Magnetite (Fe$_3$O$_4$) nanoparticles with different particle sizes (9.9–29.1 nm) were prepared by using the aerial oxidation method under light irradiation with various wavelengths at room temperature. The photocatalytic degradation of orange II using Fe$_3$O$_4$ nanoparticles as photocatalysts was evaluated. Experimental results showed that the particle sizes of Fe$_3$O$_4$ nanocrystals decreased gradually with the decreasing wavelengths of light irradiation. With the decrease in the size of the samples, the Fe$_3$O$_4$ nanoparticles exhibited a large surface area and high adsorption. Furthermore, the smallparticle-sized Fe$_3$O$_4$ sample could cause an appropriate red shift of the spectra and promote the decomposition of H$_2$O$_2$, and produce high-content ${}^{\bullet}$OH radicals, which lead to an improvement of photodegradation efficiency of orange II.

Replacing liquid electrolyte (LE) with gel polymer electrolyte (GPE) can significantly improve the safety and long-term stability of lithium-ion batteries. However, the traditional pure polymethyl methacrylate (PMMA)-based GPEafter immersion in the electrolyte has the problem of poor mechanical properties and easy damage. In this article, PMMA was used as the matrix, and low-density polyethylene particles were mixed to prepare a kind of structure-enhanced andheat-resistant GPE by phase inversion method. The prepared GPE has high ionic conductivity at room temperature. It can slow down the temperature rise of the battery by reducing the ionic conductivity at high temperatures, thereby improving the safe application performance of the battery. In addition, the symmetrical battery made of this material can maintain ultra-low over voltage and run stably for at least 800 h in the overpotential test when the current density is 1 mA cm$^{-2}$. This study provides new insights for the preparation of high-temperature safety GPE.