As the one of the core electrolyte solvents for Li-ion batteries, ethylene carbonate (EC) is still irreplaceable for its balance of ionic conductivity and interfacial stability. However, it also defines the boundary for the low-temperature performance of the battery because of its high melting point (36.4 °C). Its immediate sibling, propylene carbonate (PC), has been proposed as its convenient substitute for its much lower melting point (-48.8 °C). Unfortunately, the propylene carbonate-graphite anode interfacial problem has been a puzzle since the days before the advent of the Li-ion battery. Among various strategies to mitigate this issue, blending in selected strong solvents for Li+ to bring down propylene carbonate's presence in the solvation shell has been proven often effective but the mechanism from the interfacial chemistry perspective remains unexplored. Herein, we study a new cosolvent, N-methylpyrrolidone (NMP), for PC-based electrolyte and observe excellent reversibility that approaches the commercial standard, far beyond the similar systems in the past. To understand the mechanism, solvation chemistry analysis and in situ characterizations are undertaken to probe the interfacial chemistry from various standpoints. Based on these results and further theoretical calculation, it is proposed that N-methylpyrrolidone has mediated the reduction process of propylene carbonate to facilitate the growth of a solid electrolyte interphase (SEI) layer akin to ethylene carbonate. Finally, an electrolyte has also been successfully developed based on the NMP/PC couple to outperform the commercial electrolyte by a clear margin when tested in a LiNi0.8Co0.1Mn0.1O2-graphite cell at -30 °C.
Keywords: N-methylpyrrolidone; graphite; lithium-ion battery; propylene carbonate; solid electrolyte interphase.