Li–S batteries present great potential to realize
high-energy-density
storage, but their practical implementation is severely hampered by
the notorious polysulfide shuttling and the sluggish redox kinetics.
While rationally designed redox mediators can optimize polysulfide
conversion, the efficiency and stability of such a mediation process
still remain formidable challenges. Herein, a strategy of constructing
a “dual mediator system” is proposed for achieving efficient
and durable modulation of polysulfide conversion kinetics by coupling
well-selected solid and electrolyte-soluble mediators. Theoretical
prediction and detailed electrochemical analysis reveal the structure–activity
relationships of the two mediators in synergistically optimizing the
redox conversions of sulfur species, thus achieving a deeper mechanistic
understanding of a function-supporting mediator system design toward
sulfur electrochemistry promotion. Specifically, such a dual mediator
system realizes the bridging of full-range “electrochemical
catalysis” and strengthened “chemical reduction”
processes of sulfur species as well as greatly suppressed mediator
deactivation/loss due to the beneficial interactions between each
mediator component. Attributed to these advantageous features, the
Li–S batteries enable a slow capacity decay of 0.026% per cycle
over 1200 cycles and a desirable capacity of 8.8 mAh cm–2 with 8.2 mg cm–2 sulfur loading and lean electrolyte
condition. This work not only proposes an effective mediator system
design strategy for promoting Li–S battery performance but
also inspires its potential utilization facing other analogous sophisticated
electrochemical conversion processes.