Multiple Roles of Superoxide on Oxygen Reduction Reaction in Li⁺-Containing Nonaqueous Electrolyte: Contribution to the Formation of Oxide as Well as Peroxide

Abstract

Oxygen reduction reaction (ORR) on carbon in lithium-ioncontaining nonaqueous electrolytes is supposed to follow a three-step mechanism: oxygen (O 2 0 ) to superoxide (O 2 − or LiO 2 ) to peroxide (O 2 2− or Li 2 O 2 ) to oxide (O 2− or Li 2 O). This work attempts to solve three controversial issues: (1) whether the superoxide is really formed; (2) whether the superoxide can be stable or immediately converts to peroxide; (3) whether the formation of oxide is feasible at the typical discharge potentials of lithium−air battery or the discharge product is only peroxide. ORR on carbon was studied in LiClO 4 or LiPF 6 containing dimethyl sulfoxide. The staircase cyclic voltammetry combined with Fourier transform electrochemical impedance spectroscopy (SCV-FTEIS) was used for the in situ investigation of ORR during potential scans. Oxygen was quasi-reversibly reduced to superoxide in the first step. Superoxide showed chemical stability in electrolyte. The superoxide was further reduced to surfaceadsorbed peroxide, and the reduction proceeded to produce oxide at higher overpotentials. We identified a novel chemical route resulting in oxide formation even at not-enough overpotentials: peroxide is reduced to oxide by superoxide as an in situ formed one-electron reducing agent. The electrolyte/electrode decomposition product, Li 2 CO 3 , was observed only in electrolyte with LiClO 4 .

Superoxide stability for reversible Na-O2 electrochemistry

“…The lack of peak c 2 at slower scan rates (<20 mV s −1 ) in the PF 6 − -based electrolyte indicates that the products formed from the previous step at peak c 1 are unstable. To clearly reveal the presence of the second cathodic process (c 2 ) in PF 6 – -containing DMSO at higher scan rates, we used the combined SCV-FTEIS technique by which the transient natures of electrochemical reactions can be kinetically snapshotted while the electrochemistry with potential variation is investigated in real time 38 – 40 (Figure S4 ; Experimental details in the supporting information). A large body of impedance spectra obtained along cathodic scan was analyzed by a two-RC equivalent circuit (Figure S4b ) 41 .…”

Raman Spectroscopy in Lithium–Oxygen Battery Systems

“…Ex situ SERS results have similarly shown Li 2 O 2 formation while XRD showed no evidence of this species . On discharged carbon electrodes, Li 2 O 2 has been confirmed by Raman spectroscopy, usually alongside features attributed to LiO 2 ‐like products …”

“…Chen et al., for example, showed Raman evidence of a Li 2 CO 3 side product on a gold electrode, despite achieving 100 complete discharge and charge cycles with a DMSO‐based electrolyte and a redox mediator . Li 2 CO 3 Raman features have also been found after fixed potential discharging in DMSO solvents on a carbon cloth electrode and a glassy carbon electrode . Carbonate Raman peaks are sometimes accompanied by Li 2 O features, suggesting that these two species form through the same DMSO decomposition mechanism.…”

“…Carbonate Raman peaks are sometimes accompanied by Li 2 O features, suggesting that these two species form through the same DMSO decomposition mechanism. Dilimon et al . specifically noted that Li 2 CO 3 is a product of systems with DMSO and LiClO 4 , but not LiPF 6 , which raises the specter of decomposition mediated by salt anions.…”

Investigação do mecanismo cinético da reação de redução de oxigênio em solventes não aquosos

“…(1,52x10 -6 mol cm -2 s -1 para a Pt e 2,0x10 -6 mol cm -2 (Li et al, 2011;Lin et al, 2013;Park et al, 2013;Li et al, 2014a;Li et al, 2014b) (Mccloskey, B. D., Speidel, A., et al, 2012;Landa-Medrano et al, 2014;Dilimon et al, 2015).…”