Pseudocapacitive Sodium Storage in Mesoporous Single-Crystal-like TiO 2 - Graphene Nanocomposite Enables High- Performance Sodium-Ion Capacitors Zaiyuan Le, † Fang Liu, † Ping Nie, † Xinru Li, † Xiaoyan Liu, † Zhenfeng Bian, ‡ Gen Chen, † Hao Bin Wu,* ,† and Yunfeng Lu* ,† † Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, California 90095, United States ‡ Education Ministry Key Lab of Resource Chemistry and Shanghai Key Laboratory of Rare Earth Functional Materials, Department of Chemistry, Shanghai Normal University, Shanghai, 200234, China * S Supporting Information ABSTRACT: Sodium-ion capacitors can potentially combine the virtues of high power capability of conventional electrochemical capacitors and high energy density of batteries. However, the lack of high-performance electrode materials has been the major challenge of sodium-based energy storage devices. In this work, we report a microwave-assisted synthesis of single-crystal-like anatase TiO 2 mesocages anchored on graphene as a sodium storage material. The architecture of the nanocomposite results in pseudocapacitive charge storage behavior with fast kinetics, high reversibility, and negligible degradation to the micro/nanostructure. The nanocomposite delivers a high capacity of 268 mAh g -1 at 0.2 C, which remains 126 mAh g -1 at 10 C for over 18 000 cycles. Coupling with a carbon-based cathode, a full cell of sodium-ion capacitor successfully demonstrates a high energy density of 64.2 Wh kg -1 at 56.3 W kg -1 and 25.8 Wh kg -1 at 1357 W kg -1 , as well as an ultralong lifespan of 10 000 cycles with over 90% of capacity retention. KEYWORDS: sodium-ion capacitor, TiO 2 , nanocomposite, pseudocapacitive, energy storage E lectrochemical capacitors (ECs) hold great promise for future energy storage applications, such as grid/voltage stabilization, regenerative braking for automobile, and uninterruptible power supply (UPS). 1-3 Compared with lithium-ion batteries (LIBs), one of the dominant rechargeable battery systems, ECs deliver ∼10-fold higher power density and ∼100-fold longer cycle life; however, the relatively low energy density of ECs (1-2 times of magnitude lower than that of LIBs) is a major limitation for their practical applications. 4 Commercially available ECs are mostly electric double-layer capacitors (EDLCs) using porous carbons as electrode materials, which store charges based on double-layer ion desorption/adsorption. 1,5 Even with organic electrolytes to boost the cell voltage, the low capacitance (typically below 300 Fg -1 ) of carbon-based electrodes limits the energy density of EDLCs to ∼10 Wh g -1 . 6,7 One promising strategy to solve the problem is to build hybrid cells using electrodes with different charge storage mechanisms. 8 For example, ion capacitors based on an EDLC-type carbon electrode and an insertion-type electrode can inherit the merits of both ECs and LIBs, thus delivering high energy/power densities and long lifetime. 9 While lithium-ion capacitors (LICs) have been successfully demonstrated based on several host materials for lithium ions, developing sodium-ion capacitors (SICs) are much econom- ically favorable, especially in large-scale applications due to the wide availability of sodium source. 10-16 However, the lack of proper sodium storage materials has been the major challenge for sodium-based energy storage devices. 17-21 For example, as a remarkable anode material for LIBs, graphite is thermodynami- cally unfavorable for sodium insertion, leading to negligible capacity. 11,20,22 Hard carbon demonstrated a reversible capacity of 250 mAh g -1 , yet the low working potential leads to safety concerns. 23 High capacity anode materials based on alloying and conversion mechanisms have been under develop- Received: December 12, 2016 Accepted: March 10, 2017 Published: March 10, 2017 Article www.acsnano.org © 2017 American Chemical Society 2952 DOI: 10.1021/acsnano.6b08332 ACS Nano 2017, 11, 2952-2960