Interphases play a vital role in lithium-ion batteries (LIBs), as they protect the electrodes and the electrolyte from various decomposition reactions during ongoing charge / discharge cycling, but also influence the resistance and Li-ion transport within the cells. Especially for next-generation LIBs, interphases are crucial to cope with the newly arising challenges, e.g. a higher operational voltage, which leads to oxidative decomposition of the electrolyte and the dissolution of transition-metals into the electrolyte, resulting in “roll-over” failure. Additional challenges arise by the utilization of high-capacity electrode materials, as nickel-rich cathodes or silicon-based anodes, which undergo significant volume changes during lithiation and de-lithiation, resulting in continuous interphase reformation.
The implementation of film-forming electrolyte additives is a promising approach to facilitate the formation of effective interphases, which boosts the performance and life-time of LIBs, by preventing decomposition processes. Therefore, a deep understanding of the underlying mechanistic processes of the additive-induced interphases is necessary to systematically evaluate electrolyte additives and to develop new electrolyte formulations. Operando advanced Raman spectroscopy techniques, like shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) or surface enhanced Raman spectroscopy (SERS), are promising tools to obtain valuable information about the formation of interphases under real working conditions, due to their high surface sensitivity, high spatial resolution, set-up simplicity and ability to be combined with electrochemistry.
In the presented study we investigated different EC-based electrolyte additives regarding their influence on the performance of NMC622||AG pouch cells. In addition, we characterized the additive-induced interphases by operando SHINERS and SEM experiments. In general, it was found that the addition of the respective electrolyte additives has severely different effects on the electrochemical performance. While the addition of some additives showed an improvement of the performance, compared to the carbonate-based baseline electrolyte, the addition of other additives even led to reduced performance and a shorter cycle life. In addition, it was possible to show that the addition of the different additives has a significant influence on the morphology of the formed interphase on the anode. For the characterization of the additive-induced interphases, it was possible to identify different specific decomposition products of the electrolyte additives via SHINERS. Surprisingly, it was found that the addition of VEC to the electrolyte significantly enhances the LIBs performance, greatly surpassing the beneficial influence of widely known film-forming additives, like VC or FEC. In fact, GC-MS and SHINERS measurements even indicated that the SEI formed by VEC was rather ineffective in preventing reductive electrolyte decomposition, compared to the other investigated additives. Instead, it was found that VEC forms an interphase on the cathode side, mostly consisting of semi-carbonate type decomposition products of the electrolyte additive. This finding was proven by operando SHINERS experiments and supported by additional SEM and SERS measurements. The finding of very intense bands, which can be attributed to COO- groups, even indicate the possibility of a cross-talk between anode and cathode that leads to the observed interphase formation.