On our Limited Understanding of Electrodeposition
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.443
On our Limited Understanding of Electrodeposition Aashutosh Mistry1 and Venkat Srinivasan*,2 Argonne National Laboratory, Lemont, Illinois 60439, United States
ABSTRACT The energy density of electrodeposition reactions makes them attractive for energy storage. Although its scientific inquiries nearly date back to the inception of electrochemistry, its behavior at microscopic dimensions (relevant to battery application) is mysteriously uncontrollable. We examine experimental reports of singular spatiotemporal evolutions with a hope to identify universality in deposition patterns. We conclude that a macroscopic mass transport instability cannot account for various growth morphologies and alludes to poorly understood materials interplay at smaller scales. We summarize representative characteristics of electrodeposition to encourage mechanistic investigations.
Introduction Electrodeposition represents the formation of a solid phase from an ionic solution (i.e., electrolyte) upon passage of electricity. It is analogous to solidification as a metallurgical process. It is an interesting proposition given its unique characteristics, e.g., an electric current is easier to modulate as compared to a heat flux (amount of solid formed is proportional to the current in electrodeposition and the heat flux in solidification). It also seems lucrative from the standpoint of an energy storing reaction given apparent simplicity of reaction step(s) as compared to intercalation in solid hosts (the state-of-the-art battery chemistry). The formed solid contributes to the energy in its entirety. Thus electrodeposition reactions can provide higher energy and greater power. Consequently, electrodeposition is being pursued for futuristic energy storage technologies1. Such a scheme represents a paradigm shift from bulk energy storage to an interfacial mode. In situ, in operando and ex situ studies, however, reveal idiosyncrasies ranging from nonuniform deposition 2 to semi-reversibility3 – in general, a certain degree of uncertainty and unpredictability. 1 1 2
Email: [email protected] (AM) and [email protected] (VS) ORCID: 0000 – 0002 – 4359 – 4975 ORCID: 0000 – 0002 – 1248 – 5952
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The conventional interpretation assigns the uncontrollability to electrodeposition instability, wherein a growth front develops well-defined inhomogeneities for certain operating conditions. Such instability is analogous to the morphological instability observed for solidification 4 at fast cooling rates. Corresponding mechanistic arguments define (for constant current deposition) limiting current as a measure of growth instability. When the deposition is carried out faster than the limiting current, a planar growth front cannot be maintained and dendrit
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