Strong stress-composition coupling in lithium alloy nanoparticles

Abstract

The stress inevitably imposed during electrochemical reactions is expected to fundamentally affect the electrochemistry, phase behavior and morphology of electrodes in service. Here, we show a strong stress-composition coupling in lithium binary alloys during the lithiation of tin-tin oxide core-shell nanoparticles. Using in situ graphene liquid cell electron microscopy imaging, we visualise the generation of a non-uniform composition field in the nanoparticles during lithiation. Stress models based on density functional theory calculations show that the composition gradient is proportional to the applied stress. Based on this coupling, we demonstrate that we can directionally control the lithium distribution by applying different stresses to lithium alloy materials. Our results provide insights into stress-lithium electrochemistry coupling at the nanoscale and suggest potential applications of lithium alloy nanoparticles.

Fig. 1

Lithium (Li)-deficient phases nucleated during lithiation under stress. a–h Lithiation-induced volume changes during in situ lithiation of a Sn-SnO₂ core-shell particle. i–p The electron diffraction patterns taken at the respective (a–h) moments. The still snapshots are captured from Supplementary Movie 1. The scale bar indicates 100 nm

Fig. 2

Void-forming morphological evolution during lithiation under stress. a–j Time-series transmission electron microscopy (TEM) images from Supplementary Movie 3 showing dimensional and morphological evolutions of a Sn-SnO₂ nanoparticle during in situ lithiation for 360 s. White borderlines on each panel note the oxide shell edges. Orange curves highlight the fluctuating void surface on the Sn core. The illustrations next to the TEM images describe the morphological evolution of the particles, including swelling, shrinking, and void formation/evolution. The scale bar indicates 100 nm. k The areal changes (hexagons) and average oxide shell thicknesses (cross-shapes) plotted over the lithiation time, for three similar Sn-SnO₂ particles under two different electron-beam conditions. The error bar indicates the standard deviations obtained from 10 image frames separated by 0.5 s

Fig. 3

Time-series images of continued lithiation of a voided particle. a–c The particle repeats lithiation once shell-induced mechanical interaction is released and d self-discharge again upon sufficient lithiation. e–h As the shell tears apart, however, the core continues normal lithiation despite being partially attached to the shell. The scale bar indicates 100 nm

Fig. 4

Thermodynamic rationale for the strong stress–composition coupling. a The stress distribution within the particle during lithiation according to the relative radial position from the particle center, with the inset showing the lithiated particle geometry. The shell undergoes tensile plastic deformation while imposing compressive hydrostatic stress of 1 GPa on the core. b, c The stress effect on the equilibrium potential of the Li-Sn alloy system (b) and schematic illustrating the spontaneous core dealloying based on the stress-driven potential differences (c). The numbered circles illustrate the discrete timeframes during lithiation. d The experimentally observed stress–composition coupling in Li_xSn. The maximum volume expansion observed before dealloying initiation according to the core-to-particle radii ratio (a/b). The error bars indicate standard deviations in volume from 10 image frames separated by 1 s