcock (University of Wisconsin - Madison), M. K. Mahanthappa (University of Minnesota, University of Wisconsin - Madison), D. Morgan (University of Wisconsin - Madison), R. J. Hamers (Department of Chemistry, UW-Madison), and T. F. Kuech (University of Wisconsin - Madison)
Atomic layer deposition (ALD)
1, 2 of AlF
3 coatings for the Li-ion battery cathode LiNi
0.5Mn
0.3Co
0.2O
2 (NMC)
3, 4 was investigated using a combination of trimethylaluminum and TaF
5 precursors. ALD growth mode was dictated by a competition between temperature-limited desorption of Ta reaction byproducts and temperature-driven conversion of these byproducts into nonvolatile TaC. At
T ≥ 200°C, decomposition to TaC occurred, leading to continuous deposition and high concentrations of TaC in the resulting films. A self-limited ALD growth mode was found to occur when deposition temperature was reduced to 125 °C, and the TaF
5 exposures were followed by an extended purge. During ALD growth, vapor-surface ligand exchange reactions lead to formation of Ta(CH
3)
xF
5-x and AlF
3. Ta(CH
3)
xF
5-x desorbs from the surface or it decomposes via an intermolecular α-hydrogen abstraction,
5,6 eventually forming TaC. The extended purge times and lower temperatures reduce decomposition, while also allowing more time for Ta(CH
3)
xF
5-x desorption. NMC cathode powders were coated using these optimized conditions, and coin cells employing these coated cathode particles were characterized using electrochemical impedance spectroscopy and by cycle performance testing, which demonstrated significant improvements in charge capacity fade at high discharge rates as compared to uncoated materials (Figure 1). These improvements at high C-rates are attributed to the reduction of undesirable electrode-electrolyte reactions by protection of the cathode surface with the ALD coating through which fast Li
-ion transport occurs.
7-9Figure 1: Averaged gravimetric capacity of coin cells versus discharge cycle number during a rate test. Black squares (■) fluoride ALD coated, red circles (●) uncoated.
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