Understanding the Limits of Lithium-Air Batteries – NMR and Thermodynamic Studies
Lithium-air batteries promise to deliver exceptionally high energy density while only using common materials, such as carbon, in their cathode structure. To do this, they oxidise a metallic lithium anode to release Li+ ions, which combine with O22- ions, produced from the reduction of atmospheric oxygen. However, such batteries are yet to be commercialised due to problems in cell operation, such as their high overpotentials, poor rate capabilities and, most critically, poor cell lifetimes. This work sets out to quantify the realistic expectations that should be had of a lithium-air battery should they be realised and the cell geometry and support systems such a battery would likely need. It goes on to discuss the theory of the chemical structural motifs that promising new solvents would likely have. To aid in studying the breakdown products formed in the lithium air batteries, which limit their lifetime, operando 17O nuclear magnetic resonance was developed. This technique can non-destructively and in real time track and quantify the formation and removal of all common breakdown products, this information challenging to access by any other technique. Operando diffraction can in principle access it, however it typically requires a synchrotron and if often limits to crystalline products. Operando Raman is typically surface sensitive and chemical tests are destructive. Here 17O NMR is used to investigate the relative contributions of singlet oxygen, chemical and electrochemical breakdown to the observed decomposition products in the cell. To support this work Gaussian Process regression was utilized. It was found that Gaussian processes can also be used to denoise NMR data, matching or outperforming current denoising methods in many cases. Finally, a potential additive to the electrolyte, lithium iodide, is discussed. Lithium iodide had previously been proposed to reduce the charge overpotential and switch the discharge product in the battery to LiOH, thereby avoiding many of the corrosive species formed in the cell. Here, the thermodynamics and kinetics associated with this reaction are explored, and the range of conditions where this reaction is possible is discussed.
Batteries, Breakdown mechanism, Gaussian processes, Lithium-air, Nuclear Magnetic Resonance, Operando NMR and electrochemistry, Redox mediators, Thermodynamics
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