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

ABSTRACT

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, Li1.2Ni0.2Mn0.6O2, and Li1.2Ni0.13Mn0.54Co0.13O2 cathodes. Reversible O2-/O2n- redox takes place above 3.0 V, which is clearly distinguished from transition metal redox in the operando EPR on Li2MnO3 cathodes. O2-/O2n- redox is also observed in Li1.2Ni0.2Mn0.6O2, and Li1.2Ni0.13Mn0.54Co0.13O2 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 O2n- 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.