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Page 10 of 17 Li et al. Chem Synth 2023;3:30 https://dx.doi.org/10.20517/cs.2023.16

resolution spectrum of carbon and Se of Se@MIL-68-800 and Se@MIL-100-800 are shown in
Supplementary Figure 6A and B and Supplementary Figure 6D-F, respectively. The weight ratio of Se from
the survey scan of Se@MIL-53-800, Se@MIL-68-800, and Se@MIL-100-800 is 54.3%, 53.6%, and 45.4%,
respectively, being in very excellent consistency with the results from TGA.

Electrochemical properties
The electrochemical properties of Se@MIL-53-800, Se@MIL-68-800, and Se@MIL-100-800 cathodes are
presented in Figure 4 and Supplementary Figure 7, compared with pure Se electrode prepared as a
-1
reference. Cyclic voltammogram (CV) was collected at a scan rate of 0.1 mV s with a potential range of
1.75–2.6 V vs. Li /Li in Figure 4A and Supplementary Figure 7A-C. All the Se cathodes with MOF-derived
+
porous carbon host show two obvious reduction peaks at approximately 2.1 and 1.95 V in the first cycle,
relevant to the stepwise electrochemical reaction from Se to (Li Se ), finally to Li Se . The oxidation peak,
[65]
2
2
n
mainly at 2.25 V, and a shoulder peak at 2.35 V correspond to the Li Se back to Se with the intermediate Li 2
2
Se , respectively. Compared with pure Se cathode, the reduction of Se@MIL-(53, 68, 100)-800 occurs on
n
higher voltage in the first cycle, as the value labeled in Figure 4A and Supplementary Figure 7A-C, reflecting
the accelerated electrochemical reaction kinetics on these three cathodes. The reduction peak at 2.1 V shifts
to a higher potential of 2.21 V after six cycles of battery operation [Figure 4A]. The shift of the first cathodic
peak is due to the activation of Se particles and the formation of a stable solid electrolyte interphase (SEI)
layer [39,66,67] . The high potential shift of the first reduction peak means easier reduction reaction from Se to Li 2
Se , indicating the Se cathodes were activated in the discharge/charge process. After activation through the
n
first four cycles, the CV curves of these Se@MIL-X-800 (X = 53,68,100), overlapped well, demonstrating
improved reversibility of these batteries . The compared result of these three MOF-derived cathodes on
[68]
th
the 5 cycle is shown in Figure 4B. It can be seen that Se@MIL-68-800 possesses the smallest oxidation/
reduction potential gap (ΔV) and the highest current density than Se@MIL-53-800 and Se@MIL-100-800,
[69]
suggesting the accelerated reaction kinetics in Se@MIL-68-800 cathode . It is worth noting that the pure Se
cathode shows significantly different electrochemical behavior compared to the three MOF-derived porous
carbon materials when used as carbon hosts for the Li-Se batteries. The pure Se cathode has the lowest
reduction voltage at the first cycle due to the lack of pathways, while after five cycles, its reduction peak
shifts to a higher voltage position than the other three cathodes. The phenomenon may be attributed to the
easy diffusion and reaction of the polyselenides that have diffused out of the cathode, as they do not
encounter significant interfacial barriers. Moreover, the detailed discharge/charge curves of the different
st
th
th
electrodes at the 1 , 50 , 100 , 150 , and 200 cycles are also shown in Figure 4C and Supplementary Figure
th
th
7D-F. The discharge curves show two typical platforms, in good consistence with the cyclic voltametric
measurements containing two reduction peaks. The lowest gap between the discharge and charge platforms
(overpotential) of Se@MIL-68-800 (0.1 V) among all the other three cathodes (Se@MIL-53 with 0.12 V,
Se@MIL-100-800 with 0.17 V, and pure Se with 0.17 V) means its lowest polarization compared to the other
three batteries, indicating the smallest reaction energy barrier .
[70]
The cycling performance and CE were evaluated at 0.2 C between 1.75–2.6 V. The initial discharge capacity
of Se@MIL-53-800, Se@MIL-68-800, and Se@MIL-100-800 shows much higher values of 704.5, 648.3, and
473.7 mA h g , respectively, compared to 342.3 mA h g of pure Se cathode. The much higher initial
-1
-1
capacity than pure Se cathode indicates the much higher utilization efficiency of Se for Se@MIL-53-800,
Se@MIL-68-800, and Se@MIL-100-800. Because the highly porous carbon can lead to a large dispersion of
Se in the carbon host system, facilitating the contact between carbon and Se for better electrochemical
activity. The initial discharge capacity of Se@MIL-53-800 is higher than the theoretical value (675 mA h g ),
-1
and that of Se@MIL-68-800 is very similar to their theoretical value, while the Se@MIL-100-800 achieves a
capacity value that is far from its theoretical value. This is due to the lack of interconnected micro-
mesopores constructed electrolyte pathways for the Se@MIL-100-800 cathode, in spite of its high CE value