Graphene Wrapped FeSe2 Nano‐Microspheres with High Pseudocapacitive Contribution for Enhanced Na‐Ion Storage

Pseudocapacitance is a Faradaic process that involves surface or near surface redox reactions. Increasing the pseudocapacitive contribution is one of the most effective means to improve the rate performance of electrode materials. In this study, graphene oxide is used as a template to in situ synthesize burr globule‐like FeSe₂/graphene hybrid (B‐FeSe₂/G) using a facile one‐step hydrothermal method. Structural characterization demonstrates that graphene layers not only wrap the surfaces of FeSe₂ particles, but also stretch into the interior of these particles, as a result of which the unique nano‐microsphere structure is successfully established. When serving as anode material for Na‐ion batteries, B‐FeSe₂/G hybrid displays high electrochemical performance in the voltage range of 0.5–2.9 V. The B‐FeSe₂/G hybrid has high reversible capacity of 521.6 mAh·g^?1 at 1.0 A g^?1. Meanwhile, after 400 cycles, high discharge capacity of 496.3 mAh g^?1 is obtained at a rate of 2.5 A g^?1, with a high columbic efficiency of 96.6% and less than 1.0% loss of discharge capacity. Even at the ultrahigh rate of 10 A g^?1, a specific capacity of 316.8 mAh g^?1 can be achieved. Kinetic analyses reveal that the excellent performance of the B‐FeSe₂/G hybrid is largely attributed to the high pseudocapacitive contribution induced by the special nano‐micro structure.     

Observation of Pseudocapacitive Effect and Fast Ion Diffusion in Bimetallic Sulfides as an Advanced Sodium‐Ion Battery Anode

Sodium‐ion batteries (SIBs) are promising next‐generation alternatives due to the low cost and abundance of sodium sources. Yet developmental electrodes in SIBs such as transition metal sulfides have huge volume expansion, sluggish Na⁺ diffusion kinetics, and poor electrical conductivity. Here bimetallic sulfide (Co₉S₈/ZnS) nanocrystals embedded in hollow nitrogen‐doped carbon nanosheets are demonstrated with a high sodium diffusion coefficient, pseudocapacitive effect, and excellent reversibility. Such a unique composite structure is designed and synthesized via a facile sulfidation of the CoZn‐MOFs followed by calcination and is highly dependant on the reaction time and temperature. The optimized Co₁Zn₁‐S(600) electrode exhibits excellent sodium storage performance, including a high capacity of 542 mA h g^?1 at 0.1 A g^?1, good rate capability at 10 A g^?1, and excellent cyclic stability up to 500 cycles for half‐cell. It also shows potential in full‐cell configuration. Such capabilities will accelerate the adoption of sodium‐ion batteries for electrical energy applications.     

Unraveling the Rapid Redox Behavior of Li‐Excess 3d‐Transition Metal Oxides for High Rate Capability

Li‐excess 3d‐transition metal layered oxides are promising candidates in high‐energy‐density cathode materials for improving the mileage of electric vehicles. However, their low rate capability has hindered their practical application. The lack of understanding about the redox reactions and migration behavior at high C‐rates make it difficult to design Li‐excess materials with high rate capability. In this study, the characteristics of the atomic behavior that is predominant at fast charge/discharge are investigated by comparing cation‐ordered and cation‐disordered materials using X‐ray absorption spectroscopy (XAS). The difference in the atomic arrangement determines the dominance of the transition metal/oxygen redox reaction and the variations in transition metal–oxygen hybridization. In‐depth electrochemical analysis is combined with operando XAS analysis to reveal electronically and structurally preferred atomic behavior when a redox reaction occurs between oxygen and each transition metal under fast charge/discharge conditions. This provides a fundamental insight into the improvement of rate capability. Furthermore, this work provides guidance for identifying high‐energy‐density materials with complex structural properties.     

Graphitic Carbon Nanocage as a Stable and High Power Anode for Potassium‐Ion Batteries

As an emerging electrochemical energy storage device, potassium‐ion batteries (PIBs) have drawn growing interest due to the resource‐abundance and low cost of potassium. Graphite‐based materials, as the most common anodes for commercial Li‐ion batteries, have a very low capacity when used an anode for Na‐ion batteries, but they show reasonable capacities as anodes for PIBs. The practical application of graphitic materials in PIBs suffers from poor cyclability, however, due to the large interlayer expansion/shrinkage caused by the intercalation/deintercalation of potassium ions. Here, a highly graphitic carbon nanocage (CNC) is reported as a PIBs anode, which exhibits excellent cyclability and superior depotassiation capacity of 175 mAh g^?1 at 35 C. The potassium storage mechanism in CNC is revealed by cyclic voltammetry as due to redox reactions (intercalation/deintercalation) and double‐layer capacitance (surface adsorption/desorption). The present results give new insights into structural design for graphitic anode materials in PIBs and understanding the double‐layer capacitance effect in alkali metal ion batteries.     

Nanosheets‐Assembled CuSe Crystal Pillar as a Stable and High‐Power Anode for Sodium‐Ion and Potassium‐Ion Batteries

Thanks to low costs and the abundance of the resources, sodium‐ion (SIBs) and potassium‐ion batteries (PIBs) have emerged as leading candidates for next‐generation energy storage devices. So far, only few materials can serve as the host for both Na⁺ and K⁺ ions. Herein, a cubic phase CuSe with crystal‐pillar‐like morphology (CPL‐CuSe) assembled by the nanosheets are synthesized and its dual functionality in SIBs and PIBs is comprehensively studied. The electrochemical measurements demonstrate that CPL‐CuSe enables fast Na⁺ and K⁺ storage as well as the sufficiently long duration. Specifically, the anode delivers a specific capacity of 295 mA h g^?1 at current density of 10 A g^?1 in SIBs, while 280 mA h g^?1 at 5 A g^?1 in PIBs, as well as the high capacity retention of nearly 100% over 1200 cycles and 340 cycles, respectively. Remarkably, CPL‐CuSe exhibits a high initial coulombic efficiency of 91.0% (SIBs) and 92.4% (PIBs), superior to most existing selenide anodes. A combination of in situ X‐ray diffraction and ex situ transmission electron microscopy tests fundamentally reveal the structural transition and phase evolution of CuSe, which shows a reversible conversion reaction for both cells, while the intermediate products are different due to the sluggish K⁺ insertion reaction.     

Wet‐Chemical Processing of Phosphorus Composite Nanosheets for High‐Rate and High‐Capacity Lithium‐Ion Batteries

Phosphorus‐based materials are promising for high‐performance lithium‐ion battery (LIB) applications due to their high theoretical specific capacity. Currently, the existing physical methods render great difficulty toward rational engineering on the nanostructural phosphorus or its composites, thus limiting its high‐rate LIB applications. For the first time, a sublimation‐induced synthesis of phosphorus‐based composite nanosheets by a chemistry‐based solvothermal reaction is reported. Its formation mechanism involves solid–vapor–solid transformation driven by continuous vaporization–condensation process, as well as subsequent bottom‐up assembly growth. The proof‐of‐concept LIBs composed of the phosphorus‐based nanosheets achieve a high capacity of 630 mAh g^?1 at an ultrahigh current density of 20 A g^?1, which is attributed to efficient lithium‐ion diffusion and electron transfer. Such simple sublimation‐induced transformation opens up new prospects for rational engineering of phosphorus‐based materials for enhancing electrochemical performance.     

Silicon‐Based Nanomaterials for Lithium‐Ion Batteries: A Review

There are growing concerns over the environmental, climate, and health impacts caused by using non‐renewable fossil fuels. The utilization of green energy, including solar and wind power, is believed to be one of the most promising alternatives to support more sustainable economic growth. In this regard, lithium‐ion batteries (LIBs) can play a critically important role. To further increase the energy and power densities of LIBs, silicon anodes have been intensively explored due to their high capacity, low operation potential, environmental friendliness, and high abundance. The main challenges for the practical implementation of silicon anodes, however, are the huge volume variation during lithiation and delithiation processes and the unstable solid‐electrolyte interphase (SEI) films. Recently, significant breakthroughs have been achieved utilizing advanced nanotechnologies in terms of increasing cycle life and enhancing charging rate performance due partially to the excellent mechanical properties of nanomaterials, high surface area, and fast lithium and electron transportation. Here, the most recent advance in the applications of 0D (nanoparticles), 1D (nanowires and nanotubes), and 2D (thin film) silicon nanomaterials in LIBs are summarized. The synthetic routes and electrochemical performance of these Si nanomaterials, and the underlying reaction mechanisms are systematically described.     

Highly Tough,Li‐Metal Compatible Organic–Inorganic Double‐Network Solvate Ionogel

Ionogels are considered promising electrolytes for safe lithium‐ion batteries (LIBs) because of their low flammability, good thermal stability, and wide electrochemical stability window. Conventional ionic liquid‐based ionogels, however, face two main challenges; poor mechanical property and low Li‐ion transfer number. In this work, a novel solvate ionogel electrolyte (SIGE) based on an organic–inorganic double network (DN) is designed and fabricated through nonhydrolytic sol–gel reaction and in situ polymerization processes. The unprecedented SIGE possesses high toughness (bearing the deformation under the pressure of 80 MPa without damage), high Li‐ion transfer number of 0.43, and excellent Li‐metal compatibility. As expected, the LiFePO₄/Li cell using the newly developed SIGE delivers a high capacity retention of 95.2% over 500 cycles, and the average Coulombic efficiency is as high as 99.8%. Moreover, the Ni‐rich LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811)/Li cell based on the modified SIGE achieves a high Coulombic efficiency of 99.4%, which outperforms previous solid/quasi‐solid‐state NCM811‐based LIBs. Interestingly, the SIGE‐based pouch cells are workable under extreme conditions (e.g., severely deforming or clipping into segments). In terms of those unusual features, the as‐obtained SIGE holds great promise for next‐generation flexible and safe energy‐storage devices.     

New Binder‐Free Metal Phosphide–Carbon Felt Composite Anodes for Sodium‐Ion Battery

Metal phosphides are promising anode candidates for sodium‐ion batteries (SIBs) due to their high specific capacity and low operating potential but suffer from poor cycling stability caused by huge volume expansion and poor solid‐state ion transfer rate. Herein, a new strategy to grow a new class of mesoporous metal phosphide nanoarrays on carbon felt (CF) as binder‐free anodes for SIBs is reported. The resultant integrated electrodes demonstrate excellent cycling life up to 1000 times (>90% retention rate) and high rate capability of 535 mAh g^?1 at a current density of 4 A g^?1. Detailed characterization reveals that the synergistic effect of unique mesoporous structure for accommodating huge volume expansion during sodiation/desodiation process, ultrasmall primary particle size (≈10 nm) for providing larger electrode/electrolyte contact area and shorter ion diffusion distance, and 3D conductive networks for facilitating the electrochemical reaction, leads to the extraordinary battery performance. Remarkably, a full SIB using the new CoP₄/CF anode and a Na₃V₂(PO₄)₂F₃ cathode delivers an average operating voltage of ≈3.0 V, a reversible capacity of 553 mAh g^?1, and very high energy density of ≈280 Wh kg^?1 for SIBs. A flexible SIB with outstanding mechanical strength based on this binder‐free new anode is also demonstrated.     

Exploiting Lithium–Ether Co‐Intercalation in Graphite for High‐Power Lithium‐Ion Batteries

The intercalation of lithium ions into graphite electrode is the key underlying mechanism of modern lithium‐ion batteries. However, co‐intercalation of lithium‐ions and solvent into graphite is considered undesirable because it can trigger the exfoliation of graphene layers and destroy the graphite crystal, resulting in poor cycle life. Here, it is demonstrated that the [lithium–solvent]⁺ intercalation does not necessarily cause exfoliation of the graphite electrode and can be remarkably reversible with appropriate solvent selection. First‐principles calculations suggest that the chemical compatibility of the graphite host and [lithium–solvent]⁺ complex ion strongly affects the reversibility of the co‐intercalation, and comparative experiments confirm this phenomenon. Moreover, it is revealed that [lithium–ether]⁺ co‐intercalation of natural graphite electrode enables much higher power capability than normal lithium intercalation, without the risk of lithium metal plating, with retention of ≈87% of the theoretical capacity at current density of 1 A g^?1. This unusual high rate capability of the co‐intercalation is attributed to the (i) absence of the desolvation step, (ii) negligible formation of the solid–electrolyte interphase on graphite surface, and (iii) fast charge‐transfer kinetics. This work constitutes the first step toward the utilization of fast and reversible [lithium–solvent]⁺ complex ion intercalation chemistry in graphite for rechargeable battery technology.     

Stable Carbon–Selenium Bonds for Enhanced Performance in Tremella‐Like 2D Chalcogenide Battery Anode

2D cobalt selenide based on conversion reaction has attracted much attention due to its open layered structure and high specific capacity. However, effectively suppressing the fast capacity fade caused by the irreversible Se dissolution and serious volume change during the cycling process is still a challenge. Herein, the concentration of dispersion liquid under supercritical conditions is tuned to induce the CoSe crystal to grow along the graphene oxide (GO), and finally obtain the Tremella‐like CoSe–reduced GO (rGO) hybrid. The nature of epitaxial growth leads to the formation of stable C? Se bonds, which maintain a favorable conductive connection between CoSe and rGO as well as enhance the mechanical strength of active materials to suppress Se dissolution and volume expansion during Na/Li intercalation and deintercalation. The unique microstructural merits of the hybrid result in superior sodium/lithium storage performance (400.8 mAh g^?1 at 1 A g^?1 after 100 cycles for sodium‐ion batteries and 769.6 mAh g^?1 at 2 A g^?1 after 500 cycles for lithium‐ion batteries). Moreover, the transmission X‐ray microscopy technique is first used to directly observe the Se segregation in cobalt selenide and it being suppressed by the C? Se bonds.     

Pseudocapacitive Charge Storage in Thick Composite MoS2 Nanocrystal‐Based Electrodes

A synthesis methodology is demonstrated to produce MoS₂ nanoparticles with an expanded atomic lamellar structure that are ideal for Faradaic‐based capacitive charge storage. While much of the work on MoS₂ focuses on the high capacity conversion reaction, that process is prone to poor reversibility. The pseudocapacitive intercalation‐based charge storage reaction of MoS₂ is investigated, which is extremely fast and highly reversible. A major challenge in the field of pseudocapacitive‐based energy storage is the development of thick electrodes from nanostructured materials that can sustain the fast inherent kinetics of the active nanocrystalline material. Here a composite electrode comprised of a poly(acrylic acid) binder, carbon fibers, and carbon black additives is utilized. These electrodes deliver a specific capacity of 90 mAh g^?1 in less than 20 s and can be cycled 3000 times while retaining over 80% of the original capacity. Quantitative kinetic analysis indicates that over 80% of the charge storage in these MoS₂ nanocrystals is pseudocapacitive. Asymmetric full cell devices utilizing a MoS₂ nanocrystal‐based electrode and an activated carbon electrode achieve a maximum power density of 5.3 kW kg^?1 (with 6 Wh kg^?1 energy density) and a maximum energy density of 37 Wh kg^?1 (with 74 W kg^?1power density).     

A Novel α‐MoO3/Single‐Walled Carbon Nanohorns Composite as High‐Performance Anode Material for Fast‐Charging Lithium‐Ion Battery

Orthorhombic α‐MoO₃ is a potential anode material for lithium‐ion batteries due to its high theoretical capacity of 1100 mAh g^?1 and excellent structural stability. However, its intrinsic poor electronic conductivity and high volume expansion during the charge–discharge process impede it from achieving a high practical capacity. A novel composite of α‐MoO₃ nanobelts and single‐walled carbon nanohorns (SWCNHs) is synthesized by a facile microwave hydrothermal technique and demonstrated as a high‐performance anode material for lithium‐ion batteries. The α‐MoO₃/SWCNH composite displays superior electrochemical properties (654 mAh g^?1 at 1 C), excellent rate capability (275 mAh g^?1 at 5 C), and outstanding cycle life (capacity retention of >99% after 3000 cycles at 1 C) without any cracking of the electrode. The presence of SWCNHs in the composite enhances the electrochemical properties of α‐MoO₃ by acting as a lithium storage material, electronic conductive medium, and buffer against pulverization.     

Mesoporous MoS2 as a Transition Metal Dichalcogenide Exhibiting Pseudocapacitive Li and Na‐Ion Charge Storage

The ion insertion properties of MoS₂ continue to be of widespread interest for energy storage. While much of the current work on MoS₂ has been focused on the high capacity four‐electron reduction reaction, this process is prone to poor reversibility. Traditional ion intercalation reactions are highlighted and it is demonstrated that ordered mesoporous thin films of MoS₂ can be utilized as a pseudocapacitive energy storage material with a specific capacity of 173 mAh g^?1 for Li‐ions and 118 mAh g^?1 for Na‐ions at 1 mV s^?1. Utilizing synchrotron grazing incidence X‐ray diffraction techniques, fast electrochemical kinetics are correlated with the ordered porous structure and with an iso‐oriented crystal structure. When Li‐ions are utilized, the material can be charged and discharged in 20 seconds while still achieving a specific capacity of 140 mAh g^?1. Moreover, the nanoscale architecture of mesoporous MoS₂ retains this level of lithium capacity for 10 000 cycles. A detailed electrochemical kinetic analysis indicates that energy storage for both ions in MoS₂ is due to a pseudocapacitive mechanism.