Understanding Electrolyte Degradation Mechanisms over the Lifespan of Li-Ion Cells Using Differential Thermal Analysis (DTA) and High Precision Coulometry (HPC)

Abstract

Introduction Differential thermal analysis (DTA) is a non-destructive method designed to assess liquid electrolyte degradation mechanisms in full lithium-ion cells over their lifespan [1]. The signal obtained from a DTA measurement can be used to determine the amount of liquid electrolyte, amount of salt in the liquid phase, and the overall composition of the solvent components of the liquid electrolyte. Combined with short term high precision cycling or long-term aging studies, this technique can highlight how the salt consumption rate and chemical stability of the electrolyte correlates to the lifespan and performance of a cell. Experimental Method DTA measures the latent heat of fusion of the liquid electrolyte in a cell. This is achieved by tracking the temperature difference between the cell and an inert reference while cooling the cell down below the electrolyte freezing temperature and subsequently heating it back to room temperature. As a cell ages, the change in the amount of liquid electrolyte can be approximated by calculating the area under the curve of a DTA signal. Furthermore, the change in salt concentration of the liquid electrolyte can be determined by the peak shifts of a signal. The ability to measure the compositional changes of the liquid electrolyte over the lifespan of a cell in a non-destructive manner makes DTA a very useful tool for assessing the chemical stability of electrolyte in a full lithium-ion cell. Results A binary solvent system (EMC-DEC) was selected to demonstrate that data collected from the DTA system correlates with phase diagram measurements [2]. Figure 1a shows the signals obtained from Li-ion pouch cells filled with either pure EMC, pure DEC, or a mixture of EMC: DEC 66:33 by weight. Figure 1b shows the phase diagram obtained from a set of differential scanning calorimetry (DSC) measurements of EMC:DEC binary solutions [2]. The colored arrows provide a guide to the eye to show correlation between DTA signal melting features and phase transitions in the phase diagram. It is important to note that most DTA features occur slightly below the phase diagram temperatures as the DTA measurements were performed in full lithium-ion cells, with electrodes and pouch cell material, not just the electrolytes themselves. Results for an electrolyte blend of EC: EMC 3:7 with 1 M LiPF6 with 2% VC show that DTA signals are sensitive to very small (< 0.15 g) electrolyte mass differences (Figure 2). As more electrolyte is present in the cell, the peak position remains constant, while the intensity of each peak increases. The inset of Figure 2 shows that this change in the area under the curve correlates linearly with electrolyte mass. Each peak can be attributed to physical transitions, such as a liquidus, solidus or salt-solvent complex. Figure 3 shows how changing the concentration of salt in a solution can impact a typical DTA curve. Cells were made with variable salt concentrations (0, 0.5, 1, and 1.5 M LiPF6) in DEC. The most noticeable trend is in the liquidus feature, where the peaks shift with changing salt concentration. This presentation will show how DTA measurements can be used to help understand electrolyte degradation with aging for Li-ion cells. It will also be shown how a combination of HPC and DTA measurements can be used to better understand degradation mechanisms that will ultimately lead to cell failure. References R. P. Day, J. Xia, R. Petibon, J. Rucska, H. Wang, A. T. B. Wright, J. R. Dahn, Differential Thermal Analysis of Li-Ion Cells as an Effective Probe of Liquid Electrolyte Evolution during Aging. Journal of The Electrochemical Society. 162, A2577-A2581 (2015).S. Ding, K. Xu, S. Zhang, T.R. Jow, Liquid/Solid Phase Diagrams of Binary Carbonates for Lithium Batteries Part 2, Journal of The Electrochemical Society. 148, A299-A304 (2001). Figure 1