Dual‐Confined Sulfur Nanoparticles Encapsulated in Hollow TiO2 Spheres Wrapped with Graphene for Lithium–Sulfur Batteries

Lithium–sulfur (Li?S) batteries are attractive owing to their higher energy density and lower cost compared with the universally used lithium‐ion batteries (LIBs), but there are some problems that stop their practical use, such as low utilization and rapid capacity‐fading of the sulfur cathode, which is mainly caused by the shuttle effect, and the uncontrollable deposition of lithium sulfide species. Herein, we report the design and fabrication of dual‐confined sulfur nanoparticles that were encapsulated inside hollow TiO₂ spheres; the encapsulated nanoparticles were prepared by a facile hydrolysis process combined with acid etching, followed by “wrapping” with graphene (G?TiO₂@S). In this unique composite architecture, the hollow TiO₂ spheres acted as effective sulfur carriers by confining the polysulfides and buffering volume changes during the charge‐discharge processes by means of physical force from the hollow spheres and chemical binding between TiO₂ and the polysulfides. Moreover, the graphene‐wrapped skin provided an effective 3D conductive network to improve the electronic conductivity of the sulfur cathode and, at the same time, to further suppress the dissolution of the polysulfides. As results, the G?TiO₂@S hybrids exhibited a high and stable discharge capacity of up to 853.4 mA h g^?1 over 200 cycles at 0.5 C (1 C=1675 mA g^?1) and an excellent rate capability of 675 mA h g^?1 at a current rate of 2 C; thus, G?TiO₂@S holds great promise as a cathode material for Li?S batteries.     

石墨烯作为硫载体在锂硫电池中的研究进展

锂硫电池因其超高的理论能量密度以及硫资源丰富、成本低廉、无毒的优点,被认为是极具发展潜力与应用前景的新一代储能设备。然而,硫正极导电性差、体积膨胀以及穿梭效应严重等问题严重制约了其商业化应用。石墨烯具有高比表面积、高导电性和高柔韧性,并且易于进行表面化学修饰及组装,是一种理想的硫载体材料。本文主要综述了近年来三维石墨烯、表面化学修饰的石墨烯、石墨烯基复合材料以及石墨烯基柔性材料在锂硫电池正极中的研究现状,并展望了石墨烯作为硫载体在锂硫电池正极中的发展趋势。     

Dual Protection of Sulfur by Carbon Nanospheres and Graphene Sheets for Lithium–Sulfur Batteries

Well‐confined elemental sulfur was implanted into a stacked block of carbon nanospheres and graphene sheets through a simple solution process to create a new type of composite cathode material for lithium–sulfur batteries. Transmission electron microscopy and elemental mapping analysis confirm that the as‐prepared composite material consists of graphene‐wrapped carbon nanospheres with sulfur uniformly distributed in between, where the carbon nanospheres act as the sulfur carriers. With this structural design, the graphene contributes to direct coverage of sulfur to inhibit the mobility of polysulfides, whereas the carbon nanospheres undertake the role of carrying the sulfur into the carbon network. This composite achieves a high loading of sulfur (64.2 wt %) and gives a stable electrochemical performance with a maximum discharge capacity of 1394 mAh g^?1 at a current rate of 0.1 C as well as excellent rate capability at 1 C and 2 C. The improved electrochemical properties of this composite material are attributed to the dual functions of the carbon components, which effectively restrain the sulfur inside the carbon nano‐network for use in lithium–sulfur rechargeable batteries.     

Module-Designed Carbon-Coated Separators for High-Loading,High-Sulfur-Utilization Cathodes in Lithium–Sulfur Batteries

Lithium–sulfur batteries have great potential as next-generation energy-storage devices because of their high theoretical charge-storage capacity and the low cost of the sulfur cathode. To accelerate the development of lithium–sulfur technology, it is necessary to address the intrinsic material and extrinsic technological challenges brought about by the insulating active solid-state materials and the soluble active liquid-state materials. Herein, we report a systematic investigation of module-designed carbon-coated separators, where the carbon coating layer on the polypropylene membrane decreases the irreversible loss of dissolved polysulfides and increases the reaction kinetics of the high-loading sulfur cathode. Eight different conductive carbon coatings were considered to investigate how the materials’ characteristics contribute to the lithium–sulfur cell’s cathode performance. The cell with a nonporous-carbon-coated separator delivered an optimized peak capacity of 1112 mA∙h g⁻¹ at a cycling rate of C/10 and retained a high reversible capacity of 710 mA∙h g⁻¹ after 200 cycles under lean-electrolyte conditions. Moreover, we demonstrate the practical high specific capacity of the cathode and its commercial potential, achieving high sulfur loading and content of 4.0 mg cm⁻² and 70 wt%, respectively, and attaining high areal and gravimetric capacities of 4.45 mA∙h cm⁻² and 778 mA∙h g⁻¹, respectively.     

Cobalt oxyhydroxide/graphene oxide nanocomposite for amelioration of electrochemical performance of lithium/sulfur batteries

Cobalt oxyhydroxide combination with graphene oxide (CoOOH@GO) as a novel conductive matrix is developed for high performance lithium/sulfur batteries. Enhancement retention of polysulfide species into matrix of cobalt oxyhydroxide anchored on graphene oxide flakes by strong chemical binding of carbon-sulfur is demonstrated. Sulfur incorporated in the sheet-like morphology of CoOOH@GO delivers high initial discharge specific capacity of 1190.85 mAh/g, which raises 260 mAh/g with respect to graphene oxide/sulfur (GO/S) as a cathode material. Furthermore, CoOOH@GO/S maintains the average coulombic efficiency of 96 % after 300 cycles at 1 C rate with capacity retention of about 61 %. Good current rate capability of CoOOH@GO/S cathode reveals that the resulting composite is open platform for electrolyte diffusion and fast ion transportation leading to the improved electrochemical performance of lithium/sulfur batteries.     

Enhanced Electrochemical Kinetics on Conductive Polar Mediators for Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries have been recognized as promising substitutes for current energy‐storage technologies owing to their exceptional advantage in energy density. The main challenge in developing highly efficient and long‐life Li–S batteries is simultaneously suppressing the shuttle effect and improving the redox kinetics. Polar host materials have desirable chemisorptive properties to localize the mobile polysulfide intermediates; however, the role of their electrical conductivity in the redox kinetics of subsequent electrochemical reactions is not fully understood. Conductive polar titanium carbides (TiC) are shown to increase the intrinsic activity towards liquid–liquid polysulfide interconversion and liquid–solid precipitation of lithium sulfides more than non‐polar carbon and semiconducting titanium dioxides. The enhanced electrochemical kinetics on a polar conductor guided the design of novel hybrid host materials of TiC nanoparticles grown within a porous graphene framework (TiC@G). With a high sulfur loading of 3.5 mg cm^?2, the TiC@G/sulfur composite cathode exhibited a substantially enhanced electrochemical performance.     

Controlled synthesis of MnSn(OH)6/graphene nanocomposites and their electrochemical properties as capacitive materials

We report the synthesis of novel MnSn(OH)₆/graphene nanocomposites produced by a co-precipitation method and their potential application for electrochemical energy storage. The hydroxide decorated graphene nanocomposites display better performance over pure MnSn(OH)₆ nanoparticles because the graphene sheets act as conductive bridges improving the ionic and electronic transport. The crystallinity of MnSn(OH)₆ nanoparticles deposited on the surface of graphene sheets also impacts the capacitive properties as electrodes. The maximum capacitance of 31.2 F/g (59.4 F/g based on the mass of MnSn(OH)₆ nanoparticles) was achieved for the sample with a low degree of crystallinity. No significant degradation of capacitance occurred after 500 cycles at a current density of 1.5 A/g in 1 M Na₂SO₄ aqueous solution, indicating an excellent electrochemical stability. The results serve as an example demonstrating the potential of integrating highly conductive graphene networks into binary metal hydroxide in improving the performance of active electrode materials for electrochemical energy storage applications.     

Superior Electrochemical Performance of Sulfur/Graphene Nanocomposite Material for High‐Capacity Lithium–Sulfur Batteries

Sulfur/graphene nanocomposite material has been prepared by incorporating sulfur into the graphene frameworks through a melting process. Field‐emission scanning electron microscope analysis shows a homogeneous distribution of sulfur in the graphene nanosheet matrix. The sulfur/graphene nanocomposite exhibits a super‐high lithium‐storage capacity of 1580 mAh g^?1 and a satisfactory cycling performance in lithium–sulfur cells. The enhancement of the reversible capacity and cycle life could be attributed to the flexible graphene nanosheet matrix, which acts as a conducting medium and a physical buffer to cushion the volume change of sulfur during the lithiation and delithiation process. Graphene‐based nanocomposites can significantly improve the electrochemical performance of lithium–sulfur batteries.     

High performance lithium-sulfur batteries by facilely coating a conductive carbon nanotube or graphene layer

A dual-layer cathode electrode is constituted by facilely coating a conductive carbon nanotube or graphene layer on the pristine sulfur cathode electrode. The conductive layer can effectively improve the conductivity and suppress the polysulfide diffusion, giving rise to an enhanced electrochemical performance for Li-S batteries.     

Sandwich-type functionalized graphene sheet-sulfur nanocomposite for rechargeable lithium batteries

A functionalized graphene sheet-sulfur (FGSS) nanocomposite was synthesized as the cathode material for lithium-sulfur batteries. The structure has a layer of functionalized graphene sheets/stacks (FGS) and a layer of sulfur nanoparticles creating a three-dimensional sandwich-type architecture. This unique FGSS nanoscale layered composite has a high loading (70 wt%) of active material (S), a high tap density of ～0.92 g cm(-3), and a reversible capacity of ～505 mAh g(-1) (～464 mAh cm(-3)) at a current density of 1680 mA g(-1) (1C). When coated with a thin layer of cation exchange Nafion film, the migration of dissolved polysulfide anions from the FGSS nanocomposite was effectively reduced, leading to a good cycling stability of 75% capacity retention over 100 cycles. This sandwich-structured composite conceptually provides a new strategy for designing electrodes in energy storage applications.     

锂硫电池硫碳复合正极材料研究现状及展望

二次锂硫电池被视为最具有发展潜力的下一代高能量密度二次电池之一. 但由于正极硫的电导率低（5×10^-30 S·cm^-1）,且在放电过程中产生的中间体多硫化物易溶于有机电解液,致使锂硫电池活性物质利用率降低,溶解后的多硫化物还会迁移到负极,被还原成不溶物Li2S2/Li2S而沉积于负极锂,使电极结构遭受破坏,造成电池容量大幅衰减,循环性能差,从而限制了进一步的开发应用. 研究表明,以碳作为导电骨架的硫碳复合正极材料能在不同程度上解决上述问题,从而有效提高了锂硫电池的放电容量和循环性能. 本文综述了近年来国内外报道的各种锂硫电池正极材料的研究进展,结合作者课题组的研究,重点探讨了硫碳复合正极材料,并对其今后的发展趋势进行了展望.     

Porous carbon framework nested nickel foam as freestanding host for high energy lithium sulfur batteries

Constructing 3 D multifunctional conductive framework as stable sulfur cathode contributes to develop advanced lithium-sulfur(Li-S) batteries.Herein,a freestanding electrode with nickel foam framework and nitrogen doped porous carbon(PC) network is presented to encapsulate active sulfur for Li-S batteries.In such a mutually embedded architecture with high stability,the interconnected carbon network and nickel foam matrix can expedite ionic/electro nic tra nsport and sustain volume variations of sulfur.Furthermore,rationally designed porous structures provide sufficient internal space and large surface area for high active sulfur loading and polar polysulfides anchoring.Benefiting from the synergistic superiority,the Ni/PC-S cathode exhibits a high initial capacity of around 1200 mAh/g at 0.2 C,excelle nt rate perfo rmance,and high cycling stability with a low decay rate of 0.059% per cycle after 500 cycles.This work provides a useful strategy to exploit freestanding porous framework for diverse applications.     

Regulating the Deposition of Insoluble Sulfur Species for Room Temperature Sodium-Sulfur Batteries

Room temperature sodium-sulfur(RT-Na-S) batteries are regarded as promising candidates for next-generation high-energy-density batteries. However, in addition to the severe shuttle effect, the inhomogeneous deposition of the insoluble sulfur species generated during the discharge/charge processes also contributes to the rapid capacity fade of RT-Na-S batteries. In this work, the deposition behavior of the insoluble sulfur species in the traditional slurry-coated sulfur cathodes is investigated using microporous carbon spheres as model sulfur host materials. To achieve uniform deposition of insoluble sulfur species, a self-supporting sulfur cathode fabricated by assembling microporous carbon spheres is designed. With homogeneous sulfur distribution and favorable electron transport pathway, the self-supporting cathode delivers remarkably enhanced rate capability(509 mA·h/g at 2.5 C, 1 C=1675 mA/g), cycling stability(718 mA·h/g after 480 cycles at 0.5 C) and areal capacity(4.98 mA·h/cm² at 0.1 C), highlighting the great potential of manipulating insoluble sulfur species to fabricate high-performance RT-Na-S batteries.     

锂硫电池硫膨胀石墨正极材料的电化学性能

应用高温气相扩散沉积法由单质硫制备硫膨胀石墨.该硫膨胀石墨正极可降低反应界面电荷传递阻抗,提高扩散阻抗抑制单质硫或多硫化物在充放电过程的穿梭.其首次放电容量达到972 mAh.g-1,容量保持率为78%,循环效率在80%以上.     

Nickel–Iron Layered Double Hydroxide Hollow Polyhedrons as a Superior Sulfur Host for Lithium–Sulfur Batteries

We have designed and synthesized novel hollow Ni/Fe layered double hydroxide (LDH) polyhedrons as an advanced sulfur host for enhancing the performance of lithium–sulfur (Li–S) batteries. The Ni/Fe LDH host shows multiple advantages. First, the Ni/Fe LDH shells can provide sufficient sulfiphilic sites for chemically bonding with polysulfides. Second, the hollow architecture can provide sufficient inner space for both loading a large amount of sulfur and accommodating its large volumetric expansion. Moreover, once the active material is confined within the host, the shells could easily restrict the outward diffusion of polysulfides, guaranteeing prolonged cycle life even with high sulfur loading. As a result, the S@Ni/Fe LDH cathode has successfully solved the main issues related to sulfur electrodes, and it exhibits significantly improved electrochemical performances with prolonged life over 1000 cycles and excellent rate properties.     

Ferrocene‐Promoted Long‐Cycle Lithium–Sulfur Batteries

Confining lithium polysulfide intermediates is one of the most effective ways to alleviate the capacity fade of sulfur‐cathode materials in lithium–sulfur (Li–S) batteries. To develop long‐cycle Li–S batteries, there is an urgent need for material structures with effective polysulfide binding capability and well‐defined surface sites; thereby improving cycling stability and allowing study of molecular‐level interactions. This challenge was addressed by introducing an organometallic molecular compound, ferrocene, as a new polysulfide‐confining agent. With ferrocene molecules covalently anchored on graphene oxide, sulfur electrode materials with capacity decay as low as 0.014 % per cycle were realized, among the best of cycling stabilities reported to date. With combined spectroscopic studies and theoretical calculations, it was determined that effective polysulfide binding originates from favorable cation–π interactions between Li⁺ of lithium polysulfides and the negatively charged cyclopentadienyl ligands of ferrocene.     

Synthesis and electrochemical properties of graphene-SnS2 nanocomposites for lithium-ion batteries

Graphene-SnS₂ nanocomposites were prepared via a solvothermal method with different loading of SnS₂. The nanostructure and morphology of the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The XRD patterns revealed that hexagonal SnS₂ was obtained. SEM and TEM results indicated that SnS₂ particles distributed homogeneously on graphene sheets. The electrochemical properties of the samples as active anode materials for lithium-ion batteries were examined by constant current charge–discharge cycling. The composite with weight ratio between graphene and SnS₂ of 1:4 had the highest rate capability among all the samples and its reversible capacity after 50 cycles was 351 mAh/g, which was much higher than that of the pure SnS₂ (23 mAh/g). With graphene as conductive matrix, homogeneous distribution of SnS₂ nanoparticles can be ensured and volume changes of the nanoparticles during the charge and discharge processes can be accomodated effectively, which results in good electrochemical performance of the composites.     

百香果皮基原位氮掺杂多孔碳/硫复合正极的制备及储锂性能

以生物质百香果皮为碳源,KHCO3为活化剂,采用同步活化碳化方法制备原位氮掺杂的分级多孔碳材料,将其与单质硫复合制得多孔碳/硫正极材料。通过X射线衍射(XRD)、X射线光电子能谱(XPS)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)等表征技术对制备材料的物相组成、微观形貌、比表面积及孔结构进行研究分析。同时,利用紫外可见吸收光谱研究了多孔碳对多硫化物的吸附作用,用恒电流充放电测试了不同硫含量(60%～80%)的多孔碳/硫复合正极材料的电化学性能。结果表明,制得的多孔碳材料为无定型,具有1 093 m²·g^-1的高比表面积和0.63 cm³·g^-1的孔容;丰富的多孔结构和原位氮掺杂对多硫化物的物理化学协同吸附作用,有效降低了锂硫电池的“穿梭效应”,提高了电池的放电比容量和循环性能。硫含量为60%的多孔碳/硫复合材料,在0.05C和0.2C倍率下可释放1 057.7和763.4 mAh·g^-1的高初始放电比容量,在1C的高倍率下循环300次后的保持率为75%。     

S/In_2O_3纳米材料的制备及其在锂硫电池正极材料中的应用

以硝酸铟和蔗糖为原料,依次经水热反应和550℃碳化制得In_2O_3纳米材料(nano-In_2O_3);将硫渗入nanoIn_2O_3得S/In_2O_3,其结构和微观形貌经SEM,TEM和XRD表征。将S/In_2O_3,导电炭黑和聚偏氟乙烯按质量比8∶1∶1制成正极材料(1);将1涂覆于铝箔上,锂片作参比电极,1 mol·L~(-1)LiPF_6的DMF/DOL(V/V=1/1)溶液为电解液,组装成锂硫半电池。采用循环伏安法和恒电流充放电法研究了S/In_2O_3的电化学性能。结果表明:在1.95 V和2.3 V处有两个还原峰,2.5 V处有一个氧化峰。电流密度为335 m A·g~(-1),首次放电比容量为1 357m Ah·g~(-1),库伦效率为82.75%。经80次充放电后,放电比容量为537 m Ah·g~(-1)。     

氮硫共掺杂多孔碳材料的制备及其在锂硫电池中的应用

锂硫电池因其较高的理论容量和对环境友好等优势被视为极具发展潜力的储能装置,但是多硫化物的穿梭效应极大地限制了锂硫电池的实际应用。本文以葡萄糖为碳源,离子液体为氮源和硫源,KCl和ZnCl₂为模板剂,KOH为活化剂,通过热解工艺合成了氮硫共掺杂多孔碳（NSPC）。XPS和极性吸附实验表明N、S杂原子成功引入并且提高了碳材料对多硫化物的吸附能力,有效缓解多硫化物的穿梭效应,而较高的比表面积（1290.67 m²·g^-1）有助于提高硫负载量。负载70.1wt.%的硫后（S@NSPC）作为锂硫电池的正极材料表现出了良好的电化学性能。在167.5 mA·g^-1的电流密度下S@NSPC的首次放电容量为1229.2 mAh·g^-1,远高于S@PC的861.6 mAh·g^-1,且S@NSPC循环500圈后容量为328.1 mAh·g^-1。当电流密度从3350 mA·g^-1恢复至167.5 mA·g^-1时,可逆容量达到首圈放电比容量的80%,几乎恢复至其初始值。