Layered Li-rich Ni, Mn, Co (NMC) oxide cathodes in Li-ion batteries provide high specific capacities (>250 mAh/g) via O-redox at high voltages. However, associated high-voltage interfacial degradation processes require strategies for effective electrode surface passivation. Here, we show that an acidic surface treatment of a Li-rich NMC layered oxide cathode material leads to a substantial suppression of CO 2 and O 2 evolution, 90% and ~100% respectively, during the first charge up to 4.8 V vs. Li +/0. CO 2 suppression is related to Li 2 CO 3 removal as well as effective surface passivation against electrolyte degradation. This treatment does not result in any loss of discharge capacity and provides superior long-term cycling and rate performance compared to as-received, untreated materials. We also quantify the extent of lattice oxygen participation in charge compensation ("O-redox") during Li + removal by a novel ex-situ acid titration. Our results indicate that the peroxo-like species resulting from O-redox originate on the surface at least 300 mV earlier than the activation plateau region around 4.5 V. X-ray photoelectron spectra and Mn-L X-ray absorption spectra of the cathode powders reveal a Li + deficiency and a partial reduction of Mn ions on the surface of the acid-treated material. More interestingly, although the irreversible oxygen evolution is greatly suppressed through the surface treatment, our O K-edge resonant inelastic X-ray scattering shows the lattice O-redox behavior largely sustained. The acidic treatment, therefore, only optimizes the surface of the Li-rich material and almost eliminates the irreversible gas evolution, leading to improved cycling and rate performance. This work therefore presents a simple yet effective approach to passivate cathode surfaces against interfacial instabilities during high-voltage battery operation. File list (2) download file view on ChemRxiv McCloskey_manuscript.pdf (2.13 MiB) download file view on ChemRxiv McCloskey_SuppInfo.pdf (5.91 MiB)

The dicationic complex [CpCo(azpy)(CHCN)](ClO) 1 (azpy = phenylazopyridine) exhibits a reversible two-electron reduction at a very mild potential (-0.16 V versus Fc) in acetonitrile. This behavior is not observed with the analogous bipyridine and pyrazolylpyridine complexes (3 and 4), which display an electrochemical signature typical of Co systems: two sequential one-electron reductions to Co at -0.4 V and Co at -1.0 to -1.3 V versus Fc. The doubly reduced, neutral complex [CpCo(azpy)] 2 is isolated as an air-stable, diamagnetic solid via chemical reduction with cobaltocene. Crystallographic and spectroscopic characterization together with experimentally calibrated density functional theory calculations illuminate the key structural and electronic changes that occur upon reduction of 1 to 2. The electrochemical potential inversion observed with 1 is attributed to effective overlap between the metal d and the low-energy azo π* orbitals in the intermediary redox state and additional stabilization of 2 from structural reorganization, leading to a two-electron reduction. This result serves as a key milestone in the quest for two-electron transformations with mononuclear first-row transition metal complexes at mild potentials.

The ruthenium hydride [RuH(CNN)(dppb)] (1; CNN = 2-aminomethyl-6-tolylpyridine, dppb = 1,4-bis(diphenylphosphino)butane) reacts rapidly and irreversibly with CO2 under ambient conditions to yield the corresponding Ru formate complex 2. In contrast, the Ru hydride 1 reacts with acetone reversibly to generate the Ru isopropoxide, with the reaction free energy ΔG°(298 K) = -3.1 kcal/mol measured by (1)H NMR in tetrahydrofuran-d8. Density functional theory (DFT), calibrated to the experimentally measured free energies of ketone insertion, was used to evaluate and compare the mechanism and energetics of insertion of acetone and CO2 into the Ru-hydride bond of 1. The calculated reaction coordinate for acetone insertion involves a stepwise outer-sphere dihydrogen transfer to acetone via hydride transfer from the metal and proton transfer from the N-H group on the CNN ligand. In contrast, the lowest energy pathway calculated for CO2 insertion proceeds by an initial Ru-H hydride transfer to CO2 followed by rotation of the resulting N-H-stabilized formate to a Ru-O-bound formate. DFT calculations were used to evaluate the influence of the ancillary ligands on the thermodynamics of CO2 insertion, revealing that increasing the π acidity of the ligand cis to the hydride ligand and increasing the σ basicity of the ligand trans to it decreases the free energy of CO2 insertion, providing a strategy for the design of metal hydride systems capable of reversible, ergoneutral interconversion of CO2 and formate.

One-electron oxidation in a family of square planar nickel hydride complexes leads to facile H2 evolution.

Protonation of the Co­(I) phenylazopyridine (azpy) complex [CpCo­(azpy)] 2 occurs at the azo nitrogen of the 2-phenylazopyridine ligand to generate the cationic Co­(I) complex [CpCo­(azpyH)]+ 3 with no change in oxidation state at Co. The N–H bond of 3 exhibits diverse hydrogen transfer reactivity, as studies with a variety of organic acceptors demonstrate that 3 can act as a proton, hydrogen atom, and hydride donor. The thermodynamics of all three cleavage modes for the N–H bond (i.e., proton, hydride, and hydrogen atom) were examined both experimentally and computationally. The N–H bond of 3 exhibits a pK a of 12.1, a hydricity of ΔG°H– = 89 kcal/mol, and a bond dissociation free energy (BDFE) of ΔG°H• = 68 kcal/mol in CD3CN. Hydride transfer from 3 to the trityl cation (ΔG°H– = 99 kcal/mol) is exergonic but takes several hours to reach completion, indicating that 3 is a relatively poor hydride donor, both kinetically and thermodynamically. Hydrogen atom transfer from 3 to 2,6-di-tert-butyl-4-(4′-nitrophenyl)­phenoxyl radical (tBu2NPArO·, ΔG°H• = 77.8 kca/mol) occurs rapidly, illustrating the competence of 3 as a hydrogen atom donor.