Lithium–air battery

Originally proposed in the 1970s as a possible power source for battery electric vehicles, and hybrid electric vehicles, Li–air batteries recaptured scientific interest late in the first decade of the 2000s due to advances in materials science. Although the idea of a lithium–air battery was around long before 1996, the intrinsic poor conductivity of the charged Li2O2 species are major challenges. == Design and operation ==

In general lithium ions move between the anode and the cathode across the electrolyte. Under discharge, electrons follow the external circuit to do electric work and the lithium ions migrate to the cathode. During charge the lithium metal plates onto the anode, freeing at the cathode. The effects of pore size and pore size distribution remain poorly understood. Most Li–air battery limits are at the cathode, which is also the source of its potential advantages. Atmospheric oxygen must be present at the cathode, but contaminants such as water vapor can damage it. Electrolyte Efforts in Li–air batteries have focused on four electrolytes: aqueous acidic, aqueous alkaline, non-aqueous protic, and aprotic. In a cell with an aqueous electrolyte the reduction at the cathode can also produce lithium hydroxide: Aqueous An aqueous Li–air battery consists of a lithium metal anode, an aqueous electrolyte and a porous carbon cathode. The aqueous electrolyte combines lithium salts dissolved in water. It avoids the issue of cathode clogging because the reaction products are water-soluble. Another issue is that organic electrolytes are flammable and can ignite if the cell is damaged. However, the design required pure oxygen, rather than ambient air. Solid state A solid-state battery design is attractive for its safety, eliminating the chance of ignition from rupture. Current solid-state Li–air batteries use a lithium anode, a ceramic, glass, or glass-ceramic electrolyte, and a porous carbon cathode. The anode and cathode are typically separated from the electrolyte by polymer–ceramic composites that enhance charge transfer at the anode and electrochemically couple the cathode to the electrolyte. The polymer–ceramic composites reduce overall impedance. The main drawback of the solid-state battery design is the low conductivity of most glass-ceramic electrolytes. The ionic conductivity of current lithium fast ion conductors is lower than liquid electrolyte alternatives. == Challenges ==

As of 2013, many challenges confronted designers. Generally, they fall into either surface passivation or pore clogging, which are confronted below. Long-term battery operation requires chemical stability of all cell components. Current cell designs show poor resistance to oxidation by reaction products and intermediates. Many aqueous electrolytes are volatile and can evaporate over time. A parameter, Da, was defined to measure the variations of temperature, species concentration and potentials. In addition to the blockage of electron flow via the formation of an insulating product, cycling Li-air batteries results in the clogging of pores meant for oxygen diffusion. The chemistry of a standard Li-air battery will inevitably produce lithium peroxide, but the effects of pore size and pore size distribution remain poorly understood. However, the modulation of pore size has resulted in drastic effects on cell capacity. Catalysts have shown promise in creating preferential nucleation of over , which is irreversible with respect to lithium. Atmospheric oxygen must be present at the cathode, but contaminants such as water vapor can damage it. Electrochemistry In 2017 cell designs, the charge overpotential is much higher than the discharge overpotential. Significant charge overpotential indicates the presence of secondary reactions. Thus, electric efficiency is only around 65%. Catalysts such , Co, Pt and Au can potentially reduce the overpotentials, but the effect is poorly understood. Several catalysts improve cathode performance, notably , and the mechanism of improvement is known as surface oxygen redox providing abundant initial growth sites for lithium peroxide. It is also reported that catalysts may alter the structure of oxide deposits. Significant drops in cell capacity with increasing discharge rates are another issue. The decrease in cell capacity is attributed to kinetic charge transfer limits. Since the anodic reaction occurs very quickly, the charge transfer limits are thought to occur at the cathode. == Advancements ==

Pore size modulation The research towards deciphering the impacts of pore size and distribution remain ongoing, but some conclusions have been made, especially regarding sets of pores smaller than 100 nm. In cells using cathodes made from Super P and Ketjen Black, for example, conclusions have been made linking to discharge being stopped in Li-air batteries due to the loss of surface area near the air inlet. As the battery is used, Lithium peroxide deposits along the walls of pores, gradually sealing them. The reason for this focus on pores smaller than 100 nm is because smaller pores seem to be preferable in spite of their small size being easy to seal up with discharge products. == Applications ==

Vehicles Li–air cells are of interest for electric vehicles, because of their high theoretical specific and volumetric energy density, comparable to petrol. Electric motors provide high efficiency (95% compared to 35% for an internal combustion engine). Li–air cells could offer range equivalent to today's vehicles with a battery pack one-third the size of standard fuel tanks assuming the balance of plant required to maintain the battery was of negligible mass or volume. Grid backup In 2014, researchers from the Ohio State University announced a hybrid solar cell-battery. Up to 20% of the energy produced by conventional solar cells is lost as it travels to and charges a battery. The hybrid stores nearly 100% of the energy produced. One version of the hybrid used a potassium-ion battery using potassium–air. It offered higher energy density than conventional Li-ion batteries, cost less and avoided toxic byproducts. The latest device essentially substituted lithium for potassium. The solar cell used a mesh made from microscopic rods of titanium dioxide to allow the needed oxygen to pass through. Captured sunlight produced electrons that decompose lithium peroxide into lithium ions, thereby charging the battery. During discharge, oxygen from air replenished the lithium peroxide. == See also ==