Article Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
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Free-Standing Nitrogen-Doped Cup-Stacked Carbon Nanotube Mats for Potassium-Ion Battery Anodes Xinxin Zhao,† Yifan Tang,‡ Chaolun Ni,§ Jiangwei Wang,*,§ Alexander Star,*,‡ and Yunhua Xu*,†,∥ †
School of Materials Science and Engineering, and Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, Tianjin 300072, China ‡ Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States § Center of Electron Microscopy and State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China ∥ Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China S Supporting Information *
ABSTRACT: Free-standing nitrogen-doped cup-stacked carbon nanotube (NCSCNT) mats were synthesized and tested as anodes for potassium-ion batteries (KIBs). The edge-open structure character of the NCSCNTs allows a facile insertion of K+ ions into the carbon nanotubes. Combined with the nanosized feature and interconnected flexible structure, the NCSCNTs demonstrate impressive electrochemical performance with a reversible capacity of 323 mA h/g and a markedly improved rate capability retaining 75 mA h/g even at 1000 mA/ g. Additionally, the free-standing NCSCNT mat electrodes eliminate the utilization of nonactive components of binders and conductive agents during the battery assembly and thereby significantly enhance the total specific capacity of the electrodes. KEYWORDS: potassium-ion battery, cup-stacked carbon nanotube, nitrogen doping, free-standing electrode, energy storage
1. INTRODUCTION With the dramatically increasing demand for electric vehicles and utilization of renewable energy, high-performance and lowcost energy storage systems are highly desirable. Lithium-ion batteries (LIBs) have been viewed as one of the top choices due to their excellent electrochemical performance and wellestablished manufacturing techniques.1,2 However, the limited resources and low accessibility of lithium have raised concerns on the versatility of LIBs for the future widespread implementation.3,4 This triggers extensive exploration on alternative energy storage systems that use easily accessible materials and possess good electrochemical performance comparable with or even surpassing that of LIBs.5 As one of the most promising candidates, sodium-ion batteries (NIBs) have attracted considerable attention thanks to the abundance of sodium and their similar chemical/electrochemical nature with lithium.6,7 Nevertheless, great barriers confront its development because proper host materials are lacking, especially on the anode side.8 Recently, potassium-ion batteries (KIBs) have opened a new door toward developing an alternative energy storage technology to LIBs, and inspiring results were reported.9−12 Graphite can store K+ ions with a theoretical capacity of 279 mA h/g, thereby enabling direct transformation of the manufacturing techniques from LIBs to KIBs.13−15 However, excepted for the improved cycling stability by optimizing the binder and electrolyte systems, the graphite anodes often suffer © XXXX American Chemical Society
from poor rate performance in KIBs caused by the poor kinetics of the large K+ ions,12 which may neutralize the resource merit of potassium. One-dimensional (1D) materials, including nanofibers, nanowires, and nanotubes, have demonstrated good rate performance in LIBs and NIBs due to the reduced ion diffusion distance and interconnected conductive network.16 Recently, encouraging K+-ion storage performance was reported on amorphous carbon fiber anodes with improved rate capability.10,12 Nevertheless, the high K+-ion storage capacity of these amorphous carbon nanofibers mainly originates from the capacitive storage mechanism with sloping voltage profiles, which are not preferred for batteries. Structure-designed carbon nanotubes (CNTs) with open edge of the graphitic structure at the outer surface, i.e., cup-stacked CNTs (CSCNTs), were used to store Li ions.17 Similar intercalation behavior as that of graphite was observed but with enhanced rate performance and prolonged cycle life due to the reduced diffusion ion distance and high reversibility. Another merit of the CSCNT anodes is attributed to the network architecture and free-standing feature, which avoid the utilization of inactive components, such as binders, conductive additives, and current collectors, during battery assembly.18,19 Consequently, significantly improved Received: February 6, 2018 Accepted: March 27, 2018 Published: March 27, 2018 A
DOI: 10.1021/acsaem.8b00182 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Energy Materials total capacity can be achieved, compared with those conventional electrodes fabricated by slurry-casting technique. Although high reversible capacities (∼260 mA h/g) and good rate performance have been reported on hard carbon anodes in KIBs,20 when considering the overall weight of the conventional electrodes, the overall specific capacity of the hard carbon anodes (∼57 mA h/g) is much less than that of the graphite in on-market LIBs (∼200 mA h/g),21−23 making KIBs less competitive. In this work, free-standing nitrogen-doped cup-stacked carbon nanotube (NCSCNT) mats were fabricated by vacuum filtrating CVD-grown NCSCNTs and were directly used as anodes for KIBs.18 The open layer alignment structure of the NCSCNTs allows facile insertion/extraction of K+ ions as made evident by previous work.19 The total-mass specific capacity of 323 mA h/g was delivered and remained 236 mA h/g after 100 cycles. Impressively, the NCSCNTs retained a capacity of 75 mA h/g even at a high current rate of 1000 mA/g. In contrast, graphite anodes can only remain at 50 mA h/g at 200 mA/g.13 Our results show that the graphitic nitrogen-doped carbon nanotubes with open structure on the outer surface are favorable for potassium storage, providing a new design to seek viable KIBs.
Figure 1. Morphology characterization: (a) digital photograph and (b, c) SEM images of NCSCNT mats.
The structure of the NCSCNTs was characterized using TEM, as shown in Figure 2a. Different from the conventional
2. EXPERIMENTAL SECTION 2.1. Synthesis of the NCSCNTs. NCSCNTs were prepared using a chemical vapor deposition (CVD) method. The synthesis procedure has been described in the previous report.18 2.2. Material Characterization. The morphology and structure of the NCSCNTs were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The characterization details are the same as those described in our previous paper.12 2.3. Electrochemical Characterization. The obtained NCSCNT powder was dispersed in ethanol with a concentration of 1 mg/mL and then filtrated to fabricate the free-standing carbon nanotube mats. The free-stranding cup-stacked carbon nanotube mats were cut into circular pieces and directly used as electrodes of KIBs. The batteries were constructed using the same method as that used in our previous paper.12 The conventional multiwall carbon nanotube (MWCNT) mats were prepared using the same method and testing as NCSCNTs. Both of the NCSCNT and MWCNT electrodes were cut into round pieces in 15 mm diameter with a mass loading of ∼1 mg/cm2. The cycling stability was tested on a LAND CT-2001A (Wuhan, China) instrument in the voltage range 0−2.5 V (versus K+/K). CV scans were performed on a CHI 600E electrochemical workstation (Shanghai, China) at 0.01 mV/s between 0 and 2 V. The electrochemical impedance spectroscopy (EIS) was performed using a frequency response analyzer (1455, Solartron Analytical) coupled with an electrochemical interface (1470E, Solartron Analytical) in a frequency range from 0.01 Hz to 1 MHz and a 5 mV ac amplitude.
Figure 2. Structure characterization: (a, b) TEM images, (c) HRTEM image, and the interlayer spacing in the (d) walls and (e) bottom of the NCSCNTs.
CNTs with a tubular structure, the NCSCNTs have a cupstacked structure due to the nitrogen doping. The graphitic layers cross the CNTs from the outer edge to the inner bottom in tilted angles, which looks like the carbon cups overlap each other.18,24 The high-resolution TEM (HRTEM) images clearly illustrate the cup-stacked structure with a thickness of 15 nm in the walls and
