Numerical Investigation of Capacity Fade in Mcmb Electrodes
Authors: Rashid Muhammad , Ahamad Salahuddin , Gupta Amit 
Published: February 2020

Mesocarbon microbead (MCMB), artificial and natural graphite has been used as a commercial anode material due to its low redox potential (Li/Li+), good ionic/electronic conductivity and high structural stability during the process Li intercalation/deintercalation1. However, solid electrolyte interface (SEI) film formation due to solvent reduction at negative electrode is a major factor which is responsible for the capacity degradation in these anodes. Investigations of capacity degradation due to film formation have been carried out by various research groups by conducting experiments and simulations of MCMB/LiMn2O4 full cells2-4. In these systems, capacity loss is a concurrent effect of SEI formation with gas evolution at anode and manganese dissolution at cathode5. Hence, the contribution to capacity loss at each electrode could not be ascertained through the analysis of full cells. To fill this gap and investigate the capacity loss due to solvent reduction at anode, MCMB half cells have been studied thorough experimental measurements and numerical simulations. For this purpose, various electrochemical techniques such as galvanostatic intermittent titration (GITT), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), galvanostatic charge and discharge have been applied on MCMB half cells. For conducting the numerical study, certain parameters of ideal cell and SEI growth model have been measured and the rest calibrated with experimental findings. Using this calibrated model, numerical simulations have been carried out to quantitatively analyze the effect of SEI film and generated gas on the cycling performance of MCMB electrode with respect to cycle number.

Figure 1 (a) shows the experimentally obtained capacity from cycling of MCMB coin cell at 1C rate for first 100 cycles. The first cycle discharge capacity of the cell was 1.58 mAh, which decreases gradually as cycle number increases due to associated side reactions such as electrolyte reduction to form solid precipitates and its deposition over the active particle of the MCMB electrode. The capacity obtained after 100 cycles was 0.99 mAh which is 62.65% of 1st cycle capacity. Figure 1 (b) shows the cell potential vs. charging and discharging capacity for 1st and 100th cycle which also indicates capacity loss of 37.34% at the end of 100thcycle. The results obtained from this study will assist in quantification of the individual mechanisms of gas evolution and SEI formation on the capacity loss of cells with MCMB as the negative electrode.

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Figure 1