Nanostructured Porous Silicon - Carbon Anodes for Lithium-Ion Batteries: Two Novel Approaches

Silicon-based electrodes for Li-ion batteries (LIB) have recently attracted much attention because of their high theoretical capacity (~4000 mAh/g) and low average voltage (0.2 V) versus Li. However, the large volume change (~300%) due to Li-Si alloy formation results in poor cycling performance due to mechanical cracking and pulverization. It is believed that nanostructuring can help alleviate the effects of volume change and lead to improved cycling behaviour.

In this work, two novel approaches for composite Si-C anodes for lithium ion microbatteries are presented, based on the deposition of nanostructured porous Si films by Pulsed Laser Deposition (PLD) at room temperature and its coupling to a C layer.

In detail, the one approach is based on covering the as-deposited PLD porous Si films by a disordered graphitic C layer deposited by CVD that promotes the formation of a stable solid electrolyte interphase (SEI) layer, which is a critical step for an effective use of Si in LIB. In addition, the annealing effect induced by the CVD process on the anode is thought to be beneficial for its stability.

In the second approach, nanostructured Si is deposited by PLD on a foil made of carbon nanotubes (CNTs) that acts also as a current collector, thus making the underlying standard copper collector unnecessary. This dramatically reduces the thickness of the overall electrode, thus addressing the requirements for application in innovative microbatteries.

Introducing voids at the nanoscale in the Si film by nanostructuring is meant to accommodate the large volume expansion associated with silicon lithiation with no losses in film continuity  (in contrast, for example, with films made of Si nanoparticles), while C is well known to be the standard material for LIB anodes for its good cycle life and capability of forming stable SEI layer with commonly used electrolytes. Moreover, the as-deposited Si film exhibits an amorphous structure in high-resolution TEM (HRTEM) and this avoids its amorphization during the first cycles, turning into lower internal mechanical stresses. Structure and morphology of the C-coated porous silicon samples have been widely investigated by suitable microscopic and spectroscopic techniques and their electrochemical behaviour characterized for different current densities.

With CVD carbon coating, porous silicon promisingly performs in a very stable way, showing areal capacity of ~175µAh/cm² at 54 µA/cm², and no decay for at least 1000 cycles.

On the other hand, also porous nanostructured silicon deposited on CNTs performs promisingly, showing areal capacity of ~150µAh/cm² at 1080µA/cm² with no decay for at 1000 cycles.