Abstract
This study demonstrates the application of Al2O3 coatings for the high-voltage cathode material LiNi0.5–xMn1.5+xO4−δ (LNMO) by atomic layer deposition. The ultrathin and uniform coatings (0.6–1.7 nm) were deposited on LNMO particles and characterized by scanning transmission electron microscopy, inductively coupled plasma mass spectrometry, and X-ray photoelectron spectroscopy. Galvanostatic charge discharge cycling in half cells revealed, in contrast to many published studies, that even coatings of a thickness of 1 nm were detrimental to the cycling performance of LNMO. The complete coverage of the LNMO particles by the Al2O3 coating can form a Li-ion diffusion barrier, which leads to high overpotentials and reduced reversible capacity. Several reports on Al2O3-coated LNMO using alternative coating methods, which would lead to a less homogeneous coating, revealed the superior electrochemical properties of the Al2O3-coated LNMO, suggesting that complete coverage of the particles might in fact be a disadvantage. We show that transition metal ion dissolution during prolonged cycling at 50 °C is not hindered by the coating, resulting in Ni and Mn deposits on the Li counter electrode. The Al2O3-coated LNMO particles showed severe signs of pitting dissolution, which may be attributed to HF attack caused by side reactions between the electrolyte and the Al2O3 coating, which can lead to additional HF formation. The pitting dissolution was most severe for the thickest coating (1.7 nm). The uniform coating coverage may lead to non-uniform conduction paths for Li, where the active sites are more susceptible to HF attack. Few benefits of applications of very thin, uniform, and amorphous Al2O3 coatings could thus be verified, and the coating is not offering long-term protection from HF attack.
Highlights
Energy storage and electrification of the transport sector are critical measures for reducing global greenhouse gas emissions
Higher deposition temperatures and/or post-deposition heating steps are interesting approaches in order to change the crystallinity of the coating layer and increase the interfacial bonding strength between the LNMO substrate and the coating layer
This space group of LNMO is characterized by structural disorder where Mn and Ni cations are randomly positioned on the 16d sites and Li and O atoms are occupying 8a and 32e sites, respectively.[39]
Summary
Energy storage and electrification of the transport sector are critical measures for reducing global greenhouse gas emissions. The very uniform Al2O3 coating layer generated by ALD could lead to non-uniform conduction paths for Li, where Li ions only travel in and out of the particle at selected and electrochemically active points where the coating is thinner or absent, in particular for thicker Al2O3 coatings This immense reduction in electrochemically active surface area could explain the high polarization and very low reversible capacity of the 20 ALD sample. These active sites could be more susceptible to HF attack and eventually lead to the formation of the observed holes in the LNMO particles, as they cause very localized water generation leading to further HF generation. Similar changes in micro- and nanostructure cannot be excluded for the 5 and 10 ALD Al2O3 samples, but the more homogeneous current distribution for these samples will presumably make this effect less prominent
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