Properties and promises of nanosized insertion materials for Li-ion batteries

The substantial influence of crystallite size on the properties of Li-ion storage materials has spurred intensive research in the emerging area of nanoionics. The development of nanoscale storage materials offers a promising strategy to increase the energy storage capabilities of Li-ion batteries, potentially making them suitable for electric vehicles. Nanosizing, which increases surface area, enhances the importance of interfaces and surfaces on directly observable materials properties such as the voltage profile and the phase diagram. As a result, nanosized materials can show improved storage properties, and materials inactive at the micro size can become excellent storage materials. We suggest novel surface storage mechanisms to explain these phenomena. First-order phase transitions, which are responsible for the batteries' constant voltage output, are partially suppressed at the nanoscale. So far the morphological changes during the phase transition remain unclear. A complete understanding of the equilibrium and non-equilibrium properties of a collection of nanosized electrode particles within an actual electrode remains a formidable challenge. In this Account, we describe the efforts toward understanding the effects of nanosizing and its applications in representative insertion materials. We are particularly interested in the mechanisms and properties that will help to increase the energy storage of Li-ion batteries. We review and discuss the nanosize properties of lithium insertion materials, olivine LiFePO4, and titanium oxides. Although nanosizing intrinsically destabilizes materials, which is potentially detrimental for battery performance, the relative stability of oxide and phosphate insertion compounds makes it possible to exploit the advantages of nanosizing in these materials. The larger capacities and typical voltage profiles in nanosized materials appear to be related to the surface and interface properties that become pronounced at the nanosize, providing a potential means of tailoring the material properties by particle size and shape. The large irreversible capacity at the surface of some materials such as titanium oxides represents a disadvantage of nanosizing, but research is suggesting ways to resolve this problem. The changes in the first-order phase transition upon (de)lithiation could be related to the interface between the coexisting phases. At these interfaces, concentration gradients and strain lead to energy penalties, which significantly influence the thermodynamics of nanomaterial grains. However, it is less clear what nanoscaling effects predominate in the large collection of particles in actual electrodes. The complexity of these materials at the nanoscale and the difficulty in observing them in situ pose additional challenges. Future demands for stored electricity will require significant research progress in both nanomaterials synthesis and in situ monitoring.