Li-S Cells Research Articles

Cutting COF-like C4 N to Give Colloidal Quantum Dots: Towards Optical Encryption and Bidirectional Sulfur Chemistry via Functional Group and Edge Effects.

Herein, we report the first synthesis of colloidal C4 N quantum dots (QDs) and their functional composites and explore their optical activities and edge-selective polysulfide adsorption-catalysis. As-obtained C4 NQDs are rich in carbonyl groups and edges, allowing good solution processability and facile assembly with other moieties for creating functionalities. While C4 NQDs show normal fluorescence (FL), the QD/poly(vinyl alcohol) (PVA) composites give FL/room-temperature-phosphorescence (RTP) dual-mode emission, enabling the corresponding solution to be used as an encryption ink. The QDs anchored onto carbon nanotubes can be used as a barrier layer to decorate commercial separators, endowing a Li-S cell with excellent cycling stability, high rate capability, and large areal capacity. Computation and experiment studies show that edge sites in C4 N favor polysulfide adsorption and catalysis and the enriched edges and carbonyl groups in QDs synergically promotecatalytic conversion of sulfur species.

The Effect of Porosity of Activated Carbon Cloth Cathodes on the Cyclic Performance of Li-S Cells

The Effect of Porosity of Activated Carbon Cloth Cathodes on the Cyclic Performance of Li-S Cells

리튬-황 전지 양극재용 유기 금속 골격체 기반의 다공성 나노섬유 구조체

Metal organic framework-derived porous nanofibers were synthesized by electrospinning and subsequent heat treatment process. In this study, ZIF-8 and PS nanobeads were used as templates to form micro and meso pores in the structure. For this, ZIF-8 polyhedra were prepared by liquid-phase methode using 2-methylimidazole and zincnitrate-hexahydrate in ethylalcohol. After facile heat-treatment, ZIF-8, PS, and PVP were thermally decomposed to form pores and carbon matrix composing fiber structure. The porous carbon matrix improves the redox reaction kinetics and provides numerous chemisorption sites for the effective trapping and reuse of lithium polysulfide during cycles. As a result, the assembled Li-S cells employing unique nanostructure display superior rate capability and stable cycling performance. The Li-S cell delivered a stable discharge capacity of 655 mA h g^-1 after 100 charge-discharge cycles at 1.0 C. Thus, the uniquely nanostructured porous carbon fibers shows high potential for the development of advanced Li-S batteries utilizing pure sulfur electrodes with realistic battery parameters.

The effect of NiO-Ni3N interfaces in in-situ formed heterostructure ultrafine nanoparticles on enhanced polysulfide regulation in lithium-sulfur batteries

Inhibiting the “shuttle effect” of soluble polysulfides and improving reaction kinetics are the key factors necessary for further exploration of high-performance Li-S batteries. Herein, an effective interface engineering strategy is reported, wherein nitriding of an Ni-based precursor is controlled to enhance Li-S cell regulation. The resulting in-situ formed NiO-Ni3N heterostructure interface not only has a stronger polysulfide adsorption effect than that of monomeric NiO or Ni3N but also has a faster Li ion diffusion ability than a simple physical mixture. More importantly, this approach couples the respective advantages of NiO and Ni3N to reduce polarization and facilitate electron transfer during polysulfide reactions and synergistically catalyze polysulfide conversion. In addition, ultrafine nanoparticles are thought to effectively improve the use of additive materials. In summary, Li-S batteries based on this NiO-Ni3N heterostructure have the features of long cycle stability, rapid charging-discharging, and good performance under high sulfur loading.

The TiO2-Modified Separator Improving the Electrochemical Performance of Lithium-Sulfur Battery

Electrochemical performance of activated carbon/sulfur composite cathode in the Li-S cell with standard and TiO2-modified separator is evaluated by cyclic voltammetry and galvanostatic chronopotentiometry. The modification of the separator by TiO2 impregnation has beneficial effect on the charge capacity of the activated carbon/sulfur cathode in the Li-S cell. The specific capacity of the cathode in the cell with TiO2-modified separator is 632 mAh g-1 (calculated from cyclic voltammetry) and 673 mAh g-1 (determined from galvanostatic chronopotentiometry). Facile impregnation of the separator with nanocrystalline TiO2 results in the 10-20 % stable increase of the charge capacity of corresponding activated carbon/sulfur cathode as compared to its electrochemical performance in the system with non-modified separator.

In-situ construction of g-C3N4/carbon heterostructure on graphene nanosheet: An efficient polysulfide barrier for advanced lithium-sulfur batteries

Lithium-sulfur batteries (LSBs) have been considered as the future potential energy storage system with advantages of high energy density (2600 Wh kg−1), eco-friendliness and low cost. However, the poor conductivity and serious shuttle effects of polysulfides block their application. Herein, g-C3N4/carbon heterostructure on graphene nanosheet (PCNG) is constructed via phenyl-modified strategy and in-situ thermal polycondensation, which has a unique electron cloud distribution with excellent sulfur immobilization ability as well as good electroconductivity. As a result, Li-S cells with simple S/C cathodes and PCNG interlayers show a high initial capacity of 1192 mAh g−1 at 0.1C, and an ultra-long lifespan with a slow capacity attenuation of 0.050% per cycle after 800 cycles. The cell with PCNG/PP separator also has a stable cycle performance at high area sulfur loading of 7 mg cm−2. These findings provide a new sight on functionalizing g-C3N4 for application in Li-S electrochemistry.

The enhanced confinement effect of double shell hollow mesoporous spheres assembled with nitrogen-doped copper cobaltate nanoparticles for enhancing lithium–sulfur batteries

The construction of sulfur cathode host which can effectively prevent the diffusion of soluble polysulfides and improve the reaction dynamics is the key to the development of high-performance Li–S batteries. Herein, a nitrogen-doped CuCo2O4 double shell hollow mesoporous sphere structure assembled with ultrafine nanoparticles was reported. As a multifunctional sulfur species carrier, the unique double shell provides multiple confinement spaces, chemisorption and catalytic sites, further impeding the “shuttle effect” of polysulfides. Meanwhile, the higher surface area and abundant porosity reduce the sulfur layer thickness and shorten the charge transfer path respectively, which is conducive to the rapid redox kinetics to polysulfides. Thanks to this enhanced confinement effect, a Li-S cell with the NCuCo2O4@S as the cathode shows excellent cycle stability, high sulfur loading, and rapid charge–discharge capability. This work provides a promising strategy for achieving high performance Li–S batteries by using metal compounds with a unique hollow structure.

Single-Atom Catalyst Aggregates: Size-Matching is Critical to Electrocatalytic Performance in Sulfur Cathodes.

Electrocatalysis is critical to the performance displayed by sulfur cathodes. However, the constituent electrocatalysts and the sulfur reactants have vastly different molecular sizes, which ultimately restrict electrocatalysis efficiency and hamper device performance. Herein, the authors report that aggregates of cobalt single‐atom catalysts (SACs) attached to graphene via porphyrins can overcome the challenges associated with the catalyst/reactant size mismatch. Atomic‐resolution transmission electron microscopy and X‐ray absorption spectroscopy measurements show that the Co atoms present in the SAC aggregates exist as single atoms with spatially resolved dimensions that are commensurate the sulfur species found in sulfur cathodes and thus fully accessible to enable 100% atomic utilization efficiency in electrocatalysis. Density functional theory calculations demonstrate that the Co SAC aggregates can interact with the sulfur species in a synergistic manner that enhances the electrocatalytic effect and promote the performance of sulfur cathodes. For example, Li–S cells prepared from the Co SAC aggregates exhibit outstanding capacity retention (i.e., 505 mA h g–1 at 0.5 C after 600 cycles) and excellent rate capability (i.e., 648 mA h g−1 at 6 C). An ultrahigh area specific capacity of 12.52 mA h cm−2 is achieved at a high sulfur loading of 11.8 mg cm–2.

Practical considerations in designing solid state Li-S cells for electric aviation

The development of high-energy, high-safety, and high-power batteries beyond electric automotive standards is critical for future electric aviation applications. Non-volatile solid-state electrolytes (SSE) offer many promising advantages over traditional flammable liquid electrolytes. SSE may become an enabling technology for certain battery chemistries by preventing detrimental interactions that occur with liquid electrolytes, for example preventing dissolution of lithium-sulfur intermediates common in traditional liquid organic solvents. However, significant manufacturing challenges must be overcome before the adoption of such technology. One of the main challenges is the fabrication of sufficiently thin electrolyte layer to maximize specific energy of the solid-state cell. In this work we first show the feasibility in the assembly of an all solid-state battery via the development of composite SSE with ionic conductivity above 0.2mS/cm and thicknesses on the order of 40 micron through a tape-casting technique. With Li/Al alloy as the anode and TiS2 as the active cathode material, the resultant solid-state battery with a thin electrolyte layer and excellent cathode-electrolyte interface represents a prototype vision on a potential solid-state cell required for electric aviation. A detailed parameter analysis is further presented on the influence of the various cell metrics, such as electrolyte thickness, carbon content, active material utilization, and excess lithium, on the cell specific energy. The implications in the optimization of these required parameters to enable practical solid-state Li-S batteries for electric aviation are discussed.

Suppressing Polysulfide Shuttling in Lithium-Sulfur Batteries via a Multifunctional Conductive Binder.

Exhibiting high specific energy and low cost, lithium-sulfur batteries are considered promising candidates for the next-generation battery. However, its wide applications are limited by the insulating nature of the sulfur, dissolution of polysulfide species, and large volume change of the sulfur cathode. In this work, a conductive binder, crosslinked polyfluorene (C-PF) is synthesized and employed in Li-S batteries to enhance the overall electrochemical performance from the following three aspects: 1) possessing high electronic conductivity, C-PF facilitates lowered areal resistance for the sulfur electrode and leads to an improved rate capability; 2) owing to the cross-linked polymer structure, favorable mechanical properties of the electrode can be achieved, hence the well-preserved electrode integrity; 3) forming strong binding with various polysulfide species, C-PF manages to trap them from diffusing to the Li anode, which greatly improves the cycling stability of Li-S cells. Through designing a multifunctional binder to comprehensively enhance the Li-S cathode, this proposed approach could be broadly applied to fully harness the energy from S redox in addition to cathode material modifications.

Metal Organic Framework Derived Magnesium Oxide/Carbon Interlayer for Long-Life Lithium-Sulfur Batteries

i-S battery, with abundant S sources and high theoretical capacity, is one of the promising future battery technology. S cathode has a theoretical capacity of 1672 mAhg-1, which is an order of magnitude higher than the transition metal oxides’ of commercial lithium-ion batteries. However, the commercialization of Li-S battery is still struggling due to poor cyclic stability and limited kinetics. These problems come from the low electrical conductivity of S, LiS and Li2S, polysulfide shuttling effect and large volume change during cycling. Polysulfide shuttling effect is a phenomenon that polysulfide intermediate species, generated during charging and discharging of S and Li2S, gets dissolved into electrolyte and cause loss of capacity. Herein, the Mg-MOF-74-derived MgO/carbon interlayer is synthesized. Carbon substrate is known to provide additional electrical conductivity to S cathode, and according to Tao, X., et al, MgO is a good choice to balance between sulfide-adsorption and diffusion on the oxides. Therefore, MOF-derived MgO particles are expected to effectively mitigate polysulfide shuttling and reutilize dissolved polysulfide. Inserting MOF-derived MgO/carbon interlayer, the Li-S cell show excellent cycling stability (87.5% retention within 100 cycles) at 0.2 C with sulfur loading mass 2.6 mg cm-2, whereas the pristine and carbon interlayer sample show 60.0% and 64.3%. The capacity retention still holds at higher loading mass of sulfur electrode. Due to low electrical conductivity and polysulfide dissolution of the sulfur cathode, increasing sulfur content and loading mass of the sulfur electrode, and amount of electrolyte could be detrimental to Li-S cell performance. However, with Mg-MOF-74-derived MgO/carbon interlayer, this limitation could be overcome.

A Self-Assemble Micelle Electrolyte for Polysulfide Suppression and Li Stabilization in Li-S Battery

Li-S battery, a promising next-generation technology, has received dramatic attention recently due to high theoretical energy density (2600Wh kg-1 ) and low materials cost. However, its application has been impeded by notorious polysulfide dissolution/shuttling which causes poor cycling stability and coulombic efficiency. Herein, I will introduce a novel strategy to suppress the polysulfide dissolution by using hydrofluoroethers (HFEs) electrolyte additive. A bi-functional, surfactant-like molecule structure with a polar lithiophilic “head” attached to a fluorinated lithiophobic “tail”, was designed for these HFE solvents whereby a unique solvation mechanism is proposed: dissociated lithium ions are readily coordinated with lithiophilic “head” to induce self-assembly into micelle-like complex structures. Complex formation is verified experimentally by small angle X-ray scattering (SAXS) and is studied as a function of additive structure and concentration. The modeling work is underway to further investigate coordination between molecules and micelle structure evolution under different conditions. These HFE-based electrolytes are found to prevent polysulfide dissolution and to have excellent chemical compatibility with lithium metal, which is evidenced by superior battery performance: Li-S cells are demonstrated with 1395 mAh g-1 initial capacity and 71.9% retention over 100 cycles at >99.5% efficiency; Li||Cu stripping/plating tests reveal high coulombic efficiency (>99.5%), modest polarization, and smooth surface morphology of the uniformly deposited lithium.

Multifunctional Trilayer Separator for High-Performance Lithium-Sulfur Batteries Under Extreme Conditions

Lithium-ion Batteries (LIBs) have transformed mankind's lives beyond imagination in the past few decades. They have found their use in a variety of applications such as defense, transportation, portable electronics, and space explorations. To keep up with the demand, stress is applied for discovering materials with higher capacity and energy densities. High capacity anodes like silicon, tin, antimony, carbon composites have been identified. However, limited cathode materials are available at disposal viz., LiCoO2, LiMnNiCoO2, LiFePO4, LiMn2O4. Most of these cathodes have capacities <250 mAh g-1, and thus LIBs cannot keep up with the ever-growing demand from the applications. The high-theoretical capacity chemistry of lithium-sulfur comes to the rescue. It is the lightest element to be used as a cathode, which has attractive properties such as capacity of 1672 mAh g-1, economical (~$50 per metric tonne), and abundance in nature. However, chemistry suffers from three critical issues viz., poor sulfur conductivity, polysulfide shuttling, and lithium dendrite growth. To tackle the insulative sulfur and polysulfide shuttling effect approaches such as coating sulfur with metal sulfide or oxides, mesoporous carbon, conductive polymers, inverse vulcanization have been applied. Unfortunately, these techniques lead to a reduction in the energy density of the system, involve complex fabrication steps or raw materials are expensive. On the issue of lithium metal dendrites growth, electrolytic additives, exotic electrode coatings, and special shutdown separators are used. The downside to all of these is that they impede cell performance or are uneconomical.Here we report, the use of a multifunctional trilayer separator in Li-S battery to tackle all three issues simultaneously. Modified separator acted as a barricade for polysulfide from shuttling by trapping them preferably due to the favorable interactions. The performance of Li-S cells with and without modified separator was compared for long term cycling and rate studies. It was found that cells with modified separator outperformed those with pristine separator. Modified separator cell exhibited capacities of 925, 833, 644, 480, 326, 260, and 220 mAh g-1 at 0.1C, 0.2C, 0.5C, 1C, 2C, 3C and 4C rates, respectively with 95% capacity retention. The choice of electrolyte affects battery performance drastically. However, using 1M LiTFSI in 1:1 (v/v) 1, 3-Dioxolane (DOL): 1, 2-Dimethoxyethane (DME) with additives, provided remarkable results at low and high-temperature ranges. At 0 ℃, the cell with the modified separator yielded about 350 mAh g-1 capacity at 0.5C over 200 cycles. The performance of the cells was stellar even at 50 ℃ with 500 mAh g-1 capacity at 0.5C rate after 100 cycles. Multiple module calorimetry provided the thermal heat signature to elucidate the safety features of the Li-S system with the modified separator and compared with the existing LIBs. High-performance Li-S batteries proposed here can be valuable to a variety of applications like defense, transportation, and space explorations, where drastic conditions affect the battery functionalities, in the times ahead.

Experimental Methods for Assembly of Dendrite-free Lithium-Sulfur Batteries

To prevent dendrite formation by metallic lithium (Li) on the negative electrode in Li-S batteries, the use of a Li-intercalated carbon electrode and Li-pre-doped Li-S cell is proposed. These two cell systems achieved adequate charge-discharge performance for next-generation cell systems, even under severe input-output demands, owing to the use of a graphite negative electrode. Combination of enhanced cell safety and ease of cell production was successfully achieved.

Deciphering the role of tetrahydrofuran residue in the poly(ethylene oxide)/LiTFSI hybrid used for secondary battery electrolyte

Residual solvent in polymer electrolytes recently has attracted great attention since it influences the performance and safety of the polymer-electrolyte based batteries and electric vehicles. In this work, we did a systematic study of exploring the role of the residual solvent in lithium-ion secondary battery electrolyte beyond the traditional understanding of plasticizing effect, based on the typical polymer electrolyte system of polyethylene oxide/tetrahydrofuran/lithium bis(trifluoromethanesulfonyl)imide (PEO/THF/LiTFSI). A series of complementary characterization techniques, i.e., small-angle neutron scattering (SANS), wide-angle X-ray scattering (WAXS), Raman, infrared, nuclear magnetic resonance, thermogravimetric analysis and electrochemical impedance spectroscopy, were applied to decipher the structure-function relationship. We found that the PEO/LiTFSI hybrid formed a complex with the THF residue, which was further confirmed by the Gaussian simulation. By using the oxygen-free solvent, acetonitrile, as a reference, we demonstrated that THF provided ambient oxygen bonding sites among PEO and LiTFSI, which leaded to PEO/LiTFSI/THF clusters and endowed the hybrid a ionic conductivity value of 1.1 × 10−4 S cm−1 at 30 °C. The battery showed a discharge capacitance of 905 mAh g−1 at 0.1C in a Li-S cell. Moreover, both ionic conductivity and cell performance deteriorate significantly with vigorous drying of the polymer electrolyte membrane in vacuum oven at 120 °C and argon-purged glove box at room temperature, although there are still around 10 wt% THF residues found in the dried membranes. It seems to be that the vigorous drying caused damage to the PEO/LiTFSI/THF clusters and the ion conductive network, especially around the electrode/electrolyte interfaces. Our findings shed light on improving the ionic conductivity of polymer electrolyte for lithium-ion secondary battery by carefully tailoring the solvent residue.

Bio-inspired construction of electrocatalyst decorated hierarchical porous carbon nanoreactors with enhanced mass transfer ability towards rapid polysulfide redox reactions

Li-S batteries are considered as a highly promising candidate for the next-generation energy storage system, attributing to their tremendous energy density. However, the two-dimensional island nucleation-growth process of lithium sulfide leads to a thick insulating film covering the electrode, inducing slow electrons transfer and mass-transfer of ions and liquid sulfur species in working Li-S cells. Here, we demonstrate a bio-inspired strategy of constructing ant-nest-like hierarchical porous ultrathin carbon nanosheet networks with the implants of metallic nanoparticles electrocatalysts (HPC-MEC) as efficient nanoreactors enabling rapid mass transfer, via a simple and green NaCl template. Such nanoreactors with a large active surface area could effectively anchor polysulfides for mitigating the shuttle effect, facilitating uniformly thin Li2S film, and promoting the mass transfer for fast sulfur species conversions. This helps contribute to a continuously high sulfur utilization in Li-S batteries with the HPC-MEC reactors. As a typical exhibition, cobalt embedded hierarchical porous carbon (HPC-Co) could realize to deliver a remarkably high specific capacity of 1,540.6 mAh·g−1, an excellent rate performance of 878.8 mAh·g−1 at 2 C, and high area capacity of 11.6 mAh·cm−2 at a high sulfur load of 10 mg·cm−2 and low electrolyte/sulfur ratio of 5 µL·mg−1.

Inspiration from surfactants gives Li-S cells longer life

Inspiration from surfactants gives Li-S cells longer life

2D Zr-Fc metal-organic frameworks with highly efficient anchoring and catalytic conversion ability towards polysulfides for advanced Li-S battery

The practical viability of Li-S battery is largely hampered by undesirable shuttling behavior and sluggish conversion kinetics of polysulfides. Considering that, a multifunctional interlayer, prepared by weaving ferrocene-based two-dimensional (2D) metal-organic framework (Zr-Fc MOF) nanosheets and carbon nanotubes (CNT) via simple vacuum filtration, is proposed to achieve high-performance Li-S battery. Using a combination of in situ Raman spectroscopy, first-principles calculations and systematic electrochemical evaluation, we unveil that the Zr-Fc MOF, as a new polysulfides confinement agent, can not only restrain polysulfides shuttling via both electrostatic attraction and chemical anchoring capability, but also provide excellent electrocatalytic effect on polysulfides redox kinetics. The intertwined CNT throughout Zr-Fc MOF nanosheets facilitates electron conductivity as well as sufficient exposure of capturing and catalyzing active sites of Zr-Fc MOF. Accordingly, attributed to the synergistic entrapping-catalysis-conversion effect, we demonstrate that Li-S cell coupling with the as-prepared Zr-Fc MOF/CNT interlayer exhibits a dramatically enhanced rate performance and outstanding cyclability (1500 cycles at 1 C with ultralow capacity decay rate of 0.027% under sulfur loading of 4.11 mg cm−2). This work will inspire more intelligent 2D MOF materials simultaneously with highly efficient trapping and catalytic conversion of polysulfides for high-energy and long-life Li-S battery.

Oxygen Vacancy-Rich Mixed-Valence Cerium MOF: An Efficient Separator Coating to High-Performance Lithium-Sulfur Batteries.

Mixed-valence metal-organic frameworks (MOFs) have exhibited unique potential in fields such as catalysis and gas separation. However, it is still an open challenge to prepare mixed-valence MOFs with isolated Ce(IV, III) arrays due to the easy formation of CeIII under the synthetic conditions for MOFs. Meanwhile, the performance of Li-S batteries is greatly limited by the fatal shuttle effect and the slow transmission rate of Li+ caused by the inherent characteristics of sulfur species. Here, we report a mixed-valence cerium MOF, named CSUST-1 (CSUST stands for Changsha University of Science and Technology), with isolated Ce(IV, III) arrays and abundant oxygen vacancies (OVs), synthesized as guided by the facile and elaborate kinetic stability study of UiO-66(Ce), to work as an efficient separator coating for circumventing both issues at the same time. Benefiting from the synergistic function of the Ce(IV, III) arrays (redox couples), the abundant OVs, and the open Ce sites within CSUST-1, the CSUST-1/CNT composite, as a separator coating material in the Li-S battery, can remarkably accelerate the redox kinetics of the polysulfides and the Li+ transportation. Consequently, the Li-S cell with the CSUST-1/CNT-coated separator exhibited a high initial specific capacity of 1468 mA h/g at 0.1 C and maintained long-term stability for a capacity of 538 mA h/g after 1200 cycles at 2 C with a decay rate of only 0.037% per cycle. Even at a high sulfur loading of 8 mg/cm2, the cell with the CSUST/CNT-coated separator still demonstrated excellent performance with an initial areal capacity of 8.7 mA h/cm2 at 0.1 C and retained the areal capacity of 6.1 mA h/cm2 after 60 cycles.

Constructing multifunctional solid electrolyte interface via in-situ polymerization for dendrite-free and low N/P ratio lithium metal batteries

Stable solid electrolyte interface (SEI) is highly sought after for lithium metal batteries (LMB) owing to its efficient electrolyte consumption suppression and Li dendrite growth inhibition. However, current design strategies can hardly endow a multifunctional SEI formation due to the non-uniform, low flexible film formation and limited capability to alter Li nucleation/growth orientation, which results in unconstrained dendrite growth and short cycling stability. Herein, we present a novel strategy to employ electrolyte additives containing catechol and acrylic groups to construct a stable multifunctional SEI by in-situ anionic polymerization. This self-smoothing and robust SEI offers multiple sites for Li adsorption and steric repulsion to constrain nucleation/growth process, leading to homogenized Li nanosphere formation. This isotropic nanosphere offers non-preferred Li growth orientation, rendering uniform Li deposition to achieve a dendrite-free anode. Attributed to these superiorities, a remarkable cycling performance can be obtained, i.e., high current density up to 10 mA cm−2, ultra-long cycle life over 8500 hrs operation, high cumulative capacity over 4.25 Ah cm−2 and stable cycling under 60 °C. A prolonged lifespan can also be achieved in Li-S and Li-LiFePO4 cells under lean electrolyte content, low N/P ratio or high temperature conditions. This facile strategy also promotes the practical application of LMB and enlightens the SEI design in related fields.