Oxygen Vacancy-Rich Mixed-Valence Cerium MOF: An Efficient Separator Coating to High-Performance Lithium–Sulfur Batteries

Mixed-valence
metal–organic frameworks (MOFs) have exhibited
unique potential in fields such as catalysis and gas separation. However,
it is still an open challenge to prepare mixed-valence MOFs with isolated
Ce­(IV, III) arrays due to the easy formation of CeIII under
the synthetic conditions for MOFs. Meanwhile, the performance of Li–S
batteries is greatly limited by the fatal shuttle effect and the slow
transmission rate of Li+ caused by the inherent characteristics
of sulfur species. Here, we report a mixed-valence cerium MOF, named
CSUST-1 (CSUST stands for Changsha University of Science and Technology),
with isolated Ce­(IV, III) arrays and abundant oxygen vacancies (OVs),
synthesized as guided by the facile and elaborate kinetic stability
study of UiO-66­(Ce), to work as an efficient separator coating for
circumventing both issues at the same time. Benefiting from the synergistic
function of the Ce­(IV, III) arrays (redox couples), the abundant OVs,
and the open Ce sites within CSUST-1, the CSUST-1/CNT composite, as
a separator coating material in the Li–S battery, can remarkably
accelerate the redox kinetics of the polysulfides and the Li+ transportation. Consequently, the Li–S cell with the CSUST-1/CNT-coated
separator exhibited a high initial specific capacity of 1468 mA h/g
at 0.1 C and maintained long-term stability for a capacity of 538
mA h/g after 1200 cycles at 2 C with a decay rate of only 0.037% per
cycle. Even at a high sulfur loading of 8 mg/cm2, the cell
with the CSUST/CNT-coated separator still demonstrated excellent performance
with an initial areal capacity of 8.7 mA h/cm2 at 0.1 C
and retained the areal capacity of 6.1 mA h/cm2 after 60
cycles.