## 6 Role of Free Carbon in Si-Based Anode Materials  
### Improvement of Electrical Conductivity  
In polymer-derived ceramics (PDCs) anode materials, the presence of free carbon plays a crucial role in improving electrical conductivity. Free carbon significantly improves the conductivity of the material by forming a conductive network, thereby improving the overall performance of lithium-ion batteries [66; 67].  
First, free carbon, as a conductive agent, is able to form a continuous conductive path in PDCs. This conductive network not only improves the conductivity of the material, but also enhances the transport efficiency of lithium-ions in the electrodes. [PERSON] pointed out that by manipulating the pyrolysis parameters and microstructure of PDCs, the state of free carbon (such as content, crystallinity, and order) can be adjusted. The free carbon phase also helps to stabilize the amorphous structure of SiOC ceramics, resulting in enhanced chemical and thermal stability [68; 69]. Based on the findings of [PERSON], the reversible capacity observed during the initial discharge is attributed to the free carbon phase. The carbon phase in the non-etched silicon oxycarbide matrix can reversibly store nearly twice the amount of lithium compared to commercial graphite--723 mAh\(\cdot\)g\({}^{-1}\) versus 371 mAh\(\cdot\)g\({}^{-1}\), respectively. These values can be used to determine the x value in the LiC\({}_{\mathrm{x}}\) formula. For commercial carbon anodes, the x value is 6 (LiC\({}_{6}\)), whereas for silicon oxycarbide glasses, it is approximately 3. The enhanced reversible lithium storage capacity in the carbon phase formed in situ within the SiOC matrix may be due to the nanoscale dimensions of the carbon clusters, which allow lithium to be stored not only in the interstitial spaces of the graphite-like sp\({}^{2}\)-carbon structure but also at the edges of the graphene layers. After etching, the lithium storage capacity in the free carbon phase remains unchanged for the SiOC-10 composition, with an x value around 3 (LiC\({}_{3}\)). However, for the C-rich SiOC-200 sample, the x value increases from 3.7 in the non-etched sample to 5.4 after etching, approaching the lithium storage capacity of commercial graphite (x = 6) [70].  
Increasing the free carbon content by modifying the Si-polymer with DVB improves the electrochemical performance of the resulting SiOC and SiCN ceramics, but only in the case of intrinsically carbon-poor preceramic polymers. The introduction of carbon into carbon-rich PDCs may induce structural changes in the ceramic matrix, which stabilizes the free carbon. However, if the carbon content exceeds a certain threshold, the stabilization effect may be lost [70].  
In addition, free carbon can also be used as an active substance to store lithium-ions, increasing the reversible capacity of the material. [PERSON] [1] pointed out that although the carbon content of C/Si-O-C anode materials does not always account for the majority of the composition, the contribution of free carbon in the lithium intercalation process still accounts for most of the reversible capacity of the whole system. This shows that free carbon not only plays an important role in electrical conductivity, but also contributes significantly to improving the lithium storage performance of materials.