Bouncing Alkaline Batteries: A Basic Solution

Understanding the evolution of the electrochemical constituents of a battery during discharge can offer detailed information about state of charge as well as failure mechanisms. However, typical methods of characterizing the internal components of batteries are often only applicable post mortem. Previous work [1] has used energy-dispersive x-ray diffraction (EDXRD) spectroscopy to image discrete volumes within Zn-MnO2 “alkaline” batteries, and has shown the evolution of the internal components during discharge. Most notably, the oxidation of the anode from Zn to ZnO has been quantified as a function of state of charge. Recently, there has been popular interest [2] in the tendency of an alkaline AA battery to bounce after being dropped on its end when discharged to full capacity, compared to a flat landing with minimal bounce when the battery is as-received. This bounce test presents a non-destructive method of assessing the material properties of the battery, and thus the state of the electrochemical constituents.

In this work, we present an explanation for this bouncing, and quantify it by measuring the coefficient of restitution (COR) of alkaline AA batteries as a function of depth of discharge (DOD). The COR is shown to be constant at low DOD, but then begins to rise rapidly at 20% DOD, finally saturating at a value of 0.63 +/- 0.05 at 50% DOD, as shown in Fig. 1. We have found this rise and saturation to correlate strongly to EDXRD spectra, showing that increase in COR corresponds to the formation within the anode of a contiguous pathway of ZnO particles from the separator to the current collector. The saturation is best explained due to densification of the anode core to a porous ZnO solid. The process is outlined in Fig. 2, with Fig. 3 confirming this process through SEM micrographs of as-received and fully discharged batteries. Of note is the sensitivity of the COR to the amount of ZnO formation, which rivals the sensitivity of in situ energy-dispersive x-ray diffraction spectroscopy. Based on these results, we suggest future methods that can incorporate a transducer/detector system in which the state of charge of a cell can be measured in situ without interruption of the battery system operation.

References:

[1] J. W. Gallaway, C. K. Erdonmez, Z. Zhong, M. Croft, L. A. Sviridov, T. Z. Sholklapper, D. E. Turney, S. Banerjee, and D. A. Steingart, J. Mater. Chem. A 2, 2757 (2014).

[2] "How to test a AA battery, easiest way for any battery fast, easy!" http://www.youtube.com/watch?v=Y_m6p99l6ME (2013).

Acknowledgements:

This work was performed with financial support from the National Science Foundation CMMI 1402872, Department of Energy ARPA-E RANGE DE-AR0000400, and the Laboratory Directed Research and Development Program of Brookhaven National Laboratory (LDRD-BNL) under Contract No. DE-AC02-98CH 10866 with the U.S. Department of Energy. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886. S. B. also acknowledges the Rutgers-Princeton IGERT in Nanotechnology for Clean Energy.

Figure captions:

Fig. 1. Coefficient of restitution as a function of capacity passed. COR increases at 80% state of charge, and saturates at 50% state of charge.

Fig. 2. The progression of ZnO formation in the anode. a) The initial anode gel comprised of Zn particles in an electrolyte/cellulose matrix. b) Formation of Type I ZnO shells on Zn particles. Oxidation occurs preferentially at the separator. c) Formation of a percolation pathway. As all particles become clad in ZnO shells, a contiguous network of ZnO-clad particles forms from separator to current collector (highlighted in green). d) Densification of the anode. Type I ZnO shells grow and Zn particles oxidize to Type II ZnO.

Fig. 3. a) SEM image of as-received" cell, where the coarse zinc/electrolyte gel can be seen surrounding the current collector. b) SEM image of the same cell after full discharge (2850 mAh passed), the anode now largely converted to ZnO. A density gradient can be seen, with the region of compact growth closest to the separator. Scale bar = 1 mm