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.

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.

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.

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.

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.

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.

Facile synthesis and characterization of a SnO2-modified LiNi0.5Mn1.5O4 high-voltage cathode material with superior electrochemical performance for lithium ion batteries.

A thin-layer-SnO2 modified LiNi0.5Mn1.5O4@SnO2 material is synthesized via a facile synthetic approach. It is physically and electrochemically characterized as a high-voltage lithium ion battery cathode and compared to the pristine LiNi0.5Mn1.5O4 material prepared under similar conditions. The two materials are proved to be crystals of a well-defined disordered spinel phase with the morphology of aggregates of micron/submicron polyhedral particles. The Mn3+ ions and the inactive NixLiyO phase in the LiNi0.5Mn1.5O4@SnO2 is less than those in the LiNi0.5Mn1.5O4 due to incorporation of a very small amount of Sn2+ into the spinel structure upon high-temperature calcination of the precursor. Besides, the mean particle size of the LiNi0.5Mn1.5O4@SnO2 is obviously smaller than that of the LiNi0.5Mn1.5O4. The LiNi0.5Mn1.5O4@SnO2 demonstrates much superior electrochemical performance over the LiNi0.5Mn1.5O4 in terms of specific capacity, rate capability and cyclability. For example, the discharge capacities at current rates of 0.2C, 2C and 20C are 145.4, 139.9 and 112.2 mA h g-1, respectively. A capacity retention rate of ca. 75% is obtained after 500 cycles at 2C rate. The improved electrochemical performance is attributed to the positive effect of the surface protective SnO2 coating layer as well as the structural and morphological modifications of the spinel.

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.

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.

"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.

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.

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.

Vanadium dissolution at 30 °C and its suppression at a low temperature of -20 °C for a Na3V1.5Cr0.5(PO4)3 cathode.

Na3V1.5Cr0.5(PO4)3 cycled at 30 °C shows fast capacity decay, due to the migration of V ions into the electrolyte and the loss of V3+/V4+ redox. A low temperature of -20 °C attenuates V-ion dissolution, retains V3+/V4+ redox and improves the electrochemical performance, favorable for use in cold climates and high-altitude drones.

Distinguishing the Effects of the Space-Charge Layer and Interfacial Side Reactions on Li10GeP2S12-Based All-Solid-State Batteries with Stoichiometric-Controlled LiCoO2.

All-solid-state lithium batteries (ASSLBs) with high volumetric energy density and enhanced safety are considered one of the most promising next-generation batteries. Elucidating the capacity-fading mechanism caused by the space-charge layer (SCL) and the interfacial side reaction (ISR) is crucial for the future development of high-energy-density ASSLBs with a longer cycle life. Here, a systematic study to probe the electrochemical performance of Li10GeP2S12-based ASSLBs with stoichiometric-controlled LixCoO2 was performed with the aid of density functional theory (DFT) calculations, X-ray photoelectron spectroscopy (XPS), focused ion beam-field emission scanning electron microscopy (FIB-SEM), and solid-state nuclear magnetic resonance (NMR) spectroscopy. We discovered that the overstoichiometric Li1.042CoO2 shows a high capacity at first cycle with the smallest overpotential, but the capacity gradually decreases, which is ascribed to the weak SCL effect and strong interfacial side reactions. On the contrary, the lithium-deficient Li0.945CoO2 achieves the best cycling stability with a very low capacity associated with the strongest SCL effect and weak interfacial side reactions. The SCL effect is indeed coupled with ISR, which eventually leads to capacity fading in long-term operation. We believe that the new insights gained from this work will accelerate the future development of LiCoO2/LGPS-based ASSLBs with both a mitigated SCL effect and a longer cycle life.

Operando EPR and EPR Imaging Study on a NaCrO2 Cathode: Electronic Property and Structural Degradation with Cr Dissolution.

NaCrO2 is a potential cathode material for sodium-ion batteries due to its low cost, safety, and high power. It is necessary to further understand its electronic property during cycling in advance of practical application. In this work, operando EPR is carried out to monitor the evolution of the electronic structure for NaCrO2 cycled between 2.2-3.6 V and 2.2-4.5 V. We discover that electronic delocalization takes place at the early stage of charge, which may account for the excellent rate performance. In addition, via EPR imaging, an EPR signal associated with the irreversible phase transition at 3.8 V is located in the electrolyte, which is then attributed to the Cr5+ ions dissolved with the surface reconstruction. These findings may help researchers to better design and modify the Cr-based cathode materials.

Suppressing Singlet Oxygen Formation during the Charge Process of Li-O2 Batteries with a Co3O4 Solid Catalyst Revealed by Operando Electron Paramagnetic Resonance.

Aprotic lithium-oxygen (Li-O2) batteries promise high energy, but the cycle life has been plagued by two major obstacles, the insulating products and highly reactive singlet oxygen (1O2), which cause higher overpotential and parasitic reactions, respectively. A solid-state catalyst is known to reduce overpotential; however, it is unclear whether it affects 1O2 generation. Herein, Co3O4 was employed as the representative catalyst in Li-O2 batteries, and 1O2 generation was investigated by ex-situ and operando electron paramagnetic resonance (EPR) spectroscopy. By comparing a carbon nanotube (CNT) cathode with a Co3O4/CNT cathode, we find that 1O2 generation in the charge process can be suppressed by the Co3O4 catalyst. After carefully studying the discharge products on the two electrodes and the corresponding decomposition processes, we conclude that a LiO2-like species is responsible for the 1O2 generation during the early charge stage. The Co3O4 catalyst reduces the amount of LiO2-like species in discharge products, and thus the 1O2 formation is suppressed.

What Triggers the Voltage Hysteresis Variation beyond the First Cycle in Li-Rich 3d Layered Oxides with Reversible Cation Migration?

Herein, the structure-electrochemistry relationship of O2-Li5/6(Li0.2Ni0.2Mn0.6)O2 is deliberately studied by local-structure probes including site-sensitive 7Li pj-MATPASS NMR, quantitative 6Li magic-angle spinning NMR, and electron paramagnetic resonance (EPR). The extraction and reinsertion of LiTM (Li in the transition metal layer) during the first cycle are only partially reversible, bringing about the formation of tetrahedral LiLi (Li in the Li layer) that can be reversibly (de)intercalated after the activation cycle. The high-voltage oxygen redox process is preserved beyond the first cycle, further manifesting the structural superiority of O2 stacking over O3 stacking in bolstering oxygen redox. Moreover, the (de)lithiation process is highly reversible without pronounced structural hysteresis after the rearrangement of Li and transition metal upon the activation cycle, which can explain well the variation of voltage hysteresis from the first cycle to second cycle. These insights elucidate the imperfect structural stability of O2-type Li-rich layered oxides, which could be further improved by streamlining the returning path of LiTM.

Restraining Oxygen Loss and Boosting Reversible Oxygen Redox in a P2-Type Oxide Cathode by Trace Anion Substitution.

Oxygen redox has recently emerged as a lever to boost the specific energy density of layered sodium transition metal oxide cathode materials. However, the oxygen redox reaction is universally confronted with concomitant issues such as irreversible lattice oxygen loss and parasitical electrolyte degradation, thus debilitating cycling stability. Herein, a novel F-substituted layered structure P2-Na0.65Li0.22Mn0.78O1.99F0.01 cathode is designed, which exhibits superb capacity retention (183.6 mAh g-1 after 50 cycles at 0.05C, 87.8% of the highest discharge capacity) and rate capability (105.5 mAh g-1 at 5C) in Na half-cells. Such results are nontrivial as this system only contains the low-cost Mn transition metal element. Moreover, by systematic bulk/surface spectroscopy evidence (hard and soft X-ray absorption spectroscopy, electron paramagnetic resonance, and operando differential electrochemical mass spectrometry), we explicitly corroborate that the irreversible oxygen evolution and notorious Jahn-Teller distortion are effectively subdued by trace F-substitution. In addition, a higher oxygen vacancy formation energy for the F-substituted structure was demonstrated via density functional theory calculations. Anionic substitution could therefore be an impactful solution to boost reversible oxygen redox chemistry for layered sodium oxide cathodes.

Deciphering the Origin of High Electrochemical Performance in a Novel Ti-Substituted P2/O3 Biphasic Cathode for Sodium-Ion Batteries.

The layered Mn-based oxides (NaxMnO2), which is one of the most promising cathode families for rechargeable sodium-ion batteries, have received considerable attention because of their tunable electrochemical performances and low costs. Herein, a novel P2/O3 intergrown Li-containing Na0.8Li0.27Mn0.68Ti0.05O2 cathode material prepared by Ti-substitution into Mn-site is reported. Benefiting from the synergistic effects of the biphasic composite structure and inactive d0 element substitution, this P2/O3 electrode exhibits high initial charge/discharge capacity and excellent cycling performance. The combination of different characterization techniques including solid-state NMR, electron paramagnetic resonance, X-ray adsorption spectroscopy, and high-resolution transmission electron microscopy gives insights into the local electronic environment, the redox chemistry, and also the microstructure rigidity of these cathode materials upon cycling. On the basis of comprehensive comparison with the Ti-free P2/O3-Na0.8Li0.27Mn0.73O2, the observed improvement on the electrochemical performance is primarily attributed to the mitigation of notorious Mn3+/Mn4+ redox and the enhanced stability of the oxygen charge compensation behavior. From the viewpoint of structure evolution, Ti-substitution restrains the Li+ loss and irreversible structural degradation during cycling. This study provides an in-depth understanding of the electronic and crystal structure evolutions after inactive d0 element substitution and may shed light on the rational design of high-performance P2/O3 biphasic Mn-based layered cathodes.