The (electro)­chemical reactions between positive electrodes and electrolytes are not well understood. We examined the oxidation of a LiPF6-based electrolyte with ethylene carbonate (EC) with layered lithium nickel, manganese, and cobalt oxides (NMC). Density functional theory calculations showed that the driving force for EC dehydrogenation on oxides, yielding surface protic species, increased with greater Ni content in NMC. Ex situ infrared and Raman spectroscopy revealed experimental evidence for EC dehydrogenation on charged NMC surfaces. Protic species on charged NMC surfaces from EC dehydrogenation could further react with LiPF6 to generate less-coordinated F species such as PF3O-like and lithium nickel oxyfluoride species on charged NMC particles and HF and PF2O2 – in the electrolyte. Larger degree of salt decomposition was coupled with increasing EC dehydrogenation on charged NMC with increasing Ni or lithium deintercalation. An oxide-mediated chemical oxidation of electrolytes was proposed, providing new insights in stabilizing high-energy positive electrodes and improving Li-ion battery cycle life.

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Surface chemistry modification of positive electrodes has been used widely to decrease capacity loss during Li-ion battery cycling. Recent work shows that coupled LiPF 6 decomposition and carbonate dehydrogenation is enhanced by increased metal-oxygen covalency associated with increasing Ni and/or lithium de-intercalation in metal oxide electrode, which can be responsible for capacity fading of Ni-rich oxide electrodes. Here we examined the reactivity of lithium nickel, manganese, cobalt oxide (LiNi 0.6 Mn 0.2 Co 0.2 O 2 , NMC622) modified by coating of Al 2 O 3 , Nb 2 O 5 and TiO 2 with a 1 M LiPF 6 carbonate-based electrolyte. Cycling measurements revealed that Al 2 O 3 -coated NMC622 showed the least capacity loss during cycling to 4.6 V Li compared to Nb 2 O 5 -, TiO 2 -coated and uncoated NMC622, which was in agreement with smallest electrode impedance growth during cycling from electrochemical impedance spectroscopy (EIS). Ex-situ infrared spectroscopy of charged Nb 2 O 5 -and TiO 2 -coated NMC622 pellets (without carbon nor binder) revealed blue peak shifts of 10 cm −1 , indicative of dehydrogenation of ethylene carbonate (EC), but not for Al 2 O 3 -coated NMC622. X-ray Photoelectron Spectroscopy (XPS) of charged TiO 2 -coated NMC622 electrodes (carbon-free and binder-free) showed greater salt decomposition with the formation of lithium-nickel-titanium oxyfluoride species, which was in agreement with ex-situ infrared spectroscopy showing greater blue shifts of P-F peaks with increased charged voltages, indicative of species with less F-coordination than salt PF 6 − anion on the electrode surface. Greater salt decomposition was coupled with the increasing dehydrogenation of EC with higher coating content on the surface. This work shows that Al 2 O 3 coating on NMC622 is the most effective in reducing carbonate dehydrogenation and accompanied salt decomposition and rendering minimum capacity loss relative to TiO 2 and Nb 2 O 5 coating.

The hydrogen adsorption energetics on the surface of inorganic compounds can be used to predict electrolyte stability in Li-ion batteries and catalytic activity for selective oxidation of small molecules such as H2 and CH4. Using first-principles density functional theory (DFT), the hydrogen adsorption was found to be unfavorable on high band-gap insulators, which could be attributed to lower energy level associated with adsorbed hydrogen relative to the bottom of conduction band. In contrast, the hydrogen adsorption was shown the most favorable on metallic and semiconducting compounds, which results from an electron transfer from adsorbed hydrogen to the Fermi level or the bottom of conduction band. Of significance, computed hydrogen adsorption energetics on insulating, semiconducting and metallic oxides, phosphates, fluorides, and sulfides were decreased by lowering the ligand p band center while the energy penalty for ligand vacancy formation was increased, indicative of decreased surface reducibility. A statistical regression analysis, where 16 structural and electronic parameters such as metal-ligand distance, electronegativity difference, Bader charges, bulk and surface metal and ligand band centers, band gap, ligand band width and work function were examined, further showed that the surface ligand p band center is the most accurate single descriptor that governs the hydrogen adsorption tendency, and additional considerations of the band gap and average metal-ligand distance further reconcile the differences among compounds with different ligands/structures, which ligand bands are different in shape and width. We discuss the implications of these findings for passivating coatings and catalysts design and the need for novel theoretical methods to accurately estimate these quantities from first principles. These results establish a universal design principle for future high-throughput studies aiming to design electrode surfaces to minimize electrolyte oxidation by dehydrogenation in Li-ion batteries and enhance the H-H and C-H activation for selective oxidation catalysis.

Graphene has amazing abilities due to its unique band structure characteristics defining its enhanced electrical capabilities for a material with the highest characteristic mobility known to exist at room temperature. The high mobility of graphene occurs due to electron delocalization and weak electron-phonon interaction, making graphene an ideal material for electrical applications requiring high mobility and fast response times. In this review, we cover graphene's integration into infrared IR devices, electro-optic EO devices, and field effect transistors FETs for radio frequency RF applications. The benefits of utilizing graphene for each case are discussed, along with examples showing the current state-of-the-art solutions for these applications.Graphene has many outstanding properties due to its unique bonding and subsequently band gap characteristics, having electronic carriers act as massless DiracFermions. The material characteristics of graphene are anisotropic, having phenomenal characteristic within a single sheet and diminished material characteristics between sheet with increasing sheet number and grain boundaries. We will discuss the integration of graphene into many electronic device applications.Graphene has the highest mobility values measured in a material at room temperature, allowing integration into fast response time devices such as a high electron mobility transistor HEMT for RF applications. Graphene has shown promise in IR detectors by utilizing graphene in thermal-based detection applications.