Oxidative Substitution of Organocopper(II) by a Carbon-Centered Radical.

Copper-catalyzed coupling reactions of alkyl halides are believed to prominently involve copper(II) species and alkyl radicals as pivotal intermediates, with their exact interaction mechanism being the subject of considerable debate. In this study, a visible light-responsive fluoroalkylcopper(III) complex, [(terpy)Cu(CF3)2(CH2CO2tBu)] Trans-1, was designed to explore the mechanism. Upon exposure to blue LED irradiation, Trans-1 undergoes copper-carbon bond homolysis, generating Cu(II) species and carbon-centered radicals, where the carbon-centered radical then recombines with the Cu(II) intermediate, resulting in the formation of Cis-1, the Cis isomer of Trans-1. Beyond this, a well-defined fluoroalkylcopper(II) intermediate ligated with a sterically hindered ligand was isolated and underwent full characterization and electronic structure studies. The collective experimental, computational, and spectroscopic findings in this work strongly suggest that organocopper(II) engages with carbon-centered radicals via an "oxidative substitution" mechanism, which is likely the operational pathway for copper-catalyzed C-H bond trifluoromethylation reactions.

From Oxygen Redox to Sulfur Redox: A Paradigm for Li-Rich Layered Cathodes.

The utilization of anionic redox chemistry provides an opportunity to further improve the energy density of Li-ion batteries, particularly for Li-rich layered oxides. However, oxygen-based hosts still suffer from unfavorable structural rearrangement, including the oxygen release and transition metal (TM)-ion migration, in association with the tenuous framework rooted in the ionicity of the TM-O bonding. An intrinsic solution, by using a sulfur-based host with strong TM-S covalency, is proposed here to buffer the lattice distortion upon the highly activating sulfur redox process, and it achieves howling success in stabilizing the host frameworks. Experimental results demonstrate the prolonged preservation of the layered sulfur lattice, especially the honeycomb superlattice, during the Li+ extraction/insertion process in contrast to the large structural degeneration in Li-rich oxides. Moreover, the Li-rich sulfide cathodes exhibited a negligible overpotential of 0.08 V and a voltage drop of 0.13 mV/cycle, while maintaining a substantial reversible capacity upon cycling. These superior electrochemical performances can be unambiguously ascribed to the much shorter trajectories of sulfur in comparison to those of oxygen revealed by molecular dynamics simulations at a large scale (∼30 nm) and a long time scale (∼300 ps) via high-dimensional neural network potentials during the delithiation process. Our findings highlight the importance of stabilizing host frameworks and establish general guidance for designing Li-rich cathodes with durable anionic redox chemistry.

Na2.5Cr0.5Zr0.5Cl6: A New Halide-Based Fast Sodium-Ion Conductor.

All-solid-state batteries employing solid electrolytes (SEs) have received widespread attention due to their high safety. Recently, lithium halides are intensively investigated as promising SEs while their sodium counterparts are less studied. Herein, a new sodium-ion conductor with a chemical formula of Na2.5Cr0.5Zr0.5Cl6 is reported, which exhibits high room temperature ionic conductivity of 0.1 mS cm-1 with low migration energy barrier of ≈0.41 eV. Na2.5Cr0.5Zr0.5Cl6 has a Fm-3m structure with 41.67 mol.% of cationic vacancies owing to the occupation of Cr (8.33 mol.%) and Zr (8.33 mol.%) ions at Na sites. Supercell calculations show that the lowest columbic energy configuration has Cr/Zr/V (where V is the vacancy) clusters in the structure. Nonetheless, the clusters have mixed effects on the sodium ion conduction pathway, based on the Bond Valence Energy Landscape calculation. A global 3D Na-ion transport percolation network can be revealed in the lowest energy supercell. Effective pathways are connected through the NaCl6 and VCl6 nodes. Besides, Raman spectroscopy and 23Na solid-state nuclear magnetic resonance spectroscopy further prove the tunable structure of the SEs with different Cr to Zr ratios. The optimization between the concentration of Na+ and vacancies is crucial to create an improved network of Na+ diffusion channels.

Approaching Effective Differential Centrifugal Fractionation by Combining Image Analysis with Analytical Ultracentrifugation.

Centrifugation is one of the most commonly used methods for separation in biology and chemistry. However, effective fractionation is not always easy to obtain, as preparative centrifuge experiments are mostly conducted in an empirical way, even when it is guided by the quantitative results from analytical ultracentrifuge (AUC). Very few works have been performed to enhance the fractionation resolution of the differential centrifugation method in a swing-out rotor. This is primarily due to the absence of a characterization tool for sedimentation in the preparative centrifuge. In this study, we utilized image analysis to map the particle concentration distribution throughout the preparative centrifuge tube, revealing an unexpected and abnormal sedimentation process. By characterizing the sedimentation coefficient distributions of the fractionated product via AUC, we demonstrated that the overall sedimentation efficiency in a swing-out preparative centrifuge was significantly reduced. Furthermore, effective fractionation was confined to the intermediate phase of the entire sedimentation process. We propose that the mechanism here is a combination of the inverse Boycott effect and droplet sedimentation. The actual sedimentation process within a preparative centrifuge can be described by modifying the Lamm equation phenomenologically, which simply results in an effective sedimentation coefficient. Our work builds a foundation for determining the optimal preparative centrifugation conditions for various systems.

Achieving Long-Enduring High-Voltage Oxygen Redox in P2-Structured Layered Oxide Cathodes by Eliminating Nonlattice Oxygen Redox.

Triggering reversible lattice oxygen redox (LOR) in oxide cathodes is a paradigmatic approach to overcome the capacity ceiling determined by orthodox transition-metal (TM) redox. However, the LOR reactions in P2-structured Na-layered oxides are commonly accompanied by irreversible nonlattice oxygen redox (non-LOR) and large local structural rearrangements, bringing about capacity/voltage fading and constantly evolving charge/discharge voltage curves. Herein, a novel Na0.615 Mg0.154 Ti0.154 Mn0.615 ◻0.077 O2 (◻ = TM vacancies) cathode with both NaOMg and NaO◻ local configurations is deliberately designed. Intriguingly, the activating of oxygen redox at middle-voltage region (2.5-4.1 V) via NaO◻ configuration helps in maintaining the high-voltage plateau from LOR (≈4.38 V) and stable charge/discharge voltage curves even after 100 cycles. Hard X-ray absorption spectroscopy (hXAS), solid-state NMR, and electron paramagnetic resonance studies demonstrate that both the involvement of non-LOR at high-voltage and the structural distortions originating from Jahn-Teller distorted Mn3+ O6 at low-voltage are effectively restrained in Na0.615 Mg0.154 Ti0.154 Mn0.615 ◻0.077 O2 . Resultantly, the P2 phase is well retained in a wide electrochemical window of 1.5-4.5 V (vs Na+ /Na), resulting in an extraordinary capacity retention of 95.2% after 100 cycles. This work defines an effective approach to upgrade the lifespan of Na-ion battery with reversible high-voltage capacity provided by LOR.

Structural Transformation between a Nematic Loose Packing and a Randomly Stacked Close Packing of Granular Disks.

Packing structures of granular disks are reconstructed using magnetic resonance imaging techniques. As packing fraction increases, the packing structure transforms from a nematic loose packing to a dense packing with randomly oriented stacks. According to our model based on Edwards' volume ensemble, stack structures are statistically favored when the effective temperature decreases, which has a lower structural anisotropy than single disks, and brings down the global orientational order consequently. This mechanism identified in athermal granular materials can help us understand the nonergodic characteristics of disklike particle assemblies such as discotic mesogens and clays.

Unraveling the Strong Fluorescence Enhancement of HPBI Molecules by ZIF-8 Colloidal Suspensions via Adsorption Analysis.

Enhancing the fluorescence of organic dye by colloidal particles is one of the most promising routes to optimize fluorescence detection. However, in addition to metallic particles, which serve as the most frequently used particles and have been found to employ the plasmonic resonance to provide strong fluorescence enhancement, neither new types of colloidal particles nor new fluorescence mechanisms have been intensively explored in recent years. In this work, strongly enhanced fluorescence was observed when 2-(2-hydroxyphenyl)-1H-benzimidazole (HPBI) molecules were simply mixed with zeolitic imidazolate framework-8 (ZIF-8) colloidal suspensions. Moreover, the enhancement factor ΔI = IHPBI+ZIF-8/IHPBI does not increase accordingly with the increasing amount of HPBI. To find out how the strong fluorescence was triggered and affected by the amount of HPBI, multiple techniques were applied to analyze the adsorption behavior. By combining analytical ultracentrifugation with first-principles calculations, we proposed that HPBI molecules were adsorbed onto the surface of ZIF-8 particles coordinatively and electrostatically, depending on the concentration of HPBI molecules. The coordinative adsorption would result in a new kind of fluorescence emitter. The new fluorescence emitters tend to distribute on the outer surface of ZIF-8 particles periodically. The distance between each fluorescence emitter is fixed and much smaller than the wavelength of the excitation light. Thus, it can be concluded that collective spontaneous emission might be triggered.

Measuring T1 relaxation in paramagnetic solids with solid-state NMR: a case study on the milling induced phase transition in Li6CoO4.

Solid-state NMR has been a vital tool for the study of structural evolution of cathodes in lithium-ion and sodium-ion batteries. However, the differentiation of relaxation parameters for certain sites is difficult owing to limited spectral resolution associated with strong anisotropic hyperfine interaction. Here we propose a novel IR-pjMATPASS method that can measure T1 relaxation with site-specific resolution for paramagnetic solids. We apply this method to the characterization of ball-milling induced order-disorder phase transition in Li6CoO4 as a case study. The quasi-quantitate 7Li NMR enables the synthetic optimization of high energy ball-milling conditions to harvest a disordered cubic phase through site-specific 7Li T1 measurements. The example study shown here provides a quantitative strategy for NMR studies of paramagnetic solids.

Heavy Fluorination via Ion Exchange Achieves High-Performance Li-Mn-O-F Layered Cathode for Li-Ion Batteries.

Lithium-excess manganese layered oxide Li2 MnO3 , attracts much attention as a cathode in Li-ion batteries, due to the low cost and the ultrahigh theoretical capacity (≈460 mA h g-1 ). However, it delivers a low reversible practical capacity (<200 mA h g-1 ) due to the irreversible oxygen redox at high potentials (>4.5 V). Herein, heavy fluorination (9.5%) is successfully implemented in the layered anionic framework of a Li-Mn-O-F (LMOF) cathode through a unique ion-exchange route. F substitution with O stabilizes the layered anionic framework, completely inhibits the O2 evolution during the first cycle, and greatly enhances the reversibility of oxygen redox, delivering an ultrahigh reversible capacity of 389 mA h g-1 , which is 85% of the theoretical capacity of Li2 MnO3 . Moreover, it also induces a thin spinel shell coherently forming on the particle surface, which greatly improves the surface structure stability, making LMOF exhibit a superior cycling stability (a capacity retention of 91.8% after 120 cycles at 50 mA g-1 ) and excellent rate capability. These findings stress the importance of stabilizing the anionic framework in developing high-performance low-cost cathodes for next-generation Li-ion batteries.

Cubatic structural transformation of the packing of granular cylinders.

Packing structures of granular cylinders with the aspect ratio close to one have been reconstructed with the help of magnetic resonance imaging techniques. By controlling the container boundary conditions and preparation protocols, a structural transformation from a disordered liquid-like state to an orientationally ordered state with cubatic symmetry at a high packing fraction is observed. This ordering process is accompanied by the formation of more faceted contacts, which lower the elastic energy between jammed granular particles to drive the transformation. With the help of Edwards' volume ensemble theory, this granular structural transformation is explained using a phenomenological thermodynamic model and a self-consistent mean-field statistical mechanical model. Both models predict a sharp but continuous change of order parameter when the effective granular temperature is lowered. The intrinsic difference and connection between this granular structural transformation and the entropy-driven phase transition of conventional thermal hard-particle systems are discussed.

Quantitative and space-resolved in situ 1D EPR imaging for the detection of metallic lithium deposits.

The ability to monitor lithium deposition on the anodes in real time is becoming progressively more important due to the development of advanced anode technology. Given the fact that the detrimental Li deposits are always on the micron scale, electron paramagnetic resonance (EPR) happens to be a very effective and selective detection technology due to the skin effect. Here, quantitative in situ 1D EPR imaging is carried out with a magnetic field gradient to achieve a one-dimensional spatial resolution along the Li growth direction in a capillary cell. The quantification of Li deposits is carefully calibrated using a 1,1-diphenyl-2-picrylhydrazyl standard, and a processing method is presented to correct the double integration of the Dysonian line from the metallic Li. The Li deposition processes are compared in two different electrolytes. For the electrolyte containing fluoroethylene carbonate (FEC) additive, the fitting results of Dysonian lines suggest that the plated Li has a larger dimension of the microstructure and the stripping proceeds more uniformly. It thus accounts for the higher Coulombic efficiency in the electrolyte with FEC. In situ EPR imaging also suggests that the Sand's capacity varies with the electrolytes. The forced growth of dendritic Li is carried out at a very large current density using a derivative operando EPR method to monitor the growth locus of the Li dendrites, indicating a tip-growing mechanism. This work can be instructive for those who are engaged in the study of electro-deposited lithium using in situ EPR imaging technology.

Dual Honeycomb-Superlattice Enables Double-High Activity and Reversibility of Anion Redox for Sodium-Ion Battery Layered Cathodes.

Anion redox contributes to the anomalous capacity exceeding the theoretical limit of layered oxides. However, double-high activity and reversibility is challenging due to the structural rearrangement and potential oxygen loss. Here, we propose a strategy for constructing a dual honeycomb-superlattice structure in Na2/3 [Li1/7 Mn5/14 ][Mg1/7 Mn5/14 ]O2 to simultaneously realize high activity and reversibility of lattice O redox. Theoretical simulation and electrochemical tests show that [Li1/7 Mn5/14 ] superlattice units remarkably trigger the anion redox activity and enable the delivery of a record capacity of 285.9 mA g-1 in layered sodium-ion battery cathodes. Nuclear magnetic resonance and in situ X-ray diffraction reveal that [Mg1/7 Mn5/14 ] superlattice units are beneficial to the structure and anion redox reversibility, where Li+ reversibly shuttles between Na layers and transition-metal slabs in contrast to the absence of [Mg1/7 Mn5/14 ] units. Our findings underline the importance of multifunctional units and provide a path to advanced battery materials.

Coexistence of (O2)n- and Trapped Molecular O2 as the Oxidized Species in P2-Type Sodium 3d Layered Oxide and Stable Interface Enabled by Highly Fluorinated Electrolyte.

The interface stability of cathode/electrolyte for Na-ion layered oxides is tightly related to the oxidized species formed during the electrochemical process. Herein, we for the first time decipher the coexistence of (O2)n- and trapped molecular O2 in the (de)sodiation process of P2-Na0.66[Li0.22Mn0.78]O2 by using advanced electron paramagnetic resonance (EPR) spectroscopy. An unstable interface of cathode/electrolyte can thus be envisaged with conventional carbonate electrolyte due to the high reactivity of the oxidized O species. We therefore introduce a highly fluorinated electrolyte to tentatively construct a stable and protective interface between P2-Na0.66[Li0.22Mn0.78]O2 and the electrolyte. As expected, an even and robust NaF-rich cathode-electrolyte interphase (CEI) film is formed in the highly fluorinated electrolyte, in sharp contrast to the nonuniform and friable organic-rich CEI formed in the conventional lowly fluorinated electrolyte. The in situ formed fluorinated CEI film can significantly mitigate the local structural degeneration of P2-Na0.66[Li0.22Mn0.78]O2 by refraining the irreversible Li/Mn dissolutions and O2 release, endowing a highly reversible oxygen redox reaction. Resultantly, P2-Na0.66[Li0.22Mn0.78]O2 in highly fluorinated electrolyte achieves a high Coulombic efficiency (CE) of >99% and an impressive cycling stability in the voltage range of 2.0-4.5 V (vs Na+/Na) under room temperature (147.6 mAh g-1, 100 cycles) and at 45 °C (142.5 mAh g-1, 100 cycles). This study highlights the profound impact of oxidized oxygen species on the interfacial stability of cathode/electrolyte and carves a new path for building stable interface and enabling highly stable oxygen redox reaction.

Structural characterization and statistical properties of jammed soft ellipsoid packing.

The jamming transition and jammed packing structures of hydrogel soft ellipsoids are studied using magnetic resonance imaging techniques. As the packing fraction increases, the fluctuation of local free volume decreases and the fluctuation of particle deformation increases. Effective thermodynamic quantities are obtained by characterizing these fluctuations using k-gamma distributions based on an underlying statistical model for granular materials. Surprisingly, the two granular temperatures measuring the relative fluctuations of both free volume and particle deformation remain basically unchanged as the packing fraction increases. The total configurational entropy is also approximately constant for packing with different packing fractions. The significantly different behaviors of these effective thermodynamic quantities compared with hard sphere systems are further attributed to a statistically affine structural transformation of the packing structures along with particle deformations when the packing fraction changes.

Fast Screening of Coal Fly Ash with Potential for Rare Earth Element Recovery by Electron Paramagnetic Resonance Spectroscopy.

Rare earth elements (REYs) are in increasing global demand, but their mining is costly and environmentally destructive. Coal fly ash (CFA) is a promising alternative source of REYs, but it is necessary to identify CFA with sufficiently high REY concentrations. This study proposes the use of electron paramagnetic resonance (EPR) spectroscopy as part of a simple method to identify CFAs with adequate REY concentrations. The EPR spectra of CFA samples taken from 186 Chinese commercial coal-fired power plants were analyzed. The results suggest that CFAs without evident 6-fold resonances are worth recycling (REY concentrations of 416 ± 108 mg/kg), while those with conspicuous 6-fold resonances are not worth recycling (REY concentrations of 55 ± 26 mg/kg). This is probably due to isomorphic substitution of Ca(II) for Mn(II) and REY(III), resulting in low concentrations of Mn(II) and REY(III) in Ca-rich CFAs. This EPR evaluation method does not require specialized sample preparation, professional skills, or secondary data analysis and has potential global significance in the fast screening of CFAs with REY-recycling potential.

Superionic Conductors via Bulk Interfacial Conduction.

Superionic conductors with ionic conductivity on the order of mS cm-1 are expected to revolutionize the development of solid-state batteries (SSBs). However, currently available superionic conductors are limited to only a few structural families such as garnet oxides and sulfide-based glass/ceramic. Interfaces in composite systems such as alumina in lithium iodide have long been identified as a viable ionic conduction channel, but practical superionic conductors employing the interfacial conduction mechanism are yet to be realized. Here we report a novel method that creates continuous interfaces in the bulk of composite thin films. Ions can conduct through the interface, and consequently, the inorganic phase can be ionically insulating in this type of bulk interface superionic conductors (BISCs). Ionic conductivities of lithium, sodium, and magnesium ion BISCs have reached 1.16 mS cm-1, 0.40 mS cm-1, and 0.23 mS cm-1 at 25 °C in 25 µm thick films, corresponding to areal conductance as high as 464 mS cm-2, 160 mS cm-2, and 92 mS cm-2, respectively. Ultralow overpotential and stable long-term cycling for up to 5000 h were obtained for solid-state Li metal symmetric batteries employing Li ion BISCs. This work opens new structural space for superionic conductors and urges for future investigations on detailed conduction mechanisms and material design principles.

Revealing the Formation of Well-Dispersed Polystyrene@ZIF-8 Core-Shell Nanoparticles by Analytical Ultracentrifugation.

Significant progress has been made in the synthesis and application of core-shell nanoparticles in the past decade. But particle agglomerations are hard to avoid as the formation mechanisms of core-shell nanoparticles are still vague and not clear even for the simplest and most straightforward hard-templating method. Here, analytical ultracentrifugation (AUC) is applied to investigate the formation of polystyrene@ZIF-8 core-shell nanoparticles, as a model to understand the hard-templating method. It has been found that the concentration of the ZIF-8 precursor influences both the ZIF-8 shell formation and the agglomeration of the polystyrene (PS) template. An overdiluted ZIF-8 precursor is not suitable for ZIF-8 shell formation, while a highly concentrated ZIF-8 precursor causes strong aggregation of the PS core. By applying the optimal precursor concentration identified by AUC, well-dispersed polystyrene@ZIF-8 core-shell nanoparticles can be obtained.

A Rotorlike Supramolecular Assembly, {[K(18-crown-6)]PbI3}∞, with a Reversible Breaking-Symmetry Phase Transition near Room Temperature.

A rotorlike supramolecular crystal, {[K(18-crown-6)]PbI3}∞, is composed of a linear [PbI3]∞ chain acting as a stator and [K(18-crown-6)]+ cations fastened to the [PbI3]∞ chain and K-I bond like rotators and axes, respectively. A reversible breaking-symmetry phase transition occurs at ∼305 K. Variable-temperature 1H NMR spectra and dielectrics were used for the dynamic analysis of [K(18-crown-6)]+ cations in the crystal.

High Ethylene Selectivity in Methanol-to-Olefin (MTO) Reaction over MOR-Zeolite Nanosheets.

Precisely controlled crystal growth endows zeolites with special textural and catalytic properties. A nanosheet mordenite zeolite with a thickness of ca. 11 nm, named as MOR-NS, has been prepared using a well-designed gemini-type amphiphilic surfactant as bifunctional structure-directing agent (SDA). Its benzyl diquarternary ammonium cations structurally directed the formation of MOR topology, whereas the long and hydrophobic hexadecyl tailing group prevented the extensive crystal growth along b axis. This kind of orientated crystallization took place through the inorganic-organic interaction between silica species and SDA molecules present in the whole process. The thin MOR nanosheets, with highly exposed (010) planes and 8-membered ring (MR) windows, exhibited a much improved ethylene selectivity (42.1 %) for methanol-to-olefin (MTO) reactions when compared with conventional bulk MOR crystals (3.3 %).

Unveiling the benefits of potassium doping on the structural integrity of Li-Mn-rich layered oxides during prolonged cycling by dual-mode EPR spectroscopy.

The oxygen redox process in Li- and Mn-rich layered oxides will inevitably lead to the generation of oxygen vacancies on the surface and their subsequent injection into the bulk lattice, which incurs poor kinetics, capacity decrease, and voltage fading. Herein, this predicament is effectively alleviated by bulk doping of K+, which is intrinsically stable in the lattice to inhibit the generation of oxygen vacancies in the deep delithiated state. More importantly, the benefits of K+ doping on the structural reversibility during prolonged cycling were studied by electron paramagnetic resonance (EPR) spectroscopy in both perpendicular and parallel polarization modes and high-resolution transmission electron microscopy. The results elucidate that the migration of transition-metal ions and oxygen vacancies and the reduction of Mn-ions are mitigated after K+ doping. Consequently, the growth of Li-poor nanovoids in the bulk lattice is greatly diminished and the structural transition from layered to spinel phases is effectively delayed.