Unraveling the deposition/dissolution chemistry of MnO₂ for high-energy aqueous batteries

Abstract

Aqueous rechargeable batteries based on the deposition/dissolution of MnO2 are drawing significant attention because of their record-high theoretical capacity and redox potential in addition to their low cost and high...

Dynamic-self-catalysis as an accelerated air-cathode for rechargeable near-neutral Zn–air batteries with ultrahigh energy efficiency

“…3b, bottom), more obvious R2/O2 couples were displayed for the NN-ZAB with Cu( ii ) and Mn( ii ) additive in the electrolyte, which may be because Cu( ii ) additive in the electrolyte increases the H + concentration (pH = 3.8 of the Cu( ii ) and Mn( ii ) electrolyte and pH = 4.9 of the Mn( ii )-electrolyte) and restricts the disproportionation reaction of Mn( iii ) 2Mn( iii ) + 2H 2 O = MnO 2 + 4H + + Mn( ii )). 45 Another important observation from the CV plot of DCS-ZAB is that the Cu redox reaction occurs in the discharge voltage region in prior to the ORR (lower than 1.0 V) and the potential window for Mn redox reactions is mainly in the charge region before the OER (higher than 2.0 V), which implies that the in situ electro-reductive Cu(i) and electro-oxidative Mn(iv)O 2 may directly contribute to the ORR and OER during the discharge and charge processes, respectively (Fig. 3c).…”

Protocol in Evaluating Capacity of Zn–Mn Aqueous Batteries: A Clue of pH

“…[13,20,21] In the numerous subsequent studies, it has been unveiled that the abnormal attenuation is triggered by the inert Zn/Mn oxides (ZMO) aggregation ("dead Mn") on the electrode surface. [15,19,[22][23][24] In 2019, Chao et al proposed the latent high-voltage Mn 2+ ↔ MnO 2 (IV) electrolysis process in the conventional ZMABs, which exhibits not only high capacity ≈600 mAh g −1 but also high voltage plateau near 2 V. [14] The generally recognized ZMABs energy storage mechanisms can be summarized into two: 1) H + and/or Zn 2+ -dominated reactions; [25,26] 2) Mn 2+ -dominated electrolytic reaction (Mn 2+ to Mn n+ , 2 < n ≤ 4), [14,[27][28][29][30][31][32] as depicted in Figure 1b. These mechanisms or their combination are proposed in some cases.…”

“…[1,4] By summarizing the literature, we propose that the pH associated with proton-coupled reactions (PCRs) mainly dominates the storage mechanism, which couples with the Mn 2+ in the electrolyte to affect the capacity evolution of the ZMABs. [19,24] It is known that the Mn 2+ additives introduced to the electrolyte can suppress cathode dissolution caused by the Jahn-Teller effect. While this additive has been commonly applied in almost every ZMAB, it contributes to capacity unexpectedly.…”

“…Notably, intermediate valence manganese (III) is characterized by disproportionated risk, which triggers the recycling of by-products. [17,24,58] The disproportionation reaction might be inhibited by adding Mn 2+ ions in the electrolyte, which can balance the dissolution of MnO 2 . [18] In addition, a thorough conversion of Mn (IV, solid) to Mn (II, liquid) requires a sufficient concentration of protons at voltages above 1.6 V or even above 2.0 V. [14,18,[58][59][60] Many manganese oxides that carry zinc elements could be detected on the electrodes at low potentials, which might be the inexhaustive form originating from the first step of Zn 2+ intercalation or disproportionation, [17,[59][60][61] whereas the confirmation of the unstable MnOOH formed with intercalated H + ions is required, e.g., using advanced in situ detection methods.…”

Energy storage mechanism, advancement, challenges, and perspectives on vivid manganese redox couples

“…35–37 Furthermore, to achieve effective Mn 2+ /MnO 2 conversion, additives are introduced such as inorganic compounds, and redox mediators. 38–41…”