Dilation data are recorded for quasi-equilibrium cycling as well as for current pulses at high rates. In the quasi-equilibrium case, the staging behavior is characterized based on dilation and voltage data. By comparison with a graphite half-cell measurement, the major effects in full cell dilation are confirmed to be anode-related. In the high rate case, the dilation responses to the actual pulse and the subsequent relaxation phases are recorded systematically for different SOCs and temperatures. Positive and negative relaxation phenomena are observed as exemplified in Fig. 1. They are ascribed to both graphite staging and lithium plating processes. A model is introduced explaining the unexpected relaxation effects in all SOC ranges by a temporary coexistence of three or more staging compounds during high-rate lithiation/delithiation. These data thereby confirm the shrinking annuli model recently introduced by Heß and Novák [1].
Additionally, the effects of thermal expansion on dilation data recorded on pouch cells are investigated. In order to prevent misleading interpretations due to thermal effects, thermal expansion is quantified and a method for the thermal compensation of dilation data is developed. It is shown that for measurements at constant ambient temperature, the bias from thermal expansion is small compared to the other dilation effects. With regard to the application of dilatometry at varying temperatures, however, it is crucial to compensate for thermal expansion in the measurement data.
Based on the findings presented in this work, dilation relaxation can be used as an indicator for kinetic limitations leading to deviations from the low-rate staging course during graphite lithiation. The results provide a basis for the development of a new dilation based approach to lithium plating prevention.
Acknowledgements
This research project is supported by the Federal Ministry for Economic Affairs and Energy (BMWi) under contract number 03ET4012C. All responsibilities for this publication rest with the authors.
Figure Caption
Figure 1: Relaxation of cell thickness after current pulses at I=1C from SOC=0% to SOC=25% for different temperatures.
References
[1] Heß, M., & Novák, P. (2013). Shrinking annuli mechanism and stage-dependent rate capability of thin-layer graphite electrodes for lithium-ion batteries. Electrochimica Acta, 106, 149–158. doi:10.1016/j.electacta.2013.05.056