Nanotechnology is a rapidly progressing field with tremendous impact on fields such as materials, electronics and medicine. In the case of Li-Ion Batteries (LIBs), much effort is focused on the design of nanostructured electrodes that accelerate the charge-discharge processes, as well as prevent the fracture of the electrode caused by the volumetric expansion-contraction during the lithiation-delithiation mechanisms. This last effect is particularly important for high capacity electrodes, such as Si anodes, which can increase their volume up to four times during lithiation. Nevertheless, the synthesis of nanostructured electrodes is associated with new challenges such as reproducibility, use of harsh chemicals, aggregation and manufacturing difficulties.
Herein, we propose a new approach to synthesize nanostructured electrodes based on the use of Nanoparticle (NP) scaffolds interlayered within amorphous Si (a-Si) layers in order to exploit the advantages of both nanoparticulated and continuous Si films without suffering from their respective shortcomings. The current prototype is fully synthesized by Physical Vapor Deposition (PVD), comprising of an a-Si layer deposited by RF-sputtering and Ta NP scaffolds fabricated by Cluster Beam Deposition (CBD). This prototype anode benefits from being synthesized directly on the substrate (making it easy to incorporate to any microelectromechanical device) without the addition of any binder or chemical solvent, allowing versatile, one-pot synthesis with excellent control of thickness and size of the deposited material. The high chemical stability of the Ta combined with size selection during synthesis and post-growth behavior allow an in-depth evaluation of the NP scaffold’s effect on the structure, morphology, and nanomechanical properties of the a-Si layers, and of the electrochemical performance of the synthesized anode.
Sequential deposition of alternating Ta NP and a-Si layers provides an anode with a multilayer configuration. The a-Si layers show particular structural features such as increased surface roughness, nano-granularity and porosity that are dictated by the nanoparticle scaffolds, boosting the lithiation process due to fast Li diffusion and low electrode polarization. Consequently, the proposed anode shows a remarkable charge/discharge speed, 1200 mAh g-1 at 10C. Also, nanomechanical heterogeneity self-limits the capacity at intermediate charge/discharge rates; providing exceptional cycleability at 0.5 C, with 100% retention over 200 cycles with 700 mAh g-1. Higher capacity can be obtained when the first cycles are performed at 0.2C, due to the formation of micro-islands.
This study shows that through engineering underlayers of porous NP scaffolds it is possible to manipulate the morphological, mechanical, and electrochemical properties of the a-Si layers, providing a versatile synthesis approach with promising perspectives to customize nanostructured Si anodes.