Very sexy build photos!! I asked on their post, but I'll ask here too: "How about LiFePO4 batteries instead?" They're slightly lower energy density, but much safer that LiPoly. It's the type of batteries we preferred for big robots...
I think you didn't read too closely - that's what they chose.
>Cells
After realizing the inherent risks in some lithium chemistries and finding it difficult to source low volumes of high-power cells, we decided to use a different lithium-ion chemistry known as lithium iron phosphate (LFP or LiFePO4).
