19040
The open - ended graphene shell with defects allows fast electrochemical lithiation / delithiation , and the void space inside the graphene shell accompanied by its strong mechanical strength can effectively accommodate the volume expansion of Si upon lithiation .
 graphene, graphene, Si
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 graphene, graphene, Si