An Investigation of SEI Layer Composition in Presence of Nano-Fillers in Mcmb Electrodes
Authors: Ahamad Salahuddin , Gupta Amit 
Published: February 2020

Since the advent of first commercial Li-ion battery (LIB) in 1991 by Sony, a continuous improvement in their electrochemical performance has been realized. Research has been focused on developing anode materials having higher energy density and specific capacity to replace the existing graphitic anodes [1]. A number of materials such as SnO2 [2], Li4Ti5O12 [3], Si [4] etc. have been extensively studied. Despite displaying promising initial electrochemical characteristics, the large volume change for most of these materials causes the rupture of the solid electrolyte interface (SEI) layer. This repeated breakage and replenishing process of the SEI layer results in continuous decomposition of electrolyte and loss of cathode Li inventory thereby shortening cycle life. Moreover, some other materials such as TiO2 face the problem of low electronic conductivity, small operating voltage window and high cost [5].
Another approach to improve the long-term performance of LIB anodes, in terms of high rate performance and cycling stability, is to further improve the electrochemical performance of graphitic materials. Graphite experiences a volume change about 10% during intercalation/deintercalation [6, 7]. However, the brittle inorganic film may crack and new SEI will form if the SEI surface film is not mechanically stable to accommodate this volumetric strain. Therefore, it is critically important to build-up a thin, stable and ionically conducting surface layer that can tolerate volume change and offer low impedance to ensure a near reversible cycling performance. In this work, we explore the formation of SEI layer and its chemistry on mesocarbon microbeads (MCMB) electrode in presence of different conducting nano-fillers such as carbon black (CB) and carbon nanotubes (CNT). The SEI is characterized ex-situ using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) providing insight into its composition together with nano-scale morphological description.
To carry out the depth analysis of SEI layer, the XPS spectra were recorded after sputtering the samples for a time of 400, 2200, 4000, 5800 and 7600 sec. as shown in Fig.1a. To demonstration the effect of nano-fillers, the C1s spectra for CB as conducting additives exhibits a strong signal between 284-285 eV (C-C), 289-290 eV (Li2CO3 or alkylcarbonate) and between 285-286 eV (alkoxide) on the cycled electrode as shown in Fig. 1. As shown, the proportion of Li2CO3 steadily decreases with sputtering indicating its lower presence in inner layers of the SEI. In case of CB as conductive additives, the insignificant change in proportion of CO3-CO (Fig. 1b) suggests a mechanically weak SEI film formed on the surface. On the other hand, the surface film generated in presence of CNT as additives offers a larger CO3 proportion and significantly reduced CO (Fig. 1b). This suggests the SEI film formed on the surface of MCMB is mechanically strong due to higher proportion of carbonate. Significantly, the ratio of amount of CO3 to CO ratio is shown to lie between 0.7 to 1.5 in case of mixed CB-CNT nano-fillers as conducting additives. This finding indicates that the SEI formed in the presence of mixed nano-fillers offers (a) flexibility to accommodate partial strain generated due to volume change, and (b) is capable of conducting Li-ion as a result of a relatively higher percentage of carbonates.

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Figure 1