A Bifunctional Perovskite Promoter for Polysulfide Regulation toward Stable Lithium–Sulfur Batteries

Lithium‐sulfur (Li? S) batteries are strongly considered as the next‐generation rechargeable cells. However, both the shuttle of lithium polysulfides (LiPSs) and sluggish kinetics in random deposition of lithium sulfides (Li₂S) significantly degrade the capacity, rate performance, and cycling life of Li? S cells. Herein, bifunctional Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ perovskite nanoparticles (PrNPs) are proposed as a promoter to immobilize LiPSs and guide the deposition of Li₂S in a Li? S cell. The oxygen vacancy in PrNPs increases the metal reactivity to anchor LiPSs, and co‐existence of lithiophilic (O) and sulfiphilic (Sr) sites in PrNP favor the dual‐bonding (Li? O and Sr? S bonds) to anchor LiPSs. The high catalytic nature of PrNP facilitates the kinetics of LiPS redox reaction. The PrNP with intrinsic LiPS affinity serves as nucleation sites for Li₂S deposition and guides its uniform propagation. Therefore, the bifunctional LiPS promoter in Li? S cell yields high rate performance and ultralow capacity decay rate of 0.062% (a quarter of pristine Li? S cells). The proposed strategy to immobilize LiPSs, promotes the conversion of LiPS, and regulates deposition of Li₂S by an emerging perovskite promoter and is also expected to be applied in other energy conversion and storage devices based on multi‐electron redox reactions.     

Self‐Supported and Flexible Sulfur Cathode Enabled via Synergistic Confinement for High‐Energy‐Density Lithium–Sulfur Batteries

Lithium–sulfur (Li–S) batteries have attracted much attention in the field of electrochemical energy storage due to their high energy density and low cost. However, the “shuttle effect” of the sulfur cathode, resulting in poor cyclic performance, is a big barrier for the development of Li–S batteries. Herein, a novel sulfur cathode integrating sulfur, flexible carbon cloth, and metal–organic framework (MOF)‐derived N‐doped carbon nanoarrays with embedded CoP (CC@CoP/C) is designed. These unique flexible nanoarrays with embedded polar CoP nanoparticles not only offer enough voids for volume expansion to maintain the structural stability during the electrochemical process, but also promote the physical encapsulation and chemical entrapment of all sulfur species. Such designed CC@CoP/C cathodes with synergistic confinement (physical adsorption and chemical interactions) for soluble intermediate lithium polysulfides possess high sulfur loadings (as high as 4.17 mg cm^–2) and exhibit large specific capacities at different C‐rates. Specially, an outstanding long‐term cycling performance can be reached. For example, an ultralow decay of 0.016% per cycle during the whole 600 cycles at a high current density of 2C is displayed. The current work provides a promising design strategy for high‐energy‐density Li–S batteries.     

Greatly Improved Conductivity of Double‐Chain Polymer Network Binder for High Sulfur Loading Lithium–Sulfur Batteries with a Low Electrolyte/Sulfur Ratio

Binders have been considered to play a key role in realizing high‐energy‐density lithium–sulfur batteries. However, the accompanying problems of limited conductivity and inferior affinity of soluble polysulfide intermediates bring down their comprehensive performance for practical applications. Herein, the synthesis of a novel double‐chain polymer network (DCP) binder by polymerizing 4,4′‐biphenyldisulfonic acid connected pyrrole monomer onto viscous sodium carboxymethyl cellulose matrix, yielding a primary crystal structure is reported. Consequently, the resulted binder enables superior rate performance from 0.2 C (1326.9 mAh g^?1) to 4 C (701.4 mAh g^?1). Moreover, a high sulfur loading of 9.8 mg cm^?2 and a low electrolyte/sulfur ratio (5:1, µL mg^?1) are achieved, exhibiting a high area capacity of 9.2 mAh cm^?2. In situ X‐ray diffraction analysis is conducted to monitor the structural modifications of the cathode, confirming the occurrence of sulfur reduction/recrystallization during charge–discharge process. In addition, in situ UV–vis measurements demonstrate that DCP binder impedes the polysulfide migration, thereby giving rise to high capacity retention for 400 cycles.     

A Praline‐Like Flexible Interlayer with Highly Mounted Polysulfide Anchors for Lithium–Sulfur Batteries

The development of lithium–sulfur (Li–S) batteries is dogged by the rapid capacity decay arising from polysulfide dissolution and diffusion in organic electrolytes. To solve this critical issue, a praline‐like flexible interlayer consisting of high‐loading titanium oxide (TiO₂) nanoparticles and relatively long carbon nanofibers is fabricated. TiO₂ nanoparticles with a size gradient occupy both the external and internal of carbon fiber and serve as anchors that allow the chemical adsorption of polysulfides through a conductive nanoarchitecture. The porous conductive carbon backbone helps in the physical absorption of polysulfides and provides redox reaction sites to allow the polysulfides to be reused. More importantly, it offers enough mechanical strength to support a high load TiO₂ nanoparticle (79 wt%) that maximizes their chemical role, and can accommodate the large volume changes. Significant enhancement in cycle stability and rate capability is achieved for a readily available sulfur/multi‐walled carbon nanotube composite cathode simply by incorporating this hierarchically nanostructured interlayer. The design and synthesis of interlayers by in situ integration of metal oxides and carbon fibers via a simple route offers the potential to advance Li–S batteries for practical applications in the future.     

Yolk–Shelled C@Fe3O4 Nanoboxes as Efficient Sulfur Hosts for High‐Performance Lithium–Sulfur Batteries

Owing to the high theoretical specific capacity (1675 mA h g^?1) and low cost, lithium–sulfur (Li–S) batteries offer advantages for next‐generation energy storage. However, the polysulfide dissolution and low electronic conductivity of sulfur cathodes limit the practical application of Li–S batteries. To address such issues, well‐designed yolk–shelled carbon@Fe₃O₄ (YSC@Fe₃O₄) nanoboxes as highly efficient sulfur hosts for Li–S batteries are reported here. With both physical entrapment by carbon shells and strong chemical interaction with Fe₃O₄ cores, this unique architecture immobilizes the active material and inhibits diffusion of the polysulfide intermediates. Moreover, due to their high conductivity, the carbon shells and the polar Fe₃O₄ cores facilitate fast electron/ion transport and promote continuous reactivation of the active material during the charge/discharge process, resulting in improved electrochemical utilization and reversibility. With these merits, the S/YSC@Fe₃O₄ cathodes support high sulfur content (80 wt%) and loading (5.5 mg cm^?2) and deliver high specific capacity, excellent rate capacity, and long cycling stability. This work provides a new perspective to design a carbon/metal‐oxide‐based yolk–shelled framework as a high sulfur‐loading host for advanced Li–S batteries with superior electrochemical properties.     

Rechargeable Calcium–Sulfur Batteries Enabled by an Efficient Borate‐Based Electrolyte

Rechargeable metal–sulfur batteries show great promise for energy storage applications because of their potentially high energy and low cost. The multivalent‐metal based electrochemical system exhibits the particular advantage of the feasibility of dendrite‐free metal anode. Calcium (Ca) represents a promising anode material owing to the low reductive potential, high capacity, and abundant natural resources. However, calcium–sulfur (Ca–S) battery technology is in an early R&D stage, facing the fundamental challenge to develop a suitable electrolyte enabling reversible electrochemical Ca deposition, and at the same time, sulfur redox reactions in the system. Herein, a study of a room‐temperature Ca–S battery by employing a stable and efficient calcium tetrakis(hexafluoroisopropyloxy) borate Ca[B(hfip)₄]₂ electrolyte is presented. The Ca–S batteries exhibit a cell voltage of ≈ 2.1 V (close to its thermodynamic value) and good reversibility. The mechanistic studies hint at a redox chemistry of sulfur with polysulfide/sulfide species involved in the Ca‐based system.     

Rational Design of Statically and Dynamically Stable Lithium–Sulfur Batteries with High Sulfur Loading and Low Electrolyte/Sulfur Ratio

The primary challenge with lithium–sulfur battery research is the design of sulfur cathodes that exhibit high electrochemical efficiency and stability while keeping the sulfur content and loading high and the electrolyte/sulfur ratio low. With a systematic investigation, a novel graphene/cotton‐carbon cathode is presented here that enables sulfur loading and content as high as 46 mg cm^?2 and 70 wt% with an electrolyte/sulfur ratio of as low as only 5. The graphene/cotton‐carbon cathodes deliver peak capacities of 926 and 765 mA h g^?1, respectively, at C/10 and C/5 rates, which translate into high areal, gravimetric, and volumetric capacities of, respectively, 43 and 35 mA h cm^?2, 648 and 536 mA h g^?1, and 1067 and 881 mA h cm^?3 with a stable cyclability. They also exhibit superior cell‐storage capability with 95% capacity‐retention, a low self‐discharge constant of just 0.0012 per day, and stable poststorage cyclability after storing over a long period of six months. This work demonstrates a viable approach to develop lithium–sulfur batteries with practical energy densities exceeding that of lithium‐ion batteries.     

Stabilizing Lithium–Sulfur Batteries through Control of Sulfur Aggregation and Polysulfide Dissolution

Lithium–sulfur (Li–S) batteries are investigated intensively as a promising large‐scale energy storage system owing to their high theoretical energy density. However, the application of Li–S batteries is prevented by a series of primary problems, including low electronic conductivity, volumetric fluctuation, poor loading of sulfur, and shuttle effect caused by soluble lithium polysulfides. Here, a novel composite structure of sulfur nanoparticles attached to porous‐carbon nanotube (p‐CNT) encapsulated by hollow MnO₂ nanoflakes film to form p‐CNT@Void@MnO₂/S composite structures is reported. Benefiting from p‐CNTs and sponge‐like MnO₂ nanoflake film, p‐CNT@Void@MnO₂/S provides highly efficient pathways for the fast electron/ion transfer, fixes sulfur and Li₂S aggregation efficiently, and prevents polysulfide dissolution during cycling. Besides, the additional void inside p‐CNT@Void@MnO₂/S composite structure provides sufficient free space for the expansion of encapsulated sulfur nanoparticles. The special material composition and structural design of p‐CNT@Void@MnO₂/S composite structure with a high sulfur content endow the composite high capacity, high Coulombic efficiency, and an excellent cycling stability. The capacity of p‐CNT@Void@MnO₂/S electrode is ≈599.1 mA h g^?1 for the fourth cycle and ≈526.1 mA h g^?1 after 100 cycles, corresponding to a capacity retention of ≈87.8% at a high current density of 1.0 C.     

MoS2/Celgard Separator as Efficient Polysulfide Barrier for Long‐Life Lithium–Sulfur Batteries

A high lithium conductive MoS₂/Celgard composite separator is reported as efficient polysulfides barrier in Li–S batteries. Significantly, thanks to the high density of lithium ions on MoS₂ surface, this composite separator shows high lithium conductivity, fast lithium diffusion, and facile lithium transference. When used in Li–S batteries, the separator is proven to be highly efficient for depressing polysulfides shuttle, leading to high and long cycle stability. With 65% of sulfur loading, the device with MoS₂/Celgard separator delivers an initial capacity of 808 mAh g^?1 and a substantial capacity of 401 mAh g^?1 after 600 cycles, corresponding to only 0.083% of capacity decay per cycle that is comparable to the best reported result so far. In addition, the Coulombic efficiency remains more than 99.5% during all 600 cycles, disclosing an efficient ionic sieve preventing polysulfides migration to the anode while having negligible influence on Li⁺ ions transfer across the separator. The strategy demonstrated in this work will open the door toward developing efficient separators with flexible 2D materials beyond graphene for energy‐storage devices.     

Long‐Life Lithium–Sulfur Batteries with a Bifunctional Cathode Substrate Configured with Boron Carbide Nanowires

Developing high‐energy‐density lithium–sulfur (Li–S) batteries relies on the design of electrode substrates that can host a high sulfur loading and still attain high electrochemical utilization. Herein, a new bifunctional cathode substrate configured with boron‐carbide nanowires in situ grown on carbon nanofibers (B₄C@CNF) is established through a facile catalyst‐assisted process. The B₄C nanowires acting as chemical‐anchoring centers provide strong polysulfide adsorptivity, as validated by experimental data and first‐principle calculations. Meanwhile, the catalytic effect of B₄C also accelerates the redox kinetics of polysulfide conversion, contributing to enhanced rate capability. As a result, a remarkable capacity retention of 80% after 500 cycles as well as stable cyclability at 4C rate is accomplished with the cells employing B₄C@CNF as a cathode substrate for sulfur. Moreover, the B₄C@CNF substrate enables the cathode to achieve both high sulfur content (70 wt%) and sulfur loading (10.3 mg cm^?2), delivering a superb areal capacity of 9 mAh cm^?2. Additionally, Li–S pouch cells fabricated with the B₄C@CNF substrate are able to host a high sulfur mass of 200 mg per cathode and deliver a high discharge capacity of 125 mAh after 50 cycles.     

Pyrrolic‐Type Nitrogen‐Doped Hierarchical Macro/Mesoporous Carbon as a Bifunctional Host for High‐Performance Thick Cathodes for Lithium‐Sulfur Batteries

Lithium‐sulfur (Li‐S) batteries are highly considered as a next‐generation energy storage device due to their high theoretical energy density. For practical viability, reasonable active‐material loading of >4.0 mg cm^?2 must be employed, at a cost to the intrinsic instability of sulfur cathodes. The incursion of lithium polysulfides (LiPS) at higher sulfur loadings results in low active material utilization and poor cell cycling capability. The use of high‐surface‐area hierarchical macro/mesoporous inverse opal (IOP) carbons to investigate the effects of pore volume and surface area on the electrochemical stability of high‐loading, high‐thickness cathodes for Li‐S batteries is presented here. The IOP carbons are additionally doped with pyrrolic‐type nitrogen groups (N‐IOP) to act as a polar polysulfide mediator and enhance the active‐material reutilization. With a high sulfur loading of 6.0 mg cm^?2, the Li‐S cells assembled with IOP and N‐IOP carbons are able to attain a high specific capacity of, respectively, 1242 and 1162 mA h g^?1. The N‐IOP enables the Li‐S cells to demonstrate good electrochemical performance over 300 cycles.