It's hard to keep track of all the future battery technology candidates, but lithium-air battery technology is among the most widely-researched.
Its biggest draw is the potential to store three times the energy in batteries the same size and weight of today's electric vehicles--providing huge increases in range.
In a lithium-air battery, lithium oxidizes at the anode, forming lithium ions and electrons. Ions pass through the electrolyte to the cathode, and during recharging, the process is reversed.
Sounds simple--and with over 300 research papers published on lithium-air batteries in the last three years, its effectiveness--when perfected--is in little doubt.
The challenges are "formidable" though, according to Jun Lu, leader of a team of researchers from Argonne National Laboratory, Beijing Institute of Technology and Hanyang University.
Among the challenges published in the ACS journal, Chemical Reviews, are a need to better understand Li−O2 electrochemistry, develop new and improved cell materials, and innovate in critical aspects of cell design.
One of the main challenges facing lithium-air batteries is that lithium is very reactive, particularly in water--and as lithium-air batteries draw in fresh air for oxygen, moisture in the air can also be drawn in.
"The successful development of any aqueous Li−air batteries severely relies on the prevention of direct contact of the lithium metal electrode with water," writes Lu and his colleagues.
A main priority therefore is to find an electrolyte that's non-combustible, and research oxygen-selective membranes that ensure only necessary elements are admitted.
Four main types of electrolyte are being tested; aprotic, aqueous, solid-state, and hybrid aqueous/aprotic.
Aprotic is currently compromised by the search for a stable electrolyte, as current solutions decompose too quickly. Lithium salts are a potential stability-boost for such chemistries.
Aqueous is a real challenge, given lithium's reactivity. Here, researchers are exploring lithium-conducting glass ceramics to separate the solution--but those two have issues with fragility and resistance.
Porous metal and carbon electrodes are also being investigated, and "wonder material" Graphene has previously been explored for its excellent conductivity and strength.
The upshot--should any of these technologies prove worthwhile--is a battery with theoretical energy density of almost 11,700 Wh/kg--close to gasoline's 13,000 Wh/kg.
Usable energy density for each is much lower, but the new batteries could still get close to matching gasoline fuel--opening up the potential for electric cars with internal combustion-matching range.