Unlocking Reversible Mn2+/MnO2 Chemistry in Semisolid Slurry Electrodes for High-Performance Aqueous Zn–Mn Batteries

Abstract

Electrolytic Zn–MnO2 batteries are promising candidates for safe and sustainable energy storage owing to their high voltage, environmental benignity, and cost-effectiveness. However, practical applications are hindered by the poor conductivity and the irreversible dissolution of conventional ε-MnO2 deposits. Herein, we report a scalable semisolid slurry electrode architecture that enables stable MnO2 deposition/dissolution using a three-dimensional percolating network of carbon nanotubes (CNTs) as both conductive matrix and deposition host. The slurry system promotes the formation of highly conductive γ-MnO2 owing to enhanced charge transfer kinetics, enabling overall dissolution rather than the localized separation typically seen in traditional electrodes. The Zn–MnO2 slurry cell exhibits a reversible areal capacity approaching 60 mAh cm−2. Moreover, the flowable nature of the slurry allows electrochemically inactive MnO2 formed during dissolution to be reconnected and reactivated by CNTs in the rheological network, ensuring deep utilization and cycling stability. This work establishes a slurry electrode strategy to improve electrolytic MnO2 reactions and offers a viable pathway toward renewable aqueous batteries for grid-scale applications.

1. Introduction

The objectives of low carbon and carbon neutrality are driving researchers to pursue advanced energy storage technologies, especially rechargeable batteries with the potential in grid applications [1–3]. Lithium-ion batteries (LIBs) have dominated the markets of electric vehicles and portable electronic devices but their safety and sustainability remain a concern owing to the utilization of organic electrolytes and the scarcity of lithium resources [4–8]. In recent years, aqueous batteries with nonflammable water-based electrolytes have been booming significantly for their environmental benignity, cost-effectiveness, and facile fabrication [9–11].

Of all aqueous batteries, secondary zinc–manganese (Zn–Mn) batteries have attracted widespread attention due to their high voltage and specific capacity [12–14]. The electrolytic zinc–manganese dioxide (Zn–MnO2) batteries operated in the acid electrolyte can deliver a high theoretical capacity of 616 mAh g−1 with two-electron redox of Mn2+/Mn4+ and a high voltage (1.23 V vs. standard hydrogen electrode and 1.99 V vs. Zn/Zn2+) [15, 16]. However, the practical application of the electrolytic Zn–MnO2 batteries is hindered by sluggish charge reaction, poor electronic conductivity of MnO2, and phase transition, which contributes to the formation of electrochemically inactive MnO2 phases (dead MnO2) [17, 18].

Various strategies have been developed to address the above challenges, such as electrolyte additives, redox mediation, pH buffer solution, and polymorph modulation [19–24]. Notably, redox-mediated catalysis and atomic-level modulation of MnO2 electrolysis reactions have shown promise in improving reversibility and suppressing dead-phase accumulation [15, 16]. The reaction mechanism of electrolytic MnO2 is the liquid–solid transition process, as shown in the following equation [25]: Mn2+ + 2H2O ↔ MnO2(s) + 4H+ + 2e−. This reaction is highly dependent on the reaction interface, where a high specific surface area substrate promotes a high MnO2 deposition capacity [26]. Carbon cloth (CC) and carbon felt are currently the most commonly used host materials for MnO2 deposition [27]. However, in conventional electrolytic Zn–MnO2 (CE-MnO2) cells (Fig. 1a), MnO2 is preferentially dissolved on the highly active regions of these carbon substrates during discharge due to the poor electrical conductivity of MnO2 and the limited surface area of the substrate, resulting in unavoidable active MnO2 falling out into the electrolyte [28, 29]. It is noteworthy that the high conductivity and specific su