An Ethylene Carbonate/Propylene Carbonate Electrolyte for Silicon-Graphite/NMC811 High-Energy Lithium-Ion Battery Cells with Improved Safety and Cycle Life

Li-ion batteries (LIBs) based on silicon-carbon composite (Si-C) and Ni-rich (NMC811) layered material have demonstrated a potential to reach high energy density requirements for energy storage tools for electric vehicles [1]. However, several challenges limit their wide commercialization. Long term cycling remains a major issue related to instability of solid electrolyte interphase (SEI) layer on the electrodes. An increased the energy density also interrogates about the safety of Si-C/NMC system. This highlights the importance of proper electrolyte formulation to improve performance and safety level of this technology, which was the goal of the present study. A possibility of using a propylene carbonate (PC)/ ethylene carbonate (EC)- based electrolyte with low vapor pressure and high flash point to improve safety performance of Si-C/NMC811 cells is reported in the present work. A range of electrolyte compositions and additives was screened for Si-C/NMC811 system in coin cells to evaluate the electrochemical performance. The best performing electrolytes were validated in pouch cell tests. It was found that the EC/PC- based electrolyte (El-3) presents a much longer cycle life (+64%) compared to the reference EC/DEC/DMC-based electrolyte (El-0) and reaches higher accumulated discharge capacities of respectively 15164 mAh.g-1 vs 10505 mAh.g-1. Operando gas analysis was conducted using online electrochemical mass spectrometry (OEMS) method. The optimized electrolyte (El-3) was compared to the reference El-0 to evaluate the impact of the electrolyte composition on the gassing behavior of Si-C/NMC cell. Three cycles with different cut-off voltages were applied (two cycles in nominal conditions between 2.5V and 4.3V, one cycle in abusive conditions between 2.5V and 5V vs Li+/Li). The results show a significant decrease of hydrogen (-73%) and CO2 (-19%) generation during 3 cycles for the EC/PC based electrolyte (El-3) compared to El-0 in the Figure 1 below.