281071 Phase Changes in Secondary Manganese Dioxide Electrodes for Grid-Scale Batteries
The low cost, high availability, and safety of the alkaline electrolytic manganese dioxide (EMD) electrode make its use in large, rechargeable, stationary grid-scale batteries desirable. However, manganese dioxide is widely used only as a primary battery electrode. This is due to its propensity to form electrochemically irreversible phases while cycling, limiting its cycle life.
One strategy to extend the cycle life of manganese dioxide electrodes is to discharge in a limited window, corresponding to some fraction of one electron per manganese atom. Commercial EMD is composed of particles on the order of ten microns in diameter, and the diffusion of both protons and electrons through this crystal are required for cycling. Literature has shown that the effective diffusion coefficient for this transport is on the order of 1 × 10-15 cm2/s.1 It is believed that approaching one electron of reaction per manganese atom causes an irreversible phase change to Mn2O3 or Mn3O4.2,3 Quantifying and avoiding this change is our goal.
An added complication in studying the manganese dioxide active material is that for practical use it is always compressed in a porous electrode with graphite, which is electrically conductive, and electrolyte, which is ionically conductive. A poor distribution of current across a porous electrode can lead to disparate states of charge across the active material, presenting problems with interpreting experimental results. Electrode modeling and microscopy must be used to assure that reaction rates in the electrode are understood and accounted for.4,5
E-TEM was used to observe phase changes in the MnO2 active material. These phase changes were provoked in one of two ways: ex situ electrochemical cycling, or in situ heating. In situ X-ray diffraction (XRD) was performed on a modified manganese dioxide-zinc coin cell, which was fitted with a transparent window and a beam path through the zinc anode. These experiments were performed at the National Synchrotron Light Source (NSLS) on beamline X-14. After three shallow cycles of the cell in the range from 100% to 75% state of charge, the lattice parameters of ramsdellite were found to decrease from the charged to discharged state, while the lattice parameters for ε-MnO2 increased. This is only in partial agreement with previous literature, in which lattice parameters only increased.6 We believe a unified approach involving both spectroscopy and microscopy is important for understanding the phase changes provoked by shallow cycling.
Recent battery efforts at the CUNY Energy Institute have incorporated parallel experimentation across many cell sizes, with the target being large cell stacks.7
The authors would like to thank ARPA-E under award number DE-AR0000150 for generous support.
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