Use of Hydrogel Electrolyte in Zn-MnO 2 Rechargeable Batteries: Characterization of Safety, Performance, and Cu 2+ Ion Diffusion.

Achieving commercially acceptable Zn-MnO 2 rechargeable batteries depends on the reversibility of active zinc and manganese materials, and avoiding side reactions during the second electron reaction of MnO 2 . Typically, liquid electrolytes such as potassium hydroxide (KOH) are used for Zn-MnO 2 rechargeable batteries. However, it is known that using liquid electrolytes causes the formation of electrochemically inactive materials, such as precipitation Mn 3 O 4 or ZnMn 2 O 4 resulting from the uncontrollable reaction of Mn 3+ dissolved species with zincate ions. In this paper, hydrogel electrolytes are tested for MnO 2 electrodes undergoing two-electron cycling. Improved cell safety is achieved because the hydrogel electrolyte is non-spillable, according to standards from the US Department of Transportation (DOT). The cycling of "half cells" with advanced-formulation MnO 2 cathodes paired with commercial NiOOH electrodes is tested with hydrogel and a normal electrolyte, to detect changes to the zincate crossover and reaction from anode to cathode. These half cells achieved ≥700 cycles with 99% coulombic efficiency and 63% energy efficiency at C/3 rates based on the second electron capacity of MnO 2 . Other cycling tests with "full cells" of Zn anodes with the same MnO 2 cathodes achieved ~300 cycles until reaching 50% capacity fade, a comparable performance to cells using liquid electrolyte. Electrodes dissected after cycling showed that the liquid electrolyte allowed Cu ions to migrate more than the hydrogel electrolyte. However, measurements of the Cu diffusion coefficient showed no difference between liquid and gel electrolytes; thus, it was hypothesized that the gel electrolytes reduced the occurrence of Cu short circuits by either (a) reducing electrode physical contact to the separator or (b) reducing electro-convective electrolyte transport that may be as important as diffusive transport.