Stabilizing the Li Metal Interface: Molecular Layer Deposition for Advanced Next-Generation Energy Storage Systems
Authors: Adair Keegan , Zhao Yang , Sun Yipeng , Wang Changhong , Sun Xueliang 
Published: May 2020

The Li metal anode is sought-after for its high theoretical capacity (3860 mAh g-1) and low electrochemical potential (-3.04 V versus the standard hydrogen electrode). Nevertheless, the intrinsic instability of the Li metal towards both solid and liquid electrolytes leads to unstable solid electrolyte interphase (SEI) formation and dendritic lithium, causing safety concerns and rapid performance degradation [1]. Among the potential techniques used to protect the Li metal surface and inhibit dendrite growth, molecular layer deposition (MLD) has proven to be an invaluable tool for the development of nanoscale interfacial coatings with unique properties [2]. The layer-by-layer assembly of molecular fragments and conformal coating abilities of MLD yield the ability to tune the chemical and mechanical properties of the Li metal interface. It is believed that the exploration and adoption of new MLD films will open up new avenues for stabilizing the Li metal anode. Herein, we show several examples of high-performance Li metal anodes in liquid and solid-state systems achieved by polymeric coatings synthesized through MLD techniques [3-8]. New hybrid organic-inorganic coatings belonging to the metalcone family are shown to dramatically enhance the Coulombic efficiency and rate capability of Li metal anodes. Furthermore, the rational design and ordering of these coatings coupled with inorganic atomic layer deposition coatings reveal the importance of bilayer type structures in promoting enhanced mechanical properties for suppressing dendrite growth. The stabilizing properties of these advanced thin films are further extended to several next-generation battery systems including Li-S and Li-O2, proving effective when coupled with high energy density cathode materials. Moreover, advanced characterization techniques such as in-situ X-ray absorption spectroscopy, nanoindentation measurements, Rutherford backscattering spectrometry, and time-of-flight secondary ion mass spectrometry are used to reveal the mechanisms behind the electrochemical lithiation processes as well as cycling stability.

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