TEM reveals twin boundaries in SnO2 nanowires: the yellow streaks, highlighted by green arrows, show the direction of travel of Li-ions along twin boundaries. Credit: Reza Shahbazian-Yassar.New generation rechargeable batteries rely on the storage and transport of Li ions through the electrodes. Now researchers have confirmed that Li ions prefer to aggregate at and move along defects like twin boundaries rather than in the regular surrounding ‘perfect’ lattice [Nie et al., Nano Lett. 15 (2015) 610, http://dx.doi.org/10.1021/nl504087z].
Using in situ transmission electron microscopy (TEM) and density functional theory (DFT) modeling, Reza Shahbazian-Yassar of Michigan Technological University and the University of Illinois at Chicago, along with colleagues from King Abdullah University of Science and Technology in Saudi Arabia, has shown that Li ions energetically prefer to accumulate near twin boundaries, which act as conduits for diffusion within SnO2 nanowires.
“We observed that in nanowires with twin boundaries the transport of ions preferably happens along these boundaries instead of their standard pathways,” explains Shahbazian-Yassar.
It has been known for some time that the transport of ions can be very different at the interface of materials compared with the bulk. Scientists have hypothesized that the effects could be due to strain, which opens up the structure allowing ions to pass through, or the presence of interstitial atoms or vacancies. Differences in charge at the interface could also have an affect on the transport of ionic species. Twin boundaries, which are common in many materials, likewise are known to enhance the diffusion of impurity ions and vacancies.
Now, Shahbazian-Yassar has built on this knowledge by studying Li-ion transport on twin boundaries in SnO2 nanowires, which are likely to be representative of other Li-ion electrode materials, he believes. Aberration-corrected scanning transmission electron microscopy (STEM) and TEM analysis reveals that Li-ion transport is very different in the presence of a twin boundary. Atomic scale observations of diffusion dynamics indicates that strain develops in the lattice around the twin boundaries as a result of the transport of Li ions. The twin boundaries appear to provide a faster diffusion pathway through the lattice.
DFT calculations support the idea that it is energetically preferable for Li ions to accumulate along twin boundaries. The team’s simulations indicate that the intercalated ions take up octahedral sites along one side of the twin boundary. Shahbazian-Yassar believes the findings could pave the way for the development of new electrode materials that use twin boundary defects or other structures to facilitate Li-ion transport through the electrodes.
“This potentially can guide us to design twin boundaries as effective ion transport channels within electrode materials,” he says. “With such engineered channels, it is likely that we can improve the rate capability of Li-ion batteries.”
The researchers are now working on better control of the synthesis of SnO2 materials to guarantee the inclusion of twin boundaries and understanding how the effect works in large-scale electrode materials.
This paper was originally published in Nano Today (2015), doi:10.1016/j.nantod.2015.06.002