Effect of monomer structure on properties of polyimide as LIB separator and its mechanism study

Polyimide (PI) has remarkable thermal stability and mechanical properties, and is considered as an important candidate material for the manufacture of high-security new separators of lithium ion batteries (LIBs). However, different types of PI may exhibit performance differences in battery applications due to their various monomer structures. We synthesize four PIs with different monomers, including 1,2,4,5-pyromellitic dianhydride-4,4′-oxydianiline (PMDA-ODA), 3,3′,4,4′-biphenyltetracarboxylic dianhydride-4,4′-oxydianiline (BPDA-ODA), 3,3′,4,4′-oxydiphthalic dianhydride-4,4′-oxydianiline (ODPA-ODA) and 3,3′,4,4′-benzophenonetetracarboxylic dianhydride-4,4′-oxydianiline (BTDA-ODA), and electrospin them into nanofiber films. Those PI films are then thoroughly evaluated as the separator of LIB. Results show that the electrochemical window of those four PI films is up to 5.1 V (vs. Li+/Li), their wettability and electrolyte uptake are all related to the surface polarity and porosity. Among the four PI separators, BPDA-ODA shows the best cycle performance (95.8% @0.2C, 25 °C) and rate performance in the NCM811|Li battery system, while the BTDA-ODA is the worst in battery applications due to its surface polarity, low electrolyte uptake and the lithiation reaction occurred on the carbonyl groups between two benzene rings, and therefore is not suitable for usage in manufacturing separators.