Sulfur in Hyper-cross-linked Porous Polymer as Cathode in Lithium-Sulfur Batteries with Enhanced Electrochemical Properties

Porous Organic Polymers as Active Electrode Materials for Energy Storage Applications

Eco-friendly and efficient energy production and storage technologies are highly demanded to address the environmental and energy crises. Porous organic polymers (POPs) are a class of lightweight porous network materials covalently linked by organic building blocks, possessing high surface areas, tunable pores, and designable components and structures. Due to their unique structural and compositional advantages, POPs have recently emerged as promising electrode materials for energy storage devices, particularly in the realm of supercapacitors and ion batteries. In this work, a comprehensive overview of recent progress and applications of POPs as electrode materials in energy storage devices, including the structural features and synthesis strategies of various POPs, as well as their applications in supercapacitors, lithium batteries, sodium batteries, and potassium batteries are provided. Finally, insights are provided into the future research directions of POPs in electrochemical energy storage technologies. It is anticipated that this work can provide readers with a comprehensive background on the design of POPs-based electrode materials and ignite more research in the development of next-generation energy storage devices.

Advancing lithium-sulfur battery efficiency: utilizing a 2D/2D g-C3N4@MXene heterostructure to enhance sulfur evolution reactions and regulate polysulfides under lean electrolyte conditions

Lithium-sulfur batteries (LSBs) show promise for achieving a high energy density of 500 W h kg-1, despite challenges such as poor cycle life and low energy efficiency due to sluggish redox kinetics of lithium polysulfides (LiPSs) and sulfur's electronic insulating nature. We present a novel 2D Ti3C2 Mxene on a 2D graphitic carbon nitride (g-C3N4) heterostructure designed to enhance LiPS conversion kinetics and adsorption capacity. In a pouch cell configuration with lean electrolyte conditions (∼5 μL mg-1), the g-C3N4-Mx/S cathode exhibited excellent rate performance, delivering ∼1061 mA h g-1 at C/8 and retaining ∼773 mA h g-1 after 190 cycles with a Coulombic efficiency (CE) of 92.7%. The battery maintained a discharge capacity of 680 mA h g-1 even at 1.25 C. It operated reliably at an elevated sulfur loading of 5.9 mg cm-2, with an initial discharge capacity of ∼900 mA h g-1 and a sustained CE of over 83% throughout 190 cycles. Postmortem XPS and EIS analyses elucidated charge-discharge cycle-induced changes, highlighting the potential of this heterostructured cathode for commercial garnet LSB development.

Size Effect for Inhibiting Polysulfides Shuttle in Lithium-Sulfur Batteries

It is undeniable that the dissolution of polysulfides is beneficial in speeding up the conversion rate of sulfur in electrochemical reactions. But it also brings the bothersome "shuttle effect". Therefore, if polysulfides can be retained on the cathode side, the efficient utilization of the polysulfides can be guaranteed to achieve the excellent performance of lithium-sulfur batteries. Based on this idea, considerable methods have been developed to inhibit the shuttling of polysulfides. It is necessary to emphasize that no matter which method is used, the solvation mechanism, and existence forms of polysulfides are essential to analyze. Especially, it is important to clarify the sizes of different forms of polysulfides when using the size effect to inhibit the shuttling of polysulfides. In this review, a comprehensive summary and in-depth discussion of the solvation mechanism, the existing forms of polysulfides, and the influencing factors affecting polysulfides species are presented. Meanwhile, the size of diverse polysulfide species is sorted out for the first time. Depending on the size of polysulfides, tactics of using size effect in cathode, separator, and interlayer parts are elaborated. Finally, a design idea of materials pore size is proposed to satisfy the use of size effect to inhibit polysulfides shuttle.

Surface Engineering in Covalent Organic Polymers for High-Performance Li-S Batteries

Lithium-sulfur (Li-S) batteries are a promising energy storage technology due to their tempting high theoretical capacity and energy density. Nevertheless, the wastage of active materials that originates from the shuttling effect of polysulfides still hinders advancement of Li-S batteries. The effective design of cathode materials is extremely pivotal to solve this thorny problem. Herein, surface engineering in covalent organic polymers (COPs) has been performed to investigate the influence of pore wall polarity on the performance of COP-based cathodes used for Li-S batteries. With the assistance of experimental investigation and theoretical calculations, performance improvement by increasing pore surface polarity and a synergy effect of the polarized functionalities, along with nano-confinement effect of the COPs, are disclosed, to which the improved performance of Li-S batteries including outstanding Coulombic efficiency (99.0 %) and extremely low capacity decay (0.08 % over 425 cycles at 1.0 C) is attributed. This work not only enlightens the designable synthesis and applications of covalent polymers as polar sulfur hosts with high utilization of active materials, but also provides a feasible guide for the design of effective cathode materials for future advanced Li-S batteries.

Hypercrosslinked polymer-mediated fabrication of binary metal phosphide decorated spherical carbon as an efficient and durable bifunctional electrocatalyst for rechargeable Zn-air batteries

Bifunctional oxygen catalysts with excellent catalytic activity and durability towards both oxygen reduction and oxygen evolution reactions (ORR/OER) are pivotal for long-term rechargeable Zn-air batteries. Herein, we report a spherical carbon decorated with FeP and CoP nanoparticles (denoted as FeCoP/NPC) as an ORR/OER bifunctional electrocatalyst for rechargeable Zn-air batteries. HCTCz@Fe/Co-PA is first produced by the modification of phytic acid (PA) onto (into) a porous cross-linked polymeric sphere of poly(bis(N-carbazolyl)-1,2,4,5-tetrazine) (HCTCz), followed by chelating with metal ions (i.e., Fe³⁺ and Co²⁺). The subsequent pyrolysis yields FeCoP/NPC, which shows prominent activity and reliability for the ORR and OER due primarily to the synergistic effect of FeP and CoP active sites and N/P co-doped carbon. The aqueous Zn-air battery assembled with FeCoP/NPC provides high specific capacity and peak power density. Notably, the constructed Zn-air battery can be repetitively charged and discharged for 1200 h at 5 mA cm^-2. In addition, a flexible solid-state Zn-air battery made from FeCoP/NPC exhibits a power density of 74 mW cm^-2 and repeatedly works for 90 h at 2 mA cm^-2. This work opens up an avenue for the preparation of highly efficient bifunctional electrocatalysts for Zn-air batteries considering the extensive N-rich polymer precursors and various metal phosphide nanoparticles.

New Covalent Triazine Framework Rich in Nitrogen and Oxygen as a Host Material for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have been widely considered as the next-generation energy storage system but hindered by the soluble polysulfide intermediate-induced shuttle effect. Doping heteroatoms was confirmed to enhance the affinity of polysulfide and the carbon host, release the shuttle effect, and improve the battery performance. To enhance the Lewis acidity and reinforce the interaction between polysulfide and the carbon skeleton, a novel covalent triazine framework (CTFO) was designed and fabricated by copolymerizing 2,4,6-triphenoxy-s-triazine and 2,4,6-trichloro-1,3,5-triazine through Friedel-Crafts alkylation. Polymerization led to triazine substitution on the para-position of the phenoxy groups of 2,4,6-triphenoxy-triazine and produced two-dimensional three-connected honeycomb nanosheets. These nanosheets were confirmed to exhibit packing in the AB style through the intralayer π-π interaction to form a three-dimensional layered network with micropores of 0.5 nm. The practical and simulated results manifested the enhanced polysulfide capture capability due to the abundant N and O heteroatoms in CTFO. The unique porous polar network endowed CTFO with improved Li-S battery performance with high Coulombic efficiency, rate capability, and cycling stability. The S@CTFO cathode delivered an initial discharge capacity of 791 mAh g^-1 at 1C and retained a residual capacity of 512 mAh g^-1 after 300 charge-discharge cycles with an attenuation rate of 0.117%. The present results confirmed that multiple heteroatom doping enhances the interaction between the porous polar CTF skeleton and polysulfide intermediates to improve the Li-S battery performance.

Sulfonated Tetraphenylethylene-Based Hypercrosslinked Polymer as a Heterogeneous Catalyst for the Synthesis of Symmetrical Triarylmethanes via a Dual C–C Bond-Cleaving Path

A sulfonic acid functionalized tetraphenylethylene-based hypercrosslinked polymer (THP-SO3H) with a well-developed porous network and accessible sulfonic acid sites was synthesized and characterized by different analytical techniques. The catalytic prowess of the synthesized material THP-SO3H was investigated in a challenging dual C–C bond-breaking reaction for the synthesis of symmetrical triarylmethanes (TRAMs) in high yield. The scope of the developed metal-free method was also explored with a wide variety of substrates. The organocatalyst can be easily recovered by filtration and reused up to five consecutive cycles without substantial loss in its catalytic efficacy.

Synergetic Covalent and Spatial Confinement of Sulfur Species by Phthalazinone-Containing Covalent Triazine Frameworks for Ultrahigh Performance of Li-S Batteries

Lithium-sulfur (Li-S) batteries severely suffer from the shuttling of soluble polysulfides intermediates, insulation of sulfur and lithium sulfides, and volumetric expansion of sulfur electrodes, which result in the fast capacity decay and low utilization of active materials. To overcome these issues, a new type of porous phthalazinone-based covalent triazine frameworks (P-CTFs) with inherent N and O atoms has been in situ grown onto conductive reduced graphene oxide (rGO) by the sulfur-mediated cyclization of dinitrile monomers to afford the S/P-CTF@rGO hybrids. The well-designed structure endows the S/P-CTF@rGO composites with several features for enhanced Li-S batteries: (i) the nanoporous structure can spatially trap the sulfur species within the P-CTFs; (ii) the covalent binding of sulfur and polar groups of phthalazinone and triazine in P-CTFs exhibits strong chemical attachment and adsorption with polysulfides and further limits the diffusion of polysulfides; (iii) the conductive rGO and semiconductive P-CTFs help faster electronic transportation and accelerate the electrochemical process. Therefore, the S/P-CTF@rGO cathodes show greatly enhanced electrochemical performances with a high initial specific capacity of 1130 mAh g^-1 at 0.5C and a good capacity retention of 81.4% after 500 cycles, indicating only 0.04% degradation per cycle.

Using the Cahn–Hilliard Theory in Metastable Binary Solutions

A solution may be in one of three states: stable, unstable, or metastable. If the solution is unstable, phase separation is spontaneous and proceeds by spinodal decomposition. If the solution is metastable, the solution must overcome an activation barrier for phase separation to proceed spontaneously. This mechanism is called nucleation and growth. Manipulating morphology using phase separation has been of great research interest because of its practical use to fabricate functional materials. The Cahn–Hilliard theory, incorporating Flory–Huggins free energy, has been used widely and successfully to model phase separation by spinodal decomposition in the unstable region. This model is used in this paper to mathematically model and numerically simulate the phase separation by nucleation and growth in the metastable state for a binary solution. Our numerical results indicate that Cahn–Hilliard theory is able to predict phase separation in the metastable region but in a region near the spinodal line.

Progress Report on Phase Separation in Polymer Solutions

Polymeric porous media (PPM) are widely used as advanced materials, such as sound dampening foams, lithium-ion batteries, stretchable sensors, and biofilters. The functionality, reliability, and durability of these materials have a strong dependence on the microstructural patterns of PPM. One underlying mechanism for the formation of porosity in PPM is phase separation, which engenders polymer-rich and polymer-poor (pore) phases. Herein, the phase separation in polymer solutions is discussed from two different aspects: diffusion and hydrodynamic effects. For phase separation governed by diffusion, two novel morphological transitions are reviewed: "cluster-to-percolation" and "percolation-to-droplets," which are attributed to an effect that the polymer-rich and the solvent-rich phases reach the equilibrium states asynchronously. In the case dictated by hydrodynamics, a deterministic nature for the microstructural evolution during phase separation is scrutinized. The deterministic nature is caused by an interfacial-tension-gradient (solutal Marangoni force), which can lead to directional movement of droplets as well as hydrodynamic instabilities during phase separation.

Porous Polymers as Multifunctional Material Platforms toward Task-Specific Applications

Exploring advanced porous materials is of critical importance in the development of science and technology. Porous polymers, being famous for their all-organic components, tailored pore structures, and adjustable chemical components, have attracted an increasing level of research interest in a large number of applications, including gas adsorption/storage, separation, catalysis, environmental remediation, energy, optoelectronics, and health. Recent years have witnessed tremendous research breakthroughs in these fields thanks to the unique pore structures and versatile skeletons of porous polymers. Here, recent milestones in the diverse applications of porous polymers are presented, with an emphasis on the structural requirements or parameters that dominate their properties and functionalities. The Review covers the following applications: i) gas adsorption, ii) water treatment, iii) separation, iv) heterogeneous catalysis, v) electrochemical energy storage, vi) precursors for porous carbons, and vii) other applications (e.g., intelligent temperature control textiles, sensing, proton conduction, biomedicine, optoelectronics, and actuators). The key requirements for each application are discussed and an in-depth understanding of the structure-property relationships of these advanced materials is provided. Finally, a perspective on the future research directions and challenges in this field is presented for further studies.

Novel Non-Carbon Sulfur Hosts Based on Strong Chemisorption for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are considered as promising candidates for energy storage systems owing to their high theoretical capacity and high energy density. The application of Li-S batteries is hindered by several obstacles, however, including the shuttle effect, poor electrical conductivity, and the severe volume expansion of sulfur. The traditional method is to integrate sulfur with carbon materials. But the interaction between polysulfide intermediates and carbon is only weak physical adsorption, which easily leads to the escape of species from the framework (shuttle effect) of the material causing capacity loss. Recently, however, there has been a trend for the introduction of novel non-carbon materials as sulfur hosts based on the strong chemisorption. This review highlights recent research progress on novel non-carbon sulfur hosts based on strong chemisorption, in Li-S batteries. In comparison with carbon-based sulfur hosts, most non-carbon sulfur hosts have been demonstrated to be polar host materials that could efficiently adsorb polysulfide via strong chemisorption, mitigating their dissolution. The intrinsic mechanism associated with the role of non-carbon-based host materials in improving the performance of Li-S batteries is discussed.