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Chapter 6: Application of Electrochemical Impedance Spectroscopy and low temperature tests 6.1. Motivation of the work Existing literature contains several examples where EIS has been used to characterize Li/S cells. Indeed, it is a very powerful technique that enables the separation of electrical processes occurring in an electrochemical cell according to their specific relaxation frequencies. Therefore, important information can be elucidated from the EIS response. The literature reports on many examples where EIS technique was applied to Li/S cells, for the following purposes: characterization of the charge transport properties (conductivity measurements), investigation of insulating layer formation and dissolution207, evaluation of electrode electric response in regard to their composition128 or more generally – to study the working mechanism132,209,210,213 upon cycling. While EIS technique is relatively easy to apply, a proper interpretation of the obtained results is far more difficult. Therefore, several divergent interpretations and explanations of the same phenomena can exist in the literature. Based on obtained in situ XRD results, which allowed for the investigation of active material modifications upon battery operation (see chapter 5), we applied EIS as a complementary technique, in order to have broader and deeper insight into the electrochemical processes. Low temperature tests (cycling and EIS) were also applied, which brought fruitful information concerning the kinetics of the reactions, since the different phenomena can be separated better at low temperature than at RT. 6.2. Experimental part 6.2.1. Coin cell design All EIS experiments were performed on classical two-electrode Li/S CR2032 coin cells, fabricated as previously described (section 2.2.2). The positive electrode was casted on carbon- based current collector (so called ‘S-on-NwC’, with an average sulfur loading of ~ 4.3 mgSulfur cm-2 and reference composition of 80/10/10 wt% = S/SuperP®/PVdF 5130), the same as used inside the pouch cells used for operando XRD. Metallic lithium was used as counter electrode. 191 Chapter 6: EIS and low temperature studiesPDF Image | Accumulateur Lithium Soufre
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