The application of flexible, robust, and low-cost solid polymer electrolytes in next-generation all-solid-state lithium metal batteries has been hindered by the low room-temperature ionic conductivity of these electrolytes and the small critical current density of the batteries. Both issues stem from the low mobility of Li+ ions in the polymer and the fast lithium dendrite growth at the Li metal/electrolyte interface. Herein, Mg­(ClO4)2 is demonstrated to be an effective additive in the poly­(ethylene oxide) (PEO)-based composite electrolyte to regulate Li+ ion transport and manipulate the Li metal/electrolyte interfacial performance. By combining experimental and computational studies, we show that Mg2+ ions are immobile in a PEO host due to coordination with ether oxygen and anions of lithium salts, which enhances the mobility of Li+ ions; more importantly, an in-situ formed Li+-conducting Li2MgCl4/LiF interfacial layer homogenizes the Li+ flux during plating and increases the critical current density up to a record 2 mA cm–2. Each of these factors contributes to the assembly of competitive all-solid-state Li/Li, LiFePO4/Li, and LiNi0.8Mn0.1Co0.1O2/Li cells, demonstrating the importance of surface chemistry and interfacial engineering in the design of all-solid-state Li metal batteries for high-current-density applications.

Knowledge about degradation and failure of Li-ion batteries (LIBs) is of paramount importance especially because failure can be accompanied by severe hazards. To contribute to the understanding of such phenomena synchrotron in-line phase contrast Xray tomography was employed to investigate internal cell deformation and degradation caused by an internal short circuit (ISC). The tomographic images taken from an uncycled Li/Li cell and a short-circuited Li/Li cell reveal how lithium microstructures (LmS) develop during electrochemical stripping and plating during discharge and charge and how the three-layer separator used is damaged by growing LmSs and delaminates and melts as a consequence of an ISC. Previously unknown insights into the internal cell degradation and deformation mechanisms caused by an ISC are obtained and provide hints of how the properties of the separator could be modified in order to improve the reliability and safety of current and next-generation LIBs.

Limited understanding of the lithium (Li) nucleation and growth mechanism has hampered the implementation of Li-metal batteries. Herein, we unravel the evolution of the morphology and inner structure of Li deposits using focused ion beam scanning electron microscopy (FIB/SEM). Ball-shaped Li deposits are found to be widespread and stack up at a low current density. When the current density exceeds the diffusion-limiting current, bush-shaped deposition appears that consists of Li-balls, Li-whiskers, and bulky Li. Cryogenic transmission electron microscopy (cryo-TEM) further reveals that Li-balls are primarily amorphous, whereas the Li-whiskers are highly crystalline. Additionally, the solid electrolyte interface (SEI) layers of the Li-balls and whiskers show a difference in structure and composition, which is correlated to the underlying deposition mechanism. The revealed Li nucleation and growth mechanism and the correlation with the nanostructure and chemistry of the SEI provide insights toward the practical use of rechargeable Li-metal batteries.