Beyond Thin Films: Clarifying the Impact of c-Li₁₅Si₄ Formation in Thin Film, Nanoparticle, and Porous Si Electrodes

Abstract

The formation of the c-Li15Si4 phase has well-established detrimental effects on the capacity retention of thin film silicon electrodes. However, the role of this crystalline phase with respect to the loss of capacity is somewhat ambiguous in nanoscale morphologies. In this work, three silicon-based morphologies are examined, including planar films, porous planar films, and silicon nanoparticle composite powder electrodes. The cycling conditions are used as the lever to induce, or not induce, the formation of c-Li15Si4 through application of constant-current (CC) or constant-current constant-voltage (CCCV) steps. In this manner, the role of this phase on capacity retention and Coulombic efficiency can be determined with few other convoluting factors such as alteration of the composition or morphology of the silicon electrodes themselves. The results here confirm that the c-Li15Si4 phase increases the rate of capacity decay in planar films but has no major effect on capacity retention in half-cells based on porous silicon films or silicon nanoparticle composite powder electrodes, although this conclusion is nuanced. Besides using a constant-voltage step, formation of the c-Li15Si4 phase is influenced by the dimensions of the Si material and the lithiation cutoff voltage. Porous Si films, which, in this work, comprise primary Si particle sizes that are smaller than those in the preformed Si nanoparticle slurries, do not undergo the formation of c-Li15Si4 at 50 mV, whereas Si nanoparticle slurries are accompanied by the formation of c-Li15Si4 up to 80 mV. The solid-electrolyte interphase (SEI) formed from reaction of the c-Li15Si4 with the carbonate-based electrolyte causes polarization in both nanoparticle and porous film silicon electrodes and lowers the average Coulombic efficiency. A comparison of the cumulative irreversibilities due to SEI formation between different lithiation cutoff voltages in silicon nanoparticle slurry electrodes confirmed the connection between higher SEI buildup and formation of the c-Li15Si4 phase. This work indicates that concerns about the c-Li15Si4 phase in silicon nanoparticles and porous silicon electrodes should mainly focus on the stability of the SEI and a reduction of irreversible electrolyte reactions.

Easy Diameter Tuning of Silicon Nanowires with Low-Cost SnO2-Catalyzed Growth for Lithium-Ion Batteries

“…This plateau was also visible to a lesser extent for 30 nm–SiGt. This effect is related to the formation of the c-Li 15 Si 4 phase [ 11 , 44 , 45 ] at the end of lithiation, which is favored in bigger objects. The formation of c-Li 15 Si 4 is attested by a characteristic plateau at 0.45 V in delithiation in the voltage-capacity profiles ( Figure S9a–c ).…”

Si/CrSi2 Alloy Anodes Synthesized by a High-Energy Ball-Milling Method for Lithium-Ion Batteries: Microstructure, Electrochemistry, and Carbon Coating

“…35 Moreover, the delithiation peak of cr-Li 3.75 Si is more obscure in the following cycles (even for raw Si), which may be due to the high sensitivity to the increased current rate. 6,36,37 From the 2nd cycle, the voltage− capacity curves for all samples become more sloped and correspondingly two pairs of lithiation−delithiation peaks appear in the dQ/dV curves, where the transformation of a-Si/ a-Li 2 Si happens at ∼0.23/0.5 V and the reaction of a-Li 2 Si/a-Li 3.5 Si is assigned to ∼0.1/0.3 V. 38 On the other hand, the 1st reversible capacity and initial Coulombic efficiencies (ICEs) of the ball-milled precursors strongly rely on the added Cr ratio or formed CrSi 2 content, decreasing from 3153 mA h•g −1 /88.11% in mSi to 1323 mA h• g −1 /82.35% in Si 85 Cr 15 . The reduced capacity in Si/CrSi 2 alloys is a result of the active Si being consumed for producing the relatively inert CrSi 2 phase.…”

On the Practical Applicability of the Li Metal‐Based Thermal Evaporation Prelithiation Technique on Si Anodes for Lithium Ion Batteries

“…The fact that c‐Si at 2θ ≈ 28.6° still exists after the disappearance of c‐Li 15 Si 4 and does not transform to amorphous a‐Si is counter‐intuitive to what is typically reported in the literature. [ 42 ] This different behavior is probably because the lithiation happens only on the top layer of the electrode and cannot fully lithiate all “µ‐Si” particles within the electrode.…”

“…As previously reported, the formation of c‐Li 15 Si 4 has been linked to an increased capacity loss upon cycling due to the dynamic change and disconnection of Si materials. [ 42 ] For long‐term cycling, the formation of c‐Li 15 Si 4 entailing severe volume changes will be a critical issue. To further demonstrate the impact of cell balancing in prelithiated cells, the cell with a balancing of 1.2 shows worse Initial Coulombic efficiency (IC Eff ) and worse delithiation capacity in the first cycle as compared to the cells with a balancing of 6.8 (Figure S7, Supporting Information).…”