Microwave-Assisted Sol-Gel Synthesis of High-Voltage Ruthenium-Doped LiFePO4 Cathodes for Solid-State Lithium-Ion Batteries

Abstract

The demand for high voltage and high-capacity energy systems has grown due to increased use of portable electronics, electric vehicles, and grid energy storage systems. LiFePO4 (LFP) shows great promise as a cathode material due to its low cost, structural stability, safety characteristics, and low environmental impact. However, LFP cathodes possess a low Li-ion diffusion coefficient and poor electrical conductivity compared to oxide-based cathodes that are used currently. To make LFP competitive with oxide-based cathodes, LFP was modified to improve its characteristics. Doping of materials is a highly effective method for improving electrochemical characteristics in LFP cathodes. Ruthenium-doping of iron sites within LFP has shown positive effects in the promotion of Li-ion diffusion, reduction in band gap energies, and improved electrical conductivity. Previous studies have shown that ruthenium-doping at 0.01 moles has an increased specific capacity of 162 mAh g-1, better cycling characteristics, and reduced resistance compared to pristine LFP.1 Ruthenium doping levels above 0.02 moles have shown a decrease in specific capacity, cycling characteristics, and an increased resistance due to the formation of ruthenium oxide that disrupts the crystal lattice of LFP. In this study, LiFe1-xRuxPO4 (x= 0.01, 0.0075, and 0.005) and pristine LFP were prepared via microwave-assisted sol-gel synthesis to explore the effects of lower levels of ruthenium-doping on LFP electrochemical characteristics. Microwave synthesis was employed for reduced sintering time, beneficial effects on microstructure, and decreased cost. Samples were characterized via scanning electron microscopy, in-situ/ex-situ X-ray diffraction, energy dispersive X-ray spectroscopy, electrochemical impedance spectroscopy, and cyclic voltammetry.The authors acknowledge financial support from the NSF IUCRC program for the “Center for solid-state electric power storage” (#2052631) and support from the South Dakota Board of Regents for the “Governor’s Research Center (GRC) for electrochemical energy storage”.1.) Gao, Y.; Xiong, K.; Zhang, H.; Zhu, B. Effect of RU Doping on the Properties of Lifepo4/c Cathode Materials for Lithium-Ion Batteries. ACS Omega 2021, 6 (22), 14122–14129.