Nanostructured Silicon-Based Li-Ion Battery Anodes By Pulsed Laser Deposition

With the rapid development of portable devices such as cellular phones and notebook computers, there is a strong demand for high capacity and high energy density Li-ion batteries (LIB). Today’s LIBs are partially enough to satisfy all of these demands, but there is still room for further improvements. Particularly for the anode material, commercial graphite anode shows excellent capacity retention during battery cycling; nevertheless, despite its good cycling stability and low cost, the theoretical capacity of only 372 mAh g^-1 is clearly insufficient for the huge demands of the next generation portable electronic devices [1]. To meet the energy density requirements, many anode materials have been investigated including Si, Sn, Al, Ge and mixed compounds thereof. Because of its exceptional theoretical capacity (~4000 mAh g^-1), which means the highest energy density for next generation applications, silicon is the most promising anode candidate among these elements and has recently attracted very much attention. The main limitations to the wide spreading of its application are related to the high volume change, up to above 300%, that occurs during lithium insertion and extraction, leading to mechanical fracture and anode pulverization. Nanostructuring, introducing voids and adding less-active materials to silicon (e.g., carbon) are the main routes currently being explored to solve this problem.

In this work, mesoporous hierarchical amorphous silicon nanostructures have been grown by simple and rapid Pulsed laser Deposition (PLD), both in the form of thin quasi-transparent films and 3D grids, and tested as LIB anodes. The introduction of a controlled porosity is meant to buffer the volume expansion, while the preferential growth in the direction perpendicular to the substrate heads to favouring the electronic and ionic transport within the anode. Films were fabricated so to vary their morphology and degree of porosity and the effect of increasing porosity was studied by electrochemical testing in lithium cell configuration with liquid electrolyte under different current densities.

All the samples prepared are fully characterised from the structural-morphological and electrochemical viewpoint. The cycling performances of these nanostructured materials in lab-scale Li-based cells are presented and thoroughly discussed. High specific capacity values have been obtained upon galvanostatic charge/discharge cycling at ambient temperature with high Coulombic efficiency, thus accounting for the good cycling stability (see Fig. 1).

In order to improve the mechanical stability and thus reduce capacity losses of the anodes, silicon/carbon multilayered anodes have also been prepared in different morphologies in a single fabrication step by PLD. The addition of C is meant to stabilize the anode by promoting the formation of a passivating Solid Electrolyte Interphase and reducing the overall volume expansion.

Finally, the preliminary results of an all-solid paper-cell, comprising a nanostructured Si grown on copper grid as working electrode, a PEO-based membrane as polymer electrolyte and a lithium metal counter assembled in a flexible coffee bag envelope, are here presented, which demonstrate very interesting characteristics and prospects.

[1] Q. Si, K. Hanai, T. Ichikawa, A. Hirano, N. Imanishi, Y. Takeda, O. Yamamoto, Electrochim. Acta 195 (2010) 1720-1725.