ARTICLE pubs.acs.org/JPCC
Pore Size Effect of Carbon Electrodes on the Electrochemical Double-Layer Capacitance in LiTFSI/2-Oxazolidinone Complex Electrolyte Renjie Chen,†,‡ Zhouying He,† Li Li,*,†,‡ Feng Wu,*,†,‡ Bin Xu,§ and Man Xie† †
School of Chemical Engineering & Environment, Beijing Institute of Technology, Beijing, 100081, China National Development Center of High Technology Green Materials, Beijing 100081, China § Research Institute of Chemical Defense, Beijing 100191, China ‡
ABSTRACT: To optimize the performance of electric doublelayer capacitors (EDLCs) with ionic liquids (ILs) based on LiTFSI and 2-oxazolidinone as electrolyte, we simulated possible IL structures and ion sizes, and IL compatibility with three kinds of carbon materials was investigated. Mesoporous activated carbon (MEAC) with a surface area of 2675 m2 g1 and an average pore size of 2.5 nm was most compatible with the IL. The electrochemical performances of the EDLCs based on the MEAC electrode and the IL electrolyte were evaluated using cyclic voltammetry, AC impedance spectroscopy, galvanostatic charge/discharge test, and so on. The capacitance of EDLCs based on MEAC was as high as 175.2 F g1 at 0.1 mA (75 mA g1) and at room temperature. The capacitor also showed good rate capability and cycle performance. With temperature increased to 80 °C, the capacitance increased, and the equivalent series resistance of the EDLCs decreased dramatically, implying better performance without safety concerns.
’ INTRODUCTION Electric double-layer capacitors (EDLCs) with a higher power density and longer cycle life than batteries have attracted a great deal of attention in recent years because of their widespread application in portable devices, hybrid vehicles, backup systems, and so on.19 Because the electric energy stored in EDLCs gives rise to the separation of charged species in the electric double layer across the electrode/electrolyte interface, the performance of EDLCs depends strongly on the electrolyte and the porous carbon electrode materials. Traditional EDLCs commonly use aqueous or nonaqueous organic solutions as electrolytes. EDLCs with nonaqueous electrolytes can work over a wider range of voltage leading to a significant enhancement in energy and power densities compared with those of aqueous electrolytes. However, vapor generation, flammability, and the possible explosion of organic solvents are some concerns when using nonaqueous electrolytes at high temperatures.10,11 Room-temperature melting salts (RTMSs), also known as ionic liquids (ILs) are expected to be an alternative to traditional nonaqueous electrolytes because of their unique physical and chemical properties, such as a wide liquid range, high ionic conductivities, wide electrochemical windows, nonvolatility, nonflammability, and good safety at high temperatures.1218 More and more researchers apply ILs in EDLCs as electrolytes in recent years. Among all ILs, the type of imidazolium has a quality of low viscosities, high ionic conductivities, and low melting points. IL electrolytes composed of imidazolium-type cations have been extensively used in EDLCs, and they show excellent electrochemical stability, refined rate r 2011 American Chemical Society
capability, and cycle durability.1930 However, the low cathodic stability of imidazolium-based ILs is the most critical obstacle preventing their practical application. Other ILs have also been reported.31,32 For example, pyrrolidinium-type ILs containing different anions used in EDLCs will greatly improve the safety problem of EDLCs compared with other electrolytes.31 The above research has demonstrated that ILs are promising electrolytes for EDLCs. Electrode material is also an essential factor that affects the performances of EDLCs. Proper porous carbon structures with high surface areas are favored electrode materials for EDLCs because their capacitance is proportional to their specific surface areas. However, not all of the pores but only the surface of the pores that the ions can access will contribute to the double-layer capacitance.3338 Arbizzani et al. have identified some routes to optimize electrode materials for hybrid supercapacitor with hydrophobic IL electrolytes.37 They developed cryo/xerogel carbons of mesopore specific surface area >500 m2 g1 to achieve high capacitance with IL electrolytes. Largeot et al.38 investigated the relationship between ions and pores for EDLCs and showed that the pores with a maximum capacitance are very close to the ions’ size of the electrolytes. Moreover, from the view of rate capability, the pore size should be several times larger than the ions to facilitate ion transfer. Therefore, a good match between Received: July 28, 2011 Revised: November 10, 2011 Published: December 16, 2011 2594
dx.doi.org/10.1021/jp207232f | J. Phys. Chem. C 2012, 116, 2594–2599
The Journal of Physical Chemistry C
ARTICLE
the pore size of the carbon material and the dimensions of the ionic species is important to achieve advanced EDLCs with high capacitance and good rate capability. In our previous research, we synthesized RTMS based on LiTFSI and organic compounds with acylamino group. The complex systems all exhibit excellent thermal stability, low viscosity at room temperature, high ionic conductivity and a broad electrochemical stability.39,40 EDLCs with these complex systems as electrolytes showed good electrochemical performance at ambient and elevated temperatures. However, the capacitance of EDLCs based on certain carbon materials is not all satisfactory. To optimize the performance of EDLCs with LiTFSI-OZO as electrolyte, in this work, we simulated the possible structure and ion size of LiTFSI-OZO complex system at the molar ratio of 1:4.0, and the properties of three carbon materials—mesoporous activated carbon (MEAC), microporous activated carbon (MIAC), and carbon nanotubes (CNTs) were investigated. Then, we analyzed the compatibility between electrolyte and three electrode materials by both theoretical calculation and galvanostatic charge/discharge tests. The results indicate that the MEAC with an average pore size of 2.5 nm and a surface area of 2675 m2 g1 is well-matched with the IL electrolyte, as it gives a capacitance as high as 175.2 F g1, which is much more higher than MIAC and CNTs. Meanwhile, excellent rate performance and high-temperature performance are also presented in the Article.
’ EXPERIMENTAL SECTION Room-Temperature Complex Electrolyte. LiTFSI (3M, Inc., 99%) was dried under vacuum at 140 °C for 12 h. 2-Oxazolidinone (OZO, Acros, AP) was recrystallized using chloroform and then dried at 55 °C for 10 h in vacuum. The room-temperature complex system was prepared by simply mixing the lithium salt and OZO at a molar ratio of 1:4.0 in an argon-filled MBraun LabMaster 130 glovebox (H2O < 5 ppm). The mixing process has been reported in our previous paper.39 The water content in the complex electrolyte was determined to be
