The impact of surface chemistry on the interfacial resistance between the Li 7 La 3 Zr 2 O 12 (LLZO) solid-state electrolyte and a metallic Li electrode is revealed. Control of surface chemistry allows the interfacial resistance to be reduced to 2 Ω cm 2 , lower than that of liquid electrolytes, without the need for interlayer coatings. A mechanistic understanding of the origins of ultra-low resistance is provided by quantitatively evaluating the linkages between interfacial chemistry, Li wettability, and electrochemical phenomena. A combination of Li contact angle measurements, X-ray photoelectron spectroscopy (XPS), first-principles calculations, and impedance spectroscopy demonstrates that the presence of common LLZO surface contaminants, Li 2 CO 3 and LiOH, result in poor wettability by Li and high interfacial resistance. On the basis of this mechanism, a simple procedure for removing these surface layers is demonstrated, which results in a dramatic increase in Li wetting and the elimination of nearly all interfacial resistance. The low interfacial resistance is maintained over one-hundred cycles and suggests a straightforward pathway to achieving high energy and power density solid-state batteries.

Enabling ultra-high energy density rechargeable Li batteries would have widespread impact on society. However the critical challenges of Li metal anodes (most notably cycle life and safety) remain unsolved. This is attributed to the evolution of Li metal morphology during cycling, which leads to dendrite growth and surface pitting. Herein, we present a comprehensive understanding of the voltage variations observed during Li metal cycling, which is directly correlated to morphology evolution through the use of operando video microscopy. A custom-designed visualization cell was developed to enable operando 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 detailed 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. The 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 represents a powerful new platform for analysis.

A mechanistic analysis of voltage shape changes in lithium metal anodes explains how dead lithium causes capacity fade and failure.

Solid-state electrolytes (SSEs) have attracted substantial attention for next-generation Li-metal batteries, but Li-filament propagation at high current densities remains a significant challenge. This study probes the coupled electrochemicalmorphological-mechanical evolution of Li-metal-Li 7 La 3 Zr 2 O 12 interfaces. Quantitative analysis of synchronized electrochemistry with operando video microscopy reveals new insights into the nature of Li propagation in SSEs. Several different filament morphologies are identified, demonstrating that a singular mechanism is insufficient to describe the complexity of Li propagation pathways. The dynamic evolution of the structures is characterized, which demonstrates the relationships between current density and propagation velocity, as well as reversibility of plated Li before short-circuit occurs. Under deep discharge, void formation and dewetting are directly observed, which are directly related to evolving overpotentials during stripping. Finally, similar Li penetration behavior is observed in glassy Li 3 PS 4 , demonstrating the relevance of the new insights to SSEs more generally.

The lack of a reliable rechargeable lithium metal (Li-metal) anode is a critical bottleneck for next-generation batteries. The unique mechanical properties of lithium influence the dynamic evolution of Li-metal anodes during cycling. While recent models have aimed at understanding the coupled electrochemical-mechanical behavior of Li-metal anodes, there is a lack of rigorous experimental data on the bulk mechanical properties of Li. This work provides comprehensive mechanical measurements of Li using a combination of digital-image correlation and tensile testing in inert gas environments. The deformation of Li was measured over a wide range of strain rates and temperatures, and it was fitted to a power-law creep model. Strain hardening was only observed at high strain rates and low temperatures, and creep was the dominant deformation mechanism over a wide range of battery-relevant conditions. To contextualize the role of creep on Li-metal anode behavior, examples are discussed for solid-state batteries, "dead" Li, and protective coatings on Li anodes. This work suggests new research directions and can be used to inform future electrochemical-mechanical models of Li-metal anodes.