FFT-Impedance Spectroscopy for Silicon Microwire Anodes: Identifying Temperature and Diffusional Changes
Authors: Nöhren Sandra , Quiroga-González Enrique , Carstensen Jürgen , Adelung Rainer 
Published: February 2020

Li-ion batteries replace nowadays many devices for different applications like in automotive industries, in smart phones, medicine, aerospace and in other fields. Those applications/fields are very different and many different requirements have to be fulfilled like fast charging, high capacity, long term stability, cost effective production and temperature stability. For a wide application range, a battery has to perform under different temperatures and environmental conditions and nevertheless have a stable and high capacity.

The silicon microwire anodes exhibit a high stable capacity of 3150 mAh/g for 300 cycles with a high amount of active material (1.35 mg/cm2) at ambient temperatures. Those anodes are fabricated based on different dry and wet chemical etching processes as pre-structuring of the p-doped Silicon wafer. The main step in the micro-structuring of the silicon microwires lies in the electrochemical macropore etching. By applying a specific current-time profile, macropores of specific diameter and lengths are etched deep in the pre-structured p-doped silicon. Such a profile could be changed according to the needs of the application; changing the lengths and the shape of the pores. In this paper, wire lengths of 36 µm up to 75 µm with a constant thickness of 1.4 µm as well as wire thicknesses between 1.2 µm up to 1.8 µm with a length of 60 µm.

During the additional over-etching step, the interstices between the pores are etched away until the wires remain free-standing. In a two-step process, a copper current collector is deposited galvanically on top and in between of the wires. With this approach a good mechanical stability between the silicon wires and the current collector is provided and reduces the lateral volume expansion in only one direction. In a layer-transfer process, the anode could be separated from the silicon wafer. Compared to other silicon anode systems, this approach manages without any conductive additives and with a high amount of active material. The anodes are tested in half cells with metallic Lithium as counter and reference electrode with standard anode electrolyte LP30, provided by BASF, with lithiumhexafluorophosphate (LiPF6) as lithium salt.

In order to test the temperature influence on the electrochemical performance of the silicon microwire anodes, this study focuses on the properties inside the anode recorded via in-situ FFT-impedance spectroscopy. In a special set-up the half-cells are heated and recorded between 20 °C up to 75 °C.

This technique is a linear response analysis, where a small senoidal signal is superimposed on the dc operation point. In comparison to the standard electrochemical impedance spectroscopy (EIS), perturbations of different frequencies can be applied at the same time between 5 Hz up to 20kHz [1, 2]. The resulting three time constants and resistances can be interpreted as different electrochemical processes inside the battery anodes. Those measurements are correlated with the cyclic voltammetry measurements which are performed in a voltage regime between 1 V and 20 mV for five cycles. In previous studies [3] the size dependency of the wire dimension on the lithiation/delithiation potential was shown at ambient temperatures. Here, it showed that the charge transfer mechanism into the silicon wires of different thicknesses a “radial diffusion” process dominates.

This study illustrates the diffusional changes of the properties and its influence on the dimension if temperature is impeded on the anodes.

Figure 1a shows results of the time-dependent resistances and time constants for two different temperatures. Figure 1b shows a photo of the type of anodes used for this study. Figure 1 c shows an example of curve of the cycling performance of the silicon wire anodes at different temperatures with SEM images of the microwires after the tests.

This study focuses on the in-situFFT-Impedance spectroscopy to monitor the temperature influence on diffusional and charge transfer processes. The recorded time constants shown in Figure 1a at different temperatures show, that the charge transfer process  is much slower at high temperatures showing the slow diffusional processes due to side effects. Those results are used for process optimizations to achieve long-term stable anodes under cycling temperatures.

Figure 1: Results of electrochemical characterization a) in-situ-FFTImpedance spectroscopy at various temperatures for 75 µm long wires, b) an example of silicon wire anodes, c) cycling experiments at different temperatures.

[1]      J. Carstensen, H. Föll, A. Cojocaru, and M. Leisner, Phys. Status Solidi C6(7), 1629 (2009).

[2]      J. Carstensen and H. Föll, ECS Trans.25(3), 11 (2009).

[3]      S. Noehren, E. Quiroga-Gonzalez, J. Carstensen, and H. Foell, J.Electrochem.Soc. 163(6), A1 (2016).

Figure 1