Corrosion Inhibition of Stainless Steel in Highly Concentrated Lin(SO2F)2 Electrolyte Solutions with Organophosphate Esters
Authors: Shintani Kota , Doi Takayuki , Inaba Minoru 
Published: December 2024

Introduction

To improve the energy density of lithium-ion batteries, various kinds of electrode materials have been developed. Silicon and lithium metal are considered promising as novel high-capacity negative-electrode materials, while the practical use of them is limited due to the significant morphological changes during charging and discharging. Lithium bis(fluorosulfonyl)imide (LiFSI) has been attracting attention as an alternative salt to LiPF6 as it formed a superior solid electrolyte interphase (SEI) on the negative electrodes. However, LiFSI-based electrolyte solutions cause corrosion of cell components at high voltages; i.e., an aluminum current collector (Al) (>3.7 V)[1] and a stainless steel (SS) case used in cylindrical and coin cells (>3.8 V)[2]. In general, SS corrodes more severely than Al. The SS corrosion can be inhibited in the highly concentrated electrolyte solutions.[2] However, to our best knowledge, there are no reports on the additives and solvents that can effectively inhibit the corrosion. In this study, we focused on organophosphate ester solvents, and investigated effects of LiFSI concentration on the inhibition of SS corrosion.

Experimental

The highly concentrated electrolyte solutions used were 3.8 M LiFSI dissolved in triethyl phosphate (TEP) (TEP/Li=0.95 by mol., TEP electrolyte), and 2.2 M LiFSI dissolved in tris(2,2,2-trifluoroethyl) phosphate (TFEP) (TFEP/Li=1.7 by mol., TFEP electrolyte). 6 M LiFSI dissolved in dimethyl carbonate (DMC) (DMC/Li=1.1 by mol., DMC electrolyte) was also used as a reference. The corrosion behaver of SS electrodes was examined by cyclic voltammetry (CV) for 100 cycles in a voltage range of 3.0-4.6 V (vs. Li/Li+) using a three-electrode cell, with a SUS316L (SS316) working electrode, and lithium metal as both the counter and reference electrodes. The electrode surface after CV was observed by an optical microscope.

Result and Discussion

Figure 1 shows cyclic voltammograms (CVs) of SS316 electrodes in each electrolyte solution. In DMC electrolyte, an anodic current due to the corrosion was clearly observed in the 10th cycle at ca. 4.1-4.2 V (vs. Li/Li+), and further increased in subsequent cycles, indicating the progression of SS corrosion. In contrast, such a corrosion current peak in TEP electrolyte significantly reduced to less than one-tenth (<1.5 μA cm-2) compared to the DMC electrolyte. Moreover, no corrosion peak was observed in TFEP electrolyte. After 100 cycles in the DMC electrolyte, a number of corrosion pits and widespread corrosion products were observed, as shown in Figure 2. Similarly, corrosion pits were observed for the TEP electrolyte, while no corrosion products were seen. In contrast, neither corrosion pits nor corrosion products were obvious in the TFEP electrolyte. These results indicate that the organophosphate esters, TEP and TFEP, can effectively inhibit the corrosion of SS. More detailed analysis of the SS surface film was conducted through X-ray photoelectron spectroscopy. We will discuss how it contribute to the corrosion inhibition in our presentation.

Reference

[1] E. Svensson et al., J. Phys. Chem. C,
127, 7921-7928 (2023).

[2] C. Luo et al., Electrochem Acta.,
419, 140353 (2022).

Figure 1