Oxide Nanoparticles: Electrochemical Performance
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Although intensively studied in a variety of fields such as catalysis, magnetism, and optics, oxide nanoparticles have just been modestly considered within the field of energy storage until these last few years that have witnessed intense developments, mainly regarding Li-ion battery technology as devoted herein.
For the last 20 years, it was a common and well-accepted belief that highly divided materials could not be suitable for extended reversible redox reaction with metallic lithium. This seems quite astonishing because most primary and secondary lithium electrochemical cell devices are based on redox reactions involving interfacial reactions between a liquid organic-based electrolyte and a solid electrode material. In fact, this idea was rooted on the belief that electrochemically driven irreversible decomposition of the organic-based electrolyte occurs at the surface of the active particles while the cell is cycled, together with possible dissolution of the solids. Thus, the higher the surface of contact between the particles and the electrolyte, the higher would be the extent of this capacity loss, hence the requirement of large particles with low surface area. Along that line, the BET surface areas of powders presently used in commercial Li-ion cells do not exceed 2 m2/g. Recently, these prevailing ideas were seriously contradicted by several findings linked to the recent interest in nanopowders, as we will discuss in this paper. Owing to the present staggering trend toward nano objects in various research fields, too often driven by funding opportunities rather than true science, much confusion has surged about the real meaning of “nano.” Undoubtedly, the world of the so-called nanosciences is still waiting for a concrete and universal definition or at least an accurate, related size scale. Thus, we talk about nanopowders, nanostructures/nanocomposites, nanotextures, and nanoarchitectured electrodes by reference to materials having a single component, two or more components intimately mixed at the nanometric scale, a porous electrode having pores and components in the nanometric scale, and finally a well-designed (either by template deposition or by lithography) two-component system at the nanometric scale. Basically, we are prone to think that a given object, observed for a given property with a specific characterization mean, enters the “nanoworld” as soon as its size reaches a value below which the studied property starts to drastically differ from that of bulk. For instance, the significant decrease in Au particles' melting point as their particle size becomes smaller than 50 Å perfectly illustrates this point, and for such a phenomenon 50 Å will be the threshold value delimiting the nano/macro worlds. It is therefore unfortunate that we do not have such an equivalent to Plank's constant that neatly separates classical mechanics from quantum mechanics.
As we march from bulk materials toward systems with small particles, new electrochemical effects are recently observed and these require the formulation of a new theoretical foundation on which to base our interpretation and understanding of the experimental data. A theoretical approach was recently undertaken and the effect of surface, particle size toward the evolution of electrochemical reactions considered. A profound effect on the chemical potential–composition curves and energy sites was demonstrated. Moreover, a decrease in domain size results in a dramatic increase in grain boundaries that were shown, by acting as space charge regions, to be beneficial to ionic conductivity. However, the particle size is not the only parameter that can account for the modification of properties. Indeed, the size of the crystallized reacting domains (crystallites), the specific surface area of the powder, the porosity, and the possible confinement of the matter can have a drastic influence as well.
It is surprising that the effect of the size of the reacting domains on the electrochemical properties remains almost uninvestigated, while most of the scientific fields are now, as a whole, turning toward the nanomaterials. Given the implications of the energy storage in our evolving modern society (communication, biocompatible devices), we definitively have to consider this rich domain of opportunities that lies beyond the use of nanomaterials in this field, as illustrated by the following examples.