The potentials of LiMnPO4 cathode material for aqueous Li-ion batteries: An investigation into solid state and green chemistry approaches

The phospho-olivine LiMnPO₄ nanocrystal holds promise as a cathode material for next-generation Li-ion batteries (LIBs). In this study, we employed solid-state and green chemistry approaches to prepare LiMnPO₄ powders, with aloe vera extract serving as a capping agent for the latter approach. Selective synthesis of nanoparticles with more active sites for efficient Li⁺ intercalation-deintercalation was achieved. The phase purity, morphology, and composition of the nanoparticles were characterized using XRD, SEM/TEM, and EDS, respectively. Electrochemical performance was evaluated through cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy (EIS) in an aqueous environment. The solid-state synthesized LiMnPO₄ nanoparticles (LiMnPO₄-SS) exhibited electrolyte-dependent performance but demonstrated good reversibility of Li⁺ extraction and insertion. Specific capacity, retention of discharge capacity, and Coulombic efficiency were found to be 92.3 ± 0.5 mAhg⁻¹, 98 %, and 93 %, respectively, after 50 cycles at 0.2 C. Additionally, energy density was measured as 15.8 ± 1.4 Whkg⁻¹ at 189.2 ± 1.9 Wkg⁻¹ power density. The green-synthesized LiMnPO₄ nanoparticles (LiMnPO₄-GS) showed a specific capacity of 106.4 ± 0.9 mAhg⁻¹ (corresponding to ∼62 % of theoretical capacity) after 50 cycles, with capacity retention of ∼97 % and ∼80 % Coulombic efficiency. The energy density was 18.2 ± 1.7 Whkg⁻¹ at 218.1 ± 2.1 Wkg⁻¹ power density. EIS measurements indicated reduced charge transfer resistance for LiMnPO₄-GS, optimizing interfacial electrochemical reaction activity. The observed regular micromorphology and (200) plane of nanosized LiMnPO₄ contributed to the excellent electrochemical performance. Overall, this investigation highlights the correlation among synthesis method, calcination temperature, electrolyte, crystal structure, and morphology, providing insights for the design of next-generation LIBs.