Short-range Li diffusion vs. long-range ionic conduction in nanocrystalline lithium peroxide Li2O2-the discharge product in lithium-air batteries

Understanding charge carrier transport in Li2O2, the storage material in the non-aqueous Li-O-2 battery, is key to the development of this high-energy battery. Here, we studied ionic transport properties and Li self-diffusion in nanocrystalline Li2O2 by conductivity and temperature variable Li-7 NMR spectroscopy. Nanostructured Li2O2, characterized by a mean crystallite size of less than 50 nm as estimated from X-ray diffraction peak broadening, was prepared by high-energy ball milling of microcrystalline lithium peroxide with ism sized crystallites. At room temperature the overall conductivity a of the microcrystalline reference sample turned out to be very low (3.4 x 10(-13) S cm(-1)) which is in agreement with results from temperature-variable Li-7 NMR line shape measurements. Ball-milling, however, leads to an increase of a by approximately two orders of magnitude (1.1 x 10(-10) S cm(-1)); correspondingly, the activation energy decreases from 0.89 eV to 0.82 eV. The electronic contribution sigma(eon), however, is in the order of 9 x 10(-12) S cm(-1) which makes less than 10% of the total value. Interestingly, Li-7 NMR lines of nano-Li2O2 undergo pronounced heterogeneous motional narrowing which manifests in a two-component line shape emerging with increasing temperatures. Most likely, the enhancement in sigma can be traced back to the generation of a spin reservoir with highly mobile Li ions; these are expected to reside in the nearest neighbourhood of defects generated or near the structurally disordered and defect-rich interfacial regions formed during mechanical treatment.