Higher rate capability in Li-ion batteries is always better. Using esters as a co-solvent can significantly improve the rate capability of Li-ion batteries because of their low viscosity which leads to higher electrolyte conductivity [1, 2]. However, which ester is the best to use from a cell lifetime perspective? In this work, the four esters, methyl acetate (MA), methyl propionate (MP), ethyl acetate (EA) and methyl butyrate (MB) were compared as co-solvents in the electrolytes of Li-ion pouch cells at levels from 0% to 60 % by weight. Experiments included high temperature storage, ultra high precision coulometry, high rate charge to determine the onset of lithium plating, long term cycling and isothermal microcalorimetry. In addition, Gering’s Advanced Electrolyte Model was used to compare the conductivity and viscosity benefits associated with the use of each ester. The baseline electrolytes to which the esters were added were 1.2 M LiPF₆ in EC:EMC 30:70 or 1.2 M LiPF₆ in EC:EMC:DMC (25:5:70 by vol%). Figure 1 shows the results of 60^oC storage tests for Li-ion pouch cells containing various amounts of each of the four esters. Cells were stored at either 4.2 V, to examine the stability of the positive electrode/electrolyte interface and at 2.5 V to examine the stability of the negative electrode SEI. Figure 1 shows that of the four esters, methyl acetate is greatly preferred based on the storage testing for this Li-ion cell chemistry.

Experiments involving many Li-ion cells chemistries and the four esters will be reported. Based on this work, tradeoffs involving the use of esters have been identified and will be discussed here.

[1] M. C. Smart, B. V. Ratnakumar, K. B. Chin, L. D. Whitcanack, J. Electrochem. Soc., 157, A1361 (2010).

[2] H.-C. Shiao, D. Chua, Hsiu-ping. Lin, S. Slane, M. Salomon, J. Power Sources, 87, 167 (2000).

Figure 1. Open circuit potential versus time of Li-ion pouch cells at 60±0.1^oC with different ester contents: (a-d) cells containing MA, MP, EA and MB were precharged to 4.2V; (e-h) cells containing MA, MP, EA and MB were precharged to 2.5V.