Nano-silicon-graphite composite negative electrode material with carbon coating and aluminum metaphosphate composite modification layer on surface and preparation method thereof

A preparation method of a nano-silicon-graphite composite negative electrode material with a carbon coating and an aluminum metaphosphate composite modification layer on its surface, wherein the nano-silicon-graphite composite negative electrode material is prepared from a formulation of following components by mass percentage:

• 4-10 wt. % of an aluminum metaphosphate, 10 wt. % of an asphalt cracking carbon, 15 wt. % of a spherical nano-silicon powder, and 71-65 wt. % of a graphite powder, and the preparation method comprises the following steps:
• S1. adding a nano-silicon powder into deionized water according to the formulation, ultrasonically stirring and dispersing to obtain a nano-silicon powder suspension;
• S2. stirring the suspension of S1 at a high speed, adding a graphite powder according to the formulation, stirring and mixing well, then adding a waterborne asphalt, stirring at a high speed until uniform, and at the same time, adding deionized water to control a solid content of the suspension to be within a range of 10-15 wt. %;
• S3. spray-drying the suspension of S2 using a spray dryer to obtain a dry powder;
• S4. weighing an aluminum metaphosphate powder according to the formulation, adding to the dry powder of S3, mixing uniformly to obtain a mixture, and then adding the mixture to a mechanical fusion machine for extrusion fusion for 20 min to obtain a precursor coated with a layer of an asphalt and an aluminum metaphosphate modification on its surface;
• S5. transferring the precursor in S4 into a vacuum furnace, performing high-temperature carbonization on the precursor to obtain a carbonized precursor, and then taking the carbonized precursor out for crushing and sieving to obtain the nano-silicon-graphite composite negative electrode material with a carbon coating and an aluminum metaphosphate composite modification layer on its surface.