In the rush to rid the world of fossil fuels, there’s an incredible amount of research and development into new battery technologies being carried out at universities and research institutions around the globe.
Some make use of exotic new cell chemistries, while others focus on increasing the energy density and power capabilities of existing cell chemistries through the use of nanotechnology. Other research programs emphasise recyclability or the development of non-toxic chemistries, while others are more focused on speed charge and discharge characteristics to make the electric cars and gadgets of the future recharge in seconds rather than hours.
At the end of last week however, we had a new contender in the sphere of advanced battery research: a new breakthrough which could make it possible to produce lithium-oxygen batteries with the same energy density as gasoline. What’s more, at a fifth of the cost and a fifth of the weight of existing lithium-ion cell technology, this breakthrough could make it possible for automakers to build cars capable of driving the 410 miles between London and Edinburgh without a recharge.
Enter Materials Chemistry Professors Dr. Tao Liu and Dr. Clare Grey of Cambridge University and their team of researchers, who have developed a brand-new lithium-air battery cell technology which is not only super energy-dense but offers an incredible 93 percent energy efficiency. For those unfamiliar with that term, it means that 93 percent of all the energy put into the battery during charging can be retrieved during discharging. The best lithium-ion cells used today have energy efficiencies of somewhere between 80 and 90 percent.
As those familiar with battery technologies will tell you, lithium-air batteries aren’t new, being first proposed more than 40 years ago as a possible power source for electric vehicles. They work by using oxidation of lithium at the anode and reduction of oxygen at the cathode to induce a current flow.
Pulling in oxygen from the air outside the battery and passing it through a catalyst-enhanced cathode during discharging and then releasing oxygen back outside the battery during charging, the achilles heel of lithium-air batteries has to date been the number of charge and discharge cycles they can happily endure, since the porous cathode through which oxygen is passed can all-too easily become clogged with both Lithium peroxide from inside the battery and other elements (like nitrogen and carbon dioxide) outside the battery. This, plus other unwanted, extraneous chemical reactions inside lithium-air batteries have resulted in an extremely short cell life span, translating to batteries which become unusable after a handful of charge-discharge cycles.
The new chemistry developed by Liu and Grey’s team is different. Using sheets of highly-porous graphene just one atom thick as the cathode material combined with additives that alter the chemical reactions in the battery itself to improve energy efficiency and cell stability, the researchers say they they have a demonstrator lithium-oxygen cell which could change the way we think about energy storage.
Aside from a new cathode material and stabilizing additives, the Cambridge research team have replaced the traditional non-aqueous lithium-air battery design with one which uses lithium hydroxide rather than lithium peroxide. Inside, adding water and lithium iodine as a ‘mediator,’ and changing the chemical makeup of the electrolyte through which ions pass during charge and discharge cycles, the team discovered that the destructive chemical reactions which caused other lithium-air batteries to quickly die were dramatically reduced. Additionally, the cell performed far more efficiently and offered a far lower voltage gap between charge and discharge of the cells.
That smaller the voltage gap between a charged and discharged battery translates to a more efficient battery. Previous lithium air battery chemistries had managed to get the gap down to somewhere between 0.5 and 1.0 volts, but Liu and Grey says their battery offers a voltage gap of just 0.2 volts.
On paper, things are looking up for the research team, especially given the fact the new lithium-oxygen cell — after taking into account system losses in an electric vehicle — should offer the same kind of energy density to the wheels of an electric car as gasoline. For those who are curious, best estimates (sourced indently) equate to around 12 kWh per kilogram. While gasoline has an energy density of 13 kWh/kg, it loses much of that energy during use in an internal combustion engine vehicle due to the massive inefficiencies in the combustion cycle and drivetrain of most cars. An electric motor, being more efficient, actually results in an energy-to-the-wheels energy density of around 1.7 kWh/kg for both.
So far, so good. Sadly however, as with many other impressive technological breakthroughs, there are some pretty big hurdles Dr Liu, Dr. Grey and their team must overcome before the battery cell chemistry is ready for commercial application.
Firstly, while the highly-porous graphene electrode increases the energy capacity of the lab-based demonstration cells, it only does so at certain rates of charge and discharge. Additionally, the researchers say they need to figure out a way of protecting the metal electrode so it doesn’t form lithium dendrites with continued charge and discharge cycles. While cell cycles with this new chemistry are far superior to previous lithium-air battery chemistries, dendrites can eventually cause batteries to short circuit and explode if they get too long. For commercial use, that’s something the researchers say they need to tackle.
Finally, while the demonstrator can theoretically be used in regular air, it’s currently being cycled in pure oxygen, as the particulates and other compounds present in the air we breathe can still harm the porous graphene electrode.
Yet with the largest challenges of lithium-air technology now solved, the researchers say they are confident that solutions to these particular problems will be found.
If that happens, then it will only be a matter of time before the technology can be used in a commercial setting, assuming continued investment and of course, mass-production methods can be devised for this interesting new battery chemistry.
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