Sodium-ion and Potassium-ion Batteries
Our group focuses on sodium ion and potassium ion batteries (SIBs and PIBs) because they use the earth-abundant elements (sodium and potassium) and show promising properties as future alternatives to lithium ion batteries. In particular, our group is one of the pioneer groups in potassium ion battery research. Starting in 2016, we began to investigate potassium Prussian blue as a low-cost cathode material for potassium ion batteries (Adv. Funct. Mater. 2017, 27, 1604307; 244 citations) and achieved one of the best PIB performances in 2018 (Nature Communications 2018, 9 (1), 1720; it has received 392 citations so far). Two main strategies, geometry-dependent electrode engineering and material-dependent surface engineering, are proposed to improve the energy storage performance of sodium and potassium ion batteries.
Geometry-dependent electrode engineering: Easy ionic access from the electrolyte is a prerequisite for fast charge transfer at the electrode-electrolyte interface. Our group proposed an electrode geometry of highly ordered nanoarrays to fulfill this requirement. The units of template-prepared nanoarrays are highly oriented and strictly vertical to the substrate. It ensures an efficient space between the units through which ions in the electrolyte diffuse in a "high manner" and reach the entire surface of the units, resulting in a "dead" interface between the electrode and electrolyte. The robust array / substrate interconnect provides excellent electrode integrity over a long cycle. We have successfully demonstrated the feasibility of performance enhancement using the proposed nanoarrays through different materials in sodium ion batteries (Energy Environ. Sci. 2015, 8, 2954; Chem. Mater. 2015, 27, 4274; Adv. Energy Mater. 2016, 6, 1600448; Nano Energy 2017, 31, 514).
Material-dependent electrode engineering: Manipulating surface defects is key to material-dependent surface engineering. It is important to manipulate defects on the material surface to regulate the charge transfer behavior at the electrode-electrolyte interface. We used heteroatom doping (Nature Communications 2018, 9 (1), 1720) and oxygen defects (Angew. Chem. Int. ed. 2015, 54, 8768; Nano Energy 2017, 38, 304) to create defects on the surfaces . Nitrogen doping in carbon fibers simultaneously creates carbon defects and reactive nitrogen sites, both of which greatly increase the ion absorption energy and lead to surface-dominated potassium storage (Nature Communications 2018, 9 (1), 1720).
In addition, we have also demonstrated π-configuration expansion of organic molecules as an efficient strategy to design organic electrode materials (J. Am. Chem. Soc. 2015, 137, 3124; Adv. Funct. Mater. 2016, 26, 1777). The extended π-conjugated system facilitates electron transport and strengthens intermolecular interactions. The layer-by-layer molecular stacking forms a fast pathway for sodium ion diffusion between adjacent layers. Our work was highlighted by phys.org in the US with the title "Na-ion batteriesget closer toreplacing Li-ion batteries"(enger-li-ion.html), which is a good example of material design for organic sodium ion batteries. We have also summarized the research fields of using organic materials for rechargeable sodium ion battery applications (Mater. Today 2018, 21 (1), 60).
Our group also focuses on improving the energy storage performance of supercapacitors based on geometry-dependent electrode engineering (Adv. Sci. 2017, 4, 1700188; Small Methods 2019, 3, 1800341). Taking advantage of the structural advantages of AAO template-aligned nanostructure arrays, a complete three-dimensional nanostructured asymmetric supercapacitor with high operating voltage window based on PPy and MnO2 nanotube arrays has been realized (Nano Energy 2014, 10, 63); Self-supporting metallic nanopore arrays with highly oriented nanoporous structure were developed as current collectors to overcome the drawbacks (agglomeration and collapse) while retaining all the advantages (large specific surface area and good ion transport) of 1D nanostructure arrays to achieve high performance -power supercapacitor (Adv. Mater. 2014, 26, 7654).
Micro supercapacitors (MSCs) are of great importance as miniaturized power sources for microelectronics, but still face challenges with limited energy. Our research focuses on the design of 3D nanoelectrodes to improve the energetic performance of MSCs while still maintaining a small footprint (InfoMat 2019, 1, 74). Recently, we demonstrated a nanoelectrode design concept to achieve MSCs with high energy storage performance (Nature Communications 2020, 11 (1), 299). By mimicking the structure of natural honeycombs, we designed and fabricated honeycomb alumina nanoscaffolds starting from AAO templates and used it as a platform for the design of nanoelectrodes and the construction of MSCs. Remarkably, the areal energy density of MSCs is even comparable to that of some state-of-the-art three-dimensional microbatteries, but with much higher areal power density.