Adhesion and Surface Layers on Silicon Anodes Suppress Formation of c-Li_3.75Si and Solid-Electrolyte Interphase

Abstract

The formation of c-Li3.75Si is known to be detrimental to silicon anodes in lithium-ion batteries. To suppress the formation of this crystalline phase and improve the electrochemical performance of Si-based anodes, three approaches were amalgamated: addition of a nickel adhesion sublayer, alloying of the silicon with titanium, and addition of either carbon or TiO2 as a capping layer. The silicon-based films were analyzed by a suite of methods, including scanning electron microscopy (SEM) and a variety of electrochemical techniques, as well as X-ray photoelectron spectroscopy (XPS) to provide insights into the composition of the resulting solid-electrolyte interphase (SEI). A nickel adhesion layer decreased the extent of delamination of the silicon from the underlying copper substrate, compared to Si deposited directly on Cu, which resulted in less capacity loss. Alloying of silicon with titanium (85% silicon, 15% titanium) further increased the stability. Finally, capping these multilayer electrodes with either a thin 10 nm layer of carbon or TiO2 resulted in the best electrode behavior and lowest cumulative relative irreversible capacity. TiO2 is slightly more effective in enhancing the capacity retention, most likely due to differences in the resulting solid-electrolyte interphase (SEI). The combination of an adhesion layer, alloying, and surface coatings shows a cumulative suppression of the formation of c-Li3.75Si and SEI, resulting in the greatest improvement of capacity retention when all three are incorporated together. However, these strategies appear to only delay the onset of the c-Li3.75Si phase; eventually, the c-Li3.75Si phase will form, and at that point, the capacity degradation rate of all the electrodes becomes similar.

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

“…For x = 0.75, the amorphous peak around 0.30 V is the lowest in the second cycle, and therefore, the amount of c-Li 15 Si 4 is the highest. The doublet structure of the c-Li 15 Si 4 peak has also been observed for C/Si multilayers and Si−Ti alloy films, 17 although the spacing between the peaks is much less than the 40 mV separation we observed in Si/C multilayer structures. 47 Although the c-Li 15 Si 4 peak height increases up to cycle 10, it also narrows and a broad peak around 0.30 V, characteristic of delithiation of amorphous Li x Si, appears immediately after cycle 2 and starts to increase in height as well.…”

“…It is worth noting here that for x = 24,42 and (2) mechanical stress, which might arise as a void space is filled by SEI buildup that constrains silicon upon expansion. Although thin capping layers have been shown to provide enough stress to suppress c-Li 15 Si 4 formation, 17 the SEI is unlikely to be sufficiently mechanically robust to play this role on its own.…”

“…For instance, Xie et al showed that the onset or acceleration of capacity degradation in 100 nm thick Si films on Cu coincides with the onset of c-Li 15 Si 4 formation. 17 Suppression of c-Li 15 Si 4 to improve capacity retention can be achieved by keeping the lithiation voltage above the onset of the c-Li 15 Si 4 transition, typically above 50 mV. 12,42,43 Higher charging rates will also prevent crystallization to c-Li 15 Si 4 , and at sufficiently high rates, c-Li 15 Si 4 will not form even if the cell is discharged to 0 V vs Li.…”

“…11,12 Various strategies have been employed to mitigate fracture and disconnection, such as optimizing electrolyte composition, 13,14 including binders and conductive carbon additives, 15,16 as well as adding protective surface layers encapsulating the silicon. 17,18 Other strategies include changing the Si morphology to nanowires, 19−21 100 nm, 22−24 and introducing porosity to better accommodate volume expansion. 25−29 Targeted efforts to reduce fracture and improve capacity retention require an understanding of the alloying processes that occur as the voltage is lowered toward 0 V vs Li during lithiation and as the voltage is increased again during delithiation.…”

Stabilizing Tin Anodes in Sodium-Ion Batteries by Alloying with Silicon

“…Figure c shows the relative irreversible capacity loss for the SEI formation (RIC SEI ) for all of the studied electrodes. The RIC SEI is calculated according to the difference between the sodiation capacity at cycle n + 1, Q n +1 Sod , and delivered desodiation capacity of the previous cycle, Q n Desod , relative to the desodiation capacity at the n th cycle according to eq . − Comparing the RIC SEI for the electrodes run at a cutoff voltage of 0.8 V Na , with and without a CV step, illustrates a higher irreversible capacity loss for the formation of SEI when a CV step is applied and thus clearly illustrates the consequence of the anodic surface reaction occurring at the cutoff voltage on the electrolyte decomposition. In addition, we also calculated the relative irreversible capacity loss for the delivered capacity at the end of desodiation, RIC Desod (Figure d), according to eq . − The RIC Desod simply represents the relative capacity decay per cycle and is an approximate measure of the accumulation of surface species that isolate the active materials either electronically or ionically and/or the physical disconnection of the active materials . As shown in Figure c, all of the studied electrodes show an initial decrease in RIC SEI , which is more noticeable for the electrode cycled up to a 1 V Na cutoff voltage.…”