Alternating Silicon and Carbon Multilayer-Structured Anodes Suppress Formation of the c-Li_3.75Si Phase

Abstract

Silicon-based anodes for Li-ion batteries have been gaining a great deal of attention due to their high theoretical gravimetric energy density. Approaches for overcoming the challenge of pulverization associated with Sibased electrodes are required for efficient, reversible, and stable operation of such high energy batteries. This study focuses on addressing the source of pulverization of amorphous silicon films upon cycling, which is typically attributed to the formation of the c-Li 3.75 Si phase. Cross-sectional samples prepared by focused-ion beam milling revealed fractured sponge-like silicon structures after 150 cycles at a lithiation cutoff voltage of 5 mV Li , at which the c-Li 3.75 Si phase forms. Cycling at a higher lithiation cutoff voltage, 50 mV Li , however, resulted in a film with a higher degree of integrity, along with the absence of the c-Li 3.75 Si phase. These results clearly verify and underscore the deleterious effects of the c-Li 3.75 Si phase. Alternating carbon and silicon layers results in suppression of the formation of the c-Li 3.75 Si phase to a degree dependent upon the relative thicknesses of both the silicon and carbon layers. Best results were observed for multilayers of 8 nm Si/4 nm C, with which no evidence for the c-Li 3.75 Si phase up to 149 cycles was observed. Carbon interlayers were also found to beneficially lower the relative irreversible capacity loss due to solidelectrolyte interphase formation and associated electrical disconnection.

Adhesion and Surface Layers on Silicon Anodes Suppress Formation of c-Li3.75Si and Solid Electrolyte Interphase

“…To study the effect of the C and TiO2 coatings on the formation of the solid-electrolyte interphase (SEI), the cumulative relative irreversible capacity for SEI formation (∑RIC *+, ) throughout cycling for all of the studied samples was calculated and illustrated in Figure 4. The cumulative RICSEI is calculated as expressed below: 22,42 ∑RIC *+, = . Figure 5 shows the differential capacity plots (dQ/dV vs voltage) for all of the studied samples in Figure 3 -corresponding galvanostatic charge-discharge plots are illustrated in Figure S5.…”

“…Pores and fractures between 'islands' result in additional free volume for expansion in the electrode material, as is visible in the SEM micrographs in Figure 2 and is often observed during lithiation-delithiation cycling of silicon thin films. 22,[48][49][50] Increases in the amount of free volume available subsequently relieve the stresses in the silicon film, therefore, reducing the activation barrier for the formation of the c-Li3.75Si phase. Once a critical amount of free volume is reached the c-Li3.75Si phase can be formed, which then starts a positive feedback loop where the formation of the crystalline phase creates a differentially larger amount of fracturing and free volume; this allows a larger amount of c-Li3.75Si…”

“…20 Compressive stress arising from capping layers in contact with the silicon can similarly disfavour formation of the c-Li3.75Si phase in silicon films or silicon in sheet-like geometries. 22,29 TiN also seems effective at preventing c-Li3.75Si formation in silicon nanowires 30 . Capping layers of Al2O3, [31][32][33] C, 34 TiO2, 33,35,36 and TiN 30,33 have been shown to also alleviate the continuous growth of SEI and improve the Coulombic efficiency (CE) of the silicon electrode.…”

“…The capacity degradation has been attributed to internal stress and associated mechanical fracture that accompanies the amorphous-crystalline phase transition. [20][21][22] In addition, Han et al found that the metastable c-Li3.75Si is particularly reactive towards the electrolytes, suggesting that the formation of the c-Li3.75Si phase may exacerbate the growth of SEI on the Si electrode. [gao2017] To suppress the formation of this c-Li3.75Si phase, researchers have devised a number of approaches, all revolving around inducing stress in the material.…”

“…Application of mechanical stress has been shown to lower the lithiation voltage curve of silicon by ~100-120 mV/GPA. 23 Since c-Li3.75Si only forms below 50 mV vs Li, 19,20,22,24 sufficiently high stress can prevent this phase from forming. In one approach, Obrovac and co-workers prepared a series of alloys of silicon with transition metals including Ti, Ni, and Cu, both as powders and thin films.…”