Correlation between the Microstructure of the Carbon Protective Layer and Cycle Performance of Anode-Free All-Solid-State Lithium-Ion Batteries

Anode-free all-solid-state lithium-ion batteries (ASSBs) are among the most promising energy storage devices owing to their high energy density and safety. Protective layers, such as the Ag-C composite layer, are essential for suppressing unwanted reactions between electrodeposited Li metal and the solid-state electrolyte, thereby ensuring highly cyclable ASSBs. Although recent research efforts have focused on the composition and microstructure of carbon-based protective layers, there are a few reports on the relationship between the microstructure of carbon-based protective layers and the cycling performance of ASSBs, particularly when the pore size of the carbon layers is within the size criterion for Li Coble creep. Herein, we demonstrate that the cycling stability of anode-free ASSBs with a metal-free carbon layer can be affected by the electrical conductivity of carbon powders and the microstructure of carbon layers, especially their surface morphologies and pore volume. Among the anode-free ASSBs that exhibited relatively high capacity retention, the carbon layer with the smallest pore volume exhibited the highest capacity retention (78.8% of the initial capacity) after 300 cycles, probably because of the large volume fraction of lithiated carbon particles, which could act as Li-ion conducting media and enable uniform Li plating. These results reveal the significance of microstructural engineering of carbon-based protective layers for the long-term cycling performance of ASSBs.