As a new generation of energy devices, lithium ion batteries (LIBs) are widely regarded as the main energy storage devices in the portable electronics market due to their excellent performance (such as high energy density, high cycle stability, and no memory effect). The common electrodes of LIBs are mainly based on graphite and inorganic intercalation compounds, but these inorganic materials also raise concerns about resource availability, sustainability, and environmental friendliness. Therefore, researchers are eager to find new economical and environmentally friendly electrode materials, which should have sufficiently high energy density and unique electrolyte control characteristics, and need to be composed of elements rich in the earth.
Due to the obvious advantages of diverse structures, high natural richness, strong sustainability and environmental friendliness, organic materials stand out with their sufficient design capabilities, becoming the most powerful candidate, and providing advantages that other LIB materials cannot match. The researchers hope that by optimizing the functional groups, organic compounds with reversible ion insertion reaction and sufficient conductivity can be obtained. However, the critical dissolution of small organic molecules in ordinary electrolytes makes their cycle stability generally poor. One of the most common and effective strategies is to polymerize small active molecules into polymers so that they can remain insoluble in the electrolyte. In various polymer systems, aromatic polyimide, as a class of organic carbonyl compounds, seems to be a promising electrode material with its satisfactory capacity, excellent mechanical strength and good thermal stability. The electrochemical active site of aromatic polyimide mainly comes from the carbonyl group of the dianhydride, and each repeating unit includes two single-electron reduction steps. In addition, some studies have shown that the voltage curve of polyimide depends on dianhydride and diamine, and the capacity can be adjusted by choosing different monomers.
Generally speaking, increasing the ratio of the number of transferred electrons per molecular unit of the polymer to the molecular weight is beneficial to obtain a high theoretical specific capacity. Therefore, it is possible to improve the theoretical specific capacity of polyimide by embedding more carbonyl groups in the polymer chain of the low molecular weight link. However, the steric effect caused by the close proximity of the benzoquinone and imide structures inhibits the redox behavior of the quinone unit, resulting in a low specific capacity. Therefore, effective structural design should balance the theoretical specific capacity and cycle stability to obtain organic electrode materials with high capacity, stability, and long cycle life. In addition, the interaction between the electrode and the electrolyte is also a key factor in determining the electrochemical performance of the electrode material.
Here, Professor Zhang Qinghua and Associate Researcher Zhao Xin of Donghua University synthesized a new diamine monomer AQPDA containing a benzoquinone structure through Michael addition reaction, and polymerized it with two different dianhydrides PMDA and NTCDA Two polyimide cathode materials PMAQ and NTAQ. Related work was published in ACS APPLIED MATERIALS & INTERFACES with the title of "Benzoquinone-Based Polyimide Derivatives as High-Capacity and Stable Organic Cathodes for Lithium-Ion Batteries". Studies have shown that because the structural design introduces additional active sites with large π-conjugated main chains, the average reversible capacity of PMAQ and NTAQ can reach 170 mAh / g and 145 mAh / g at a rate of 0.1 C, respectively. In contrast, the NTAQ electrode showed better cycle stability than PMAQ, maintaining a higher rate performance at 108 mAh / g at a current rate of 1 C, which may be due to the strong conjugation of naphthalene. In addition, NTAQ can provide excellent long-term cycle stability. After 1000 cycles at 0.5C, the capacity retention rate is as high as 80.3%. The researchers studied the possible lithiation mechanism of NTAQ through theoretical calculations, and also explored the interaction between electrodes and electrolytes in ether-based and carbonate-based electrolytes.