Lithium Squarate as Sacrificing Electrolyte Additive for Prelithiation: Case Study in Zero-Excess Lithium Metal Batteries

Among different approaches, the prelithiation via sacrificing additives is at least technically readily applicable. In this work, lithium squarate (Li₂C₄O₄) as a literature-known representative is investigated in zero-excess lithium metal batteries. These batteries 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). However, the gaseous evolution of CO and CO 2 visually rupture the cathode composite. Interestingly, when incorporated via electrolyte, the additive oxidation is absent and is attributed to its reductive depletion in course of solid electrolyte interphase (SEI) formation. In line with computational analysis its reduction is indeed before the reduction of other electrolyte components according to lowest unoccupied molecular orbital (LUMO). Hence, the squarate is impractical as an additive in electrolytes combined with anodes, which in operando form the SEI ( e.g., graphite, Si). In Li metal cells, i.e., with an already existing “preformed” native SEI, the squarate oxidation can be seen again, but the oxidation onset sensitively depends on the cathode type and electrolyte formulation. Figure 1