Understanding composition and morphology of solid-electrolyte interphase in mesocarbon microbeads electrodes with nano-conducting additives

Solid-electrolyte interface (SEI) between the active material and the electrolyte is critically important to the electrochemical performance of a lithium-ion battery. In this work, a depth profile measurement of the SEI layer after prolonged electrochemical cycling is reported in order to quantify the influence of conducting additives in the development of this layer in mesocarbon microbeads (MCMB) electrodes. It is shown that the SEI in the electrodes having multiwalled carbon nanotubes (CNT) as conductive additive is primarily a multi-layered structure consisting of an outer organic layer near the electrolyte/SEI interface and an inner inorganic layer near the SEI/electrode interface. The chemical analysis of such electrodes exhibits that the inorganic layer is made up of compounds such as Li2O, LiF and Li2CO3, while the outer layer consists mainly of organic carbonates, oligomers, traces of randomly distributed LixPOyF, and undissociated LiPF6. The chemical analysis of electrodes with carbon black (CB) as conducting additive reveals a similar composition of the outer layer, whereas the inner layer shows a relatively uniform molar amount of organic and inorganic carbonates and species like LixPOyF, and LixPOy, resulting in a porous SEI layer which as confirmed in AFM and SEM characterizations. On the other hand, the electrodes containing hybrid (CB-CNT) conducting additives show a relatively higher oligomer content than the electrodes with CNT only. The SEI formed on electrodes with CB-CNT as additives is shown to offer increased flexibility to accommodate partial strain generated due to volume change with narrow cracks exhibited in the SEM micrographs. Further, the variation in surface features and roughness augment the XPS findings. (C) 2020 Elsevier Ltd. All rights reserved.