Fabrication of nitrogen doped carbon encapsulated ZnO particle and its application in a lithium ion conversion supercapa

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Dong Zheng and Joshua Harris Department of Mechanical Engineering, College of Engineering and Applied Science, University of Wisconsin Milwaukee, Milwaukee, WI 53211

Dan Liu, Lu Wang, Zhizhong Xie, Haolin Tang,a) and Liang Xiao Department of Chemistry, State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, Hubei, People’s Republic of China

Deyang Qub) Department of Mechanical Engineering, College of Engineering and Applied Science, University of Wisconsin Milwaukee, Milwaukee, WI 53211 (Received 14 October 2016; accepted 13 December 2016)

A new lithium ion hybrid supercapacitor is reported, in which the negative electrode was made from ZnO nano-crystals coated with a nitrogen doped carbon, and a positive electrode composed of activated carbon. The ZnO nano-crystals were highly dispersed in a nitrogen doped carbon matrix through a bio-inspired route. Dopamine, used as the nitrogen and carbon source, self-polymerized and deposited onto the surface of ZnO nano-crystal. After pyrolysis, a nitrogen doped amorphous carbon coated ZnO nano-crystal materials were obtained. The characteristics of the synthesized carbon coated ZnO nano-crystal electrode as well as the electrochemical performance of the hybrid device were investigated. The ZnO nano-crystal structure was preserved in the course of the carbon coating. The lithium ion supercapacitor demonstrated a high capacity and good cycling stability. Such good performance can be attributed to improved conductivity, the prevention of ZnO nano particles from pulverization and the high degree of crystallinity of the ZnO material.

I. INTRODUCTION

A hybrid supercapacitor combines a secondary battery electrode with a double-layer electrode. It aims to fill the energy density gap between a rechargeable battery and double-layer supercapacitor while retaining a reasonable power capability. Various rechargeable battery electrodes have been studied, including Ni(OH)2, MnO2, MoO3, V2O5, TiO2, LiMn2O4, Li4Ti5O12, and graphite.1–10 The hybrid device comprising a lithium ion battery electrode is also referred to as a lithium ion supercapacitor (LIC). ZnO has been widely used in solar cells, semiconductor lasers, photocatalysts, gas sensors and the Zn/Ni secondary battery.11–15 It is now considered as a promising conversion Li-ion cathode material due to its relatively high theoretical capacity, environmental benignity, low cost, easy preparation and morphologic variety.16–25 In this study, a ZnO electrode was investigated in a LIC. In this unique configuration, the ZnO was used as the Contributing Editor: Chongmin Wang Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2016.512

negative electrode, in which the conversion reaction (ZnO 1 2Li1 1 2e 5 Zn 1 Li2O) and the alloying/ dealloying process (Zn 1 Li1 1 e 5 LiZn) were the energy storage and conversion mechanisms. A porous carbon was used as the positive electrode where nonfaradic double-layer proces