How do high-voltage cathode and PEO electrolyte get along well? EIS analysis mechanism & potentiometric control strategy

Abstract

PEO-based all-solid-state electrolytes are extensively utilized and researched owing to their exceptional safety, low-mass-density, and cost-effectiveness. However, the low oxidation potential of PEO makes the interface problem with the high-voltage cathode extremely severe. In this work, the impedance of PEO-based all-solid-state batteries with high-voltage cathode (NCM811) was studied at different potentials. The Nyquist plots displayed a gyrate arc at low-frequencies for NCM811/PEO interface. Based on the kinetic modeling, it was deduced that there is a decomposition reaction of PEO-matrix in addition to de-embedded reaction of NCM811, and the PEO intermediate product (dehydra-PEO) adsorbed on the electrode surface leading to low-frequency inductive arcs. Furthermore, the distribution of relaxation time shows the dehydra-PEO results in the kinetic tardiness of the charge transfer process in the temporal dimension. Hence, an artificial interface layer (CEIx) was modified on the surface of NCM811 to regulate the potential of cathode/electrolyte interface to prevent the high-voltage deterioration of PEO. NCM/CEIx/PEO batteries exhibit capacity retentions of 96.0%, 84.6%, and 76.8% after undergoing 100 cycles at cut-off voltages of 4.1, 4.2, and 4.3 V, respectively. Therefore, here the failure mechanism of high-voltage PEO electrolyte is investigated by EIS and a proposed solving strategy is presented.