Elastic Restraint Induced by Carbon Coating Enhances Potassium-Ion Batteries' Performance via Phase Transition.

Potassium-ion batteries (PIBs) possess great potential in the next generation of large-scale energy storage due to their abundant sources and suitable operating voltage. However, the serious volume expansion resulting from the large radius of K+ makes it difficult to insert and extract, which greatly limits the development of PIBs. Herein, tin phosphide coated with carbon (Sn4P3@C) is designed for the PIB anode material by in situ construction of robust physical barriers of carbonaceous materials to accommodate the strain induced by volume expansion. Furthermore, the unique elastic restraint induced by the carbon coating in Sn4P3@C blocks the phase transition of α-Sn to ß-Sn during the process of potassiation. Meanwhile, the existence of α-Sn facilitates K+ diffusion dynamics, endowing the Sn4P3@C electrode with high reversible discharge ability, good circularity, and a low discharge plateau. Moreover, the electrode can maintain a capacity of 187 mAh g-1 over repeated 1500 cycles at 1 A g-1. This work not only explores the chemical kinetics of K+ in Sn4P3 but also provides a new idea for basic research of tin-based anode materials.

Regulating the PO4 and TiO6 Polyhedral Building Blocks in TiP2O7 Boosts the Potassium Ion Diffusion Kinetics.

Achieving fast and durable potassiation/depotassiation of anode materials for potassium ion batteries (PIB) still remains an elusive yet fascinating goal. Herein, we challenge the conventional wisdom in synthesizing the TiP2O7 superstructure and report a nanocarbon coating on TiP2O7 (TiP2O7/C) using layered MXene as a Ti source to realize an effective tuning in the TiO6 and PO4 building blocks for boosting the K+ diffusion kinetics in PIB. Experimental investigations coupled with systematic theoretical simulations indicate that the interface interaction between TiP2O7 and coated nanocarbon could induce internal adjustment in individual Ti-O bonding and relieve the local distortions of TiO6 octahedra, which endows the TiP2O7/C with favorable regulation in a K+ hopping manner and significantly reduces the K+ diffusion barrier via the diffusion propagation along PO4 blocks with dominant coordination between O/P and K+. Consequently, the TiP2O7/C anode could retain 230 mA h g-1 even after 2200 long-term cycles with an ultralow degradation rate of 0.005%.

In situ construction of a hierarchical TiO2/Ti3C2 hybrid via water steam etching for high-performance potassium-ion batteries.

Potassium ion batteries (PIBs) have attracted great research interest in new-generation large-scale energy storage considering their abundant source, low cost, and suitable working potential. Herein, a hierarchical TiO2/Ti3C2 hybrid is developed via a green, facile water steam etching method for realizing an efficient and durable anode material for PIBs. In this hierarchical assembly, the TiO2 nanoparticles anchored on the Ti3C2 surface contribute a high pseudocapacitance while mitigating the restacking of the Ti3C2 MXene skeleton, which ensures mechanical robustness to accommodate large K+ ions. Benefiting from the amalgamation of structural properties and the synergistic effects stemming from the individual constituents, the optimized TiO2/Ti3C2 anode harvests remarkable performance in the potassium ion storage, including a high reversible capacity of ∼255 mA h g-1 at 0.2 A g-1 after 1300 cycles as well as an outstanding long-term cycling performance and rate capability (a high capacity of ∼230 mA h g-1 even after intensive 10 000 cycles at 2 A g-1). The excellent TiO2/Ti3C2 anode enables the assembled pouch-cell coupling PTCDA cathode to deliver a capacity of ∼173 mA h g-1 at 0.05 A g-1 and retain 120 mA h g-1 after 30 cycles. The employment of the pouch-cell in successfully powering the LED module showcases its application prospect for advanced PIBs.

Deciphering the Sb4O5Cl2-MXene Hybrid as a Potential Anode Material for Advanced Potassium-Ion Batteries.

Potassium-ion batteries (PIBs) possess great potential in new-generation large-scale energy storage. However, their applications are plagued by large volume change and sluggish reaction kinetics of the electrode materials during the repeated charge/discharge processes. Guided by computerization modeling, we, herein, report the atomic-scale interfacial regulation of Sb4O5Cl2 coupled with structural engineering for the robust anode material of PIBs via simple MXene hybridization using a microwave-assisted hydrothermal method. Benefiting from the ostensive interfacial interplay between Sb4O5Cl2 and Ti3C2, MXene hybridization induces a favorable variation in spin polarization densities and the coordination of Sb atoms in Sb4O5Cl2, which are effective in optimizing the K+ ion diffusion path, thus resulting in a significantly reduced K+ ion diffusion barrier and promoted K+ insertion/extraction kinetics. The as-prepared Sb4O5Cl2-MXene anodes exhibit a highly reversible discharge capacity and decent cyclability, in addition to the low discharge plateau and promising full cell performance. This work is pivotal for not only paving the way for the exploration of anode materials for high-performance PIBs but also shedding light on the fundamental research on K+ ion storage in antimony oxychloride.

Fluorine-Free Fabrication of MXene via Photo-Fenton Approach for Advanced Lithium-Sulfur Batteries.

The mainstream synthesis method for MXene is using aqueous fluorine-containing acidic solutions to eliminate the A-element layers from their MAX phases. However, this strategy is environmentally hazardous and impairs the material performance (e.g., supercapacitor and Li-S batteries) owing to the presence of -F terminations. Herein, we exploit a low-temperature "soft chemistry" approach based on photo-Fenton (P.F.) reaction for the fabrication of F-free Ti3C2 (Ff-Ti3C2) with high purity of 95%. It is confirmed that the continuous generation of highly reactive oxygen species (HO• and O2•- radicals) during the P.F. reaction weakens the metallic Ti-Al bonds in the MAX phase and promotes the formation of high concentration OH- anions, which are conducive to the sequential topochemical deintercalation of Al layers. Moreover, the strengthened charge accumulation on the Ff-Ti3C2 surface creates rich electron "reservoirs" for actuating the Li-S chemistry, which not only strengthens the host-guest interactions but also propels the kinetics of the polysulfide conversion. Taking advantage of the superior mechanical robustness, better electrolyte wettability, and improved electrocatalytic activity, the resultant Ff-Ti3C2 can be used as an ideal sulfur host and Li-S chemistry mediator for advanced flexible Li-S batteries.

Hierarchical Assembly of CNTs-VSe2-VOx/S for Flexible Lithium-Sulfur Batteries.

Constructing highly conductive compact sulfur cathode composites with shorter ion diffusivity lengths is vital for achieving comprehensively superior electrochemical performance under a high mass loading condition. Guided by computerization modeling, we, herein, report self-supporting CNTs-VSe2-VOx/S assembly with balanced tortuosity and porosity for flexible Li-S batteries. The resultant hybrid sulfur cathode with a tortuosity of 2.42 and a porosity of 0.44 delivers prominent rate performance and cycling stability with a medium sulfur loading. More importantly, we demonstrate that the pouch cells with a high sulfur loading of 6.0 mg cm-2 and a low electrolyte to sulfur ratio of 4.2 µL mg-1 could synchronously deliver high gravimetric/volumetric energy densities of 424.1 Wh kg-1 and 469.2 Wh L-1, as well as decent cycling behavior under arbitrary bending conditions, which provides a ponderable reference for future flexible and wearable electronic devices.

Electronic Structure and d-Band Center Control Engineering over Ni-Doped CoP3 Nanowall Arrays for Boosting Hydrogen Production.

To address the challenge of highly efficient water splitting into H2, successful fabrication of novel porous three-dimensional Ni-doped CoP3 nanowall arrays on carbon cloth was realized, resulting in an effective self-supported electrode for the electrocatalytic hydrogen-evolution reaction. The synthesized samples exhibit rough, curly, and porous structures, which are beneficial for gaseous transfer and diffusion during the electrocatalytic process. As expected, the obtained Ni-doped CoP3 nanowall arrays with a doping concentration of 7% exhibit the promoted electrocatalytic activity. The achieved overpotentials of 176 mV for the hydrogen-evolution reaction afford a current density of 100 mA cm-2, which indicates that electrocatalytic performance can be dramatically enhanced via Ni doping. The Ni-doped CoP3 electrocatalysts with increasing catalytic activity should have significant potential in the field of water splitting into H2. This study also opens an avenue for further enhancement of electrocatalytic performance through tuning of electronic structure and d-band center by doping.

Intralayered Ostwald Ripening-Induced Self-Catalyzed Growth of CNTs on MXene for Robust Lithium-Sulfur Batteries.

The distinguishable physicochemical properties of MXenes render them attractive in electrochemical energy storage. However, the strong tendency to self-restack owing to the van der Waals interactions between the MXene layers incurs a massive decrease in surface area and blocking of ions transfer and electrolytes penetration. Here, in situ generated Ti3 C2 Tx MXene-carbon nanotubes (Ti3 C2 Tx -CNTs) hybrids are reported via low-temperature self-catalyzing growth of CNTs on Ti3 C2 Tx nanosheets without the addition of any catalyst precursors. With combined spectroscopic studies and theoretical calculation results, it is certified that the intralayered Ostwald ripening-induced Ti3 C2 Tx nanomesh structure contributes to the uniform precipitation of ultrafine metal Ti catalysts on Ti3 C2 Tx , thus giving rise to the in situ CNTs formation on the surface of Ti3 C2 Tx with high integrity. Taking advantages of intimate electrolyte penetration, unobstructed 3D Li+ /e transport, and rich electroactive sites, the Ti3 C2 Tx -CNTs hybrids are confirmed to be ideal 3D scaffolds for accommodating sulfur and regulating the polysulfides conversion for high-loaded lithium-sulfur batteries.

Selective S/Li2S Conversion via in-Built Crystal Facet Self-Mediation: Toward High Volumetric Energy Density Lithium-Sulfur Batteries.

The gravimetric, areal, and volumetric capacities pose important influences on market penetration for secondary batteries. Carbonaceous materials take a leading stand for the improvement of gravimetric and areal capacity in lithium-sulfur batteries; however, they exhibit some intrinsic deficiencies, including insufficient fixation on lithium polysulfides (LiPS) and low tap density, incurring poor volumetric performance and inferior cycling behavior. Here, we report a sulfur cathode based on highly conductive ZrB2 nanoflakes with only 2 wt % conductive carbon. The resultant closely packed ZrB2-S electrode delivers a high areal capacity of 8.5 mAh cm-2 and cell-level volumetric energy density of 533 Wh L-1 with a high sulfur loading of 7.8 mg cm-2 and an ultralow electrolyte dosage. With combined spectroscopic studies and theoretical calculation results, it was confirmed that an in-built Janus crystal facet self-mediation is on-site constructed by the exposed B and Zr atoms for an effective bonding and selective conversion on LiPS upon charge-discharge processes.

Enhanced Chemisorption and Catalytic Effects toward Polysulfides by Modulating Hollow Nanoarchitectures for Long-Life Lithium-Sulfur Batteries.

Hollow nanostructures with intricate interior and catalytic effects hold great promise for the construction of advanced lithium-sulfur batteries. Herein, a double-shelled hollow polyhedron with inlaid cobalt nanoparticles encapsulated by nitrogen-doped carbon (Co/NC) nanodots (Co-NC@Co9 S8 /NPC) is reported, which is acquired by using imidazolium-based ionic-polymer-encapsulated zeolitic imidazolate framework-67 as a core-shelled precursor. The Co/NC nanodots promote redox kinetics and chemical adsorbability toward polysulfides, while the interconnected double shells serve as a nanoscale electrochemical reaction chamber, which effectively suppresses the polysulfide shuttling and accelerates ion/electron transport. Benefiting from structural engineering and reaction kinetics modulation, the Co-NC@Co9 S8 /NPC-S electrode exhibits high cycling stability with a low capacity decay of 0.011% per cycle within 2000 cycles at 2 C. The electrode still shows high rate performance and cyclability over 500 cycles even in the case of high sulfur loading of 4.5 mg cm-2 and 75 wt% sulfur content. This work provides one type of new hollow nanoarchitecture for the development of advanced Li-S batteries and other energy storage systems.

Titanium silicalite as a radical-redox mediator for high-energy-density lithium-sulfur batteries.

Lithium-sulfur (Li-S) batteries are receiving intense interest owing to their high energy densities, cost effectiveness, and the natural abundance of sulfur. However, practical applications are still limited by rapid capacity decay caused by multielectron redox reactions and complex phase transformations. Here, we include commercially available titanium silicalite-1 (TS-1) in carbon/sulfur cathodes, to introduce strong chemical interactions between the lithium polysulfides (LiPS) and TS-1 in a working Li-S battery. In situ UV-visible spectroscopy together with other experimental results confirm that incorporation of TS-1 mediators enables direct conversion between S82- and S3*- radicals during the discharge process, which effectively promotes the kinetic behaviors of soluble LiPS and regulates uniform nucleation and growth of solid sulfide precipitates. These features give our TS-1 engineered sulfur cathode an ultrahigh initial capacity of 1459 mA h g-1 at 0.1C. Moreover, the system has an impressively high areal capacity (3.84 mA h cm-2) and long cycling stability with a high sulfur loading of 4.9 mg cm-2. This novel and low-cost fabrication procedure is readily scalable and provides a promising avenue for potential industrial applications.

Ultrafine Ti3C2 MXene Nanodots-Interspersed Nanosheet for High-Energy-Density Lithium-Sulfur Batteries.

Nanostructured carbon materials have been extensively used for encapsulating sulfur and improving cyclic stability of lithium-sulfur (Li-S) batteries, but high carbon content and low packing density greatly limit their volumetric energy density. Herein, we present MXene-based Ti3C2T x (T x stands for the surface terminations) nanodots-interspersed Ti3C2T x nanosheet (TCD-TCS) to accomplish spatial immobilization and conversion of high-loaded sulfur species. Rich surface polar sites in TCD-TCS enhance structural integrity of the resultant electrode, while the absence of the carbon-based materials and conductive additives results in high tap density of the cathode materials. The TCD-TCS/S electrode exhibits an almost theoretical discharge capacity at a medium sulfur loading of 1.8 mg cm-2. Notably, ultrahigh volumetric capacity (1957 mAh cm-3) and high areal capacity (13.7 mAh cm-2) are synchronously achieved at a high sulfur loading of 13.8 mg cm-2. The mechanism study of sulfur evolution during discharge process highlights the importance of the integration of MXene-based nanodots and nanosheets in Li-S batteries. This proposed methodology holds great promise for the development of various high-performance energy storage materials.

Synchronous Gains of Areal and Volumetric Capacities in Lithium-Sulfur Batteries Promised by Flower-like Porous Ti3C2T x Matrix.

The areal and volumetric capacities are important metrics in practical deployment of advanced energy storage systems with the imposed constraints of device volume and chip area. Conductive carbons are promising sulfur host materials for improving areal capacity in lithium-sulfur (Li-S) batteries, but they face a few congenital deficiencies, such as low tap density and weak polysulfide entrapment ability, resulting in poor volumetric performance. Here, we report one type of cathode system based on flower-like porous Ti3C2T x (FLPT) without the incorporation of any carbon hosts or conductive additives. The resultant FLPT-S electrode synchronously acquires a high areal capacity of 10.04 mAh cm-2 and ultrahigh volumetric capacity of 2009 mAh cm-3. Furthermore, ex situ electron paramagnetic resonance and UV-visible spectra have demonstrated that FLPT enables a fast dynamic equilibrium between S62- anion and S3•- radical during cycling, which promotes the redox reactions of sulfur species.

Sandwich-Type NbS2@S@I-Doped Graphene for High-Sulfur-Loaded, Ultrahigh-Rate, and Long-Life Lithium-Sulfur Batteries.

Lithium-sulfur batteries practically suffer from short cycling life, low sulfur utilization, and safety concerns, particularly at ultrahigh rates and high sulfur loading. To address these problems, we have designed and synthesized a ternary NbS2@S@IG composite consisting of sandwich-type NbS2@S enveloped by iodine-doped graphene (IG). The sandwich-type structure provides an interconnected conductive network and plane-to-point intimate contact between layered NbS2 (or IG) and sulfur particles, enabling sulfur species to be efficiently entrapped and utilized at ultrahigh rates, while the structural integrity is well maintained. NbS2@S@IG exhibits prominent high-power charge/discharge performances. Reversible capacities of 195, 107, and 74 mA h g-1 (1.05 mg cm-2) have been achieved after 2000 cycles at ultrahigh rates of 20, 30, and 40 C, respectively, and the corresponding average decay rates per cycle are 0.022%, 0.031% and 0.033%, respectively. When the area sulfur loading is increased to 3.25 mg cm-2, the electrode still maintains a high discharge capacity of 405 mAh g-1 after 600 cycles at 1 C. Three half-cells in series assembled with NbS2@S@IG can drive 60 indicators of LED modules after only 18 s of charging. The instantaneous current and power of the device reach 196.9 A g-1 and 1369.7 W g-1, respectively.

Highly Conductive Porous Transition Metal Dichalcogenides via Water Steam Etching for High-Performance Lithium-Sulfur Batteries.

Lithium-sulfur (Li-S) batteries show significant advantages for next-generation energy storage systems owing to their high energy density and cost effectiveness. The main challenge in the development of long-life and high-performance Li-S batteries is to simultaneously facilitate the redox kinetics of sulfur species and suppress the shuttle effect of polysulfides. In this contribution, we present a general and green water-steam-etched approach for the fabrication of H- and O-incorporated porous TiS2 (HOPT). The conductivity, porosity, chemisorptive capability, and electrocatalytic activity of HOPT are enhanced significantly when compared with those of raw TiS2. The synthetic method can be expanded to the fabrication of other highly conductive transition metal dichalcogenides such as porous NbS2 and CoS2. The as-obtained HOPT can serve as both a substitute of conductive agents and an additive of interlayer materials. The optimal electrode delivers discharge capacities of 950 mA h g-1 after 300 cycles at 0.5 C and 374 mA h g-1 after 1000 cycles at 10 C. Impressively, an unprecedented reversible capacity of 172 mA h g-1 is achieved after 2500 cycles at 30 C, and the average capacity fading rate per cycle is as low as 0.015%. Importantly, four half-cells based on this electrode in series could drive 60 light-emitting diode indicator modules (the nominal power 3 W) after 20 s of charging. The instantaneous current and power of this device on reaching 275 A g-1 and 2611 W g-1, respectively, indicate outstanding high-power discharge performance and potential applications in electric vehicles and other large-scale energy storage systems.

Sulfur-nitrogen co-doped three-dimensional carbon foams with hierarchical pore structures as efficient metal-free electrocatalysts for oxygen reduction reactions.

Despite the good progress in developing doped carbon catalysts for oxygen-reduction reaction (ORR), the current metal-free carbon catalysts are still far from satisfactory for large-scale applications of fuel cell. Developing new metal free doped carbon materials with abundance active sites as well as excellent electron transfer and reactant transport rate towards ORR may be a potential solution. Herein, we develop a novel three-dimensional (3D) sulfur-nitrogen co-doped carbon foams (S-N-CF) with hierarchical pore structures, using a convenient, economical, and scalable method. The experimental results have demonstrated that the obtained 3D S-N-CF exhibited better catalytic activity, longer-term stability and higher methanol tolerance than a commercial Pt/C catalyst. Such excellent performances may be attributed to the synergistic effect, which includes high catalytic sites for ORR provided by high S-N heteroatom loading, excellent reactant transport caused by hierarchical pore structures and high electron transfer rate provided by 3D continuous networks. Our results not only develop a new type of catalysts with excellent electrocatalytic performance by a commercially valid route, but also provide useful information for further clarification of the relationship between the microstructures of metal-free carbon materials and catalyst properties for ORR. More importantly, the idea to design hierarchical pore structures could be applied to other catalytic materials and serve as a general strategy for improving the activity of various ORR catalysts.