Physical vapor deposited coatings on high Ni content NMC811 Li-ion battery cathode powder

The storage capacity and reliability of Lithium-ion batteries has increased at an amazing rate in the past years. They are operated in millions of electrically powered devices, but still their components are subject to continuous optimization.

In this paper we focus on the cathode material of the system, which is based on powders of Nickel-Manganese-Cobalt (NMC) mixed oxides. They store Lithium-ions by intercalation between their oxidic planes. A current trend in the development of these mixed oxides is to increase the Ni content. This would reduce the need for Co, an expensive material mined under hazardous conditions. However, a high Ni content has several drawbacks, like the irreversible occupation of Li sites by Ni, structural phase transformations upon Li removal or surface contaminations caused by the chemical reactivity of Ni.

We address the last point by passivating the surface of the powder with a thin layer of inert oxide. Alumina (Al₂O₃) or Zirconia (ZrO₂) with an approximate thickness of 0.2–1.6 nm were deposited by reactive magnetron sputtering on high Ni content NMC 811 powder with an average particle size of approx. 10 μm using a rotating and tumbling powder container. The average thickness and uniformity of the coating could be correlated to the electrical resistance of the powder, which was determined by a custom-built system for powder resistance measurement under variable compression force. These measurements were confirmed by imaging the coatings on selected powder particles using low voltage SEM. Generally, the coatings were found to be uniform on both, individual grains and large ensembles of powder particles. The impact of the coating on the release of Li-ions was checked by Li leaching experiments. The coating reduced the Li release rate by approximately 10 %, which could be tolerated in a battery.