Single-atom catalysts based on C2N for sulfur cathodes in Na-S batteries: a first-principles study.

Several major roadblocks, including the "shuttle effect" caused by the dissolved higher-order sodium polysulfides (NaPSs), extremely poor conductivity of sulfur cathodes, and sluggish conversion kinetics of charging-discharging reactions, have hindered the commercialization of sodium-sulfur batteries (NaSBs). In our study, representative C2N-based single-atom catalysts (SACs), TM@C2N (TM = Fe, Ni and V), are proposed to improve the comprehensive performance of NaSBs. Based on first-principles calculations, we first discuss in detail the anchoring behavior of all adsorption systems, TM@C2N/(S8 and NaPSs). The results indicate that compared to pristine C2N, TM@C2N substrates exhibit a stronger capability to capture S8/NaPSs clusters through physical/chemical binding, with V@C2N showing the most outstanding capability ranging from -2.37 to -5.03 eV. The density of states analysis reveals that metallic properties can be well maintained before and after adsorption of polysulfides. More importantly, TM@C2N configurations can greatly reduce the energy barriers of charging and discharging reactions, thereby accelerating the conversion efficiency of NaSBs. It is worth mentioning that V@C2N has lower charge-discharge energy barriers and Na ion migration rates, since the embedded TM atom weakens the strong binding of Na+ in the N6 cavity of C2N. The intrinsic mechanism analysis reveals that the interaction between the d orbitals of V and the p orbitals of S leads to the weakening of Na-S bonds, which can not only effectively inhibit the shuttle effect, but also promote the dissociation of Na2S. Overall, this work not only offers excellent catalytic materials, but also provides vital guidance for designing SACs in NaSBs.

Regulating the coordination environment of single atom catalysts anchored on C3N monolayer for Li-S battery by first-principles calculations.

Owing to the extremely high theoretical specific capacity and energy density, the catalytic materials of lithium-sulfur (Li-S) batteries are widely explored. The "shuttle effect", poor electrode conductivity, and slow charge-discharge reaction dynamics are some of the key issues that have seriously hampered their commercialization process. Herein, based on the density-functional-theory (DFT), the catalytic performances of a series of single-atom catalysts (SACs) designed by regulating the N-content around coordination center in C3N (TM@N2C2/N3C/N4-C3N (TM = Ti, V, Fe, Co, Ni)), are systematically analyzed and evaluated. Among all the constructed SACs, Ti-centered configurations with fewer d electrons, especially for the Ti@N2C2-C3N, have the remarkable catalytic effect in improving the electron conductivity, trapping soluble polysulfides and accelerating the redox reaction. The in-depth mechanism indicates that the interaction between d orbital of Ti, mainly the splitting [Formula: see text] , and p orbital of S is the key factor for achieving high-effective adsorption. More importantly, the integral value of crystal orbital Hamiltonian population (ICOHP) of the Li-S bond in the adsorbed Li2S can serve as an excellent descriptor for evaluating the overall catalytic ability of substrates. Our work has vital guiding significance for designing high-performance SACs of Li-S batteries.

Rectangular Transition Metal-rTCNQ Organic Frameworks Enabling Polysulfide Anchoring and Fast Electrocatalytic Activity in Li-Sulfur Batteries: A Density Functional Theory Perspective.

Two-dimensional metal-organic frameworks (MOFs) have shown great development po-tential in the field of lithium-sulfur (Li-S) batteries. In this theoretical research work, we propose a novel 3d transition metals (TM)-embedded rectangular tetracyanoquinodimethane (TM-rTCNQ) as a potential high-performance sulfur host. The calculated results show that all TM-rTCNQ structures have excellent structural stability and metallic properties. Through exploring different adsorption patterns, we discovered that TM-rTCNQ (TM = V, Cr, Mn, Fe and Co) monolayers possess moderate adsorption strength for all polysulfide species, which is mainly due to the existence of the TM-N4 active center in these frame systems. Especially for the non-synthesized V-rCTNQ, the theoretical calculation fully predicts that the material has the most suitable adsorption strength for polysul-fides, excellent charging-discharging reaction and Li-ion diffusion performance. Additionally, Mn-rTCNQ, which has been synthesized experimentally, is also suitable for further experimental con-firmation. These findings not only provide novel MOFs for promoting the commercialization of Li-S batteries, but also provide unique insights for fully understanding their catalytic reaction mecha-nism.

Structural screening and descriptor exploration of black phosphorus carbide supported bifunctional catalysts for lithium-sulfur batteries.

Developing optimal catalysts, to suppress the shuttling of lithium polysulfides (LiPSs), and serving as bifunctional catalyst with fast discharge-charge reaction kinetics, are essential for the practical applications of Li-S batteries. Herein, based on density functional theory (DFT) calculations, single-atom catalysts formed by embedding 3d transition metals (TMs) into the nitrogen doped defective black phosphorus carbide (TM@N4-CP) are systematically explored toward fast kinetics in Li-S batteries. Remarkably, V@N4-CP, possessing excellent metallic features, outstanding structural stability, suitable binding and easy diffusion for LiPSs, eventually stands out as the promising bifunctional electrocatalyst. Our results unveil that d-p orbital hybridization between transition metal (TM) atom and sulfur species is accompanied by weakened surrounding Li-S bonds. Consequently, the formation of TM-S bonds not only ensures inhibition of LiPSs shuttling, but also promotes the dissociation of Li2S. With the analysis of correlation map of key parameters, the ICOHP values of TM-S bonds and adsorption energy of *Li2S are identified and proposed as descriptors for fast screening towards fast reaction kinetics. Our work shows a feasible strategy for the rational design and retrieval of the decisive feature of active catalysts for Li-S batteries.

Transition metal decorated phthalocyanine as a potential host material for lithium polysulfides: a first-principles study.

The shuttle effect caused by the soluble long-chain lithium polysulfides greatly hinders the practical application of lithium-sulfur (Li-S) batteries. Therefore, the introduction of suitable anchoring materials is more effective to mitigate this problem. Transition metal phthalocyanines (TMPc) are regarded as a new class of sulfur host materials. Here, 4d transition metal (Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd) decorated phthalocyanines are designed and systematically researched for the performance analysis of anchoring S8/LiPSs by first-principles calculations. The results reveal that the bonding strength of LiPSs can be well adjusted by introducing suitable 4d transition metals into the phthalocyanine structure. The electronic structure analysis indicates the formation of TM-S bonds between the TMPc substrate materials and the LiPSs, which is essential to weaken the Li-S bonds and hence slow down the shuttle effect of LiPSs. ZrPc and NbPc both exhibit excellent potential and thermal stability for facilitating the conversion of LiPSs, as well as a better promoting effect for the sulfur reduction reactions (SRR) with a reduced Gibbs free energy in the rate-determining step (*Li2S2 → *Li2S) during the discharge reaction process. These findings in our work may encourage further experimental and theoretical research for anchoring LiPSs with TMPc as a host material.