*3.3. Doped Composite*

The Silicon–amorphous carbon–graphite three elements Si@C composite system is mainly prepared by ball-milling and high-temperature pyrolysis. In this system, the chemical properties of the material can be improved by modifying the porous structure of silicon [36]. The existence of silicon contributes to the increase of capacity, graphite contributes to the improvement of the dispersion of silicon particles [37], and the amorphous carbon plays the role of the binder. In this composite, the type of carbon is changeable. The mesoporous carbon@ carbon nanotubes@ amorphous carbon materials are prepared using magnesium thermal reduction and CVD method. After 400 cycles, the reversible capacity remains 710 mAhg−1. Wang et al. [38] reported Si@flake-graphite–amorphous-carbon by using ball-milling and spray drying. In this system, silicon and graphite were coupled by a PVP (polyvinyl pyrrolidone) binder, which was also used as a carbon source to form the coated layer. After high temperature treatment, a porous spherical shape was obtained. At a current density of 5 Ag−1, the capacity of 200 mAhg−<sup>1</sup> was maintained. After 300 cycles, the gravimetric capacity of up to 400 mAhg−<sup>1</sup> was obtained. Wu et al. [39] used electrostatic spray deposition and heat treatment to synthesize a Si–graphene–porous carbon composite that had a layer-by-layer porous carbon framework to suppress the volume effect of silicon and a flexible graphene layer to facilitate electron transport and maintain integrity of the system. The material displayed a reversible capacity of 1020 mAhg−<sup>1</sup> after 100 cycles at a current density of 200 mAg−1. Agyeman et al. [40] synthesized the sandwich structure of Si@C-rGO by using stirring and vacuum filtration. Owing to the sandwich structure, with strong covalent and hydrogen bonding, the materials presented an excellent rate capacity of 767 mAhg−<sup>1</sup> at a current density of 3 Ag−1, high gravimetric capacity of 1001 mAhg−<sup>1</sup> at 300 mAg−1, and a grea<sup>t</sup> cyclic stability.

#### **4. Conclusions and Prospect**

The cycle stability and the reversible capacity retention rate are two important properties in the development of lithium ion batteries. It also restricts the commercialization of silicon carbon composites. By exploring the essential factors that affect its performance, the structure of composite materials is optimized to solve the problem of performance. In this paper, the recent research progress

of in-situ synthesis of silicon carbon composites is described. The following suggestions are put forward for the future development of anode materials for lithium ion batteries: (1) Seeking a monomer with such kind of advantages, including the inorganic core and the organic shell of siliceous material; (2) optimizing the microstructure of composite material by a more effective preparation method; (3) combining the nano size with the mesoporous structure by the ternary system; (4) the combination of multiple forms achieves multiple performance capability.

In the system of in-situ electrochemical synthesis of Si@C anode materials, a new concept where the lithium-intercalated silicon alloy is uniformly dispersed in the carbon matrix materials has been proposed, which can make its capacity retention rate steady in a low voltage window without affecting the total energy density. By dispersing the lithium-intercalated alloys in different states into the matrix, and by preparing the anode materials with different contents of component, the electrochemical properties of the materials under different voltage windows can be investigated. Furthermore, it can also explain the lithium-intercalation mechanism of the anode materials in the process of charging–discharging. This method provides a new possibility for the preparation of anode materials and on the application and production with further development. In the system of in-situ solid-state synthesis, through the in-situ growth of graphene, the one-step solid-state synthesis of Si@C composites can be achieved. The composite takes full advantage of the excellent properties of graphene, which has high conductivity and can effectively regulate the charge–discharge cycle. This self-assembly method is simple, safe, and low-cost. It is helpful to synthesize graphene-based composites with different functions that can be applied to the lithium-ion batteries, supercapacitors, and fuel cells. In the system of in-situ carbothermal synthesis of Si@C anode materials, the raw materials are natural rice husk materials and the composites are prepared by a typical large-scale carbothermal reduction treatment. It is an environmentally friendly preparation method, which has the potential of large-scale production. Looking at the whole lithium-ion battery anode material industry, Si@C anode is still in the infancy stage, and its commercial application still needs further development. Nevertheless, in-situ synthesis provides a new direction for the preparation of Si@C anode materials. In the process of seeking high performance, the mechanism of internal electrochemical changes can also be studied, providing possibility for the improvement of the performance and industrialization.

**Author Contributions:** Conceptualization, W.T.; validation, H.Z.; investigation, Z.D.; writing—original draft preparation, Z.B.; writing—review and editing, H.T.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
