Lithium ion batteries have been widely investigated and used due to their high energy density, long cycle life, environmentally friendly character and so on. Recently, their application has been expanded to the field of electric vehicles, and the perspective is deemed to be broad.

However, a series of electric car fire accidents happened which indicates that safety is still the main restricting factor for the development of high power lithium ion batteries and the electric vehicles.Separators are critical components in lithium ion batteries, which could prevent physical contact between the electrodes and enable lithium ionic transport. The properties of separators are tied up with battery safety. Thus, to develop separators with enhanced thermal stability and excellent electrochemical performances has become an important research direction for high powerand high safety lithium ion battery.

Prof. Zhaoping Liu’s group at the Ningbo Institute of Materials Technology Engineering (CNITECH) designed and fabricated a kind of polyimide (PI) matrix enhanced cross-linked gel separator by a simple dip-coating and heat treatment method (Fig. 1). The PI nonwoven substrate could provide high-temperature thermal stability for the gel separator (Fig. 1) to prevent short circuit of lithium ion battery under high temperature, and the crosslinked gel part could yield enhanced affinity with liquid electrolyte. Besides, the cross-linked polymer network could solve the limitation of long-term durability of the composite separator in battery. The gel separator shows better cyclability and rate capability than the traditional PP separator, implying a promising potential application in high power and high safety lithium ion batteries (Fig. 2). The preparation process is compatible with the traditional manufacture process of the nonwoven membrane and is easily converted to continuously produce on industrial scale.  

 Fig. 1 The preparation process and the thermal stability of the gel separator

Fig. 2 The electrochemical performances of the gel separator and the commercial PP separator

This work was published recently in J. Mater. Chem. A, DOI:10.1039/C4TA00808A.

Key words: Lithium ion battery; nonwoven; composite separator; crosslink; satety