Abstract
Batteries for electrical storage are central to any future alternative energy paradigm. The ability to probe the redox mechanisms occurring at electrodes during their operation is essential to improve battery performances. Here we present the first report on Electron Paramagnetic Resonance operando spectroscopy and in situ imaging of a Li-ion battery using Li2Ru0.75Sn0.25O3, a high-capacity (>270 mAh g−1) Li-rich layered oxide, as positive electrode. By monitoring operando the electron paramagnetic resonance signals of Ru5+ and paramagnetic oxygen species, we unambiguously prove the formation of reversible (O2)n− species that contribute to their high capacity. In addition, we visualize by imaging with micrometric resolution the plating/stripping of Li at the negative electrode and highlight the zones of nucleation and growth of Ru5+/oxygen species at the positive electrode. This efficient way to locate ‘electron’-related phenomena opens a new area in the field of battery characterization that should enable future breakthroughs in battery research.
Highlights
Batteries for electrical storage are central to any future alternative energy paradigm
Spectacular images of dendrites growing at the negative electrode–electrolyte interface upon cycling were observed very early by in situ SEM9 and the possible extension of this approach towards in situ TEM experiments has just been demonstrated in ref. 10 exploring the mechanistic of the Li uptake/removal in Si in an ionic liquid medium
Classical positive electrodes for the Li-ion technology operate mainly via an insertion–deinsertion redox process involving cationic species. This is no longer the case with the development of Li-rich layered Li(LixNiyCozMn1 ÀxÀyÀz)O2 phases, referred to as Li-rich NMC13–15, for which we previously demonstrated the redox activity occurring on the anionic network with the reversible formation of peroxo/superoxo-like groups (O2 À -O2n À where 3ZnZ1) to be responsible for their staggering capacities (280 mAh g À 1). This was accomplished via a game-changing chemical approach coupled with X-ray photoemission spectroscopy (XPS) and electron paramagnetic resonance (EPR) measurements[16,17]
Summary
EPR electrochemical cell was shown to sustain extended cycling, confirming its air tightness as well as the stability/compatibility of the Kel-F polymer towards electrolyte. The EPR spectrum is featureless, consistent with the fact that neither the active material Li2Ru0.75Sn0.25O3 (Ru4 þ is EPR-silent) in the positive electrode nor the cell components contain unpaired electrons. The bulk Li foil at the negative electrode should exhibit a broad EPR signal because of unpaired electron spins at the top of the Li Fermi level. The featureless EPR spectrum/difficulty in observing the bulk Li EPR signal can be because of both the weak Pauli paramagnetism (low EPR intensity) and the skin depth effect, which limits the penetration of the microwave field to roughly a micrometre into the bulk
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