Effect of Low-Temperature Al2O3 ALD Coating on Ni-Rich Layered Oxide Composite Cathode on the Long-Term Cycling Performance of Lithium-Ion Batteries.

Conformal coating of nm-thick Al2O3 layers on electrode material is an effective strategy for improving the longevity of rechargeable batteries. However, solid understanding of how and why surface coatings work the way they do has yet to be established. In this article, we report on low-temperature atomic layer deposition (ALD) of Al2O3 on practical, ready-to-use composite cathodes of NCM622 (60% Ni), a technologically important material for lithium-ion battery applications. Capacity retention and performance of Al2O3-coated cathodes (≤10 ALD growth cycles) are significantly improved over uncoated NCM622 reference cathodes, even under moderate cycling conditions. Notably, the Al2O3 surface shell is preserved after cycling in full-cell configuration for 1400 cycles as revealed by advanced electron microscopy and elemental mapping. While there are no significant differences in terms of bulk lattice structure and transition-metal leaching among the coated and uncoated NCM622 materials, the surface of the latter is found to be corroded to a much greater extent. In particular, detachment of active material from the secondary particles and side reactions with the electrolyte appear to lower the electrochemical activity, thereby leading to accelerated capacity degradation.

Room temperature, liquid-phase Al2O3 surface coating approach for Ni-rich layered oxide cathode material.

A room temperature, atomic-layer-deposition-like coating strategy for NCM811 (80% Ni) is reported. Trimethylaluminum is shown to readily react with adsorbed moisture, leading both to Al2O3 surface layer formation on NCM811 and to trace H2O removal in a single treatment step. Even more importantly, the cycling performance of pouch cells at 45 °C is greatly improved.

Molecular Surface Modification of NCM622 Cathode Material Using Organophosphates for Improved Li-Ion Battery Full-Cells.

Surface coating is a viable strategy for improving the cyclability of Li1+ x(Ni1- y- zCo yMn z)1- xO2 (NCM) cathode active materials for lithium-ion battery cells. However, both gaining synthetic control over thickness and accurate characterization of the surface shell, which is typically only a few nm thick, are considerably challenging. Here, we report on a new molecular surface modification route for NCM622 (60% Ni) using organophosphates, specifically tris(4-nitrophenyl) phosphate (TNPP) and tris(trimethylsilyl) phosphate. The functionalized NCM622 was thoroughly characterized by state-of-the-art surface and bulk techniques, such as attenuated total reflection infrared spectroscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry (ToF-SIMS), to name a few. The comprehensive ToF-SIMS-based study comprised surface imaging, depth profiling, and three-dimensional visualization. In particular, tomography is a powerful tool to analyze the nature and morphology of thin coatings and is applied, to our knowledge, for the first time, to a practical cathode active material. It provides valuable information about relatively large areas (over several secondary particles) at high lateral and mass resolution. The electrochemical performance of the different NCM622 materials was evaluated in long-term cycling experiments of full-cells with a graphite anode. The effect of surface modification on the transition-metal leaching was studied ex situ via inductively coupled plasma optical emission spectroscopy. TNPP@NCM622 showed reduced transition-metal dissolution and much improved cycling performance. Taken together, with this study, we contribute to optimization of an industrially relevant cathode active material for application in high-energy-density lithium-ion batteries.

Foot of the Wave Analysis for Mechanistic Elucidation and Benchmarking Applications in Molecular Water Oxidation Catalysis.

The description of the foot of the wave analysis (FOWA) applied to the electrocatalytic oxidation of water to dioxygen is reported for cases where the rate determining step is first order and second order with regard to catalyst concentration, coinciding mechanistically with the so-called water nucleophilic attack (WNA) and the interaction of two M-O units (I2M, where M represents the metal center of the catalyst), respectively. The newly adapted equations are applied to a range of relevant molecular catalysts, both in homogeneous and heterogeneous phase, and the kinetic parameters are determined, including apparent rate constants and turnover frequencies. In this respect, the application of FOWA at different catalyst concentrations allows elucidation of the reaction mechanism that operates in each case. In addition, catalytic Tafel plots are used for assessing the performance of several molecular water oxidation catalysts (WOCs) as a function of overpotential under analogous conditions, and thus can be used for benchmarking purposes. This analysis was carried out earlier for oxide-based WOCs; however, this is the first report using molecular WOCs.

Establishing the Family of Diruthenium Water Oxidation Catalysts Based on the Bis(bipyridyl)pyrazolate Ligand System.

A bis(bipyridyl)pyrazolate ((Me)bbp(-)) has recently been introduced as a rugged dinucleating, bis(tridentate) ligand for the formation of efficient diruthenium water oxidation catalysts (J. Am. Chem. Soc. 2014, 136, 24-27). Now, detailed protocols for the synthesis of a whole family of such dinuclear ruthenium complexes [{Ru(pyR(2))2}2(µ-(R1)bbp)(X,Y)](2+) based on the bis(bipyridyl)pyrazolate scaffold are reported. The isolation of a synthetic key intermediate allowed the straightforward introduction of different pyridines as axial ligands. Thereby, a set of complexes with different substituents at the pyrazolate backbone (R(1) = Br, H, Me), different pyridines as axial ligand (R(2) = H, NMe2, SO3), and different (non)bridging units in the in,in-position (X,Y = Cl, H2O, OAc) has been prepared and thoroughly characterized. Complexes of the type [{Ru(pyR(2))2}2(µ-(R1)bbp)(µ-OAc)](2+), with an exogenous acetato bridge, have been used as catalyst precursors in catalytic water oxidation experiments with a sacrificial oxidant. The effect of substitution on the pyrazole core of the (R1)bbp(-) ligand as well as on the pyridine ligands on both electrochemistry and catalytic activity has been systematically investigated. The catalyst stability, reflected by the turnover number, is crucially determined by the substituent at the pyrazolate ligand (R(1) = Me > H > Br). In contrast, the axial pyridine ligands modulate the rate of the catalytic process, expressed by the initial turnover frequency (R(2) = H > NMe2H(+)).

Efficient Light-Driven Water Oxidation Catalysis by Dinuclear Ruthenium Complexes.

Mastering the light-induced four-electron oxidation of water to molecular oxygen is a key step towards the achievement of overall water splitting to produce alternative solar fuels. In this work, we report two rugged molecular pyrazolate-based diruthenium complexes that efficiently catalyze visible-light-driven water oxidation. These complexes were fully characterized both in the solid state (by X-ray diffraction analysis) and in solution (spectroscopically and electrochemically). Benchmark performances for homogeneous oxygen production have been obtained for both catalysts in the presence of a photosensitizer and a sacrificial electron acceptor at pH 7, and a turnover frequency of up to 11.1 s(-1) and a turnover number of 5300 were obtained after three successive catalytic runs. Under the same experimental conditions with the same setup, the pyrazolate-based diruthenium complexes outperform other well-known water oxidation catalysts owing to both electrochemical and mechanistic aspects.

One-electron-mediated rearrangements of 2,3-disiladicarbene.

A disiladicarbene, (Cy-cAAC)2Si2 (2), was synthesized by reduction of Cy-cAAC:SiCl4 adduct with KC8. The dark-colored compound 2 is stable at room temperature for a year under an inert atmosphere. Moreover, it is stable up to 190 °C and also can be characterized by electron ionization mass spectrometry. Theoretical and Raman studies reveal the existence of a Si═Si double bond with a partial double bond between each carbene carbon atom and silicon atom. Cyclic voltammetry suggests that 2 can quasi-reversibly accept an electron to produce a very reactive radical anion, 2(•-), as an intermediate species. Thus, reduction of 2 with potassium metal at room temperature led to the isolation of an isomeric neutral rearranged product and an anionic dimer of a potassium salt via the formation of 2(•-).

New powerful and oxidatively rugged dinuclear Ru water oxidation catalyst: control of mechanistic pathways by tailored ligand design.

A new powerful and oxidatively rugged pyrazolate-based water oxidation catalyst of formula {[Ru(II)(py-SO3)2(H2O)]2(µ-Mebbp)}(-), 1(H2O)2(-), has been prepared and thoroughly characterized spectroscopically and electrochemically. This new catalyst has been conceived based on a specific ligand tailoring design, so that its performance has been systematically improved. It was also demonstrated how subtle ligand modifications cause a change in the O-O bond formation mechanism, thus revealing the close activation energy barriers associated with each pathway.

Mixed-spin [2 × 2] Fe4 grid complex optimized for quantum cellular automata.

The new pyrazolate-bridged proligand 4-methyl-3,5-bis{6-(2,2'-bipyridyl)}pyrazole ((Me)LH) has been synthesized. Similar to its congener that lacks the backbone methyl substituent ((H)LH) it forms a robust Fe(II)4 grid complex, [(Me)L4Fe(II)4](BF4)4. The molecular structure of [(Me)L4Fe(II)4](BF4)4·2MeCN has been elucidated by X-ray diffraction, revealing two high-spin (HS) and two low-spin (LS) ferrous ions at opposite corners of the rhombic metal ion arrangement. SQUID and (57)Fe Mössbauer data for solid material showed that this [HS-LS-HS-LS] configuration persists over a wide temperature range, between 7 and 250 K, while spin-crossover sets in only above 250 K. According to Mössbauer spectroscopy a [1HS-3LS] configuration is present in solution at 80 K. Thus, the methyl substituent in [(Me)L](-) leads to a stronger ligand field compared to parent [(H)L](-) and hence to a higher LS fraction both in the solid state and in solution. Cyclic voltammetry of [(Me)L4Fe(II)4](BF4)4 reveals four sequential oxidations coming in two pairs with pronounced stability of the di-mixed-valence species [(Me)L4Fe(II)2Fe(III)2](6+) (K(C) = 3.35 × 10(8)). The particular [HS-LS-HS-LS] configuration as well as the di-mixed-valence configuration, both with identical spin or redox states at diagonally opposed vertices of the grid, make this system attractive as a molecular component for quantum cellular automata.