Fracture of crystalline silicon nanopillars during electrochemical lithium insertion

Abstract

From surface hardening of steels to doping of semiconductors, atom insertion in solids plays an important role in modifying chemical, physical, and electronic properties of materials for a variety of applications. High densities of atomic insertion in a solid can result in dramatic structural transformations and associated changes in mechanical behavior: This is particularly evident during electrochemical cycling of novel battery electrodes, such as alloying anodes, conversion oxides, and sulfur and oxygen cathodes. Silicon, which undergoes 400% volume expansion when alloying with lithium, is an extreme case and represents an excellent model system for study. Here, we show that fracture locations are highly anisotropic for lithiation of crystalline Si nanopillars and that fracture is strongly correlated with previously discovered anisotropic expansion. Contrary to earlier theoretical models based on diffusion-induced stresses where fracture is predicted to occur in the core of the pillars during lithiation, the observed cracks are present only in the amorphous lithiated shell. We also show that the critical fracture size is between about 240 and 360 nm and that it depends on the electrochemical reaction rate.anisotropy | lithium ion battery | plasticity | silicon anode I n modern high-energy density battery systems, the primary mechanism for energy storage is the insertion of secondary species into solid electrodes, as opposed to the surface reactions that occur in many traditional electrochemical systems (1, 2). In these batteries, understanding how the inserted species interacts with and changes the original material is vital for good performance. For long-term battery cycling with good capacity retention, cyclical insertion and extraction of secondary species during battery charge and discharge must occur with minimal irreversible structural changes that degrade storage capacity in the solid electrode material. Most commercial Li-ion batteries employ positive and negative electrode materials that react through an intercalation mechanism in which Li atoms are inserted and extracted from layered host structures with only small associated strains and structural changes (1, 2). These well-understood intercalation reactions allow for capacity retention over many cycles, but the specific capacity of intercalation materials is limited due to the weight of the atomic framework. Negative electrode materials that react with Li through an alloying mechanism have a much higher specific capacity, but large volume changes during lithium insertion/extraction can cause capacity fade with cycling due to fracture of the electrode materials (3, 4). Therefore, the control of structural and volume changes during Li insertion/extraction in these alloying electrode materials is essential for good performance.Silicon, a Li-alloy negative electrode material, has an especially high theoretical lithium storage capacity of 4;200 mA hg −1 (approximately 10 times that of conventional graphite negative electrodes) (5-7). Capacity fade due ...

Microstructural evolution induced by micro-cracking during fast lithiation of single-crystalline silicon

“…1e) indicates a different physical picture, namely a nonplanar reaction front, which can lead to a more complicated state of stress and thereby generate surface cracks. Anisotropic reaction kinetics in crystalline silicon have been observed in a number of studies [26,29,33]. It is possible that these kinetics can result in an instability such as non-planar growth along the direction with the fastest reaction rate.…”

Revealing lithium–silicide phase transformations in nano-structured silicon-based lithium ion batteries via in situ NMR spectroscopy

“…However, this high capacity is associated with large volume changes (of up to 300%) 1,2 , resulting in fracture, capacity loss [3][4][5][6][7] and cell design issues. Recently, various nano/micro-sized Si powders 3,[8][9][10][11][12] , nano-Si composites 8,[13][14][15][16] and Si nanowire (SiNW)-based LIBs have been reported 9,[17][18][19] , which can help accommodate the volume expansion.…”

“…Here we develop a new strategy for performing in situ, in operando 7 Li NMR spectroscopy, which allows us to study in unprecedented detail the kinetics of the electrochemical lithiation and delithiation reactions that occur in technologically relevant nm-sized Si based anodes under realistic cycling conditions (with careful voltage/current controls) over multiple cycles. We note that poor electrochemistry was observed beyond the 1st discharge in our previous in situ NMR studies of micron-sized Si 29 , in part due to problems associated with maintaining electrical contacts between the Si particles and the current collector within earlier anode designs.…”

“…In situ 7 Li NMR experiments on SiNW-CFGDL composites were performed with two different electrochemical schedules: galvanostatic (C/30; rate defined based on the total active material, 3,579 mAh g À 1 for Si and 372 mAh g À 1 for C) and potentiostatic steps (that is, stepped potential electrochemical spectroscopy (SPECS), see Methods). A schematic of the measurement set-up is shown in Fig.…”

“…In situ NMR analysis was first performed on bare CFGDL to identify 7 Li signals originating from lithiated CFGDL. Due to the relatively long spin lattice relaxation times of the signals from the Li x C phases (at 180 to -80 p.p.m.)…”