In situ monitoring of quantitative mechanical properties in batteries

The operando techniques play a crucial role in monitoring the comprehensive properties of battery electrode materials during operation, which promotes the in-depth understandings of their intrinsic structure-property relationships. Recently, Prof. Jun Lou’s group at Rice University have developed an in-situ tensile test method performed in the scanning electron microscope to quantitatively study the mechanical properties of lithiated and delithiated SnO₂ nanowires (NWs) (Song et al. Nano Energy 53 (2018) 277–285. doi: org/10.1016/j.nanoen.2018.08.057). SnO₂ is considered as a promising anode material in lithium-ion batteries because of its low cost, abundance, environmental benignity, and high theoretical capacity. “For practical applications, the biggest bottleneck is the large volume expansion (~300 %) of SnO₂ with the structural transition from pristine to amorphous structures. Although great progress on mechanical characterizations has been acquired in previously reported works, it is still difficult to accurately estimate mechanical properties of lithiated SnO₂ nanomaterials.” says Prof. Jun Lou, the corresponding author of this study.

 The researchers creatively designed a nanomechanical device equipped in SEM to achieve quantitative in-situ tensile test of individual SnO₂ NWs. As is shown in Figure 1, at the center of the device there is a pair of shuttles supported by four symmetrical thin cantilevers. The samples were placed across the gap between the two shuttles. Using this device, the mechanical properties of the pristine, 1st- and 3rd-cycle lithiation-delithiation SnO₂ NWs were systematically investigated. For pristine SnO₂ NWs, the fracture strength σf and Young's modulus E are calculated to be 2.53 ± 0.66 GPa and 91.74 ± 22.78 GPa, respectively. The σf and E of 1st-lithiated NWs were determined to be 0.65 ± 0.36 GPa and 41.31 ± 28.87 GPa, respectively, a decrease of ~74.30% and ~42.65% when compared with pristine NWs. However, when the electrochemical process enters delithiation stages, the fracture strength and Young's modulus have an obvious increase as compared with lithiated ones.

In addition, the effect of SnO₂ NW structures resulted from the electrochemical process on mechanical properties was clearly revealed by the transmission electron microscope (TEM) characterizations and finite element (FEA) analysis. After full lithiation, the single-crystal lattice structure of pristine SnO₂ NWs undergoes a drastic chemical reaction with an obvious crystal-to-glass transition. Furthermore, based on linear elastic and elasto-plastic hardening law, the theoretical results from a practical FEA modelare in good agreement with experimental ones. “It is found that the lithiation-delithiation processes can cause a phase transition from crystalline to  the composite structure, leading to an obvious increase in fracture strain accompanied by plastic deformation, as compared to pristine SnO₂ NWs.” says Bill Song, the first author of the study.

This study represents a step forward towards gaining a fundamental understanding of mechanical properties of lithiated and delithiated SnO₂ that will allow for optimized designs for NW-structured anodes for next generation LIBs. In addition, this novel technique also opens up a new avenue to give an in-depth understanding of the quantitative mechanical properties of functional materials, showing great potential for more frontier fields.