The Role of Catholyte Modulation in Suppressing the Initial Capacity Fade of Zinc Electrolytic Manganese Dioxide Coin Cells

Despite its potential for zinc-manganese oxide batteries, electrolytic manganese dioxide (EMD) can experience capacity fade due to a deficiency in the Mn²⁺ supply at the cathode electrolyte interphase (CEI) from side reactions, even in the presence of an electrolyte additive. In this work, electrolyte loading modulation at the cathode electrolyte interface (CEI) was correlated with Zn∥EMD cell capacity retention and cycling performance, as a proposed measure to curb the initial capacity fade observed in EMD. Initial galvanostatic charge/discharge cycling, with varied electrolyte loading, revealed severe capacity fade (from ∼188 to 10 mAh g^-1 for the highest loading of 200 μL) within the first 15 cycles. Such a decrease in cell capacity is correlated with the formation of a Mn⁴⁺ deposit on the current collector and consequently, the Mn²⁺ depletion at CEI, as was supported by elemental and Raman analyses. Interestingly, confinement of the electrolyte to the CEI at a lower (≤15 μL) electrolyte loading mitigated Mn⁴⁺ side-deposition, maintaining the cell capacity at >80% over the first 15 cycles. Interfacial Mn supply/depletion could be monitored via voltammetric analysis based on changes of the Zn²⁺ insertion-reduction peak. Additional galvanostatic experiments corroborated the voltammetric interpretation and the proposed degradation pathway in the studied cell conditions. The outcomes of this work provide practical insight into coin-cell design and configuration strategies for developing Zn∥EMD batteries.