Researchers tailor lithiation behavior in Li-ion anode material using nanoscale interface and bandgap engineering
A team from Sandia National Laboratories and Los Alamos National Laboratory has demonstrated that the lithiation behavior of a nanostructure can be controlled using interface and band gap engineering. In a paper published in the ACS journal NanoLetters, they report the first direct observation of a “dramatic” change in the lithiation behavior of a Ge nanowire caused by such nanoscale engineering—specifically by applying an epitaxial ultra-thin Si shell layer.
The team suggests that the work highlights the potential importance of materials design of lithium-ion battery electrodes and proves a new and effective way to control the volume expansion of high-energy anode materials such as silicon (Si) and germanium (Ge).
High-energy anode materials are of great interest because of their potential to boost Li-ion battery energy density to meet the ever-increasing demand of higher battery performance. One of the challenges with these materials is the huge volume change they undergo during the lithium insertion and extraction cycles, which cause rapid degradation of the electrodes.
Accordingly, many research teams are exploring a range of nanostructured and nanocomposite materials—such as nanoparticles, nanotubes, and nanowires—because they offer facile strain relaxation, fast electron and ion transport paths at the nanoscale, and large surface areas for functionalization, the researchers note.
Parallel to the efforts of seeking for new materials and device architectures, understanding of the underlying reaction mechanisms is crucially important for the rational design of high performance Li-ion batteries. The electrochemical reactions in an electrode material, denoted as M, involve lithiation that is defined as the alloying reaction between Li and M to form a lithiated phase, LixM, or the reverse process of delithiation. The lithiation behavior of M is usually found to be intrinsic to the specific material itself, such as preferential insertion routes along certain crystallographic directions, and is less sensitive to the test conditions.
...Batteries, as electronic devices, require both mass and charge transport in a harmonious manner during their operation. The flow of charges has huge impacts on the materials. For instance, the direction of volume expansion is perpendicular to the lithiation reaction front (i.e., the interface between LixM and M), as if the Li ions are digging into the anode material and pushing the lithiated products in their wake. Therefore, the volume expansion could be also controlled by defining the Li insertion direction. As the electrons and Li ions must meet to initiate lithiation, this requires manipulation of the charged particles (electrons, Li ions, or both) to flow in a controlled manner. In the past two decades, it has been shown that ionic transport properties can be dominated by interfaces at the nanoscale, which provides the possibility to control the Li diffusion pathways and to modify the volume expansion direction by introducing heterojunctions (namely chemical and structural discontinuities).
Moreover, if the materials across the junction interface are properly chosen the energy band-edge of the heterostructure can be further controlled to tailor the properties of the material. Such bandgap engineering is key to tailoring the performance of electronic devices, such as high-mobility field effect transistors, laser diodes, high-performance thermoelectric materials, and high-efficiency water-splitting devices.
Similarly, potential barriers can be introduced into the Li-ion battery electrode materials via bandgap engineering, which can hinder the Li ions or electrons transport in certain directions while promoting transport in other directions. Therefore, as an electrochemical device, it is anticipated that the lithiation behavior of a battery electrode may also be controlled by band structure engineering through creation of suitable interfaces to modulate the movement of charged particles.—Liu et al.
In their experiments, the team observed the structural evolution of a pure Ge nanowire upon lithiation. The reaction front propagated progressively along the nanowire; Li ions diffused along the surface and were simultaneously inserted into the Ge nanowire from the radial directions, resulting in a lithiated LixGe shell on a tapered, unreacted crystalline Ge core (shown in the top element in the schematic above). The lithiated nanowire swelled along both axial and radial directions.
They then deposited a conformal, epitaxial, and ultra-thin (thickness between 1 and 5 nm) silicon (Si) shell on the  Ge nanowires. The Si segment grew along the  direction, thus formed a 19.5° kink with respect to the  Ge wire. The thin Si shell layer on the Ge nanowire drastically changed the lithiation behavior, as illustrated in the bottom element of the schematic above. The radial lithiation and swelling were completely suppressed, as if the Li ion transport and insertion could only occur along the axial direction. The reaction front was transformed to the cross section of the nanowire.
They determined that the axial lithiation in the radially heterostructured nanowires was of a chemical potential origin.
...an ultra-thin layer of Si as small as 1 nm is dramatically effective in controlling the Li ion transport through and reaction with the bulk Ge nanowire, an effect almost impossible to achieve with a purely mechanical confining layer.
...The control of volume expansion is extremely difficult for high-energy anode materials such as Si, Ge, Sn and their compounds, which may exhibit isotropic or orientation- dependent expansions up to 300% upon lithiation. Thus mechanical confinement has demonstrated limited success. Our experiments demonstrate an alternative strategy, that is, tailoring the electronic and interface properties of materials, which can be highly effective in defining the Li transport and electrode reactions with minimum added weight.—Liu et al.
Yang Liu, Xiao Hua Liu, Binh-Minh Nguyen, Jinkyoung Yoo, John P. Sullivan, S. Tom Picraux, Jian Yu Huang, and Shadi A. Dayeh (2013) Tailoring Lithiation Behavior by Interface and Bandgap Engineering at the Nanoscale. Nano Letters doi: 10.1021/nl4027549