Kinetics Of Redox Research Articles

Enhancing Reversibility and Kinetics of Anionic Redox in O3‐NaLi1/3Mn2/3O2 through Controlled P2 Intergrowth

AbstractAnionic redox chemistry can surpass theoretical limits of conventional layered oxide cathodes in energy density. A recent model system of sodium‐ion batteries, O3‐NaLi1/3Mn2/3O2, demonstrated full anionic redox capacity but is limited in reversibility and kinetics due to irreversible structural rearrangement and oxygen loss. Solutions to these issues are missing due to the challenging synthesis. Here, we harness the unique structural richness of sodium layered oxides and realize a controlled ratio of P2 structural intergrowth in this model compound with the overall composition maintained. The resulted O3 with 27 % P2 intergrowth structure delivers an excellent initial Coulombic efficiency of 87 %, comparable to the state‐of‐the‐art Li‐rich NMCs. This improvement is attributed to the effective suppression of irreversible oxygen release and structural changes, evidenced by operando Differential Electrochemical Mass Spectroscopy and X‐ray Diffraction. The as‐prepared intergrowth material, based on the environmentally benign Mn, exhibits a reversible capacity of 226 mAh g−1 at C/20 rate with excellent cycling stability stemming from the redox reactions of oxygen and manganese. Our work isolates the role of P2 structural intergrowth and thereby introduces a novel strategy to enhance the reversibility and kinetics of anionic redox reactions in sodium layered cathodes without compromising capacity.

Enhancing Reversibility and Kinetics of Anionic Redox in O3-NaLi1/3Mn2/3O2 through Controlled P2 Intergrowth.

Anionic redox chemistry can surpass theoretical limits of conventional layered oxide cathodes in energy density. A recent model system of sodium-ion batteries, O3-NaLi1/3Mn2/3O2, demonstrated full anionic redox capacity but is limited in reversibility and kinetics due to irreversible structural rearrangement and oxygen loss. Solutions to these issues are missing due to the challenging synthesis. Here, we harness the unique structural richness of sodium layered oxides and realize a controlled ratio of P2 structural intergrowth in this model compound with the overall composition maintained. The resulted O3 with 27 % P2 intergrowth structure delivers an excellent initial Coulombic efficiency of 87 %, comparable to the state-of-the-art Li-rich NMCs. This improvement is attributed to the effective suppression of irreversible oxygen release and structural changes, evidenced by operando Differential Electrochemical Mass Spectroscopy and X-ray Diffraction. The as-prepared intergrowth material, based on the environmentally benign Mn, exhibits a reversible capacity of 226 mAh g-1 at C/20 rate with excellent cycling stability stemming from the redox reactions of oxygen and manganese. Our work isolates the role of P2 structural intergrowth and thereby introduces a novel strategy to enhance the reversibility and kinetics of anionic redox reactions in sodium layered cathodes without compromising capacity.

Efficient Ni2V2O7 catalyst for high-performance Li–S batteries

Although Li–S batteries have a high theoretical capacity, their application are significantly hindered by the complex shuttle effect of polysulfides. To address the issues, in this paper, Ni2V2O7 prepared by a simple method is proposed as an efficient catalyst for S cathodes. Ni2V2O7 composed of Ni–O octahedra and V–O tetrahedra has a specific electronic structure. The Ni and V atoms interacted, enabling a fast kinetics of polysulfide redox. Moreover, Ni2V2O7 contains metal sites (Ni and V) and oxygen vacancies, providing a high adsorption capacity and abundant active sites for polysulfide redox. Therefore, Li–S batteries consisting of Ni2V2O7-based cathodes exhibit an excellent cycling stability with a capacity of 1061 mAh g−1 and capacity retention of 87.6% after 150 cycles at 0.2 C. Moreover, at 1 C rate, the batteries exhibit a capacity of 706 mAh g−1 after 300 cycles with a minimal capacity decay of only 0.08% per cycle.

Structure and defect engineering synergistically boost Mo6+/Mo4+ circulation of Sv-MoS2 based photo-Fenton-like system for efficient levofloxacin degradation

The kinetics of redox couples based exclusively on peroxymonosulfate (PMS) are notably slow and inefficient. In this study, urchin-like Sv-MoS2 were in situ deposited onto defect-rich NBC-C3N5 surfaces using a hydrothermal method. The inherent defects in NBC-C3N5@Sv-MoS2 (NCM) along with sulfur vacancies (Sv) directionally inject electrons into Mo sites through a “stabilize-then-accelerate” strategy, culminating in the self-regeneration of Mo sites. Both experimental data and Density Functional Theory (DFT) calculations confirm that NCM effectively activates PMS, generating hydroxyl radicals (•OH) and sulfate radicals (SO4•-) for the degradation of Levofloxacin (LFX) under visible light. The presence of Sv-MoS2 notably extends the peroxy bond (O-O) length in PMS, enhancing electron flux and thereby facilitating superior radical generation compared to singlet oxygen (1O2). Furthermore, experimental assessments of reactors and stirring devices within the NCM system underscore its potential applications in water environmental remediation.

Achieving new insights on rational design and application of double perovskite Y2CrMnO6 nanostructures as potential materials for electrochemical hydrogen storage performance

Fossil fuel consumption and environmental concerns have long motivated the use of electric vehicles around the world. To meet the global energy demand, hydrogen has been proposed as a clean energy carrier with carbon-free combustion. There is, however, a great need for high-performance electrode materials in hydrogen storage applications in order to achieve the desired goals of long life and superior specific capacity. The purpose of our study is to investigate how novel double perovskite Y2CrMnO6 (YCMO) nanostructures can be used as highly active electrocatalysts in hydrogen storage activity by kinetics of redox and physisorption mechanisms. For the first time, spherical-shaped YCMO nanoparticles were fabricated via sol-gel auto-combustion using grape juice as fuel and complexing agent. A series of physic-chemical and electrochemical techniques was used to analyze the electrochemical performance of resultant YCMO samples based on their uniformity, size, and phase purity, including X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FE-SEM), Fourier-Transform Infrared Spectroscopy (FT-IR), Brunauer-Emmett-Teller (BET), Cyclic Voltammetry (CV), chronopotentiometry, electrochemical impedance spectroscopy (EIS), and Tafel polarization. At a constant current of 1 mA in a three electrode set-up, the 15th profile of hydrogen storage capacity of YCMO-15, synthesized with 15 mL of grape juice, reached a maximum value of 465 mAh g−1 at a constant current of 1 mA, which is higher than that of YCMO-5 (355 mAh g−1) and YCMO-10 (350 mAh g−1). The results presented here can also lead to a new plan for developing Cr–Mn-based double perovskite oxides with hydrogen storage properties.

Facile Morphology Modulation to Enhance Stability and Fast Kinetics of Anionic Redox for Li-Rich Layered Oxides Cathodes

Facile Morphology Modulation to Enhance Stability and Fast Kinetics of Anionic Redox for Li-Rich Layered Oxides Cathodes

Reduced graphene oxide/MXene hybrid decorated graphite felt as an effective electrode for vanadium redox flow battery.

Vanadium redox flow battery (VRFB) is a highly suitable technology for energy storage and conversion in the application of decoupling energy and power generation. However, the sluggish reaction kinetics of redox couples is one of the bottlenecks hindering the commercialization of VFFBs. Developing efficient electrode is a promising method to improve the battery performance. In this work, a reduced graphene oxide/Mxene hybrid-decorated graphite felt (rGO/Mxene@GF) is designed to facilitate the kinetics of redox reaction. The electrocatalytic activity and mass transfer of the prepared electrode are investigated through experiment and simulation methods. The results indicate that the favorable mass transfer and the synergistic effect between rGO and Ti3C2Tx Mxene remarkably improve the performance of electrode. The flow cell with rGO/Mxene@GF delivers a good stability up to 100 cycles with a coulombic, voltage, and energy efficiency of 91.6%, 82.7%, and 75.8%, respectively, at a current density of 80 mA cm-2. These findings suggest that the as-prepared rGO/Mxene@GF holds a good application potential in VRFB and provides a promising approach to design efficient electrode for electrochemical devices.

(Invited) Relationship of Pseudo-Capacitive Current in Sulphuric Acid and Vanadium Flow Battery Reaction Kinetics at Carbon Electrodes

Flow battery technologies are promising solutions to the need for electricity storage driven by the increasing demand for energy supply at the same time as the electricity grid becomes more and more dependent on renewable sources such as solar, wind and ocean energy. Vanadium flow batteries (VFBs) are perhaps the most promising of these technologies and a growing number of commercial systems are in operation worldwide.The energy efficiency of vanadium flow batteries (VFBs) largely depends upon the performance of the electrodes. Poor electrode kinetics cause large overpotentials, which reduce the voltage efficiency. They can also lead to side reactions, such as hydrogen evolution at the negative electrode and oxidation of the positive electrode, which reduce the coulombic efficiency and the overall performance and durability.There is considerable variation in the data regarding the kinetics of vanadium redox reactions at carbon electrodes. We have previously reported that cathodic treatment enhances the kinetics of the positive (VIV-VV) electrode in VFBs but inhibits the kinetics of the negative (VII-VIII) electrode, while anodic treatment inhibits the kinetics of the positive electrode but enhances the kinetics of the negative electrode1-3. We also showed that the activity of carbon-based material is strongly dependent on the surface history, in particular the most positive and the most negative potential used to treat an electrode4. In this presentation we will further investigate the effect of electrochemical treatment of the carbon surface and show the relationship between the observed pseudo-capacitive behavior and the kinetics of the different vanadium redox couples. References Bourke, M. A. Miller, R. P. Lynch, X. Gao, J. Landon, J. S. Wainright, R. F. Savinell and D. N. Buckley, J. Electrochem. Soc. 163, A5097 (2016). A. Bourke, M. A. Miller, R. P. Lynch, J. S. Wainright, R. F. Savinell and D. N. Buckley, J. Electrochem. Soc., 162, A1547 (2015). M. A. Miller, A. Bourke, N. Quill, J. S. Wainright, R. P. Lynch, D. N. Buckley and R. F. Savinell, J. Electrochem. Soc., 163, A2095 (2016). M. Al Hajji Safi, A. Bourke, D. N. Buckley, R. P. Lynch, ECS Trans., 109, 67-84 (2022).

Investigation on the Origin of Sluggish Anionic Redox Kinetics in Cation-Disordered Cathode

Cation-disordered rock salt (DRX) cathodes exhibit high specific capacity due to the simultaneous use of anionic and cationic redox reactions. However, DRX systems face severe challenges that limit their practical applications; a most important challenge is their poor rate performance. In this work, the structure and morphology of Li1.17Ti0.58Ni0.25O2 (LTNO) were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), etc. In combination with various electrochemical characterizations, we found that the sluggish kinetics of anionic redox within LTNO can be the key reason for the inferior rate performance. By sample relaxation at moderate temperature and X-ray absorption near edge structure (XANES), the ligand-to-metal charge transfer process is verified to occur between O and Ni and exhibits a prolonged characteristic time of 113.8 min. This time-consuming charge transfer process is verified to be the very fundamental origin of the slow kinetics of oxygen oxidation and reduction. This claim is further supported by the galvanostatic intermittent titration technique (GITT) at different temperatures. These findings provide essential guidance for understanding and further optimizing cathodes with anion redox reactions not only in the context of DRX cathodes but also conventional Li-rich cathodes.

Does single-crystallization a feasible direction for designing Li-rich layered cathodes?

Although the synthesis technology of popular single-crystal electrode materials is mature, the large enough, greatly dispersed Li-rich single-crystal cathodes have been rarely reported. In this study, we overcome the challenge of elevated Mn-O breaking activation energy so that achieve micro-sized (∼3 μm) single-crystal Mn-Li-rich particles with extreme dispersity by regulating Li content and high temperature residence time step by step. However, the modified parameters during the sintering process severely intensify the Li/Ni mixing, leading to unavoidable dynamical hysteresis and structural damage. In addition, the long-range diffusion pathway in single-crystal Li-rich grains not only limits the effective Li⁺ transfer but also significantly weakens the kinetics of anionic redox. Hence, whether the single-crystal strategy is seriously applicable to Mn-based Li-rich cathodes remains a matter of debate, with promising outlets focusing on the enhancement of the dynamics in Li-rich cathodes.

Reducing Co/O Band Overlap through Spin State Modulation for Stabilized High Capability of 4.6 V LiCoO2

High-voltage LiCoO2 (LCO) attracts great interest because of its large specific capacity, but it suffers from oxygen release, structural degradation, and quick capacity drop. These daunting issues root from the inferior thermodynamics and kinetics of the triggered oxygen anion redox (OAR) at high voltages. Herein, a tuned redox mechanism with almost only Co redox is demonstrated by atomically engineered high-spin LCO. The high-spin Co network reduces the Co/O band overlap, eliminates the adverse phase transition of O3 → H1-3, delays the exceeding of the O 2p band over the Fermi level, and suppresses excessive O → Co charge transfer at high voltages. This function intrinsically promotes Co redox and restrains O redox, fundamentally addressing the issues of O2 release and coupled detrimental Co reduction. Moreover, the chemomechanical heterogeneity caused by different kinetics of Co/O redox centers and the inferior rate performance limited by slow O redox kinetics is simultaneously improved owing to the suppression of slow OAR and the excitation of fast Co redox. The modulated LCO delivers ultrahigh rate capacities of 216 mAh g-1 (1C) and 195 mAh g-1(5C), as well as high capacity retentions of 90.4% (@100 cycles) and 86.9% (@500 cycles). This work sheds new light on the design for a wide range of O redox cathodes.

Effect of Pore Formation on Redox-Driven Phase Transformation.

When solid-state redox-driven phase transformations are associated with mass loss, vacancies are produced that develop into pores. These pores can influence the kinetics of certain redox and phase transformation steps. We investigated the structural and chemical mechanisms in and at pores in a combined experimental-theoretical study, using the reduction of iron oxide by hydrogen as a model system. The redox product (water) accumulates inside the pores and shifts the local equilibrium at the already reduced material back toward reoxidation into cubic Fe_{1-x}O (where x refers to Fe deficiency, space group Fm3[over ¯]m). This effect helps us to understand the sluggish reduction of cubic Fe_{1-x}O by hydrogen, a key process for future sustainable steelmaking.

Nanoscale transition metal catalysts anchored on perovskite oxide enabling enhanced kinetics of lithium polysulfide redox in lithium-sulfur batteries

To obtain high-performance lithium-sulfur (Li-S) batteries, it is necessary to rationally design electrocatalytic materials that can promote efficient sulfur electrochemical reactions. Herein, the robust heterostructured material of nanoscale transition metal anchored on perovskite oxide was designed for efficient catalytic kinetics of the oxidation and reduction reactions of lithium polysulphide (LiPSs), and verified by density functional theory (DFT) calculations and experimental characterizations. Due to the strong interaction of nanoscale transition metals with LiPSs through chemical coupling, heterostructured materials (STO@M) (M = Fe, Ni, Cu) exhibit excellent catalytic activity for redox reactions of LiPSs. The bifunctional heterostructure material STO@Fe exhibits good rate performance and cycling stability as the cathode host, realizing a high-performance Li-S battery that can maintain stable cycling under rapid charge–discharge cycling. This study presents a novel approach to designing electrocatalytic materials for redox reactions of LiPSs, which promotes the development of fast charge–discharge Li-S batteries.

Interfacially engineered induced nickel-based heterostructures as efficient catalysts for Li-O2 batteries

Lithium-oxygen batteries have much attracted attention due to their ultra-high theoretical energy density. However, its practical application is still hampered by many issues, which are mainly attributed to the slow kinetics of oxygen redox reaction/oxygen evolution reaction (ORR/OER) dynamics. Aimed to this issue, in this work, NiO nanosheets loaded on carbon cloth (NiO/CC) were first synthesized, and then the Se@NiO/CC heterostructure was obtained by selenization. The synthesized Se@NiO/CC with distinct heterointerface is beneficial to form a built-in electric field during charge/discharge process, which endows the cathode with a fast charge transfer rate and reaction kinetics. Furthermore, the formation of heterointerfaces facilitates the deformation of localized fine atomic arrays, which can serve as additional active sites for ORR/OER. Experimental and theoretical results show that the heterogeneous interface of NiO and NiSe can adjust the adsorption energy of intermediate LiO2 and optimize the morphology of the discharge product. The interaction between Se@NiO/CC and the intermediate during discharge/charge is conducive to electron transfer. The round-trip efficiency, cycle stability and discharge capacity of Li-O2 batteries are greatly improved thanks to the synergistic effect of heterostructures.

(Invited) Effect of Electrochemical Treatment and pH on VIV/VV Electrode Kinetics

Flow batteries are a promising technology for flexible large-scale energy storage systems to tackle the problem of intermittency with renewable energy sources. Several flow battery systems have been successfully developed and deployed to date, including all-vanadium, zinc-iron, zinc-bromine, and all-iron flow batteries.1 Among these, vanadium flow batteries (VFBs) have received a lot of attention in the large-scale energy storage sector due to their excellent characteristics such as long cycle life with almost zero electrode and electrolyte degradation, safe and non-flammable aqueous electrolytes, short response times, and the ability to operate at reasonable temperatures.2 The operation of flow batteries involves simultaneous oxidation and reduction reactions at the anodic and cathodic compartments. In the VFB, the redox couple at the anode is VII-VIII and at the cathode is VIV-VV.Voltage inefficiencies at the electrode-electrolyte interface are a source of concern for the operation of these systems.3 Electrode polarization, slow kinetics, and iR losses associated with contact and ionic resistances all contribute to battery voltage inefficiencies. It is widely acknowledged that the electrode material and pretreatments (chemical, thermal, and electrochemical) have significant impacts on the electrode kinetics and hence the performance of the VFB system.4 Investigation of the electrode kinetics is thus important in order to better understand and improve the performance of VFBs.Previous research from our group has shown that electrochemical treatments of carbon electrodes at different potentials significantly affect the electrode kinetics of VIV-VV and VII-VIII oxidation-reduction reactions. The electrode kinetics of VIV-VV are enhanced by cathodic treatment of the electrode and inhibited by anodic treatment. In contrast, the electrode kinetics of VII-VIII are enhanced by anodic treatment of the electrode and inhibited by cathodic treatment. Furthermore, the carbon electrode can be toggled repeatedly and reproducibly between the enhanced (activated) and the inhibited (deactivated) state.5,6,7 Other factors affecting the electrode activity include ageing of the electrode, the final treatment potential, the redox rest potential, and the treatment potential window.8 This previous research involves treatments in strong acid, low pH electrolytes.In this study, we investigate the electrochemical treatment of glassy carbon electrodes in electrolytes at a range of pH. The resulting electrode kinetics of the vanadium redox couples at each treatment potential are studied using electrochemical characterization techniques including cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Acknowledgements Varsha Sasikumar S P would like to thank the Irish Research Council (IRC) for providing funding for this research work. References Xinyou Ke et al., Chemical Society Reviews, 47, 8721-8743 (2018).R. Chalamala et al., Proceedings of the IEEE, 102(6), 976-999, (2014).A. H. Whitehead and M. Harrer, Journal of Power Sources, 230, 271–276 (2013).Ki Jae Kim, et al., Journal of Materials Chemistry A, 33(3), 16913-16933 (2015).A. Bourke et al., Journal of Electrochemical Society, 163, A5097 (2016).A. Bourke et al., Journal of Electrochemical Society, 162, A1547 (2015).M. A. Miller, et al., Journal of Electrochemical Society, 163, A2095-A2102, (2016).D. N. Buckley, et al., ECS Transactions, 98(9), 223-239 (2020).

Comparison of Zinc Bromine and Zinc Iodine Flow Batteries: From Electrolde to Electrolyte

Research in flow batteries and their application in large scale energy storage has received a growing amount of attention and promise over the past two decades. Although the energy density of flow batteries is low relative to the Li-ion battery, their comparatively lower costs, preferred safety, and ease of scalability has made flow batteries some of the most promising contenders for large-scale stationary energy storage, and are currently commercially available for this purpose. The zinc-bromine flow battery (ZBFB), despite being one of the first proposed flow batteries in the 1980s, has only recently gained enough traction to compete with the well established all-vanadium redox flow batteries. This is largely due to the high solubility of the bromine redox species in aqueous electrolytes, which has allowed the ZBFB is achieve double the energy density of the all-vanadium technology. Recently, an analogue to the zinc-bromine flow battery was introduced: the zinc-iodine flow battery (ZIFB). Similar to the ZBFB, the main advantages of this technology arose from the high solubility of the electroactive species in the electrolyte (iodine/tri-iodide). The solubility of the iodine redox species is even higher than that of analogous bromine electrolytes, and accordingly, the highest energy densities of all aqueous flow batteries to date has been for the ZIFB.Despite the similarities between the two technologies, they are held back by different issues, and so different approaches have been taken to improving the performances of the ZBFB and ZIFB. The ZBFB primarily suffers from a low power-density due to the sluggish kinetics of the bromine redox couple. Therefore, a majority of research on the ZBFB has focused on identifying new, low cost electrode materials that minimize kinetic losses at the bromine half-cell. In contrast, a majority of research on the ZIFB has been on improvements to the electrolyte composition. The ZIFB is plagued by issues of a thick, high impedance iodine film that forms at the positive electrode on charge. Due to the strong Lewis acid nature of the iodine species, a variety of charge-transfer complexes can be formed in the electrolyte, having a variety of effects on the battery performance.This presentation provides an overview on the similarities and differences between the ZBFB and ZIFB technologies. We performed a variety of half-cell and flow battery tests varying the electrode and electrolyte compositions. A number of low cost carbon materials are used as electrode materials, along with a variety of modifications to the bromine and iodine electrolytes. Through the use of high-surface area carbon blacks, the exchange current of the bromine redox couple is able to be increased by two orders to magnitude in comparison to glassy carbon. Additions of MSA or other acids to the ZBFB increases the oxidation kinetics greatly, and accordingly the overall energy efficiency of the ZBFB. For the ZIFB, the presence of high surface area catalysts have little to no effect on the overall performance. We found that in aqueous electrolytes, the iodine electrode is largely held back by the iodine film that forms on charge. Therefore, by adding ions to the electrolyte such as Br-, Cl-, and SO4 2-, we were able to increase the solubility of the iodine film and the reversibility of the battery, and accordingly its efficiency. Although the ZIFB initially performs better than the ZBFB, after making systematic adjustments to both the electrode and electrolyte compositions, the discrepancy between their performances is largely minimized, demonstrating both can be viable for the future of large-scale energy storage. Figure 1

Heat‐Triggered Ferri‐to‐Paramagnetic Transition Accelerates Redox Couple‐Mediated Electrocatalytic Water Oxidation

AbstractRedox couple oxidation as an initial step of water oxidation may be key for high electricity consumption in the electrochemical oxygen evolution reaction (OER). Here, a heat‐induced magnetic transition strategy is reported to speed up the oxidation kinetics of redox couples. The activation energy of Ni2+/Ni3+ redox couple oxidation is sharply decreased by heating the Ni0.67Fe0.33OxHy catalyst above a Curie temperature (Tc) of 70 °C. In such a strategy, heat instead of electricity drives the spin flipping of Ni2+/Ni3+ oxidation through heat‐sensitive ferri‐to‐paramagnetic spin state changes. As a result of magnetic transition‐assisted efficient heat‐electricity coupling, Ni0.67Fe0.33OxHy exhibits the lowest OER overpotential of 221 mV at 100 mA cm−2 at 90 °C in alkaline electrolytes, outperforming the benchmark IrO2 catalyst. The findings provide new insights into the design of efficient heat–electricity complementary OER devices.

An individual sandwich hybrid nanostructure of cobalt disulfide in-situ grown on N doped carbon layer wrapped on multi-walled carbon nanotubes for high-efficiency lithium sulfur batteries

Binding and trapping of lithium polysulfide (LPS) are being conceived as the most effective strategies to improve lithium-sulfur (Li-S) battery performance. Therefore, exploiting a simple but cost-effective approach for the absorption and conversion of LPS and the transfer of electrons and Li+ ions is of paramount importance. Herein, sandwich structure MWCNTs@N-doped-C@CoS2 integrated with multiple nanostructures of zero-dimensional (0D) CoS2 nanoparticles, 1D carbon nanotubes (CNTs), and 2D N-doped amorphous carbon layer was obtained, where MWCNTs was firstly uniformly attached with a polydopamine (PDA) of excellent adhesion, followed by hydrothermal method, the Co2+ nanoparticles were in-situ grown on the PDA by the formation of complex compound of Co2+ and N atoms in PDA, and then the CoS2 nanoparticles were in-situ grown on CNTs in a point-surface contact way by a bridging of N-doped amorphous carbon layer derived from the carbonization of attached PDA after the vulcanization at 500 °C under Ar atmosphere. The multifunction synergism of absorption, conductivity, and the kinetics of LPS redox is significantly improved, consequently effectively suppressing the shuttle effect and tremendously increasing the utilization rate of active substance. For the Li-S battery assembled with MWCNTs@N-doped-C@CoS2-modified separator, its rate capacity and cycling performance can be greatly enhanced. It can exhibit a high initial discharge capacity of 1590 mAh g−1 at 0.1 C, a stable long-term cycling performance with a relatively low capacity decay of 0.07% per cycle during 500 cycles at 1 C, and a reversible capacity of 772 mAh g−1 and a capacity decay of 0.04% per cycle during 250 cycles at 2 C. Even at a large current density of 4 C, an initial specific discharge capacity of 634 mAh g−1 can still be delivered. With a high sulfur loading of 5.0 mg cm−2, additionally, an outstanding cycling stability can also be well maintained at 685 mAh g−1 at 0.1 C after 50 cycles. This work provides a novel and simple but effective strategy to develop such sandwich hybrid materials comprised of polar metal sulfides and conductive networks via an effective bridging to help realize durable and stable Li-S battery.

The Effect of the Upper and Lower Electrochemical Treatment Potentials on the Electrode Activity Towards Vanadium Redox Couples

Vanadium flow batteries (VFBs) are a promising energy storage technology. They have long cycle lives and are essentially immune to cross contamination through their ion-exchange membranes1. Battery energy efficiency depends on voltage and coulombic efficiency. Sluggish electrode kinetics reduces voltage efficiency by increasing overpotential at the electrodes and thereby can result in an increase in the fraction of current associated with unwanted side reactions at the electrodes, reducing columbic efficiency. It follows that study of the VFB electrode kinetics is very important.The kinetics of the redox couples at the positive and negative electrodes have been reported in flow batteries and three-electrode cell experiments for a range of carbon materials. The reports show that the kinetics of these reactions are not fully understood and discrepancies exist regarding which half-cell redox couple has slower kinetics. Many researchers2-4 have concluded that the redox couple has slower kinetics but some4-6 have concluded that the redox couple has slower kinetics.Carbon electrodes are very sensitive to changes in their environment which can subsequently alter the kinetics of redox couples.10 Thus, comparing the reported kinetics in the literature is complicated. We have previously reported7-9for a range of carbon materials – including carbon fibres, glassy carbon, reticulated vitreous carbon (RVC) and carbon paper – that the kinetics of are enhanced by cathodic treatment of the electrode and inhibited by anodic treatment. In contrast, the electrode kinetics of are enhanced by anodic treatment of the electrode and inhibited by cathodic treatment. Therefore, it is more reasonable when comparing the electrode kinetics towards different redox couples to compare electrodes that have been treated in the same manner.We showed previously7-9 that the optimised kinetics at the positive electrode is faster than at the negative electrode. In this presentation we further investigate the effect of the electrochemical treatment on the electrode kinetics by controlling the upper and lower treatment potentials experienced by the electrode. We will present the resulting dependency of electrode kinetics on the range of the electrochemical treatment window experienced by the electrode and will show, as can be seen in Fig. 1, that determining which redox couple has the fastest kinetics at an electrode is highly dependent on the range of potentials experienced by the electrode. Fig.1: The activity of VIV-VV (circular markers) and VII-VIII (triangular markers) electrodes plotted as a function of the anodic (red markers) and cathodic (blue markers) treatment potentials for two different potential windows/ranges of maximum upper and minimum lower potential. It is clearly shown that the optimised kinetics of VII-VIII exceed those of VIV-VV for a window of upper and lower potentials of –1.2 V and +1.2 V, respectively (graph on left), while the optimised kinetics of VIV-VV exceed those of VII-VIII for a window of upper and lower potentials of –1.2 V and +1.5 V, respectively (graph on right).

Cotton pad derived 3D lithiophilic carbon host for robust Li metal anode: In-situ generated ionic conductive Li3N protective decoration

Lithium metal anode is considered as one of the most promising candidates for the next-generation batteries with high specific energy density. However, several thorny problems encompassing uncontrollable Li dendritic growth and wild volume variation during cycling, accompanied by the short lifespan and alarming safety concerns, have hindered the commercial viability of Li metal-based batteries. In this contribution, we designed a Li composite anode fabricated via Li infusion into N, O co-doped and Ag coated 3D carbon host from simple treatments of commercial cotton pads, referred as Ag-NOCP@Li. The incorporation of multi lithiophilic atoms can significantly improve the affinity of 3D carbon host towards Li. More importantly, during molten Li infiltration, the composite anode can in-situ generate Li3N decoration, an excellent Li+ conductor and electron insulator. The first-principles calculations further revealed that the active sites for the Li3N generation most likely are pyrrolic nitrogen sites. The Li3N with favorable mechanical strength and ultra-fast Li+ diffusion rate can effectively boost the kinetics of Li transport and redox, as well as inhibit the dendritic generation. Besides, the Ag-NOCP with hierarchical pores and multi-microchannel within the nanofibers, allows the rapid Li+ diffusion and buffers the volume change over long cycling. Therefore, such Ag-NOCP@Li electrode could maintain a stable cycling for 1400 h at 1.0 mA cm−2/1.0 mAh cm−2. The full cells using Ag-NOCP@Li anode paired with LiFePO4 and LiNi0.5Co0.2Mn0.3O2 cathodes, displayed impressive long-term cyclic stability up to 400cycles at 0.5 and 1.0C, respectively. This work paves new way for rational design of 3D lithiophilic host towards durable Li anode.