Balancing salt concentration and fluorinated cosolvent for graphite cathode-based dual-ion batteries.

High-concentration electrolytes (HCEs) are favored in graphite cathode-based dual-ion batteries (DIBs) due to their high oxidative stability and good interphase compatibility with both anode and cathode. Although a localized high-concentration electrolyte (LHCE) with the similar solvation structure as HCE is proposed for graphite cathode by adding weakly solvated fluorinated cosolvents, the balance between salt concentration and fluorinated cosolvent has not been investigated. Herein, various electrolytes were prepared by introducing tris(2,2,2-trifluoroethyl) phosphate (FTEP) into the HCE composed of potassium bis(fluorosulfonyl)imide (KFSI) and triethyl phosphate (TEP), and the moderate concentrated electrolyte at the TEP/FTEP volume ratio of 5:1 can stabilize K/graphite DIBs with the capacity of 45 mAh g−1 after 300 cycles at 0.5 A g−1. The abundant ion–solvent complexes as HCE and the entrance of FTEP contribute to building stable FSI−-derived cathode/electrolyte interphase with the KF-rich inner layer. Importantly, FTEP replaces partial TEP solvents to coordinate with FSI−, enhancing the electrolyte oxidative stability, whereas their weak interaction further suppresses the solvent co-intercalation. As a proof, this electrolyte enables high output voltage (3.75 V) and good cyclability of dual-ion full cells coupled with tellurium anode. This work will promote the development of fluorinated cosolvent-modified LHCEs for high-performance DIBs. The balance between salt concentration and fluorinated cosolvent is discovered for graphite cathode with high-rate capacity and good cyclability. Abundant ion–solvent complexes and the participation of fluorinated cosolvent facilitate building anion-derived cathode/electrolyte interphase. The coordination of fluorinated cosolvent with anions to replace some solvent molecules enhances the electrolyte oxidative stability, whereas the weak interaction further suppresses the solvent co-intercalation. [Display omitted] • Graphite cathode exhibits high-rate capacity by introducing FTEP as fluorinated cosolvent in HCE. • Abundant ion–solvent complexes and FTEP's participation help build anion-derived and F-rich CEI. • Weak interaction between FTEP and anion suppresses solvent co-intercalation into graphite cathode. [ABSTRACT FROM AUTHOR]