Calcination Temperature-Controlled Electrochemical Lithium Adsorption Performance of Prussian Blue-Derived Iron Oxides

To investigate the effect of calcination temperature on the electrochemical lithium-adsorption performance of Prussian blue-derived iron oxides, Prussian blue (PB) synthesized via a hydrothermal method was used as the precursor. A series of iron-oxide materials were obtained by calcination at 350, 550, and 650 °C in air. Scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FT-IR), and N2 adsorption–desorption isotherms were employed to characterize the morphology, crystal phase, and pore structure of the prepared materials. Their electrochemical behaviors, Li+ adsorption capacity, adsorption kinetics, and cycling stability were evaluated in LiCl electrolyte. The results reveal that calcination temperature exerts a significant influence on the pore-structure evolution and crystal-phase transformation of the adsorbents. PB-Fe2O3 (350 °C) delivers the largest specific surface area of 67.52 m2·g−1; PB-Fe2O3 (550 °C) forms a mixed β/γ-Fe2O3 phase while preserving the mesoporous structure; obvious particle agglomeration and pore-structure shrinkage occur for PB-Fe2O3 (650 °C). At −0.9 V (vs. saturated calomel electrode, SCE), the maximum experimentally determined Li+ adsorption capacity of PB-Fe2O3 (550 °C) reaches 132.23 mg·g−1, which is higher than those of pristine PB, PB-Fe2O3 (350 °C), and PB-Fe2O3 (650 °C). Kinetic analyses demonstrate that the Li+ adsorption processes of pristine PB and PB-Fe2O3 (550 °C) can be well described by the pseudo-first-order kinetic model. After 10 charge–discharge cycles, PB-Fe2O3 (550 °C) maintains a capacity retention above 93%, exhibiting favorable short-term cycling stability. Moreover, in a binary Li+/Mg2+ simulated brine, PB-Fe2O3 (550 °C) exhibited preferential Li+ uptake with a Li+/Mg2+ separation factor was 4.07, demonstrating its potential for selective lithium recovery from Mg2+-rich brines. Appropriate calcination can harmonize the pore-structure, crystal-phase, and oxygen-vacancy characteristics of materials, thus improving their electrochemical lithium-adsorption performance. This work provides experimental support for structural regulation and performance optimization of Prussian blue-derived iron oxides for electrochemical lithium adsorption.