Effect of electrolyte additives on the LiNi0.5Mn0.3Co0.2O2 surface film formation with lithium and graphite negative electrodes

The effect of various electrolyte additives (fluoroethylene carbonate (FEC), vinylene carbonate (VC), and propane sultone (PS)) on the performance of LiNi0.5Mn0.3Co0.2O2 (NMC532) electrodes in combination with lithium (half-cell) and graphite (full-cell) negative electrodes, is herein reported. The cathode/electrolyte interface (CEI) layer formed on the NMC532 electrode cycled up to 4.5 V versus Li+/Li is investigated by X-ray photoelectron spectroscopy and scanning electron microscopy. This allows correlating the electrochemical performance of the electrodes to the CEI chemical composition, thickness, and morphology. All the investigated electrolyte additives exhibit beneficial effects in half- and full-cell systems, confirming the effective passivation layer formation protecting the electrolyte from further decomposition at high voltages. It is found that the thickness of the CEI layer forming on the NMC532 electrodes in half- and full-cell configuration is different. VC and PS are found to be the best additives to enhance the performance mainly due to their positive contribution to the CEI formation. The solid/electrolyte interphase (SEI) formed on the graphite electrode is also investigated and compared to the CEI layer formed on the cathode. The detection of typical SEI reduction products on the positive electrode surface confirms the occurrence of cross-talking between the two electrodes.