Carbon Modified Lithium Nickel Phosphate Perovskites as Conductive Materials for Electrochemical Devices

Abstract

LiNiPO4 (LNP) composition has been carefully selected to provide optimum processing and electrical characteristics as a cathode in electrochemical devices, mainly in lithium ion battery (LIBs). This study demonstrates the synthesis of olivine LiNiPO4/C composite (LNP/C) for use as cathode materials in LIBs. LiNiPO4 was synthesized at 450, 550 and 650 °C under argon atmosphere and 600, 700 and 800 °C under air atmosphere via non aqueous sol gel process using ethylene glycol. In the first series of experiments the effect of the calcination temperature on phase, morphology, and sintering characteristics was studied using XRD, SEM, EDX, FTIR, TGA, DSC, PSA, and BET techniques. All the morphological, physical and electrochemical properties of the composites have been prominently affected by the synthetic conditions. In the second series of experiments the LNP/C-composites were fabricated through high energy ball milling with different ratios of carbon (LNP-823-C15, LNP-923-C15, LNP 923-C25; prepared in Argon). The sharp diffraction peaks of LiNiPO4/C composite (LNP/C) revealed that the pure olivine phase was maintained after carbon coating. The LNP/C-composites possessed large surface areas as verified by BET and porous structures with uniformly distributed carbon (as inferred by SEM and PSA). LIBs application indicated that compositing carbon to LiNiPO4 has significantly improved its electronic conductivity and cyclic stability as compared to bare LiNiPO4. Discharge capacities of LNP-923-C25 (25 % carbon) were 175 mA h g-1 , 150 mA h g -1 and 125 mA h g-1 with corresponding capacity retention of 82.7%, 84.1% and 82.2% after 100 cycles at the C-rate of 0.05C, 0.1C and 1C, respectively. Electrochemical impedance spectra (EIS) corresponded to decreased charge transfer resistance with increased electronic conductivity and minimum cell polarization for the LNP/C composites. Additionally, the inflow of lithium ion flux in cathode particle was studied by using phase field modeling which indicated the coexistence of Li-poor and Li-rich phases during charging and discharging processes. The findings are significant for the development of optimal battery electrode materials as the used methodology and insights are readily transferable to other ion-insertion based electrodes