NiS-filled Carbon Nanotubes and Dynamic In-Situ Lithiation

In order to replace graphite for improved charge storage capacity and performance, energy storage applications need advanced anode materials.

In recent research, CNTs, or nickel sulfide (Ni3S2)-filled carbon nanotubes to give them their full title, have been proposed as an anode material that is an optimal metallic conductor for both improved capacity and enhanced ion conduction, thanks to nanostructuring of the material.

Despite the fact that bulk electrochemical testing has demonstrated the strong performance of these anodes for lithium cycling, the lithiation and deformation mechanisms are not well understood.

Simultaneous low-dose imaging is delivered by the K3® IS camera through the use of real-time electron counting, a large field of view and fast continuous data capture.

Methods and Materials

A silicon substrate was used on which Ni3S2-filled CNTs were synthesized. First, the nanowires were scraped from the substrate using a razor blade. These were then subsequently affixed using conductive epoxy to an Al wire.

The sample was first loaded in a Nanofactory STM-TEM vs. Li metal on a tungsten probe and subsequently observed in a Titan ETEM. A K3 IS camera was used to visualize the lithiation and deformation mechanisms.

A series of 5760 x 4092 TEM images at 37.5 frames per second at 1.9 Å pixel size were captured by the camera, at a low electron dose rate of just 2.1 e- /Å2/s, which is significantly less than that of a typical high-resolution TEM image.

In-situ imaging. Lithiation progressed under -6 V of potential, where the Li-ions traveled down the inner wall of the CNT and alloyed with the NiS. Lithiation progressed from the perimeter of the Ni3S2 into the core, where the lithiation front progressed quickly down the CNT inner walls. As the Ni3S2 alloyed with lithium, the volume expansion caused strain on the CNT enclosure, which eventually caused the fracture of the CNT. Each frame displayed here represents 40 original frames that were summed, leading to minor blurring not present in the raw data due to sample rotation. The dose displayed is the cumulative dose since the start of this video.

 Figure 1. In-situ imaging. Lithiation progressed under -6 V of potential, where the Li-ions traveled down the inner wall of the CNT and alloyed with the NiS. Lithiation progressed from the perimeter of the Ni3S2 into the core, where the lithiation front progressed quickly down the CNT inner walls. As the Ni3S2 alloyed with lithium, the volume expansion caused strain on the CNT enclosure, which eventually caused the fracture of the CNT. Each frame displayed here represents 40 original frames that were summed, leading to minor blurring not present in the raw data due to sample rotation. The dose displayed is the cumulative dose since the start of this video. Image Credit: Gatan Inc.

Summary

It was identified by the dynamic changes in the lithiation events that the rate of lithiation caused huge strain to the walls of the CNT, which culminated in a fracture.

The lithiation front was seen to move the quickest along the CNT interface with the Ni3S2 filler. This is contrary to the expected mechanism for Ni3S2 volume increase during lithiation, in the sense that volume expansion would be limited to the transverse direction of the CNT.

This fast reaction was captured at a high temporal and spatial resolution by the K3 IS, without interfering with the beam-sensitive lithiated materials.

Credit(s)

Gratitude is owed to Integrated Nanotechnologies (CINT) and Katherine L. Jungjohann. Dr. Wenzhi Li’s research group in the Department of Physics at Florida International University synthesized the materials.

Gatan, Inc. is the leading global manufacturer of instrumentation and software which is used to improve and extend electron microscopes — ranging from specimen preparation and manipulation to imaging and analysis.

This information has been sourced, reviewed and adapted from materials provided by Gatan Inc.

For more information on this source, please visit Gatan Inc.

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