*2.2. Synthesis of P*−*Si NPs*

In the typical MR process, magnesium powders (Mg 99%) and the obtained SiO2 microspheres (Mg:SiO2 = 1:1) were uniformly mixed well and placed on the end of a graphite boat. Sodium chloride (NaCl: SiO2: = 9:1) was placed on the other side. The graphite boat was placed in the center of the quartz tube of the tube furnace (Kejing, Shenzhen, China), heated to 750 ◦C at a rate of 5 ◦C min−<sup>1</sup> in an Ar/H2 (5 % H2) atmosphere, and maintained for 6 h. After cooling to room temperature, 1 mol L−<sup>1</sup> hydrochloric acid (HCl) was used to remove NaCl, magnesium oxide (MgO), and other impurities. Finally, the P−Si powders were collected by centrifuging three times with ethanol and vacuum drying for 12 h.
