Lithium insertion in nanostructured titanates

Upon nano-sizing of insertion compounds several significant changes in Li-insertion behavior have been observed for sizes below approximately 50 nm. Although the origins of the phenomena are interrelated, the changes can be divided in three main observations. (1) The formation of new phases, leading to enhanced reactivity and extended capacities, which is most likely due to the lesser role of kinetic restrictions, and easier accommodation of strain in nanoscale compounds. (2) Thermodynamic changes due to the relatively increased impact of both surface and interface energy, and (3), the excess Li-storage on the surface of the particles in the form of Li2O. These three phenomena have a positive effect on the performance of the electrode material in a Li-ion battery application. The higher Li-capacities provide the electrode material with a higher energy density. Furthermore, the enhanced extend of the solid solution regime observed at lower Li-capacities, due to changes in the particle’s thermodynamics, supports better ionic mobility in the absence of a rate-limiting phase boundary. However, the relatively increase op the surface also enhances the negative effects, such as the formation of a solid electrolyte interface (SEI). Also, a Li-rich surface layer inherently shows poor Li-ion mobility, and effectively acts as a block for further intercalation of the bulk of the particle. The detailed insight in the nano-size related surface storage mechanisms discussed in this thesis forms an important basis for understanding the properties of nano-sized insertion materials. This knowledge supports future design of nano-sized electrode materials for Li-ion batteries and H-storage devices, potentially paving the way for the manufacturing of environmentally clean and highly efficient full-electrical cars.Radiation, Radionuclides & ReactorsApplied Science

Size Effects in the Li4+xTi5O12 Spinel

The nanosized Li(4+x)Ti(5)O(12) 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 Li(4+x)Ti(5)O(12) 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

Dynamic Solubility Limits in Nanosized Olivine LiFePO(4)

Because of its stability, nanosized olivine LiFePO(4) opens the door toward high-power Li-ion battery technology for large-scale applications as required for plug-in hybrid vehicles. Here, we reveal that the thermodynamics of first-order phase transitions in nanoinsertion materials is distinctly different from bulk materials as demonstrated by the decreasing miscibility gap that appears to be strongly dependent on the overall composition in LiFePO(4). In contrast to our common thermodynamic knowledge, that dictates solubility limits to be independent of the overall composition, combined neutron and X-ray diffraction reveals strongly varying solubility limits below particle sizes of 35 nm. A rationale is found based on modeling of the diffuse interface. Size confinement of the lithium concentration gradient, which exists at the phase boundary, competes with the in bulk energetically favorable compositions. Consequently, temperature and size diagrams of nanomaterials require complete reconsideration, being strongly dependent on the overall composition. This is vital knowledge for the future nanoarchitecturing of superior energy storage devices as the performance will heavily depend on the disclosed nanoionic properties

Size Effects in Li4+xTi5O12 Spinel

RRR/Radiation, Radionuclides and ReactorsApplied Science