Due to their great success for portable electronic devices, lithium-based batteries are becoming increasingly important for powering (hybrid) electric vehicles. Yet, in order to achieve driving ranges comparable to those provided by combustion engines, substantial improvements regarding their energy density are required. Concerning the anode side, most commercial lithium-ion batteries use carbonaceous materials and silicon material seems promising due to its high theoretical specific capacity (up to 3580 mAh.g^-1) and its relatively low operational potential. However, the huge volume expansion of silicon alloys upon lithiation (up to 300%) leads to electrical contact loss and pulverization of the material, as well as a continuous consumption of lithium due to the decomposition of the electrolyte on the renewed fresh surface of Si exposed by the cracks of the particles volume expansion. Thus, it leads to poor cycling stability and rapid capacity fading. These effects can be partially counteracted by using nanometer sized Si particles (Si-NPs) that can better sustain the mechanical strains induced by the heterogeneous volume changes. In addition, the particle expansion could be limited by carbon shell coating onto the nanoparticles, which can enhance the electronic conductivity and stabilize the initially formed solid-electrolyte interphase (SEI).

In an attempt to combine both approaches, we have synthetized using a laser pyrolysis technique carbon-coated (Si@C-NPs) or non-coated Si nanoparticles (Si-NPs) [1] [2]. As it was shown that both surface state and crystallinity of the Si-NPs govern the electrochemical behavior of the materials [1] [2], both amorphous and crystalline cores were prepared. Transmission electron microscopy (TEM) was performed on native NPs (see figure 2c and d for illustration on crystalline Si NPs) and revealed the core-shell structure. Electrodes with the various NPs samples (see table 1) were processed and tested in coin cell configuration versus lithium foil. The electrochemical behavior was monitored and it was shown that carbon coating leads to improved capacity upon cycling (see figure 1).

To characterize the morphology of the various electrodes upon cycling and its impact on performances, we have carried out in-depth microstructural investigation of NPs (shape, size, core-shell structure) using ex situ small-angle neutron scattering (SANS) at the Institut Laue-Langevin (Neutron reactor-Grenoble). The SANS technique is particularly suited to investigate such complex systems, because neutrons are highly sensitive to lithium (by contrast to X-rays) and can provide detailed chemical/microstructural information hardly attainable using TEM on cycled electrodes. Taking advantage of the contrast variation method (i.e. use of specific isotopic mixtures of solvent to selectively match a compound in multicomponent systems) we were able to unravel the effect of carbon coating on the NPS morphology at different (de-)lithiation stages (see figure 2a and b for results on electrodes with crystalline Si NPs). In particular, we show that Si@C NPs exhibit a different mechanism of Li insertion compared to non-coated NPs. This work elucidates the microstructural evolution of NPs used in battery electrodes and therefore helps in designing innovative nanomaterials for high density Si-based anodes.

References:

[1] Sourice, J.; Quinsac, A.; Leconte, Y. ; Sublemontier, O. ; Porcher, W. ; Haon, C. ; Bordes, A. ; De Vito, E. ; Boulineau, A. ; Jouanneau Si Larbi, S. ; Herlin-Boine, N. ; Reynaud, C., Appl. Mater. Interfaces, 2015, 7, 6637-6644.

[2] Sourice, J.; Chimie. University of Paris-Sud, 2015. (Thesis)