*2.1. Materials*

Mg(OH)2 was obtained from Aladdin Chemical Co., Ltd., Shanghai, China. KNO3, NaNO3 and LiNO3 were commercially supplied by Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. All chemicals are of analytical grade.

## *2.2. Preparation*

The first step is the preparation of lithium nitrate–potassium nitrate–sodium nitrate eutectic nitrate. Many scholars have studied the ternary eutectic point of the LiNO3– NaNO3–KNO3 ternary system. Zhong et al. [36] predicted the phase diagram of the LiNO3–NaNO3–KNO3 ternary system and experimentally verified the predicted ternary invariant points. This result is similar to the report by Coscia et al. [37]. The mass ratio of the LiNO3–NaNO3–KNO3 ternary system is 29:58:13 (wt %). The ternary nitrate with the mass balance was ground in a mortar for 1 h; then, it was transferred to a crucible and placed in a resistance furnace at 300 ◦C for 5 h. Then, we took out the crucible, cooled it, and ground it to obtain ternary eutectic nitrate.

According to the ratio in Table 1, the magnesium hydroxide precursor was added to the ternary eutectic nitrate. Then, we ground it in an agate mortar for 30 min. After that, the mixture was transferred to a ceramic crucible and placed in a resistance furnace at 400 ◦C for 2 h to ensure complete decomposition of the magnesium hydroxide precursor. After taking it out, it was quickly cooled in air and ground to obtain a sample. The process is shown in Figure 1.


**Figure 1.** Preparation of modified salt.

To further test the material, it is necessary to obtain the nanoparticles synthesized in situ. The modified salt was washed with deionized water and then centrifuged. After the nitrate is washed away, the nanoparticles are dispersed by ultrasound. Finally, the resulting nanoparticles are dried.
