Cathodic electrolyte engineering toward durable Zn–Mn aqueous batteries

Zn–Mn aqueous batteries (ZMABs) present potential for grid-scale energy storage with the benefits of low cost, high safety and eco-friendliness 1. Since 1866 (Leclanché wet cell), we have witnessed the prosperity of ZMABs in the primary battery market and an increasing interest in rechargeable ZMABs. In the last 5 years, achievements have been made in high-capacity MnO2 cathodes, dendrite-free Zn metal anodes and functionalized electrolytes 2–4, which push rechargeable ZMABs a step closer to practical applications. In particular, electrolyte regulation has been regarded as most important in stabilizing the interface but it remains challenging. Recently, writing in National Science Review, Liang, Fang and co-workers reported a lean-water quasi-eutectic electrolyte (QEE) that has been shown to be beneficial in facilitating the reversible interfacial deposition and reaction kinetic of Mn-based cathodes in a long cycle process 5. The development of rechargeable ZMABs is still in the primary stage. The competitiveness of ZMABs is mainly beset by their unsatisfied energy density and poor lifespan. The energy density is affected by two aspects: the specific capacity and the output voltage. The capacity of the MnO2 cathode is dependent on the electrolyte. For instance, the pH of the electrolyte determines the working mechanism of a MnO2 cathode with 1 or 2 e– transfer. Considering the narrow electrochemical stability window (ESW) of water (∼1.23 V), electrolytes may directly limit the output voltage. Recent reports have shown that regulating pH and using water-in-salt electrolytes are effective in widening the ESW 1,6, which can endow ZMABs with a high voltage of >2 V. As for the lifespan, the instability of the MnO2 cathode can be caused by three main reasons (illustrated in Fig. 1a): the irreversible MnO2 → Mn2+ dissolution, the passivation of the formed zinc hydroxide sulfate (Zn4(SO4)(OH)6·nH2O, ZHS) byproduct and the accumulation of dead MnO2 2,3,7. Importantly, these failure processes are closely related to the electrolyte. For instance, the dead MnO2 stems from the increase in pH and the generation of ZHS needs to consume ions in the electrolyte including Zn2+, SO42– and OH–. Therefore, electrolyte optimization is of pivotal importance. Typical mitigation strategies include pH balance, solvation regulation and redox mediation for durable ZMABs, which are to be elaborated as follows. Summary of challenges and electrolyte engineering of MnO2 cathodes toward durable Zn–MnO2 aqueous batteries. (a) Illustration of the three major failure mechanisms of the MnO2 cathode. (b) Summary of pH regulation for changing the reaction mechanism of the MnO2 cathode. Reprinted with permission from 2. (c) Diagram of solvation structure regulation by using a urea-based eutectic electrolyte at the cathode interface. Reprinted with permission from 5. (d) Schematic illustration of the elimination of dead MnO2 using a redox mediator (RM). The current consensus of energy-storage mechanisms in ZMABs focuses on H+ and/or Zn2+-dominated insertion/deinsertion and Mn2+-dominated dissolution/deposition reactions that are associated with a pH clue (proton-coupled reaction) 2. As summarized in Fig. 1b, in mild pH (3–6) electrolytes, typically in 2 M ZnSO4 electrolyte, the Mn2+ deposition/dissolution (usually 1 e– transfer) would mix with Zn2+ deinsertion/insertion and the corresponding dominant reaction changes with the variation in pH values, causing abnormal capacity fluctuations. The possible factors that cause the pH increase include an irreversible H+ de-intercalation reaction and MnOOH disproportionation reaction in the cathode, hydrogen evolution reaction and corrosion, side reactions of the Zn anode and a backward hydrolysis reaction due to Zn2+/Mn2+ consumption after cycles 2. The appearance of ZHS can work as a buffer layer to stabilize the pH. However, ZHS is poor in both electrical and ionic conductivity, greatly restraining the charge transport 7. As a result, introducing pH buffer electrolytes or additives, such as H2PO4– and CH3COO–, can act as a proton buffer reservoir to maintain the pH value toward durable ZMABs 2. In addition, further decreasing the pH to acid (pH 50 cycles 9. Despite its beneficial role in boosting the performance of redox-flow ZMABs, the metrics of the RM design also need to be evaluated regarding its solubility, electrochemical reversibility, kinetics and stability. To conclude, electrolyte optimizations via pH balance, solvation regulation and redox mediation are available and effective for addressing the challenges of dissolution, ZHS passivation and dead Mn in MnO2 cathodes for durable ZMABs. Looking into future, we propose the following research directions: Design novel pH buffer electrolytes or additives that can maintain stable pH and restrain the passivation of the cathode. Regulate a Mn2+ solvation structure that accelerates and stabilizes the deposition/dissolution reaction of the MnO2 cathode without dead Mn. Develop stable redox mediators with a low crossover that can facilitate reversible reactions of the MnO2 cathode with a small potential gap. Develop wide-temperature electrolytes that endow durable ZMABs with excellent low- and high-temperature performances. Conflict of interest statement. None declared.