Rational design of electrolytes operating at low temperatures: Does the co-solvent with a lower melting point correspond to better performance?

To achieve desirable performance of lithium-ion batteries at low temperatures, using co-solvents with low melting points, especially carboxylic esters, is an efficient strategy and thus, there is a general question: does the co-solvent with a lower melting point correspond to the composite electrolytes with better performance? Herein, we chose two constitutional isomers with 22 °C of difference in melting point, ethyl propionate and propyl acetate, as the co-solvent in the formula of two low-temperature electrolytes for comparison. At low temperatures, the Li||LiNi0.8Co0.1Mn0.1O2 coin cells and graphite||LiNi0.8Co0.1Mn0.1O2 pouch cells using the two electrolytes show close capacity retention, suggesting that the co-solvent with a lower melting point does not necessarily result in better low-temperature performance. Molecular dynamics simulations indicate that the similar solvation structure of contact ion pair for the two electrolytes is mainly responsible for their similar low-temperature performance. Furthermore, the Li||LiNi0.8Co0.1Mn0.1O2 and graphite||LiNi0.8Co0.1Mn0.1O2 cells with the two electrolytes at ambient temperature show capacity retention of 84.1%, 84.3% after 200 cycles and 88.2%, 89.6% after 360 cycles, respectively, demonstrating the reliability of the electrolytes. The results of this study provide general guidance for the development of low-temperature electrolyte regarding the choice of co-solvents.