Limitations of Ultrathin Al₂O₃ Coatings on LNMO Cathodes

Abstract

This study demonstrates the application of Al2O3 coatings for the high-voltage cathode material LiNi0.5–x Mn1.5+x O4−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.

Improved Electrochemical Performance for High-Voltage Spinel LiNi0.5Mn1.5O4 Modified by Supercritical Fluid Chemical Deposition

“…The O 1s spectrum of the coating material prepared as a reference in SFCD conditions shows only three contributions with the most intense at 531.9 eV. This one is attributed to Al−O bonding energies, 55 while the others could correspond not only to the C−O-type bonds but also to the O−H-type bonds as it is highlighted in FT-IR. In the case of the coated LNMO materials (SFCD 2.5% and DIS 2.5%), the ratio of intensities between the O 1s peak at 529.8 eV (Mn−O, Ni−O) and the one at 531.9 eV (Al−O) appears significantly dependent on the process used and is thus a very good criterion to determine the efficiency of the process to prepare a covering coating: the higher the ratio, the less covered is the active material (Figure 8a).…”

“…For the O 1s core level (Figure 8d), bare LNMO shows the main contribution at 529.7 eV associated with M−O bonds in LNMO (M = Ni, Mn). Minor 55 but also to OH bonds and adsorbed water. 56 Carbon pollution is at the origin of some of these peaks.…”

Soft Carbon Integration for Prolonging the Cycle Life of LiNi0.5Mn1.5O4 Cathode

“…It acts as a physical barrier between the electrode surface and the electrolyte without affecting Li + intercalation–deintercalation, thereby minimizing parasitic reactions. Previous literature reports demonstrate the utilization of metals, metal oxides, metal fluorides, metal phosphates, and so forth, as the coating layers. – The application of direct metal as the coating layer brings in limited improvement in performance. The metal oxide and metal fluoride coatings provide excellent oxidation resistance and mechanical protection.…”

On the Durability of Protective Titania Coatings on High‐Voltage Spinel Cathodes

“…This flexibility enables cost‐efficient implementation of the coating, as it is not necessary to choose complicated and expensive coating techniques and/or precursors. Many oxide coating materials have, however, limited Li‐ion and electric conductivity and can form a Li diffusion barrier that increases the impedance of the cell [13,18] . A merit of TiO 2 as a coating material candidate is the relatively high Li‐ion conductivity compared to, for example, Al 2 O 3.…”

“…Many oxide coating materials have, however, limited Li-ion and electric conductivity and can form a Li diffusion barrier that increases the impedance of the cell. [13,18] A merit of TiO 2 as a coating material candidate is the relatively high Li-ion conductivity compared to, for example, Al 2 O 3. The rutile and anatase phases have been reported to have diffusion coefficients of 1 × 10 À 6 cm 2 s À 1 and 4.7 × 10 À 12 cm 2 s À 1 at 300 K, respectively.…”