Modulating the Coordination Chemistry of Cobalt Catalytic Sites by Ruthenium Species to Accelerate the Polysulfide Conversion Kinetics in Lithium-Sulfur Batteries

The performance of lithium-sulfur batteries is compromised by the loss of sulfur as dissolved polysulfides in the electrolyte and consequently the polysulfide redox shutting effect. Accelerating the conversion kinetics of polysulfide intermediates into sulfur or lithium sulfide through electrocatalysis has emerged as a root-cause solution. Co-N-C composite electrocatalyst is commonly used for this purpose. It is illustrated here that how the effectiveness can be improved by modulating the coordination chemistry of Co-N-C catalytic sites through introducing Ru species (RuCo-NC). The well-dispersed Ru in the Co-NC carbon matrix altered the total charge distribution over the Co-N-C catalytic sites and led to the formation of electron-rich Co-N, which is highly active for the polysulfide conversion reactions. Using Ru to modulate the electronic structure in the Co-N-C configuration and the additional catalytic sites over the Ru-Nx species can manifest optimal adsorption behavior of polysulfides. Consequently, the sulfur cathode with RuCo-NC can reduce the capacity fade rate from 0.11 % per cycle without catalyst (initial capacity of 701 mAh g-1) to 0.054 % per cycle (initial capacity of 1074 mAh g-1) over 400 cycles at 0.2 C rate. The results of this study provide the evidence for a feasible catalyst modification strategy for the polysulfide electrocatalysis.

Diminishing Self-Discharge of High-Loading Li-S Batteries with Oxygen-Rich Biomass Carbon Interlayers

Lithium-sulfur batteries (Li-S) have possessed gratifying development in the past decade due to their high theoretical energy density. However, the severe polysulfide shuttling provokes undesirable self-discharge effect, leading to low energy efficiency in Li-S batteries. Herein, an interlayer composed of oxygen-rich carbon nanosheets (OCN) derived from bagasse is elaborated to suppress the shuttle effect and reduce the resultant self-discharge effect. The OCN interlayer is able to physically block the shuttling behavior of polysulfides and its oxygen-rich functional groups can strongly interact with polysulfides via O-S bonds to chemically immobilize mobile polysulfides. The self-discharge test for seven days further shows that the self-discahrge rate is diminished by impressive 93 %. As a result, Li-S batteries with the OCN interlayer achieve an ultrahigh discharge specific capacity of 710 mAh g-1 at a high mass loading of 7.18 mg. The work provides a facile method for designing functional interlayers and opens a new avenue for realizing Li-S batteries with high energy efficiency.

Polyoxomolybdate-Based Metal-Organic Framework-Derived Cu-Embedded Molybdenum Dioxide Hybrid Nanoparticles as Highly Efficient Electrocatalysts for Al-S Batteries

High-performance rechargeable aluminum-sulfur batteries (RASB) have great potential for various applications owing to their high theoretical capacity, abundant sulfur resources, and good safety. Nevertheless, the practical application of RASB still faces several challenges, including the polysulfide shuttle phenomenon and low sulfur utilization efficiency. Here, we first developed a synergistic copper heterogeneous metal oxide MoO2 derived from polymolybdate-based metal-organic framework as an efficient catalyst for mitigating polysulfide diffusion. This composite enhances sulfur utilization and electrical conductivity of the cathode. DFT calculations and experimental results reveal the catalyst Cu/MoO2@C not only effectively anchors aluminum polysulfides (AlPSs) to mitigate the shuttle effect, but also significantly promotes the catalytic conversion of AlPSs on the sulfur cathode side during charging and discharging. The unique nanostructure contains abundant electrocatalytic active sites of oxide nanoparticles and Cu clusters, resulting in excellent electrochemical performance. Consequently, the established RASB exhibits an initial capacity of 875 mAh g-1 at 500 mA g-1 and maintains a capacity of 967 mAh g-1 even at a high temperature of 50 °C.

Tandem Catalysis inside Double-Shelled Nanocages with Separated and Tunable Atomic Catalyst Sites for High Performance Lithium-Sulfur Batteries

Single-atomic catalysts are effective in mitigating the shuttling effect and slow redox kinetics of lithium polysulfides (LiPSs) in lithium-sulfur (Li-S) batteries, but their ideal performance has yet to be achieved due to the multi-step conversion of LiPSs requiring multifunctional active sites for tandem catalysis. Here double-shelled nano-cages (DSNCs) have been developed to address this challenge, featuring separated and tunable single-atom sites as nano reactors that trigger tandem catalysis and promote the efficient electrochemical conversion of LiPSs. This enables high capacity and durable Li-S batteries. The DSNCs, with inner Co-N4 and outer Zn-N4 sites (S/CoNC@ZnNC DSNCs), exhibit a high specific capacity of 1186 mAh g-1 at 1 C, along with a low capacity fading rate of 0.063% per cycle over 500 cycles. Even with a high sulfur loading (4.2 mg cm-2) and a low E/S ratio (6 µL mg-1), the cell displays excellent cycling stability. Moreover, the Li-S pouch cells are capable of stable cycling for more than 160 cycles. These results demonstrate the feasibility of driving successive sulfur conversion reactions with separated active sites, and are expected to inspire further catalyst design for high performance Li-S batteries.

Bivalent Cobalt as Efficient Catalyst Intercalation Layer Improves Polysulfide Conversion in Lithium-Sulfur Batteries

Herein, we investigated in detail the effect of metal valences in different cobalt-based organic framework compounds on the kinetics of sulfur reaction in lithium-sulfur batteries (LSBs). On this basis, two organic framework compounds of zeolite-imidazole-based cobalt organic framework compound (Co-ZIF) and tetrakis(4-benzoic acid) porphyrinato-Co^III chloride [Co-TBP(III)] with different valences were constructed as the functional intercalation separators of LSBs, and explored the effects of different valences on improving the reaction kinetics of polysulfides and inhibiting the shuttle effect. Experiments and theoretical calculations prove that Co^II exhibits the best catalytic activity. This is mainly due to the fact that +2 valence shows a strong adsorption energy for polysulfides and a higher Fermi level compared with +3 valence, thus improving the efficiency of the rapid catalytic conversion of sulfur species. As expected, the discharge specific capacity of Co-ZIF as the catalytic layer of the LSBs reached 772.7 mAh g^-1 at a high current density of 5 C. More importantly, the initial specific capacity is 839.6 mAh g^-1 at high current 3 C, and after 720 cycles, the attenuation rate of per cycle is only 0.092 %, and the coulombic efficiency remains above 92 %.

Flower-Like NiS2 /WS2 Heterojunction as Polysulfide/sulfide Bidirectional Catalytic Layer for High-Performance Lithium-Sulfur Batteries

The slow sulfur oxidation-reduction kinetics are one of the key factors hindering the widespread use of lithium-sulfur batteries (LSBs). Herein, flower-shaped NiS₂ -WS₂ heterojunction as the functional intercalation of LSBs is successfully prepared, and effectively improved the reaction kinetics of sulfur. Flower-like nanospheres composed of ultra-thin nanosheets (≤10 nm) enhance quickly transfer of mass and charge. Meanwhile, the heterostructures simultaneously serve as an electron receptor and a donor, thereby simultaneously accelerating the bidirectional catalytic activity of reduction and oxidation reactions in the LSBs. In addition, the adsorption experiment, chemical state analysis of elements before and after the reaction and theoretical calculation have effectively verified that NiS₂ -WS₂ heterojunction nanospheres optimize the adsorption capacity and bidirectional catalytic effect of polysulfides. The results show that the initial discharge capacity of NiS₂ -WS₂ functional intercalation is as high as 1518.7 mAh g^-1 at 0.2 C. Even at a high current density of 5 C, it still shows a discharge specific capacity of 615.7 mAh g^-1 , showing excellent rate performance. More importantly, the capacity is 258.9 mAh g^-1 after 1500 cycles at 5 C, and the attenuation per cycle is only 0.039%, and the Coulomb efficiency remains above 95%.

Using sp2 N atom anchoring effect to prepare ultrafine vanadium nitride particles on porous nitrogen-doped carbon as cathode for lithium-sulfur battery

Porous carbon-supported transition metals and their compounds have attracted much attention as sulfur host materials for cathodes of lithium-sulfur batteries, due to their high chemisorption capacity and ability to catalyze the conversion of polysulfides. However, actual activity of these materials is not very high because of low specific surface areas of transition metal compounds synthesized at high temperatures. In this study, ultra-fine vanadium nitride particles with an average particle size of ca. 4 nm (VN/M/NC) are successfully grown on the surface of nitrogen-doped three-dimensional carbon using sp² nitrogen atoms, resulting from melamine pyrolysis in the presence of ammonium metavanadate, as anchor points to lock vanadium atoms in the VN/M/NC material. When used as a cathode for lithium-sulfur battery, VN/M/NC demonstrates initial discharge specific capacity of 1080 mAh g^-1 at 0.2 C, and retains a discharge capacity of 475 mAh g^-1 at a high rate of 2 C. With capacity attenuation of only 0.037% per cycle after 500 cycles at 1 C, the newly obtained VN/M/NC can be a promising cathode material for lithium-sulfur batteries.

CoS2@montmorillonite as an efficient separator coating for high-performance lithium-sulfur batteries

The shuttle effect and sluggish redox kinetic of polysulfides still hinder the large-scale application of lithium-sulfur (Li-S) batteries. Herein, we adopt a CoS2-intercalated/coated-montmorillonite (CoS2@montmorillonite) composite to work as an efficient...

Flexible electrodes with high areal capacity based on electrospun fiber mats

The ever-growing portable, flexible, and wearable devices impose new requirements from power sources. In contrast to gravitational metrics, areal metrics are more reliable performance indicators of energy storage systems for portable and wearable devices. For energy storage devices with high areal metrics, a high mass loading of the active species is generally required, which imposes formidable challenges on the current electrode fabrication technology. In this regard, integrated electrodes made by electrospinning technology have attracted increasing attention due to their high controllability, excellent mechanical strength, and flexibility. In addition, electrospun electrodes avoid the use of current collectors, conductive additives, and polymer binders, which can essentially increase the content of the active species in the electrodes as well as reduce the unnecessary physically contacted interfaces. In this review, the electrospinning technology for fabricating flexible and high areal capacity electrodes is first highlighted by comparing with the typical methods for this purpose. Then, the principles of electrospinning technology and the recent progress of electrospun electrodes with high areal capacity and flexibility are elaborately discussed. Finally, we address the future perspectives for the construction of high areal capacity electrodes using electrospinning technology to meet the increasing demands of flexible energy storage systems.

Fast polysulfide catalytic conversion and self-repairing ability for high loading lithium-sulfur batteries using a permselective coating layer modified separator

Li-S batteries are considered as one of the most promising battery systems because of their large theoretical capacity and high energy density. However, the "shuttle effect" of soluble polysulfides and sluggish electrochemical redox kinetics of Li-S batteries could cause a broken electrode structure and poor electrochemical performance. Herein, a high-performance and stable Li-S battery has been demonstrated by employing organo-polysulfide chain modified acetylene black (ABPS) as the coating layer on the separator. In addition to the traditional advantages of fast electron transport and polysulfide-interception ability of the carbon coating layer, the grafted organo-polysulfide chain endows the ABPS coating layer with permselectivity for lithium ion against polysulfides, electrocatalytic ability for the sluggish redox kinetics and self-repairing ability for the broken electrode. Hence, the battery prepared using an ABPS-coated separator delivers the best cycling performance (970 mA h g^-1 at 0.2 C after 100 cycles) and rate performance (805 mA h g^-1 at 2 C) as compared to the cells using acetylene black (AB)-coated or Celgard separators. Moreover, the Li-S battery prepared using an ABPS-coated separator exhibits a stable cycling performance at 1 C over 500 cycles with a low degradation of 0.04% per cycle, and a high coulombic efficiency (near 100%). Furthermore, as the sulfur loading was increased to 6.8 mg cm^-2, the Li-S battery using the ABPS-coated separator still could deliver a high areal capacity of 6.03 mA h cm^-2 with a low electrolyte/sulfur ratio (E/S = 4, μL_electrolyte mg_S^-1) after 170 cycles. Significantly, ABPS is an effective coating layer material for improving and stabilizing Li-S batteries.

Self-branched Nb2O5 nanoarrays as “electron-ion reservoirs” to enhance the conversion of polysulfides in flexible Li–S batteries

A novel sulfur matrix integrating self-branched Nb2O5 nanoarrays and flexible carbon cloth (NBCC) is designed. Intriguingly, the selected Nb2O5 nanoarrays can work as “electron-ion reservoirs” to accelerate the conversion reaction of LiPSs.