A Porous Air Electrode for Li-O2 Batteries Based on a Green Method
Authors: Xia Guofeng , Shen Shuiyun , Zhu Fengjuan , Wu Aiming , Wu Ruofei , Zhang Junliang 
Published: February 2020

Since Li-oxygen (Li-O2) battery was first roported in 1996 by Abraham and Jiang 1, it had been attracting increasingly attention due to its remarkably high theoretical specific energy of 11400 Wh/kg, which is very close to that of gasoline (13000 Wh/kg) and is regarded as the most promising alternative to fossil fuel based energy2. However, the practical capacity of Li-O2 battery is always far below the theoretical value. The prime reason is that, up discharging, the insoluble discharge product, namely Li2O2, will block the paths for both oxygen and the electrolyte and result in an early termination of the discharge process. So the specific capacity of Li-O2 electrode strongly depends on the porosity of the electrode. And it is very crucial to build porous air electrodes with plenty of voids to accommodate Li2O2 and to facilitate the transportation of oxygen and electrolyte. Up to now, various mesoporous materials or nano-structured materials have been explored to make porous electrodes for Li-O2batteries and have promoted the electrochemical performance significantly.
However, present electrode manufacturing prefer to use polyvinyliden-di-fluoride (PVdF) as the binder and use volatile organic solvents such as N-methyl-pyrrolidone (NMP). Most of these solvents cost a lot and lead to the harmful environment pollution during the electrode manufacturing process. In addition, PVdF is also a relatively costly polymer and is necessary to be substitued by an economic binder.
Herein, we proposed a green method to build a porous air electrode for Li-O2 batteries by a freeze-dry process with carboxymethyl cellulose (CMC) as the binder. The electrode is characterized with abundant voids for discharge product, which also provide plenty of effective passages for both oxygen and electrolyte.
This electrode demonstrate not only a high specific capacity but also an excellent rechargeable ability by combination with high conductive MWCNT. As shown in Figure 1a, at a current density of 100 mA g-1, the porous air cathode demonstrates an initial discharge capacity of 6,702 mAh g-1 with an average discharge potential of about 2.70 V. The increased discharge capacity can be ascribed to the fact that the plenty of room in the porous electrode accommodate more discharge product. Meanwhile, the abundant space facilitates the transportation of oxygen and electrolyte, thus extending the discharge process. The high conductivity of MWCNT also promotes the electrochemical performance of Li-O2batteries. By all the virtues above, the porous air cathode shows an enhanced cycling life as high as 53cycles (Figure 1b).

Acknowledgement

This work was supported by Science Foundation of Ministry of Education of China (grant 413064), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.

Reference

1. K. M. Abraham, J. Electrochem. Soc. 1996, 143, 1-5.
2. Z. Q. Peng, S. A. Freunberger, Y. H. Chen, P. G. Bruce, Science, 2012, 337, 563 – 566.

Figure 1