The lithiation onset of amorphous silicon thin-film electrodes

Abstract

The lithiation onset of amorphous silicon (a-silicon) films up to 10% state of charge (SOC) is characterized by a Li+-uptake region around 0.5 V vs a Li reference electrode. In the literature, this is commonly attributed to surface processes such as the formation of a solid electrolyte interphase layer and/or the reduction of the surface native oxide, and more seldom to bulk processes such as reduction of oxygen contaminations inside the silicon film and to silicon lithiation. This work presents evidence that this process is associated with the lithiation of elemental silicon using electrochemical and non-electrochemical measurements and including a discussion of literature data. Cyclic voltammetry performed on pre-lithiated a-silicon and a-silicon films with different oxygen concentrations and with different film thicknesses reveals a lithiation process that is not associated with the reduction of oxygen. Elemental depth-profiling with secondary ion mass spectrometry (SIMS) reveals a two-phase lithiation mechanism at the lithiation onset, which sharply delimits non-lithiated silicon from a Li-poor phase of constant Li content (Li0.3Si). The published operando neutron reflectometry data suggest that this is also the case for oxygen-free Si single crystal wafers. SIMS measurements further show that the whole a-silicon thin film is converted into the Li-poor phase. This phase remains stable in the vicinity of the current collector even at 100% SOC, indicating that mechanical stress may play an important role.

Nano-FTIR Spectroscopy of the Solid Electrolyte Interphase Layer on a Thin-Film Silicon Li-Ion Anode

“…The thickness of the SEI layer on the 0.3 V electrode is much less compared to the cycled electrode with the crossover between F and Si occurring at 70 s suggesting further growth after the initial passivation of the surface at 0.417 V. The Li content also decreases as the bulk Si is detected and reached 25 at % suggesting that there was also Li insertion into the bulk Si during the reduction peak. This result is supported by secondary ion mass spectroscopy (SIMS) and XPS depth profiling experiments that revealed a composition of Li 0.3 Si after the cathodic peak around 0.4 V in sputtered amorphous Si, suggesting that the onset of the lithiation of amorphous Si first occurs during this cathodic peak. , In addition to the bulk lithiation of the Si, we also find that the onset of SEI film formation also occurs during this 0.417 V peak, suggesting that the first steps of SEI film formation is concomitant with the initial Li insertion into the bulk Si.…”

“…30 The first cycle shows a sharp peak at 0.425 V, which is not present in the subsequent cycles suggesting it is related to passivation of the surface. Based on the similar CV features of sputtered Si thin films as reported by Huger et al, 31 we believe that the peak at 0.425 V is associated with the initial Li insertion into amorphous Si and the beginning of the SEI layer formation, which we will demonstrate in Section 3.3. Figure 2b shows the current response to the long-term hold at 0.05 V and the steady-state parasitic current of 0.3 μA/cm 2 after 12 h. The high initial current (70 μA/cm 2 ) is attributed to lithiation of the Si film at the beginning of the 0.05 V hold while the slowly decreasing current (i.e., parasitic current) is attributed to the irreversible electrolyte reduction and the SEI layer's gradual growth and reformation.…”

“…This result is supported by secondary ion mass spectroscopy (SIMS) and XPS depth profiling experiments that revealed a composition of Li 0.3 Si after the cathodic peak around 0.4 V in sputtered amorphous Si, suggesting that the onset of the lithiation of amorphous Si first occurs during this cathodic peak. 31,60 In addition to the bulk lithiation of the Si, we also find that the onset of SEI film formation also occurs during this 0.417 V peak, suggesting that the first steps of SEI film formation is concomitant with the initial Li insertion into the bulk Si.…”

Lithium Niobate for Fast Cycling in Li-ion Batteries: Review and New Experimental Results

“…Figure 1a shows a scheme of commercial batteries where the negative electrode (called also anode) contains graphite [27,28] as the electrochemically active material. More charge can be stored if the negative electrode encompasses silicon [29,30] (Figure 1b). Figure 1c schematically shows a LIB whose negative electrode contains various kinds of LiNbO 3 entities mentioned in this review as suitable for LIB operation.…”

“…to cite only some of the work on positive electrodes). Carbon (e.g., graphite) [27,28] and silicon [29,30] are utilized as Li + storage media, i.e., as electroactive material for negative electrodes. Since recently, the introduction of lithium niobate (LiNbO 3 ) in LIB is considered to boost stability (integrity) and fast operation even for high-voltage (i.e., towards 5 V) LIBs, as it is further outlined in the next section.…”