Optimizing AlF3 atomic layer deposition using trimethylaluminum and TaF5: Application to high voltage Li-ion battery cathodes

Atomic layer deposition (ALD) of conformal AlF3 coatings onto both flat silicon substrates and high-voltage LiNi0.5Mn0.3Co0.2O2 (NMC) Li-ion battery cathode powders was investigated using a Al(CH3)3/TaF5 precursor combination. This optimized approach employs easily handled ALD precursors, while also obviating the use of highly toxic HF(g). In studies conducted on planar Si wafers, the film's growth mode was dictated by a competition between the desorption and decomposition of Ta reaction byproducts. At T ≥ 200 °C, a rapid decomposition of the Ta reaction byproducts to TaC led to continuous deposition and high concentrations of TaC in the films. A self-limited ALD growth mode was found to occur when the deposition temperature was reduced to 125 °C, and the TaF5 exposures were followed by an extended purge. The lower temperature process suppressed conversion of TaFx(CH3)5−x to nonvolatile TaC, and the long purges enabled nearly complete TaFx(CH3)5−x desorption, leaving behind the AlF3 thin films. NMC cathode powders were coated using these optimized conditions, and coin cells employing these coated cathode particles exhibited significant improvements in charge capacity fade at high discharge rates.