Evaluation of Electrolyte Resolution Behavior of 18650 Type Lithium Ion Battery By NMR Measurements

Li-ion batteries are currently used in portable electronic and power tool and electric vehicles and grid energy applications due to their high energy density. The development of higher power, higher capacity and longer life batteries becomes more pressing. Current cycle life may be insufficient. One method to developing longer life cells is to use electrolyte additives such as vinylene carbonate to electrolyte solution which has been shown to significantly increase lifetime [1].In this report, the effects of vinylene carbonate an electrolyte additive were studied to improve battery life. VC was added to electrolyte solution to examine the impact in the solution structure change before / after cycling tests by 18650-type battery cells. All solutions were extracted from cycled 18650- type cells to detect solution composition by nuclear magnetic resonance (NMR) techniques.Compared with the case of no additives, VC that was included at 1 wt% concentration in the electrolyte at the initial state was reduced to half after only one cycle. Then it was completely consumed after 100 cycles due to be reacted at the interphase of positive/negative electrodes. Then the electrolyte composition change in until 100 cycles was slightly different with the addition of VC detected by NMR measurement. However, after 1000 cycles both electrolytes showed substantially the same composition. Ethyl methyl carbonate (EMC) which was one component of the electrolyte solution was decomposed and degradation products were produced in electrolyte solution. And the products were detected dimethyl carbonate and diethyl carbonate as the ester exchange reaction of EMC [2]. Furthermore NMR results showed poly-carbonate species from the polymeric reaction ethylene carbonate with non cyclic carbonate. Those were revealed by 1D as 1 H or 13 C NMR and 2D HMQC techniques. We will discuss relations of the resolution mechanism of the electrolyte with the degradation of the battery. [1] J. Self, C.P. Aiken, R. Petibon, J.R. Dahn, J. Electrochem. Soc. 162(2015) A796-A802. [2] E. S. Takeuchi, H. Gan, M. Palazzo, R. A. Leising, S. M. Davis, J. Electrochem. Soc. 144(1997) 1944-1948.