Currently, the development of lithium ion batteries (LIBs) is toward large-scale ones because electric vehicles (EVs), hybrid electric vehicles (HEVs) and stationary energy storage systems for smart grid community are considered necessary from the viewpoint of clean energy and sustainability. However, the large-scale electronic applications are more likely used in high-temperature environment, thus increasing the risk of battery failure. To overcome this issue, much efforts are put on the development of electrolyte additives, among which vinylene carbonate (VC) is known as an effective one in enhancing the thermal stability of carbonaceous anode material. [1] Nevertheless, the film-forming mechanism induced by VC on the carbonaceous anode surface, particularly at elevated temperature, is not well understood. To clarify the contribution of VC to the formation of thermally resistant film, we use LC-MS together with direct analysis in real time mass spectrometry (DART-MS) to analyze the electrolyte and electrode surface.

Experimental

The test cell was CR2032 coin cell, in which LiFePO₄ and carbonaceous material, namely graphite or hard carbon, are respectively used as cathode and anode materials. For the preparation of cathode, LiFePO₄ powders were uniformly mixed with conductive agents and water-based acrylic binder. Similarly, the carbonaceous material was mixed with conductive agents and water-based acrylic binder to make the anode. In addition, the electrolyte was 1 M LiPF₆ in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ration of 1:1, and VC was used as an additive. To evaluate the cell’s thermal stability, the charge/discharge cycling tests were performed at 60 °C. For the analysis, the cycled test cells were decomposed, followed by collecting the electrolyte and electrode under an environment with an ultra-low humidity. Next, the collected electrolyte and electrode were respectively moved to the LC-MS and DART-MS apparatuses to analyze the electrolyte and electrode surface.

Results and Discussion

The test cell with the VC additive shows better cycle performance than that without the additive. Since LiFePO₄ is a thermally stable cathode material, the difference in cycle performance can thus be attributed to the interaction between the electrolyte and the carbonaceous anode material. According to the LC-MS analysis, VC is found to suppress the decomposition of lithium salt and solvent, leading to reduced organophosphates as well as carbonate oligomers. It is also found from the DART-MS analysis that VC can assist the formation of thermally resistant organophosphates on the anode surface, preventing further degradation of electrolyte at elevated temperature. This should be the reason for the improved cycle performance for the cell with the VC additive.

Reference

1. M. Herstedt, H. Rensmo, H. Siegbahn, and K. Edströma, Electrochim, Acta, 49, 2351–2359 (2004).