SiO₂ Coated Electrospun Polyimide Nanofiberous Membrane As Separator For Lithium-ion Batteries

Recently, rechargeable lithium-ion batteries (LIBs) have been drawing considerable attention to large groups of researchers due to their powerful energy capacities and long cycle life. In LIBs system, the separator is considered as a key component, because of its primary function of physically preventing electric contact of the electrodes. The most commonly used commercial separators material for LIBs is polyolefin. Though these separators have good performance in chemical stability, thickness and mechanical, their low porosity and unsatisfied wettability with polar liquid electrolyte will hinder the performance of the rechargeable Li-ion batteries. Furthermore, polyolefin-based separator suffers from poor thermal shrinkage, which may lead to short circuit even explosion. In order to find the ideal separator for lithium-ion battery, researchers proposed to fabricate nonwovens and composite membranes.Polyimide (PI) material has been applied in many advanced field due to their thermal stability, good chemical resistance, which is known as a famous engineering polymer. Electrospinning polyimide nanofiber membranes possess high porosities, large specific surface areas and unique pore structure, and as a result, they have the capability of soak large amounts of electrolyte and conducting more ions. However, polyimide nanofiber-based nonwovens cannot meet the mechanic requirement of LIBs assembly process. To overcome this disadvantage, a facile strategy is studied to synthesize a SiO2 coated polyimide nanofiber mat to improve the mechanical property, thermal properties and electrolyte wettability.Polyimide nanofiber membranes coated with SiO2 are deeply studied and fabricated via electrospinning for separators of LIBs in this research. It is found that the thermal and mechanical properties of SiO2 coated PI nanofiber-based membranes with unique morphology have been significantly improved. The tensile modulus of silica coated PI nonwoven can reach 41.77MPa, although there is a certain gap with commercial Celgard, but it can meet the basic needs of battery encapsulation. The ionic conductivity of the silica coated PI separator (0.94 mS cm-1) is nearly twice of Celgard (0.54 mS cm-1). The cells content SiO2 coated PI separator shows ideal electrochemical performance.When the current rate reaches 5C, the discharge capacities of SiO2 coated PI separator holds at 84.44% of the discharge capacity at 0.1C. While Celgard and PI separators present values of 62.23% and 77.47%, respectively. After 100 cycles under the constant current charge/discharge (1C/1C) conditions, the discharge capacities retention of the cell with SiO2 coated PI nonwoven is over are more than 98.00%, higher than that of the Celgard separator (96.44%). Results demonstrated that SiO2 coated PI nanofiber-based membranes are promising separator candidate for high performance Lithium-ion batteries.