Yes, diamonds.
Super Capacitor Could Power Phone, Laptop for Days
"If commercialized, the new super capacitor could be fully charged in a second and, coupled to a normal battery, provide enough energy to power a cell phone for weeks or a laptop battery for days."
"The researchers then transformed those nanodiamonds into dozens or even hundreds of graphene layers, all nestled inside one another like little Russian dolls."
Now we need to get the ereader of our choice down below $50 and lower than 3 oz.