Octahedral high voltage LiNi0.5Mn1.5O4 spinel cathode: enhanced capacity retention of hybrid aqueous capacitors with nitrogen doped graphene

Abstract

A new type of aqueous hybrid supercapacitor has been constructed using electrospun octahedral high voltage LiNi_0.5Mn_1.5O₄ spinel as the cathode material and nitrogen doped graphene (NDG) as the anode material. The structural and morphological changes of the products are investigated by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM) and thermogravimetric analysis (TGA/DTA). The XRD pattern indicates that the LiNi_0.5Mn_1.5O₄ spinel is highly crystalline in nature. The electrochemical performance of LiNi_0.5Mn_1.5O₄ and NDG was evaluated by cyclic voltammetry, galvanostatic constant current charge–discharge cycling and impedance analysis in 3 M LiNO₃. The specific capacitance of the LiNi_0.5Mn_1.5O₄/NDG asymmetric hybrid supercapacitor (HSC) cell was found to be 72 F g⁻¹ at a current density of 0.5 mA cm⁻². The HSC delivered a maximum energy density of 15 W h kg⁻¹ at a power density of 110 W kg⁻¹. Furthermore, it was shown that an expanded voltage window of 1.3 V could be achieved when combining a composite LiNi_0.5Mn_1.5O₄ (cathode) and NDG (anode) in a charge balanced asymmetric capacitor. Moreover, the HSC exhibits remarkable capacity retention upon cycling indicating the significant impact on the morphology of LiNi_0.5Mn_1.5O₄ and NDG.