Nanoscale materials have recently shown major breakthroughs in insertion materials with their stability against diffusion-induced stress [1-3]. Designed nanomaterials exhibit stable Li insertion/extraction over a wide composition range without developing much stress [4-6]. However, our understanding of stress-composition coupling has been strongly one-sided, since most work on applying nanomaterials into insertion materials has been focused on how to mediate the diffusion-induced stress via nanomaterials. How stress affects the composition in nanomaterials has remained a realm to be explored, despite the promising novel applications in sensors, energy harvesters, and actuators.  

In this work, we showed that there exists a proportional relationship between the applied stress and the composition in nanoscale lithium alloys. Using graphene liquid cell electron microscopy[7], we directly observed the phase evolution and Kirkendall void formation in a single nanoparticle during in situ galvanostatic lithiation of core-shell Sn-SnO₂ nanoparticles (see figure below). 

The observed stress-composition coupling was rationalized thermodynamically with the stress contributed chemical potential differences. Based on this coupling mechanism, we were able to intentionally drive directional lithium flow and demonstrate a high-efficiency mechanical energy harvester.

The insights from this work enhance our knowledge of stress-matter interaction in the nanoscale. With fast-diffusing components, nanoparticles may dynamically adjust their composition under stress, given a nearby particle for the diffusing components to migrate into. This result provides scientific insights on why the practical capacity for battery electrodes is remarkably different from their theoretical capacity or how Kirkendall void or pulverization initiates during battery cycling.

Technologically, we believe this work opens the opportunity in utilizing nanoscale stress-composition coupling at room temperature, such as mechanically rechargeable batteries or electrochemical non-volatile actuators. Moreover, the experimental advancements and theoretical results in this work represent an important advance in the field of nanoscale stress-matter interaction.

References

[1] C.K. Chan et al. Nat. Nanotechnol. 3, 31-35 (2008)
[2] C. Wang et al. Nat. Chem. 5 1042-1048 (2013)
[3] N. Liu et al. Nat. Nanotechnol. 9 187-192 (2014)
[4] H. Wu et al. Nat. Nanotechnol. 7, 310-315 (2012) 
[5] Z.W. Seh et al. Nat. Comms. 4 1331 (2013)
[6] G. Zheng et al. Nat. Nanotechnol. 9 618-623 (2014)
[7] J.M. Yuk et al. Science 336, 61-64 (2012)