PREPARATION AND CHARACTERISTICS OF NOVEL HYPERBRANCHED PEU-BASED GEL POLYMER ELECTROLYTES

Abstract

Gel polymer electrolytes (GPEs) based on hyperbranched poly (ether urethane) (PHEU) were prepared from the reaction of the hyperbranched polyether (PHEMO) with different diisocyanates in liquid electrolyte. The ionic conductivities and transference number of the GPEs were investigated by alternating current (AC) impedance and multi-potential steps. It is found that the ionic conductivities of GPEs were influenced by diisocyanate structure, lithium salt concentration and addition amount of liquid electrolyte. Sample PUM-0.5M-80% originated from 4, 4-diphenylmethane diisocyanate (MDI) as the hard segment shows the highest ionic conductivity of 2.41×10-3 Scm-1 at ambient temperature. The lithium ion transference number is 0.7. In addition, small molecules of liquid electrolyte were perfectly entrapped by polymer matrix, and liquid leakage in lithium ion battery was avoided. 5 . The hard segment provides good mechanical strength, while the soft segment improves the ionic conductivity. The rubbery soft segments can dissolve alkali metal salts, and help achieving high ion mobility, which originates from the interaction of the ether oxygen with the alkali metal ions. In the previous report 6 , the hyperbranched polyether (PHEMO) without stiff groups is selected as soft segment, as well as its amorphous state inherently derived from the hyperbranched molecular structure. Therefore, PHEMO is expected to improve the ionic conductivity of polymer electrolyte 7 . Due to their good mechanical properties, diisocyanates are chosen to act as hard segment domains contributing to the dimensional stability of the polymer electrolytes 8 . In this paper, GPEs based on PHEU were prepared with hyperbranched polyether (PHEMO) and diisocyanates in liquid electrolyte. Diisocyanates with different structures, such as aliphatic isocyanates of hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI), or aromatic isocyanates of 4, 4-diphenylmethane diisocyanate (MDI) and 2, 4-tolune diisocyanate (TDI) were served as hard segments, respectively. The effects of diisocyanate structure, lithium salt concentration and addition amount of electrolyte on the ionic conductivities of the as-prepared GPEs were discussed and compared.