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Inorganics 2017, 5, 25 10 of 17 3.3. Olivine-Like Materials LiMPO4 (M = Fe, Mn) olivine materials used as electrodes of high-power sources (in hybrid Inorganics 2017, 5, 25 10 of 17 electric vehicles, for instance) request an increase of rate capability that could be achieved by reducing the particle size of electrochemically active materials. The decrease as much as possible of the size of the particle size of electrochemically active materials. The decrease as much as possible of the size of the particles has two effects; first, it increases the effective electrode-electrolyte contact that is the active the particles has two effects; first, it increases the effective electrode-electrolyte contact that is the interface for electrochemical reactions, secondly, it reduces the pathway for electrons and lithium ions active interface for electrochemical reactions, secondly, it reduces the pathway for electrons and inside the bulk. Consequently, the electronic and ionic conductivity are small [40], this reduction is lithium ions inside the bulk. Consequently, the electronic and ionic conductivity are small [40], this expected to be beneficial to the performance, especially at high C-rates. The experimental results, reduction is expected to be beneficial to the performance, especially at high C-rates. The experimental however, are not as simple as one might have expected because the reduction in size implies that results, however, are not as simple as one might have expected because the reduction in size implies surface effects become more important, and the surface layer does not necessarily have the same that surface effects become more important, and the surface layer does not necessarily have the same properties as the bulk, which impacts the electrochemical properties. The bulk properties (i.e., physical properties as the bulk, which impacts the electrochemical properties. The bulk properties (i.e., and chemical properties of particles big enough so that surface effects are negligible) are now well physical and chemical properties of particles big enough so that surface effects are negligible) are understood. This is not the case, yet, for surface effects that are still under debate. now well understood. This is not the case, yet, for surface effects that are still under debate. Several experiments have provided evidence of the existence of a disordered layer (DSL) at the Several experiments have provided evidence of the existence of a disordered layer (DSL) at the surface of particles of oxide [41], typically in a surface layer a few nanometers thick, which modified surface of particles of oxide [41], typically in a surface layer a few nanometers thick, which modified the intrinsic properties of electrode material for Li-ion batteries [41,42]. However, the quality of the the intrinsic properties of electrode material for Li-ion batteries [41,42]. However, the quality of the particle surface state and particle morphology are primordial, because the surface-to-volume ratio particle surface state and particle morphology are primordial, because the surface-to-volume ratio increases for nanomaterials (see Figure 1) that prevents the surface to act as a barrier for lithium increases for nanomaterials (see Figure 1) that prevents the surface to act as a barrier for lithium ions ions and electrons during the charge and discharge process of lithium batteries. Figure 11 depicts and electrons during the charge and discharge process of lithium batteries. Figure 11 depicts a simple a simple model of the shell-core volume ratio, R model of the shell-core volume ratio, RSC, for a 5-nm thick surface layer: in the case of a big enough , for a 5-nm thick surface layer: in the case of a big = 3%, while it becomes 49% for 25 nm diameter nanoparticle. SC enough particle (0.5 μm diameter) R particle (0.5 μm diameter) RSC = 3%, while it becomes 49% for 25 nm diameter nanoparticle. In this SC In this context, it is obvious that the nanoparticle behaves differently than the big one. The strong context, it is obvious that the nanoparticle behaves differently than the big one. The strong dependence of the surface chemistry of active particles on the capacity retention of cathode materials dependence of the surface chemistry of active particles on the capacity retention of cathode materials has been suggested by Aurbach et al. [43]. It was states that any particles of the insertion compound, has been suggested by Aurbach et al. [43]. It was states that any particles of the insertion compound, are always covered by a surface film that limits the migration of Li-ions and the charge transfer across are always covered by a surface film that limits the migration of Li-ions and the charge transfer across the active interface. This can be evidenced by high-resolution transmission electronic microscope the active interface. This can be evidenced by high-resolution transmission electronic microscope (HRTEM) images, which show a surface layer (SL) of about 2 nm thick. Generally, the SL is strongly (HRTEM) images, which show a surface layer (SL) of about 2 nm thick. Generally, the SL is strongly disordered, but not amorphous [41]. HRTEM experiments display the core of the particles below the disordered, but not amorphous [41]. HRTEM experiments display the core of the particles below the surface that has a size similar to the coherence length deduced from the XRD analysis. Therefore, surface that has a size similar to the coherence length deduced from the XRD analysis. Therefore, the the particles are well crystallized surrounded by a disordered surface layer (DSL). Figure 11 displays particles are well crystallized surrounded by a disordered surface layer (DSL). Figure 11 displays the the scanning electron microscopy (SEM) images of two types of LFP particles: one was synthesized scanning electron microscopy (SEM) images of two types of LFP particles: one was synthesized by by solid-state reaction via the polymer-precursor method showing particles of 2–5 μm size, and the solid-state reaction via the polymer-precursor method showing particles of 2–5 μm size, and the other other one was synthesized by hydrothermal method showing particles with an average size of 300 nm. one was synthesized by hydrothermal method showing particles with an average size of 300 nm. The The modified Peukert plots for these two electrode materials are shown in Figure 12. It is remarkable modified Peukert plots for these two electrode materials are shown in Figure 12. It is remarkable that that the power-grade powders retained 75% of the initial capacity at 10 C-rate. the power-grade powders retained 75% of the initial capacity at 10 C-rate. Figure 11. SEM images of LiFePO4 powders. (a) energy grade and (b) power grade powders. Figure 11. SEM images of LiFePO4 powders. (a) energy grade and (b) power grade powders.PDF Image | Nanotechnology of Positive Electrodes for Li-Ion Batteries
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