Large Impact of Particle Size on Insertion Reactions. A Case for Anatase Li<i>_x</i>TiO₂

TLDR

Insertion reactions are critical for Li‑ion and hydrogen storage materials and energy‑storage devices. Surface strain between intercalated phases becomes energetically prohibitive in small particles, explaining the size effect. Smaller anatase TiO₂ particles show higher Li capacity and solubility, with the phase diagram shifting at ~40 nm, resulting in solid‑solution behavior that deviates from bulk phase separation.

Abstract

Insertion reactions are of key importance for Li ion and hydrogen storage materials and energy storage devices. The particle size dependence of insertion reactions has been investigated for lithiated anatase TiO2, revealing progressively increasing Li capacity and Li-ion solubility for decreasing particle sizes, strongly deviating from the expected Li-rich and Li-poor phase separation as occurs in the bulk material. The phase diagram alters significantly, changing the materials properties already at sizes as large as 40 nm. A rationale is found in the surface strain that occurs between the different intercalated phases, which becomes energetically too costly in small particles. In particular the observed particle size-induced solid solution behavior is expected to have fundamental and practical implications for two-phase lithium or hydrogen insertion reactions.