Recently, the research team led by Professor Cong Huaiping and Yu Shuhong from the School of Chemistry and Chemical Engineering has made big leap in the structure design of macro-assembly material equipped with micro-nano hierarchical structure and the preparation of high-performance lithium-ion battery cathode material. On September 6, the research progress was published online on Angewandte Chemie International Edition, a top Journal in international chemical field, with the title of "Combining Nitrogen-Doped Graphene Sheets and MoS2: A Unique Film-Foam-Film Structure for Enhanced Lithium Storage”, (Angew. Chem. Int. Ed. 2016, DOI: 10.1002 / anie.201606870. Impact Factor 11.709), which was selected as VIP paper by the Journal. Shan Tingtian, a postgraduate student admitted in 2014, is the first author.
As a typical transition metal sulfide, molybdenum disulfide (MoS2) has a promising application prospect as a cathode material for Li-ion batteries because of its unique S-Mo-S two-dimensional layered structure and high active sulfur content. MoS2 stores lithium in the manner of “ intercalation-transformation”, and its theoretical specific capacity (~ 670 mA h g-1) of MoS2 is twice as that of the graphite cathode currently in wide use. However, its low conductivity and unstable electrochemistry are not conducive to its cycle and rate performance. In addition, after the first cycle, it is easy to form biggish crystal phase particles during the rearrangement of MoS2’s stratified structure, which causes Mos2’s insufficient reaction in the subsequent lithiation/de-lithiation process, and furthermore results in rapid attenuation of the material capacity and its practical applications in batteries.
To solve the abovementioned problems, the team developed the self-assembly design and amplification preparation of self-supporting molybdenum disulfide-graphene composite film . The basic structure unit of the film consists of nitrogen-doped graphene (NG) and honeycomb-like nano-MoS2 (NG-MoS2, Fig. a) and presents the top-down macroscopic-microscopic-nanoscopic grading of "film-foam-film" (Fig. b, c). When used in cathode materials for lithium-ion batteries, this new structure guarantees high compaction density of the composite material and rapid transport of lithium ions and electrons within the material. It can also accommodate volume change of sulfide materials in the intercalation/deintercalation of lithium. For its obvious structural advantages, the composite material NG-MoS2 exhibits excellent electrochemical performance in the cathode of Li-ion batteries. Its reversible lithium storage capacity reaches 1200 mAh g-1 at 0.1 Ag-1 current density , the specific capacity still 700 mA h g-1 when the current density reaches 5.0 Ag-1 and 980 mA h g-1 after 400 cycles at 1.0 Ag-1 current density. Compared with traditional lithium cathode materials like graphite, MoS2 has significant advantages in capacity. In view of this, NG-MoS2 will have a good prospect in the following generation of energy storage system represented by lithium ion battery and will be beneficial for developing future-oriented sustainable energy technologies.
In addition, the team explored new methods for easy preparation of high-performance energy storage materials in macroscopic quantity by using the design concept of new hierarchical nano materials. Recently, they proposed a biomimetic template method for the design and synthesis of pyrolytic carbon sphere of which the graphite coated hollow resin (Fig. 2), improved the graphitizing degree of resin pyrolytic carbon by chemical activation resin pyrolytic carbon, and developed a new graphitized hollow carbon sphere coated by graphene, which has excellent electrochemical performance in the application of lithium ion batteries, sodium ion batteries and supercapacitor electrode materials. When it is used as a cathode material for lithium ion batteries, its reversible specific capacity keeps as high as 935 mA h g-1 after 100 cycles at a current density of 0.1 Ag-1, after a lapse of 1000 cycles at a current density of 1 Ag-1 still 595 mA h g-1; when used for supercapacitor electrode materials, the specific capacitance reaches 189 Fg-1 at a current density of 0.5 Ag-1, and the capacity retention rate is 96% after 5000 cycles. The paper on the abovementioned findings is published on ChemNanoMat 2016, 2 (6), 540-546 as the Inside Cover. Wiley Publishers Materials View highlights this work titled "Bioinspired hollow spheres for energy storage". Materials View China also reported this breakthrough.
The team also studied on the design of lithium-sulfur battery electrode materials. At present, it is difficult to meet the actual working requirements of the battery with the low conductivity of sulfur anode in lithium-sulfur battery. At the same time, soluble polysulfide is formed and the shuttle effect is caused in the process of charging-discharging, leading to the rapid attenuation of capacity. These problems seriously hinder the practical application of lithium battery. Therefore, the research team proposed an easy-to-enlarge synthesis method to prepare low-cost but high-performance sulfur anode. In this method, Hierarchical porous carbon (HPC) was obtained by direct pyrolysis of polysaccharide-sodium alginate, a common biomass, and sulfur was further loaded into it to prepare composite sulfur anode material (Fig. 3). The macroporous structure, the base of HPC, can effectively accommodate the volume change of the sulfur during the cycle. The mesoporous and microporous structures are favorable to keep the soluble polysulfide on the positive side, thus effectively improving the cycling stability of the sulfur positive electrode. Due to poor conductivity of sulfur anode, researchers further embedded multi-walled carbon nanotube (CNT) into the HPC to form a conductive network; and further introduced sulfur into the preparation of sulfur-carbon composite anode material S / (CNT @ HPC) Channel. So it is helpful to significantly improve the conductivity of sulfur anode and to enhance the structural rigidity of composite materials. To overcome the electrochemical barrier of the discharge product Li2S during de-lithiation, the researchers added one constant-voltage charging process at the end of the charging process and found that the electrochemical reaction activity was markedly improved in the de-lithiation of the sulfur anode. Hence it is beneficial for improving reversible capacity and the capacity retention rate of the positive electrode material and reducing its polarization. This work provides a new and easily applicable approach for the structural design and performance optimization of lithium-sulfur battery anode materials. The paper is published on ChemNanoMat 2016, 2 (7), 712-718. Wiley Publishers Materials View and Energy Harvesting Journal highlights this work ,titled respectively “A biomass-derived porous carbon matrix with carbon nanotubes for enhanced Li-S battery performance” and “Carbon nanotubes for enhanced Li- S battery performance”.
The above research was funded by the National Natural Science Foundation of China, the Ministry of education "New Century Talent Supporting Project" and the Outstanding Youth Training Program of Hefei University of Technology, etc.