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Abstract Li₂MnO₃has been contemplated as a high‐capacity cathode candidate for Li‐ion batteries; however, it evolves oxygen during battery charging under ambient conditions, which hinders a reversible reaction. However, it is unclear if this irreversible process still holds under subambient conditions. Here, the low‐temperature electrochemical properties of Li₂MnO₃in an aqueous LiCl electrolyte are evaluated and a reversible discharge capacity of 302 mAh g⁻¹at a potential of 1.0 V versus Ag/AgCl at −78 °C with good rate capability and stable cycling performance, in sharp contrast to the findings in a typical Li₂MnO₃cell cycled at room temperature, is observed. However, the results reveal that the capacity does not originate from the reversible oxygen oxidation in Li₂MnO₃but the reversible Cl₂(l)/Cl⁻(aq.) redox from the electrolyte. The results demonstrate the good catalytic properties of Li₂MnO₃to promote the Cl₂/Cl⁻redox at low temperatures. 

Abstract It remains a challenge to design aqueous electrolytes to secure the complete reversibility of zinc metal anodes. The concentrated water‐in‐salt electrolytes, e.g., 30 m ZnCl₂, are promising candidates to address the challenges of the Zn metal anode. However, the pure 30 m ZnCl₂electrolyte fails to deliver a smooth surface morphology and a practically relevant Coulombic efficiency. Herein, it is reported that a small concentration of vanillin, 5 mg mL_water⁻¹, added to 30 m ZnCl₂transforms the reversibility of Zn metal anode by eliminating dendrites, lowering the Hammett acidity, and forming an effective solid electrolyte interphase. The presence of vanillin in the electrolyte enables the Zn metal anode to exhibit a high Coulombic efficiency of 99.34% at a low current density of 0.2 mA cm⁻², at which the impacts of the hydrogen evolution reaction are allowed to play out. Using this new electrolyte, a full cell Zn metal battery with an anode/cathode capacity (N/P) ratio of 2:1 demonstrates no capacity fading over 800 cycles. 

Abstract Iron ion batteries using Fe²⁺as a charge carrier have yet to be widely explored, and they lack high‐performing Fe²⁺hosting cathode materials to couple with the iron metal anode. Here, it is demonstrated that VOPO₄∙2H₂O can reversibly host Fe²⁺with a high specific capacity of 100 mAh g⁻¹and stable cycling performance, where 68% of the initial capacity is retained over 800 cycles. In sharp contrast, VOPO₄∙2H₂O's capacity of hosting Zn²⁺fades precipitously over tens of cycles. VOPO₄∙2H₂O stores Fe²⁺with a unique mechanism, where upon contacting the electrolyte by the VOPO₄∙2H₂O electrode, Fe²⁺ions from the electrolyte get oxidized to Fe³⁺ions that are inserted and trapped in the VOPO₄∙2H₂O structure in an electroless redox reaction. The trapped Fe³⁺ions, thus, bolt the layered structure of VOPO₄∙2H₂O, which prevents it from dissolution into the electrolyte during (de)insertion of Fe²⁺. The findings offer a new strategy to use a redox‐active ion charge carrier to stabilize the layered electrode materials. 

Abstract Proton conduction underlies many important electrochemical technologies. A family of new proton electrolytes is reported: acid‐in‐clay electrolyte (AiCE) prepared by integrating fast proton carriers in a natural phyllosilicate clay network, which can be made into thin‐film (tens of micrometers) fluid‐impervious membranes. The chosen example systems (sepiolite–phosphoric acid) rank top among the solid proton conductors in terms of proton conductivities (15 mS cm⁻¹at 25 °C, 0.023 mS cm⁻¹at −82 °C), electrochemical stability window (3.35 V), and reduced chemical reactivity. A proton battery is assembled using AiCE as the solid electrolyte membrane. Benefitting from the wider electrochemical stability window, reduced corrosivity, and excellent ionic selectivity of AiCE, the two main problems (gassing and cyclability) of proton batteries are successfully solved. This work draws attention to the element cross‐over problem in proton batteries and the generic “acid‐in‐clay” solid electrolyte approach with superfast proton transport, outstanding selectivity, and improved stability for room‐ to cryogenic‐temperature protonic applications.