Rechargeable lithium batteries for energy storage in smart grids

Nanotechnology of Positive Electrodes for Li-Ion Batteries

“…The same material prepared by a green technique using a mixed chelator composed by bio-reagents such as those that exhibit a smaller crystallite size of 6.4 nm, which results in primary nanorods of~17 nm in diameter and 84 nm in length. Slight modifications of the synthesis recipe induce different morphologies of nanostructured α-MnO 2 ; for instance, sea-urchin-like shape, with a diameter of~3 µm, were prepared in acidic conditions [13], while a neutral medium provided a caddice-clew-like MnO 2 showing lower electrochemical performance [14]. The use of mixed chelator has been successful for the growth of Li-riche layered compounds.…”

“…Thus, at high current densities, i.e., J > 1C rate (the rate is denoted C/n, where C is the theoretical cathode capacity and a full discharge occurs in n hours), the poor electrochemical performance is attributed to the slow electron transport of the material and the sluggish Li-ion kinetics within the grains. The currently adopted approach to get high rate capability is to reduce the diffusion path length of charge species by minimizing the particle size of the active phase [2,3]. The smaller the particle size, the larger the surface area over volume ratio, as shown in Figure 1 in which a 3 × 3 cube is compared to a single element.…”

“…In this case, the surface/volume ratio is increased from 2 to 6. The characteristic time τ for an ionic species i (in practice Li + ions in the present case) to reach the surface of an active particle of dimension L is given by the relation [2]: The first equation is the Fick's law that applies when the motion of the ion is a diffusion process, with D* is the chemical diffusion coefficient of moving ions. Such is the case when the chemical reaction proceeds by a single phase (sp) process, i.e., within a solid solution.…”

“…On another hand, in a two-phase (tp) process, where there is a separation between a Li-rich and Li-poor phase instead of a solid solution, the chemical reaction proceeds by the nucleation of the phases and motion of the propagation of the boundaries that separate the two phases. In that case, the characteristic time is given by the Equation (2), where Vm is the molar mass of the active compound, σ i the ionic conductivity and Δμ i the difference of the chemical potential of ions between the two phases. In both cases, however, τ is proportional to L 2 .…”

“…In addition to the effect of the particle size and particle size distribution on the insertion mechanism, we must also consider the effect of the high specific surface area of nanoparticles (safety problems) and the minimization of the volume expansion upon Li-ion insertion. The characteristic time τ for an ionic species i (in practice Li + ions in the present case) to reach the surface of an active particle of dimension L is given by the relation [2]:…”