Metal-air batteries are capable of packing far more energy into a battery than lithium-ion batteries and, as a result, solve the two leading issues hindering the dissemination of electric vehicles: low energy density and cost.
Compared to their combustion counterparts, electric motors are lightweight, powerful, and provide full rotational force starting from zero rpm. These specs may be excellent on paper, but the low energy density of lithium-ion batteries limits storage to 250 Wh/kg at best, where gasoline condenses up to 13,000 watt-hours per kilogram. On the flipside, the energy density of metal-air batteries ranges from the 1,200 Wh/kg of iron-air, to the 13,300 Wh/kg of beryllium-air.
With the average range being 5 or 6 km/kWh and current lithium-ion batteries priced at $200 to $300 per kilowatt-hour, an 800 km (500 miles) journey will require a 150-kWh battery costing $30,000 to $45,000. Considering that high-performance combustion engine vehicles like Ford Mustang GT fall below $40,000, the cost per kilowatt-hour must drop below $100 for electric vehicles to gain a wider competitive foothold. The $35,000 Tesla Model 3 is a step in the right direction, but much of the cost is taken up by the battery.
The most promising solution for this predicament comes in the form of metal-air batteries and their ability to charge and recharge using an electrochemical reaction, instead of the intercalation process that takes place within lithium-ion batteries. In lithium-ion batteries, the lithium ions travel from one electrode to the other across a porous polymeric separator embedded within the electrolyte. The direction of the ions depends on whether the battery is charging or discharging, but ions crossing the electrolyte are squeezed between the layers of the electrode in a process called intercalation.
How a lithium-air battery works
By contrast, metal-air batteries do not ferry lithium ions through the porous polymeric separator occupying the electrolyte, but instead perform a true electrochemical reaction. Lithium ions released by the metallic lithium anode travel through the electrolyte and react with oxygen at the cathode, forming lithium peroxide (Li2O2) on its surface. The accumulation of the peroxide reverses itself once recharging occurs. For this reason, the capacity of lithium-air batteries is commensurate with the size of the cathode’s surface area, not the volume of the anode and cathode as is the case in lithium-ion. This results in a much higher energy density with less mass.
The commercialization of metal-air batteries is expected to take at least a couple of years, with a few early contenders already in the process of testing their proof of concept. Whether or not electric vehicle manufacturers embrace the technology remains to be seen, especially given that big players like Tesla Motors are heavily invested in the manufacturing of Lithium-ion batteries.