Two-electron mediation enables fast alkali peroxide cycling and suppresses side reactions in metal-O2 cells

Technical Summary: Two-electron mediation enables fast alkali peroxide cycling and suppresses side reactions in metal-O2 cells

Problem Statement
Aprotic alkali metal–O2 batteries (specifically Li-O2 and Na-O2) offer high theoretical energy density due to the reversible formation of metal peroxides (M2O2) involving two electrons per O2 moiety. However, practical realization is hindered by two primary challenges: the insulating nature of solid peroxide products, which leads to slow reaction kinetics and high overpotentials, and severe parasitic chemistry that degrades cell components and limits cycle life.

The root of the parasitic chemistry is linked to the mechanism of the second electron transfer. In both mediated and non-mediated systems, the formation and decomposition of peroxides typically proceed via a superoxide intermediate (MO2). The second electron transfer is dominated by the disproportionation (DISP) of this superoxide (2 MO2 → O2 + M2O2). This DISP process is problematic because it generates highly reactive singlet oxygen (1O2), which attacks organic electrolytes and cell components. Furthermore, the DISP mechanism relies on the diffusion and encounter of two superoxide species, which is kinetically slow and difficult to control, leading to long-lived intermediates that facilitate side reactions. While redox mediators (RMs) have been used to improve kinetics, conventional one-electron mediators fail to suppress superoxide DISP, as they still rely on sequential encounters that allow DISP to occur between electron transfer steps.

Methodology
The authors propose and investigate "two-electron mediators" (2e– RMs) designed to transfer two electrons consecutively to a single O2 moiety (or from a single peroxide site) during a single molecular encounter. This approach aims to bypass the superoxide disproportionation step entirely.

• Mediator Design:
  • Discharge (Reduction): The study utilizes 3,3',5,5'-tetra-tert-butyldiphenoquinone (DPQ). In Li+ electrolytes, strong ion pairing causes the two reduction waves of the quinone to merge into a single reversible two-electron wave. This allows the doubly reduced species (DPQ2–) to interact with O2.
  • Charge (Oxidation): The study employs DiTEMPO, an ethylene glycol-bridged dimer of two TEMPO moieties. This molecule exhibits a single reversible two-electron oxidation wave, forming a doubly oxidized species (DiTEMPO2+).
• Characterization Techniques:
  • Cyclic Voltammetry (CV): Used to verify the merging of redox waves into a single 2e– process in the presence of Li+ ions compared to TBA+ or Na+.
  • Singlet Oxygen Detection: Near-infrared (NIR) emission at 1270 nm was measured to detect 1O2 generation during the reaction of reduced mediators with O2.
  • Scanning Electrochemical Microscopy (SECM): Utilized to measure heterogeneous electron transfer kinetics between mediators and solid Li2O2 surfaces. Repeated approach curves were used to assess surface passivation.
  • In-situ Surface-Enhanced Raman Spectroscopy (SERS): Used to monitor the presence of LiO2 intermediates during the oxidation of Li2O2 by mediators.
  • Cell Cycling: Li-O2 cells were assembled with porous carbon cathodes and electrolytes containing DPQ (discharge mediator) and DiTEMPO (charge mediator) to evaluate capacity, overpotential, and cycling stability.

Key Contributions and Results

1. Mechanism of Two-Electron Mediation:
   The study demonstrates that a single encounter between a two-electron mediator and an O2 moiety (or peroxide site) is sufficient to transfer two electrons consecutively.

   • Discharge: The doubly reduced DPQ2– reacts with O2 to form a complex that undergoes rapid, consecutive electron transfers to form Li2O2 directly, avoiding the accumulation of the superoxide intermediate.
   • Charge: The doubly oxidized DiTEMPO2+ encounters a Li2O2 surface, extracts a first electron to form LiO2, and immediately extracts a second electron from the same site to evolve O2. Because the second transfer occurs from the same mediator molecule in the immediate vicinity, the lifetime of the LiO2 intermediate is minimized.

2. Suppression of Disproportionation and Side Reactions:

   • Absence of 1O2: NIR measurements showed that while the conventional one-electron mediator DBBQ generates a strong 1O2 signal (indicating superoxide DISP), the two-electron mediator DPQ shows nearly absent 1O2 signals.
   • Suppression of Passivation: SECM experiments revealed that conventional mediators (like TEMPO) cause rapid passivation of the Li2O2 surface due to side reactions (likely 1O2 or electrolyte decomposition by superoxide). In contrast, DiTEMPO maintained stable kinetics over repeated approaches, indicating no surface passivation.
   • SERS Evidence: In-situ SERS spectra of Li2O2 oxidation by TEMPO+ showed a distinct LiO2 intermediate band, confirming the accumulation of superoxide. Conversely, oxidation by DiTEMPO2+ showed no significant LiO2 signal, confirming that the intermediate is consumed immediately by the second electron transfer.

3. Performance Metrics:

   • Kinetics: The apparent rate constant for Li2O2 oxidation by DiTEMPO was approximately double that of TEMPO, consistent with a mechanism where every first electron transfer is immediately followed by a second, faster transfer.
   • Capacity and Yield: Li-O2 cells using DPQ and DiTEMPO achieved significantly higher discharge capacities (doubled compared to DBBQ at low current densities) and higher Li2O2 yields with minimal Li2CO3 impurities.
   • Cyclability: Cells cycled with these mediators showed virtually identical voltage profiles over 100 cycles, demonstrating sustained cyclability at high depths of discharge.

Significance and Claims
The paper claims that two-electron mediation represents a fundamental shift in controlling metal-O2 battery chemistry. By transforming the reaction pathway from an EC mechanism (where the chemical step, disproportionation, is difficult to control) to a mediated EE mechanism (where both electron transfers are controlled by the mediator), the authors achieve:

• Direct Control: The ability to control the kinetics and products of the second electron transfer, which was previously eluded.
• Parasitic Suppression: The effective elimination of superoxide disproportionation, thereby suppressing the generation of singlet oxygen and the associated electrolyte degradation.
• Enhanced Performance: The realization of faster reaction kinetics, larger reversible capacities, and improved cycle life in Li-O2 cells.

The authors conclude that this strategy offers a general solution to the challenges of alkali peroxide cycling, moving beyond the limitations of one-electron mediators that inadvertently facilitate the very side reactions they aim to mitigate.