High‐Performance Lithium‐Oxygen Battery Electrolyte Derived from Optimum Combination of Solvent and Lithium Salt

Abstract To fabricate a sustainable lithium‐oxygen (Li‐O2) battery, it is crucial to identify an optimum electrolyte. Herein, it is found that tetramethylene sulfone (TMS) and lithium nitrate (LiNO3) form the optimum electrolyte, which greatly reduces the overpotential at charge, exhibits superior oxygen efficiency, and allows stable cycling for 100 cycles. Linear sweep voltammetry (LSV) and differential electrochemical mass spectrometry (DEMS) analyses reveal that neat TMS is stable to oxidative decomposition and exhibit good compatibility with a lithium metal. But, when TMS is combined with typical lithium salts, its performance is far from satisfactory. However, the TMS electrolyte containing LiNO3 exhibits a very low overpotential, which minimizes the side reactions and shows high oxygen efficiency. LSV‐DEMS study confirms that the TMS‐LiNO3 electrolyte efficiently produces NO2 −, which initiates a redox shuttle reaction. Interestingly, this NO2 −/NO2 redox reaction derived from the LiNO3 salt is not very effective in solvents other than TMS. Compared with other common Li‐O2 solvents, TMS seems optimum solvent for the efficient use of LiNO3 salt. Good compatibility with lithium metal, high dielectric constant, and low donicity of TMS are considered to be highly favorable to an efficient NO2 −/NO2 redox reaction, which results in a high‐performance Li‐O2 battery.

from Sigma Aldrich. Ketjen black ® and PTFE were combined with an isopropyl alcohol/water mixture to produce a slurry. This slurry was coated onto a gas diffusion layer (TGP-H030, Toray Co.) and dried at 150 °C under vacuum to produce the cell cathode. The typical loading of the KB in the cathode electrode was ~0.5 mg/cm 2 .
Coin-type Li-O 2 cells with a top SUS mesh were assembled in an argon-filled glove box (MBraun, H 2 O & O 2 <1 ppm) for subsequent battery experiments. The Li-O 2 cell was composed of a Li metal anode, a glass microfiber membrane separator, and a KB cathode. The coin cell was inserted in a cell holder with two attached SUS capillaries (outside diameter: 1/16 inch) for gas to flow in and out of the cell. The cell holder with a Li-O 2 coin test cell was transferred from the glove box to a battery cycling test system with gas capillaries tightly capped. In the cycling test system, high-purity oxygen gas (>99.999%) was fed through the inlet capillary attached to the upper side of the cell holder into the KB cathode. The gas exited from the cathode through a second capillary (outlet) that allowed the exhausted oxygen to flow from the cell. The oxygen pressure on the cathode was maintained at ~1.5 bar during the cell cycling, with the inlet capillary kept open and the outlet closed. The cycling of the Li-O 2 cells was controlled by a VMP3 potentiostat (Biologic Science Instrument). In a typical Li-O 2 cell cycle, a current of 200 mA g c -1 (g c : weight of KB in the cathode) was applied for 5 h for both the discharging and charging of the cell with a cut-off potential of 2.0 V for the discharge and 5.0 V for the charge. All the potentials reported in this paper were displayed relative to the voltage of the Li/Li + couple, unless otherwise stated.

In situ differential electrochemical mass spectrometry (DEMS) analysis
The consumption of O 2 during the cell discharge, and evolution of O 2 and other gaseous products during the cell charge were quantitatively measured using in situ DEMS analysis. The pressure drop in the hermetically sealed Li-O 2 cell during discharge was recorded to quantitatively measure the quantity of oxygen consumed.
Following discharge, the oxygen in the cell was flushed out and replaced with argon.
During the charging process, any evolved gases in the isolated cell were accumulated during a programmed interval (e.g., 10 min) and were then transferred by argon carrier gas into the mass spectrometer to identify and quantify the gases. The quantity of the gases was determined by comparing the intensity of the peaks of the gases recorded by the mass spectrometer analysis to the intensity of MS peaks generated by carrier gas argon.
Several of the parameters measured by the DEMS analysis are defined as follows.
The discharge oxygen efficiency (η O2, dis ) is the ratio of the quantity of O 2 consumed during cell discharge compared with the amount of O 2 consumed in an ideal discharge reaction as represented by, 2Li + + 2e -+ O 2 → Li 2 O 2 . The charge oxygen efficiency (η O2 , ch ) is the ratio of the quantity of O 2 evolved during the charging of the cell compared with that in the ideal charge reaction, (Li 2 O 2 → 2Li + + 2e -+ O 2 ). The CO 2 gas ratio (r CO2 ) is defined as the amount of CO 2 produced on charge normalized to the quantity of O 2 produced in the ideal charge reaction. Energy efficiency (η energy ) on cycling is defined by the energy (energy is the product of current-time and potential) produced during discharge to the energy consumed during charge.

Characterization methods
After discharge or full cycling, the cells were disassembled, and the cathode was