Exploring Cycling Behavior of Lifsi-Bearing Electrolytes in Li1.03(Ni0.5Mn0.3Co0.2)0.97O2//Graphite cells

Introduction

There is increasing interest in finding alternatives to the LiPF₆ electrolyte salt currently used in lithium-ion cells. Among the various candidates, LiFSI (Lithium bis(fluorosulfonyl)imide) has attracted much recent attention because it is reported to have higher ionic conductivity, better high temperature stability, and improved stability towards hydrolysis than LiPF₆.^1-4 In addition LiFSI is reported to be compatible with graphite and silicon based negative electrodes creating stable solid electrode interphase layers that enhance cell life. In this work we compare the cycling behavior of cells with Li_1.03(Ni_0.5Mn_0.3Co_0.2)_0.97O₂–based and graphite-based electrodes, and full cells with these electrodes, in LiFSI- and LiPF₆-bearing electrolytes. Results from such studies are expected to determine the suitability and limits of LiFSI-bearing electrolytes in lithium-ion cells for transportation applications.

Materials and Cells

All electrochemical tests were conducted in 2032-type coin cells (1.6 cm² area electrodes). Al clad parts were used for the positive electrode half cells and for full cell tests; non Al-clad parts were used for the negative electrode half cell tests. Positive and negative electrodes were obtained from the Cell Analysis, Modeling and Prototyping (CAMP) Facility at Argonne. The positive electrodes contained 90 wt% Li_1.03(Ni_0.5Mn_0.3Co_0.2)_0.97O₂ (Toda NCM523), 5 wt% C45 carbon (Timcal), and 5 wt% PVdF (Solvay 5130). The negative electrodes contained ~92 wt% graphite (Superior Graphite SLC1520P), 6 wt% PVdF (Kureha KF 9300) and 2 wt% SuperP (Timcal). The LiFSI salt was purchased from Sarchem Laboratories, EC and EMC solvents from BASF, and FEC from Solvay. Various Celgard separators (2325, 2400, 3501) were examined to determine their compatibility with LiFSI.

Results

Fig. 1 shows that the first two cycles of full cells with EC:EMC (3:7, w/w) + 1.2M LiPF₆ (aka LiPF₆) or 1.2M LiFSI (aka LiFSI) electrolytes are very similar. This similarity is also seen in the differential capacity plots (Fig. 2). However, Fig. 2 inset, which is an expanded view of the 1.5-3.5 V first charge shows differences between the SEI formation characteristics of LiPF₆and LiFSI electrolytes; i.e., the LiFSI salt alters interaction of the solvent with the graphite electrode. Other observations from our experiments include the following:

• The Celgard separator type did not affect the data; similar data were obtained for all cells.
• Initial impedance behavior is similar for cells with the LiPF₆ and LiFSI electrolytes.
• Significant self-discharge is observed for LiFSI-bearing full cells cycled at >4.2V; the upper cutoff voltage (UCV) for these cells should be ≤4.1 V. In contrast, LiPF₆-bearing full cells can be cycled at voltages ≤4.5V.

More information on cell performance and performance degradation, including data from self-discharge, capacity retention, and impedance measurements at 30° and 55°C will be discussed during our presentation. Strategies to overcome the current limitations of LiFSI-bearing electrolytes will also be highlighted. 

Acknowledgements

We acknowledge support from the U.S. Department of Energy’s Office of Vehicle Technologies. We are grateful to S. Trask, B. Polzin, and A. Jansen, from the CAMP Facility (Argonne) for the electrodes, and to M. Kras from the Materials Engineering Research Facility (MERF) for assembling the coin cells, used in this work. These facilities are fully supported within the core funding of the Applied Battery Research (ABR) for Transportation Program.

References

(1)  http://www.boulderionics.com

(2)  Han et al.; J. Power Sources, 2011, 196, 3623.

(3)  Phillipe et al.; J. Am. Chem. Soc, 2013, 135, 9829.

(4)  Shkrob et al.; J.Phys. Chem. C, 2014, 118, 19661.