Improvement of LiNi0.8Co0.1Mn0.1O2 Positive-Electrode Properties by Introducing Fluorinated Propionic Acid Ester-Based Solvent to Highly Concentrated Lin(SO2F)2 Electrolyte Solutions
Authors: Kang Yoojin , Inaba Minoru , Doi Takayuki 
Published: December 2024

Introduction
Further improvement in energy density and safety of lithium-ion batteries is required to expand the spread of EVs. A LiNi0.8Co0.1Mn0.1O2 (NCM811) positive-electrode is drawing attention because it can deliver a high discharge capacity of about 220 mAh g-1 or more by raising the charging upper limit potential to 4.6 V (vs. Li/Li+). In general, however, the electrolyte solution is oxidized and decomposed at about 4 V (vs. Li/Li+) or more, and cracks of the NCM811 particles occur, which results in a significant decrease in charge and discharge performance. Accordingly, highly concentrated electrolyte solutions are attracting attention because of their high stability against oxidation and thermal stability1, while the problem is that the viscosity is high and the ion conductivity is low.
In this study, flame-retardant tris (2,2,2-trifluoroethyl) phosphate (TFEP)-based electrolyte solutions containing high concentrations of LiN(SO2F)2 (LiFSI) were prepared, and methyl 2,3,3,3-tetrafluoropropionate (4FMP) as a co-solvent and methyl perfluoropropionate (5FMP) as a diluent were introduced to reduce the viscosity and improve the ion conductivity. The charge-discharge characteristics of NCM811 positive-electrodes were investigated by conducting charge-discharge tests over 100 cycles in a voltage range of 3.0-4.6 V using a half-cell with an NCM811 working electrode and a lithium metal counter electrode.
Experimental
The nearly saturated 2.2 M LiFSI/TFEP (TFEP/Li+=1.7 by mol) electrolyte solution, and the highly concentrated LiFSI/TFEP+4FMP (TFEP:4FMP=1:1 and 2:1 by vol.) and LiFSI/TFEP+5FMP (TFEP/Li+=1.7 by mol, TFEP:5FMP=1:1, 2:1 and 3:1 by vol.) electrolyte solutions were prepared. An NCM811 composite electrode (13 mmf) was used as a working electrode and a Li foil (14 mmf) was used as a counter electrode, and each electrolyte solution was used to prepare a Li|NCM811 cell. Charge-discharge tests were conducted between 3.0 and 4.6 V. The charge/discharge rate in the first cycle was set to C/10, which corresponds to a current that can theoretically complete each charge (discharge) process in 10 h under the presumption that the theoretical specific capacity of NCM811 is 278 mAh/g. In addition, the NCM811 working electrode after 100 cycles was washed with dimethyl carbonate, dried, and then analyzed by a field emission scanning electron microscope (FE-SEM)/energy dispersive X-ray spectroscopy (EDX). To evaluate the oxidation resistance of the electrolyte solutions, linear sweep voltammetry (LSV) was performed by using three-electrode cell having a Pt working electrode (12 mmf).
Results and Discussion
Figure 1 shows the changes in discharge capacity of Li|NCM811 cells in 100 cycles. 2.1 M LiFSI/TFEP+4FMP (TFEP: 4FMP=2:1 by vol., (TFEP+4FMP)/Li+=2.3 by mol., 4FMP electrolyte) and 1.6 M LiFSI/TFEP+5FMP (TFEP: 5FMP=2:1 by vol., 5FMP-diluted electrolyte) showed the highest performance among the 4FMP- and 5FMP-diluted electrolyte solutions, respectively. After 100 cycles, the discharge capacity retention was 92.5% and 93.1% in the 4FMP- and 5FMP-diluted electrolytes, respectively. In addition, the average Coulomb efficiency reached 99.0% and 99.3%, respectively. The 4FMP electrolyte showed the highest discharge capacity in the 100th cycle (209.0 mAh/g), and the 5FMP-diluted electrolyte showed the highest discharge capacity retention and average Coulomb efficiency. After the 100 charge-discharge cycles, the F atomic concentrations on the surface of NCM811 were 11.6% and 10.1% in 4FMP- and 5FMP-diluted electrolytes, respectively, which was the lowest among the 4FMP and 5FMP-diluted electrolyte solutions. LSV of Pt showed a large anodic current due to the oxidative decomposition of the 4FMP electrolyte, suggesting that the charge-discharge performance of NCM811 is improved by forming a thin and uniform protective film on it. On the other hand, since the oxidation decomposition current of the 5FMP-diluted electrolyte was kept at the lowest level, the charge-discharge performance of NCM811 should be improved by the high stability against oxidation of the electrolyte solution.

Figure 1