C and N Co-Doped CuO Hollow Microspheres Derived from Cu/Cu2O Assembles As High Performance Anode Materials for Lithium Ion Batteries
Authors: Yu Sicen , Lu Zhouguang , Lyu Fucong , Wu Shaofei , Nan Bo , Wang Zhenyu , Cao Lujie , Sun Zhifang , Li Mingchan , Yang Mingyang , Wang Wenxi , Shang Chaoqun 
Published: February 2020

A green, energy-efficient and simple route was design to synthesis the hollow structure of precursor electrode materials for high-performance lithium-ion batteries through in situ reduction self-assembly metallic composites in supramolecular hydrogel. By annealing the precursor at the appropriate temperature, we successfully obtained hollow CuO spheres with many self-assembly nanoparticles and highly homogeneous carbon distribution and N dopant as well. The hollow CuO spheres exhibit remarkably enhanced cycling performance and rate performance when being used as anode materials in lithium-ion batteries. This process provides a general route to the synthesis of hollow nanostructures of a large number of compounds for fields such as smart devices and other energy store systems.
Figure 1a presents SEM of CuO-500. As can be seen, the microsphere is apparently having a hollow structure. To investigate the compositional distributions of Cu, O, N and C in CuO-500, we carried out elemental mapping using EDS coupled with HAADF/STEM. The images revealed that these components in the hollow sphere, along with the N and C atoms, were distributed uniformly with the Cu and O atoms. The intensity of these elements components was slightly higher at the sphere rims, similarly strong confirmed the formation of a hollow structure.
As anodes in Li-ion batteries, Figure 1b, the specific capacity of as-prepared CuO was found to be 750 mAhg−1 after 10 cycles and can retain almost 100% of its original capacity after 100 cycles. The outstanding electrochemical performance of the CuO-500 composite could be attributed to the unique advantageous hollow structure. These can be explained by the following aspects: Firstly, the chitosan was successfully grafted to the Cu/Cu2O particles during the in situ reduction and self-assembly process, this special method formed a highly homogeneous carbon distribution and N dopant after the carbonization of chitosan. These carbon particles not only helped provide good electrical connection to the poor conductive CuO particles and provided channels for fast electron transport, but also hindered the aggregation of CuO nanoparticles. Secondly, the unique hollow structure supplied a large void space which can largely endure the volume expansion of CuO particles without changing the secondary particle size during the lithium insertion. And the carbon around the CuO particles also reinforced strength of the hollow structure and played an important role in preventing the cracking of the electrode during the volume change process. This particular structure can stay the original big-ball morphology even after 138 cycles. Thirdly, the assemble CuO particles lowered the electrode–electrolyte contact area, limited most SEI formation to the outer secondary particle surface instead of on individual nanoparticles, thus increased the stability of the SEI films1

Acknowledgments
This work was financially supported by the Shenzhen Peacock Plan (KQCX20140522150815065), the Natural Science Foundation of Shenzhen (JCYJ20150331101823677) and the Science and Technology Innovation Foundation for the Undergraduates of SUSTC (2015x19 and 2015x12).

Reference

Liu, N. et al. A pomegranate-inspired nanoscale design for large-volume-change lithium battery anodes. Nature nanotechnology
9, 187-192, doi:10.1038/nnano.2014.6 (2014).

Figure 1