How\nDoes Li<sub>2</sub>C<sub>4</sub>O<sub>4</sub> Prelithiation\nAdditive Influence the Solid Electrolyte Interphase of Dual Carbon\nLithium-Ion Capacitors?

Prelithiation\nis a critical step in dual carbon lithium-ion capacitors\n(LICs) due to the lack of Li<sup>+</sup> in the system, which needs\nto be incorporated externally to avoid electrolyte depletion. Several\nprelithiation techniques have been developed over the years, and recently,\ndilithium squarate (Li<sub>2</sub>C<sub>4</sub>O<sub>4</sub>) has\nbeen reported as an air-stable, easy to synthesize, safe, and cost-effective\nprelithiation reagent for LICs. Li<sub>2</sub>C<sub>4</sub>O<sub>4</sub> has successfully been used in a wide range of chemistries, and its\nintegration into positive electrodes has been scaled up to roll-to-roll\nprocessing and demonstrated in multilayer pouch cells. However, its\ninfluence in the solid electrolyte interphase (SEI) has not yet been\nstudied. In this work, the SEI formed on the hard carbon (HC) negative\nelectrode when using Li<sub>2</sub>C<sub>4</sub>O<sub>4</sub> as a\nprelithiation agent has been studied by X-ray photoelectron spectroscopy\n(XPS). The electrode surface has been analyzed in the lithiated and\ndelithiated states along the first lithiation cycle, as well as at\nthe end of the prelithiation protocol, to gain insight into the SEI\nformation and evolution during the prelithiation process. In addition,\nan aging test has been carried out to study the long-term SEI stability.\nWe have observed that the use of Li<sub>2</sub>C<sub>4</sub>O<sub>4</sub> induces a chemical modification in the composition of the\nSEI with respect to the SEI that forms by using a standard electrochemical\nprelithiation process, resulting in a less soluble interface. Therefore,\nthe chemical composition of the SEI is stable over cycling. Those\nfindings confer to Li<sub>2</sub>C<sub>4</sub>O<sub>4</sub> the ability\nto tune the SEI of the devices, enabling its use in LICs and LIBs\nnot only as a prelithiation agent but also as a film-forming additive.