The poor performance and safety concerns of Li metal anodes represent a critical challenge to enable high energy density rechargeable batteries. This is attributed to the evolution of Li metal morphology during cycling, which leads to dendrite growth and surface pitting. However, there is a lack of fundamental knowledge on the origins of dendrite nucleation, the morphological evolution of non-planar Li during growth and dissolution, and the exact mechanisms that lead to cell failure. As a result, the majority of research has focused on approaches that mitigate the symptoms of poor performance (dendrite formation, electrolyte consumption, Coulombic efficiency of plating/stripping from a current collector), rather than understanding or addressing the root causes [1].

Herein, a comprehensive understanding of the voltage variations observed during Li metal cycling is presented, which is directly correlated to morphology evolution through the use of operando video microscopy. A custom-designed visualization cell was developed to enable in situ synchronized observation of Li metal electrode morphology and electrochemical behavior during cycling [2]. 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, (4) kinetic parameters that dictate overpotential as the electrode morphology evolves, and (5) how this understanding can be applied to evaluate 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 of Li metal anodes.

It is further shown that after extended cycling, mass transport effects arise as a result of dead Li accumulation at the Li metal electrode, which introduces a tortuous pathway for Li-ion transport [3]. In Li–Li symmetric cells, mass transport effects cause the shape of the galvanostatic voltage response to change from “peaking” to “arcing”, along with an increase in total electrode overpotential. The continued accumulation of dead Li is also conclusively shown to directly cause capacity fade and rapid “failure” of Li–LCO full cells containing Li metal anodes. These results provide 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 is a powerful new platform for analysis. Furthermore, this work helps underscore the potential of Li–Li symmetric cells as a powerful analytical tool for understanding the effects of Li metal electrodes in full cell batteries.

[1] K. N. Wood, M. Noked, N. P. Dasgupta ACS Energy Lett. 2, 664 (2017)
[2] 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)
[3] K.-H. Chen, K. N. Wood, E. Kazyak, W. S. LePage, A. L. Davis, A. J. Sanchez, N. P. Dasgupta, J. Mater. Chem. A 5, 11671 (2017)