Abstract
A facile scalable strategy is reported for the synthesis of a hybrid of lithium titanate (\(Li_4Ti_5O_{12}\) or LTO) and 1-pyrenecarboxylic acid decorated multiwalled carbon nanotubes (PCA@CNTs). LTO platelets comprising of quasi-spherical nanoparticles afford short diffusion paths for electrolyte ions. PCA@CNTs, enhance the electrical conductivity of the nearly insulating LTO by 3 orders of magnitude, thus maximizing the ion-uptake capability of the hybrid. Symmetric and asymmetric supercapacitors with the LTO/PCA@CNTs hybrid supported over Ni foam substrates are assembled with a novel \(Li^+\) conducting alginate gel, in air without any inert conditions that are typically used for all LTO based devices. The gel shows an average ionic conductivity of ~\(8.4 \text{ mS cm}^{-1}\) at room temperature, and is found to be electrochemically stable over a wide operational voltage window of ~\(2.5 \text{ V}\). Benefitting from the synergy of electrical double layer (EDL) storage afforded by PCA@CNTs, ion-storage by LTO through a redox reaction and EDL, and the ease ion-movement across the cell due to the open architecture of CNTs, the asymmetric LTO/PCA@CNTs hybrid cell outperforms the symmetric cells by a large margin. The best areal specific capacitance (SC), volumetric SC and energy density are ~\(54 \text{ mF cm}^{-2}\), ~\(4.3 \text{ F cm}^{-3}\) (at \(0.5 \text{ mA cm}^{-2}\)) and ~\(3.7 \text{ mWh cm}^{-3}\) (at a power density of \(49.6 \text{ mW cm}^{-3}\)) significantly enhanced for the asymmetric LTO/PCA@CNTs hybrid cell, compared to the symmetric- PCA@CNTs and hybrid cells. The design is simple to implement and can serve as a prototype to develop a range of yet unexplored LTO/carbon nanomaterial based supercapacitors.