Electro-Chemo-Mechanical Coupling Effects of Al ₂ O ₃ Coatings on Separators in High Energy Density Lithium Metal Batteries

Al₂O₃-coated separators (e.g., Al₂O₃/PE) are widely utilized in lithium batteries to enhance battery performance, however, the electrochemical role of the Al₂O₃ coating remains unclear. Here, we reveal that the generally conceived "inert" Al₂O₃ coating layer is electrochemically active, and it is lithiated to form a LiAlO₂ thin layer of 20 nm on the surface of Al₂O₃ particles. The Al₂O₃ coating and in situ generated LiAlO₂ spatially homogenize Li⁺ transport and regulate Li⁺ flux to enable uniform Li deposition. Moreover, the Al₂O₃ layer helps to anchor solvent molecules via Lewis acid-base interaction, thereby facilitating the formation of a solid electrolyte interphase enriched with inorganic components derived from anions, particularly lithium fluoride (LiF). The regulation of Li⁺ flux only acts in the battery with the Al₂O₃ layer facing the anode, while it is absent when the Al₂O₃ layer faces the cathode. The effect of anchoring solvent also becomes weaker for the Al₂O₃ layer facing the cathode than facing the anode. Consequently, a 4.48 Ah LiNi0.8Co0.1Mn0.1O₂||Li pouch cell with Al₂O₃/PE separator and porous polymer electrolyte demonstrates stable cycling over 250 cycles at a high energy density of 452 Wh kg⁻¹. A 29 Ah LiNi0.9Co0.05Mn0.05O₂||Li pouch cell with an energy density of 528.9 Wh kg⁻¹ is also demonstrated. These findings uncover the critical electro-chemo-mechanical roles of the generally thought "inert" Al₂O₃ coating in enhancing the electrochemical performance of lithium metal batteries (LMBs), which provide scientific bases for utilizing ceramic coatings on conventional separators to boost the energy density of LMBs.