Cobalt Phosphide Anchored on Graphene Naonosheet As High-Capacity Anodes for Li-Ion Battery
Authors: Yang Yang , Ma Zi-Feng , Alamgir Faisal M. 
Published: February 2020

Traditional anodes for Li-ion batteries (LIBs) including graphite, TiO2 and Li4Ti5O12, generally have specific capacities less than 200 mAh g-1, which no longer meet rapidly increasing commercial demands. In recent years, various novel anode materials, such as metals, alloys, silicon, transition metal oxides (TMOs) and sulfides, have been widely studied because they can achieve much higher capacity than traditional anodes.[1]

Transition metal phosphides (TMPs) have been investigated extensively owing to their high theoretical capacities and relatively low intercalation potentials vs Li/Li+.[2] In particular, cobalt phosphide (CoP), with a theoretical capacity of ∼894 mAh g-1 , has been proven to be one of the most promising candidates for LIB anodes.[3] To date, various CoP materials have been investigated,[4, 5] but their specific capacity and stability are still unsatisfactory due to low electric conductivity and fast structural degradation during high-rate or long-term charge/discharge processes.

Herein, we report on a robust high-capacity anode based on CoP/reduced graphene oxide (rGO) nanocomposite.[6] We conduct a facile and versatile strategy involving an oil bath, freeze drying and phosphidation processes, allowing nanostructured CoP particles to be uniformly embedded in rGO nanosheet network. The resulting CoP/rGO nanocomposite can exhibit enough surface area and porosity, which can improve the electrolyte diffusion. Meanwhile, the rGO network can enhance the electrical conductivity and structural stability of active CoP anodes. Electrochemical measurements indicate that the CoP/rGO nanocomposite shows a high specific capacity over 1100 mAh g-1 at a current density of 100 mA g-1. A capacity retention of ~840 mAh g-1 is obtained when the current density increases to 2 A g-1, which reveals an excellent rate capability. The nanocomposite also shows a ultralong cycle life of 2000 stable cycles at a high current density of 2 A g-1. These results indicate that our strategy is very effective and versatile to improve TMP-based anodes for the development of state-of-the-art LIBs.

[1] N. Nitta, F. Wu, J.T. Lee, G. Yushin, Li-ion battery materials: present and future, Mater. Today, 18 (2015) 252-264.

[2] L. Ji, Z. Lin, M. Alcoutlabi, X. Zhang, Recent developments in nanostructured anode materials for rechargeable lithium-ion batteries, Energy Environ. Sci., 4 (2011) 2682-2699.

[3] J. Yang, Y. Zhang, C. Sun, H. Liu, L. Li, W. Si, W. Huang, Q. Yan, X. Dong, Graphene and cobalt phosphide nanowire composite as an anode material for high performance lithium-ion batteries, Nano Res., 9 (2016) 612-621.

[4] W. Wang, J. Li, M. Bi, Y. Zhao, M. Chen, Z. Fang, Dual function flower-like CoP/C nanosheets: High stability lithium-ion anode and excellent hydrogen evolution reaction catalyst, Electrochim. Acta, 259 (2018) 822-829.

[5] X. Xu, J. Liu, R. Hu, J. Liu, L. Ouyang, M. Zhu, Self-Supported CoP Nanorod Arrays Grafted on Stainless Steel as an Advanced Integrated Anode for Stable and Long-Life Lithium-Ion Batteries, Chem. Eur. J., 23 (2017) 5198-5204.

[6] Y. Yang, Y. Jiang, W. Fu, X. Liao, Y. He, W. Tang, F. Alamgir, Z.-F. Ma, Cobalt phosphide embedded in graphene nanosheet network as a high-performance anode for Li-ion batteries, Dalton Trans., (2019) DOI: 10.1039/C1039DT01240K