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Page 12 of 17 Li et al. Chem Synth 2023;3:30 https://dx.doi.org/10.20517/cs.2023.16

[73]
utilization of Se . The Se@MIL-68-800 and Se@MIL-53-800 exhibit very similar high discharge capacities,
with the current density increasing from 0.1 C to 5 C due to the high proportion of mesopores and
-1
macropores in the corresponding two host materials. The capacity value of Se@MIL-68-800 (307 mA h g )
-1
is higher than that of Se@MIL-53-800 (292 mA h g ) at the current density of 5 C, reflecting its better pore
size cooperation to accelerate the transportation of electrons/ions. When the current density returns to 0.1
C, a high reversible capacity of 544 mA h g is achieved for Se@MIL-68-800, indicating fast electrode
-1
reaction kinetics. The best rate capability of Se@MIL-68-800 is probably stemmed from its large proportion
of large mesopores, which can ensure the rapid electrolyte transportation and shorten the distance between
active Se in the cathode. The capacity of Se@MIL-100-800 and pure Se cathode can be only 220 and 130 mA
-1
-1
h g at 5 C and the reversible capacities of 380 and 295 mA h g at 0.1 C, respectively, demonstrating
deteriorated rate performance because of inappropriate porous structures. From the cycling performance,
CE, and rate capability test, it is clearly seen that Se@MIL-68-800 performs better than Se@MIL-53-800,
Se@MIL-100-800, and pure Se in Li-Se battery. Moreover, compared with the reported papers with similar
Se host materials [74-76] , the Se@MIL-68-800 cathode achieves a promising electrochemical performance, as
shown in Supplementary Table 3.

To obtain further insight into the mechanism of improved cycle and rate performance, EIS analysis was
carried out [Figure 4F]. The curves are composed of a semicircle at high frequency followed by a straight
line at low frequency. The start point corresponds to the Ohmic resistance (R ) of the whole battery [77-79] ,
Ω
while the diameter of the semicircle corresponds to the interface resistance between electrode and
electrolyte (also called charge transfer resistance, R ) [80-82] . The fitted circuit diagram is shown in the inset.
ct
The interface resistance has a huge influence on the Li-ion and electron transportation. The R values of
ct
Se@MIL-53-800, Se@MIL-68-800, Se@MIL-100-800, and pure Se cathode were 95, 48, 93, and 162 Ω,
respectively. The smallest of Se@MIL-68-800 indicates its fastest reaction kinetics in the discharge/charge
process.

Figure 5 schematically illustrates the mechanism of the MIL-53-800, MIL-68-800, and MIL-100-800 as hosts
for Se. For the Se@MIL-53-800 composites (I), there are some interconnected large mesopores that traverse
the whole particles, along with a large number of small mesopores and numerous micropores [Figure 2C]
to carry selenium. This designed structure could ensure a high rate of electrolyte transfer and enough space
for Se loading and reaction sites. For Se@MIL-68-800 (II), the difference in pores distribution compared to
Se@MIL-53-800 is that the large mesopores occupy a larger proportion, while small mesopores are less
abundant. At the same time, the number of micropores becomes a little bit less [Figure 2C], but it is still
enough for Se loading. The enlarged pathways accelerate the mass transfer of ions and electrolytes. In the
case of Se@MIL-100-800 (III), although there are plentiful micropores for loading Se, the lack of pathway of
mesopores leads to slow mass transfer and inadequate reaction. The compromise between the efficient
adsorption of polyselenides, fast electrolyte transfer, and fast Li-ion transportation should be well
considered. Therefore, the balance of micropores (providing space for Se loading, reaction sites, and strong
adsorption to polyselenides) and mesopores (pathways for Se loading and ions/electrons transportation) is
critical for high reaction kinetics achievement. Se@MIL-68-800 cathode achieved the best electrochemical
performances due to the optimized distribution and ratio of micropores and mesopores. The excellent
electrochemical performances of Se@MIL-68-800 can be attributed to the following reasons: (1) High
specific surface area and pore volume of high conductivity carbon materials are necessary to achieve good
performance due to better Se dispersion, high utilization, and volume expansion suppression; (2) The
rational range of micropores that provide Se loading space and interconnected with micropores-mesopores
that shorten the pathways of the electrolyte can maximize the battery performance; (3) Most importantly,
the favorable MOF-derived hierarchically porous carbon including the optimized ratio of micropores and