Neoteric Polyimide Nanofiber Encapsulated by the TiO₂ Armor As the Tough, Highly Wettable, and Flame-Retardant Separator for Advanced Lithium-Ion Batteries

Abstract

Nowadays, separators with superior properties have drawn widespread attention for the development of advanced and safe large-scale lithium-ion batteries (LIBs). Yet it is still a great challenge for improving overall the thermostability, flame endurance, wetting property, and ion-transport resistance of the polymer-based separators. Herein, a novel and green strategy is reported to address the aforementioned issue by means of the advanced nanostructured surface configuration design in which polyimide (PI) nanofibers are encapsulated by titania (TiO2) nanolayer via dipping in the titanium oxysulfate (TiOSO4) solution, which serves as the source of TiO2. Unlike conventional ceramics-coating methods, this distinctive TiO2@PI core–shell nanostructure is fabricated by the in situ hydrolysis deposition process, and the TiO2 nanoshell thickness can be controlled via simply changing the soaking time in TiOSO4 solution. After being encapsulated by the uniform TiO2 nanolayer, the PI-TiO2 core–shell separator manifests superior flame resistance, outstanding wettability for electrolyte, great thermal dimensional stability at 300 °C, higher glass transition temperature at 400 °C, and better ionic conductivity. Moreover, the cell installed in the PI-TiO2 core–shell separator displays brilliant cycling durability at 120 °C and high-rate property with as high as 82% capacity retention under 5 C (135 mAh g–1), which is superior to the cell using Celgard PP (60%, 90 mAh g–1) and PI nonwoven (75%, 123 mAh g–1). All the admirable features make PI-TiO2 nanofibrous membrane advanced and secure separator for large-scale power LIBs.

Simultaneously Blocking Chemical Crosstalk and Internal Short Circuit via Gel‐Stretching Derived Nanoporous Non‐Shrinkage Separator for Safe Lithium‐Ion Batteries

“…The reason why GS‐PI separator led to a better cycling performance at elevated temperatures mainly lies in its excellent compatibility and thermal stability. [ 22 ]…”

“…[17] However, the coating layer cannot fundamentally improve heat-resistance of the base separators. In addition to polyolefins, many heat resistant materials including cellulose, [18] poly(m-phenylene isophthalamide), [19] and polyimide (PI), [20][21][22] have been also fabricated as battery separators through electrospinning or phase inversion technique. Nevertheless, these separators either have inhomogeneous macroporous structures or insufficient mechanical strength, making them less effective in securing battery.…”

Polyimide separators for rechargeable batteries

“…With a thin Al 2 O 3 layer, the thermal stability and electrochemical performance of PE separators can be significantly improved [85]. The hybridization of inorganic components with PI has also been well investigated [86][87][88]. Electrospun PI/ceramic composite nanofibers can be obtained either by electrospinning a mixture of PAA and other components (e.g., inorganic particles and their precursors) or by adopting post-treatment and/or surface modification of PI nanofibers after electrospinning.…”

“…As shown in Fig. 10(c), TiO 2 nanolayer-coated PI nanofibers were fabricated by in-situ hydrolysis deposition [87]. The uniform TiO 2 coating not only improved the physical properties of the electrospun mats/membranes, including porosity, liquid electrolyte uptake, mechanical properties, and thermal dimensional stability but also provided superior ionic conductivity, thus resulting in good electrochemical stability and excellent rate capability.…”

Safer Lithium‐Ion Batteries from the Separator Aspect: Development and Future Perspectives

“…To overcome this drawback, covalently bonding inorganic particles on the separator is an efficient approach to prevent the particles shedding from the separator. The nanoparticles can be chemically grafted on the polymer membrane by electron beam radiation, [ 66 ] in situ hydrolysis deposition, [ 67‐69 ] layer‐by‐layer self‐assembly process, [ 70,71 ] chemical cross‐linking [ 72‐74 ] ( Figure ), and so on. The high‐energy electron beam radiation can produce free radicals to uniformly initiate grafting reactions.…”