Field of Expertise: Advanced Material Science
| |
Field of Expertise: Advanced Material Science | |
Research on Li-air Batteries: Ionic Conductivity of Nanocrystalline Lithium Peroxide Li2O2 Lithium air batteries cause tremendous interest not only from the part of scientific community but also from an industrial point of view. The specific energy of lithium air batteries is 5 to 10 times that of a conventional lithium-ion battery [1,2]. Key applications in the long term include electric vehicles as well as the storage of electricity from intermittent sources such as wind, solar, and tidal. On discharge, at the positive electrode O2 from the atmosphere enters the porous electrode, dissolves in the electrolyte within the pores, and is reduced at the electrode surface forming a solid product, Li2O2, that can be oxidised upon charging [2,3]. So far, these cells do not reach their theoretical capacity limit, and achieve only moderate rates due to the insulating nature of Li2O2. This is because the growing Li2O2 film at the porous carbon electrolyte interface may hamper charge transport from a critical film thickness onwards. There is a lively debate on charge transport properties in Li2O2 with partly contra¬dicting views stemming from theory which predicts either low polaron migration barriers or metallic surface conduction in same crystal faces [4-6]. In this contribution, charge transport in both microcrystalline and nanocrystalline Li2O2 was studied by impedance spectroscopy over a large frequency range (10 Hz to 20 MHz) and temperature range (– 150 °C to 300 °C). The nanocrystalline sample, characterised by a mean crystallite size of less than 20 nm as estimated from X-ray diffraction line broadening, was prepared by high-energy ball milling of the microcrystalline source material consisting of µm-sized crystallites. The overall con-ductivity σ of the microcrystalline sample turned out to be very low which is in agreement with results from temperature-variable 7Li NMR line shape measurements. However, ball-milling leads to an increase of σ by approximately two orders of magnitude; correspondingly, the activation energy decreases from 0.89 eV to 0.82 eV. |