Impedance Characterization of Porous Electrodes in Lithium-Sulfur Batteries

Kurzfassung

Lithium-sulfur batteries are among the most promising alternatives as the next generation of energy storage systems. Highly porous carbon materials are commonly employed in composite cathodes for lithium-sulfur batteries to address issues such as the poor conductivity of sulfur and the significant volume expansion caused by the conversion reaction of sulfur to Li2S2/Li2S. This would indeed have considerable effects not only on the conductivity of the cathode, but also on the diffusion and charge transport parameters as well as polysulfide retention. In the presented study, porous carbonbased electrodes are systematically investigated using electrochemical impedance spectroscopy. By applying symmetrical cell configurations, the contribution of the lithium anode can be eliminated. Three carbon materials are used for this purpose, including commercially available Ketjenblack and two micro- and micro-mesoporous carbon aerogels with different pore size distributions. Transmission line models are used to understand and evaluate in detail the underlying processes including the porous characteristics of the cathode materials. Taking impedance spectroscopy and related processes into account, the overall impedance could be expressed by the following elements: electrolyte bulk resistance, electrolyte resistance in the pores, inter-particle contact resistance and charge transfer resistance at the electrode/electrolyte interface. The dependence of these resistances in open circuit voltage on the fraction of the active material, the porosity of utilized carbon and thickness of the electrode is investigated. It is shown that the electrolyte resistance in the pore increases with decreasing pore size, increasing proportion of active material and electrode thickness. Furthermore, particular emphasis is placed on the impact of sulfur-carbon composite preparation method. To illuminate the thermodynamics and kinetics of involved processes in relation to the cathode, the impedance at different depths of discharge/states of charge is recorded. With further investigations of recorded impedance and extracted DRT spectra at different discharge depths, individual charge transfer processes and their relaxation times can be identified. For the gas-infiltration method, which is implemented for ultra-microporous carbon aerogels, the solid-solid charge transfer process at deeper depths of discharge is defined and quantified. Unlike gas-infiltrated carbon aerogels, the cathodes prepared using mechanical mixing as well as state of art melt-infiltration reveal other charge transfer processes with substantial lower relaxation time constants which indicates faster kinetics. These faster charge transfers are assigned to the long- and short-chain polysulfide reduction in the solution phase. It is noted that the solution of the sulfur in the electrolyte (S8 → S2 8−) has similar relaxation times to that of the porous response, resulting in an overlap in the impedance spectra. Although transmission line theory is mainly used here to model the porous properties of cathode materials, the high number of processes with similar relaxation times called for extended transmission line models or connections of different equivalent circuits for single responses in series.It is further shown that microporous carbons are unsuitable if sulfur is not infiltrated but mixed. In contrast, no quasi-solid-state charge transfer can be achieved when using mesoporous carbons, regardless of the infiltration method. Moreover, the melt-infiltration of mesoporous carbons shows negative influences with regard to undesired side reactions and the cycle stability of the cell which suggest mixing of the cathode components instead of infiltration. The evaluations thus confirm that the size distribution of pores in carbon as well as the infiltration method play a decisive role in the charge transfer reaction path and the performance of the cell. It is further shown that the modified reaction path of infiltrated, microporous electrodes allows the use of carbon-based electrolytes. Additionally, the indication of the self-discharge and polysulfide shuttle in impedance spectroscopy is discussed elaborately. In summary, this work successfully identified the implication of different processes and phenomena related to the composite sulfur cathode in the recorded impedance spectra.