Direct Observation of Lattice Aluminum Environments in Li Ion Cathodes LiNi_{1–<i>y</i>–<i>z</i>}Co_<i>y</i>Al_<i>z</i>O₂ and Al-Doped LiNi_<i>x</i>Mn_<i>y</i>Co_<i>z</i>O₂ via ²⁷Al MAS NMR Spectroscopy

Direct observations of local lattice aluminum environments have been a major challenge for aluminum-bearing Li ion battery materials, such as LiNi_{1–<i>y</i>–<i>z</i>}Co_<i>y</i>Al_<i>z</i>O₂ (NCA) and aluminum-doped LiNi_<i>x</i>Mn_<i>y</i>Co_<i>z</i>O₂ (NMC). ²⁷Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy is the <i>only</i> structural probe currently available that can <i>qualitatively</i> and <i>quantitatively</i> characterize lattice and nonlattice (i.e., surface, coatings, segregation, secondary phase etc.) aluminum coordination and provide information that helps discern its effect in the lattice. In the present study, we use NMR to gain new insights into transition metal (TM)–O–Al coordination and evolution of lattice aluminum sites upon cycling. With the aid of first-principles DFT calculations, we show direct evidence of lattice Al sites, nonpreferential Ni/Co–O–Al ordering in NCA, and the lack of bulk lattice aluminum in aluminum-“doped” NMC. Aluminum coordination of the paramagnetic (lattice) and diamagnetic (nonlattice) nature is investigated for Al-doped NMC and NCA. For the latter, the evolution of the lattice site(s) upon cycling is also studied. A clear reordering of lattice aluminum environments due to nickel migration is observed in NCA upon extended cycling

First-Cycle Evolution of Local Structure in Electrochemically Activated Li₂MnO₃

X-ray absorption spectroscopy is utilized to determine changes in the local structure of Li₂MnO₃ resulting from electrochemical extraction and insertion of lithium. Specially prepared electrodes allow for a first-cycle charge close to 100% of theoretical, even in crystalline Li₂MnO₃ prepared at 850 °C. After full delithiation to ∼5 V, the local Mn environments are similar to those of the freshly prepared Li₂MnO₃ electrodes but with increased disorder. On discharge, significant reduction of Mn is observed accompanied by rearrangement of Mn resulting in Jahn–Teller distorted, Mn³⁺ local environments. It is also shown, as expected, that the electrochemical and structural behavior of pure Li₂MnO₃ is markedly different than that observed for the structurally similar Li₂MnO₃ component of lithium- and manganese-rich composite cathodes

Rhodium Catechol Containing Porous Organic Polymers: Defined Catalysis for Single-Site and Supported Nanoparticulate Materials

A single-site, rhodium­(I) catecholate containing porous organic polymer was prepared and utilized as an active catalyst for the hydrogenation of olefins in both liquid-phase and gas-phase reactors. Liquid-phase, batch hydrogenation reactions at 50 psi and ambient temperatures result in the formation of rhodium metal nanoparticles supported within the polymer framework. Surprisingly, the Rh­(I) complex is catalytically active and stable for propene hydrogenation at ambient temperatures under gas-phase conditions, where reduction of the Rh­(I) centers to Rh­(0) nanoparticles requires at least 200–250 °C under a flow of hydrogen gas. After high-temperature treatment, the Rh(0) nanoparticles are active arene hydrogenation catalysts that convert toluene to methylcyclohexadiene at a rate of 9.3 × 10^–3 mol g^–1 h^–1 of rhodium metal at room temperature. Conversely, single-site Rh­(I) is an active and stable catalyst for the hydrogenation of propylene (but not toluene) under gas-phase conditions at room temperature

Understanding the Role of Temperature and Cathode Composition on Interface and Bulk: Optimizing Aluminum Oxide Coatings for Li-Ion Cathodes

Surface coating of cathode materials with Al₂O₃ has been shown to be a promising method for cathode stabilization and improved cycling performance at high operating voltages. However, a detailed understanding on how coating process and cathode composition change the chemical composition, morphology, and distribution of coating within the cathode interface and bulk lattice is still missing. In this study, we use a wet-chemical method to synthesize a series of Al₂O₃-coated LiNi_0.5Co_0.2Mn_0.3O₂ and LiCoO₂ cathodes treated under various annealing temperatures and a combination of structural characterization techniques to understand the composition, homogeneity, and morphology of the coating layer and the bulk cathode. Nuclear magnetic resonance and electron microscopy results reveal that the nature of the interface is highly dependent on the annealing temperature and cathode composition. For Al₂O₃-coated LiNi_0.5Co_0.2Mn_0.3O₂, higher annealing temperature leads to more homogeneous and more closely attached coating on cathode materials, corresponding to better electrochemical performance. Lower Al₂O₃ coating content is found to be helpful to further improve the initial capacity and cyclability, which can greatly outperform the pristine cathode material. For Al₂O₃-coated LiCoO₂, the incorporation of Al into the cathode lattice is observed after annealing at high temperatures, implying the transformation from “surface coatings” to “dopants”, which is not observed for LiNi_0.5Co_0.2Mn_0.3O₂. As a result, Al₂O₃-coated LiCoO₂ annealed at higher temperature shows similar initial capacity but lower retention compared to that annealed at a lower temperature, due to the intercalation of surface alumina into the bulk layered structure forming a solid solution

Effect of Cooling Rates on Phase Separation in 0.5Li₂MnO₃·0.5LiCoO₂ Electrode Materials for Li-Ion Batteries

The results of a detailed structural investigation on the influence of cooling rates in the synthesis of lithium- and manganese-rich 0.5Li₂MnO₃·0.5LiCoO₂ composite electrode materials, which are of interest for Li-ion battery applications, are presented. It is shown that a low-temperature, intermediate firing step, often employed in cathode synthesis, yields a minor secondary component representing a polydisperse distribution of lattice parameters, not found in the absence of low-temperature treatments. However, regardless of the heating and cooling conditions employed, all samples present two distinctly different local environments as evidenced by X-ray absorption fine structure spectroscopy (XAFS) and nuclear magnetic resonance (NMR) analysis. Transmission electron microscopy (TEM) data show discrete domain structures that are consistent with the XAFS and NMR findings. Furthermore, high resolution synchrotron X-ray diffraction (HR-XRD), as well as the XAFS and NMR data show no discernible differences between sample sets heated in similar fashion and subsequently cooled at different rates. The results contradict recent reports, using X-ray diffraction, that rapidly quenched samples of the same composition are true solid solutions. This apparent discrepancy is assigned, in part, to the inherent nature of conventional diffraction, which firmly elucidates the average long-range structure but does not capture the local domain microstructure of these nanocomposite materials. The combined use of HR-XRD, XAFS, NMR, and TEM data indicate that charge ordering, which is initiated at relatively low temperatures, is the dominant force that produces a nanoscale, inhomogeneous composite structure, irrespective of the cooling rate