2, Georgia Institute of Technology, Atlanta, Georgia, United States
State-of-the-art Li-ion battery electrodes provide stable cycling performance at the cost of necessitating a host matrix to create interstitial sites for Li-ion insertion. Such intercalation mechanism essentially limits their energy density in terms of host to Li atom ratio. Next generation electrode materials based on alloying or conversion reactions could potentially overcome this limitation by allowing each host atom to accommodate multiple Li ions. However, these new reaction mechanisms generally lead to significant volume expansion after complete lithiation, which results in poor reversibility caused by mechanical disintegration, with the prominent example of Si anodes decrepitating after cycling. Nano-structuring Si can alleviate this problem for each nanoscale element but the traditional slurry-based electrode fabrication method does not provide efficient and reliable assembly of the nanoscale building blocks and leads to problems like capacity degradation, sluggish kinetics and low active material loading.
Recent advances in additive manufacturing present an opportunity to rationally design 3D electrode architecture with periodic lattice geometry and nanoscale feature sizes that could potentially resolve the volume expansion problem for high energy density electrode materials. We fabricate 3D-architected Si electrodes by depositing a conformal layer of 250nm a-Si on an interconnected polymer-Cu core-shell scaffold, where local mechanical stability is enabled by the enhanced ductility of nanoscale Si, and structural robustness of the electrode is reinforced by lattice architecture design. Good transport kinetics in these nano-architected electrodes is maintained by the conductive scaffold backbone and the low-tortuosity periodic structure immersed in liquid electrolyte. The high mechanical strain induced by the lithiation of Si stretches and elongates lattice beams with a relatively compliant polymer-Cu composite backbone, which results in unfavorable global expansion in the 3D-acrhitected electrodes. We demonstrate that by rationally designing the lattice architecture with built-in instability, buckling of the lattice beams could accommodate Si volume expansion and internalizes lattice beam elongation turning the square lattice into an auxetic lattice, as directly visualized by a custom-made in operando optical cell. We analyze this mechanical instability-driven mechanism using finite element modelling to reveal insights on the critical role of buckling for stress-relief and strain accommodation during Si lithiation. Special attention is paid to how the buckling orientation of each lattice beam interacts with its neighbors and how introducing artificial defects can control the buckling orientation collectively in the 3D-architected nanolattice electrode.