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 LiNi_0.5Mn_0.3Co_0.2O₂ (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 TaF_x(CH₃)_5-x to nonvolatile TaC, and the long purges enabled nearly complete TaF_x(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.