posted on 2023-01-03, 14:03authored byIsabelle
P. Gordon, Wei Xu, Sophia Randak, T. Richard Jow, Nicholas P. Stadie
Phosphorus-doped silicon has been reported to exhibit
improved
cycling stability and/or higher capacity retention than pure silicon
as the anode in lithium-ion batteries. However, crystallite size and
particle morphology are difficult to decouple from compositional tuning
during chemical modification. In this work, we explore direct solid-state
routes to phosphorus doping of silicon powders relevant to electrochemical
applications. A wide range of compositions are assessed, from 0.05
to 3.0 at % P, as well as a wide range of silicon starting materials
of varying crystallinity, particle size, and particle morphology.
Successful incorporation of phosphorus into the silicon lattice is
best confirmed by X-ray diffraction; the Si(111) reflection shifts
to higher angles as consistent with the known lattice contraction
of 0.002 Å per 1 at % phosphorus. The addition of phosphorus
to Si nanoparticles (100–500 nm) in the high doping regime
causes grain coarsening and catalyzes an increase in crystallinity.
On the other hand, dilute doping of phosphorus can be carried out
without great alteration of the nanoparticulate morphology. The opposite
effect occurs for very large microparticles (>10 μm), whereby
the doping is concomitant with a disruption of the crystal lattice
and reduction of the crystallite size. These effects are borne out
in both the electrochemical stability over long-term cycling in a
lithium-ion half-cell as well as in the thermal stability under high-temperature
decomposition. By comparison across a wide range of pure and P-doped
materials of varying particle and crystallite sizes, the independent
effects of doping and structure on thermal and electrochemical stability
are able to be decoupled herein. A stabilizing effect is most significant
when phosphorus doping is dilute and heterogeneous (surface-enriched)
within the silicon nanoparticles.