Joint Research Projects

The world's new energy policy is to reduce CO2 emissions and transition to low-carbon energy production, and thus green alternative energy and electric vehicles must be a top priority. This demands for large-scale energy storage systems, and therefore superior rechargeable batteries in terms of energy/power density are now required at a substantially lower cost. The next-generation batteries need to have high specific capacity and operating voltage, fast-charge ability, high safety, wide operating temperatures, long cycle life, and most importantly, environmental friendliness. Increasing needs in high energy density and superior power density will require the design and development of novel functional redox-active materials as electrodes. The limited excess to lithium resources will also require the discovery of new electrochemistry beyond lithium-ion battery (LIB) technology. New post-Li-ion batteries (PLIBs) based on the monovalent and multivalent charge carriers, such as Na⁺, Mg²⁺, and Zn²⁺, have the promise to deliver higher specific capacity and energy density at a lower cost. However, in order to materialize these new energy storage technologies, several challenges must be overcome and complete understanding of redox mechanisms must be achieved.

To meet the demand of large-scale energy storage, new materials that are abundant, inexpensive, and more environmentally friendly, and can deliver higher energy and power density will need to be discovered. Efforts are being made towards eliminating or reducing the amount of toxic and/or expensive transition metals (such as Co and Ni) in the electrode materials. The specific objective of this project is to design and develop advanced functional materials with complex nanostructures which are far superior to the transition-metal-based compounds currently used in today’s Li-ion batteries. These materials can be applied to not only traditional lithium-based batteries but also new rechargeable battery systems which are safer and cheaper such as sodium-, zinc-, and other multivalent-based systems. The findings from this project will address a broad range of research challenges, from new materials discovery to energy storage. Our project will uncover the relationships between materials properties, electrode architectures, and electrochemical mechanisms.

Organic electrode materials possess many advantages such as low toxicity, sustainability, and chemical/structural tunability toward high energy density. However, to compete with inorganic-based compounds, crucial aspects such as redox potential, capacity, cycling stability, and electronic conductivity need to be improved. As an alternative for transition-metal oxides, organic-based compounds can be designed to incorporate reversible redox-active functional groups, comprising naturally abundant elements without expensive and toxic transition metals. In addition to low cost and environmental friendliness, organic moieties can contain a large number of redox active sites capable of undergoing a multielectron-transfer process at a potential which is properly fine-tuned, thus leading to a high energy density. We have designed a small organic electron-acceptor-molecule — hexaazatrianthranylene (HATA) embedded quinone (HATAQ). By introducing conjugated quinone moieties into the electron-deficient hexaazatriphenylene-derivative core, HATAQ with highly extended π-conjugation can yield extra-high capacity for metal-ion storage.

Figure 1. (a) Chemical structure of HATAQ which has been newly-designed and synthesized in this project by using cheap and readily-available starting materials. (b) The 2D sheet of HATAQ molecules connected by C—H···O bonds (green dashed lines). The unique unconventional hydrogen bonds in the solid state facilitate favorable supramolecular layered arrangement (confirmed by single-crystal X-ray diffraction, SXRD analysis) which enhances electrode cycling stability.

Among different types of metal anodes which can be used in PLIBs, Zn has several advantages over Li in terms of natural abundance and low cost. It is also non-toxic and can be used in both organic and aqueous electrolytes. Inspired by the excellent electrochemical performance of HATAQ in LIBs, we have investigated the compound as cathode in rechargeable aqueous Zn-ion batteries (AZIBs) with 1 M ZnSO₄ solution as electrolyte. As shown in Figure 2, the large number of redox active sites and extended - conjugation allow HATAQ to deliver extra-high capacity of 492 mAh g⁻¹ at 50 mA g⁻¹ and excellent rate capability up to 20 A g⁻¹ with 199 mAh g⁻¹ reversible capacity corresponding to 99% retention after 1000 cycles. The performance from HATAQ is found to be among the best ever reported for AZIBs. Several characterization techniques have been used to elucidate the redox mechanism of HATAQ in AZIBs which is found to be a simultaneous Zn-ion and proton co-insertion.

Figure 2. (a, b) The molecule and single crystal structure showing packing of HATAQ (c, d, e, f) Electrochemical properties of HATAQ in AZIBs which include voltage profiles at different rates, capacity retention, and rate capability.