Copolymer-assisted Polypropylene Separator for Fast and Uniform Lithium Ion Transport in Lithium-ion Batteries

In lithium-ion batteries (LIBs), separators play a vital role in lithium-ion (Li+) transport, and thus affect rate performance, battery life, and safety. Here, a new kind of multifunctional copolymer poly(acrylonitrile- co -lithium acrylate- co -butyl acrylate) (PAAB-Li) is synthesized through soap-free emulsion polymerization, and is used to form homogeneous-covered separator based on PP matrix by a simple dip-annealing process. Compared to the bare PP separator, the modified separators with PAAB-Li enable higher ionic conductivity, higher lithium ion transference number (increased from 0.360 to 0.525), and lower interface impedance (reduced from 155 Ω to 34 Ω). It has been indicated that PAAB-Li functional layer significantly promotes the fast transport of Li + and improves the compatibility of the separator/electrolyte-electrode interface. The LiCoO 2 /graphite cells with the PAAB-Li-assisted separator demonstrate excellent cycle stability and rate performance. In addition, the Li symmetric cells with the modified separator stably cycle over 800 h, indicating the functional layer effectively suppresses the lithium dendrite growth. This facile strategy can be easily applied to LIBs requiring high safety and even be scalable to Li metal batteries. Moreover, the possible mechanism of the PAAB-Li functional layer promoting fast and uniform Li+ transport is discussed in this paper.