Operando EPR for Simultaneous Monitoring of Anionic and Cationic Redox Processes in Li-Rich Metal Oxide Cathodes

Anionic redox chemistry offers a transformative approach for significantly increasing specific energy capacities of cathodes for rechargeable Li-ion batteries. This study employs operando electron paramagnetic resonance (EPR) to simultaneously monitor the evolution of both transition metal and oxygen redox reactions, as well as their intertwined couplings in Li₂MnO₃, Li_1.2Ni_0.2Mn_0.6O₂, and Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ cathodes. Reversible O^2–/O₂^<i>n</i>– redox takes place above 3.0 V, which is clearly distinguished from transition metal redox in the operando EPR on Li₂MnO₃ cathodes. O^2–/O₂^<i>n</i>– redox is also observed in Li_1.2Ni_0.2Mn_0.6O₂, and Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ cathodes, albeit its overlapping potential ranges with Ni redox. This study further reveals the stabilization of the reversible O redox by Mn and e^– hole delocalization within the Mn–O complex. The interactions within the cation–anion pairs are essential for preventing O₂^<i>n</i>– from recombination into gaseous O₂ and prove to activate Mn for its increasing participation in redox reactions. Operando EPR helps to establish a fundamental understanding of reversible anionic redox chemistry. The gained insights will support the search for structural factors that promote desirable O redox reactions

Lithiation and Delithiation Dynamics of Different Li Sites in Li-Rich Battery Cathodes Studied by <i>Operando</i> Nuclear Magnetic Resonance

Li in Li-rich cathodes mostly resides at octahedral sites in both Li layers (Li_Li) and transition metal layers (Li_TM). Extraction and insertion of Li_Li and Li_TM are strongly influenced by surrounding transition metals. pjMATPASS and <i>operando</i> Li nuclear magnetic resonance are combined to achieve both high spectral and temporal resolution for quantitative real time monitoring of lithiation and delithiation at Li_Li and Li_TM sites in Li₂MnO₃, Li_1.2Ni_0.2Mn_0.6O₂, and Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ cathodes. The results have revealed that Li_TM are preferentially extracted for the first 20% of charge and then Li_Li and Li_TM are removed at the same rate. No preferential insertion or extraction of Li_Li and Li_TM is observed beyond the first charge. Ni and Co promote faster and more complete removal of Li_TM. The recovery of the removed Li is <60% for Li_TM and >80% for Li_Li upon first discharge. The study sheds light on the activity of Li_Li and Li_TM during electrochemical processes as well as their respective contributions to cathode capacity

Li Distribution Heterogeneity in Solid Electrolyte Li₁₀GeP₂S₁₂ upon Electrochemical Cycling Probed by ⁷Li MRI

All-solid-state rechargeable batteries embody the promise for high energy density, increased stability, and improved safety. However, their success is impeded by high resistance for mass and charge transfer at electrode–electrolyte interfaces. Li deficiency has been proposed as a major culprit for interfacial resistance, yet experimental evidence is elusive due to the challenges associated with noninvasively probing the Li distribution in solid electrolytes. In this Letter, three-dimensional ⁷Li magnetic resonance imaging (MRI) is employed to examine Li distribution homogeneity in solid electrolyte Li₁₀GeP₂S₁₂ within symmetric Li/Li₁₀GeP₂S₁₂/Li batteries. ⁷Li MRI and the derived histograms reveal Li depletion from the electrode–electrolyte interfaces and increased heterogeneity of Li distribution upon electrochemical cycling. Significant Li loss at interfaces is mitigated via facile modification with a poly­(ethylene oxide)/bis­(trifluoromethane)­sulfonimide Li salt thin film. This study demonstrates a powerful tool for noninvasively monitoring the Li distribution at the interfaces and in the bulk of all-solid-state batteries as well as a convenient strategy for improving interfacial stability

Li Distribution Heterogeneity in Solid Electrolyte Li₁₀GeP₂S₁₂ upon Electrochemical Cycling Probed by ⁷Li MRI

All-solid-state rechargeable batteries embody the promise for high energy density, increased stability, and improved safety. However, their success is impeded by high resistance for mass and charge transfer at electrode–electrolyte interfaces. Li deficiency has been proposed as a major culprit for interfacial resistance, yet experimental evidence is elusive due to the challenges associated with noninvasively probing the Li distribution in solid electrolytes. In this Letter, three-dimensional ⁷Li magnetic resonance imaging (MRI) is employed to examine Li distribution homogeneity in solid electrolyte Li₁₀GeP₂S₁₂ within symmetric Li/Li₁₀GeP₂S₁₂/Li batteries. ⁷Li MRI and the derived histograms reveal Li depletion from the electrode–electrolyte interfaces and increased heterogeneity of Li distribution upon electrochemical cycling. Significant Li loss at interfaces is mitigated via facile modification with a poly­(ethylene oxide)/bis­(trifluoromethane)­sulfonimide Li salt thin film. This study demonstrates a powerful tool for noninvasively monitoring the Li distribution at the interfaces and in the bulk of all-solid-state batteries as well as a convenient strategy for improving interfacial stability

Role of Electrolyte Anions in the Na–O₂ Battery: Implications for NaO₂ Solvation and the Stability of the Sodium Solid Electrolyte Interphase in Glyme Ethers

Herein we investigate the influence of the sodium salt anion on the performance of Na–O₂ batteries. To illustrate the solvent–solute interactions in various solvents, we use ²³Na-NMR to probe the environment of Na⁺ in the presence of different anions (ClO₄^–, PF₆^–, OTf^–, or TFSi^–). Strong solvation of either the Na⁺ or the anion leads to solvent-separated ions where the anion has no measurable impact on the Na⁺ chemical shift. Contrarily, in weakly solvating solvents the increasing interaction of the anion (ClO₄^– < PF₆^– < OTf^– < TFSi^–) can indeed stabilize the Na⁺ due to formation of contact ion pairs. However, by employing these electrolytes in Na–O₂ cells, we demonstrate that changing from weakly interacting anions (ClO₄^–) to TFSi does not result in elevated battery performance. Nevertheless, a strong dependence of the solid electrolyte interphase (SEI) stability on the choice of sodium salt was found. By correlation of the physical properties of the electrolyte with the chemical SEI composition, the crucial role of the anion in the SEI formation process is revealed. The remarkable differences and consequences for long-term stability are further established by cycling Na coin cells, where electrolytes using NaTFSi are absolutely detrimental for metallic sodium, employing NaOTF and NaClO₄ leads to short-term stability, and only the combination of 1,2-dimethoxyethane with NaPF₆ allows for high efficiency and performance

Solid-State NMR of the Family of Positive Electrode Materials Li₂Ru_1–<i>y</i>Sn_<i>y</i>O₃ for Lithium-Ion Batteries

The possibilities offered by ex situ and in situ operando ⁷Li solid-state nuclear magnetic resonance (NMR) are explored for the Li₂Ru_1–<i>y</i>Sn_<i>y</i>O₃ family (0 < <i>y</i> < 1), shown previously to display cationic and anionic redox activity when used as a positive electrode for Li ion batteries. Ex situ NMR spectroscopic studies indicate a nonrandom Sn/Ru substitution in the family. In the first charge, an increased metallicity at 4 V is deduced from the NMR spectra. Surprisingly, no striking difference is observed at 4.6 V compared to the pristine electrode, although the electronic structure is expected to be very different and the local cation environment to be distorted. For in situ operando measurements, we designed a new electrochemical cell that is compatible with NMR spectroscopy and one-dimensional magnetic resonance imaging (MRI). These operando measurements validate the ex situ observations and indicate that the environment formed at 4 V is specific of the initial charge and that there is little, if no, electrolyte decomposition, even at 4.6 V. This is another attractive feature of these compounds

Solid-State NMR of the Family of Positive Electrode Materials Li₂Ru_1–<i>y</i>Sn_<i>y</i>O₃ for Lithium-Ion Batteries

The possibilities offered by ex situ and in situ operando ⁷Li solid-state nuclear magnetic resonance (NMR) are explored for the Li₂Ru_1–<i>y</i>Sn_<i>y</i>O₃ family (0 < <i>y</i> < 1), shown previously to display cationic and anionic redox activity when used as a positive electrode for Li ion batteries. Ex situ NMR spectroscopic studies indicate a nonrandom Sn/Ru substitution in the family. In the first charge, an increased metallicity at 4 V is deduced from the NMR spectra. Surprisingly, no striking difference is observed at 4.6 V compared to the pristine electrode, although the electronic structure is expected to be very different and the local cation environment to be distorted. For in situ operando measurements, we designed a new electrochemical cell that is compatible with NMR spectroscopy and one-dimensional magnetic resonance imaging (MRI). These operando measurements validate the ex situ observations and indicate that the environment formed at 4 V is specific of the initial charge and that there is little, if no, electrolyte decomposition, even at 4.6 V. This is another attractive feature of these compounds

From Biomass to Functional Crystalline Diamond Nanothread: Pressure-Induced Polymerization of 2,5-Furandicarboxylic Acid

2,5-Furandicarboxylic acid (FDCA) is one of the top-12 value-added chemicals from sugar. Besides the wide application in chemical industry, here we found that solid FDCA polymerized to form an atomic-scale ordered sp3-carbon nanothread (CNTh) upon compression. With the help of perfectly aligned π–π stacked molecules and strong intermolecular hydrogen bonds, crystalline poly-FDCA CNTh with uniform syn-configuration was obtained above 11 GPa, with the crystal structure determined by Rietveld refinement of the X-ray diffraction (XRD). The in situ XRD and theoretical simulation results show that the FDCA experienced continuous [4 + 2] Diels–Alder reactions along the stacking direction at the threshold C···C distance of ∼2.8 Å. Benefiting from the abundant carbonyl groups, the poly-FDCA shows a high specific capacity of 375 mAh g–1 as an anode material of a lithium battery with excellent Coulombic efficiency and rate performance. This is the first time a three-dimensional crystalline CNTh is obtained, and we demonstrated it is the hydrogen bonds that lead to the formation of the crystalline material with a unique configuration. It also provides a new method to move biomass compounds toward advanced functional carbon materials

Single-Atom Catalysts with Unsaturated Co–N₂ Active Sites Based on a C₂N 2D-Organic Framework for Efficient Sulfur Redox Reaction

The lithium–sulfur battery (LSB) is a viable option for the next generation of energy storage systems. However, the shuttle effect of lithium polysulfides (LiPS) and the poor electrical conductivity of sulfur and lithium sulfides limit its deployment. Here, we report on a 2D-organic framework, C2N, with a high loading of low-coordination cobalt single atoms (Co-SAs/C2N) as an effective sulfur host in LSB cathodes. Experimental and computational results reveal that unsaturated Co–N2 active sites with an asymmetric electron distribution act as effective polysulfide traps, accommodating electrons from polysulfide ions to form strong Sx2––Co–N bonds. Additionally, charge transfer between LiPS and unsaturated Co–N2 active sites endows immobilized LiPS with low free energy and low electrochemical decomposition energy barriers, thus accelerating the kinetic conversion of LiPS. As a result, S@Co-SAs/C2N-based cathodes exhibit superior rate performance, impressive cycling stability, and good areal capacity at high sulfur loading, 2-fold that of commercial lithium-ion batteries. This work emphasizes the potential capabilities and promising prospects of single-atom catalysts with unsaturated coordination in LSBs