Combining Solid Solution Strengthening and Second Phase Strengthening for Thinning Li Metal Foils

Thin lithium (Li) metal foils have been proved to be indispensable yet elusive for practical high-energy-density lithium batteries. Currently, the realization of such thin foils (<50 μm) is impeded by the inferior mechanical processability of metallic Li. In this work, we demonstrate that the combination of solid solution strengthening and second phase strengthening, achieved by the addition of silver fluoride (AgF) to Li metal, can substantially enhance both the strength and ductility of metallic Li. Benefiting from the improved machinability, we succeed in fabricating an ultrathin (down to 5 μm), freestanding, and mechanically robust Li-AgF composite foil. More interestingly, the in situ-formed LixAg-LiF skeleton in the composite facilitates Li diffusion kinetics and uniform Li deposition, where the thin Li-AgF electrode displays a prolonged cycle life over 500 h at 1 mA cm–2 and 1 mAh cm–2 in a carbonate electrolyte. Coupled with a commercial LiCoO2 cathode (3.4 mAh cm–2), the LiCoO2||Li-AgF cell delivers a notable capacity retention of ∼90% over 100 cycles at 0.5 C with a low negative/positive ratio of 2.5.