Core/Shell Cu-TiO2 Nanoparticles As Efficient Anode Material for Li-Ion Capacitors

Abstract

Lithium-ion capacitor (LIC) belongs to a specific category of energy storage materials [1]. It is a combination of an electrode from a lithium-ion battery (LIB) with an electrode from an electrical double layer capacitor (EDLC). A usage of these two electrodes in one system results in formation of a hybrid energy device which can exhibit desired properties, such as: charge/discharge rapid capability, long cycle life and high energy density. However, the widescale usage of LICs is still limited due to the inherent low concentration of Li-ions in this system [2]. Typically, to improve it, an electrode needs to be pre-lithiated. There are many pre-lithiation methods that have been developed and implemented, but with varying level of success [3]. Therefore, the most optimal solution is to remove completely the need of pre-lithiation. In this work, we propose an approach which eliminates the requirement of pre-lithiation by designing a core-shell structure for the anodic active material. Herein, the shell is the material responsible for electrochemical activity and the core facilitates electronic conductivity.Titanium dioxide (α-TiO2), a well-researched anode material, was used as the shell, while copper (Cu), a highly conductive metal, was used as a core. Cu@TiO2 core-shell structures were synthesized by a wet-chemistry approach. The microscopic studies of chemical composition and morphology confirmed that the formation of TiO2 on the surface of copper was successful. Electrochemical measurements in the half cell of the electrode with Cu@TiO2 as the active material showed a specific capacity of around 156 mAh g-1. Next, the Cu@TiO2 anode was tested in a LIC against cathode with activated carbon as the active material. The resulting LIC showed showed high specific capacity and stable cycling behavior up to a 1000 cycles at 2.5 A g-1, with a coulombic efficiency of around 98%. Rate capability tests also showed relatively high stability of the anode material at low to high current rates.The results of this work open up potential avenues for further research focused on designing anode structures for LICs. The core-shell structure with Cu as the core and α-TiO2 as the shell has been demonstrated to enhance the stability of the capacitor and remove the need for pre-lithiation. The combination of low-cost and environmentally friendly materials for anode, and ease of LIC assembly, makes this a promising material for LICs.[1] Amatucci, G.G.; Badway, F.; du Pasquier, A.; Zheng, T. An Asymmetric Hybrid Nonaqueous Energy Storage Cell. J. Electrochem. Soc. 2001, 148, A930[2] Wang, H.; Zhu, C.; Chao, D.; Yan, Q.; Fan, H.J. Nonaqueous hybrid lithium-ion and sodium-ion capacitors. Adv. Mater. 2017, 29, 1702093[3] Holtstiege, F.; Bärmann, P.; Nölle, R.; Winter, M.; Placke, T. Pre-Lithiation Strategies for Rechargeable Energy Storage Technologies: Concepts, Promises and Challenges. Batteries 2018, 4, 4