In Situ Constructing a Catalytic Shell for Sulfur Cathode via Electrochemical Oxidative Polymerization.

Sluggish multiphase reaction kinetics and severe shuttle effect of lithium polysulfides (LiPSs) are two major challenges facing lithium-sulfur (Li-S) batteries, which largely prevent them from becoming a reality. Herein, a shell with catalytic function for sulfur cathode is in situ constructed through an ingenious electrochemical oxidative polymerization strategy by introducing hexafluorocyclotriphosphazene (HFPN) as additives, which suppresses the shuttle effect and promotes efficient sulfur conversion. The shell with abundant heteroatoms effectively confines polysulfides to the cathode matrix by chemically interacting with them to eliminate capacity degradation. Moreover, the shell exhibits high catalytic activities, which turns Li2S(2) into an activated state and facilitates its dissociation. The functionalized shell substantially advances the performance of Li-S batteries, thanks to efficient lithium-ion transportation and abundant adsorption-catalytic sites. As a result, Li-S batteries demonstrate superb resistance to self-discharge, ultrastable cycle performance, and greatly enhanced rate capability. Impressively, the batteries show an ultralow capacity decay rate of 0.034% throughout 700 cycles at 2C. They deliver a capacity of 517 mAh g-1 even at a 4C rate, exhibiting relieved electrochemical polarization and excellent sulfur utilization. This work provides an ingenious strategy to construct adsorption-catalytic nets for next-generation Li-S batteries with enhanced lifespan and electrochemical performance.

Thin Nano Cages with Limited Hollow Space for Ultrahigh Sulfur Loading Lithium-Sulfur Batteries.

Owning to its various advantages, the lithium-sulfur battery is one of the research hot spots for new energy storage systems. Diverse hollow structures with specific morphologies have been used as the sulfur host materials to adsorb or/and catalyze the polysulfides, and can in particular concurrently inhibit the volume expansion during electrochemical processes in lithium-sulfur batteries. However, hollow space with a large volume will restrict the performance of the cell under high sulfur area loading, which is a very important indicator for the practical applications of the lithium-sulfur battery. Here, we report a nano thin cage cobalt acid zinc (ZnCo2O4) with limited hollow space as the cathode catalyst for lithium-sulfur batteries, which greatly reduces the electrode volume occupied by the hollow structure. The hollow volume of these thin cages is much smaller than those of the normally reported hollow materials in the literatue. The electrochemical performance of lithium-sulfur batteries with ZnCo2O4 thin cages could greatly improve due to the unique structure and the synergistic adsorption/catalytic effect of Zn/Co sites, especially at an ultrahigh S area load. Under a high S loading of 8 mg cm-2, the cell could keep a reversible capacity of 600 mAh g-1 after 500 cycles. Even at a sulfur loading of 10 mg cm-2, the cell still releases a discharge capacity of 1000 mAh g-1 which is equivalent of an area capacity of 10 mAh cm-2. This work provides a feasible way to develop lithium sulfur batteries with a high area sulfur load. This idea provides a possible solution to develop a Li-S battery at high area S loading and move one step closer to the practical applications.

Single-dispersed polyoxometalate clusters embedded on multilayer graphene as a bifunctional electrocatalyst for efficient Li-S batteries.

The redox reactions occurring in the Li-S battery positive electrode conceal various and critical electrocatalytic processes, which strongly influence the performances of this electrochemical energy storage system. Here, we report the development of a single-dispersed molecular cluster catalyst composite comprising of a polyoxometalate framework ([Co4(PW9O34)2]10-) and multilayer reduced graphene oxide. Due to the interfacial charge transfer and exposure of unsaturated cobalt sites, the composite demonstrates efficient polysulfides adsorption and reduced activation energy for polysulfides conversion, thus serving as a bifunctional electrocatalyst. When tested in full Li-S coin cell configuration, the composite allows for a long-term Li-S battery cycling with a capacity fading of 0.015% per cycle after 1000 cycles at 2 C (i.e., 3.36 A g-1). An areal capacity of 4.55 mAh cm-2 is also achieved with a sulfur loading of 5.6 mg cm-2 and E/S ratio of 4.5 µL mg-1. Moreover, Li-S single-electrode pouch cells tested with the bifunctional electrocatalyst demonstrate a specific capacity of about 800 mAh g-1 at a sulfur loading of 3.6 mg cm-2 for 100 cycles at 0.2 C (i.e., 336 mA g-1) with E/S ratio of 5 µL mg-1.

A carbon-based material with a hierarchical structure and intrinsic heteroatom sites for sodium-ion storage with ultrahigh rate and capacity.

The storage of sodium ions with carbon materials has huge potential for large-scale application due to its resource-rich and environmental advantages. However, how to realize high power density, high energy density and long cycle life are the bottlenecks restricting its development. Herein, by using a facile synthesis strategy, a carbon-based framework with a hierarchical structure and intrinsic heteroatom sites which are the characteristics contributing to ultrahigh rate and capacity has been achieved. As a result, the hierarchical carbon-based material exhibits excellent performance when used as both the anode and cathode for sodium-ion capacitors (SICs), which can deliver a high energy density of 224 W h kg-1 (at 180 W kg-1), an ultrahigh power density of 17 160 W kg-1 (at 128 W h kg-1) and ultralong cycle life (91% capacity retention after 10 000 cycles at 2 A g-1), outperforming most of the previously reported SICs with other configurations.

The Intrinsic Charge Carrier Behaviors and Applications of Polyoxometalate Clusters Based Materials.

Polyoxometalates (POMs) are a series of molecular metal oxide clusters, which span the two domains of solutes and solid metal oxides. The unique characters of POMs in structure, geometry, and adjustable redox properties have attracted widespread attention in functional material synthesis, catalysis, electronic devices, and electrochemical energy storage and conversion. This review is focused on the links between the intrinsic charge carrier behaviors of POMs from a chemistry-oriented view and their recent ground-breaking developments in related areas. First, the advantageous charge transfer behaviors of POMs in molecular-level electronic devices are summarized. Solar-driven, thermal-driven, and electrochemical-driven charge carrier behaviors of POMs in energy generation, conversion and storage systems are also discussed. Finally, present challenges and fundamental insights are discussed as to the advanced design of functional systems based upon POM building blocks for their possible emerging application areas.

A Carbon Foam with Sodiophilic Surface for Highly Reversible, Ultra-Long Cycle Sodium Metal Anode.

Sodium metal anodes combine low redox potential (-2.71 V versus SHE) and high theoretical capacity (1165 mAh g-1), becoming a promising anode material for sodium-ion batteries. Due to the infinite volume change, unstable SEI films, and Na dendrite growth, it is arduous to achieve a long lifespan. Herein, an oxygen-doped carbon foam (OCF) derived from starch is reported. Heteroatom doping can significantly reduce the nucleation resistance of sodium metal; combined with its rich pore structure and large specific surface area, OCF provides abundant nucleation sites to effectively guide the nucleation and subsequent growth of sodium metal, and the nature of this foam can accommodate the deposited sodium. Furthermore, a more uniform, robust, and stable SEI layer is observed on the surface of OCF electrode, so it can maintain ultra-high reversibility and excellent integrity for a long time without dendritic growth. As a result, when the current density is 10 mA cm-2, the electrode can maintain stable 2000 cycles and the coulombic efficiency can reach to 99.83%. Na@OCF||Na3V2(PO4)3 full cell also has extremely high capacity retention of about 97.53% over 150 cycles. These results provide a simple but effective method for achieving the safety and commercialization of sodium metal anode.

A Holey Graphene Additive for Boosting Performance of Electric Double-Layer Supercapacitors.

We demonstrate a facile and effective method, which is low-cost and easy to scale up, to fabricate holey graphene nanosheets (HGNSs) via ultrafast heating during synthesis. Various heating temperatures are used to modify the material properties of HGNSs. First, we use HGNSs as the electrode active materials for electric double-layer capacitors (EDLCs). A synthesis temperature of 900 °C seems to be optimal, i.e., the conductivity and adhesion of HGNSs reach a compromise. The gravimetric capacitance of this HGNS sample (namely HGNS-900) is 56 F·g-1. However, the volumetric capacitance is low, which hinders its practical application. Secondly, we incorporate activated carbon (AC) into HGNS-900 to make a composite EDLC material. The effect of the AC:HGNS-900 ratio on the capacitance, high-rate performance, and cycling stability are systematically investigated. With a proper amount of HGNS-900, both the electrode gravimetric and volumetric capacitances at high rate charging/discharging are clearly higher than those of plain AC electrodes. The AC/HGNS-900 composite is a promising electrode material for nonaqueous EDLC applications.

ZIF-8-Based Quasi-Solid-State Electrolyte for Lithium Batteries.

The quasi-solid-state electrolytes (QSSEs) with an inorganic skeleton, a solid-liquid composite material combining their respective merits, exhibit high ionic conductivity and mechanical strength. However, most quasi-solid electrolytes prepared by immobilizing ionic liquid (IL) or organic liquid electrolyte in inorganic scaffold generally have poor interface compatibility and low lithium ion migration number, which limits its application. Herein, we design and prepare a ZIF-8-based QSSE (ZIF-8 QSSE) in which the ZIF-8 has a special cage structure and interaction with the guest electrolyte to form a composite electrolyte with good ionic conductivity about 1.05 × 10-4 S cm-1 and a higher lithium-ion transference number of about 0.52. With the ZIF-8 QSSE, a protype lithium battery coupled with LiCoO2 cathode shows good electrochemical performances with an initial discharge capacity of 135 mAh g-1 at 50 mA g-1 and a remaining capacity of 119 mAh g-1 after 100 cycles, only 0.119% capacity degradation per cycle. It is worth noting that the ZIF-8-based QSSEs have good thermal stability up to 350 °C that does not show thermal runaway, which is significantly higher than that of a conventional organic liquid battery system.

Composition Modulation of Ionic Liquid Hybrid Electrolyte for 5 V Lithium-Ion Batteries.

Electrolyte is a key component in high-voltage lithium-ion batteries (LIBs). Bis(trifluoromethanesulfonyl)imide-based ionic liquid (IL)/organic carbonate hybrid electrolytes have been a research focus owing to their excellent balance of safety and ionic conductivity. Nevertheless, corrosion of Al current collectors at high potentials usually happens for this kind of electrolyte. In this study, this long-standing problem is solved via the modulation of the IL/carbonate ratio and LiPF6 concentration in the hybrid electrolyte. The proposed electrolyte suppresses Al dissolution and electrolyte oxidation at 5 V (vs Li+/Li) and thus allows for ideal lithiation/delithiation performance of a high-voltage LiNi0.5Mn1.5O4 (LNMO) cathode even at 55 °C. The underlying mechanism is examined in this work. Excellent cycling stability (97% capacity retention) for an LNMO cathode after 300 cycles is achieved. This electrolyte shows good wettability toward a polyethylene separator and low flammability. In addition, satisfactory compatibility with both graphite and Si-based anodes is confirmed. The proposed electrolyte design strategies have great potential for applications in high-voltage LIBs.

Tuning Redox Active Polyoxometalates for Efficient Electron-Coupled Proton-Buffer-Mediated Water Splitting.

We present strategies to tune the redox properties of polyoxometalate clusters to enhance the electron-coupled proton-buffer-mediated water splitting process, in which the evolution of hydrogen and oxygen can occur in different forms and is separated in time and space. By substituting the heteroatom template in the Keggin-type polyoxometalate cluster, H6 ZnW12 O40 , it is possible to double the number of electrons and protonation in the redox reactions (from two to four). This increase can be achieved with better matching of the energy levels as indicated by the redox potentials, compared to the ones of well-studied H3 PW12 O40 and H4 SiW12 O40 . This means that H6 ZnW12 O40 can act as a high-performance redox mediator in an electrolytic cell for the on-demand generation of hydrogen with a high decoupling efficiency of 95.5 % and an electrochemical energy efficiency of 83.3 %. Furthermore, the H6 ZnW12 O40 cluster also exhibits an excellent cycling behaviour and redox reversibility with almost 100 % H2 -mediated capacity retention during 200 cycles and a high coulombic efficiency >92 % each cycle at 30 mA cm-2 .

Multifunctional Ion-Sieve Constructed by 2D Materials as an Interlayer for Li-S Batteries.

For Li-S batteries, the interlayer between the separator and sulfur cathode preventing lithium polysulfide (LiPS) travel across the membrane is a research hotspot. The good blocking ability for LiPSs indicates that these interlayers can promote the electrochemistry performance with high S loading. However, most of these interlayers are just used as a simple blocking wall. Such a blocking wall, for example, the lower Li+ ion conductivity, would often reduce the electrochemical performance, especially under large current density. Here, we report a multifunctional ion-sieve made by three two-dimensional (2D) sheets, graphitic carbon nitride (g-C3N4), boron nitride (BN), and graphene. A g-C3N4 sheet which possesses orderly channels with a size of 3 Å in the crystalline structure can effectively prevent polysulfides from passing through but allow lithium ions to pass freely, whereas a BN sheet acts as an excellent catalyst for sulfur redox, and graphene acts as an extended collector, which can promote the conductivity of the sulfur electrode region. Benefiting from the synergistic effect among these 2D materials, the ion-sieve interlayer makes the Li-S battery show excellent performance at a large rate with both high sulfur loadings and high sulfur content. In addition, the host materials are not necessary in these cells. The ion-sieve liberated a discharge capacity of about 600 mA h g-1 after 500 cycles at 1 C, and the capacity attenuation was less than 0.01% per cycle with a 6 mg cm-2 areal S-loading (pure S as the active material). The reversible capacity could be maintained at more than 400 mA h g-1 at 2 C, which amounts to an area current density of 26.88 mA cm-2.

Stable Na Plating and Stripping Electrochemistry Promoted by In Situ Construction of an Alloy-Based Sodiophilic Interphase.

Sodium metal anodes are poor due to the reversibility of Na plating/stripping, which hinders their practical applications. A strategy to form a sodiophilic Au-Na alloy interphase on a Cu current collector, involving a sputtered Au thin layer, is shown to enable efficient Na plating/stripping for a certain period of time. Herein, electrochemical behaviors of Na plating on different substrates are explored, and it is revealed that the sodiophilic interphase can be achieved universally by in situ formation of M-Na (M = Au, Sn, and Sb) alloys during Na plating prior to Na bulk deposition in the initial cycle. Moreover, it is found that repetitive alloying-dealloying leads to falling-off of thin film sodiophilic materials and thus limits the lifespan of efficient Na cycling. Therefore, an approach is further developed by employing particles of sodiophilic materials combined with the control over the cutoff potential, which significantly improves the stability of Na plating/stripping process. Especially, the low-cost Cu@Sn-NPs and Cu@Sb-MPs composite current collectors allow Na plating and stripping to cycle for 2000 and 1700 times with the average efficiency of 99.9% at 2 mA cm-2 .

Supercapacitive Properties of Micropore- and Mesopore-Rich Activated Carbon in Ionic-Liquid Electrolytes with Various Constituent Ions.

Ionic-liquid (IL) electrolytes, characterized by large potential windows, intrinsic ionic conductivity, low environmental hazard, and high safety, are used for micropore- and mesopore-rich activated-carbon (ACmicro and ACmeso ) supercapacitors. IL electrolytes consisting of various cations [1-ethyl-3-methylimidazolium (EMI+ ), N-propyl-N-methylpyrrolidinium (PMP+ ), and N-butyl-N-methylpyrrolidinium (BMP+ )] and various anions [bis(trifluoromethylsulfonyl)imide (TFSI- ), BF4 - , and bis(fluorosulfonyl)imide (FSI- )] are investigated. The electrolyte conductivity, viscosity, and ion transport properties at the ACmicro and ACmeso electrodes are studied. In addition, the capacitance, rate capability, and cycling stability of the two types of AC electrodes are systematically examined and post-mortem material analyses are conducted. The effects of IL composition on the charge-discharge capacitances of the ACmicro electrodes are more pronounced than those for the ACmeso electrodes. The FSI-based IL electrolytes, for which electrochemical properties are cation dependent, are found to be promising. Incorporating EMI+ with FSI- results in a low electrolyte viscosity and a fast ion transport, giving rise to optimized electrode capacitance and rate capability. Replacing EMI+ with PMP+ increases the cell voltage (to 3.5 V) and maximum energy density (to 42 Wh kg-1 ) of the ACmicro cell at the cost of cycling stability.

Lithiophilic Faceted Cu(100) Surfaces: High Utilization of Host Surface and Cavities for Lithium Metal Anodes.

Lithium metal anodes suffer from poor cycling stability and potential safety hazards. To alleviate these problems, Li thin-film anodes prepared on current collectors (CCs) and Li-free types of anodes that involve direct Li plating on CCs have received increasing attention. In this study, the atomic-scale design of Cu-CC surface lithiophilicity based on surface lattice matching of the bcc Li(110) and fcc Cu(100) faces as well as electrochemical achievement of Cu(100)-preferred surfaces for smooth Li deposition with a low nucleation barrier is reported. Additionally, a purposely designed solid-electrolyte interphase is created for Li anodes prepared on CCs. Not only is a smooth planar Li thin film prepared, but a uniform Li plating/stripping on the skeleton of 3D CCs is achieved as well by high utilization of the surface and cavities of the 3D CCs. This work demonstrates surface electrochemistry approaches to construct stable Li metal-electrolyte interphases towards practical applications of Li anodes prepared on CCs.

Enhanced Adsorptions to Polysulfides on Graphene-Supported BN Nanosheets with Excellent Li-S Battery Performance in a Wide Temperature Range.

For Li-S batteries, the catalysis for S redox reaction is indispensable. A lot of multifunctional sulfur electrode support materials with have been investigated widely. However, most of these studies were carried out at room temperature, and the interaction between different components in the matrix is not often paid enough attention. Here, we report a graphene supported BN nanosheet composite in which the synergistic effect between BN and graphene greatly enhanced the adsorption for polysulfides, thus leading to excellent performance in a wide temperature range. When used as a host material of sulfur, it can make the Li-S battery apply to a wide range of temperatures, from -40 to 70 °C, delivering a high utilization of sulfur, an excellent rate capability, and outstanding cycling life. The capacity can stabilized at 888 mAh g-1 at 2 C after 300 cycles with a capacity attenuation of <0.04% per cycle at 70 °C, and the battery can deliver a capacity above 650 mAh g-1 at -40 °C.

Designable ultra-smooth ultra-thin solid-electrolyte interphases of three alkali metal anodes.

Dendrite growth of alkali metal anodes limited their lifetime for charge/discharge cycling. Here, we report near-perfect anodes of lithium, sodium, and potassium metals achieved by electrochemical polishing, which removes microscopic defects and creates ultra-smooth ultra-thin solid-electrolyte interphase layers at metal surfaces for providing a homogeneous environment. Precise characterizations by AFM force probing with corroborative in-depth XPS profile analysis reveal that the ultra-smooth ultra-thin solid-electrolyte interphase can be designed to have alternating inorganic-rich and organic-rich/mixed multi-layered structure, which offers mechanical property of coupled rigidity and elasticity. The polished metal anodes exhibit significantly enhanced cycling stability, specifically the lithium anodes can cycle for over 200 times at a real current density of 2 mA cm-2 with 100% depth of discharge. Our work illustrates that an ultra-smooth ultra-thin solid-electrolyte interphase may be robust enough to suppress dendrite growth and thus serve as an initial layer for further improved protection of alkali metal anodes.

Strategies to Explore and Develop Reversible Redox Reactions of Li-S in Electrode Architectures Using Silver-Polyoxometalate Clusters.

Investigations of the Ag (I)-substituted Keggin K3[H3AgIPW11O39] as a bifunctional Lewis acidic and basic catalyst are reported that explore the stabilization of Li2Sn moieties so that reversible redox reactions in S-based electrodes would be possible. Spectroscopic investigations showed that the Li2Sn-moieties can be strongly adsorbed on the {AgIPW11O39} cluster, where the Ag(I) ion can act as a Lewis acid site to further enhance the adsorption of the S-moieties, and these interactions were investigated and rationalized using DFT. These results were used to construct an electrode for use in a Li-S battery with a very high S utilization of 94%, and a coulometric capacity of 1580 mAh g-1. This means, as a result of using the AgPOM, both a high active S content, as well as a high areal S mass loading, is achieved in the composite electrode giving a highly stable battery with cycling performance at high rates (1050 and 810 mAh g-1 at 1C and 2C over 100 to 300 cycles, respectively).

Electrolyte Engineering: Optimizing High-Rate Double-Layer Capacitances of Micropore- and Mesopore-Rich Activated Carbon.

Various types of electrolyte cations as well as binary cations are used to optimize the capacitive performance of activated carbon (AC) with different pore structures. The high-rate capability of micropore-rich AC, governed by the mobility of desolvated cations, can outperform that of mesopore-rich AC, which essentially depends on the electrolyte conductivity.

Co4N Nanosheet Assembled Mesoporous Sphere as a Matrix for Ultrahigh Sulfur Content Lithium-Sulfur Batteries.

High utilization and loading of sulfur in cathodes holds the key in the realization of Li-S batteries. We here synthesized a Co4N mesoporous sphere, which was made up of nanosheets, via an easy and convenient method. This material presents high affinity, speedy trapping, and absorbing capacity for polysulfides and acts as a bifunctional catalysis for sulfur redox processes; therefore it is an ideal matrix for S active material. With such a mesoporous sphere used as a sulfur host in Li-S batteries, extraordinary electrochemistry performance has been achieved. With a sulfur content of 72.3 wt % in the composite, the Co4N@S delivered a high specific discharge capacity of 1659 mAh g-1 at 0.1 C, almost reaching its theoretic capacity. Also, the battery exhibited a large reversible capacity of about 1100 mAh g-1 at 0.5 C and 1000 mAh g-1 at 1 C after 100 cycles. At a high rate of 2 C and 5 C, after 300 cycles, the discharge capacity finally stabilized at 805 and 585 mAh g-1. Even at a 94.88% sulfur content, the cathode can still deliver an extremely high specific discharge capacity of 1259 mAh g-1 with good cycle performance.

High-Performance Polyoxometalate-Based Cathode Materials for Rechargeable Lithium-Ion Batteries.

The polyoxovanadate cluster Li7[V15O36(CO3)] is shown to be an active cathode material in Li-ion batteries, delivering a capacity of 250 mA h g(-1) at 50 mA g(-1) and 140 mA h g(-1) at 10 A g(-1). Li-ion diffusion is rapid in this material and gives rise to an impressive maximum power density output of 25.7 kW kg(-1) (55 kW L(-1)).