(400f) Fundamental Drivers and Mechanisms for Dendritic Zn Growth, Electrolyte Leakage and Hydrogen Gassing in Zn-MnO2 Batteries

Fundamental Drivers and Mechanisms for Dendritic
Zn Growth, Electrolyte Leakage and Hydrogen Gassing in Zn-MnO₂ Batteries

Ehsan Faegh¹,
Travis Omasta^1,2, Matthew Hull³, Michael Zuraw³
and William E. Mustain¹

¹ Department of Chemical Engineering, University of South
Carolina, Columbia, South Carolina 29208, USA

² Department of Chemical & Biomolecular Engineering,
University of Connecticut, Storrs, Connecticut 06269, USA

³ Duracell, Bethel, Connecticut 06801, USA

The leading cause for safety vent rupture
in alkaline cells is the intrinsic instability of Zn in the highly alkaline
reacting environment. Zn and aqueous KOH react in a parasitic process to
generate hydrogen gas, which can rupture the seal and vent the hydrogen along
with small amounts of electrolyte, and thus, damage consumer devices. Under
most circumstances, cells are engineered to slow the hydrogen evolution
process, and its effects are never apparent to the end user. However, abusive
conditions, particularly deep discharge, are known to accelerate the rate of this
hydrogen “gassing” phenomena.

In order to investigate the fundamental
drivers and mechanisms for such leakage and gassing behavior, we have designed
isolation and operando cells. Using the isolation cell, the discharge behavior
of isolated Zn particles is investigated and the transport mechanism for
electrons, ions and Zn is elucidated. Operando cells enable us to have a
cross-sectional view of a cylindrical Zn-MnO₂ primary alkaline
battery throughout its discharge and recovery. There also exists version of the
cell that can actively measure the in-cell pressure, and hence corrosion rates.
It is shown that steep concentration gradients of zincate ions emerge during
the cell discharge through a redox electrolyte mechanism, leading to the
formation of high surface area black Zn deposits that experience rapid
corrosion when the cell rests to its open circuit voltage. Such corrosion is coupled
with the release of hydrogen and high cell pressure – eventually leading to
cell rupture. The morphological changes of Zn due to the appearance of such
steep concentration gradients of zincate is one of the main reasons that both
causes gassing and limits the rechargeability of the Zn-MnO₂
battery. Therefore, relaxing the zincate concentration gradients is a key
factor for mitigating hydrogen gassing in primary and improving cycleability in
secondary Zn-MnO₂ batteries.