Impedance of Nonelectroneutral Solid Electrolyte Interphases With Nanopores: A Theoretical Model

Solid-electrolyte interphases (SEI) in lithium batteries reportedly possess a charged solid-solid interface and, in some cases, nanopores, while the effects of these two nonidealities on the impedance are ambiguous. Herein, we employ physical models to calculate local reaction conditions and resulting impedance spectra of nonelectroneutral and nanoporous SEIs under both nonreactive and reactive conditions. The calculated impedance is compared with existing experimental data. Under nonreactive conditions, low-frequency constant-phase element (CPE) phenomenon, which is prevalent in measurements yet remains puzzling, can be attributed to the nonelectroneutral local conditions in the SEI, because no CPE phenomenon is observed under electroneutral conditions. Under reactive conditions, the charge transfer resistance could grow, unexpectedly, with increasing overpotential during lithium stripping due to unfavorable local reaction environment. The structural parameters of nanopores within the inner layer markedly impact the impedance response, which cannot be captured by simple equivalent circuit models; physical models accounting for nanoconfined interfaces in the nanopores are needed, instead.