  Varying the amounts of silicon and carbon, different composites have been
prepared by ball milling of Si, Ni$_{3.4}$Sn$_4$, Al and C. Silicon and carbon
contents are varied from 10 to 30 wt.% Si, and 0 to 20 wt.% C. The
microstructural and electrochemical properties of the composites have been
investigated by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and
electrochemical galvanostatic cycling up to 1000 cycles. Impact of silicon and
carbon contents on the phase occurrence, electrochemical capacity and
cycle-life are compared and discussed. For C-content comprised between 9 and 13
wt.% and Si-content >= 20 wt.%, Si nanoparticles are embedded in a
Ni$_{3.4}$Sn$_4$-Al-C matrix which is chemically homogeneous at the micrometric
scale. For other carbon contents and low Si-amount (10 wt.%), no homogeneous
matrix is formed around Si nanoparticles. When homogenous matrix is formed,
both Ni$_3$Sn$_4$ and Si participate to the reversible lithiation mechanism,
whereas no reaction between Ni$_3$Sn$_4$ and Li is observed for no homogenous
matrix. Moreover, best cycle-life performances are obtained when Si
nanoparticles are embedded in a homogenous matrix and Si-content is moderate
(<= 20 wt.%). Composites with carbon in the 9-13 wt.% range and 20 wt.% silicon
lead to the best balance between capacity and life duration upon cycling. This
work experimentally demonstrates that embedding Si in an intermetallic/carbon
matrix allows to efficiently accommodate Si volume changes on cycling to ensure
long cycle-life.
