Dilithium squarate: A game-changing sacrificial salt for pre-lithiation and interphace stabilization in non-SEI forming electrolytes

Abstract

Lithium-ion capacitors (LICs) represent a promising hybrid energy storage technology, merging the high energy density of lithium-ion batteries with the high power density of supercapacitors. However, the absence of lithium in conventional carbon-based LIC electrodes imposes a pre-lithiation step to introduce the Li⁺ required to properly form the solid electrolyte interphase (SEI). This work introduces the role of dilithium squarate (Li₂C₄O₄) sacrificial salt as a breakthrough lithium agent integrated in the positive electrode, that together with non-conventional and beyond carbonate based electrolytes facilitates pre-lithiation and stable SEI formation in LICs. The universality of this approach is demonstrated with three electrolytes with chemically distinct solvents: 1 M LiPF₆ in 3-cyanopropionic acid methyl ester (CPAME), ethyl isopropyl sulfone (EiPS), and γ-valerolactone (GVL). The Li₂C₄O₄ decomposition, and the release of CO₂ and CO decomposition products, lead to robust SEI formation independent of the solvent chemistry. Electrochemical characterization revealed significant enhancements in cell performance and stability, underscoring its compatibility across diverse solvents. This multifunctional additive simplifies LIC design by eliminating the need for SEI-forming additives, reducing costs. Thus, enabling the integration of advanced electrolytes in lithium-ion capacitors and lithium-ion batteries, achieving enhanced properties that include extended electrochemical stability window, high thermal stability or improved safety and sustainability. These findings establish Li₂C₄O₄ as a pivotal enabler for next-generation energy storage technologies.