Manganese dioxide (MnO₂) and zinc (Zn) are one of the most abundant, safest and cheapest materials available. Together, they are found in common household batteries like Duracell, Energizer, etc. as small cylindrical alkaline cells. These cells or batteries are used as primary batteries, i.e., as single use batteries, where the entire capacity of the battery is delivered once and then discarded. The disadvantage of primary batteries is that it takes a lot of energy to produce the battery than the energy that can be actually obtained from it, and also, it creates environmental waste. However, the manufacturing of primary cells has still been rampant due to the cost of manufacturing MnO₂-Zn cells being very cheap. In terms of improving the overall energy efficiency, reducing waste and maintaining its cost advantage, it makes good sense, economically and environmentally, to make MnO₂-Zn cells rechargeable. However, the main deterrent to this direction has been the fundamental material and chemical problems of the main raw components, i.e., MnO₂ and Zn.

Manganese dioxide can theoretically deliver a capacity of approximately 617mAh/g. It delivers this capacity through a 2 electron electrochemical reaction (each electron providing around 308mAh/g). MnO₂ has been found to be rechargeable when the capacity has been limited to around 5-10% of the 617mAh/g. It suffers a crystal structure breakdown as more of the capacity is accessed, and it inherently forms electrochemical irreversible phases. If the entire 2 electron capacity can be accessed then theoretically it can reach energy density numbers near lithium-ion batteries. Similar problems are associated with the zinc electrode, where higher utilization of its capacity causes dendrite formation, shape change and formation of inactive zinc oxides that ultimately lead to electrode failure. These are the main deterrent to a cheap and safe battery that could be a disruptive technology in the energy storage field.

In this presentation, we report the breakthrough of reversibly accessing the 2^nd electron capacity of MnO₂ by using its layered polymorph called birnessite mixed with bismuth oxide (Bi-birnessite) and intercalating the layers with Cu ions (1). Bi-birnessite’s undergo conversion reactions in alkaline electrolyte and ultimately form electro-inactive hausmannite (Mn₃O₄) because of its poor charge transfer characteristics. Intercalating the layers of Bi-birnessite with Cu ions is shown to improve its charge transfer characteristics dramatically and regenerate its layered structure reversibly for thousands of cycles. We also present a case of Cu-intercalated Bi-birnessite’s applicability in practical batteries by cycling the material at high areal capacities (10-29mAh/cm²) for thousands of cycles at C-rates that are of interest in the battery community.

For true applicability in practical energy dense batteries its pairing with a Zn anode is essential. The use of Zn anodes has also presented problems as it is the source of zincate ions in electrolyte that react with the cathode, MnO₂, to form electro-inactive phase called haeterolite (ZnMn₂O₄). The best reported cycle life data for high depth-of-discharge (DOD) birnessite cathodes with Zn anodes had been 50 cycles till our recent publication, which showed over 90 cycles achieving 140Wh/L. In this presentation, we also report the effect of zincate ions on the Cu-intercalated Bi-birnessite cathodes beyond 100 cycles (2). The Cu-intercalated Bi-birnessite cathodes when paired with Zn anodes are shown to deliver 160Wh/L and cycle reversibly for over 100 cycles. The Cu ions play an important role in mitigating the detrimental effect of zincate ions in the 100 cycles; however, the zincate ions eventually poison the cathode to form ZnMn₂O₄. The mechanism through which ZnMn₂O₄ is formed is presented in detail with the aid of electroanalytical and spectroscopic methods. A solution of trapping the zincate ions is also presented, where the membrane that is used successfully traps the zincate ions from interacting with the cathode and thus, extend cycle life to over 900 cycles. This is the best reported cycle life data with a manganese dioxide cathode accessing the near 2^nd electron capacity paired with Zn anodes.

1] Yadav, G. G.; Gallaway, J. W.; Turney, D. E.; Nyce, M.; Huang, J.; Wei, X.; Banerjee, S. “Regenerable Cu-intercalated MnO₂ layered cathode for highly cyclable energy dense batteries” Nat. Commun. 8, 14424 (2017).

2] Yadav, G. G.; Wei, X.; Huang, J.; Gallaway, J. W.; Turney, D. E.; Nyce, M.; Secor, J.; Banerjee, S., J. Mater. Chem. A, 2017, 5 (30), 15845-15854