Enhanced Ionic and Electronic Transport in Nano-TiO2/Sheared CNT Composite Electrode for High Performance Na+ Insertion-Based Pseudocapacitors

Abstract

Ion-insertion based pseudocapacitors show great promise to bridge the gap in performance between capacitors of high power densities and batteries of high energy densities. While Li+ based insertion pseudocapacitors (LICs) have been extensively studied, the exceedingly more abundant and inexpensive Na+ based pseudocapacitors (SICs) have not. Owing to the significantly larger size of Na+ than Li+, finding suitable low-voltage insertion materials for Na+ has been challenging. In this presentation, we will report a new low-voltage Na+ insertion electrode consisting of TiO2 nanocrystals anchored onto N-doped, sheared-carbon nanotubes (SCNTs) that overcomes the poor kinetics, low storage capacity, and limited stability often associated with Na+ based intercalation materials. In a half cell configuration, our anode shows a large capacity of 267 mAh g-1 at a 1C cycling rate and 120 mAh g-1 at a 10C rate while maintaining 87% of the initial capacity after 1000 cycles. When coupled with activated carbon (AC) in a full cell configuration, a remarkable energy density and power density of 54.9 Wh kg-1 and 1410 W kg-1, respectively, can be achieved while maintaining 90% capacity retention over 5000 cycles. The exceptional rate capability, energy and power density, and stability of our SICs are attributed to the dramatically enhanced electronic and Na+ ion conductivity of the TiO2/SCNTs composite electrode with unique architecture created during our synthesis process. The electrode designed in the current work presents an advancement in Na+ insertion electrode design that will ultimately result in cheap, high energy and power density energy storage devices for commercial use.