The combination of greater theoretical Li-capacity (~4200 mAhg^-1) and improved safety aspects with respect to graphitic carbon renders silicon a potential candidate for replacement of the presently used graphitic carbon based anode materials in Li-ion batteries. However, enormous volume changes that occur during electrochemical cycling (i.e., Li-insertion/removal) lead to severe stress developments, resulting in mechanical degradation of the Si-based anode materials (cracking/fracture/delamination/loss of electric contacts/and continued SEI formation) and concomitantly drastic fade in capacity [1]. It is widely believed that use of ‘buffer’ interlayers may reduce the stresses or at least the impact of such stresses [1-3]. However, the literature base lacked direct experimental investigation/assessment of the stress developments during lithiation/delithiation of Si in the presence/absence of such ‘buffer’ interlayers. Against this backdrop, we have monitored the stress developments in-situ during electrochemical cycling of continuous amorphous Si (a-Si) films, in the presence and absence of suitable ‘buffer’ interlayers between the active electrode film and the current collector. The effects of presence of such interlayers on the mechanical integrity during the course of lithiation/delithiation cycles have also been thoroughly monitored. Such comprehensive experimental work, along with in-depth analysis, has led to better understandings of the mechanistic aspects related to the effects of the presence of ‘buffer’ interlayers and also the interfacial properties.

In the present work, two different types of ‘buffer’ interlayers, viz. CVD-grown well-ordered multi-layered graphene (MLG; ~9-10 layers) and NiTi shape memory alloy (SMA; grown via PLD; 150 nm thick), have been used (sandwiched between ~250 nm thick a-Si and ~100 nm thick Ni current collector films). While interlayer sliding of the individual graphene layers may be expected in the case of MLG in response to the expansion/contraction of the a-Si, NiTi SMA is expected to exhibit pseudo-elasticity (viz., reversible stress induced phase transformation; B2 <=> B19’) [3]. The multi-layered film electrodes, on stiff circular quartz substrates (~500 µm thick) were assembled in custom-made electrochemical cell and cycled galvanostatically against Li-metal (counter/reference electrode). Multi-beam optical stress sensor was used for real-time monitoring of the stress developments in the film electrodes, similar to our previously reported works [1,4-8].

The presence of both the buffer interlayers improved not only the cyclic stability, but also the Li-capacity possibly because of reducing the magnitude of compressive stresses developed during lithiation. In addition to reducing the stress magnitudes (when normalized by the corresponding Li-capacities), the stress profiles recorded in real-times tend to indicate that while MLG supressed the ‘flow’ of a-Si during lithiation, stress development was accommodated by preferential plastic deformation of NiTi. Top-view and cross-section SEM observations at different stages of the cycling indicated that, even though the a-Si films cracked in the absence, as well as in the presence of the buffer ‘interlayers’, the cracked ‘islands’ maintained considerably improved integrity upon further cycling in the presence of the ‘buffer’ interlayers (as compared to directly on Ni current collector). Scratch tests indicated weaker interfacial adhesion strengths in the presence of the ‘interlayers’. Along with the reduced magnitudes of the lithiation induced stresses, engineering of the adhesion strengths, in combination with the observed cracked ‘island’ sizes, concomitant suppression of flow of cracked a-Si ‘islands’ (due to weaker adhesion in the presence of the interlayers), and possible incremental plastic deformation of the Ni current collector in the absence of the ‘interlayers’, were associated with the observed stress profiles and improved mechanical integrity in presence of the ‘buffer’ interlayers.

Keywords: amorphous Si, patterned film, multi-layered graphene, NiTi shape memory alloy, in-situ stress monitoring.

References:

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