Muon studies of Li+ diffusion in LiFePO4 nanoparticles of different polymorphs

Abstract

The lithium diffusion in nanostructured olivine LiFePO₄ has been investigated for the first time using muon spectroscopy (μSR). A microwave-assisted approach has been employed for nanoparticle preparation, where the choice of solvent is shown to play an important role in determining particle morphology and crystal chemistry. Two phases have been obtained: Pnma LiFePO₄ and the high pressure Cmcm phase. The Li⁺ diffusion behaviour is strikingly different in both phases, with D_Li of 6.25 × 10⁻¹⁰ cm² s⁻¹ obtained for Pnma LiFePO₄ in good agreement with measurements of bulk materials. In contrast, Li⁺ diffusion is impeded with the addition of the high pressure Cmcm phase, with a lower D_Li of 3.96 × 10⁻¹⁰ cm² s⁻¹ noted. We have demonstrated an efficient microwave route to nanoparticle synthesis of positive electrode materials and we have also shown μSR measurements to be a powerful probe of Li⁺ diffusion behaviour in nanoparticles.