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

Abstract

Understanding charge carrier transport in Li₂O₂, the storage material in the non-aqueous Li-O₂ 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 ⁷Li 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 μm sized crystallites. At room temperature the overall conductivity σ of the microcrystalline reference sample turned out to be very low (3.4 × 10⁻¹³ S cm⁻¹) which is in agreement with results from temperature-variable ⁷Li NMR line shape measurements. Ball-milling, however, leads to an increase of σ by approximately two orders of magnitude (1.1 × 10⁻¹⁰ S cm⁻¹); correspondingly, the activation energy decreases from 0.89 eV to 0.82 eV. The electronic contribution σ_eon, however, is in the order of 9 × 10⁻¹² S cm⁻¹ which makes less than 10% of the total value. Interestingly, ⁷Li 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 σ 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.