Strain Anisotropies and Self‐Limiting Capacities in Single‐Crystalline 3D Silicon Microstructures: Models for High Energy Density Lithium‐Ion Battery Anodes

Abstract

This study examines the crystallographic anisotropy of strain evolution in model, single-crystalline silicon anode microstructures on electrochemical intercalation of lithium atoms. The 3D hierarchically patterned singlecrystalline silicon microstructures used as model anodes were prepared using combined methods of photolithography and anisotropic dry and wet chemical etching. Silicon anodes, which possesses theoretically ten times the energy density by weight compared to conventional carbon anodes, reveal highly anisotropic but more importantly, variably recoverable crystallographic strains during cycling. Model strain-limiting silicon anode architectures that mitigate these impacts are highlighted. By selecting a specifi c design for the silicon anode microstructure, and exploiting the crystallographic anisotropy of strain evolution upon lithium intercalation to control the direction of volumetric expansion, the volume available for expansion and thus the charging capacity of these structures can be broadly varied. We highlight exemplary design rules for this self-strain-limited charging in which an anode can be variably optimized between capacity and stability. Strain-limited capacities ranging from 677 mAhg − 1 to 2833 mAhg − 1 were achieved by constraining the area available for volumetric expansion via the design rules of the microstructures.

Fracture of crystalline silicon nanopillars during electrochemical lithium insertion

“…These models have revealed that high stresses are possible and have also suggested a critical size below which Si nanostructures will avoid fracture; in one study, experimental evidence of fracture in Si nanowires corroborated theoretical predictions (18). However, recent experimental work has shown anisotropic volume expansion behavior along h110i crystalline directions during lithiation of crystalline Si nanostructures such as nanopillars, nanowires, and fabricated microstructures (21)(22)(23). These experimental observations possibly indicate the presence of more complicated mechanical stress states and different fracture behavior than has previously been modeled.…”

The influence of carrier density and doping type on lithium insertion and extraction processes at silicon surfaces

“…[63,87] Raman scattering was previously used to demonstrate the transition from crystalline to amorphous Si in a carbon composite by monitoring the TO c-Si phonon mode [87]. Recently it was shown using Raman scattering that Li inserts anisotropically into single crystalline Si [88]. Figures 7(a,b) show the Raman spectra of pristine p-type Si, of p-type Si after 1 CV cycle and after 30 CV cycles at 1 mV s -1 .…”

Chemomechanical modeling of lithiation-induced failure in high-volume-change electrode materials for lithium ion batteries

“…7,8,82,83 It was rationalized 9,67 and experimentally verified later 81 that the anisotropic swelling arises due to the crystallographic orientation dependent lithiation rate, i.e., the chemical reaction rate is much higher along <110> directions than other directions. To simulate the anisotropic swelling of c-Si upon lithiation, Yang et al set different lithiation rates (modeled by different diffusivity D 0 at the reaction front) at several low-index orientations, 9,67 and interpolated the lithiation rates in other orientations by smooth functions.…”