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 ...

The time-independent and time-dependent mechanical behavior of electrochemically lithiated silicon was studied with nanoindentation. As indentation was performed with continuous stiffness measurements during loading and load-hold, new insight into the deformation behavior of lithiated silicon is furnished. Supporting other research, Young's modulus and the hardness of lithiated silicon are found to decline with increasing lithium content. However, the results of this study indicate that Young's modulus of the fully lithiated phase, at 41 GPa, is in fact somewhat larger than reported in some other studies. Nanoindentation creep experiments demonstrate that lithiated silicon creeps readily, with the observed viscoplastic flow governed by power law creep with large stress exponents (>20). Flow is thought to occur via local, shear-driven rearrangement at the scale of the Li 15 Si 4 molecular unit volume. This research emphasizes the importance of incorporating viscoplasticity into lithiation/delithiation models. Additionally, more broadly, the work offers insight into nanoindentation creep methodology.

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The high theoretical specific capacity of Si as an anode material is attractive in lithium‐ion batteries, although the issues caused by large volume changes during cycling have been a major challenge. Efforts have been devoted to understanding how diffusion‐induced stresses cause fracture, but recent observations of anisotropic volume expansion in single‐crystalline Si nanostructures require new theoretical considerations of expansion behavior during lithiation. Further experimental investigation is also necessary to better understand the anisotropy of the lithiation process. Here, we present a method to reveal the crystalline core of partially lithiated Si nanopillars with three different crystallographic orientations by using methanol to dissolve the Li atoms from the amorphous Li‐Si alloy. The exposed crystalline cores have flat {110} surfaces at the pillar sidewalls; these surfaces represent the position of the reaction front between the crystalline core and the amorphous Li‐Si alloy. It was also found that an amorphous Si structure remained on the flat surfaces of the crystalline core after dissolution of the Li, which was presumed to be caused by the accumulation of Si atoms left over from the removal of Li from the Li‐Si alloy.