Study of Sulfur-Based Electrodes By in Situ Characterization Techniques

Abstract

The lithium-sulfur (Li/S) battery system is one of the most promising candidates to become the successor of the current Li-ion battery. However, the insufficient cycle life of the Li/S batteries needs to be resolved before envisioning their commercial use. Several phenomena are reported to be involved in the capacity fading of the Li/S cells upon cycling, among which important structural changes in the sulfur electrode. Hence the constant need for understanding of those mechanisms drives the use of innovative in situ characterization techniques. In the present study, operando acoustic emission (AE) technique and in-situ synchrotron X-ray tomography and diffraction are used to study the degradation mechanism of sulphur-based electrodes depending of their formulation. To date, AE technique has been used to characterize the degradation mechanisms (crack growth, friction, delamination, matrix cracking, corrosion, etc) of various materials subjected to a stress by mechanical, pressure or thermal means. In the present work, AE technique is used for the first time to monitor the mechanical degradation of sulfur electrodes depending on the nature of the binder (polyvinylidene difluoride ( PVdF) compared to carboxymethylcellulose (CMC)) and the geometry of the current collector (flat Al foil compared to 3D porous carbon paper (CP)). In all cases, AE signals are mainly detected during the 1st plateau of the 1st discharge, related to the initial dissolution of elemental sulfur into the electrolyte, causing a collapse of the electrode network. At the end of charge, the formation of micrometric sulfur on the surface of the electrode can be acoustically detected, in particular in the case of the CMC/Al formulation, where signals are also detected during subsequent cycles. This is believed to be a result of the good adhesion of CMC-based electrode to the current collector, which allows acoustic waves to propagate through the electrode to the AE sensor more easily. However, with CP current collector, no AE activity is detected upon charge, reflecting a lower mechanical stress attributed to a more homogeneous growth of the S particles in the electrode. It is also noted that inefficient electrode formulation/elaboration can be detected via AE, as AE signals are emitted at the end of the discharge when the cell polarizes more than usually observed. The use of CP as a current collector greatly improves the electrochemical performance of the cells, especially when combined with the better adhesion/cohesion strengths of CMC binder, reaching an initial capacity of 1180 mAh g-1 (~4.5 mAh cm-2) stabilizing around 860 mAh g-1 after 50 cycles. X-Ray computed tomography (XRCT) is certainly one of the most powerful analytical tools enabling non-destructive 3D imaging of objects with complex and porous morphologies such as S-based electrodes. Using appropriate image processing, segmentation and analysis procedures, quantitative parameters can be extracted such as the volume fraction, the size distribution, the connectivity and the geometrical tortuosity of the constitutive phases of the sulfur electrodes. A spatial resolution of a few tens of nm can be reached with a synchrotron X-ray source. This technique, coupled with in situ synchrotron X-ray diffraction, enables to follow in great details the evolution of the different species inside the tested Li/S batteries and get invaluable information on their degradation mechanisms. In the present study, in-situ synchrotron X-ray tomography and diffraction is used to investigate the structural changes upon cycling of sulfur electrodes integrating a poly-electrolyte binder (PEB) likely to confine the polysulfide chains and prevent sulfur loss over cycling. Its behaviour is compared to a classic CMC-binder based electrode.