Lithium-air batteries are superior to gasoline and lithium-ion batteries with regards to energy density. Energy density is the amount of energy that can be stored in a given system per unit of mass or volume. Lithium-air batteries achieve a higher specific energy density using oxygen from the environment rather than from an internal oxidizer. Lithium-air batteries utilize a chemical reaction between the surface of the lithium and the air at the anode to create lithium ions and electrons, which flow across an electrolyte to reduce oxygen at the cathode and create an electrical current. In this process, lithium peroxide (Li2O2) usually deposits on the cathode as a result of the reaction and causes profound degradation of the battery, negatively affecting the ability of the battery to recharge. In lithium-air batteries, titanium dioxide (TiO2) nanotube arrays, with their large surface area and transport channels, accommodate Li2O2 precipitates and aid in the reversal of Li2O2 formation and decomposition that occurs during the discharge and recharge phases of the battery. Adding TiO2 nanotubes to pre-existing graphene cathodes also helps to limit side reactions within the cathode and prevent the decomposition of carbon, which increases the longevity of the cell. With battery technology heralded as the rate-limiting factor for moving the automobile fleet away from internal combustion engines that emit carbon, smog forming compounds, and other air pollutants regulated by the US Environmental Protection Agency, Lithium-air batteries’ improvement to battery viability make electric and hybrid vehicles better able to facilitate this automobile fleet transition.
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Using a lithium anode and carbon based cathode with TiO2 nanotubes enhance the production of electrical current in lithium-air batteries. The metal-air exchange generates higher amounts of energy (theoretical 43.2 MJ/kg v. liquid gasoline 36.4-49.6 MJ/kg) by eliminating the need for internal oxidizers. TiO2 nanotubes improve the integrity of the cell by collecting Li2O2 deposits and decreasing side reactions in the cathode and electrolyte solution.
The addition of TiO2 nanotubes to the cathode of lithium-air batteries preserves the cell by collecting Li2O2 precipitate that could otherwise degrade the porous cathode, and also aids in the reversal of Li2O2 formation and decomposition to prevent deposits. TiO2 nanotubes are more stable, environmentally friendly, easily manufactured, and cost efficient as compared to nanoporous gold particles. The bio-compatibility of these nanotubes due to their transport channels also allows for the introduction of additional catalysts into the cathode, which aid in the oxygen evolution reaction and disrupt the crystalline structure of Li2O2 precipitates.
The predominant design of lithium-air batteries is the aprotic-aqueous model, in which ions and electrons pass through an electrolyte to the cathode. Aprotic (liquid organic electrolyte) and aqueous (lithium salts dissolved in water) solutions are flammable and can rupture and ignite, making the design more conductive than solid-state models, but at increased safety risk.