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 additivesfluoroethylene
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.
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