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Page 2 of 17 Li et al. Chem Synth 2023;3:30 https://dx.doi.org/10.20517/cs.2023.16

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reversible capacity of 544 mA h g when current density returns to 0.1 C. The present invention not only provides a
facile way to obtain hierarchically porous carbon materials from MOFs but also gives insights on tailoring
micropores and mesopores proportion to maximize Li-Se battery performance for their practical industrial
implementation.

Keywords: Metal-organic frameworks (MOFs), hierarchically porous carbon host, lithium-selenium (Li-Se) battery,
physical adsorption

INTRODUCTION
Selenium (Se), from the same group as sulfur (S), has much higher electronic conductivity (1 × 10 S m )
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-3 [1,2]
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compared with S (5 × 10 S m ) and comparable volumetric capacity (3253 mA h cm ) . This makes Li-
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Se one of the most promising candidates and has attracted growing attention. However, the soluble
intermediates lithium polyselenides (Li Se , 4 ≤ n ≤ 8) in Li-Se battery still induce shuttle effects; in addition,
2
n
the Se particles undergo volume expansion, and the final lithium selenide (Li Se) products are not
2
conductive, leading to quick capacity decay, poor cycle performance, and low Coulombic efficiency (CE).
Confining Se in porous carbon materials is the most widely adopted and very efficient strategy to address
these drawbacks. The porous carbon materials have significant influences on the final battery performances
via highly improved conductivity of the electrode and increased reaction areas to adsorb the soluble
polyselenides to relieve the shuttle effect and the volume expansion . The porous carbon materials with
[3-5]
[8]
[6]
[7]
diverse morphologies and architectures, such as spheres , 1D nanotubes , 2D graphene , 3D hierarchical
structures , hollow nanostructures , and core-shell structures , have been applied and shown improved
[11]
[9]
[10]
performance of Li-Se batteries. According to the theoretical calculations using density functional theory
[12]
(DFT), the size of the cyclo-Se is determined to be 0.726 nm . The pores around this size are necessary for
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effectively accommodating and maintaining the loaded active Se in a well-infiltrated short chain-like
amorphous state. Therefore, tailoring the pore size of porous carbon materials plays an important role in Li-
Se batteries. Xin et al. reported microporous carbon coated on carbon nanotube (CNT) (CNT@MPC)
composite consisting of numerous short-range-ordered slit pores of approximately 0.5 nm to confine Se in
order to only form active small Se molecules . The electrochemical behavior of these confined Se chains
[13]
revealed a reversible one-step reaction with lithium (Li) to yield highly active small Se molecules after the
initial discharge. This significantly reduces the shuttle effect in the battery. Liu et al. prepared a microporous
carbon polyhedral with a pore size of 1.1 nm to confine Se . The enlarged microporous carbon
[14]
[6]
nanospheres with a pore size of around 1.3 nm were also utilized to confine Se . Excellent cycling stability
has been observed because the micropores are beneficial for efficient polyselenides adsorption during the
reaction. In a study on the effect of pore size on Li-S battery, Hippauf et al. experimentally demonstrated
that the ultramicropores (less than 0.7 nm), supermicropores (between 0.7 and 2 nm), and mesopores
behave quite differently to polysulfide adsorption in liquid phase with the help of UV/vis absorption
spectroscopy . They reported that the ultramicroporous materials are up to eight times more efficient than
[15]
mesoporous ones in adsorbing polysulfide. However, the solely microporous carbon is limited to the high
proportion of Se loading, while it is detrimental to fast electrolyte diffusion. The typical ordered
mesoporous carbon CMK-3 with a pore size of 3 nm and a high pore volume (1.276 cm g ) was proposed
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to confine Se [16,17] . The mesoporous carbon can confine a higher amount of Se and promote quick electrolyte
penetration. However, the cycling performance fades quickly because of polyselenides formation and easy
dissolution in mesoporous carbons. Moreover, Liu et al. synthesized mesoporous carbon microspheres with
different average pore sizes of 3.8, 5, 6.5, and 9.5 nm, using resorcinol-formaldehyde as a carbon precursor
and silica sol as a hard template. The battery cycling performance decreases with increasing sizes of
mesopores, and they proved that the size of mesoporous carbon host plays a key role in Se