High-Capacity Si Microwire Anode with Enhanced Conductivity

As a potential anode for Li ion batteries silicon has a theoretical capacity of 4200 mAh/g, more than ten times that of standard graphite anodes with a capacity of 370 mAh/g. Microstructured Si in wire-shape overcomes problems caused by its four-fold volume expansion during its lithiation, allowing capacity stability over hundreds of cycles [1].

We have developed a new concept of Si microwire anodes that consists of an array of Si microwires embedded at one end in a Cu current collector [2]. The capacity of the anodes is very stable over 100 cycles [3], and breaks all the records when considering the capacity per area (mAh/cm²) [4]. The mechanical stability of the wires is surprising, since their diameter (» 1 µm) is far larger than what was deemed reasonable (< 300 nm) for avoiding cracking. However, it has been observed that the resistance of the wires is large and may increase after tens of cycles, probably due to a porosification process. Due to the large resistance the voltage limits of the anode (at which the galvanostatic charging/discharging is stopped) are reached fast. The anodes are cycled in a standard way, changing to a potentiostatic mode once the voltage limits are reached, stopping until the current decreases to 10 % of the initial value. The potentiostatic mode is slower than the galvanostatic one, thus the longer the galvanostatic mode lasts, the faster is the cycling. For many applications it is desired to have the possibility of charging/discharging fast.

By chemically coating the wires with a thin granular Cu film of about 20 nm (see Fig.1a), their resistance is decreased. The galvanostatic lithiation is low just for the first cycle, and then it is greatly enhanced from the second cycle (see Fig.1b). The low value for the first cycle occurs because during the first cycle the wires are amorphized, and a solid electrolyte interface is formed, which are slow processes. When the wires are uncoated, it takes more cycles to reach a high value. Further details of the enhancement of conductivity and its effect on the performance of the anodes will be shown in the full paper and at the conference.

[1]      C.K. Chan, H. Peng, G. Liu, K. McIlwrath, X.F. Zhang, R.A. Huggins, Y. Cui, Nat. Nanotechnol., 3, 31 (2008).

[2]      E. Quiroga-González, E. Ossei-Wusu, J. Carstensen, H. Föll, J. Electrochem. Soc., 158, E119 (2011).

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

[4]      E. Quiroga-González, J. Carstensen, H. Föll, Energies, 6, 5145 (2013).