Changes at the Li Ion Battery‘s Solid Electrolyte Interphase: from Cradle to Grave
Li ion batteries are actual enablers for electromobility and grid stabilisation. A major challenge in the quest for high energy density, long-life Li ion batteries is the instability of the electrolyte, which reacts at the negative electrode, usually carbon, to form the solid-electrolyte interphase (SEI). Whereas this decreases the available Li for reaction and thus battery capacity, a stable, thin and ion conductive SEI acts as a protective layer which slows down degradation and thus enables to reach thousands of charge-discharge cycles. The end of life of a battery is usually connected to too high losses related to the SEI. Initial formation of this nm thin, stable layer and keeping it stable over a wide operating range and time, also during misuse, is thus essential for save and economic batteries. This in turn requires in-depth understanding of the state and properties as well as growth and decomposition of the layer.
This talk will give a model-assisted insight into the SEI from cradle to grave. The formation process is analysed using a coupled continuum-kinetic-Monte-Carlo model; it gives insight into the initial phase, where desorption of reactants hinders a rapid surface passivation, and the following phase where -depending on the applied current – different SEI compositions and structures are formed. It is further shown how during the life time of a battery, the degradation state including SEI thickness can be monitored by reproducing experimental impedance spectra with a continuum model. Finally, an insight into the feared thermal runaway of batteries, i.e. the uncontrollable self-heating of batteries when maltreated, is given with a physico-chemical model. It reveals the crucial interaction of SEI decomposition or formation reactions and related phase changes.
All in all, the analysis and models not only give a deeper understanding of this crucial battery component – they also open up the possibility for model-supported improvement of capacity, life time and safety.