A Ceramic/Lithiophilic Metal Dual Protective Layer for Stable Lithium Metal Anodes Fabricated via Transfer-Printing Methods
Authors: Choi Junyoung , Lee Myeong Hwan , Suk Jungdon 
Published: August 2024

Lithium metal has attracted researcher’s attention because of its superior theoretical capacity (3860mAh g-1 or 2061 mAh cm-3), which is nearly 10 times higher than commercialized anode materials, for example, graphite (372 mAh g-1). However, lithium metal is not currently used in commercial lithium metal batteries due to safety and efficiency issues that arise during long-term battery operation. The high reactivity of lithium metal causes almost all available electrolytes to be spontaneously reduced on the Li surface to form a solid-electrolyte interface (SEI) layer. Unfortunately, this passivating SEI layer is typically too fragile to withstand significant mechanical deformation of the electrode during cycling, resulting in crack formation. These cracks expose fresh Li underneath, which locally enhances Li-ion flux and promotes morphological deformation of the lithium metal surface. This often leads to dendritic Li deposition that can cause internal short circuits and compromise battery safety. Additionally, repeated failures and repairs of SEIs result in the continued loss of both Li and electrolyte, leading to low coulombic efficiency (CE) and rapid capacity decrease.

To mitigate these problems, researchers have attempted to suppress lithium dendrite growth by slurry coating on the lithium metal surface such as ceramic layer and lithiophilic metal. These materials and conventional lithium metal protection coating methods partially inhibit the growth of lithium dendrites; however, they come with several significant drawbacks. The ceramic layer possesses high mechanical properties that physically inhibit the growth of lithium dendrites. However, it exhibits poor interfacial characteristics with lithium metal, leading to increased resistance. This heightened resistance results in the formation of uneven lithium deposition layers at high current densities, causing cracks in the ceramic layer. Lithiophilic metals exhibit an advantage in reducing nucleation resistance through alloying reactions with lithium prior to nucleation, thereby facilitating uniform lithium ion deposition. However, their low mechanical properties limit their potential for enhancing electrochemical performance. Furthermore, the conventional coating method used for lithium metal protection involves the direct application of solvent-containing slurries onto the lithium metal surface using slurry coating techniques. Given the inherent reactivity of lithium metal, the presence of residual moisture and impurities within the solvent may induce sub-reactions, ultimately compromising the stability of lithium metal.

In this study, we propose an approach to improve the cycling stability of lithium metal through Ceramic/lithiophilic metal dual protective layer fabricated via transfer-printing methods. The dual lithium metal protection layer, consisting of Ceramic/lithiophilic, involves coating a lithiophilic metal layer between the ceramic layer and the lithium metal. This method aims to maximize electrochemical performance improvement through a synergistic effect that utilizes the high mechanical properties of ceramics and solves the problem of interfacial resistance between the ceramic layer and lithium metal through the lithiophilic metal. Furthermore, we propose a coating process for lithium metal protection layers using a method based on transfer printing. This method involves applying pressure to the lithium metal surface through transfer printing of pre-finished composite protective layers on a film, rather than directly applying slurry onto the lithium metal surface. Transfer printing offers the advantage of maintaining the stability of lithium metal by avoiding direct application of slurry onto the lithium metal surface, thereby ensuring the integrity of the protection layer.