Reduced Graphene Oxide-Wrapped FeS2 Composite as Anode for High-Performance Sodium-Ion Batteries

Nanohybrids of BCN-Fe1-xS for Sodium and Lithium Hybrid Ion Capacitors

Hybrid ion capacitors (HIC) are receiving a lot of attention due to their potential to achieve high energy and power densities, but they remain insufficient. It is imperative to explore outstanding and environmentally benign electrode materials to achieve high-performing HIC systems. Here, a unique boron carbon nitride (BCN)-based HIC system that comprises a microporous BCN structure and Fe1-xS nanoparticle incorporated BCN nanosheets (BNF) as cathode and anode, respectively is reported. The BNF is prepared through a facile one-pot calcination process using dithiooxamide (DTO), boric acid, and iron source. In situ, crystal growth of Fe1-xS facilitates the formation of BCN structure through the creation of holes/defects in the polymeric structure. The first principle density functional (DFT) theory simulations demonstrate the structural and electronic properties of the hybrid of BCN and Fe1-xS as compelling anode materials for HIC applications. The DFT calculations reveal that both BCN and BNF structures have excellent metallic characters with Li+ storage capacities of 128.4 and 1021.38 mAh g-1 respectively. These findings are confirmed experimentally where the BCN-based HIC system delivers exceptional energy and power densities of 267.5 Wh kg-1/749.5 W kg-1 toward Li+ storage, which outweighs previous HIC performances and demonstrates favorable performance for Li+ and Na+ storages.

Achieving High-Rate and Stable Sodium-Ion Storage by Constructing Okra-Like NiS2/FeS2@Multichannel Carbon Nanofibers

Transition metal sulfides (TMSs) are considered as promising anode materials for sodium-ion batteries (SIBs) due to their high theoretical capacities. However, the relatively low electrical conductivity, large volume variation, and easy aggregation/pulverization of active materials seriously hinder their practical application. Herein, okra-like NiS2/FeS2 particles encapsulated in multichannel N-doped carbon nanofibers (NiS2/FeS2@MCNFs) are fabricated by a coprecipitation, electrospinning, and carbonization/sulfurization strategy. The combined advantages arising from the hollow multichannel structure in carbon skeleton and heterogeneous NiS2/FeS2 particles with rich interfaces can provide facile ion/electron transfer paths, ensure boosted reaction kinetics, and help maintain the structural integrity, thereby resulting in a high reversible capacity (457 mA h g-1 at 1 A g-1), excellent rate performance (350 mA h g-1 at 5 A g-1), and outstanding long-term cycling stability (93.5% retention after 1100 cycles). This work provides a facile and efficient synthetic strategy to develop TMS-based heterostructured anode materials with high-rate and stable sodium storage properties.

Review of Transition Metal Chalcogenides and Halides as Electrode Materials for Thermal Batteries and Secondary Energy Storage Systems

Transition metal chalcogenides and halides (TMCs and TMHs) have been extensively used and reported as electrode materials in diverse primary and secondary batteries. This review summarizes the suitability of TMCs and TMHs as electrode materials focusing on thermal batteries (utilized for defense applications) and energy storage systems like mono- and multivalent rechargeable batteries. The report also identifies the specific physicochemical properties that need to be achieved for the same materials to be employed as cathode materials in thermal batteries and anode materials in monovalent rechargeable systems. For example, thermal stability of the materials plays a crucial role in delivering the performance of the thermal battery system, whereas the electrical conductivity and layered structure of similar materials play a vital role in enhancing the electrochemical performance of the mono- and multivalent rechargeable batteries. It can be summarized that nonlayered CoS2, FeS2, NiS2, and WS2 were found to be ideal as cathode materials for thermal batteries primarily due to their better thermal stability, whereas the layered structures of these materials with a coating of carbon allotrope (CNT, graphene, rGO) were found to be suitable as anode materials for monovalent alkali metal ion rechargeable batteries. On the other hand, vanadium, titanium, molybdenum, tin, and antimony based chalcogenides were found to be suitable as cathode materials for multivalent rechargeable batteries due to the high oxidation state of cathode materials which resists the stronger field produced during the interaction of di- and trivalent ions with the cathode material facilitating higher energy density with minimal structural and volume changes at a high rate of discharge.

Three-dimensional micro-nanostructures based on binary transitional metal sulfides with doped carbon protector enabled high-performance and safe batteries

The lack of suitable Li⁺ reservoirs and the risk of thermal runaway have hindered the extended use of lithium-ion batteries. Although utilizing Li₄Ti₅O₁₂ or TiO₂ can improve the thermal safety, their low theoretical capacities compromise the electrochemical performance of the cell. In this study, a three-dimensional micro-nanostructure based on binary transitional metal sulfides (TMSs) with a doped carbon protector (SnS/Co₉S₈@HC) is designed. When operating at 0.1-1 A g^-1, the SnS/Co₉S₈@HC cell exhibits a high inceptive capacity of 1104.8 mAh g^-1 with a high coulomb efficiency of 97.1%. Even after 1000 cycles, it delivers a relatively-high capacity of 450.3 mAh g^-1, indicating a low capacity decay rate of 0.033% per cycle (from the 2nd to the 1000th cycle). The thermal runaway actions of the cells with graphite and SnS/Co₉S₈@HC anodes are investigated. The results demonstrate that the cell with the SnS/Co₉S₈@HC anode exhibits a significantly reduced maximum thermal runaway temperature of 473.5 ± 6.2℃ and maximum temperature increasing rate of 15.1 ± 0.6 °C min^-1 compared to the graphite cell. This indicates that SnS/Co₉S₈@HC cell holds higher thermal safety. The potential of SnS/Co₉S₈@HC as sodium ion batteries anode is also investigated. The results indicate an initial capacity of 631.7 mAh g^-1, with a low capacity decay rate of 0.063% per cycle when operating at 2 A g^-1. This work may be enlightening for constructing multi-phase TMSs based hierarchical structure towards superior and safe energy storage.

Is there a common reaction pathway for chromium sulfides as anodes in sodium-ion batteries? A case study about sodium storage properties of MCr2S4 (M = Cr, Ti, Fe)

We present new insights into the electrochemical properties of three metal sulfides MCr2S4 (M = Cr, Ti, Fe) probed as anode materials in sodium-ion batteries for the first time. The electrodes deliver decent reversible capacities and good long-term cycle stability, e.g., 470, 375, and 524 mAh g−1 are obtained after 200 cycles applying 0.5 A g−1 for M = Cr, Ti, and Fe, respectively. The reaction mechanisms are investigated via synchrotron-based X-ray powder diffraction and pair distribution function analyses. The highly crystalline educts are decomposed into Na2S nanoparticles and ultra-small metal particles during initial discharge without formation of intermediate NaCrS2 domains as previously reported for CuCrS2 and NiCr2S4. After a full cycle, the structural integrity of MCr2S4 (M = Cr, Ti, Fe) is not recovered. Thus, the Na storage properties are attributed to redox reactions between nanoscopic to X-ray amorphous conversion products with only local atomic correlations M···S/S···S in the charged and M···M/Na···S in the discharged state.

Graphical Abstract

Synthetically Produced Isocubanite as an Anode Material for Sodium-Ion Batteries: Understanding the Reaction Mechanism During Sodium Uptake and Release

Bulk isocubanite (CuFe₂S₃) was synthesized via a multistep high-temperature synthesis and was investigated as an anode material for sodium-ion batteries. CuFe₂S₃ exhibits an excellent electrochemical performance with a capacity retention of 422 mA h g^-1 for more than 1000 cycles at a current rate of 0.5 A g^-1 (0.85 C). The complex reaction mechanism of the first cycle was investigated via PXRD and X-ray absorption spectroscopy. At the early stages of Na uptake, CuFe₂S₃ is converted to form crystalline CuFeS₂ and nanocrystalline NaFe_1.5S₂ simultaneously. By increasing the Na content, Cu⁺ is reduced to nanocrystalline Cu, followed by the reduction of Fe²⁺ to amorphous Fe⁰ while reflections of nanocrystalline Na₂S appear. During charging up to -5 Na/f.u., the intermediate NaFe_1.5S₂ appears again, which transforms in the last step of charging to a new unknown phase. This unknown phase together with NaFe_1.5S₂ plays a key role in the mechanism for the following cycles, evidenced by the PXRD investigation of the second cycle. Even after 400 cycles, the occurrence of nanocrystalline phases made it possible to gain insights into the alteration of the mechanism, which shows that Cu_xS phases play an important role in the region of constant specific capacity.

Reacquainting the Electrochemical Conversion Mechanism of FeS2 Sodium-Ion Batteries by Operando Magnetometry

In spite of the excellent electrochemical performance in lithium-ion batteries (LIBs), transition-metal compounds usually show inferior capacity and cyclability in sodium-ion batteries (SIBs), implying different reaction schemes between these two types of systems. Herein, coupling operando magnetometry with electrochemical measurement, we peformed a comprehensive investigation on the intrinsic relationship between the ion-embedding mechanisms and the electrochemical properties of the typical FeS₂/Na (Li) cells. Operando magnetometry together with ex-situ transmission electron microscopy (TEM) measurement reveal that only part of FeS₂ is involved in the conversion reaction process, while the unreactive parts form "inactive cores" that lead to the low capacity. Through quantification with Langevin fitting, we further show that the size of the iron grains produced by the conversion reaction are much smaller in SIBs than that in LIBs, which may lead to more serious pulverization, thereby resulting in worse cycle performance. The underlying reason for the above two above phenomena in SIBs is the sluggish kinetics caused by the larger Na-ion radius. Our work paves a new way for the investigation of novel SIB materials with high capacity and long durability.

A Partial Sulfuration Strategy Derived Multi-Yolk-Shell Structure for Ultra-Stable K/Na/Li-ion Storage

Metal sulfides are attractive anodes for alkali metal ion batteries due to the high theoretical capacity, while their practical implementation is hampered by the inherent poor conductivity and vast volume variation during cycles. Approaching rational designed microstructures with good stability and fast charge transfer is of great importance in response to these issues. Herein, a partial sulfuration strategy for the rational construction of multi-yolk-shell (m-Y-S) structures, from which multiple Fe_{1- x} S nanoparticles are confined within hollow carbon nanosheet with tunable interior void space is reported. As anode materials, the m-Y-S Fe_{1- x} S@C composite can display high capacity and excellent rate capability (134, 365, and 447 mA h g^-1 for K⁺ , Na⁺ , and Li⁺ storage at 20 A g^-1 ). Remarkably, it exhibits ultra-stable potassium storage up to 1200, 6000, and 20 000 cycles under current densities of 0.1, 0.5, and 1 A g^-1 , which is much superior to previous yolk-shell structures and metal-sulfide anodes. Based on comprehensive experimental analysis and theoretical calculations, the exceptional performance of m-Y-S structure can be ascribed to the optimized interior void space for good structure stability, as well as the multiple connection points and conductive carbon layer for superior electron/ion transportation.

CuFeS2 as a Very Stable High-Capacity Anode Material for Sodium-Ion Batteries: A Multimethod Approach for Elucidation of the Complex Reaction Mechanisms during Discharge and Charge Processes

Highly crystalline CuFeS₂ containing earth-abundant and environmentally friendly elements prepared via a high-temperature synthesis exhibits an excellent electrochemical performance as an anode material in sodium-ion batteries. The initial specific capacity of 460 mAh g^-1 increases to 512 mAh g^-1 in the 150th cycle and then decreases to a still very high value of 444 mAh g^-1 at 0.5 A g^-1 in the remaining 550 cycles. Even for a large current density, a pronounced cycling stability is observed. Here, we demonstrate that combining the results of X-ray powder diffraction experiments, pair distribution function analysis, and ²³Na NMR and Mössbauer spectroscopy investigations performed at different stages of discharging and charging processes allows elucidation of very complex reaction mechanisms. In the first step after uptake of 1 Na/CuFeS₂, nanocrystalline NaCuFeS₂ is formed as an intermediate phase, which surprisingly could be recovered during charging. On increasing the Na content, Cu⁺ is reduced to nanocrystalline Cu, while nanocrystalline Na₂S and nanosized elemental Fe are formed in the discharged state. After charging, the main crystalline phase is NaCuFeS₂. At the 150th cycle, the mechanisms clearly changed, and in the charged state, nanocrystalline Cu_xS phases are observed. At later stages of cycling, the mechanisms are altered again: NaF, Cu₂S, and Cu_7.2S₄ appeared in the discharged state, while NaF and Cu₅FeS₄ are observed in the charged state. In contrast to a typical conversion reaction, nanocrystalline phases play the dominant role, which are responsible for the high reversible capacity and long-term stability.

Stabilization and Utilization of Pyrite under Light Irradiation: Discussion of Photocorrosion Resistance

The control of pyrite (FeS₂) oxidation from a source is a problem of great concern on treatment of acid mine drainage (AMD). Compared with air and water, the effect of light on pyrite oxidation has not attracted enough attention. However, we found that pyrite photocorrosion in the light promoted the oxidation of pyrite. Herein, we introduce a method of coating pyrite with graphene oxide (GO), which can inhibit the oxidation and photocorrosion of pyrite while it can also degrade organic pollutants. The characterization results show that a covalent bond forms between the GO and pyrite. The stable and uniform GO coating prevents the permeation of O₂ and H₂O and promotes the transfer of photogenerated electrons. Moreover, it changes the conduction band (CB) and valence band (VB) levels of GO-pyrite. All of these are vital for preventing the corrosion of pyrite and promoting its photocatalytic ability. More importantly, the effect of CB and VB levels on the oxidized species was discussed. The inhibition of photocorrosion is achieved by the reaction of GO with the photoinduced h⁺, ^•OH, and ^•O₂ ^-. The study provides insights for source treatment of AMD under light and the reuse of massive abandoned pyrite.

Effect of Nitrogen Doping on the Performance of Mesoporous CMK-8 Carbon Anodes for Li-Ion Batteries

Designing carbonaceous materials with heightened attention to the structural properties such as porosity, and to the functionalization of the surface, is a growing topic in the lithium-ion batteries (LIBs) field. Using a mesoporous silica KIT-6 hard template, mesoporous carbons belonging to the OMCs (ordered mesoporous carbons) family, namely 3D cubic CMK-8 and N-CMK-8 were synthesized and thoroughly structurally characterized. XPS analysis confirmed the successful introduction of nitrogen, highlighting the nature of the different nitrogen atoms incorporated in the structure. The work aims at evaluating the electrochemical performance of N-doped ordered mesoporous carbons as an anode in LIBs, underlining the effect of the nitrogen functionalization. The N-CMK-8 electrode reveals higher reversible capacity, better cycling stability, and rate capability, as compared to the CMK-8 electrode. Coupling the 3D channel network with the functional N-doping increased the reversible capacity to ~1000 mAh·g−1 for the N-CMK-8 from ~450 mAh·g−1 for the undoped CMK-8 electrode. A full Li-ion cell was built using N-CMK-8 as an anode, commercial LiFePO4, a cathode, and LP30 commercial electrolyte, showing stable performance for 100 cycles. The combination of nitrogen functionalization and ordered porosity is promising for the development of high performing functional anodes.

Rational Design of Unique ZnO/ZnS@N-C Heterostructures for High-Performance Lithium-Ion Batteries

Conversion-type anodes with high theoretical capacity have attracted enormous interest for lithium storage, although their extremely poor conductivity and volume variations during lithiation-delithiation processes seriously limit their practical applications. Herein, a facile strategy to fabricate ZnO/ZnS@N-C heterostructures decorated on carbon nanotubes (ZnO/ZnS@N-C/CNTs) with metal-organic framework assistance is developed. The as-prepared anodes display higher reversible capacity of 1020.6 mAh g^-1 at 100 mA g^-1 after 200 cycles and excellent high-cyclability with 386.6 mAh g^-1 at 1000 mA g^-1 over 400 cycles. The conductive CNT network and N-doped carbon shell could successfully improve the electrical conductivity and avoid the aggregation of ultrasmall ZnO/ZnS nanoparticles. The results calculated from density functional theory also suggest that the ZnO/ZnS heterostructures could promote electron-transfer capability.

Rational design of Ni/Ni2P heterostructures encapsulated in 3D porous carbon networks for improved lithium storage

Nickel phosphides are considered to be a promising lithium storage host due to their high theoretical capacities. However, the volume change during the charge-discharge process and inherent poor reaction kinetics limit their electrochemical performance. To solve these problems, Ni/Ni₂P heterostructures encapsulated in 3D porous carbon networks are fabricated. The macro/micro-pores-rich carbon networks are in situ constructed via a freeze-drying method and subsequent pyrolysis route using NaCl as a template. In the following phosphorization process, Ni/Ni₂P nanoparticles are homogenously embedded in the carbon matrix. When used as anodes for lithium ion batteries, the Ni/Ni₂P/porous carbon networks deliver high discharge capacity, good cycling stability as well as good rate performance. It is believed that metallic Ni and porous carbon networks significantly improve the conductivity of electrodes. Moreover, the 3D conductive matrix can not only alleviate the volume change, but also prevent the aggregation and pulverization of Ni₂P nanoparticles during the charge-discharge process.

Turbostratic carbon-localised FeS2 nanocrystals as anodes for high-performance sodium-ion batteries

Low-cost metal sulfides are promising anode materials for sodium-ion batteries (SIBs); however, they suffer from sluggish kinetics and large volume expansion upon cycling. Here, a strategy to grow FeS₂ on turbostratic carbon (t-carbon) assisted by chemical interactions between Fe and C electrons was realized via a simple and scalable mechanical alloying (MA) approach with a trace amount of CNTs. The structural change in CNTs synchronized with the in situ growth of FeS₂ on the transformed t-carbon during the MA process, forming localised FeS₂ nanocrystals wrapped in the frameworks of t-carbon. This intertwined structure within a grain consisted of chemical bonding by electronic hybridization between FeS₂ and its adjacent carbon layer, resulting in enhanced structural stability upon cycling. Moreover, the localised FeS₂ nanocrystals with an ultrasmall diameter of 10 nm encapsulated in the nanoframeworks of t-carbon could effectively shorten the diffusion paths of electrons/ions and withstand the volume expansion. The as-synthesized FeS₂-C hybrid composite electrode exhibited a pseudocapacitive diffusion behavior with high specific capacity, good cycling stability, and remarkable rate capability. This strategy is a facile, scalable, and low-cost route toward high-performance metal sulfide anode materials for the commercial utilization of SIBs.

Ultrathin Ti3C2Tx (MXene) Nanosheet-Wrapped NiSe2 Octahedral Crystal for Enhanced Supercapacitor Performance and Synergetic Electrocatalytic Water Splitting

Metal selenides, such as NiSe₂, have exhibited great potentials as multifunctional materials for energy storage and conversation. However, the utilization of pure NiSe₂ as electrode materials is limited by its poor cycling stability, low electrical conductivity, and insufficient electrochemically active sites. To remedy these defects, herein, a novel NiSe₂/Ti₃C₂T_x hybrid with strong interfacial interaction and electrical properties is fabricated, by wrapping NiSe₂ octahedral crystal with ultrathin Ti₃C₂T_x MXene nanosheet. The NiSe₂/Ti₃C₂T_x hybrid exhibits excellent electrochemical performance, with a high specific capacitance of 531.2 F g^-1 at 1 A g^-1 for supercapacitor, low overpotential of 200 mV at 10 mA g^-1, and small Tafel slope of 37.7 mV dec^-1 for hydrogen evolution reaction (HER). Furthermore, greater cycling stabilities for NiSe₂/Ti₃C₂T_x hybrid in both supercapacitor and HER have also been achieved. These significant improvements compared with unmodified NiSe₂ should be owing to the strong interfacial interaction between NiSe₂ octahedral crystal and Ti₃C₂T_x MXene, which provides enhanced conductivity, fast charge transfer as well as abundant active sites, and highlight the promising potentials in combinations of MXene with metal selenides for multifunctional applications such as energy storage and conversion.

Improved Na+/K+ Storage Properties of ReSe2-Carbon Nanofibers Based on Graphene Modifications

Rhenium diselenide (ReSe₂) has caused considerable concerns in the field of energy storage because the compound and its composites still suffer from low specific capacity and inferior cyclic stability. In this study, ReSe₂ nanoparticles encapsulated in carbon nanofibers were synthesized successfully with simple electrospinning and heat treatment. It was found that graphene modifications could affect considerably the microstructure and electrochemical properties of ReSe₂-carbon nanofibers. Accordingly, the modified compound maintained a capacity of 227 mAh g^-1 after 500 cycles at 200 mA g^-1 for Na⁺ storage, 230 mAh g^-1 after 200 cycles at 200 mA g^-1, 212 mAh g^-1 after 150 cycles at 500 mA g^-1 for K⁺ storage, which corresponded to the capacity retention ratios of 89%, 97%, and 86%, respectively. Even in Na⁺ full cells, its capacity was maintained to 82% after 200 cycles at 1C (117 mA g^-1). The superior stability of ReSe₂-carbon nanofibers benefitted from the extremely weak van der Waals interactions and large interlayer spacing of ReSe₂, in association with the role of graphene-modified carbon nanofibers, in terms of the shortening of electron/ion transport paths and the improvement of structural support. This study may provide a new route for a broadened range of applications of other rhenium-based compounds.

Bi2Se3/C Nanocomposite as a New Sodium-Ion Battery Anode Material

Bi₂Se₃ was studied as a novel sodium-ion battery anode material because of its high theoretical capacity and high intrinsic conductivity. Integrated with carbon, Bi₂Se₃/C composite shows excellent cyclic performance and rate capability. For instance, the Bi₂Se₃/C anode delivers an initial capacity of 527 mAh g^-1 at 0.1 A g^-1 and maintains 89% of this capacity over 100 cycles. The phase change and sodium storage mechanism are also carefully investigated.