Adiponitrile (ADN): A Stabilizer for the LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) Electrode/Electrolyte Interface of a Graphite/NCM811 Li-Ion Cell

Adiponitrile (ADN) is studied as an additive of commercial carbonate electrolyte for a graphite/NCM811 Li-ion cell in this work. Compared with the cell using a baseline electrolyte, the cycling stability of the cells using 0.2, 0.5, 1, and 2 wt % ADN-added electrolytes is significantly improved. Among them, the effect is most prominent when the ADN content is 0.5 wt %, and the capacity retention rate of the cell after 200 cycles at 0.3 C is increased by 17.6%. Scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy analyses confirm that ADN can make the coating layer formed on the cathode and anode surfaces of the cell denser and uniform. The formation of the cathodic coating layer does not obey the usual mechanism of adsorption between the nitrile group and metal ions, but it may be due to the polymerization of ADN initiated by carbocations from the oxidation of solvent molecules (ethylene carbonate/ethyl methyl carbonate) in the electrolyte. The cathodic coating layer greatly decreases the oxidation of the electrolyte so that a stable coating layer (also called solid electrolyte interface/SEI) can be formed on the anode, which is the reason for improvement of the cycling performance of the cell. However, the formation of the anode SEI layer is almost independent of ADN, and it still depends on the common reduction reaction of the electrolyte.