Silicon@Graphite/Carbon composite anodes: Influence of various pitch derived carbon coatings, Si sizes and concentrations on performance

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In the present study the influence of various compositional parameters for a core-shell structured nano-silicon graphite composite (Si@Gr/C) was studied. At first, the stability of the composite particles was evaluated at different silicon contents. It could be observed that an increase in a higher amount of silicon led to reduced composite stability and a detachment of silicon nanoparticles upon ultrasonic stressing. Furthermore, a linear increase in specific surface area was evident with increasing silicon content. A reduction of the silicon particle size below the known critical particle size of 150 nm resulted on the one hand in an exponential increase of the specific surface area but on the other hand in an improved cycling stability. The capacity retention could be enhanced from 82.2 % at 160 nm to 91.8 % at 120nm after 125 cycles. A similar effect could be obtained after a pitch derived carbon coating (Si@Gr/C), which led to a capacity retention of 91.9 % even for silicon particle sizes of 160 nm. Furthermore, the carbon coating significantly decreased the specific surface area by a factor of three (10 m2 g-1 to 3 m2 g-1), which could be one of the reasons behind the better electrochemical performance. As a result of the improved performance, a study of pitch materials with different particle sizes and softening points have been conducted. It could be shown that the capacity retention is mainly attributed to the softening point, whereas the initial capacity might be more related to the initial capacity. Higher softening points and larger particle sizes (around 7-20 µm) led to the best performance. Finally, a calendering study was carried out in order to evaluate the effect of a carbon coating after densification. The uncoated composite anode suffered from higher capacity fading and reached a capacity retention of only 72.8 % after 125 cycles. In contrast to this, the carbon coating of the Si@Gr/C composite anode was able to stabilize the electrode structure and maintain a good cycling performance reaching a capacity retention of 91.9 %. All in all, the study revealed the effects of silicon particle size, silicon content and a pitch derived carbon coating on particulate properties and performance. A smaller silicon particle size is beneficial in terms of cycling stability, but over-proportionally increases the specific surface are. In order to decrease the specific surface area, a carbon coating could be used, which significantly improved the electrochemical performance, even at high electrode densities.

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