Silicon Hysteresis and Voltage Relaxation Phenomena: Implications on Overpotential Analysis
Authors: Berg Clara Marie , Morasch Robert , Gasteiger Hubert A. 
Published: September 2023

Currently, many strategies are being developed to enable a successful implementation of silicon anode active material into commercial lithium-ion cells with the goal of increasing their specific capacities and energy densities. Compared to the commonly used graphite active material, silicon gives rise to a number of challenges owing to its large voltage hysteresis. Next to a decrease in energy efficiency and a significant corresponding heat evolution during cycling,1 the path-dependency of the potential and the resulting ambiguous voltage vs. SOC correlation complicates the development of battery control systems. While the origin of the voltage hysteresis is not yet fully understood, several studies suggest that it can, to a large part, be explained by compressive and tensile stress on the silicon material induced by its lithiation and delithiation respectively.2, 3 Another hysteresis effect which may also be explained by changes in mechanical stress is the unusual voltage relaxation of silicon at open-circuit voltage (OCV) conditions: the voltage undergoes significant changes at open-circuit and the voltage relaxation is still significant over extended periods of time.4 The implications of this voltage relaxation/hysteresis behavior on the commonly used experimental methods for the analysis of overpotentials (EIS, GITT, DCIR) have, to the best of our knowledge, not been analyzed in the literature.
In this study we characterize the hysteresis and voltage relaxation behavior of a silicon-dominant electrode containing 70 wt% micro-scale silicon particles (Wacker Chemie AG) for which the amorphous silicon fraction is reversibly lithiated to the composition Li2Si (reversible capacity: 1200 mAh/gSi).5, 6 We find an offset of ~60 mV between the voltage after a 2 h OCV phase compared to the voltage at a very slow rate of C/20, which is surprisingly large compared to the voltage differences observed between different C-rates (e.g., ~10 mV between C/20 and C/2, see Fig. 1). This suggests that a form of structural relaxation may occur in the silicon material once there is no ongoing (de-)lithiation. As a result of this, the offset between the potential under load and at open-circuit cannot be used to calculate a resistance or a diffusion coefficient, as would typically be done for other active materials.7 By comparing charge/discharge potential curves recorded at different C-rates for thin silicon electrodes (1.0 mAh/cm2), effective cell resistances are estimated. Based on these results, methods used for the characterization of the kinetics of silicon are critically analyzed for their significance. We compare galvanostatic impedance (GEIS) measurements performed under load to potentiostatic impedance (PEIS) measurements performed at OCV. Further, we compare ΔE/ΔI values calculated at different points in time after the current has been turned off or on in order to analyze the impact and time-scale of the current-independent OCV relaxation. All results are compared to the same measurements performed for a graphite electrode. We ultimately show that the analysis of overpotentials of silicon active materials, when done incorrectly, can yield parameters which are physically meaningless and misleading for the characterization of the kinetics of silicon electrodes.

Acknowledgements

We gratefully acknowledge the German Federal Ministry of Education and Research (BMBF) for its financial support within the AQua HysKaDi project (03XP0321B). We also thank Wacker Chemie AG for kindly providing the silicon material.

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Figure 1 Silicon electrode potentials measured against a lithium reference electrode in a half-cell (Swagelok® T-cell setup) plotted against the estimated composition of lithium in the amorphous silicon fraction. (De-)lithiation potentials are shown for the C-rates of C/20 and C/2 and are compared to potentials recorded after 2 hours at OCV. The gray inset shows the offset between the two C-rates and the OCV at the estimated composition of ~Li1Si. Loading: 0.8 mgSi/cm2; areal capacity: 1.0 mAh/cm2; electrolyte: 1M LiPF6 in FEC/DEC (2:8 v:v); separator: two glassfiber separators (VWR); cycling: CC capacity limited lithiation, CCCV delithiation to 1.0 V.

Figure 1