Ultrathin Oxide Films by Atomic Layer Deposition on Graphene

In this paper, a method is presented to create and characterize mechanically robust, free-standing, ultrathin, oxide films with controlled, nanometer-scale thickness using atomic layer deposition (ALD) on graphene. Aluminum oxide films were deposited onto suspended graphene membranes using ALD. Subsequent etching of the graphene left pure aluminum oxide films only a few atoms in thickness. A pressurized blister test was used to determine that these ultrathin films have a Young’s modulus of 154 ± 13 GPa. This Young’s modulus is comparable to much thicker alumina ALD films. This behavior indicates that these ultrathin two-dimensional films have excellent mechanical integrity. The films are also impermeable to standard gases suggesting they are pinhole-free. These continuous ultrathin films are expected to enable new applications in fields such as thin film coatings, membranes, and flexible electronics

Effect of Al₂O₃ Coating on Stabilizing LiNi_0.4Mn_0.4Co_0.2O₂ Cathodes

Using atomic layer deposition of Al₂O₃ coating, improved high-voltage cycling stability has been demonstrated for the layered nickel–manganese–cobalt pseudoternary oxide, LiNi_0.4Mn_0.4Co_0.2O₂. To understand the effect of the Al₂O₃ coating, we have utilized electrochemical impedance spectroscopy, operando synchrotron-based X-ray diffraction, and operando X-ray absorption near edge fine structure spectroscopy to characterize the structure and chemistry evolution of the LiNi_0.4Mn_0.4Co_0.2O₂ cathode during cycling. Using this combination of techniques, we show that the Al₂O₃ coating successfully mitigates the strong side reactions of the active material with the electrolyte at higher voltages (>4.4 V), without restricting the uptake and release of Li ions. The impact of the Al₂O₃ coating is also revealed at beginning of lithium deintercalation, with an observed delay in the evolution of oxidation and coordination environment for the Co and Mn ions in the coated electrode due to protection of the surface. This protection prevents the competing side reactions of the electrolyte with the highly active Ni oxide sites, promoting charge compensation via the oxidation of Ni and enabling high-voltage cycling stability