Enhanced lithiation and fracture behavior of silicon mesoscale pillars via atomic layer coatings and geometry design
Introduction
The large lithium storage capacity (∼3579 mAh g−1) and industry scalable manufacturing capability of various silicon materials (including micro- and nano-scale structures) have inspired intense research in these materials as anodes for lithium-ion batteries (LIBs) [1], [2]. The enormous potential of silicon as energy storage materials has however been counteracted by several known challenges, including a rather large volume expansion (VE ∼300%) during lithiation that inevitably degrades the structural integrity of silicon electrodes during the cycling, highly anisotropic lithiation/delithiation behavior witnessed in single crystalline silicon, and poor solid electrolyte interphase (SEI) layer formation. To overcome the first two shortcomings, a wide variety of nanostructures or amorphous silicon (e.g., nanowires, nanotubes, nanoparticles, and nanoporous structures) have been intensively investigated where much valuable information has been garnered [3], [4], [5], [6], [7]. Unfortunately nanostructured silicon is expensive to scale-up and also suffers from intrinsically low tap density (leading to low volumetric capacity). Furthermore, high surface area nanostructures inevitably induce worse SEI layers that are known to degrade the performance of LIBs [8]. In comparison, the electrochemical and electromechanical behavior of mesoscale (e.g., micrometer-sized) silicon is less well understood. To date a limited amount of experiments have been conducted [9], [10]. Because of the clear size, stress and subsequent phase boundary curvature differences, the lithiation kinetics of microsized silicon is expected to be different from that of nanostructures. Such information could bear critical relevance to the commercial applications due to the high energy density needs that call for thick electrodes (e.g., commercial electrodes are typically over 100 μm thick).
Another critical challenge yet little understood to the long cycle life of silicon-based LIBs is to overcome the poor SEI layers that are intrinsically associated with large volume change electrodes. Such SEI layers are unstable both mechanically and thermally as the SEI layer is an organic/inorganic composite (e.g., containing Li2CO3, LiF, (CH2OCO2Li)2, polycarbonates) [8], [11] that could decompose at a relatively low temperature. The continuous re-exposure of fresh silicon to electrolyte due to the instability of SEI leads to low Coulombic efficiency and may promote subsequent exothermal reactions that lead to “thermal runaway” and cause fire and explosion of LIBs due to the chain reactions of oxidative cathode materials (if a full cell configuration is used). As such, the thermal and mechanical stability of SEI layers on anodes is of importance to the safety of LIBs. To this end, few studies have been performed to address these SEI issues, with existing effort focused on carbon or silicon oxide coatings as the potential front-runner solutions. The former is electrically conductive such that it may not be able to impede the growth of SEI at low potentials [10], while the latter has low fracture toughness, Table 1 [12], [13], [14], [15], [16], [17], [18], and can be reactive to fluoride species, and thus requires strict structural designs [1]. Another important class of coating materials is metal oxides, which can not only offer high thermal stability, but also possess other beneficial properties such as high mechanical strength and fracture toughness, low electrical conductivity, and high lithium diffusivity (Table 1 [19]). These unique properties render them as excellent surface protection materials for anodes (as well as cathodes). Enhanced cycling performance and high Coulombic efficiency have indeed been reported in Al2O3-coated silicon nanostructures [20], [21]. Nonetheless, there exists limited understanding of the impact of metal oxide coatings on the lithiation and fracture/failure behavior of silicon materials.
By using atomic layer deposition (ALD), here we report on the substantially enhanced lithiation and fracture behavior of silicon micropillar arrays that are ALD-ed with an ultrathin layer (<1 nm) of Al2O3 and TiO2, respectively. Silicon micropillars for this study were directly fabricated from (100) n-type silicon wafers with a diameter of 2 μm and a height of 50 μm, yielding a height/diameter aspect ratio of 25:1. To our knowledge, this is the highest aspect ratio silicon micropillars reported so far for investigation of lithiation behavior, which mechanistically ensures plane strain condition near the pillar top without having to take into account the substrate confinement effect. Similar pillars have been popularly used as thermal neutron detector materials with excellent performance [22]. The penetration ability of ALD technique to very high aspect ratio structures further makes these studies possible. We investigate two types of conformal coatings; i.e., 0.43-nm-thick Al2O3 and 0.75-nm-thick TiO2, respectively (both thicknesses are nominal). To explore the initial pillar geometry effect on the lithiation/fracture behavior, square micropillars were also fabricated. Systematic and comparison experiments were performed on the bare silicon circular micropillars (bare-Circular-Si), Al2O3-coated (Al2O3-ALD-Circular-Si) and TiO2-coated (TiO2-ALD-Circular-Si) silicon circular micropillars, and TiO2-coated square micropillars (TiO2-ALD-Square-Si). For the square-shaped pillars, the orientation of four sides is oriented along {110} crystallographic planes, which is considered as the fastest Li diffusion direction in silicon. Earlier studies have generally revealed that circular shape nanowires exhibit strong anisotropic expansion, leading to grooving and fracture. It is thus interesting and of technological importance whether similar behavior occurs in mesoscale pillars and whether one can take advantage of geometrical design to mitigate or even completely annihilate such anisotropic failure behavior.
Section snippets
Preparation of Si micropillars
Bulk n-type (100) silicon wafers with the conductivity of 2 S cm−1 (determined by a four point probe method) were selected for micropillar fabrication. The pillar diameter and spacing were defined lithographically, followed by inductively coupled plasma etching. The etching process used a Bosch Process (also known as pulsed or time-multiplexed etching), alternating repeatedly between two modes to achieve vertical structures. This was accomplished by alternating between isotropically etching the
Unexpected lithiation behavior before and after ALD
With a half-cell configuration shown in Fig. 1g, we investigated the initial lithiation behavior of above silicon micropillars, including Li uptake, SEI formation, VE, and fracture behavior. The lithiation time for all pillars is fixed at 20 h, and the total Li uptake is estimated from the current profile for three types of circular pillars, as illustrated in Fig. 1h. By using the data in the figure, and assuming that the Li intake is mainly due to the silicon micropillars, we found that the
Theoretical framework
To understand qualitatively the effect of ALD coating and initial geometry on the VE and stress evolution of silicon micropillars upon lithiation, a two-phase model is developed, following the literature [27], [28]. The model assumes that the rate-limiting processes during the lithiation involve the bulk diffusion of lithium ion through the pillars and the solid state reaction at the interface (i.e., other rate-controlling mechanisms such as surface kinetics are not considered [29], [30]. As
Modeling results and discussion
The key purpose of this phenomenological model is to help understand very different fracture behavior observed with and without ALD coatings, and the strong sample geometry effect observed in our work. Note that earlier in-situ TEM experiments revealed a strong orientation-dependent interface mobility during lithiation of silicon nanowires [2], [38], which has been the basis of many existing modeling effort [38], [39]. Our experiments here on the bare silicon pillars however indicate that the
Conclusions and outlook
In summary, we have investigated the initial lithiation behavior of (100) n-type silicon micropillars in three different forms: bare circular silicon, ALD-coated circular silicon, and ALD-coated square silicon pillars. In contrast to what has been reported in the literature on nanostructures, the bare silicon micropillars studied here exhibit a relatively uniform VE behavior before fracturing along somewhat stochastic directions, likely due to the regulation effect of SEI layers in controlling
Acknowledgments
The authors would like to thank C.E. Reinhardt and N. Teslich for experimental assistance. Helpful discussions with B.C. Wood, J. Lee and M.D. Merrill are acknowledged. The work was performed under the auspices of the US Department of Energy by LLNL under contract No. DE-AC52-07NA27344. The project is supported by the Laboratory Directed Research and Development (LDRD) programs of LLNL (12-ERD-053 and 13-LW-031). HJ acknowledges the support from NSF CMMI-1067947 and CMMI-1162619.
References (42)
- et al.
J. Power Sources
(2011) - et al.
J. Power Sources
(2012) - et al.
J. Power Sources
(2012) - et al.
J. Power Sources
(2009) - et al.
Solid State Ionics
(2009) - et al.
Appl. Surf. Sci.
(2001) - et al.
Acta Mater.
(2003) - et al.
J. Mech. Phys. Solids
(2013) - et al.
Electrochem. Commun.
(2011) - et al.
Nat. Nanotech.
(2012)