Metal–air electrochemical cell

A metal–air electrochemical cell is an electrochemical cell that uses an anode made from pure metal and an external cathode of ambient air, typically with an aqueous or aprotic electrolyte. During discharging of a metal–air electrochemical cell, a reduction reaction occurs in the ambient air cathode while the metal anode is oxidized. The specific capacity and energy density of metal–air electrochemical cells is higher than that of lithium-ion batteries, making them a prime candidate for use in electric vehicles. While there are some commercial applications, complications associated with the metal anodes, catalysts, and electrolytes have hindered development and implementation of metal–air batteries. Lithium–air battery The remarkably high energy density of lithium metal (up to 3458 Wh/kg) inspired the design of lithium–air batteries. A lithium–air battery consists of a solid lithium electrode, an electrolyte surrounding this electrode, and an ambient air electrode containing oxygen. Current lithium–air batteries can be divided into four subcategories based on the electrolyte used and the subsequent electrochemical cell architecture. These electrolyte categories are aprotic, aqueous, mixed aqueous/aprotic, and solid state, all of which offer their own distinct advantages and disadvantages. Nonetheless, efficiency of lithium–air batteries is still limited by incomplete discharge at the cathode, charging overpotential exceeding discharge overpotential, and component stability. During discharge of lithium–air batteries, the superoxide ion (O_2^−) formed will react with the electrolyte or other cell components and will prevent the battery from being rechargeable. Sodium–air batteries were proposed with the hopes of overcoming the battery instability associated with superoxide in lithium–air batteries. Sodium, with an energy density of 1605 Wh/kg, does not boast as high an energy density as lithium. However, it can form a stable superoxide (NaO_2) as opposed to the superoxide undergoing detrimental secondary reactions.