In Situ Observation of Single-Phase Lithium Intercalation in Sub-25-nm Nanoparticles

Many lithium-storage materials operate via first-order phase transformations with slow kinetics largely restricted by the nucleation and growth of a new phase. Due to the energy penalties associated with interfaces between coexisting phases, the tendency for a single-phase solid-solution pathway with exceptional reaction kinetics has been predicted to increase with decreasing particle size. Unfortunately, phase evolutions inside such small particles (tens of nanometers) are often shrouded by electrode-scale inhomogeneous reactions containing millions of particles, leading to intensive debate over the size-dependent microscopic reaction mechanisms. This study provides a generally applicable methodology capable of tracking lithiation pathways in individual nanoparticles and unambiguously reveals that lithiation of anatase TiO₂, previously long believed to be biphasic, converts to a single-phase reaction when particle size reaches ≈25 nm. These results imply the prevalence of such a size-dependent transition in lithiation mechanism among intercalation compounds and provide important guidelines for designing high-power electrodes, especially cathodes.

An unexpected single-phase solid-solution lithiation pathway is directly revealed in sub-25-nm anatase TiO₂ nanoparticles by tracking the electron diffraction pattern of individual particles in a multiparticle system, which demonstrates much faster kinetics compared to that in a two-phase reaction. These results imply the prevalence of a size-dependent transition in the reaction pathway among electrode materials exhibiting poor two-phase reaction kinetics.

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