Electrolyte plays a critical role in lithium ion batteries (LIBs) as it determines the ion conductivity and dictates the formation of solid electrolyte interphase (SEI)¹. Typically it consists of cyclic carbonate and linear carbonate, and it is believed via combination of cyclic carbonate possessing high polarity and dielectric constant but high viscosity while linear carbonate with low polarity, low dielectric constant and lower viscosity, the mixed solvent will provide satisfactory performance.

Nowadays urgent needs for high performance LIBs from electrical vehicles (EVs) industry propel the development of LIBs capable of fast charging, and addition of esters possessing lower viscosity^2,3 into the commonly used liquid electrolyte as cosolvents is considered as a possible solution to this problem. Herein performance of esters including Ethyl Acetate (EA), Methyl Acetate (MeOAC) and Ethyl Formate (EF) are investigated via cycling tests.

Firstly, physical properties (viscosity, ion conductivity and transfer numbers) of the blended electrolyte systems, consisting of 1.2 mol/L LiPF₆ solvated in EC/EMC/Esters (30:50:20 wt%) are studied. Experimental results clearly show the improvement of ionic conductivity with adding the four esters compared to baseline electrolyte (1.2 mol/L LiPF₆ in EC/EMC. For viscosity, the electrolytes with ester co-solvents all have lower viscosity than the baseline electrolyte over all the salt concentrations. Cycling tests are also performed for 520mAh Li(Ni_0.6Mn_0.2Co_0.2)O₂/graphite pouch cells between 2.8V-4.2V at 30°C. The cells were charged at 4C (charging ends at 15 minute) and discharged at 1C, and the result is shown in Figure 1. The cell performance and analysis will be presented during the meeting.

References:

[1] Xu, Kang. "Electrolytes and interphases in Li-ion batteries and beyond." Chemical reviews 114.23 (2014): 11503-11618.

[2] Ma, Xiaowei, et al. "A study of highly conductive ester co-solvents in Li [Ni_0.5Mn_0.3Co_0.2] O₂/Graphite pouch cells." Electrochimica Acta 270 (2018): 215-223.

[3] Smart, M. C., B. V. Ratnakumar, and S. Surampudi. "Use of organic esters as cosolvents in electrolytes for lithium-ion batteries with improved low temperature performance." Journal of The Electrochemical Society 149.4 (2002): A361-A370.

Figure 1. Cycling performance for the 4 blended electrolyte using EF, MeOAC, EtOAC and DMC