3D Architected Si-Cu Core-Shell Nanolattices As Mechanically Robust, Binder-Free Li-Ion Battery Electrodes
Authors: Xia Xiaoxing , Gu X. Wendy , 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 larger-sized electrodes is difficult because of Si’s poor electronic conductivity and the loss of electrical contact after cycling [3]. Si particles tend to lose contact with surrounding particles or binding materials during lithiation and delithiation cycles. Meanwhile, due to the large surface to volume ratio of nanoparticles, solid-electrolyte interface (SEI) formation on particle surface reduces the amount of electrochemically active materials. 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 is then deposited over Cu by plasma-enhanced chemical vapor deposition (PECVD). The Si layer is kept to hundreds of nanometers in thickness in order to take advantage of the enhanced ductility in Si. Amorphous Si is used to reduce internal fracture 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 SEM 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 shrinks in volume entirely. Si volume expansion is calculated to be about 200% yet the total expansion of the electrode structure is minimal. To better study how the electrode will be cycled in real batteries where electrode is fully submerged in 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 cycling is being conducted to examine capacity and cyclability performance.
Solid mechanics analysis of the volume expansion and stress evolution of the Si-Cu core-shell nanolattices during lithitation is also being conducted using finite element analysis (FEA) software. The core-shell configuration significantly reduces the shear interfacial stress at the Si-Cu interface comparing to Si thin film on Cu current collector. Electrochemically facilitated plastic flow of Si expansion bonded by the outermost lithiated Si shell [4] induces a compressive normal stress at the Si-Cu interface, which further eliminates the risk of Si delamination, a key mechanism for Si electrode failure. Future work will be put into optimizing nanolattice geometries with different unit cell types, unit cell parameters, and Cu/Si thicknesses and predicting the most favorable geometry to withstand dramatic volume expansion during lithiation.
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