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.

Evident structural anisotropies arising from near-zero particle asphericity in granular spherocylinder packings.

With magnetic resonance imaging experiments, we study packings of granular spherocylinders with merely 2% asphericity. Evident structural anisotropies across all length scales are identified. Most interestingly, the global nematic order decreases with increasing packing fraction, while the local contact anisotropy shows an opposing trend. We attribute this counterintuitive phenomenon to a competition between gravity-driven ordering aided by frictional contacts and a geometric frustration effect at the marginally jammed state. It is also surprising to notice that such slight particle asphericity can trigger non-negligible correlations between contact-level and mesoscale structures, manifested in drastically different nonaffine structural rearrangements upon compaction from that of granular spheres. These observations can help improve statistical mechanical models for the orientational order transformation of nonspherical granular particle packings, which involves complex interplays between particle shape, frictional contacts, and external force field.

Interplay between Intermediate Anionic and Cationic Species and the Reflection to Voltage Hysteresis of Oxygen-Redox Battery Electrodes: A Holistic Picture.

Ligand-to-metal charge transfer (LMCT) is conceived as a universal theory to account for voltage hysteresis in oxygen-redox battery electrodes. However, the influence of oxygen anionic species on mediating LMCT and its reflection to voltage hysteresis remain poorly understood. Herein, we demonstrate a close interplay between the chemical states of oxidized oxygen species, the cationic species, and the kinetics of LMCT and forcefully identify their influence on the magnitude of voltage hysteresis. Combining electrochemical/spectroscopic evidence and first-principles calculations, we clarify two distinct kinds of dynamic LMCT processes─that is, the formation of trapped molecular O2 accompanied by the reduction of Ni4+/Ni3+ to Ni2+ (fast LMCT) during relaxation in Li-rich cation-disordered rock-salt (DRX) Li1.3Ni0.27Ta0.43O2 with extremely large voltage hysteresis, the formation of O-O dimers, and the partial reduction of Mn4+ to Mn3+ (slow LMCT) in DRX-Li1.3Mn0.4Ta0.3O2 with medium hysteresis. We further validate the maintenance of both cationic (Mn4+) and anionic (O-•) species during relaxation in Na2Mn3O7, reconciling its nonhysteretic behavior to the absence of LMCT. This study highlights the critical role of intermediate anionic species in mediating LMCT and provides a causal explanation of various voltage hysteresis signatures of oxygen-redox materials.

Destabilization of Oxidized Lattice Oxygen in Layered Oxide Cathode.

Integrating anion-redox capacity with orthodox cation-redox capacity is deemed as a promising solution for high-energy-density battery cathodes surmounting the present technical bottlenecks. However, the evolution of oxidized oxygen species during the electrochemical or chemical process easily jeopardizes the reversibility of oxygen redox and remains poorly understood. Herein, we showcase the gradual conversion of the π-interacting oxygen (localized hole states on O) to the σ-interacting oxygen upon resting at a high voltage for P3-type Na0.6Li0.2Mn0.8O2 with nominally stable ribbon-like superstructure, accompanied by the O-O dimerization and the local structural reorganization. We further pinpoint an abnormal Li+ migration process from the alkali-metal layer to the transition-metal layer for desodiated P3-Na0.6Li0.2Mn0.8O2, thereby leading to a partial reconstruction of the ribbon superstructure. The high-voltage plateau of oxygen-redox cathodes is concluded to be exclusively controlled by the oxygen stabilization mechanism rather than the superstructure ordering. In addition, there exists a kinetic competition between π and σ interaction during the uninterrupted electrochemical process.

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.

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.

Sustainable Anionic Redox by Inhibiting Li Cross-Layer Migration in Na-Based Layered Oxide Cathodes.

The irrational utilization of an anionic electron often accompanies structural degradation with an irreversible cation migration process upon cycling in sodium-layered oxide cathodes. Moreover, the insufficient understanding of the anionic redox involved cation migration makes the design strategies of high energy density electrodes even less effective. Herein, a P3-Na0.67Li0.2Fe0.2Mn0.6O2 (P3-NLFM) cathode is proposed with the in-plane disordered Li distribution after an in-depth remolding of the Li ribbon-ordered P3-Na0.6Li0.2Mn0.8O2 (P3-NLM) layered oxide. The disordered Li sublattice in the transition metal slab of P3-NLFM leads to the dispersed |O2p orbitals, the lowered charge transfer gap, and the suppressed phase transition at high voltages. Then the enhanced Mn-O interaction and electronic stability are disclosed by the crystal orbital Hamilton population (COHP) analysis at high voltage in P3-NLFM. Furthermore, ab initio molecular dynamics (AIMD) simulation suggests the order/disorder of the transition metal layer is highly correlated with the stability of the Li sublattice. The cross-layer migration and loss of Li in P3-NLM are suppressed in P3-NLFM to enable the high reversibility upon cycling. As a result, the P3-NLFM delivers a high capacity of 163 mAh g-1 without oxygen release and an enhanced capacity retention of 81.9% (vs 42.9% in P3-NLM) after 200 cycles, which constitutes a promising approach for sustainable oxygen redox in rechargeable batteries.

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.

Solvent-pair surfactants enabled assembly of clusters and copolymers towards programmed mesoporous metal oxides.

Organic-inorganic molecular assembly has led to numerous nano/mesostructured materials with fantastic properties, but it is dependent on and limited to the direct interaction between host organic structure-directing molecules and guest inorganic species. Here, we report a "solvent-pair surfactants" enabled assembly (SPEA) method to achieve a general synthesis of mesostructured materials requiring no direct host-guest interaction. Taking the synthesis of mesoporous metal oxides as an example, the dimethylformamide/water solvent pairs behave as surfactants and induce the formation of mesostructured polyoxometalates/copolymers nanocomposites, which can be converted into metal oxides. This SPEA method enables the synthesis of functional ordered mesoporous metal oxides with different pore sizes, structures, compositions and tailored pore-wall microenvironments that are difficult to access via conventional direct organic-inorganic assembly. Typically, nitrogen-doped mesoporous ε-WO3 with high specific surface area, uniform mesopores and stable framework is obtained and exhibits great application potentials such as gas sensing.

Miniature line-scanned dual-axis confocal microscope for versatile clinical use.

A miniature optical-sectioning fluorescence microscope with high sensitivity and resolution would enable non-invasive and real-time tissue inspection, with potential use cases including early disease detection and intraoperative guidance. Previously, we developed a miniature MEMS-based dual-axis confocal (DAC) microscope that enabled video-rate optically sectioned in vivo microscopy of human tissues. However, the device's clinical utility was limited due to a small field of view, a non-adjustable working distance, and a lack of a sterilization strategy. In our latest design, we have made improvements to achieve a 2x increase in the field of view (600 × 300 µm) and an adjustable working distance range of 150 µm over a wide range of excitation/emission wavelengths (488-750 nm), all while maintaining a high frame rate of 15 frames per second (fps). Furthermore, the device is designed to image through a disposable sterile plastic drape for convenient clinical use. We rigorously characterize the performance of the device and show example images of ex vivo tissues to demonstrate the optical performance of our new design, including fixed mouse skin and human prostate, as well as fresh mouse kidney, mouse intestine, and human head and neck surgical specimens with corresponding H&E histology. These improvements will facilitate clinical testing and translation.

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.

4d Lithium-Rich Cathode System Reinvestigated with Electron Paramagnetic Resonance: Correlation between Ionicity, Oxygen Dimers, and Molecular O2.

Layered lithium-rich (Li-rich) oxide cathodes with additional capacity contribution via oxygen redox are promising high energy density cathodes for next generation Li-ion batteries. However, the chemical states of the oxidized oxygen in charged materials are under fierce debate, including the O2- with stable electron holes, O-O dimer (O2)n- (n > 0), molecular O2, and oxygen π redox. Here, we show using electron paramagnetic resonance (EPR) spectroscopy that in the 4d Li-rich ruthenate compounds, Li2Ru0.75Sn0.25O3 and Li2Ru0.5Sn0.5O3, strong covalency between 4d transition metal and oxygen can inhibit the formation of trapped molecular O2 but not suppress the formation of O-O dimer. As the covalent bond of Ru-O weakens and the ionic bond Sn-O becomes dominant in Li2Ru0.25Sn0.75O3, (O2)- will detach from Sn4+, eventually leading to the formation of trapped molecular O2 during the deep oxygen redox. We propose two possible evolution paths of oxidized oxygen as (1) oxygen electron holes → Ru-(O2)m- (m > 1) → Ru-(O2)- or (2) oxygen electron holes → Sn-(O2)m- (m > 1) → Sn-(O2)- → O2, and the species to which they will evolve are related to which metal (O2)- bonds to and whether the ionicity dominates.

"Win-Win" Modification of LiCoO2 Enables Stable and Long-Life Cycling of Sulfide-Based All Solid-State Batteries.

Interfacial side reactions and space charge layers between the oxide cathode material and the sulfide solid-state electrolytes (SSEs), along with the structural degradation of the active material, significantly compromise the electrochemical performance of all-solid-state batteries (ASSLBs). Surface coating and bulk doping of the cathodes are considered the most effective approaches to mitigate the interface issues between the cathode and SSEs and enhance the structural integrity of composite cathodes. Here, a one-step low-cost means is ingeniously designed to modify LiCoO2 (LCO) with heterogeneous Li2 TiO3 /Li(TiMg)1/2 O2 surface coating and bulk gradient Mg doping. When applied in Li10 GeP2 S12 -based ASSLBs, the Li2 TiO3 and Li(TiMg)1/2 O2 coating layers effectively suppress interfacial side reactions and weaken space charge layer effect. Furthermore, gradient Mg doping stabilizes the bulk structure to mitigate the formation of spinel-like phases during local overcharging caused by solid-solid contact. The modified LCO cathodes exhibit excellent cycle performance with a capacity retention of 80 % after 870 cycles. This dual-functional strategy provides the possibility for large-scale commercial implementation of cathodes modification in sulfide based ASSLBs in the future.

Mitigating the Formation of Tetrahedral Zn in Layered Oxides Enables Reversible Lattice Oxygen Redox Triggering by the Na-O-Zn Configuration.

Na-ion layered oxides with Na-O-A' local configurations (A' represents nonredox active cations such as Li+, Na+, Mg2+, Zn2+) are attractive cathode choices for energy-dense Na-ion batteries owing to the accumulation of cationic and anionic redox activities. However, the migration of A' would degrade the stability of the Na-O-A' configuration, bringing about drastic capacity decay and local structural distortions upon cycling. Herein, we uncover the close interplay between irreversible Zn migration and the inactivation of lattice oxygen redox (LOR) for layered oxides based on Na-O-Zn configuration by 23Na solid-state NMR and Zn K-edge EXAFS techniques. We further design a Na2/3Zn0.18Ti0.10Mn0.72O2 cathode in which irreversible Zn migration is effectively prevented, and the LOR reversibility is significantly enhanced. Theoretical insights demonstrate that the migrated Zn2+ is more inclined to occupy the tetrahedral site rather than the prismatic site and can be effectively minimized by incorporation of Ti4+ into the transition-metal layer. Our findings substantiate that the Na-O-Zn configuration can be utilized as an appropriate structure to achieve stable LOR by the cautious manipulating of intralayer cation arrangements.

Probing the degradation of LiCoO2 in batteries subjected to high-voltage cycling with 17O solid-state NMR spectroscopy.

The long-term cycling stability of LiCoO2 under high-voltage operation in lithium-ion batteries is still not satisfactory and the mechanism of capacity decay is not well understood. Here we mainly apply 17O MAS NMR spectroscopy to probe the phase transformation of cycled LiCoO2 cathodes in both liquid cells and solid cells. It turns out that deterioration into the spinel phase is the main cause.

All-Solid-State Batteries with Pre-plated Ultrathin Lithium Metal Enabled by No Pressure Holding and Characterized by In Situ Electron Paramagnetic Resonance Imaging.

All-solid-state batteries with ultrathin lithium metal, close to the anode-free solid-state batteries, could achieve high energy density. However, it is not trivial to plate an ultrathin lithium metal layer on a Cu current collector (Cu-CC). In situ electron paramagnetic resonance (EPR) imaging showed that the pre-plating on Cu-CC in a Li-In|Li6PS5Cl|Cu cell without pressure holding produced a lithium metal layer with a small area. However, when a Li-In|Li6PS5Cl|Cu cell was held under a considerable pressure, this cell would produce a lithium metal layer with a large area. The Cu-CC with the ultrathin lithium metal layer of a small surface could be extracted and then assembled with LiNiO2 and Li6PS5Cl to achieve better electrochemical performance compared to that of a large surface, because a small area of the lithium metal layer would lead to fewer side reactions and less consumption of lithium ions. This work suggests the importance of the control of the lithium plating and the observation of lithium plating by in situ EPR imaging.

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.

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.

Highly Reversible Local Structural Transformation Enabled by Native Vacancies in O2-Type Li-Rich Layered Oxides with Anion Redox Activity.

A novel O2-phase Li1.033Ni0.2[□0.1Mn0.5]O2 cathode with native vacancies (denoted as "□") was delicately designed. By a combination of noninvasive 7Li pj-MATPASS NMR and electron paramagnetic resonance measurements, it is unequivocally shown that the reservation of native vacancies enables the fully reversible local structural transformation without the formation of Li in the Li layer (Litet) in Li1.033Ni0.2[□0.1Mn0.5]O2 during the initial and subsequent cycling. In addition, the pernicious in-plane Mn migration that would result in the generation of trapped molecular O2 is effectively mitigated in Li1.033Ni0.2[□0.1Mn0.5]O2. As a result, the cycle stability of Li1.033Ni0.2[□0.1Mn0.5]O2 is significantly enhanced compared to that of the vacancy-free Li1.033Ni0.2Mn0.6O2, showing an extraordinary capacity retention of 102.31% after 50 cycles at a rate of 0.1C (1C = 100 mA g-1). This study defines an efficacious strategy for upgrading the structural stability of O2-type Li-rich layered oxide cathodes with reversible high-voltage anion redox activity.

Oxygen Redox Activation at Initial Cycle to Improve Cycling Stability for the Na0.83Li0.12Ni0.22Mn0.66O2 System.

Oxygen reactions are commonly used to increase the specific capacities of Na-ion batteries, especially for the NaxLiyTMO2 systems. Previous research focused on improving the stabilities of oxygen reactions to enhance cycling stability. However, the effects of oxygen reactions on the distribution of Li ions in the transition metal (TM) and alkali metal (AM) layers for the Na-ion battery are relatively unexplored and rarely employed. In this study, we employ a layered P2-Na0.83Li0.12Ni0.22Mn0.66O2 cathode to control the effects of the oxygen reactions on the distributions of Li ions in two layers. With oxygen-redox-activation-at-first-cycle (ORAFIC)-cycling, which cycled first within 2.0-4.6 V to activate oxygen redox and then cycled within 2.0-4.2 V, this cathode exhibited better cycling stability compared to low-voltage (LV)-cycling of 2.0-4.2 V and high-voltage (HV)-cycling of 2.0-4.6 V. Using nuclear magnetic resonance spectroscopy, electron paramagnetic resonance, inductively coupled plasma experiments, and X-ray diffraction, it is confirmed that ORAFIC-cycling stabilizes the crystal structure and distributions of Li ions in the TM and AM layers and reduces Li-ion loss, thus improving the cycling stability.