No abstract

Sulfurized polyacrylonitrile (S-cPAN) has been recognized as a particularly promising cathode material for lithium–sulfur (Li–S) batteries due to its ultra-stable cycling performance and high degree of sulfur utilization. Though the synthetic conditions and routes for modification of S-cPAN have been extensively studied, details of the molecular structure of S-cPAN remain yet unclear. Herein, a more reasonable molecular structure consisting of pyridinic/pyrrolic nitrogen (NPD/NPL) is proposed, based on the analysis of combined X-ray photoelectron spectroscopy, 13C/15N solid-state nuclear magnetic resonance, and density functional theory data. The coexistence of vicinal NPD/NPL entities plays a vital role in attracting S2 molecules and facilitating N–S bond formation apart from the generally accepted C–S bond in S-cPAN, which could explain the extraordinary electrochemical features of S-cPAN among various nitrogen-containing sulfurized polymers. This study provides new insights and a better understanding of structural details and relevant bond formation mechanisms in S-cPAN, providing a foundation for the design of new types of sulfurized cathode materials suitable for application in next-generation high-performance Li–S batteries.

Because of the high specific capacity and low redox potential, lithium metal constitutes a promising material and might be an option for high energy density next-generation battery technologies, though application of lithium metal batteries (LMBs) is currently limited by poor long-term performance and severe safety issues when liquid electrolytes are used. These challenges arise from formation of “dead” lithium or inhomogeneous lithium deposits as well as ineffective solid electrolyte interphase (SEI) layers on lithium metal electrodes. Notably, lithium consumed by SEI formation and fractions of “dead” lithium was derived from in situ 7Li nuclear magnetic resonance (NMR) of pouch-type cells, while 19F 1D magnetic resonance imaging (MRI) profiling along with operando optical microscopy analysis revealed the nature of lithium deposits, considering the impact of electrode kinetics on the occurrence of dendritic lithium microstructures, governed by processes of electrodeposition and electrodissolution. Various electrolyte formulations were compared in view of different cell configurations, including Li||Li symmetric cells as well Li||Cu cells, Cu||NMC cells, and finally NMC||Li full cell systems, establishing the origin and likely contributions to irreversible capacity losses while systematically evaluating different active materials (including electrolyte formulations, cathode material, and lithium metal anodes). Indeed, a mixture of film-forming additivesfluoroethylene carbonate (FEC) and lithium difluorophosphate (LiPO2F2)was demonstrated to afford both “better” interfacial/interphasial properties and more homogeneous lithium deposition, thus exhibiting promising electrochemical performance.