A High Energy Density Lithium Ion Capacitor with CNT Anchored on Carbon Nano Plate As a New Anode Material

Abstract

1. Introduction Lithium ion Capacitors (LIC), hybrid of electric double layer capacitors (EDLC) and lithium ion batteries (LIB), have been widely developed to realize high power and energy density application [1,2]. However, current status of LIC (Hard carbon materials) has been limited to apply in primary power sources due to their lower energy density than that of LIB as well as lower power density than that of EDLC. Graphite anode materials, for high energy density of LIC, have been widely studied owing to their higher theoretical capacity and wider operating voltage range than those of hard carbon. However, slow kinetics of graphite electrode is one of the major limitation for use in LIC anode. To overcome these drawbacks of graphite electrode, a new additional material is needed. Recently, CNP (carbon nano plate) is one of the promising candidate for improving graphite electrode of LIC because of those large theoretic specific surface area, high electronic conductivity, and good capacitance effect [3]. Nevertheless, it is necessary to develop a new structure of CNP based materials to overcome degradation of cell performance caused by inappropriate dimensional orientation as well as poor handling problems. Herein, we designed a new type of carbon nano-plates (CNP) with vertically grown carbon nanotube (CNT). The characteristics of the CNP with vertically grown CNT (CNP-CNT) were studied in terms of physicochemical properties as well as electrochemical properties as anode materials for LICs. 2. Experiment To vertically grow the CNTs on the CNP surface, pristine CNP powders were pre-treated with Pd/Sn components and readily decorated with Fe nanoparticles via electroless plating. After the process of metal-catalyst-loading on the CNP surface, CNTs were spontaneously synthesized on a CNP surface such as downy hairs in all directions through the CVD process. The electrochemical measurements were performed using pouch type single full cells, which were composed of the anodes (graphite mixed with prepared CNP-CNTs), the cathode (activated carbon), porous polyolefin separator, and liquid electrolyte (1.3M LiPF6in ethylene carbonate and dimethyl carbonate). 3. Results and Discussion During the CVD process, hydrocarbons decomposed on the top surface of metal catalyst, and carbons diffused down through the metal. Then, CNT precipitated out across the metal bottom, pushing the whole metal catalyst of the CNP surface. Fig. 1(a) presents the SEM image of the CNTs with directly anchored on CNP surface. To investigate more detailed junction points between them, we additionally took a TEM image (Fig. 1(b)) and found the direct anchoring points in an inset of Fig. 1(b). The CNP-CNTs are well synthesized by CVD process. The synthesized CNP with vertically grown carbon nanotubes (CNP-CNT) were applied to anode materials for LIC by mixing the graphite. The Fig. 1(c) compared voltage profiles of the LIC cells as assembled with graphite with 10 wt% of CNP-CNT as well as graphite only as anode material. The cell with graphite and CNP-CNT showed reduced voltage overpotential as well as increased charge and discharge capacitance at 10 C-rate condition. The cell containing CNP-CNT shows ca.15% higher energy density than that of graphite only. The cell assembled with mixture of CNP-CNT and graphite shows higher cell performance in comparison with graphite only, which is supposed to be due to the high electronic conductivity and capacitance of CNP-CNT. Furthermore, the LIC with CNP-CNT mixed with graphite shows good cycle retention than that of graphite only. Therefore, the mixture of CNP-CNT and graphite materials is one of promising candidate for anode materials of LIC to increase energy density as well as power density. References 1) J. R. Miller, P. Simon, Science2008, 321, 651 2) L. Hao, J. Ning, B. Luo, B. Wang, Y. Zhang, Z. Tang, J. Yang, A. Thomas, L. Zhi, J. Am. Chem. Soc.2015, 137, 219 3) Tao Chen, Xiong Zhang, Peng Yu, Tanwei Ma, J. power sources2010, 195, 3031 Figure 1