Multi-Functional Polyimide Separators for Electrochemical Capacitor and Lithium-Ion Cell Applications

Abstract

Achieving both higher energy and power densities are significant targets for next-generation energy storage devices. This will involve the development of new materials and chemistries, but also optimisation and improvements of all the cell components, such as separators, electrolytes and current collectors. Optimisation of cell separators to move beyond the present industry standard of polyolefin materials has received wider attention in the past few years. Polyolefin materials have poor wettability with electrolytes, resulting in low ionic conductivity and affecting the cell performance at high current densities. Moreover, polyolefin class of separators present a higher safety risk due to their poor thermal-dimensional stability that cannot effectively terminate the thermal runaway. The present solution to improve safety is achieved via a sandwich structure (polypropylene (PP)/polyethene (PE)/polypropylene (PP)). However, this leads to a decrease in the ionic conductivity, sacrificing cell performance.Herein, the polyimide-based separators with superior characteristics, including high thermo-dimensional stability, excellent electrolyte affinity, and ionic conductivity, are fabricated by electrospinning and applied within electrochemical capacitors. The specific capacitance of the cells with this composite separator is approximately 13 F/g at 50 A/g, which is double the achievable value using commercial cellulose separators (TF4030). Furthermore, a thermal shutdown functionality can be added to the composite film after spin-coating a layer of low-density polyethene on the surface to apply within lithium-ion cells. Due to the superior physical properties of polyimide, the composite film can endure high temperature without any shrinkage. Additionally, the specific capacity of the cells using a composite separator is approximately 20 mAh/g higher than that of a commercial separator (Celgard2325, PP/PE/PP). The highly porous polyimide (PI) with controllable thickness and high electrolyte wettability is also applied to lithium-free cells to decrease the overpotentials for nucleation and growth of lithium (Reduced from 175 mV to 130 mV in Cu//Li cells). As a result, the coulombic efficiency of the cells with PI separators is 20% higher than via using PE in 1.2 M LiPF6 (EC/DMC=1/1) electrolyte without any additives. The SEM images also present granular lithium deposition (15~30 μm in diameter) rather than dendritic lithium when the electrospun PI separator is applied to the cells. Due to the relationship between the morphology of lithium deposition and the overpotential required for lithium nucleation/growth from the literature. Experiments confirm that the low overpotential can facilitate large lithium deposition rather than dendritic lithium and this is achieved via improvements in the separator.