Enhancing the performance of lithium-ion batteries is a hot topic. Within a lithium ion battery cell, the solid-electrolyte interphase (SEI) layer composition and change during use is of keen interest. The SEI is a by-product of the charge and discharge process is the formation of the on the anode. Understanding the SEI layer is essential to controlling and therefore improving cell performance.
Lithium, however, is very sensitive to air and moisture, so it cannot be studied in ambient conditions. In order to detect photoelectrons at a material’s surface, X-ray photoelectron spectroscopy requires high vacuum conditions. This presents a challenge: How to study the surface chemistry of lithium-ion batteries with XPS?
The trick is to prepare and analyze a material to ensure the surface experiences no changes during analysis. In this regard XPS can now determine the key interface layer before use and after multiple cycle times. Capturing two critical points in time during battery use – the first use and at some point after cycling – is analogous to watching the degradation of an automobile tire right off the factory floor and after being driven for 10,000 miles.
In order to complete an XPS analysis of the SEI from a used and unused battery, researchers used a Vacuum Transfer Module (VTM) in a glove box. The VTM was evacuated in the glove box antechamber, and then transported to the XPS system. As the VTM is held together by air pressure, it automatically opens during the pump-down cycle in the system load-lock and is therefore integrated into the standard, automated, sample transfer process.
A Thermo Scientific™ Nexsa™ XPS System and the Vacuum Transfer Module were used to analyze the surface of lithium ion battery electrodes. The results of the XPS analysis of the two SEI layers are discussed in the following application note.