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Nanostructured Electrode Enabling Fast and Fully Reversible MnO2-to-Mn2+ Conversion in Mild Buffered Aqueous Electrolytes

On account of their low-cost, earth abundance,
eco-sustainability, and high theoretical charge storage capacity, MnO2
cathodes have attracted a renewed interest in the development of rechargeable
aqueous batteries. However, they currently suffer from limited gravimetric
capacities when operating under the preferred mild aqueous conditions, which
leads to lower performance as compared to similar devices operating in strongly
acidic or basic conditions. Here, we demonstrate how to overcome this limitation
by combining a well-defined 3D nanostructured conductive electrode, which
ensures an efficient reversible MnO2-to-Mn2+ conversion
reaction, with a mild acid buffered electrolyte (pH 5). A reversible
gravimetric capacity of 560 mA·h·g-1
(close to the maximal theoretical capacity of 574 mA·h·g-1
estimated from the MnO2 average oxidation state of 3.86)
was obtained over rates ranging from 1 to 10 A·g-1. The
rate capability was also remarkable, demonstrating a capacity retention of 435 mA·h·g-1 at a
rate of 110 A·g-1. These
good performances have been attributed to optimal regulation of the mass
transport and electronic transfer between the three process actors, i.e. the 3D conductive scaffold, the MnO2
active material filling it, and the soluble species involved in the reversible
conversion reaction. Additionally, the high reversibility and cycling stability
of this conversion reaction is demonstrated over 900 cycles with a Coulombic
efficiency > 99.4 % at a rate of 44 A·g-1.
Besides these good performances, also demonstrated in a Zn/MnO2 cell
configuration, we discuss the key parameters governing the efficiency of the
MnO2-to-Mn2+ conversion. Overall, the present study
provides a comprehensive framework for the rational design and optimization of
MnO2 cathodes involved in rechargeable mild aqueous batteries.