Stabilizing Effects of Phosphorus-Doped Silicon Nanoparticle Anodes in Lithium-Ion Batteries

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.