Carbon nanotubes have promised some incredible advances in power efficiency, but one of the most promising (and most realistic) is boosting the capacity of lithium-ion batteries by a factor of two in the near term, and eventually by five. Suddenly, your electronics that last all day will be lasting all week instead.
Lithium-ion batteries use anodes made of graphite to store charge. Graphite works fine for this, but only a sixth of the carbon atoms are able to hang on to a lithium ion, which makes for an inefficient energy density. Using something like silicon, on the other hand, gets you four lithium ions binding to every one silicon atom, boosting energy density by up to 10 times. Superb! Wonderful! Fabulous!
So where's my lithium-silicon battery, huh? Problem is, when silicon atoms suck up lithium ions like that, they swell up to four times their initial volume and the entire battery pack expands. It shrinks again when you discharge it, but this expansion and contraction rapidly causes permanent damage to the battery and renders it useless. Bad times.
Like almost every other problem, this issue can be solved with nanotechnology. Researchers at SLAC (the Stanford Linear Accelerator Center, now called the Stanford National Accelerator Laboratory or SNAL, I guess) have made a silicon battery anode out of custom-designed double-walled silicon nanotubes coated with silicon oxide. As this nanotube anode charges up, the silicon atoms in the nanotube start to expand, but the silicon oxide coating forces them to expand inward into the hollow center of the tube where they can't do any damage. This solves the entire problem, and testing has shown that batteries with this technology inside can easily survive 6,000 charge/discharge cycles with 85% capacity remaining.
The big hurdle now is to find a cheap and efficient way to manufacture the nanotubes, but Stanford has already licensed the relevant patents out to a company (founded by the researchers themselves) called Amprius, which has a "near-term goal" of producing a battery with double the energy density of the batteries that we're all using right now. Additional research into high capacity nanotech anodes should boost that density figure to an increase of five times, but that's probably a bit father off. And anyway, wouldn't you be more than happy with having all of your electronics last twice as long as they do now? Sure you would!