Effect of chelating agent on the sol-gel thermolysis synthesis of LiNiPO4 and its electrochemical properties for hybrid capacitors

The present work reports the preparation of olivine structured LiNiPO₄ nanoparticles through sol-gel thermolysis method using three different chelating agents of citric acid (LiNi-C), L-ascorbic acid (LiNi-A) and D-sorbitol (LiNi-S). The flame temperature and enthalpy change of each reaction using the chelating agent is calculated. Further, the sharp X-ray diffraction peak reveals the phase pure and high crystalline nature of the prepared LiNiPO₄ nanoparticles with the space group of Pnma (62) irrespective of the chelating agents. The four possible fundamental vibrations of phosphate anion PO₄^3- are revealed through Fourier Transform Infrared Spectroscopy (FTIR) studies. The presence of elements such as Ni, P and O and its valence state is identified through X-Ray Photoelectron Spectroscopy analysis. The spherical shape particles with the uniform size distribution of LiNi -S is observed than the LiNi-C, LiNi-A particles through FESEM analysis. The redox peaks and plateau regions in the cyclic voltammetry (CV) and Galvanostatic charge-discharge (GCD) profiles infer the dominance of battery-type charge process rather than a capacitive mechanism. As a result, LiNiPO₄ exhibits a maximum specific capacitance of 417 F g^-1 at 2 mV s^-1 and 357 F g^-1 at 1 mA cm^-2 in 1 M LiOH, which enables as a suitable cathode material for hybrid supercapacitor. Also, the assembled hybrid supercapacitor delivered a high energy density of 12.5 Wh kg^-1 at 200 W kg^-1 as well as a longer cycle life of 89% at a current density of 1 mA cm^-2 over 2000 cycles is noticed. These results infer that LiNiPO₄ could be used as a novel electrode material for hybrid supercapacitor application.