## 6 Role of Free Carbon in Si-Based Anode Materials
### Optimization of Electrochemical Performance  
Free carbon in polymer-derived ceramics (PDCs) anode materials has a significant impact on their electrochemical properties. First, the presence of free carbon can significantly improve the cyclic stability and rate performance of the material. [PERSON] et al. [47] has shown that carbon-rich PDCs exhibit excellent stability during cycling, especially under high-temperature pyrolysis conditions, where the content and structure of free carbon have a direct impact on the electrochemical properties of the materials. In addition, [PERSON]'s research [69] pointed out that the state of free carbon can be optimized by adjusting the pyrolysis temperature and microstructure, thereby significantly improving the electrochemical performance of PDCs anode materials. For example, by maintaining a high degree of disorder of free carbon, the cycle stability and rate performance of materials can be effectively improved.  
[PERSON] et al. suggested that an increase in free carbon content can significantly increase the carbon content of SiCN and SiOC ceramics, thereby improving the lithium-ion storage capacity and charge-discharge efficiency [63]. Their study found that as DVB was added, the free carbon content increased significantly, resulting in an increase in the reversible capacity of the material from 136 mAh\(\cdot\)g\({}^{-1}\) to 574 mAh\(\cdot\)g\({}^{-1}\) in the first cycle, while the coulombic efficiency increased by 10%. This view is also supported by [PERSON]'s research, pointing out that free carbon can not only be used as an active material to store lithium-ions, but also as a conductive agent to improve the conductivity of C/Si-O-C systems [1]. His research shows that even at low free carbon content, the C/Si-O-C anode material still shows a reversible capacity much higher than the theoretical capacity of graphite, suggesting that the contribution of the relative reversible capacity of Si-O-C glass is also very important.  
[PERSON] et al. [31] found that the treatment temperature has a significant effect on the electrochemical properties of SiOC anode materials. For samples treated at 1000 \({}^{\circ}\)C, with an amorphous phase of 51 wt% and a free carbon of 49 wt%, it exhibits the highest reversible capacity and the best cycling stability. However, in high-carbon samples treated at 1300 \({}^{\circ}\)C, the reduction of free carbon leads to a decrease in reversible capacity and cycling stability. It shows that the free carbon is influenced by treatment temperature, and it is necessary to control treatment temperature to improve the performance of lithium-ion batteries.  
Free carbon deeply influences the performance of materials. The performance of these materials is shown in Table 1[58]:  
In summary, the role of free carbon in PDCs anode materials is not only limited to providing lithium-ion storage sites, but more importantly, it effectively alleviates the volume change and improves the structural stability of the materials through its disordered structure and excellent electrical conductivity. These properties make free carbon a key factor in improving the performance of anode materials for PDCs. The role of free carbon in PDCs anode materials cannot be ignored. By optimizing the content and structure of free carbon, the electrochemical properties of the material can be significantly improved, including cycling stability and rate performance. These research results provide an important theoretical basis and experimental support for the future development of high-performance anode materials for lithium-ion batteries.