Li₂0₂ in Li-0₂ batteries : catalytic enhancement of electrochemical oxidation and thermophysical transformations

Abstract

Electrification of transportation in the United States is of importance in reducing dependence on foreign oil and curtailing global warming. However, optimal market penetration of electric vehicles is confronted with the prohibitive cost and limited energy capacity of current state of the art lithium-ion battery packs, factors which limit range below 300 miles. Lithium-air (Li-air or Li-0 2) batteries could deliver more than three times the gravimetric energy of Li-ion batteries at potentially reduced cost by replacing transition metal oxide cathode with formation of lithium oxides (Li20 2 and Li 20). Being in its infancy, the Li-0 2 technology faces multiple challenges such as inadequate round trip efficiency (below 80%), low power capability, poor cycle life (less than 100 cycles) and thermal safety concerns. This thesis is concerned with the poor oxidation kinetics of the discharge product Li20 2, root cause of poor round trip efficiency, and the thermal stability of the candidate discharge products Li 20 2 and Li20. Catalysis of the Li20 2-oxidation by LaCrO 3, Bao.5Sro.5Coo.8Feo.20 3 (BSCF), LaNiO3, LaMnO3+8, and LaFeO3 was systematically investigated. It was found that LaCrO3, reported with the lowest activity in aqueous OER, shows a threefold higher activity compared to BSCF, reported with two orders of magnitude higher activity in aqueous OER. We postulate that efficient catalysts affect the surface energy landscape of Li20 2 at interfaces to result in larger proportions of low oxidation-overpotential surface orientations and, therefore, enhanced Li2O2-oxidation at lower overpotentials. Regarding the thermal stability of Li20 2 and Li20, X-ray diffraction revealed significant decrease in the c/a ratio of the lattice parameters of Li 20 2 from 280 'C to 700 'C, which are attributed to the transformation of Li 20 2 to Li202-6 . Upon further heating, a lithiumdeficient Li2-6O phase appeared at 300 'C and gradually became stoichiometric upon further heating to ~550 'C. XPS measurements showed growth of Li2CO3 on surfaces of Li20 2 and Li20 at 250 'C attributable to chemical reactions between Li 20 2/Li 2O and carbon-containing species.