The formation of a protecting nanolayer, so-called solid
electrolyte interphase (SEI), on the negative electrode of Li-ion batteries
(LIBs) from product precipitation of the cathodic decomposition of the
electrolyte is a blessing since the electrically insulating nature of this
nanolayer protects the electrode surface, preventing continuous electrolyte decomposition and enabling the large nominal cell voltage of LIBs,
e.g., 3.3−3.8 V. Thus, the protection performance of the nanolayer SEI is
essential for LIBs to achieve a long cycle life. Unfortunately, the
evaluation of this critical property of the SEI is not trivial. Herein, a new,
cheap, and easily implementable methodology, the redox-mediated
enhanced coulometry, is presented to estimate the protecting quality of
the SEI. The key element of the methodology is the addition of a redox
mediator in the electrolyte during the degassing step (after the SEI
formation cycle). The redox mediator leads to an internal self-discharge process that is inversely proportional to the protecting
character of the SEI. Also, the self-discharge process results in an easily measurable decrease in Coulombic efficiency. The influence
of vinylene carbonate as an electrolyte additive in the resulting SEI is used as a case study to showcase the potential of the proposed
methodology.
