The large volume changes upon cycling imply that the SEI can be broken as the nanostructure shrinks during delithiation. This re-exposes the fresh Si surface to the electrolyte and more SEI forms, resulting in a thicker and thicker SEI film upon charge/discharge cycling and, thus, aging [
85]. Stabilizing the SEI is thus crucial. This is partly done by the appropriate choice of the binder. The best one for Si-anodes is alginate which is helpful in building a deformable and stable SEI [
86]. In addition, coating the silicon structure with a protective element helps in the formation of a stable SEI layer [
87,
88]. A thick shell is preferred to avert fracture of the nanoparticles, but a thin shell reduces the loss of weight. Therefore, careful engineering of the core and shell materials is required to obtain an optimal balance, not only to optimize (and thus control) the thickness of the shell, but also the choice of the material that must have the appropriate Young’s modulus [
89,
90].
The traditional process for any active material used in Li-ion batteries, whether it is used on the cathode or the anode side, is to coat the particles with conductive carbon. The same was tried with Si particles. Si-C composite particles in which silicon nanoparticles are embedded in porous carbon particles [
91] and porous Si-C composite nanospheres [
92] were synthesized. A successful control of the SEI growth of porous nanotubes was obtained by coating them with rigid carbon [
93]. Embedding Si nanowires in a network of carbon nanotubes is also a way to improve the conductivity to improve the overall electric conductivity of the anode; moreover, the resulting anode is flexible and self-standing [
94]. In [
95], Si nanoparticles were completely sealed inside thin, self-supporting carbon shells, with void space in between the particles and the shells. Due to the well-defined void space, the Si particles can expand freely without breaking the outer carbon shell that stabilize the SEI on the shell surface. This yolk-shell structured Si electrode exhibits high capacity (2800 mAh·g
−1 at
C/10), long cycle life (1000 cycles with 74% capacity retention), and high coulombic efficiency (99.84%). With 10 nm thick carbon coating on Si nanowires of 90 nm in diameter, the first cycle coulombic efficiency was greatly enhanced from 70% (without coating) to 83%, in addition to the increased capacity from 3125 mAh·g
−1 (without coating) to 3702 mAh·g
−1, and cycling stability (5% capacity retention after 15 cycles) [
96]. However, replacing the carbon coating with 10 nm thick Cu film can further improve the coulombic efficiency of the initial cycle up to 90.3%, and improve capacity retention up to 86% after 15 cycles [
97]. We have already mentioned the results obtained on a Si-C nanocomposite in [
83], but in this work too, the coating with Cu gives better results (
Figure 6). These results illustrate that the traditional slurry coating method exploiting conductive carbon may not be best in the case of Si nanoparticles. To explain this result, one can invoke the fact that Cu is more conductive than carbon; it is also possible that the Cu coat is more protective and stabilizes the SEI more efficiently. In any case, this has been the motivation for trying different coats. In particular, Al-coating proved to be efficient to obtain a more stable mechanical structure of electrodes [
98,
99]. Si nanowires coated with
a ≈ 100 nm layer of Ag/poly(3,4-ethylenedioxythiophene) (PEDOT) exhibited an improvement of the capacity retention from 30% after few cycles, to 80% after 100 cycles, with respect to the same wires before coating [
100]. These coats have in common the fact that they are metallic,
i.e., conducting. However, Al
2O
3 coatings (<10 nm) obtained by ALD have been tested successfully on Si thin films [
101,
102] and Si Nws [
103]. Upon the first lithiation, Al
2O
3 transforms into an Al-Li-O glass [
104], which is a good ionic conductor and an electronic insulator, thus exhibiting the attributes of a good SEI substitute. Indeed, the Al
2O
3 coating resulted in a 45% increase of the anode lifetime, and 1280 cycles at 1
C have been obtained with Al
2O
3 coated Si Nws [
103]. Double walled Si nanotubes have been obtained by coating the Si nanotube with SiO
x [
105]. This coat is rigid enough and mechanically strong, so that it can successfully prevent the Si from expanding outward during lithiation, while still allowing lithium ions to pass through. As a result, the SiO
x-coated Si nanotubes in [
93] demonstrated a long cycle life (6000 cycles with 88% capacity retention), high capacity (2970 mAh·g
−1 at
C/5; 1000 mAh·g
−1 at 12
C), and fast charging/discharging rates (up to 20
C). This result shows that it is now possible to obtain Si anodes with high capacity and good capacity retention for thousands of cycles at the laboratory scale.
Figure 6.
Top: SEM images of the silicon nanowires (Si Nws) (
a) with and (
b) without copper-coating after 100 cycles at the rate of 0.5
C; Bottom: capacity–cycle number curves for Si nanowires (
a) without and (
b) with copper-coating at different rates. Reproduced with permission from [
97]. Copyright Elsevier, 2011.