A Concentrated Electrolyte of LiTFSI and Dimethyl Carbonate for High-Voltage Li Batteries

Realizing the high energy density of lithium batteries depends significantly on creating an electrolyte suited for high-voltage operation and compatible with lithium metal. Generally, a high concentration of solvents within diluted electrolytes may readily form thick and susceptible solid electrolyte interphases, especially when encountering high-voltage operation and a lithium-metal anode. Herein, we present an electrolyte design by significantly increasing the LiTFSI salt concentration within the dimethyl carbonate (DMC) solvent to solve the interfacial incompatibility. The increased salt concentration not only leads to the ion association mode's gradual evolution from a solvent-separated ion pair into a contact ion pair (CIP) but also enhances the oxidation stability. Although a CIP structure generally results in a relatively low ionic conductivity, the lack of free solvent molecules can enhance the compatibility between the designed electrolyte and electrodes. Furthermore, the concentrated 3.5 M LiTFSI electrolyte with DMC as the solvent still maintains a relatively low viscosity, e.g., ∼4.8 mPa s at 30 °C. As a result, LiFePO4 (LFP)//Li and LiNi0.5Co0.2Mn0.3O2 (NCM)//Li cells with the 3.5 M LiTFSI electrolyte exhibit exceptional electrochemical performance. A constructed LFP//Li battery operates stably at 2.0 C for 200 cycles, with a retained discharge capacity of 115.4 mAh g–1, and a NCM//Li cell cycles stably from 2.0 to 4.2 V for 100 cycles. Even after raising the top cutoff voltage to 4.4 V vs Li/Li+, a NCM//Li cell still exhibits a discharge capacity of 89.3 mAh g–1 after 50 cycles at 1.0 C. Therefore, this LiTFSI/DMC-based concentrated electrolyte is promising for high-energy-density lithium-metal batteries.