Jiewei Yin

Study on the preparation and properties of copolyimides based on hexafluoroisopropylidene bis(3,4-phthalic anhydride) and 1,12-di(4-aminophenoxy)dodecane

Abstract Copolyimides were prepared from hexafluoroisopropylidene bis(3,4-phthalic anhydride) (6FDA), 1,12-di(4-aminophenoxy)dodecane and 4,4′-diaminodiphenylether through a solution co-polycondensation reaction followed by a chemical imidization reaction. It was observed that copolyimides prepared with any diamine molar ratio studied (1,12-di(4-aminophenoxy)dodecane/4,4′-diaminodiphenylether=7/3∽1/9) were soluble in polar organic solvents such as N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMA), dimethylformamide (DMF) and m-cresol, tetrahydrofuran (THF) and chloroform, while the corresponding homopolyimides were insoluble. Wide angle X-ray diffraction (WAXD) results show a clear relationship between the crystallinity tendency and the solubility of polyimides. High crystallinity tendency leads to low organo-solubility. The glass transition temperature of a copolyimide was in good agreement with Fox’s equation for random copolymers. The long flexible chains in the backbone of copolyimides are the least thermally stable part.

Facile preparation of unique three-dimensional (3D) α-MnO2/MWCNTs macroporous hybrid as the high-performance cathode of rechargeable Li-O2 batteries

Undoubtedly, there remains an urgent prerequisite to achieve significant advances in both the specific capacity and cyclability of Li-O2 batteries for their practical application. In this work, a series of unique three-dimensional (3D) α-MnO2/MWCNTs hybrids are successfully prepared using a facile lyophilization method and investigated as the cathode of Li-O2 batteries. Thereinto, cross-linked α-MnO2/MWCNTs nanocomposites are first synthesized via a modified chemical route. Results demonstrate that MnO2 nanorods in the nanocomposites have a length of 100–400 nm and a diameter ranging from 5 to 10 nm, and more attractively, the as-lyophilized 3D MnO2/MWCNTs hybrids is uniquely constructed with large amounts of interconnected macroporous channels. The Li-O2 battery with the 3D macroporous hybrid cathode that has a mass percentage of 50% of α-MnO2 delivers a high discharge specific capacity of 8,643 mAh·g−1 at 100 mA·g−1, and maintains over 90 cycles before the discharge voltage drops to 2.0 V under a controlled specific capacity of 1,000 mAh·g−1. It is observed that when being recharged, the product of toroidal Li2O2 particles disappears and electrode surfaces are well recovered, thus confirming a good reversibility. The excellent performance of Li-O2 battery with the 3D α-MnO2/MWCNTs macroporous hybrid cathode is ascribed to a synergistic combination between the unique macroporous architecture and highly efficient bi-functional α-MnO2/MWCNTs electrocatalyst.