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Lithium-sulfur batteries (li -sulfur batteries) have received much attention as one of the next-generation energy storage systems due to their high energy density (2600 Wh Kg-1), low cost, and environmental friendliness. However, the slow conversion of intermediate lithium polysulfides (LiPSs, also known as Li2Sn, 4 ≤ n ≤ 8) to discharge products (Li2S2/Li2S) leads to the accumulation of LiPSs in the organic electrolyte, where soluble LiPSs tend to migrate between electrodes under a concentration gradient, resulting in the so-called “shuttle effect “ problem. This leads to low sulfur utilization and rapid capacity decay, hindering the practical application of Li-S batteries.
To address the above challenges, physical adsorption and chemical anchoring strategies have been used to confine or block LiPSs on the cathode side, both of which are essentially passive solutions as they cannot effectively inhibit the dissolution and accumulation of LiPSs During the solid-liquid-solid sulfur reduction reaction (SRR), the kinetic differences of the various steps result in the inability of the generated LiPSs to be rapidly converted to insoluble Li2S2 /Li2S, which is the root cause of the shuttle effect. Recently, a p-charge descriptor was obtained to predict the SRR activity and the maximum p-charge gain obtained for Bi2S3 has the lowest energy barrier in the rate-determining step (RDS, conversion of lips to Li2S2/Li2S) of p-block metal sulfides, which greatly improves the performance of the cell Therefore, finding a high-activity catalyst to lower the energy barrier of RDS is the key to suppressing the shuttle effect The catalytic conversion of SRR involves an adsorption-conversion-deposition process, which remains a challenge for single-component catalysts.
The main point of this paper
Characterization of metal oxides and nitrides:
Metal oxides (e.g., TiO2, VO2) have strong adsorption capacity for LiPSs (lithium polysulfides), but limited catalytic conversion activity.
Metal nitrides (e.g. TiN, VN) have high catalytic activity for LiPSs, but weak adsorption capacity.
Application of heterogeneous structures:
Heterostructures with two or more components (e.g., WS2-WO3, TiO2-TiN, MoN-VN, Co-MoN, and Bi-Bi2O3) were screened to balance the adsorption-conversion-deposition process and inhibit the shuttle effect.
Advantages of tungsten catalysts:
The tungsten catalyst exhibits excellent catalytic activity and promotes the nucleation and decomposition of Li2S, conferring excellent multiplicative performance of Li-S batteries.
Role of polar tungsten carbide (W2C):
W2C has a strong adsorption capacity for soluble LiPSs, avoiding their diffusion into the electrolyte and inhibiting the shuttle effect.
Synthesis of W-W2C heterostructures:
The W-W2C heterostructure (W-W2C/G) catalyst was rapidly synthesized using a flash Joule heating method.
Application of W-W2C/G catalysts:
As a catalytic intermediate layer coating on CNTs/S@W-W2C/G cathode, intercepting LiPSs, accelerating the RDS (rate-determining step) process, and promoting the nucleation and growth of Li2S.
Role of carbon nanotubes (CNTs):
CNTs are used as sulfur carriers due to their high electrical conductivity and high specific surface area
Shuttle effect problem:
Accumulation of lithium polysulfide in the electrolyte leads to a barrier to the practical application of lithium-sulfur batteries.
Catalyst design:
The design of effective catalysts is necessary to improve the conversion of polysulfides.
Synthesis of W-W2C heterostructures:
A one-step flash Joule heating route was used to synthesize W-W2C heterostructures (W-W2C/G) on graphene substrates as catalytic interlayers.
Theoretical calculations:
The difference in the figure of merit between W (5.08 eV) and W2C (6.31 eV) generates an internal electric field at the interface of the heterostructures, which accelerates the motion of electrons and ions and promotes the sulfur reduction reaction (SRR) process.
In situ Raman analysis:
The high catalytic activity of W-W2C/G is confirmed by the reduction of activation energy and the inhibition of polysulfide shuttling.
Li-S battery performance:
With the W-W2C/G intercalation, the Li-S battery demonstrated excellent rate performance (665 mAh g-1 at 5.0 C) and stable cycling performance.
Cycling 1000 cycles at a high rate of 3.0 C with a low decay rate of 0.06%
In summary, this study presents a general method for synthesizing heterostructured catalysts such as W-W2C/G and Mo-Mo2C/G heterostructures using ultrafast flash Joule heating and examines their effectiveness as catalytic interlayers for lithium-ion batteries. A series of experimental measurements including in situ Raman properties and theoretical calculations verified that the W-W2C heterostructures have a strong adsorption capacity for lithium-ion batteries and generate an internal electric field at the interface of the heterostructures, which ensures a fast charge transfer, greatly suppresses the shuttle effect in lithium-ion batteries, enhances the kinetic performance of the SRRs and improves electrochemical performances.The W-W2C intercalation allows the The W-W2C intercalation enables Li-ion batteries to achieve a high capacity retention of 80.6% after 100 cycles at 0.2 C, even with a high sulfur loading of 7.9 mg cm-2 , and excellent stability after 1000 cycles at 3.0 C. The W-W2C intercalation provides a high capacity retention of 80.6% even with a high sulfur loading of 7.9 mg cm-2 .
