Polysiloxane-Coated PI Nonwoven Separators with Higher Thermal and Electrochemical Stability for Lithium Ion Battery Application

Abstract

Polyolefin separators are widely used for most of commercial application owing to their excellent mechanical properties, electrical insulation, and internal porous structures. The raw polyolefin materials have economic viability and possess facile mechanical extension suitable for processing as a film [1]. However, the inherent thermal property of polyolefin inevitably results in the severe thermal deformation of the separators upon the elevation of LiB temperature that has been pointed out as a major cause of fire in LiBs with the possibility of short circuits caused by lithium dendrites [2]. There have been trials to secure cycle characteristics by preventing LiB separators from shrinkage. But, even the coated polyolefin separator couldn’t have thermal stability at high temperatures above 180 oC, since the polyolefin was the base film material.Many studies have been performed to achieve high temperature stability with engineering plastics, including polyimide (PI) [1]. As PI has superior thermal stability, it has been widely applied as nonwoven separators. PI nonwoven fabric separators exhibit stability usually above 200 oC. As PI nonwoven separators have also high ionic conductivity and good electrolyte wettability, it is therefore expected to show excellent rate capability and cyclability [3]. Despite those advantages, however, PI nonwoven separators still have several issues to resolve before application of them to LiBs. One of the problems is the existence of large pores [4]. Pores over a certain size is thought to render particles in cathode active materials pass through or Li dendrites penetrate upon overcharging. Then, it can result in leakage current and internal short circuit eventually. Various studies such as particle-coating or ceramic-coating have been studied in order to figure out this problem [26].Other than pore-size distribution or existence of large pores, it must be the electrochemical stability of PI nonwoven separator and the electrochemical cell performance with the separator at high voltage that needs to be studied and examined for the potential application to LiBs [3]. In accordance with the strong demand on high power LiB development, study on the electrochemical resistance of materials and components is highly required [5]. As PI has polar chemical structure in contrast to non-polar chemical structure of polyolefin, it would be worthy to investigate the electrochemical nature and stability under high current or high voltage. In this study, a PI nonwoven separator is modified by coating of polysiloxane to improve its electrochemical stability and properties as well as its porous structure. As the coated PI nonwoven separator should be stable up to 200 oC due to the robust PI frame and the modification via polysilicon-coating is rather simple, the feasibility of this chemical approach is demonstrated and studied including its electrochemical behaviors. Ionic conductivity, electrolyte wettability, and gas permeability of the modified separator is also examined as a set of tests for LiB application. It was electrochemically stable during LSV even at 5 V vs Li+/Li. The polysiloxane-coating maintains or improves the excellent thermal and electrical stability of the PI nonwoven separators. The full cell test demonstrated that the polysiloxane-coating enabled a cyclability of 98.6% after 100 cycles, while the PI nonwoven could not be charged due to an internal short circuit.[1] H. Lee, M. Yanilmaz, O. Toprakci, K. Fu, X. Zhang, A review of recent developments in membrane separators for rechargeable lithium-ion batteries, Energy Environ. Sci, 7, 2014, 3857.[2] X. Zhang, E. Sahraei, K. Wang, Li-ion Battery Separators, Mechanical Integrity and Failure Mechanisms Leading to Soft and Hard Internal Shorts, Sci. Rep., 6, 2016, 32578.[3] Z. Lu, F. Sui, Y. Miao, G. Liu, C. Li, W. Dong, J. Cui, T. Liu, J. Wu, C. Yang, Polyimide separators for rechargeable batteries, J. Energy Chem., 58, 2021, 170.[4] G. Dong, B. Liu, G. Sun, G. Tian, S. Qi, D. Wu, TiO2 nanoshell@polyimide nanofiber membrane prepared via a surface-alkaline-etching and in-situ complexation-hydrolysis strategy for advanced and safe LIB separator, J. Membr. Sci., 577, 2019, 249.[5] Y. Xiang, J. Li, J. Lei, D. Liu, Z. Xie, D. Qu, K. Li, T. Deng, H. Tang, Advanced Separators for Lithium-Ion and Lithium-Sulfur Batteries: A Review of Recent Progress, ChemSusChem, 9, 2016, 1. Figure 1