Use of Pulsed RF Glow Discharge Optical Emission Spectroscopy for the Study of Elemental Distribution of Li-NMC Cathode at Various State of Charge in Li-Ion Batteries

Abstract

Glow discharge optical emission spectroscopy (GDOES) is an analytical technique that provides both a surface and depth profile of the elemental composition and distribution of solid or porous samples. Although GDOES is a destructive technique, it provides the advantage of being a high throughput technique that is highly sensitive to all elements from H to U and can provide high resolution (from nm to µm) depth profiles of elemental distributions in practically any sample. Previous works have investigated and optimized the operating parameters of using GDOES to study negative electrode materials and from the data gathered therein [1] we used 5% H2 in Ar gas in this study to increase sputtering rate of Li-NMC cathode materials and to increase sensitivity to Li in the sample to provide a more detailed elemental distribution. This study investigates the use of GDOES as a method for analyzing elemental and specifically lithium distribution in Li-NMC cathode materials in extreme depth and with validation from other techniques such as x-ray photoelectron spectroscopy (XPS) and x-ray fluorescence (XRF) and with additional morphological analysis of cells using x-ray computed tomography (X-ray CT).In this study coin cells were assembled in an Ar filled glovebox using a 15 mm diameter double sided graphite anode coated on a Cu current collector, 14 mm double sided NMC 111 cathode coated on an Al current collector, either a Celgard 3401 or 5550 separator, and 75 µL of 1M LiPF6–EC:EMC (3:7) electrolyte. The coin cells were then brought outside and EIS data was taken on the freshly made cells. They were then allowed to rest for 12 hours before going through a formation cycle at a C-rate of C/20. After the formation cycle, all cells went through 10 charge/discharge cycles at a C-rate of C/5. After these cycles EIS data was taken again and the cells were then charged to different states of charge (SOC) of 0, 25, 75, and 100% and again EIS data was taken after this. The cells were then brought back into the glovebox where they were de-crimped and disassembled. The electrodes were then rinsed in DMC and dried. They were then analyzed using XPS for surface analysis, XRF for elemental mapping, and X-ray CT for analysis of electrode morphology. Finally, they were analyzed using GDOES and the through thickness Li atomic distribution was plotted for the SOCs listed. An example of atomic concentration distribution of various elements for a SOC of 0% is shown by Figure 1. This study aims to validate the use of GDOES to generate accurate Li distribution profiles and elemental distributions in Li-NMC cathode materials.[1] H. Takahara, A. Kojyo, K. Kodama, T. Nakamura, K. Shono, Y. Kobayashi, M. Shikano, and H. Kobayashi, J. Anal. At. Spectrom., 29, 95–104 (2014). Figure 1