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.