Carbonate swollen Lithiated Nafion Electrolyte for Lithium-Sulfur Batteries

Kurzfassung

The formation of polysulfide intermediates during operation of lithium-sulfur battery is prevented by confining sulfur into ultramicroporous carbon.1 By size restriction physical entrance of typically used electrolyte solvents is hindered.2 That minimizes direct contact of sulfuric species with electrolyte solvent to the entrance of pores. Due to the reduced sulfur-solvent contact, carbonate solvents can be used with such sulfur cathodes. Electrophilic carbonates are harmless and non-toxic compared to typical ether electrolyte solvents. But they react irreversibly with nucleophilic polysulfides (PS of lower order) to form thioethers (cyclic carbonates) or sulfonium (linear carbonates).3 Their use in conventional sulfur cathodes is therefore not possible. Gas-infiltrated micro/ultramicoporous carbon sulfur composite cathodes show excellent cyclability and stability of capacitance.4 The use of carbonate solvent further creates an advantageous protective layer on the pore opening, preventing solvent penetration at the limit of pore size <0.7nm (UMP)/>0.7nm (supermicropores). However, active material is also irreversibly lost, which is not available as an electron carrier and limits the specific capacity of the sulfur cathode. Single lithium-ion conducting solid polymer electrolytes (SLIC-SPEs) based on lithiated Nafion membrane have been investigated, utilizing the chemical and thermal stability of this kind of ionomers. A mixture of ethylene carbonate (EC) and propylene carbonate (PC) was tested as membrane swelling solvent and was beneficial compared to glycol ether or linear carbonates in terms of high ion mobility while avoiding phase separation issues and undesired leaching. The combination of Li-Nafion_EC:PC membrane with sulfur-infiltrated ultramicroporous carbon composite cathodes facilitating a quasi-solid-state lithium-sulfur full cell. Comprising carbonate compatibility and high active material utilization thanks to reduced amount of solvent in the cell and spatial concentration in the membrane and its interfaces. Electrochemical performance of the full cell was investigated elaborately and compared with a reference system using solely the liquid EC:PC electrolyte. While rapid capacity fading can be observed for the liquid electrolyte cell, an enduring high capacity is found in the lithiated Nafion system. For better understanding of these effects, cycling analysis was accompanied by implementing impedance spectroscopy upon galvanostatic cycling. The electrochemical characterization indicates the formation of a stable film and high degree of reversibility during cycling of the lithiated-Nafion cell whereas an increased film resistance due to sulfur cross over and electrolyte decomposition was observed within the liquid electrolyte system. 1 Helen, M.; Diemant, T.; Schindler, S.; Behm, R. J.; Danzer, M.; Kaiser, U.; Fichtner, M.; Anji Reddy, M. Insight into Sulfur Confined in Ultramicroporous Carbon. ACS Omega 2018, 3, 11290-11299. 2 Maria Joseph, H.; Fichtner, M.; Munnangi, A. R. Perspective on ultramicroporous carbon as sulphur host for Li–S batteries. Journal of Energy Chemistry 2021, 59, 242-256. 3 Yim, T.; Park, M.-S.; Yu, J.-S.; Kim, K. J.; Im, K. Y.; Kim, J.-H.; Jeong, G.; Jo, Y. N.; Woo, S.-G.; Kang, K. S.; Lee, I.; Kim, Y.-J. Effect of chemical reactivity of polysulfide toward carbonate-based electrolyte on the electrochemical performance of Li–S batteries. Electrochimica Acta 2013, 107, 454-460. 4 Nojabaee, M.; Sievert, B.; Schwan, M.; Schettler, J.; Warth, F.; Wagner, N.; Milow, B.; Friedrich, K. A. Ultramicroporous carbon aerogels encapsulating sulfur as the cathode for lithium–sulfur batteries. Journal of Materials Chemistry A 2021, 9, 6508-6519.