Abstract
Nanomaterials undergoing cyclic swelling-deswelling benefit from inner void spaces that help accommodate significant volumetric changes. Such flexibility, however, typically comes at a price of reduced mechanical stability, which leads to component deterioration and, eventually, failure. Here, we identify an optimised building block for silicon-based lithium-ion battery (LIB) anodes, fabricate it with a ligand- and effluent-free cluster beam deposition method, and investigate its robustness by atomistic computer simulations. A columnar amorphous-silicon film was grown on a tantalum-nanoparticle scaffold due to its shadowing effect. PeakForce quantitative nanomechanical mapping revealed a critical change in mechanical behaviour when columns touched forming a vaulted structure. The resulting maximisation of measured elastic modulus (~120 GPa) is ascribed to arch action, a well-known civil engineering concept. The vaulted nanostructure displays a sealed surface resistant to deformation that results in reduced electrode-electrolyte interface and increased Coulombic efficiency. More importantly, its vertical repetition in a double-layered aqueduct-like structure improves both the capacity retention and Coulombic efficiency of the LIB.
Highlights
Nanomaterials undergoing cyclic swelling-deswelling benefit from inner void spaces that help accommodate significant volumetric changes
Low mechanical stability of both solid electrolyte interfaces (SEIs) and the entire electrodes is an impediment to the commercialisation of lithium-ion battery (LIB) with silicon anodes, but composite anodes containing silicon additives are already being marketed
A pronounced peak in the elastic modulus exactly when columns contact each other marks the sealing of the anode surface and indicates the creation of a vault-like structure
Summary
Nanomaterials undergoing cyclic swelling-deswelling benefit from inner void spaces that help accommodate significant volumetric changes. When this compressive stress exceeds the yield strength[9,10], the electrode deforms to accommodate the volume change[11,12] For this reason, mechanical property studies by in situ-modulated optical solid-state spectrometry and nanoindentation have increased over the last few years, showing the variation in elastic modulus (defined as displacement divided by stress, E = σ/ε) with respect to the structure[13], the state of charge[14,15], or the binder[16]. Silicon anodes with vaulted structures simultaneously show good electrochemical performance combined with high mechanical stability and low lithium consumption during formation of SEI, addressing the two main challenges for silicon anode commercialisation This structural unit of optimal electrochemical performance is identified by a distinct transition in mechanical behaviour exactly when individual silicon columns merge to form closed arches (but not beyond that point, with further growth of amorphous-silicon film on top); most importantly, its mechanical stability can be improved further if reiterated vertically, due to clamping effect. The introduction of vaulted structure and arch action brings many new possibilities in the design of new materials for batteries, and for other applications in which the surface is under strong and variable stress action
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