Investigating SEI Formation Methods on the Graphite Anode Interface Using Nuclear Reaction Analysis

Observing and managing the functional characteristics of solid electrolyte interphases (SEIs) has been a central topic in lithium-ion battery research for many years. Recently, Antonopoulos et al. reported that forming SEIs on composite graphite electrodes in a high-potential region (1.2 - 0.78 V vs. Li/Li+) has a significant influence on rate capability in half cell formats. [1] In our work, we reviewed several formation methods in full cell devices and studied their impact on the SEI via Li nuclear reaction analysis (Li-NRA) measurements on post-mortem electrodes. This novel technique was previously used to study Li-ion electrode interfaces by Schultz et al. (2017). By detecting gamma emissions from the reaction [7Li (p,γ)8Be] between incident hydrogen ions of specific energies and Li in the sample, we obtained a qualified Li content profile. The NRA results were compared with secondary ion mass spectrometry (SIMS) depth profiles and X-ray photoelectron spectra (XPS). These profiles showed relatively high levels of lithium in the surface region of the anode (~150 nm deep) with variation among electrodes using different formation procedures. This finding, connected with a reduced rate capability for some cells, implies that the buildup of Li at the anode interface may be an important factor influencing the full cell performance after formation.