Sustainable Prelithiation Strategy: Enhancing Energy Density and Lifespan with Ultrathin Li‐Mg‐Al Alloy Foil

Abstract Prelithiation is a well‐established strategy for enhancing battery energy density. However, traditional prelithiation approaches have primarily addressed compensating for the initial active lithium loss (ALL) while overlooking the ALL during extended cycling. In response, a novel method is introduced by increasing the prelithiation degree and pre‐storing stable LiC x within the anode. This innovation facilitates sustainable lithium replenishment, resulting in a significant improvement in battery cycle life and energy density. Moreover, challenges associated with using pure Li foils to realize this strategy in contact prelithiation are revealed, such as difficulties in thinning to less than 5 µm, and the loss of the electronic pathway during prelithiation, resulting in low lithium utilization rates and numerous residues. Additionally, the significantly accelerated capacity fading caused by these residues, typically emerging after hundreds of cycles, has been overlooked. To overcome these challenges, an ultrathin Li‐Mg‐Al alloy foil is developed with significantly improved mechanical properties and delithiation behavior. During prelithiation, the 96Li2Mg2Al alloy maintains a complete film structure with numerous micropores, avoiding randomly distributed debris. This structure ensures high utilization, unimpeded electronic pathways, and efficient electrolyte filtration. By employing a 5‐µm 96Li2Mg2Al foil for sustainable prelithiation, a substantial improvement in energy density is achieved and tripled the battery's lifespan.