(Invited) Fabrication and Characterization of Silicon Microwire Anodes by Electrochemical Etching Techniques

Silicon with a gravimetrical capacity of 4200 mAh/g for Li storage is optimal for Li ion battery anodes. Unfortunately neither bulk silicon nor silicon micro particles (mixed in pastes together with e.g. carbon black, CNT's) up to now allowed for stable battery anodes. In contrast highly ordered silicon microwire arrays galvanically connected to a Cu electrode shows good cycling stability, combined with nearly optimal gravimetric capacity taking into account the strong expansion of silicon when loaded with Li. In this paper several aspects of the completely (electro-) chemical process sequence of the Si microwire anode will be discussed including macropore ethching with sophisticated pore diameter modulation (cf. Fig. 1), post etching for wire formation and galvanic Cu deposition for optimal embedding of the Si microwires into a Cu foil electrode [1, 2].

FFT impedance (FFT-IS) analysis has been used for the in-situ characterization and optimization as well of the electrochemical process steps as for the performance of the battery anode. Several examples will be presented. Optimal cycling stability has been found for a Li charging up to 75% of the full capacitance while the wire length can be tuned to capacitance per area of the cathode, e.g. microwires with a length of 70 µm ,with an active loading of 1.35 mg/cm²show a stable capacity of 3150 mAh/g for more than 100 cycles [1].

Especially the comparison of FFT-IS data for standard paste electrodes containing silicon particles and for the silicon microwire anodes clearly indicates that the highly symmetric (closed packed) arrangement of the microwires increases the cycling stability: when charging the anode the expansion of hexagonally closed packed silicon wires leads to a mechanical stabilization between the wires. In addition no passive material is forced to move and the electrical contact to the Cu electrode is not affected while the silicon wires enlarge in size. Further improvements for silicon containing anodes will be discussed.

[1]      E. Quiroga-González, J. Carstensen, and H. Föll, Electrochim. Acta 101, 93 (2013).

[2]      E. Quiroga-González, J. Carstensen, and H. Föll, Materials 6, 626 (2013).