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

Al 2 O 3 -coated separators ( e.g ., Al 2 O 3 /PE) are widely utilized in lithium batteries to enhance battery performance, however, the electrochemical role of the Al 2 O 3 coating remains unclear. Here, we reveal that the generally conceived “inert” Al 2 O 3 coating layer is electrochemically active, and it is lithiated to form a LiAlO 2 thin layer of 20 nm on the surface of Al 2 O 3 particles. The Al 2 O 3 coating and in situ generated LiAlO 2 spatially homogenize Li + transport and regulate Li + flux to enable uniform Li deposition. Moreover, the Al 2 O 3 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 Al 2 O 3 layer facing the anode, while it is absent when the Al 2 O 3 layer faces the cathode. The effect of anchoring solvent also becomes weaker for the Al 2 O 3 layer facing the cathode than facing the anode. Consequently, a 4.48 Ah LiNi 0.8 Co 0.1 Mn 0.1 O 2 ||Li pouch cell with Al 2 O 3 /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 LiNi 0.9 Co 0.05 Mn 0.05 O 2 ||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” Al 2 O 3 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.