Further In Situ EDXRD Studies of the MnO2 Cathode

Tuesday, 7 October 2014: 17:00
Sunrise, 2nd Floor, Star Ballroom 2 (Moon Palace Resort)
D. A. Steingart (MAE/ACEE Princeton University), J. W. Gallaway (CUNY Energy Institute at the City College of New York), B. J. Hertzberg (Department of Mechanical & Aerospace Engineering, Princeton University), and C. Erdonmez (Brookhaven National Laboratory)
Alkaline Zn/MnO2 batteries employ low-cost, safe materials but typically target single-use applications due to irreversible phase transformations of electrode materials when recharged. We have developed shallow cycling protocols, limiting energy stored each cycle, but allowing hundreds of cycles of operation. This approach makes Zn/MnO2 a viable path for large scale grid-storage. We have used energy dispersive diffraction (EDXRD) to map crystalline phases and track their transformations in both fresh and extended shallow-cycled alkaline batteries. By comparing, in situ, phase content/distribution among batteries undergoing different cycling protocols, we have learn much empirically about degradation mechanisms and their dependence on cycling parameters. By comparing phase distributions in batteries aged to different cycle numbers, have determined more about the fundamental feasibility of shallow-cycled alkaline cells for grid applications and identify specific processes setting cycling life limits under current protocols.

The electrical grid has little storage, necessitating rigorous controls for continually matching electrical generationand demand. Intermittent generation sources (solar or wind power) will further amplify the need for grid-storage. Alkaline Zn-MnO2 cells are traditionally employed as primary batteries, as electrode materials develop stability problems upon recharge. The cathode material, MnO2 undergoes numerous phase transformations during discharge, some being irreversible. Reactions involving less than 0.5 electrons per Mn maintain the reversible MnO2 structure needed for recharging, but staying within this limit can be difficult in practice: rate distributions exist in commercially feasible, cm-scale cells. By shallow cycling, we have demonstrated the lifetimes of hundreds of cycles in alkaline batteries. Thus, overbuilding capacity and limiting rates within the battery may prolong Zn/MnO2 cell lifetime sufficiently for grid-storage. By studying in operando phase distributions and cycling-induced transformations in alkaline batteries, we can examine multiple failure modes within a real system simulatenously. Cells with extended cycling histories have been studied, revealing cumulative effects. We have also demonstrated that the EDXRD at X17B1 can spatially map, in situ, crystalline phases as a function of cycling parameters in a non-destructive way across many batteries in a single experiment, creating an application driven phase map.

We have observed the structural state of MnO2 is a strong function of not only state of charge, but cycling protocol, and we believe we have identified previously unseen "over strain" conditions in which one full electron is removed from a standard MnO2 electrode without the typical Mn2O3 "phase lock".  The attached figure illustrates a time resolved EDXRD of MnO2 at the separator interface of a Duracell LR6 ("AA") Zn/MnO2 battery.