Li Metal Storage in Porous Carbon Frameworks: Effect of Li–Substrate Interaction
Authors: Yun Jonghyeok , Shin Hong Rim , Won Eun-Seo , Lee Jong-Won 
Published: July 2022

Lithium-metal electrodes are of particular interest for next-generation rechargeable batteries because of their high specific capacity and low redox potential. Therefore, Li-metal-based batteries may afford higher energy densities than commercially available Li-ion batteries with graphite anodes. However, various bottlenecks have hampered the commercial development of these batteries, including uncontrolled Li dendrite formation and huge volume changes during cycling. Consequently, Li-metal batteries suffer from low Coulombic efficiency and poor cycling stability. In recent years, there has been extensive research on the design and construction of three-dimensional (3D) porous electrodes that can host metallic Li. However, the low pore utilization and uneven Li plating remain crucial issues. This can be understood in terms of electronic and ionic transport through the framework electrode. The carbon frameworks exhibit high electronic conductance; however, large resistance to Li+ migration in the electrolytes of internal and interparticle pores inhibits the penetration of Li+ deep into the electrode. In this work, we demonstrate that a strong interaction between Li and a lithiophilic nanolayer on a substrate plays a critical role in enhancing pore utilization in carbon framework electrodes. As a model architecture, we examine a Li storage process in a framework electrode consisting of porous carbon derived from metal-organic frameworks (MOFs) and a galvanically displaced Ag layer on a Cu substrate (Cu@Ag). The electrochemical experiments combined with operando XRD measurements and microstructural characterizations suggest that a lithiophilic Ag on the Cu substrate preferentially reacts with Li+ to form Li
x
Ag during the initial stage of Li plating. This Li
x
Ag phase acts as a seed that can regulate the subsequent Li plating, promoting confined Li storage in the carbon framework electrode while suppressing top plating. Because of these advantages, the MOF-C framework electrode on Cu@Ag exhibits better cycling stability (>250 cycles) than the MOF-C framework electrode on Cu (140cycles). However, when the thickness of the MOF-C framework is increased to 90 μm, the diffraction peak for Ag remains dominant throughout Li plating-stripping, and the formation of Li
x
Ag alloys is not clearly detectable in the diffraction patterns, suggesting that only a limited amount of Ag is involved in the alloying reaction with Li+. Based on the computational studies, the efficacy of lithiophilic layers toward improving pore utilization is discussed in terms of the kinetic competition between Li+ transport through porous channels and the interfacial reaction of Li+ with the substrate. This study conveys an important message that the Li-substrate interaction plays a vital role in promoting the confined Li storage; hence, it should be considered a key design factor for porous carbon frameworks with high capacity and long cycle lifetime.

References

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