Rigid and Flexible SEI Layer Formed Over a Cross‐Linked Polymer for Enhanced Ultrathin Li Metal Anode Performance

Li metal has been attracting considerable attention as the most promising anode material for application in next‐generation Li rechargeable batteries. However, the instability of the formed solid electrolyte interphase (SEI) in the Li metal anode leads to a low coulombic efficiency (CE). Here, two kinds of synthesized polymer materials with different molecular configurations (chain and cross‐linked), which are grafted as skins on Cu foils (current collectors), are reported. The interaction between the polymers and electrolyte solvent reduces the amount of the solvent used in the formation of the SEI layer, while facilitating the decomposition of Li salts; this results in inorganic‐component‐dominated SEI layers with excellent stability. Furthermore, the Cu foil grafted with the polymer has a cross‐linked structure that results in an SEI layer that exhibits a combination of rigidity and flexibility. The SEI layer improves the average CE of the Li/Cu batteries to 98.7% over 550 cycles. A Li/Li symmetric battery assembled with 10 µm ultrathin Li anodes achieves prolonged cycling for 1300 h at a current density of 3 mA cm−2. The ultrathin Li anodes can be applied to Li/S batteries, highlighting a new approach to the design of future batteries.