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