Dendrites and Pits: Untangling the Complex Behavior of Li Metal Anodes through Operando Video Microscopy
Authors: Wood Kevin N. , Kazyak Eric , Chadwick Alexander F. , Chen Kuan-Hung , Zhang Ji-Guang , Thornton Katsuyo , Dasgupta Neil P. 
Published: February 2020

The poor performance and safety concerns of Li metal anodes represents a critical challenge to enable high energy density rechargeable batteries beyond Li-ion. This is attributed to the evolution of Li metal morphology during cycling, which leads to dendrite growth and surface pitting. However, while the prevailing description of dendrite nucleation and growth behavior has long been proposed to be related to surface inhomogeneities, direct visualization of dendrite nucleation and growth in realistic cell geometries has been largely lacking. This limits our ability to correlate the dynamic morphological evolution of lithium metal surfaces and dendrites to electrochemical signatures observed during cycling. For example, during galvanostatic cycling of Li metal, cell polarization is often observed to vary significantly during each half-cycle, with a characteristic “peaking” voltage profile during early cycles. While a qualitative description of these voltage variations is often described in the context of electrode “stability”, a quantitative framework for correlating polarization changes to morphology during cycling has been lacking. This limits our ability to interpret voltage “signatures” during cycling, which could provide valuable insight into the mechanistic origins of cell performance in cell geometries (i.e. coin, pouch cells) where we are typically “blind” to morphology.

To address these limitations, we present a comprehensive understanding of the voltage variations observed during initial Li metal cycling, which is directly correlated to morphology evolution through the use of operando video microscopy [1]. A custom-designed visualization cell was developed to enable in situ synchronized observation of Li metal electrode morphology and electrochemical behavior during cycling. A mechanistic understanding of the complex behavior of these electrodes is gained through correlation with continuum-scale modeling, which provides insight into the dominant surface kinetics. This work provides a comprehensive explanation of (1) when dendrite nucleation occurs, (2) how those dendrites evolve as a function of time, (3) when surface pitting occurs during Li electrodissolution, (4) kinetic parameters that dictate overpotential as the electrode morphology evolves, and (5) how this understanding can be applied to evaluate electrode performance in a variety of electrolytes. An understanding of how the impedance of reaction pathways change during cycling is used to provide predictive insight into the behavior lithium metal anodes. This provides detailed insight into the interplay between morphology and the dominant electrochemical processes occurring on the Li electrode surface through an improved understanding of changes in cell voltage, which represents a powerful new platform for analysis.

References:

1. K. N. Wood, E. Kazyak, A. F. Chadwick, K.-H. Chen, J.-G. Zhang, K. Thornton, N. P. Dasgupta, ACS Central Science
2, 790 (2016).