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

Mingxue Tang; Annalisa Dalzini; Xiang Li; Xuyong Feng; Po-Hsiu Chien; Likai Song; Yan-Yan Hu

doi:10.1021/acs.jpclett.7b01425

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

J Phys Chem Lett. 2017 Sep 7;8(17):4009-4016. doi: 10.1021/acs.jpclett.7b01425. Epub 2017 Aug 14.

Authors

Mingxue Tang^{1

2}, Annalisa Dalzini², Xiang Li¹, Xuyong Feng¹, Po-Hsiu Chien¹, Likai Song², Yan-Yan Hu^{1

2}

Affiliations

¹ Department of Chemistry and Biochemistry, Florida State University , Tallahassee, Florida 32306, United States.
² National High Magnetic Field Laboratory , 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, United States.

PMID: 28796514
DOI: 10.1021/acs.jpclett.7b01425

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, Li_1.2Ni_0.2Mn_0.6O₂, and Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ cathodes. Reversible O^2-/O₂^n- 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^n- 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^n- 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.