Lithium Metal Anodes: Toward an Improved Understanding of Coupled Morphological, Electrochemical, and Mechanical Behavior

Li metal anodes are often considered a "holy grail" in the field of rechargeable batteries. Accordingly, the research community continuously seeks new strategies to improve their cyclability and reduce interfacial degradation. However, many recent reports focus on approaches that mitigate the symptoms of poor performance due to dendrites without addressing the underlying root cause of why they form and how they evolve. We propose that an emphasis on purely performance-based metrics has diluted the community's understanding of why a certain methodology is (un)successful. Furthermore, the lack of consistent protocols for reporting cell performance and inconsistent terminology for describing physical phenomena that occur at the Li anode make quantitative comparison difficult. The goal of this Perspective is to motivate the need for more consistent and fundamental research on the interfacial electrochemistry on Li metal anodes. Herein we provide an overview of: 1) recent advances in understanding the fundamental behavior of Li metal 2) the different "dendrite" morphologies (needle, mossy, fractal) often observed during cycling 3) the corresponding electrochemical and mechanical signatures of these various dendrites during cycling 4) the various failure modes of Li metal anodes and 5) how these failure modes are related to interactions at the electrode/electrolyte interface. As a result of these discussion points, five major questions are proposed that should be addressed through fundamental research in order to formulate design rules for mitigating deleterious performance of Li metal anodes, and standard experimental conditions are proposed that should be taken into account when reporting new strategies for Li stabilization.