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The electrochemical performance of Li-ion batteries (LIBs) including a marine-biochar electrode and ionic-liquid-based hybrid electrolyte has been investigated. The formation of micro/macro-ordered porosity in the biochar structure after pyrolysis provides a three-dimensional (3D) olive-shaped architecture for facile diffusion of electroactive species within the electrode. Three imidazolium-based ionic liquids, namely 1-ethyl-3-methylimidazolium hexafluorophosphate (EMImPF6), 1-ethyl-2,3 dimethylimidazolium hexafluorophosphate (EDImPF6), and 1,3-dimethoxy-2-methylimidazolium hexafluorophosphate [(OM)2MImPF6], were used to fabricate hybrid electrolytes and investigate the effect of the imidazolium cation structure on the safety and electrochemical performance of marine-biomass-based LIBs at various temperatures. Electrochemical characterization was carried out using galvanostatic charge–discharge measurements and electrochemical impedance spectroscopy (EIS). It was found that mixing 40 wt.% (OM)2MImPF6 IL with the organic electrolyte (modified electrolyte) remarkably improved the capacity, cyclability, and coulombic efficiency (CE) of the marine-biochar electrode. After 100 charge–discharge cycles, the capacity retention of the cell containing 40 wt.% (OM)2MImPF6 IL was 85%, 84%, and 81% at 25°C, 45°C, and 65°C, respectively, whereas capacity fading of 35%, 45%, and 68% was observed for the cell without modified electrolyte in this condition. According to EIS analysis, Li+ transfer at the electrode–electrolyte interface was significantly improved in the presence of the modified hybrid electrolyte compared with the other cells. Moreover, the results of thermal and scanning electron microscopy (SEM) analyses proved that this IL could be an appropriate electrolyte to improve the thermal stability and the solid electrolyte interphase (SEI) formation on the marine-biochar surface, respectively.