Atomic Layer Deposition of Nitrogen-Doped Al Phosphate Coatings for Li-Ion Battery Applications

In situ nitrogen doping of aluminum phosphate has been investigated in two different plasma-enhanced atomic layer deposition (PE-ALD) processes. The first method consisted of the combination of trimethyl phosphate plasma (TMP*) with a nitrogen plasma and trimethyl aluminum (TMA), that is, TMP*-N₂*-TMA. The second method replaces TMP* with a diethylphosphoramidate plasma (i.e., DEPA*-N₂*-TMA) of which the amine group could further aid nitrogen doping and/or eliminate the need for a nitrogen plasma step. At a substrate temperature of 320 °C, the TMP*-based process showed saturated growth (0.8 nm/cycle) of a nitrogen-doped (approximately 8 atom %) Al phosphate, while the process using DEPA* showed a similar amount of nitrogen but a significantly higher growth rate (1.4 nm/cycle). In the latter case, nitrogen doping could also be achieved without the nitrogen plasma, but this leads to a high level of carbon contamination. Both films were amorphous as-deposited, while X-ray diffraction peaks related to AlPO₄ appeared after annealing in a He atmosphere. For high coating thickness (>2 nm), a significant increase in the Li-ion transmittance was found after nitrogen doping, although the coating has to be electrochemically activated. At lower thickness scales, such activation was not needed and nitrogen doping was found to double the effective transversal electronic conductivity. For the effective transversal ionic conductivity, no conclusive difference was found. When a lithium nickel manganese cobalt oxide (NMC) powder is coated with one ALD cycle of N-doped Al phosphate, the rate capability and the energy efficiency of the electrode improves.