Size Effects in the Li4+xTi5O12 Spinel

The nanosized Li4+xTi5O12 spinel is investigated by electrochemical (dis)charging and neutron diffraction. The near-surface environment of the nanosized particles allows higher Li ion occupancies, leading to a larger storage capacity. However, too high surface lithium storage leads to irreversible capacity loss, most likely due to surface reconstruction or mechanical failure. A mechanism where the large near-surface capacity ultimately leads to surface reconstruction rationalizes the existence of an optimal particle size. Recent literature attributes the curved voltage profiles, leading to a reduced length of the voltage plateau, of nanosized electrode particles to strain and interface energy from the coexisting end members. However, the unique zero-strain property of the Li4+xTi5O12 spinel implies a different origin of the curved voltage profiles observed for its nanosized crystallites. It is proposed to be the consequence of different structural environments in the near-surface region, depending on the distance from the surface and surface orientation, leading to a distribution of redox potentials in the near-surface area. This phenomenon may be expected to play a significant role in all nanoinsertion materials displaying the typical curved voltage curves with reduced length of the constant-voltage plateau.