In the case of silicon nanoparticles, the role of particle size with regards to suppression of the c-Li15Si4 phase and improving capacity retention has been more contentious. It is now well established that nanoparticles below 60 nm form the c-Li15Si4 phase less readily than larger particles. It is easy to confuse the fracture effects of the initial c-Si → a-LixSi transition, which also diminish with decreasing size, and those associated with formation of c-Li15Si4. However, the c-Li15Si4 phase is still formed when fracture during the first lithiation is eliminated by using material below a critical size (~150 nm for c-Si nanoparticles and ~300 nm for c-Si nanowires). The multiple impacts of particle size complicate experiments to determine the intrinsic effects of the c-Li15Si4 phase in silicon nanoparticles. For example, Gao et al. demonstrated that capacity loss with 120 nm Si nanoparticles was associated with formation of the Li15Si4 phase at 0 mV vs. Li. However, the difference in the quantity of c-Li15Si4 formed was correlated with the use of fluoroethylene carbonate as an electrolyte additive, which is known to be beneficial in and of itself. Smaller 60 nm silicon nanoparticles did not show any formation of the c-Li15Si4 phase, even at 0 mV in ex-situ diffraction measurements on fully lithiated material. It was assumed that the increased surface area compared to larger particles prevented the detection of the c-Li15Si4 phase because of the enhanced reactivity of c-Li15Si4 compared to a-LixSi as derived from static leakage current measurements. Obrovac et al. also pointed out "polarization induced c-Li15Si4 suppression" in the case of nanosized electrodes, which showed no formation of the c-Li15Si4 phase near 0 V. Schott et al. observed minimal loss of capacity retention associated with the appearance of c-Li15Si4 in silicon/graphite electrodes, when using 5-10 wt% silicon relative to graphite in a blended electrode. For the smallest nanoparticles, 30-50 nm, the influence of the c-Li15Si4 phase on capacity retention, tested by comparing CC vs. CCCV lithiation, was minimal. In fact, CC cycling consistently showed worse capacity retention, irrespective of cut-off voltage and particle size, which means the CV step likely helps to reduce the effects of increasing polarization. In the present work, we attempt to establish a generalization of the role of the formation of the c-Li15Si4 phase on capacity retention for three different Si morphologies: planar films, porous films, and commercially available Si nanoparticles. From the literature, the detrimental effect of c-Li15Si4 is well-established for thin films, but its effect is less clear for nanoparticles of silicon and porous films. We confirm that the c-Li15Si4 phase has a significant negative impact on capacity retention of planar films, but has only a small impact on capacity retention in silicon nanoparticles. The upper voltage limit for c-Li15Si4 formation is ~80 mV in Si nanoparticles and is dependent on size. We find evidence for higher reactivity of c-Li15Si4 with the electrolyte compared to a-LixSi by comparing cumulative irreversibilities associated with SEI build-up in silicon nanoparticle electrodes with different lithiation cut-off voltages.