Interfacial Impedance Growth in Si-Alloy/Graphite Blended Electrodes
Authors: Yan Zilai , Wei Congxiao , Obrovac Mark N 
Published: February 2020

Active material blended electrodes are comprised of physical mixtures of two or more lithium insertion compounds. Blended electrodes can have superior balanced performance compared to electrodes with an individual active compound.1 For instance, Si and Si-based materials have been successfully blended into graphite electrodes in lithium-ion batteries.2,3 Si-alloy/graphite blended electrodes result in higher cell energy density than graphite electrodes and pure Si-alloy electrodes and also have higher capacity retention than pure Si-alloy electrodes.3 Some properties of active material blended electrodes are difficult to predict simply from the individual active materials, such as cell lifetime.4 Electrochemical impedance spectroscopy (EIS) provides diagnostic information concerning the state of health of electrodes and properties of the solid-electrolyte interphase (SEI).5 The electrode/electrolyte interfacial impedance growth of Si-alloy/graphite electrodes will be discussed here.

Figure 1 (a-c) shows measured Nyquist plots of graphite, Si-alloy, and graphite/Si-alloy blended electrodes at different cycles. The interfacial resistance (Rir) is highest for the graphite and Si-alloy electrodes and is lowest for the graphite/Si blended alloy electrode. Changes in Rir for these electrodes are not monotonic and do not at first seem related. The trends in Rir as a function of cycle number for these electrodes are summarized in Figure 1 (d), where the Rir data were normalized with respect to the first cycle. After normalization relationships between the coatings appear. For all three electrodes, Rir increases from the 1st cycle to the 5th cycle and then decreases from cycle 5 to 10. After 10 cycles, Rir increases for graphite electrodes, while Si-alloy and the blended electrodes have a constant Rir. The relative Rir values of the graphite/Si-alloy blended electrodes have very similar behavior to the pure Si-alloy electrode. Therefore, it appears that the Si-alloy component is dominant with regard to the behavior of Rir. To verify this, the relative Rir at the 20th cycle of electrodes with various ratios of Si-alloy/graphite blended electrodes are shown in Figure 2. All blended electrodes have relative Rir values that are different from pure graphite, but have similar relative Rir values to pure Si-alloy electrodes, regardless of the graphite content. In this presentation, an explanation of this behavior in the interfacial resistance of graphite/alloy blended electrodes will be proposed.

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Figure
1 Nyquist plots for (a) graphite, (b) Si-alloy, and (c) Si-alloy/graphite (60:28 wt/wt) blended electrodes measured at different cycles. (d) the relative interfaical resistance vs. cycle number of these three electrodes. Relative interfaical resistance values were measured as the diameter of the semicircles in the Nyquist plots, normalized on the basis of first cycle interfacial resistance. The error bars were calculated as the range of two samples. The cells were cycled at 30.0 ± 0.1 ºC and EIS spectra were measured at 10 ºC.

Figure
2 The relative interfaical resistances of graphite/Si-alloy blended electrodes with various compositions, measured at 20th cycle. The error bars were calculated as the range of two samples.The cells were cycled at 30.0 ± 0.1 ºC and EIS spectra were measured at 10 ºC.

Figure 1