Lithium Squarate as Sacrificing <i>Electrolyte</i> Additive for Prelithiation: Case Study in Zero‐Excess Lithium Metal Batteries

Among different approaches, the prelithiation via sacrificing additives can be technically relatively easy applied. In this work, lithium squarate (Li₂C₄O₄) as a literature-known representative is investigated in zero-excess lithium metal batteries, which are simple in handling and have high active lithium loss (ALL), thus ideal for R&D of sacrificing additives. When incorporated via cathode, Li₂C₄O₄ oxidizes at a relatively low cathode potential (≈4.5 V vs Li|Li⁺) and provides the aimed active Li (= capacity), but ruptures the cathode via gaseous evolution of CO and CO₂. Interestingly, when incorporated via electrolyte, the additive oxidation is absent. This can be correlated with its reductive depletion in the course of solid electrolyte interphase (SEI) formation, as hinted via computational analysis by its relatively low energetic level of lowest unoccupied molecular orbital (LUMO), as well as by energy dispersive x-ray spectroscopy, and decreased gas evolution. Hence, the squarate is concluded to be impractical as an additive in electrolytes when combined with anodes, which in operando form the SEI (e.g., graphite, Si). In Li metal cells, i.e., with an already existing and passivating "preformed" native SEI, the squarate oxidation can be seen again, but the oxidation onset sensitively depends on the cathode type and electrolyte formulation.