Dependence of the Lithiation/Delithiation Potentials of Silicon Microwire Anodes on Their State of Charge and Sizes

Silicon microwires are a good alternative to graphite as active material in anodes for Li ion batteries. Besides a high gravimetric capacity of 4200 mAh/g, self-standing Si microwire anodes exhibit large cycle life (> 100) with a coulomb efficiency of 96 % without suffering of irreversible losses [1, 2]. It has been observed that for obtaining this exceptional performance it is necessary to charge the anodes within specific voltage limits depending on the dimensions of the Si microwires [1].

Four peaks have been found in voltammetry measurements which indicate that two lithiation and two delithiation events occur in the Li-Si system. In a previous study on Si mWs, a dependency of the lithiation and delithiation peaks during voltammetry studies on the dimensions of the Si microwires was observed [3]. In the present work the dependency of the lithiation/delithiation susceptibility of the anodes on their state of charge (SOC) has been studied. Si microwire anodes were charged to 0 %, 25%, 50 % and 75 % of their maximum capacity and analyzed by cyclic voltammetry. Single-crystalline Si wires with a lengths of 36, 46, 60, and 75 µm and widths of 1.2, 1.4, 1.6, and 1.8 µm were used for the study. For the test, paste electrodes composed of Si mWs, carbon black and CMC, coated on Cu foil were prepared. The current amplitude as well as the potential of the voltammetric peaks (lithiation/delithiation potentials) show a shift depending on the pre-lithiation levels. During the full-lithiation event, the current amplitude as well as the potential increase steadily when increasing the SOC, independently of the length of the wires (see Fig. 1a). The same effect is observed for wires of different thicknesses. The delithiation events are also affected by the SOC. They mainly decrease in potential when increasing the SOC, but at SOCs larger higher than 50 % the tendency is not so clear. According to the results, it can be inferred that the lithiation and delithiation are facilitated when increasing the SOC, probably until the pre-charge limit of 50 %.

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

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

[3]     S. Nöhren, E. Quiroga-González, H. Föll, Size dependency study of Si mircowire anodes by cyclic voltammetry,
          Kraftwerk Batterie 2013, Feb. 25-26, 2013, Aachen, Germany.