Structure engineering of cathode host materials for Li-S batteries

Although lithium-sulfur batteries are one of the favorable candidates for next-generation energy storage devices,a few key challenges that have not been addressed have limited its commercialization.These challenges include lithium dendrite growth in the anode side,volume change of the active material,poor electrical conductivity,dissolution and migration of poly sulfides,and slow rate of solid-state reactions in the cathode side.Since the electrochemical performance of lithium-sulfur batteries is greatly affected by the design of the cathode host material,it has also been widely discussed in addressing the abovementioned issues.In this paper,three design ideas of cathode host materials in terms of microstructure,crystal structure and electronic structure are introduced and summarized.Crucially,the current progress of these three structural design strategies and their effects on the electrochemical performance of lithium-sulfur batteries are discussed in detail.Finally,future directions in the structural design of cathode materials for lithium-sulfur batteries are discussed and further perspectives are provided.

Linker aggregation engineering of TADF materials to tune carrierbalance for highly efficient organic LEDs with long operationallifetime

Thermally activated delayedfluorescence(TADF)molecules are regarded as promis-ing materials for realizing high-performance organic light-emitting diodes(OLEDs).The connecting groups between donor(D)and acceptor(A)units in D–A type TADF molecules could affect the charge transfer and luminescence performance of TADF materials in aggregated states.In this work,we design and synthesize four TADF molecules using planar and twisted linkers to connect the aza-azulene donor(D)and triazine acceptor(A).Compared with planar linkers,the twisted ones(Az-NP-T and Az-NN-T)can enhance A–A aggregation interaction between adjacent molecules to balance hole and electron density.As a result,highly efficient and stable deep-red top-emission OLEDs with a high electroluminescence efficiency of 57.3%and an impressive long operational lifetime(LT_(95)∼30,000 h,initial luminance of 1000 cd m^(-2))are obtained.This study provides a new strategy for designing more effi-cient and stable electroluminescent devices through linker aggregation engineering in donor–acceptor molecules.

Stable immobilization of lithium polysulfides using three-dimensional ordered mesoporous Mn_(2)O_(3) as the host material in lithium-sulfur batteries

Lithium-sulfur batteries(LSBs)have drawn significant attention owing to their high theoretical discharge capacity and energy density.However,the dissolution of long-chain polysulfides into the electrolyte during the charge and discharge process(“shuttle effect”)results in fast capacity fading and inferior electrochemical performance.In this study,Mn_(2)O_(3)with an ordered mesoporous structure(OM-Mn_(2)O_(3))was designed as a cathode host for LSBs via KIT-6 hard templating,to effectively inhibit the polysulfide shuttle effect.OM-Mn_(2)O_(3)offers numerous pores to confine sulfur and tightly anchor the dissolved polysulfides through the combined effects of strong polar-polar interactions,polysulfides,and sulfur chain catenation.The OM-Mn_(2)O_(3)/S composite electrode delivered a discharge capacity of 561 mAh g^(-1) after 250 cycles at 0.5 C owing to the excellent performance of OM-Mn_(2)O_(3).Furthermore,it retained a discharge capacity of 628mA h g^(-1) even at a rate of 2 C,which was significantly higher than that of a pristine sulfur electrode(206mA h g^(-1)).These findings provide a prospective strategy for designing cathode materials for high-performance LSBs.

Synthesis,characterization and applications of vinylsilafluorene copolymers:New host materials for electroluminescent devices

Vinylsilafluorene(VSiF) was successfully synthesized and copolymerized with vinylcarbazole and methyl methacrylate via free radical copolymerization for the first time.The synthesis,photophysical properties,computational modeling studies,and organic light-emitting devices of the VSiF copolymers were presented.The good coordinated photoluminescent(PL) spectra with the absorption of blue light-emitting materials and the high energy band-gap of the VSiF copolymers were observed.Higher triplet band gap(3Eg) to host the blue phosphorescent emitters and better HOMO and LUMO than PVK for electron and hole injection and transportation of the VSiF model compounds were revealed by density functional theory(DFT) calculations.The preliminary device results in applications of these copolymers as host materials for green phosphorescent emitters demonstrate the copolymers of VSiF and vinylcarbazole have comparable device performance of polyvinylcarazole(PVK),suggesting a bright future of VSiF as building blocks for host materials.

Triphenyl Phosphine Oxide and Carbazole-based Polymer Host Materials for Green Phosphorescent Organic Light-emitting Diodes

Four novel polymers, poly（3,6-9-decyl-carbazole-alt-1,3-benzene） （PB13CZ）, poly（3,6-9-decyl-carbazole-alt- bis（4-phenyl） （phenyl） phosphine oxide） （PTPPO38CZ）, poly（3,6-9-decyl-carbazole-alt-2,4-phenyl（diphenyl） phosphine oxide） （PTPPO13CZ） and poly（3,6-9-decyl-carbazole-alt-bis（3-phenyl） （phenyl） phosphine oxide） （PTTPO27CZ） were synthesized, and their thermal, photophysical properties and device applications were further investigated to correlate the chemical structures with the photoelectric performance of bipolar host materials for phosphorescent organic light emitting diodes. All of them show high thermal stability as revealed by their high glass transition temperatures and thermal decomposition temperatures at 5% weight loss. These polymers have wide band gaps and relatively high triplet energy levels. As a result, the spin coating method was used to prepare the green phosphorescent organic light emitting diodes with polymers PTPPO38CZ, PTPPO13CZ and PTTPO27CZ as the typical host materials. The green device of polymer PTPPO38CZ as host material shows electroluminescent performance with maximum current efficiency of 2.16 cd.A-1, maximum external quantum efficiency of 0.7%, maximum brightness of 1475 cd.m-2 and reduced efficiency roll-off of 7.14% at 600 cd.m-2, which are much better than those of the same devices hosted by polymers PTTPO27CZ and PTPPO13CZ.

Silafluorene moieties as promising building blocks for constructing wide-energy-gap host materials of blue phosphorescent organic light-emitting devices

In this article, we reported the synthesis and characterization of a novel silafluorene-based host material, 1,3-bis（5-methyl-5H- dibenzo[b,d]silol-5-yl）benzene （Me-DBSiB）, for blue phosphorescent organic light-emitting devices （PHOLEDs）. The Me- DBSiB was constructed by linking 9-methyl-9-silafluorene units to the phenyl framework through the sp3-hybridized silica atom to maintain high singlet and triplet energy, as well as to enhance thermal and photo-stability. The calculated result shows that the phenyl core does not contribute to both the highest occupied molecular orbital and lowest unoccupied molecular orbital. Wide optical energy gap of 4.1 eV was achieved. When the Me-DBSiB was used as the host and iridium （Ⅲ） bis[（4,6-difluorophenyl）pyridinato-N,C2＇]picolate （Firpic） as the guest, a maximum current efficiency was 14.8 cd/A, lower than the counterpart of 1,3-bis（9-carbazolyl）benzene （28 cd/A）. The unbalanced barrier for electron and hole injection to host layer may be responsible for low efficiency. Even so, our results show that silafluorene moieties are promising building blocks for constructing wide-energy-gap host materials.

Synthesis and characterization of carbazole-based dendrimers as bipolar host materials for green phosphorescent organic light emitting diodes

A group of novel, carbazole-based dendrimers comprised of the electron-accepting dibenzothiophene core and the electron-donating oligo-carbazole dendrons, namely G1 SF and G2 SF, are synthesized utilizing the Ullmann C–N coupling reaction. The dendrimers are designed in such a way to show good solubility in common organic solvents, excellent thermochemical stability with decomposition temperatures（Td） up to430 8C, and high HOMO levels in a range from 5.45 e V to 5.37 e V. Results of density functional theory calculations（DFT） indicate G2 SF has an almost complete separation of HOMO and LUMO levels at the holeand electron-transporting moieties; while G1 SF exhibits only partial separation of the HOMO and LUMO levels possibly due to intramolecular charge transfer. Green phosphorescent OLEDs were fabricated by the spin coating method with the dendrimers as hosts and traditional green iridium phosphor as doped emitter. Under ambient conditions, a maximum luminance efficiency（hL） of 19.83 cd A^-1and a maximum external quantum efficiency of 5.85% are achieved for G1 SF, and 15.50 cd A ^-1and 4.57% for G2 SF.

Three-dimensional topological insulators:A review on host materials

In recent years,three-dimensional topological insulators(3DTI) as a novel state of quantum matter have become a hot topic in the fields of condensed matter physics and materials sciences.To fulfill many spectacularly novel quantum phenomena predicted in 3DTI,real host materials are of crucial importance.In this review paper,we first introduce general methods of searching for new 3DTI based on the density-functional theory.Then,we review the recent progress on materials realization of 3DTI including simple elements,binary compounds,ternary compounds,and quaternary compounds.In these potential host materials,some of them have already been confirmed by experiments while the others are not yet.The 3DTI discussed here does not contain the materials with strong electron-electron correlation.Lastly,we give a brief summary and some outlooks in further studies.

Preparation, Characterization and Optical Properties of Host-guest Nanocomposite Material Mordenite-silver Iodide

Silver iodide nanoclusters were successfully prepared in the channels of mordenite by a heat diffusion method. Powder X ray diffraction, adsorption technique and infrared spectroscopy were used to characterize the prepared materials, which showed that the guest silver iodide had been encapsulated in the channels of mordenite. The optical properties of the solid phase diffuse reflectance absorption of nanocomposite material NaM AgI were studied, showing that the absorption bands of the diffuse reflectance absorption of the prepared material moved to the region of high energy. The absorption peak of the material prepared shifted to the region of high energy. Namely, blue shift was caused. This has demonstrated the incorporation of silver iodide into the channels of the zeolite. We observed the luminescence and surface photovoltage spectra of NaM AgI sample, proposing the mechanisms of the photoluminescence and photovoltaic responses.

Stabilizing lithium deposition within bimodal porous SiO_(2)-TiO_(2) microspheres as 3D host structure

Three-dimensional (3D) host materials for lithium metal anodes (LMAs) have gained attention because they can mitigate volume expansion and local current density through their large surface area and suppress the dendritic growth of lithium. Recent research on 3D host materials has focused on conductive materials;however, the benefits of 3D host materials cannot be fully utilized because lithium deposition begins at the top of the structure. Herein, we fabricate SiO_(2)-TiO_(2) composite microspheres with bimodal pore structures (bi-SiTiO) by simple spray pyrolysis. These microspheres effectively store lithium within the structure from the bottom of the electrode while preventing lithium dendrite formation. Focused ion beam-scanning transmission electron microscopy (FIB-STEM) analysis reveals that the lithiophilic properties of composite microspheres enhanced their effectiveness in storing lithium, with small pores acting as “lithium-ion sieves” for a uniform lithium-ion flux and large pores that provide sufficient volume for lithium deposition. The bi-SiTiO composite microspheres exhibit a high Coulombic efficiency of 98.5% over 200 cycles at 2.0 mA·cm^(−2) when operated in a lithium half-cell. With a high lithium loading of 5.0 mAh·cm^(−2), the symmetrical cell of the bi-SiTiO electrode sustains more than 900 h. A full cell coupled with an LiNi_(0.8)Co_(0.1)Mn_(0.1)O_(2) (NCM811) cathode also exhibits enhanced electrochemical properties in terms of cycling stability and rate capability.

Modularized sulfur storage achieved by 100% space utilization host for high performance lithium-sulfur batteries

Popularization of lithium-sulfur batteries(LSBs) is still hindered by shuttle effect and volume expansion.Herein, a new modularized sulfur storage strategy is proposed to solve above problems and accomplished via employing 100% space utilization host material of cobalt loaded carbon nanoparticles derived from ZIF-67. The modular dispersed storage of sulfur not only greatly increases the proportion of active sulfur,but also inhibits the occurrence of volume expansion. Meanwhile, 100% space utilization host material can greatly improve the conductivity of the cathode, provide a larger electrolyte wetting interface and effectively suppress the shuttle effect. Moreover, loaded cobalt particles have high catalytic activity for electrochemical reaction and can effectively improve the redox kinetics. The cell with new cathode host material carbonized at 650 ℃(ZIF-67(650 ℃)) exhibits superior rate performance and can maintain a high specific capacity of 950 m Ah/g after 100 cycles at 0.2 C, showing a good cycle stability.

A silane-based host material with improved electron transport properties for phosphorescent OLEDs with high efficiency and low efficiency roll-off

A dibenzosilole-based host material was designed and characterized.The host material,9,9'-(5,5-diphenyl-5H-dibenzo[b,d]silole-2,8-diyl)bis(9H-carbazole)(SSiCz),was designed to enhance the electron transport properties and rigidity by coupling two phenyl units of the tetraphenyl silane of a strong hole transport type bis(4-(9Hcarbazol-9-yl)phenyl)diphenylsilane host.The device efficiency roll-off was improved considerably by balancing the carriers in the phosphorescent organic light-emitting diodes(PhOLEDs),and the driving voltage at high luminance was reduced.The red and green PhOLEDs exhibited high external quantum efficiencies(EQEs)of 23.8%and 24.9%,respectively,and a relieved efficiency roll-off.In addition,S-SiCz was used as an electron transport type host with a hole transport type3,3'-di(9H-carbazol-9-yl)-1,1'-biphenyl host.The maximum EQEs of the red and green PhOLEDs using the mixed host were 26.0%and 25.5%,respectively,and EQE roll-off values were 18%and 6%,respectively.Therefore,the planarization design strategy of the host is effective for better device performance.

Enhancing lithium-sulfur battery performance with In_(2)O_(3)-In_(2)S_(3)@NSC heterostructures:Synergistic effects of double barrier and catalytic transformation

The sluggish redox reaction kinetics of lithium polysulfides(LiPSs)are considered the main obstacle to the commercial application of lithium-sulfur(Li-S)batteries.To accelerate the conversion by catalysis and inhibit the shuttling of soluble LiPSs in Li-S batteries,a solution is proposed in this study.The solution involves fabrication of N,S co-doped carbon coated In_(2)O_(3)/In_(2)S_(3)heterostructure(In_(2)O_(3)-In_(2)S_(3)@NSC)as a multifunctional host material for the cathode.The In_(2)O_(3)-In_(2)S_(3)@NSC composite can reduce the Gibbs free energy for the conversion reactions of LiPSs,which results in superior performance.The synergy between different components in In_(2)O_(3)-In_(2)S_(3)@NSC and the unique 3D structure facilitate ion and electron transport in Li-S batteries.The In_(2)O_(3)-In_(2)S_(3)@NSC/Li 2 S 6 cathode exhibits excellent rate capacity,with a capacity of 599 mAh g−1 at 5.5 C,and good cycle stability,with a capacity of 436 mAh g^(−1)after 1000 cycles at 1 C.Overall,this study proposes a promising solution to improve the energy storage properties of Li-S batteries,which could potentially facilitate the commercialization of Li-S batteries.

An orderly arranged dual-role MIL-53(Al)nanorods array rooted on carbon nanotube film for long-life and stable lithium-sulfur batteries

It is of great value to synchronously resolve the critical issues of the polysulfide shuttle and dendrite growth in lithium-sulfur(Li-S)batteries.Herein,a bifunctional Al-based Material of Institute Lavoisier-53(MIL-53(Al))/carbon nanotube(MIL-53/CNT)composite is reported for this matter,which was constructed by growing an ordered MIL-53(Al)nanorods array on the CNT film.For the sulfur cathode,the proposed structure serves as a multifunctional interlayer to block polysulfides and accelerate their catalytic conversion,thus efficaciously inhibiting the shuttle effect.Meanwhile,when applied as the anode host material(Li@MIL-53/CNT),the flexible CNT film serves as a self-standing framework to accommodate Li metal and alleviate the volume expansion,while the uniform ion channels built by the MIL-53(Al)nanorods array along with the abundant oxygen groups can homogenize Li ion diffusion,enabling a steady Li plating/stripping behavior and limiting the dendrite growth.Not surprisingly,Li-S full battery with MIL-53/CNT interlayer and Li@MIL-53/CNT anode delivers an appreciable specific capacity of 735 mAh·g^(–1)and excellent cycle durability at 5 C,presenting a limited capacity decay of 0.03%per cycle in 500 cycles.Besides,an impressive cycle stability and rate capability are also achieved at high-sulfur loading and lean electrolyte conditions.

Recent advances in cathodes for all-solid-state lithium-sulfur batteries

Lithium-sulfur(Li-S)batteries have been regarded as the candidate for the next-generation energy storage system due to the high theoretical specific capacity(1675 m Ah/g), energy density(2600 Wh/kg)and the abundance of elemental sulfur, but the application of Li-S batteries is impeded by a series of problems. Recently, all-solid-state Li-S batteries(ASSLSBs) have drawn great attention because many drawbacks such as safety issues caused by metallic lithium anodes and organic liquid electrolytes can be overcome through the use of solid-state electrolytes(SEs). However, not only the problems brought by sulfur cathodes still exist, but more trouble arouses from the interfaces between SEs and cathodes, hampering the practical application of ASSLSBs. Therefore, in order to deal with the problems, enormous endeavors have been done on ASSLSB cathodes during the past few decades, including engineering of cathode active materials, cathode host materials, cathode binder materials and cathode structures. In this review, the electrochemical mechanism and existing problems of ASSLSBs are briefly introduced. Subsequently, the strategies for developing cathode materials and designing cathode structures are presented. Then there follows a brief discussion of SE problems and expectations, and finally, the challenges and perspectives of ASSLSBs are summarized.

Designing principles of advanced sulfur cathodes toward practical lithium-sulfur batteries

As one of the most promising candidates for next-generation energy storage systems,lithium-sulfur(Li-S)batteries have gained wide attention owing to their ultrahigh theoretical energy density and low cost.Nevertheless,their road to commercial application is still full of thorns due to the inherent sluggish redox kinetics and severe polysulfides shuttle.Incorporating sulfur cathodes with adsorbents/catalysts has been proposed to be an effective strategy to address the foregoing challenges,whereas the complexity of sulfur cathodes resulting from the intricate design parameters greatly influences the corresponding energy density,which has been frequently ignored.In this review,the recent progress in design strategies of advanced sulfur cathodes is summarized and the significance of compatible regulation among sulfur active materials,tailored hosts,and elaborate cathode configuration is clarified,aiming to bridge the gap between fundamental research and practical application of Li-S batteries.The representative strategies classified by sulfur encapsulation,host architecture,and cathode configuration are first highlighted to illustrate their synergetic contribution to the electrochemical performance improvement.Feasible integration principles are also proposed to guide the practical design of advanced sulfur cathodes.Finally,prospects and future directions are provided to realize high energy density and long-life Li-S batteries.