Fabrication and in Situ Scanning Electron Microscope Lithiation of Mechanically Robust 3D Architected Si-Cu Core-Shell Nanolattices
Authors: Xia Xiaoxing , Di Leo Claudio V , Greer Julia R 
Published: February 2020

Silicon has been intensely studied as an anode material in Li batteries because of its high storage capacity. A major drawback is that Si suffers from mechanical degradation because of the large volumetric expansion during lithiation. Nano-structuring Si can alleviate the problem for each nanoscale element such as nanowires and nanoparticles [1, 2] because of the size-induced ductility and the availability of open space for Si to expand into. However, incorporating such nanoscale elements into device-sized electrodes is difficult. Si particles assembled using the traditional slurry method, which is designed for intercalation type electrodes, tend to lose contact with the surrounding binder after repeated lithiation and delithiation cycles [3]. Therefore, we propose 3D architected Si-Cu core-shell nanolattices as interconnected, binder-free Li-ion battery electrodes with the combined benefits of size-induced ductility, fast electron and ion transport, and mechanically robust structural geometries.

To fabricate architected Si-Cu core-shell electrodes, a polymer nanolattice mold is first made from positive photoresist on a glass substrate covered by metal thin film via two-photon lithography. Cu is then electroplated into the mold, and free-standing solid Cu nanolattices are made by removing the remaining photoresist. Amorphous Si (a-Si) is then deposited over Cu by plasma-enhanced chemical vapor deposition (PECVD). The Si layer is ~250nm in thickness, and amorphous Si is used to reduce cracking due to anisotropic stress during Si expansion.

For in situ battery testing, an electrochemical half-cell is built inside of a scanning electron microscope (SEM) using a lithium electrode and the nano-architected Si-Cu electrode. First, Li is brought into contact with Si-Cu nanolattices directly with an oxidized layer of Li2O on Li as a solid electrolyte, and a constant voltage bias is applied during electrochemical cycling. In situ observation shows that during the first lithiation a reaction front propagates from the top of the electrode where it is contacting solid Li2O electrolyte to the bottom as each lattice beam expands in volume and bows out. During delithiation, no clear reaction front is observed as the structure contracts in volume entirely. Si volume expansion is calculated to be about 200% yet the total expansion of the electrode structure is minimal with no cracks formed on the beams at a rate of 0.25C-1C. To better study how the electrode will be cycled in practical batteries where electrode is fully submerged in liquid electrolyte, 10wt% LiTFSI in P14TFSI ionic liquid electrolyte is used to connect the Li electrode and the Si-Cu lattices inside SEM. Cyclic voltammetry confirms the electrochemical reaction occurs between Si and Li. Galvanostatic discharge indicates a first cycle capacity of ~3000mAh/g normalized by the mass of Si, and post-lithiation imaging indicates a 250% Si volume expansion with no cracks observed. Coin cells with high Si loading are being fabricated and tested for long term cycling performance.

To better understand the evolution of local stresses within the Si-Cu nanolattices, we apply the theory and numerical capability developed by Di Leo, et al [4] to model the a-Si shell lithiation. The fully-coupled diffusion-deformation theory accounts for transient diffusion of Li, large elastic-plastic deformations, and the effect of mechanical stress on the diffusion of Li. The simulation shows that the core-shell beam geometry and the plastic deformation of the lithiated Si help relieve stresses inside Si and at the Si-Cu interface. Interfacial delamination is unlikely because the calculated tolerable critical flaw size is much larger than the observed void size at the Si-Cu interface using a transmission electron microscope (TEM).

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