Operando Visual and Electrochemical Correlation of Dendrite Evolution in Li Metal Electrodes
Authors: Wood Kevin N. , Kazyak Eric , Chen Kuan-Hung , Dasgupta Neil P. 
Published: February 2020

Thermodynamic and kinetic limitations in battery materials have prevented the cost effective development of a 300 mile/per charge electric vehicle using current lithium-ion battery technology. One of the most attractive strategies to increase energy density is the use of a Li metal anode instead of graphite.  Unfortunately, significant technological hurdles including low Coulombic efficiency, poor cycle life, and safety concerns are preventing widespread Li metal anode commercialization in rechargeable batteries. These hurdles can all be traced back to uncontrolled dendrite formation and growth.   Hundreds of researchers have tried to solve this problem over the last 50 years, but little knowledge exists on the cause, mechanism, and continued propagation of dendritic growth.  Recently, in-situ microscopy has provided great insight into some aspects of general Li dendrite growth1–3, however many questions remain on the effects of dendrites in real systems and specifically in regard to growth of Li dendrites on a bulk Li metal surface.   

To this end we explore the effects of current density, substrate, electrolyte, and protective coatings on the transient morphological evolution of pitting and dendrite growth by operandooptical visualization of Li metal.  This insight allows for the direct correlation between the evolution of dendrite morphology and the chronopotentiometric traces resulting from galvanostatic charge/discharge cycles.  These voltage traces appear in a variety of battery architectures employing Li metal anodes (coin cells, Swagelok cells) and allows for an in-depth understanding of dendritic development as function of applied current. 

Our results show that a detailed quantification of the amount of Li lost during each individual half cycle on lithium metal can be determined through careful interpretation of potential traces as a function of time.   Furthermore, by varying cell operating parameters, electrolyte composition, and introduction of surface passivation layers directly onto Li metal anodes using Atomic Layer Deposition (ALD), we are able to gain a mechanistic insight into lithium metal electrode performance, including loss of active lithium and variations in Coulombic Efficiency. In particular, ultrathin (~2nm) Al2O3 coatings by ALD were shown to double the lifetime of Li anodes before failure,4 and the operando visualization cell allows for an improved mechanistic understanding of the coupled morphological and electrochemical behavior of these ALD protected electrodes during operation. The consequences of these results can be applied to a wide range of systems, and provide a platform for quantitative comparison of a wide variety of Li metal anode protection strategies. 

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