An All-Fluorinated Ester Electrolyte for Stable High-Voltage Li Metal Batteries Capable of Ultra-Low-Temperature Operation

Abstract

Improving the energy output of batteries at sub-zero temperatures is crucial to the long-term application of advanced electronics in extreme environments. This can generally be accomplished by employing high-voltage cathodes, applying Li metal anodes, and improving the electrolyte chemistry to provide facile kinetics at ultralow temperature. However, systems capable of all three of these have seldom been studied. Herein, we demonstrate the design of such a system through solvent fluorination, applying a 1 M LiPF6 in a methyl 3,3,3-trifluoropionate (MTFP)/fluoroethylene carbonate (FEC) (9:1) electrolyte that simultaneously provided high-voltage cathode and Li metal anode reversibility at room temperature. This performance was attributed to the production of fluorine-rich interphases formed in the MTFP-based system, which was investigated by X-ray photoelectron spectroscopy (XPS). Furthermore, the all-fluorinated electrolyte provided 161, 149, and 133 mAh g–1 when discharged at −40, −50, and −60 °C, respectively, far exceeding the performance of the commercial electrolyte. This work provides new design principles for high-voltage batteries capable of ultra-low-temperature operation.

Role of Li‐Ion Depletion on Electrode Surface: Underlying Mechanism for Electrodeposition Behavior of Lithium Metal Anode

“…[ 6 b] Thus, regulation of electrodeposition behavior of Li to obtain a dense microstructure with minimum porosity is an essential route to improve the Coulombic efficiency and to address safety issues for practical Li‐metal anodes. [ 7 ] To realize the goal of dendrite‐free Li anode, a plenty of the effective strategies have been developed to tackle the non‐uniform electrodeposition of Li during plating/stripping process, including the interphase engineering, [ 8 ] the optimization of electrolytes, [ 9 ] the integrated composite electrode, [ 10 ] the utilization of solid‐state electrolyte, [ 11 ] and thermally driven suppression of Li dendrite. [ 12 ] Meanwhile, to reveal the dendrite growth from various aspects like reaction kinetics, mass transport and electrochemical conditions, considerable computational studies were performed by using phase‐field, kinetic Monte Carlo and hydrodynamic methods.…”

“…Factors like Li‐salt concentration, temperature and applied current density have previously been independently investigated with respect to regulating electrodeposition of Li. [ 9 c, 9 d, 15,18 ] However, a comprehensive view of how these key factors of the mass transfer process jointly affect the electrodeposition behavior of Li is still lacking.…”

Designing Advanced Lithium‐Based Batteries for Low‐Temperature Conditions

“…For instance, Holoubek et al employed a 1 m LiPF 6 in a methyl 3,3,3-trifluoropropionate (MTFP): fluoroethylene carbonate (FEC) (9: 1) electrolyte. [55] The MTFP acts as a fluorinated analogue of methyl propionate, which along with other esters has been utilized as a low-temperature co-solvent in much of the lithium-ion low-temperature literature. [56,57] Impressively, the fluorinated ester formulation maintains a high ionic conductivity of 0.75 mS cm −1 at −60 °C compared to 0.005 mS cm −1 for 1 M LiPF 6 in EC/DEC, meeting the requirement for low-temperature phase stability.…”

Design aspects of electrolytes for fast charge of Li‐ion batteries

“…The carbonate solvents are thermodynamically unstable against Li metal, lithiated graphite, delithiated layered cathode, and at extreme potentials (ie, low potentials at the anode and high potentials at the cathode) in the Li-ion batteries of the charged state. For these problems, fluorinated electrolytes [12][13][14][15][16][17][18] and super-concentrated electrolytes [19][20][21][22] have been studied to enhance the kinetic stability of carbonate solvents. The former is formulated by partially replacing carbonate with a fluorinated carbonate [12][13][14][15]17 or a fluorinated ether 13,15,18,[23][24][25] based on the concept that in the presence of Li + ions, the fluorinated solvents can be sparingly decomposed to form a LiF-rich SEI.…”

“…For these problems, fluorinated electrolytes 12‐18 and super‐concentrated electrolytes 19‐22 have been studied to enhance the kinetic stability of carbonate solvents. The former is formulated by partially replacing carbonate with a fluorinated carbonate 12‐15,17 or a fluorinated ether 13,15,18,23‐25 based on the concept that in the presence of Li + ions, the fluorinated solvents can be sparingly decomposed to form a LiF‐rich SEI. The primary disadvantage of such electrolytes is high cost.…”