Mixed aqueous/aprotic

Schematic of a mixed aqueous-aprotic type Li-Air battery design

The aqueous/aprotic or mixed Li-air battery design is an attempt to unite advantages of both the aprotic and aqueous battery designs. Although there exist multiple hybrid designs, the common feature of these designs is a separated two-part (one part aqueous and one part aprotic) electrolyte connected by a lithium-conductingmembrane. A lithium metal anode is in contact with the aprotic side of the electrolyte while the porous cathode is in contact with the aqueous side. A lithium-conducting ceramic is typically employed as the membrane joining the two electrolytes.

Solid state

Schematic of solid-state type Li-Air battery design

The solid-state battery design eliminates problems at the anode/cathode interfaces associated with using a liquid electrolyte. It is also attractive from a safety standpoint as organic solvents, currently used in lithium-ion batteries (and employed in the aprotic Li-air battery design), are flammable and at high temperatures the use of an organic electrolyte can lead to rupture and ignition of the battery. 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 which enhance charge transfer at the anode and electrochemically couple the cathode to the electrolyte. The polymer-ceramic composites serve to reduce the overall impedance of the solid-state Li-air battery. The main drawback of the solid-state battery design is the low conductivity of most glass-ceramic electrolytes. Lithium aluminum germanium phosphate has been found to be an effective electrolyte, but the ionic conductivity of current lithium fast ion conductors are still lower than liquid electrolyte alternatives.

Challenges

There are many challenges facing the design of Li-air batteries, which currently limits their use to the laboratory. One of the largest challenges lies in keeping the battery protected from the environment. Atmospheric oxygen must be present at the cathode, but the cathode can be degraded by humidity.

Cathode

Most of the current limitations in Li-air battery development are seen at the cathode. One of the problems seen is incomplete discharge due to blockage of the porous carbon cathode with discharge product. The effect of pore size and pore size distribution is still poorly understood. Production of a cathode with high a pore size and ability to hold a large amount of Li₂O₂ is essential to Li-air battery development. Catalysts have shown promise in creating preferential nucleation of Li₂O₂ over Li₂O, which is irreversible with respect to lithium.

Anode

The current anode of choice in Li-air batteries is metallic Li, as Li offers the highest energy density. Due to the reactive nature of Li, the main challenge in anode development is preventing the anode from reacting. New interfacial materials or solid-state electrolytes will prevent anode degradation. Another area of concern when using metallic lithium cathodes is dendrite formation, which will lead to a short circuit within the battery.

Electrochemical

In current cell designs, the charge overpotential is much higher than the discharge overpotential. The presence of a significant charge overpotential indicates secondary reactions, besides recharging, are occurring. As a result, the electrical efficiency is only around 65%. There is some indication that catalysts, such as MnO₂, Co, Pt, and Au can reduce the overpotentials, but the effect is still poorly understood. In addition, significant drops in cell capacity with increasing discharge rates have been observed by many researchers. The decrease in cell capacity is attributed to kinetic charge transfer limitations. Since the anodic reaction occurs very quickly, the charge transfer limitations are thought to occur at the cathode. Again, electrocatalysts could improve the charge transfer rate.

Environmental

Long term battery operation requires chemical stability of all the components of the cell. Current cell designs show poor resistance to oxidation by the reaction products and intermediates. Many aqueous electrolytes are also volatile, and can be lost over time. One of the largest barriers to fully operable commercial cells is the development of effective environmental interfaces. Atmospheric oxygen is intrinsically required for cell operation, but the cell must be shielded from the environment, as water vapor can rapidly degrade the system.

Applications

The primary application for Li-air battery development is in automotive. The high specific energy densities and volumetric energy densities required for next-generation hybrid and electric vehicles are beyond current battery designs. Li-air batteries are attractive for any application where weight is a primary concern, such as in mobile devices. Flow batteries, such as the vanadium redox battery may offer better performance for applications such as off-grid power storagе.

Date: 2015-12-11; view: 778