[Nanosized Insertion Materials for Li-Ion Batteries]Introduction: Energy Storage in Li-Ion

Electrochemical storage is attractive, having very high storage efficiencies typically exceeding 90%, as well as relatively high energy densities

Schematic principle of a Li-ion battery

Only Li ions can flow through the electrolyte, and the charge compensation electrons have
to follow the Li ions via the external circuit, which can be used to power an application

By application of a higher electrical potential than the spontaneous equilibrium open circuit polarization, the process can be reversed. High energy density requires a large specific capacity of ions in both electrodes and a large difference in chemical potential. High power requires both electrons and Li ions to be highly mobile throughout the electrode materials and electrolyte

 - Recent research has focused on nanosizing of electrode materials holding the promise of larger (dis)charge rates because it reduces the length of the rate-limiting diffusion pathway of Li-ions and electrons through the electrode material.

• The downside of the large surface area of nanostructured materials is the relative instability of nanomaterials promoting electrode dissolution and the increased reactivity toward electrolytes at voltages below 1 V vs Li/Li+, which may adversely affect the Li-ion battery performance
• Another potential disadvantage is the less dense packing leading to lower volumetric energy densities. Among the materials that benefit from the possibilities of nanosizing are the relatively stable transition metal oxides and phosphates operating well within the stability window of the electrolyte

Voltage profiles of different particle sizes for LixFePO4, anatase LixTiO2, and spinel Lix+4Ti5O12

The structural impact of nanosizing in the various insertion materials determined by neutron diffraction. (a) Calculated solubility limits in olivine LixFePO4 based on the diffuse interface in excellent agreement with the diffraction data. (b) Solubility limits in anatase LixTiO2where α,β, and γ represent anatase, lithium-titanate, and LiTiO2respectively. (α)+(β) and (α+β) refer to the situation that each particle either has phase α or β and that both phases coexist within one particle, respectively. (c) Li occupancy of the 8a (closed symbols) and 16c (open symbols) sublattices in spinel Li4+xTi5O12.

The fundamental question is: are these changes simply due to the more abundant surface area and the trivial shorter diffusion distances, or does nanosizing additionally alter critical materials properties such as defect chemistry and thermodynamics in a nontrivial way?