A Volcano Correlation between Catalytic Activity for Sulfur Reduction Reaction and Fe Atom Count in Metal Center.

Sulfur reduction reaction (SRR) facilitates up to 16 electrons, which endows lithium-sulfur (Li-S) batteries with a high energy density that is twice that of typical Li-ion batteries. However, its sluggish reaction kinetics render batteries with only a low capacity and cycling life, thus remaining the main challenge to practical Li-S batteries, which require efficient electrocatalysts of balanced atom utilization and site-specific requirements toward highly efficient SRR, calling for an in-depth understanding of the atomic structural sensitivity for the catalytic active sites. Herein, we manipulated the number of Fe atoms in iron assemblies, ranging from single Fe atom to diatomic and triatomic Fe atom groupings, all embedded within a carbon matrix. This led to the revelation of a "volcano peak" correlation between SRR catalytic activity and the count of Fe atoms at the active sites. Utilizing operando X-ray absorption and X-ray diffraction spectroscopies, we observed that polysulfide adsorption-desorption and electrochemical conversion kinetics varied up and down with the incremental addition of even a single iron atom to the catalyst's metal center. Our results demonstrate that the metal center with exactly two iron atoms represents the optimal configuration, maximizing atom utility and adeptly handling the conversion of varied intermediate sulfur species, rendering the Li-S battery with a high areal capacity of 23.8 mAh cm-2 at a high sulfur loading of 21.8 mg cm-2. Our results illuminate the pivotal balance between atom utilization and site-specific requirements for optimal electrocatalytic performance in SRR and diverse electrocatalytic reactions.

Breaking Low-Strain and Deep-Potassiation Trade-Off in Alloy Anodes via Bonding Modulation for High-Performance K-Ion Batteries.

Alloy anode materials have garnered unprecedented attention for potassium storage due to their high theoretical capacity. However, the substantial structural strain associated with deep potassiation results in serious electrode fragmentation and inadequate K-alloying reactions. Effectively reconciling the trade-off between low-strain and deep-potassiation in alloy anodes poses a considerable challenge due to the larger size of K-ions compared to Li/Na-ions. In this study, we propose a chemical bonding modulation strategy through single-atom modification to address the volume expansion of alloy anodes during potassiation. Using black phosphorus (BP) as a representative and generalizing to other alloy anodes, we established a robust P-S covalent bonding network via sulfur doping. This network exhibits sustained stability across discharge-charge cycles, elevating the modulus of K-P compounds by 74%, effectively withstanding the high strain induced by the potassiation process. Additionally, the bonding modulation reduces the formation energies of potassium phosphides, facilitating a deeper potassiation of the BP anode. As a result, the modified BP anode exhibits a high reversible capacity and extended operational lifespan, coupled with a high areal capacity. This work introduces a new perspective on overcoming the trade-off between low-strain and deep-potassiation in alloy anodes for the development of high-energy and stable potassium-ion batteries.

Li3PO4-Enriched SEI on Graphite Anode Boosts Li+ De-Solvation Enabling Fast-Charging and Low-Temperature Lithium-Ion Batteries.

Li+ de-solvation at solid-electrolyte interphase (SEI)-electrolyte interface stands as a pivotal step that imposes limitations on the fast-charging capability and low-temperature performance of lithium-ion batteries (LIBs). Unraveling the contributions of key constituents in the SEI that facilitate Li+ de-solvation and deciphering their mechanisms, as a design principle for the interfacial structure of anode materials, is still a challenge. Herein, we conducted a systematic exploration of the influence exerted by various inorganic components (Li2CO3, LiF, Li3PO4) found in the SEI on their role in promoting the Li+ de-solvation. The findings highlight that Li3PO4-enriched SEI effectively reduces the de-solvation energy due to its ability to attenuate the Li+-solvent interaction, thereby expediting the de-solvation process. Building on this, we engineer Li3PO4 interphase on graphite (LPO-Gr) anode via a simple solid-phase coating, facilitating the Li+ de-solvation and building an inorganic-rich SEI, resulting in accelerated Li+ transport crossing the electrode interfaces and interphases. Full cells using the LPO-Gr anode can replenish its 80 % capacity in 6.5 minutes, while still retaining 70 % of the room temperature capacity even at -20 °C. Our strategy establishes connection between the de-solvation characteristics of the SEI components and the interfacial structure design of anode materials for high performance LIBs.

Solid-Solution or Intermetallic Compounds: Phase Dependence of the Li-Alloying Reactions for Li-Metal Batteries.

Electrochemical Li-alloying reactions with Li-rich alloy phases render a much higher theoretical capacity that is critical for high-energy batteries, and the accompanying phase transition determines the alloying/dealloying reversibility and cycling stability. However, the influence of phase-transition characteristics upon the thermodynamic properties and diffusion kinetic mechanisms among the two categories of alloys, solid-solutions and intermetallic compounds, remains incomplete. Here we investigated three representative Li-alloys: Li-Ag alloy of extended solid-solution regions; Li-Zn alloy of an intermetallic compound with a solid-solution phase of a very narrow window in Li atom concentration; and Li-Al alloy of an intermetallic compound. Solid-solution phases undertake a much lower phase-transition energy barrier than the intermetallic compounds, leading to a considerably higher Li-alloying/dealloying reversibility and cycling stability, which is due to the subtle structural change and chemical potential gradient built up inside of the solid-solution phases. These two effects enable the Li atoms to enter the bulk of the Li-Ag alloy to form a homogeneous alloy phase. The pouch cell of the Li-rich Li20Ag alloy pairs with a LiNi0.8Co0.1Mn0.1O2 cathode under an areal capacity of 3.5 mAh cm-2 can retain 87% of its initial capacity after 250 cycles with an enhanced Coulombic efficiency of 99.8 ± 0.1%. While Li-alloying reactions and the alloy phase transitions have always been tightly linked in past studies, our findings provide important guidelines for the intelligent design of components for secondary metal batteries.

The Vital Role of Electrolyte Reduction Potential in Forming a Stable SEI in Phosphorus-Based Anodes.

In view of their high lithium storage capability, phosphorus-based anodes are promising for lithium-ion batteries. However, the low reduction potential (0.74 V versus Li+ /Li) of the commonly used ethylene carbonate-based electrolyte does not allow the early formation of a solid electrolyte interphase (SEI) prior to the initial phosphorus alloying reaction (1.5 V versus Li+ /Li). In the absence of a protective SEI, the phosphorus anode develops cracks, creating additional P/electrolyte interfaces. This results in the loss of P and the formation of a discontinuous SEI, all of which greatly reduce the electrochemical performance of the anode. Here, the effect of solvent reduction potential on the structure of the SEI is investigated. It is found that solvents with a high reduction potential, such as fluoroethylene carbonate, decompose to form an SEI concomitantly with the P alloying reaction. This results in a continuous, mechanically robust, and Li3 PO4 -rich SEI with improved Li-ion conductivity. These attributes significantly improve the cyclic stability and rate performance of the phosphorus-based anode.

A Superior Two-Dimensional Phosphorus Flame Retardant: Few-Layer Black Phosphorus.

The usage of flame retardants in flammable polymers has been an effective way to protect both lives and material goods from accidental fires. Phosphorus flame retardants have the potential to be follow-on flame retardants after halogenated variants, because of their low toxicity, high efficiency and compatibility. Recently, the emerging allotrope of phosphorus, two-dimensional black phosphorus, as a flame retardant has been developed. To further understand its performance in flame-retardant efficiency among phosphorus flame retardants, in this work, we built model materials to compare the flame-retardant performances of few-layer black phosphorus, red phosphorus nanoparticles, and triphenyl phosphate as flame-retardant additives in cellulose and polyacrylonitrile. Aside from the superior flame retardancy in polyacrylonitrile, few-layer black phosphorus in cellulose showed the superior flame-retardant efficiency in self-extinguishing, ~1.8 and ~4.4 times that of red phosphorus nanoparticles and triphenyl phosphate with similar lateral size and mass load (2.5~4.8 wt%), respectively. The char layer in cellulose coated with the few-layer black phosphorus after combustion was more continuous and smoother than that with red phosphorus nanoparticles, triphenyl phosphate and blank, and the amount of residues of cellulose coated with the few-layer black phosphorus in thermogravimetric analysis were 10 wt%, 14 wt% and 14 wt% more than that with red phosphorus nanoparticles, triphenyl phosphate and blank, respectively. In addition, although exothermic reactions, the combustion enthalpy changes in the few-layer black phosphorus (-127.1 kJ mol-1) are one third of that of red phosphorus nanoparticles (-381.3 kJ mol-1). Based on a joint thermodynamic, spectroscopic, and microscopic analysis, the superior flame retardancy of the few-layer black phosphorus was attributed to superior combustion reaction suppression from the two-dimensional structure and thermal nature of the few-layer black phosphorus.

Regulating Frozen Electrolyte Structure with Colloidal Dispersion for Low Temperature Aqueous Batteries.

Electrolyte freezing under low temperatures is a critical challenge for the development of aqueous batteries (ABs). While lowering the freezing point of the electrolyte has caught major research efforts, limited attention has been paid to the structural evolution during the electrolyte freezing process and regulating the frozen electrolyte structure for low temperature ABs. Here, we reveal the formation process of interconnected liquid regions for ion transport in frozen electrolytes with various in situ variable-temperature technologies. More importantly, the low-temperature performance of ABs was significantly improved with the colloidal electrolyte design using graphene oxide quantum dots (GOQDs), which effectively inhibits the growth of ice crystals and expands the interconnected liquid regions for facial ion transport. This work provides new insights and a promising strategy for the electrolyte design of low-temperature ABs.

Non-trivial Contribution of Carbon Hybridization in Carbon-based Substrates to Electrocatalytic Activities in Li-S Batteries.

Appling an electrochemical catalyst is an efficient strategy for inhibiting the shuttle effect and enhancing the S utilization of Li-S batteries. Carbon-based materials are the most common conductive agents and catalyst supports used in Li-S batteries, but the correlation between the diversity of hybridizations and sulfur reduction reaction (SRR) catalytic activity remains unclear. Here, by establishing two forms of carbon models, i.e., graphitic carbon (GC) and amorphous carbon (AC), we observe that the nitrogen atom doped in the GC possesses a higher local charge density and a lower Gibbs free energy towards the formation of polysulfides than in the AC. And the GC-based electrode consistently inherits considerably enhanced SRR kinetics and superior cycling stability and rate capability in Li-S batteries. Therefore, the function of carbon in Li-S batteries is not only limited as conductive support but also plays an unignorable contribution to the electrocatalytic activities of SRR.

Regulating Sodium Deposition through Gradiently-Graphitized Framework for Dendrite-Free Na Metal Anode.

Na metal anode (NMA) is one of the most promising candidate materials for next-generation low-cost sodium metal batteries. However, the preferred deposition of Na metal at the anode/separator interface increases the risk of dendrite penetration of the separator, consequently, reduces safety and life of batteries with NMA. In this study, a Na deposition-regulating strategy is shown by designing a gradiently graphitized 3D carbon fiber (CF) framework as host (grad-CF), whereby Na is guided to deposit preferentially at the bottom of the anode, safely away from the separator. The obtained Na anode significantly reduces the probability of dendrite-induced short circuits. The grad-CF host enables NMA stable cycling at a high current density of 6 mA cm-2 . When the Na@grad-CF is applied as anode in full cells pared with Na3 V2 (PO4 )3 (NVP) cathode, it exhibits a reversible capacity of 73 mA h g-1 after 500 cycles with a low decay rate of 0.13%.

Role of the Metal Atom in a Carbon-Based Single-Atom Electrocatalyst for LiS Redox Reactions.

Carbon-based single metal atom catalysts (SACs) are being extensively investigated to improve the kinetics of the Li-S redox reaction, which is greatly important for batteries with cell-level energy densities >500 W h kg-1 . However, there are contradictory reports regarding the electrocatalytic activities of the different metal atoms and the role of the metal atom in LiS chemistry still remains unclear. This is due to the complex relationship between the catalytic behavior and the structure of carbon-based SACs. Here, the catalytic behavior and active-site geometry, oxidation state, and the electronic structure of different metal centers (Fe/Co/Ni) embedded in nitrogen-doped graphene, and having similar physicochemical characteristics, are studied. Combining X-ray absorption spectroscopy, density functional theory calculations, and electrochemical analysis, it is revealed that the coordination-geometry and oxidation state of the metal atoms are modified when interacting with sulfur species. This interaction is strongly dependent on the hybridization of metal 3d and S p-orbitals. A moderate hybridization with the Fermi level crossing the metal 3d band is more favorable for LiS redox reactions. This study thus provides a fundamental understanding of how metal atoms in SACs impact LiS redox behavior and offers new guidelines to develop highly active catalytic materials for high-performance LiS batteries.

Solid-Solution-Based Metal Alloy Phase for Highly Reversible Lithium Metal Anode.

Lithium metal batteries are vital devices for high-energy-density energy storage, but the Li metal anode is highly reactive with electrolyte and forms uncontrolled dendrite that can cause undesirable parasitic reactions and, thus, poor cycling stability and raise safety concerns. Despite remarkable progress to partially solve these issues, the Li metal still plates at the electrode/electrolyte interface where the parasitic reactions and dendrite formation invariably occur. Here, we demonstrate the inward-growth plating of Li atoms into a metal foil of thickness of tens of micrometers while avoiding surface deposition, which is driven by the reversible solid-solution-based alloy phase change. Lithiation of the solid-solution alloy phase allows the freshly generated Li atoms at the surface to sink into the metal foil, while the reversible alloy phase change is companied by the dealloying reaction during delithiation, which extracts Li atoms from inside of the metal foil. The yielded dendrite free Li anode produces an enhanced Coulombic efficiency of 99.5 ± 0.2% with a reversible capacity of 1660 mA h g-1 (3.3 mA h cm-2).

Graphene foil as a current collector for NCM material-based cathodes.

When used as a current collector, aluminum foil (AF) is vulnerable to local anodic corrosion during the charge/discharge process, which can lead to the deterioration of lithium-ion batteries (LIBs). Herein, a graphene foil (GF) with high electrical conductivity (∼5800 S cm-1) and low mass density (1.80 g cm-3) was prepared by reduction of graphene oxide foil with ultra-high temperature (2800 °C) annealing, and it exhibited significantly anodic corrosion resistance when serving as a current collector. Moreover, a LiNi0.5Co0.2Mn0.3O2 (NCM523) cathode using GF as a current collector (NCM523/GF) demonstrated a gravimetric capacity of 137.3 mAh g-1 at 0.5 C based on the mass of the whole electrode consisting of the active material, carbon black, binder, and the current collector, which is 44.5% higher than that of the NCM523/AF electrode. Furthermore, the NCM523/GF electrode retains higher capacity at relatively faster rates, from 0.1 C to 5.0 C. Therefore, GF, a lightweight corrosion-resistant current collector, is expected to replace the current commercial metal current collectors in LIBs and to achieve high energy-density batteries.

Rechargeable Aluminium-Sulfur Battery with Improved Electrochemical Performance by Cobalt-Containing Electrocatalyst.

The rechargeable aluminium-sulfur (Al-S) battery is regarded as a potential alternative beyond lithium-ion battery system owing to its safety, promising energy density, and the high earth abundance of the constituent electrode materials, however, sluggish kinetic response and short life-span are the major issues that limit the battery development towards applications. In this article, we report CoII,III as an electrochemical catalyst in the sulfur cathode that renders a reduced discharge-charge voltage hysteresis and improved capacity retention and rate capability for Al-S batteries. The structural and electrochemical analysis suggest that the catalytic effect of CoII,III is closely associated with the formation of cobalt sulfides and the changes in the valence states of the CoII,III during the electrochemical reactions of the sulfur species, which lead to improved reaction kinetics and sulfur utilization in the cathode. The Al-S battery, assembled with the cathode consisting of CoII,III decorated carbon matrix, demonstrates a considerably reduced voltage hysteresis of 0.8 V, a reversible specific capacity of ≈500 mAh g-1 at 1 A g-1 after 200 discharge-charge cycles and of ≈300 mAh g-1 at 3 A g-1 .

A Black Phosphorus-Graphite Composite Anode for Li-/Na-/K-Ion Batteries.

Black phosphorus (BP) is a desirable anode material for alkali metal ion storage owing to its high electronic/ionic conductivity and theoretical capacity. In-depth understanding of the redox reactions between BP and the alkali metal ions is key to reveal the potential and limitations of BP, and thus to guide the design of BP-based composites for high-performance alkali metal ion batteries. Comparative studies of the electrochemical reactions of Li+ , Na+ , and K+ with BP were performed. Ex situ X-ray absorption near-edge spectroscopy combined with theoretical calculation reveal the lowest utilization of BP for K+ storage than for Na+ and Li+ , which is ascribed to the highest formation energy and the lowest ion diffusion coefficient of the final potassiation product K3 P, compared with Li3 P and Na3 P. As a result, restricting the formation of K3 P by limiting the discharge voltage achieves a gravimetric capacity of 1300 mAh g-1 which retains at 600 mAh g-1 after 50 cycles at 0.25 A g-1 .

Cobalt in Nitrogen-Doped Graphene as Single-Atom Catalyst for High-Sulfur Content Lithium-Sulfur Batteries.

Because of their high theoretical energy density and low cost, lithium-sulfur (Li-S) batteries are promising next-generation energy storage devices. The electrochemical performance of Li-S batteries largely depends on the efficient reversible conversion of Li polysulfides to Li2S in discharge and to elemental S during charging. Here, we report on our discovery that monodisperse cobalt atoms embedded in nitrogen-doped graphene (Co-N/G) can trigger the surface-mediated reaction of Li polysulfides. Using a combination of operando X-ray absorption spectroscopy and first-principles calculation, we reveal that the Co-N-C coordination center serves as a bifunctional electrocatalyst to facilitate both the formation and the decomposition of Li2S in discharge and charge processes, respectively. The S@Co-N/G composite, with a high S mass ratio of 90 wt %, can deliver a gravimetric capacity of 1210 mAh g-1, and it exhibits an areal capacity of 5.1 mAh cm-2 with capacity fading rate of 0.029% per cycle over 100 cycles at 0.2 C at S loading of 6.0 mg cm-2 on the electrode disk.

Azide Passivation of Black Phosphorus Nanosheets: Covalent Functionalization Affords Ambient Stability Enhancement.

Two-dimensional (2D) black phosphorus (BP) has a unique band structure, but it suffers from low ambient stability owing to its high reactivity to oxygen. Covalent functionalization has been demonstrated to passivate the reactive BP effectively, however the reported covalent functionalization methods are quite limited to aryl diazonium and nucleophilic additions affording P-C and P-O-C single bonds, for which the retaining of one unpaired electron in the Group 15 phosphorus atom hampers the passivation effect. Now, covalent azide functionalization of BP nanosheets (BPNSs) is reported, leading to significant enhancement of the ambient stability of BP as confirmed by UV/Vis spectroscopic studies. The most stable configuration of the azide functionalized BPNSs (f-BPNSs) is predicted by theoretical calculations, featuring the grafting of benzoic acid moiety onto BPNSs via the unprecedented P=N double bonds formed through in situ nitrene as a reactive intermediate.

Origin of the Overpotential for the Oxygen Evolution Reaction on a Well-Defined Graphene Electrode Probed by in Situ Sum Frequency Generation Vibrational Spectroscopy.

To develop an efficient material for the cathode of the lithium-oxygen (Li-O2) secondary battery, the oxygen reduction and evolution reactions (ORR and OER) on a well-defined graphene electrode have been investigated in a typical organic solvent, dimethyl sulfoxide (DMSO). The adsorption and desorption behaviors of the solvents on the graphene electrode surface were evaluated by an intrinsically surface-selective vibrational spectroscopy of sum frequency generation (SFG) during the ORR and OER. After the initial ORR depositing lithium peroxide (Li2O2) on the graphene electrode surface in a LiClO4/DMSO solution, the SFG spectroscopy revealed that the subsequent OER oxidizing the Li2O2 preferentially proceeds at the interface between the Li2O2 and graphene rather than that between the Li2O2 and bulk solution. Therefore, the OER tends to reduce the electric conductivity between the Li2O2 and graphene by decreasing their contact area before a large part of the deposited Li2O2 was oxidized, which elucidates the origin of the high overpotential for the OER.

Degradation Chemistry and Stabilization of Exfoliated Few-Layer Black Phosphorus in Water.

Exfoliated black phosphorus (BP), as a monolayer or few-layer material, has attracted tremendous attention owing to its unique physical properties for applications ranging from optoelectronics to photocatalytic hydrogen production. Approaching intrinsic properties has been, however, challenged by chemical reactions and structure degradation of BP under ambient conditions. Surface passivation by capping agents has been proposed to extend the processing time window, yet contamination or structure damage rise challenges for BP applications. Here, we report experiments combined with first-principle calculations that address the degradation chemistry of BP. Our results show that BP reacts with oxygen in water even without light illumination. The reaction follows a pseudo-first-order parallel reaction kinetics, produces PO23-, PO33-, and PO43- with reaction rate constants of 0.019, 0.034, and 0.023 per day, respectively, and occurs preferentially from the P atoms locating at BP edges, which yields structural decay from the nanoflake edges in water. In addition, a negligible decay ratio (0.9 ± 0.3 mol %) and preserved photocatalytic activity of BP are observed after storage in deoxygenated water for 15 days without surface passivation under ambient light. Our results reveal the chemistry of BP degradation and provide a practical approach for exfoliation, delivery, and application of BP.

Hot-Roll-Pressing Mediated Transfer of Chemical Vapor Deposition Graphene for Transparent and Flexible Touch Screen with Low Sheet-Resistance.

Obstacles associated with graphene as transparent conductive films mainly consist of the difficulties in high-quality graphene synthesis, efficient transfer and doping of samples with lateral size of tens of centimeters for practical applications. Herein we demonstrate a hot-roll-pressing transfer technique followed by wet-chemical doping of large area graphene film grown on copper foil by chemical vapor deposition (CVD). This method enabled cost-effective and ultraclean transfer of single-layer graphene with an arbitrary size onto transparent ethylene vinyl acetate/polyethylene terephthalate (EVA/PET) substrate without any polymer residues. The sheet resistance of the single-layer graphene covered EVA/PET (graphene/EVA/PET) reached 200 Ω/sq with optical transparency of 87.3%. The graphene/EVA/PET film can be bent over 10000 cycles at a radius of 2 mm with ∼0.02% increase in sheet resistance, showing excellent mechanical flexibility for bendable electronics which was demonstrated by a capacitive-type touch screen based on the graphene/EVA/PET transparent conducting film.

Amorphous Molybdenum Sulfide/Carbon Nanotubes Hybrid Nanospheres Prepared by Ultrasonic Spray Pyrolysis for Electrocatalytic Hydrogen Evolution.

Developing cost-effective electrocatalysts with high activity and stability for hydrogen evolution reaction (HER) plays an important role in modern hydrogen economy. Amorphous molybdenum sulfide (MoSx ) has recently emerged as one of the most promising alternatives to Pt-based catalysts in HER, especially in acidic electrolytes. Here this study reports a simple ultrasonic spray pyrolysis method to synthesize hybrid HER catalysts composed of MoSx firmly attached on entangled carbon nanotube nanospheres (MoSx /CNTs). This synthetic process is fast, continuous, highly durable, and amenable to high-volume production with high yields and exceptional quality. The MoSx /CNTs hybrid catalyst prepared at 300 °C exhibits a low overpotential of 168 mV at the current density of 10 mA cm-2 with a small Tafel slope of 36 mV dec-1 . Electrochemical measurements and X-ray photoelectron spectroscopy analyses reveal that the CNT network not only promotes the charge transfer in corresponding HER process but also enhances the stability of the active sites in MoSx . This work demonstrates that ultrasonic spray pyrolysis is a reliable and versatile approach for synthesizing amorphous MoSx -based HER catalysts.