Electrochemical Performances of Li-Ion Batteries Based on LiFePO4 Cathodes Supported by Bio-Sourced Activated Carbon from Millet Cob (MC) and Water Hyacinth (WH)

The electrochemical performance of Li-ion batteries employing LiFePO4 (LFP) cathodes supported by bio-sourced activated carbon derived from millet cob (MC) and water hyacinth (WH) were systematically investigated. Carbon activation was carried out using potassium hydroxide (KOH) at varying mass ratios of KOH to precursor material: 1:1, 2:1, and 5:1 for both WH and MC-derived carbon. The physical properties (X-ray diffraction patterns, BET surface area, micropore and mesopore volume, conductivity, etc.) and electrochemical performance (specific capacity, discharge at various current rates, electrochemical impedance measurement, etc.) were determined. Material characterization revealed that the activated carbon derived from MC exhibits an amorphous structure, whereas that obtained from WH is predominantly crystalline. High specific surface areas were achieved with activated carbons synthesized using a low KOH-to-carbon mass ratio (1:1), reaching 413.03 m2·g−1 for WH and 216.34 m2·g−1 for MC. However, larger average pore diameters were observed at higher activation ratios (5:1), measuring 8.38 nm for KOH/WH and 5.28 nm for KOH/MC. For both biomass-derived carbons, optimal electrical conductivity was obtained at a 2:1 activation ratio, with values of 14.7 × 10−3 S·cm−1 for KOH/WH and 8.42 × 10−3 S·cm−1 for KOH/MC. The electrochemical performance of coin cells based on cathodes composed of 85% LiFePO4, 8% of these activated carbons, and 7% polyvinylidene fluoride (PVDF) as a binder, with lithium metal as the anode were studied. The LiFePO4/C (LFP/C) cathodes exhibited specific capacities of up to 160 mAh·g−1 at a current rate of C/12 and 110 mAh·g−1 at 5C. Both LFP/MC and LFP/WH cathodes exhibit optimal energy density at specific values of pore size, pore volume, charge transfer resistance (Rct), and diffusion coefficient (DLi), reflecting a favorable balance between ionic transport, accessible surface area, and charge conduction. Maximum energy densities relative to active mass were recorded at 544 mWh·g−1 for LFP/MC 2:1, 554 mWh·g−1 for LFP/WH 2:1, and 568 mWh·g−1 for the reference LFP/graphite system. These performance results demonstrate that the development of high-performing bio-sourced activated carbon depends on the optimization of various parameters, including chemical composition, specific surface area, pore volume and size distribution, as well as electrical conductivity.