Rational Design of 3-D Current Collectors and Interfaces for Stabilized Li Metal Anodes
Authors: Chen Kuan-Hung , Sanchez Adrian J. , Dasgupta Neil P. 
Published: February 2020

Improving the performance of rechargeable Li metal anodes is a critical bottleneck to enable next-generation battery systems beyond Li-ion. Unfortunately, Li dendrite growth originating from undesirable electrode/electrolyte interactions results in poor Coulombic efficiency, which has restricted the development of rechargeable Li metal batteries.
Current collectors play a critical role in determining Li metal anode performance because their geometry affects the current density distribution, thus directly impacting Li plating/stripping during cycling. Conventional planar Cu current collectors suffer from uncontrolled Li dendrite growth due to inhomogeneous Li-ion flux along the electrode surface, preventing long-term cycling of Li metal anodes. Recently, there has been a dramatic increase in the number of publications that synthesize 3-D current collectors to suppress dendrite growth and promote a uniform Li plating/stripping during cell cycling.1 While these studies have reported that 3-D current collectors can reduce the local effective current density and accommodate Li deposition, they often compare a planar control electrode to a disordered 3-D structure, rather than rationally controlling geometric parameters in highly ordered structures to achieve an optimal performance. Also, the majority of these papers have not explored surface modifications of 3-D current collectors to further tune the interfacial chemistry, while those that have included surface coatings do not typically provide a mechanistic insight into decoupling the role of geometry and surface chemistry.
In this work, we first demonstrate a bottom-up fabrication process of vertically-aligned Cu pillars with control over the pillar diameter, orientation, and spacing. This is achieved by templated electrodeposition2, which is a highly tunable and scalable approach that can be integrated into industrial battery manufacturing. With ordered pillar arrays as a model platform, we systematically study the effect of pillar diameter on Li plating/stripping morphology upon cycling, spanning length scales from nanometers to micrometers. It was observed that with the optimized pillar size and geometric parameters, compact and uniform Li deposits can be achieved, suppressing the highly-branched dendrite structures observed on that of planar Cu foils. To better characterize the uniformity of Li deposition/dissolution on the 3-D current collector, a customized operando electrochemical cell was also developed, which allows for a real-time observation of the dendrite evolution during cycling.3

In addition, we further demonstrate surface modifications of the optimized 3-D Cu microstructures by atomic layer deposition (ALD). An ultrathin ALD layer was deposited as an interfacial layer to facilitate the initial Li nucleation, further regulating the subsequent Li plating/stripping on the Cu pillars. With the synergistic effects of the optimized geometry and interface modification, Li metal anodes are demonstrated high Coulombic efficiency > 99% for over 200 cycles, providing new insights into rational design of 3-D current collectors.
References
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2006, 5 (7), 567–573.
(3) Wood, K. N.; Kazyak, E.; Chadwick, A. F.; Chen, K.-H.; Zhang, J.-G.; Thornton, K.; Dasgupta, N. P. Dendrites and Pits: Untangling the Complex Behavior of Lithium Metal Anodes through Operando Video Microscopy. ACS Cent. Sci.
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