Looked at from an automotive perspective, Li-air cells could power electric cars for more than 400 miles on a single charge using a battery pack that’s a fifth of the weight of today’s EV batteries. This would thrust electric cars into the same ballpark as gasoline-powered vehicles in terms of the amount of energy derived from a given weight of fuel, erasing range concerns and permitting practical, widespread use of fully electric vehicles.
Today batteries in electric cars can’t compare with gasoline in the amount of energy derived from a given weight of fuel. The Li-air battery has a theoretical specific energy (energy per unit mass) of 3.5 kWh/kg (kilowatt hours per kilogram). By comparison, Li-ion batteries have only 105 Wh/kg (watt hours per kilogram) at the pack level, limiting fully electric cars to about 150 km of driving range. The energy density of gasoline is roughly 13 kWh/kg, of which 1.7 kWh/kg of energy is provided to the wheels after losses.
Accounting for the weight of a full Li-air battery pack (casing, materials, etc.) the energy density will be considerably lower but estimates range up to 1.7kWh/kg to the wheels – right there with gasoline. Given the high efficiency of electric motors that should be sufficient to deliver much longer driving on a single charge than is now possible.
Add to that the issue of lithium-ion batteries overheating and possibly igniting if they’re charged too quickly or charged past their designed capacity. These reasons alone have spurred worldwide efforts to find a better solution.
Though different, Li-air isn’t a full starboard tack away from Li-ion batteries. A Li-ion battery has three basic parts: a positive terminal (cathode), a negative terminal (anode) and an electrolyte. In the batteries we use in our smartphones and laptops, a metal oxide such as lithium cobalt oxide is used to form the cathode. The anode can be formed from materials such as a graphite mixture and the electrolyte is usually a lithium salt dissolved in an organic solvent. A thin separator keeps the electrodes from touching and allows lithium-ions to pass though during charging or discharging. The energy of the battery depends on the movement of these ions between terminals, which generates a current.
While Li-ion batteries can store energy in a small container, their capacity deteriorates with age, and their low energy density means they need to be recharged frequently. Like Li-ion, a Li-air battery also has an anode and a cathode and uses oxidation of lithium at the anode and reduction of oxygen at the cathode to induce a current fl ow. Here, however, the anode is made of lithium metal but the cathode is oxygen pulled from the air. As with the Li-ion battery, the electric current fl ows between the anode and the cathode via a liquid electrolyte.
Realizing the enormous potential of Li-air, however, is a very challenging scientific problem, mostly centered on achieving a high percentage of the theoretical energy density, improving the electrical efficiency of recharging, increasing the number of times the battery can be cycled, and optimizing the complex chemical reactions that occur during charge and discharge to boost battery performance.
But there have been major breakthroughs moving Li-air battery technology from a laboratory curiosity closer to practical implementation. Here are a few examples.
Commercial batteries are self-contained but this has not been the case for lithium-air batteries, early versions of which adopted a so-called “open-cell” design to get enough air in to the reaction point. This oxygen was then released again to the atmosphere during the reverse reaction in the charging cycle. Open systems require the consistent intake of oxygen from the environment, while closed systems do not — making them safer and more efficient.
In the open process, the oxygen also changes states from gaseous to solid and back again during charging and discharging, resulting in huge volume changes (gases occupy more volume than solids) and placing a great deal of mechanical stress on the cell, disrupting electrical conduction paths and possibly causing it to fail prematurely.
The Li-air batteries developed in the lab since the 1970s also wasted much of the injected energy as heat and degraded relatively quickly. To avoid damaging the cell they required expensive extra components to scrub out water from the humid incoming air and filter carbon dioxide from the air that fed the batteries.
By making cells that contain their own oxygen, rather than relying on the proportion of oxygen in the air, MIT Professor Ju Li and his team of researchers (including members from Argonne National Laboratory and Peking University in Beijing), created a much more practical self-contained design. In the MIT approach the oxygen remains inside the cell and in a solid state at all times – the oxygen becomes bound to the lithium to form a glass-like material. These molecules are encased in a matrix of cobalt oxide, forming what researchers call “nanolithia” – nanostructures that act as a catalyst for the chemical reactions that take place in the cell.
This reduces the voltage loss by a factor of five, according to the researchers, so only eight percent of the electrical energy is turned into heat. This development could lead to faster charging for cars, as heat removal from the battery pack becomes less of a safety concern.
In cycling tests, a lab version of the new battery was put through 120 charging-discharging cycles, and showed less than a two percent loss of capacity, indicating that such batteries could have a long useful lifetime. And because these batteries could be installed and operated just like conventional solid lithium-ion batteries, without any of the auxiliary components needed in the past, they could be easily adapted to existing installations or conventional battery pack designs for cars, electronics or even grid-scale power storage.
The MIT team expects to move from this lab-scale proof of concept to a practical prototype within about a year. Another challenge for those exploring Li-air battery chemistries is after a few charging cycles the carbon electrode would become corroded and the electrolytic fluid would decompose. But no one knew exactly why. Now scientists from the Munich Technical University (TUM) and the Jülich Research Center have found the reason for this behavior: during the charging process, highly reactive “singlet oxygen” an extremely reactive substance, is generated. This oxygen corrodes surrounding materials within fraction of seconds, the researchers discovered. It also decomposes electrolytic fluid.
The researchers hope to find a mechanism to prevent the formation of singlet oxygen during charging. One of the most promising approaches is by replacing the lithium cobalt oxide cathode with carbon particles.
Another team, led by Dr. Kyeongjae Cho, a professor of materials science and engineering in the Erik Jonsson School of Engineering and Computer Science at the University of Texas, Dallas, along with his graduate student Yongping Zheng, discovered new catalyst materials for Li-air batteries that could jumpstart efforts at expanding battery capacity.
Their research, published in the journal Nature Energy, focuses on the electrolyte catalysts inside the battery, which, when combined with oxygen, create chemical reactions that form the basis of the battery’s energy capacity. They report that soluble-type catalysts possess significant advantages over conventional solid catalysts, exhibiting much higher efficiency. They found that only certain organic materials can be utilized as a soluble catalyst. Cho and Zheng have collaborated with researchers at Seoul National University in Korea to create a new catalyst for the Li-air battery called dimethylphenazine, which possesses higher stability and increased voltage efficiency.
The researchers say with this new catalyst the Li-air battery should become a more practical energy storage solution. But it could take five to 10 years before their research translates into new batteries that can be used in consumer devices and electric vehicles.
Another new study published in the Journal of The Electrochemical Society (JES) also focuses on catalyst materials and could represent a fundamental step forward in advancing Li-air technology through more efficient electrode reactions in the battery. The study details a cathode catalyst composed of three transition metals (manganese, nickel, and cobalt), which can create the right oxidation state during battery cycling. K.M. Abraham of Northeastern University, co-author of the study, says it represents a fundamental step forward in advancing Li-air that could lead to more efficient electrode reactions.
Li-air is highly researched, but there are still barriers to be overcome before it can go into full-fledged practical use, Abraham notes. He thinks there will be some limited use in the near future in specialty applications, but it could be five years or more before it becomes a fully used technology. Just in time for 5G phones and autonomous electric vehicles.
The liquid inside most lithium-ion batteries is highly flammable. If the battery short-circuits, either by puncturing the thin sheet of plastic separating the positive and negative sides of the battery or because of impurities – it can heat up the liquid electrolyte – and possibly, result in a fire.
Overcharging is another culprit. Lithium-ion batteries have circuitry inside to prevent overcharging and short circuits, but if this circuitry is damaged, the battery could become overheated, resulting in a process known as “thermal runaway.” Basically, the battery generates heat, causing reactions that generate more heat, until it erupts in flames. One advantage of the MIT Li-air variation is that the battery cannot be overcharged, because the reaction is self-limiting. The new battery is protected because when overcharged, the reaction shifts to a di erent form that prevents further activity. The researchers claim they have overcharged the battery for 15 days, to 100 times its capacity, without any damage.
There is growing interest in wearable sensors and flexible displays that need power. Traditional lithium-ion batteries, which are hard and rigid, do not fit the bill. Until recently, neither did Li-air batteries since conventional Li-air cathodes are typically made of rigid materials, such as ceramics encased in fiberglass. If the battery is flexed, the electrolyte—a liquid—would leak out. This problem is compounded when oxygen reacts with lithium at the cathode to produce lithium peroxide, a solid that builds up and pushes out the electrolyte, killing the battery cell.
But if researchers at the Changchun Institute of Applied Chemistry in China have their way, next generation Li-air batteries could power everything from clothing packed with light-emitting diodes (LEDs) to roll-up tablets.
Unlike conventional Li-air batteries, consisting of an anode and cathode made of hard materials, they made the anode from concentric lithium layers and the cathode was constructed using a flexible porous mesh of carbon and nickel. The liquid electrolyte used in conventional batteries was replaced by a soft flexible polymer gel. When this assembly was complete, the researchers gently heated the outer rubber layer, causing it to shrink wrap the rest of the components and packing the layers in close electrical contact.
This prototype battery is reported to have withstood more than 1,000 bending and twisting actions while powering a set of LEDs. They claim these batteries can be recharged 90 times and as a bonus, it is waterproof, working up to five minutes while submerged.
The research team is working to improve the electrodes to limit corrosion, on the configuration of cell assemblies and to optimize the structure of the battery.
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