Local Transmittance Measurements As Advanced New Diagnostic Tool for the Optimization of Porous Si Foils for Li-Battery Anodes
Authors: Schütt Andreas , Nöhren Sandra , Carstensen Jürgen , Adelung Rainer 
Published: February 2020

The solar cell local characterization (CELLO) technique is a common tool for the characterization of solar cells and solar cell modules [1]. In this paper, we modify a standard CELLO measurement, the photocurrent measurement of a solar cell minimodule, in order to characterize porous silicon (PorSi) foils that were fabricated for the use as anodes in Li-Ion Batteries. In this modified set-up, the transmittance of various different laser wavelengths (401 nm, 650 nm, 830 nm, and 934 nm) of the PorSi-foil is measured and analyzed. From these measurements informations about the foil thickness and porosity can be extracted which are needed for the battery cycling and for the optimization of the PorSi formation.
Compared to commercially available anode and cathode materials, silicon with its high gravimetrical capacity of 4200 mAh/g is the material of choice for high-capacity batteries. Due to the large volume expansion of over 400 %, the material has to be nano- or microstructured to avoid pulverization. Porous silicon foils benefit of their light weight and high amount of active material in a battery without increasing the weight of the battery itself. The porous silicon foils, presented in this paper, are fabricated in a “top-down” approach by large-scale (“in-line”) electrochemical etching process. The cost effective and fast production allows large-scale batteries for different kinds of applications. Furthermore, for this approach cost-effective solar Si material as anode material is used which is produced without curfloss, thus saving roughly 50% of classical wafering costs.
The etching is performed in a two step-process: a) a low porous layer with pores of 50 - 150 nm width is etched; b) in a second step a highly-porous zip or releasing layer is etched in a 50 wt. % hydrofluoric acid solution. This zip or releasing layer is used in a layer-transfer process to separate the anode with the galvanically deposited Cu current collector from the p-doped silicon wafer (0.02 Ω*cm, (100)). In order to analyze the porous silicon foil with the CELLO technique, a PET tape with adhesive is used in a layer-transfer-process to separate the foil from the silicon wafer. The porous silicon foil anodes are tested in half-cells with metallic Lithium as counter and reference electrodes via cyclic voltammetry measurements and long-term cycling tests to guarantee a long-term stable anode. In order to cycle those anodes successfully, the thickness and porosity of such porous foils are crucial parameters. Due to the difficulties during the etching process and the problems of characterizing those films via standard characterization tools like SEM, AFM etc., this study focuses on the application of a new diagnostic tool (CELLO).
For common CELLO measurements an intensity modulated laser beam is scanned with a piezoelectric mirror across a solar cell. The small signal response of the solar cell is mapped using a lock-in routine. For the characterization of the porous silicon films, the setup is slightly changed: a solar cell minimodule that is an encapsulated solar cell with front cover glass is used instead of a solar cell. This allows to place the transmittance sample, in our case a transparent PET-tape adhered to the porous silicon foil, directly on the mini-module glass. As reference for the calculation of a transmittance map, a photocurrent map with sample is recorded and subtracted from a photocurrent map without the sample, resulting in a transmittance map in ‰, shown in Figure 1 where different regions are highlighted and discussed in this paper. Figure 1 shows the transmittance map of the analyzed sample with different regions of interest I. areas with different porosities (tree rings, Fig. 1a), II. terraces (areas with different film thickness).
Figure 1b shows first cycling experiments by galvanostatic/potentiostatic steps and a capacity limitation of 75 % of the maximum silicon capacity in order to reduce the volume expansion and stresses during cycling. It shows a stable capacity of over 600 mAh/g thus being a promising candidate for long time stable anodes.
This study illustrates the promising new battery concept basing on a porous silicon film with a directly galvanically deposited Cu collector (Figure 1c) in order to reduce the lateral volume expansions of the anode and having a high amount of active material. Furthermore, this study emphasizes the need and development of a new diagnostic tool to extract film thicknesses and porosity out of the measurement data.
References
[1] J. Carstensen, A. Schütt, G. Popkirov, and H. Föll, "CELLO measurement technique for local identification and characterization of various types of solar cell defects", Phys. Status Solidi C8(4), 1342 (2011).

Figure 1