LiBF4-ethyl methyl carbonate (EMC)-based solutions have not been successfully employed in dual-ion bateries (DIBs) mainly on account of few solvated BF4- intercalations into graphite positive electrodes. In this article, trimethyl phosphate (TMP) is introduced into 1 M LiBF4-EMC solution as the electrolyte solution for DIBs, thus revealing a synergetic effect in which the discharge capacity for the anion storage using 1 M BF4-EMC/TMP (8:2 by vol) (ca. 26.7 mA h g(-1)) is far superior to that for the batteries using 1 M LiBF4-EMC (ca. 2.7 mA h g(-1)) or 1 M BF4--TMP (ca. 1.1 mA h g(-1)). In this case, the effects of TMP on LiBF4 in 1 M BF4-EMC/TMP electrolyte solutions are explored by conventional electrochemical tests, ex situ X-ray diffraction, in situ Raman spectroscopy, and nuclear magnetic resonance.