Hierarchical Free‐Standing Carbon‐Nanotube Paper Electrodes with Ultrahigh Sulfur‐Loading for Lithium–Sulfur Batteries

The rational combination of conductive nanocarbon with sulfur leads to the formation of composite cathodes that can take full advantage of each building block; this is an effective way to construct cathode materials for lithium–sulfur (Li–S) batteries with high energy density. Generally, the areal sulfur‐loading amount is less than 2.0 mg cm^?2, resulting in a low areal capacity far below the acceptable value for practical applications. In this contribution, a hierarchical free‐standing carbon nanotube (CNT)‐S paper electrode with an ultrahigh sulfur‐loading of 6.3 mg cm^?2 is fabricated using a facile bottom–up strategy. In the CNT–S paper electrode, short multi‐walled CNTs are employed as the short‐range electrical conductive framework for sulfur accommodation, while the super‐long CNTs serve as both the long‐range conductive network and the intercrossed mechanical scaffold. An initial discharge capacity of 6.2 mA·h cm^?2 (995 mA·h g^?1), a 60% utilization of sulfur, and a slow cyclic fading rate of 0.20%/cycle within the initial 150 cycles at a low current density of 0.05 C are achieved. The areal capacity can be further increased to 15.1 mA·h cm^?2 by stacking three CNT–S paper electrodes—resulting in an areal sulfur‐loading of 17.3 mg cm^?2—for the cathode of a Li–S cell. The as‐obtained free‐standing paper electrode are of low cost and provide high energy density, making them promising for flexible electronic devices based on Li–S batteries.     

3D Interconnected Porous Carbon Aerogels as Sulfur Immobilizers for Sulfur Impregnation for Lithium‐Sulfur Batteries with High Rate Capability and Cycling Stability

To eliminate capacity‐fading effects due to the loss of sulfur cathode materials as a result of polysulfide dissolution in lithium–sulfur (Li–S) cells, 3D carbon aerogel (CA) materials with abundant narrow micropores can be utilized as an immobilizer host for sulfur impregnation. The effects of S incorporation on microstructure, surface area, pore size distribution, and pore volume of the S/CA hybrids are studied. The electrochemical performance of the S/CA hybrids is investigated using electrochemical impedance spectroscopy, galvanostatical charge–discharge, and cyclic voltammetry techniques. The 3D porous S/CA hybrids exhibit significantly improved reversible capacity, high‐rate capability, and excellent cycling performance as a cathode electrode for Li–S batteries. The S/CA hybrid with an optimal incorporating content of 27% S shows an excellent reversible capacity of 820 mAhg^?1 after 50 cycles at a current density of 100 mAg^?1. Even at a current density of 3.2C (5280 mAg^?1), the reversible capacity of 27%S/CA hybrid can still maintain at 521 mAhg^?1 after 50 cycles. This strategy for the S/CA hybrids as cathode materials to utilize the abundant micropores for sulfur immobilizers for sulfur impregnation for Li–S battery offers a new way to solve the long‐term reversibility obstacle and provides guidelines for designing cathode electrode architectures.     

High‐Performance Carbon‐LiMnPO4 Nanocomposite Cathode for Lithium Batteries

A cathode material of an electrically conducting carbon‐LiMnPO₄ nanocomposite is synthesized by ultrasonic spray pyrolysis followed by ball milling. The effect of the carbon content on the physicochemical and electrochemical properties of this material is extensively studied. A LiMnPO₄ electrode with 30 wt% acetylene black (AB) carbon exhibits an excellent rate capability and good cycle life in cell tests at 55 and 25 °C. This electrode delivers a discharge capacity of 158 mAh g^?1 at 1/20 C, 126 mAh g^?1 at 1 C, and 107 mAh g^?1 at 2 C rate, which are the highest capacities reported so far for this type of electrode. Transmission electron microscopy and Mn dissolution results confirm that the carbon particles surrounding the LiMnPO₄ protect the electrode from HF attack, and thus lead to a reduction of the Mn dissolution that usually occurs with this electrode. The improved electrochemical properties of the C‐LiMnPO₄ electrode are also verified by electrochemical impedance spectroscopy.     

Mechanistic Investigation of Polymer-Based All-Solid-State Lithium/Sulfur Battery

Although employing solid polymer electrolyte (SPE) in all-solid-state lithium/sulfur (ASSLS) batteries is a promising approach to obtain a power source with both high energy density and safety, the actual performance of SPE-ASSLS batteries still lag behind conventional lithium/sulfur batteries with liquid ether electrolyte. In this work, combining characterization methods of X-ray photoelectron spectroscopy, in situ optical microscopy, and three-electrode measurement, a direct comparison between these two battery systems is made to reveal the mechanism behind their performance differences. In addition to polysulfides, it is found that the initial elemental sulfur can also dissolve into and diffuse through the SPE to reach the anode. Different from the shuttle effect that causes uniform corrosion on the anode in a liquid electrolyte, dissolved sulfur species in SPE unevenly passivate the anode surface and lead to the inhomogeneous Li⁺ plating/stripping at the anode/SPE solid-solid interface. Such inhomogeneity eventually causes void formation at the interface, which leads to the failure of SPE-ASSLS batteries. Based on this understanding, a protection interlayer is designed to inhibit the shuttling of sulfur species, and the modified SPE-ASSLS batteries show much-improved performance in cycle life.     

Bifunctional Separator with a Light‐Weight Carbon‐Coating for Dynamically and Statically Stable Lithium‐Sulfur Batteries

Sulfur is appealing as a high‐capacity cathode for rechargeable lithium batteries as it offers a high theoretical capacity of 1672 mA h g^?1 and is abundant. However, the commercialization of Li‐S batteries is hampered by fast capacity fade during both dynamic cell cycling and static cell resting. The poor electrochemical stability is due to polysulfide diffusion, leading to a short cycle life and severe self‐discharge. Here, we present the design of a bifunctional separator with a light‐weight carbon‐coating that integrates the two necessary components already inside the cell: the conductive carbon and the separator. With no extra additives, this bifunctional carbon‐coated separator allows the use of pure sulfur cathodes involving no complex composite synthesis process, provides a high initial discharge capacity of 1389 mA h g^?1 with excellent dynamic stability, and facilitates a high reversible capacity of 828 mA h g^?1 after 200 cycles. In addition, the static stability is evidenced by low self‐discharge and excellent capacity retention after a 3 month rest period.     

Effect of the Anion Activity on the Stability of Li Metal Anodes in Lithium‐Sulfur Batteries

With the significant progress made in the development of cathodes in lithium‐sulfur (Li‐S) batteries, the stability of Li metal anodes becomes a more urgent challenge in these batteries. Here the systematic investigation of the stability of the anode/electrolyte interface in Li‐S batteries with concentrated electrolytes containing various lithium salts is reported. It is found that Li‐S batteries using LiTFSI‐based electrolytes are more stable than those using LiFSI‐based electrolytes. The decreased stability is because the N–S bond in the FSI^? anion is fairly weak and the scission of this bond leads to the formation of lithium sulfate (LiSO_x) in the presence of polysulfide species. In contrast, in the LiTFSI‐based electrolyte, the lithium metal anode tends to react with polysulfide to form lithium sulfide (LiS_x), which is more reversible than LiSO_x formed in the LiFSI‐based electrolyte. This fundamental difference in the bond strength of the salt anions in the presence of polysulfide species leads to a large difference in the stability of the anode‐electrolyte interface and performance of the Li‐S batteries with electrolytes composed of these salts. Therefore, anion selection is one of the key parameters in the search for new electrolytes for stable operation of Li‐S batteries.     

Lithium Tritelluride as an Electrolyte Additive for Stabilizing Lithium Deposition and Enhancing Sulfur Utilization in Anode-Free Lithium–Sulfur Batteries

Despite the potential to become the next-generation energy storage technology, practical lithium–sulfur (Li–S) batteries are still plagued by the poor cyclability of the lithium-metal anode and sluggish conversion kinetics of S species. In this study, lithium tritelluride (LiTe₃), synthesized with a simple one-step process, is introduced as a novel electrolyte additive for Li–S batteries. LiTe₃ quickly reacts with lithium polysulfides and functions as a redox mediator to greatly improve the cathode kinetics and the utilization of active materials in the cathode. Moreover, the formation of a Li₂TeS₃/Li₂Te-enriched interphase layer on the anode surface enhances ionic transport and stabilizes Li deposition. By regulating the chemistry on both the anode and cathode sides, this additive enables a stable operation of anode-free Li–S batteries with only 0.1 m concentration in conventional ether-based electrolytes. The cell with the LiTe₃ additive retains 71% of the initial capacity after 100 cycles, while the control cell retains only 23%. More importantly, with high utilization of Te, the additive enables significantly better cyclability of anode-free pouch full-cells under lean electrolyte conditions.     

Nitrogen‐Doped Mesoporous Carbon Promoted Chemical Adsorption of Sulfur and Fabrication of High‐Areal‐Capacity Sulfur Cathode with Exceptional Cycling Stability for Lithium‐Sulfur Batteries

As one important component of sulfur cathodes, the carbon host plays a key role in the electrochemical performance of lithium‐sulfur (Li‐S) batteries. In this paper, a mesoporous nitrogen‐doped carbon (MPNC)‐sulfur nanocomposite is reported as a novel cathode for advanced Li‐S batteries. The nitrogen doping in the MPNC material can effectively promote chemical adsorption between sulfur atoms and oxygen functional groups on the carbon, as verified by X‐ray absorption near edge structure spectroscopy, and the mechanism by which nitrogen enables the behavior is further revealed by density functional theory calculations. Based on the advantages of the porous structure and nitrogen doping, the MPNC‐sulfur cathodes show excellent cycling stability (95% retention within 100 cycles) at a high current density of 0.7 mAh cm^‐2 with a high sulfur loading (4.2 mg S cm^‐2) and a sulfur content (70 wt%). A high areal capacity (≈3.3 mAh cm^‐2) is demonstrated by using the novel cathode, which is crucial for the practical application of Li‐S batteries. It is believed that the important role of nitrogen doping promoted chemical adsorption can be extended for development of other high performance carbon‐sulfur composite cathodes for Li‐S batteries.     

Nanoengineered Polypyrrole‐Coated Fe2O3@C Multifunctional Composites with an Improved Cycle Stability as Lithium‐Ion Anodes

Novel multifunctional composites composed of highly dispersed nanosized Fe₂O₃ particles, a tubular mesoporous carbon host, and a conductive polypyrrole (PPy) sealing layer are hierarchically assembled via two facile processes, including bottom‐up introduction of Fe₂O₃ nanoparticles in tubular mesoporous carbons, followed by in situ surface sealing with the PPy coating. Fe₂O₃ particles are well‐dispersed within the carbon matrix and PPy is spatially and selectively coated onto the external surface and the pore entrances of the Fe₂O₃@C composite, thereby bridging the composite particles together into a larger unit. As an anode material for Li‐ion batteries (LIBs), the PPy‐coated Fe₂O₃@C composite exhibits stable cycle performance. Additionally, the PPy‐coated Fe₂O₃@C composite also possesses fast electrode reaction kinetics, high Fe₂O₃ use efficiency, and large volumetric capacity. The excellent electrochemical performance is associated with a synergistic effect of the highly porous carbon matrix and the conducting PPy sealing layer. Such multifunctional configuration prevents the aggregation of NPs and maintains the structural integrity of active materials, in addition to effectively enhancing the electronic conductivity and warranting the stability of as‐formed solid electrolyte interface (SEI) films. This nanoengineering strategy might open new avenues for the design of other multifunctional composite architectures as electrode materials in order to achieve high‐performance LIBs.     

Gas Pickering Emulsion Templated Hollow Carbon for High Rate Performance Lithium Sulfur Batteries

A CO₂ in water nanoparticle stabilized Pickering emulsion is used to template micrometer sized hollow porous nitrogen doped carbon particles for high rate performance lithium sulfur battery. For the first time, nanoparticles serve the dual role of an emulsion stabilizer and a pore template for the shell, directly utilizing in situ generated CO₂ bubbles as template for the core. The minimalistic nature of this method does not require expensive surfactants or additional core templates. Upon polymerization of melamine formaldehyde onto CO₂, a robust polymer/silica composite shell is formed and transformed into a porous shell upon washing. The micrometer‐sized hollow morphology in combination with its nitrogen rich porous shell demonstrates impressive rate capabilities of 670 and 500 mAh g^?1 even at a high rate of 7C and 9C, respectively. This material also possesses excellent cycle durability, exhibiting a low capacity decay of 0.088%/cycle over 300 cycles. Measurement of the shuttle current and impedance provides interesting insight into the polysulfide mass transfer mechanism of hollow structured sulfur hosts.     

Magnesium Anodes with Extended Cycling Stability for Lithium‐Ion Batteries

Magnesium as a promising alloy‐type anode material for lithium‐ion batteries features both high theoretical specific capacity (2150 mAh g^?1) and stack energy density (1032 Wh L^?1). However, the poor cycling performance of Mg‐based anodes severely limits their application, mainly because high‐impedance films can grow easily on the surface of Mg and cause diminished electrochemical activity. As a result, the capacities of reported Mg anodes fade quickly in less than 100 cycles. To improve the stability of Mg anodes, 3D Cu@Mg@C structures are prepared by depositing Mg/C composite on 3D Cu current collectors. The resulting 3D Cu@Mg@C anodes can deliver an initial capacity of 1392 mAh g^?1. With a second‐cycle capacity of 1255 mAh g^?1, 91% can be retained after 1000 cycles at 0.5 C. When cycled at 2 C, the initial capacity can be maintained for 4000 cycles. This remarkably improved cycling performance can be attributed to both the 3D structure and the embedded carbon layers of the 3D Cu@Mg@C electrodes that facilitate electrical contact and prevent the growth of high‐impedance films during cycling. With 3D Cu@Mg@C anodes and LiFePO₄ cathodes, full cells are assembled and charging by a rotating triboelectric nanogenerator that can harvest mechanical energy is demonstrated.     

Functional Mesoporous Carbon‐Coated Separator for Long‐Life,High‐Energy Lithium–Sulfur Batteries

The lithium–sulfur (Li–S) battery is regarded as the most promising rechargeable energy storage technology for the increasing applications of clean energy transportation systems due to its remarkable high theoretical energy density of 2.6 kWh kg^?1, considerably outperforming today's lithium‐ion batteries. Additionally, the use of sulfur as active cathode material has the advantages of being inexpensive, environmentally benign, and naturally abundant. However, the insulating nature of sulfur, the fast capacity fading, and the short lifespan of Li–S batteries have been hampered their commercialization. In this paper, a functional mesoporous carbon‐coated separator is presented for improving the overall performance of Li–S batteries. A straightforward coating modification of the commercial polypropylene separator allows the integration of a conductive mesoporous carbon layer which offers a physical place to localize dissolved polysulfide intermediates and retain them as active material within the cathode side. Despite the use of a simple sulfur–carbon black mixture as cathode, the Li–S cell with a mesoporous carbon‐coated separator offers outstanding performance with an initial capacity of 1378 mAh g^?1 at 0.2 C, and high reversible capacity of 723 mAh g^?1, and degradation rate of only 0.081% per cycle, after 500 cycles at 0.5 C.     

Nanoarchitectured Graphene/CNT@Porous Carbon with Extraordinary Electrical Conductivity and Interconnected Micro/Mesopores for Lithium‐Sulfur Batteries

The sp²‐hybridized nanocarbon (e.g., carbon nanotubes (CNTs) and graphene) exhibits extraordinary mechanical strength and electrical conductivity but limited external accessible surface area and a small amount of pores, while nanostructured porous carbon affords a huge surface area and abundant pore structures but very poor electrical conductance. Herein the rational hybridization of the sp² nanocarbon and nanostructured porous carbon into hierarchical all‐carbon nanoarchitectures is demonstrated, with full inherited advantages of the component materials. The sp² graphene/CNT interlinked networks give the composites good electrical conductivity and a robust framework, while the meso‐/microporous carbon and the interlamellar compartment between the opposite graphene accommodate sulfur and polysulfides. The strong confinement induced by micro‐/mesopores of all‐carbon nanoarchitectures renders the transformation of S₈ crystal into amorphous cyclo‐S₈ molecular clusters, restraining the shuttle phenomenon for high capacity retention of a lithium‐sulfur cell. Therefore, the composite cathode with an ultrahigh specific capacity of 1121 mAh g^?1 at 0.5 C, a favorable high‐rate capability of 809 mAh g^?1 at 10 C, a very low capacity decay of 0.12% per cycle, and an impressive cycling stability of 877 mAh g^?1 after 150 cycles at 1 C. As sulfur loading increases from 50 wt% to 77 wt%, high capacities of 970, 914, and 613 mAh g^?1 are still available at current densities of 0.5, 1, and 5 C, respectively. Based on the total mass of packaged devices, gravimetric energy density of GSH@APC‐S//Li cell is expected to be 400 Wh kg^?1 at a power density of 10 000 W kg^?1, matching the level of engine driven systems.     

Carbonized Polyacrylonitrile‐Stabilized SeSx Cathodes for Long Cycle Life and High Power Density Lithium Ion Batteries

A facile synthesis of selenium sulfide (SeS_x)/carbonized polyacrylonitrile (CPAN) composites is achieved by annealing the mixture of SeS₂ and polyacrylonitrile (PAN) at 600 °C under vacuum. The SeS_x molecules are confined by N‐containing carbon (ring) structures in the carbonized PAN to mitigate the dissolution of polysulfide and polyselenide intermediates in carbonate‐based electrolyte. In addition, formation of solid electrolyte interphase (SEI) on the surface of SeS_x/CPAN electrode in the first cycle further prevents polysulfide and polyselenide intermediates from dissolution. The synergic restriction of SeS_x by both CPAN matrix and SEI layer allows SeS_x/CPAN composites to be charged and discharged in a low‐cost carbonate‐based electrolyte (LiPF₆ in EC/DEC) with long cycling stability and high rate capability. At a current density of 600 mA g^?1, it maintains a reversible capacity of 780 mAh g^?1 for 1200 cycles. Moreover, it retains 50% of the capacity at 60 mA g^?1 even when the current density increases to 6 A g^?1. The superior electrochemical performance of SeS_x/CPAN composite demonstrates that it is a promising cathode material for long cycle life and high power density lithium ion batteries. This is the first report on long cycling stability and high rate capability of selenium sulfide‐based cathode material.     

Challenges and Solutions for Lithium–Sulfur Batteries with Lean Electrolyte

Lithium–sulfur (Li–S) batteries have high theoretical energy density and are regarded as next-generation batteries. However, their practical energy density is much lower than the theoretical value. In previous studies, the increase of the areal capacity of the cathode and the decrease of the negative/positive ratio can be well achieved, yet the energy density shows no corresponding increase. The main reason is the difficulty in decreasing electrolyte dosage because lean electrolyte inevitably causes the deterioration of reaction kinetics and sulfur utilization. Thus, the electrolyte/active material ratio in the reported works is usually higher than 10 µL mg⁻¹, much higher than that in Li-ion batteries (usually lower than ≈0.3 µL mg⁻¹ for cathode). Although many works have focused on this topic, a systematic discussion is still rare. This review systematically discusses the key challenges and solutions for assembling high-performance lean-electrolyte Li–S batteries. First, the key challenges arising from lean-electrolyte conditions are discussed in detail. Then, the approaches and the recent progress to reduce electrolyte usage, including optimization of electrode porosity and ion conduction, the introduction of electrocatalysis, exploration of new active materials, electrolyte regulation, and Li metal protection are reviewed. Finally, future research directions in lean-electrolyte Li–S batteries are proposed.     

Long Cycle Life Lithium Metal Batteries Enabled with Upright Lithium Anode

The lithium metal anode is the holy grail of the battery field due to its lowest reduction potential and high specific capacity; however, its application is hindered by severe safety hazards and inferior cyclic stability due to dendrites and unstable solid electrolyte interphase (SEI). Aiming at these problems, a coiled Li anode with a unique upright structure is proposed. The upright structure endows coiled Li anode with abundant inner reaction interface/space/mass for lithium deposit/storage/transport, which can induce the inner growth of Li dendrites and SEI. The Li⁺ transport/deposit behavior and mechanism of coiled Li anode are clarified via in situ observation and numerical simulation. Benefiting from the small volume expansion and sufficient Li⁺ transport, the coiled Li anodes combined with Li₄Ti₅O₁₂ cathodes achieve a long life of over 2000 cycles at 5C with a reversible capacity of 129 mAh g^?1 and 100% Coulombic efficiency.     

Toward High Performance Thiophene‐Containing Conjugated Microporous Polymer Anodes for Lithium‐Ion Batteries through Structure Design

Poly(thiophene) as a kind of n‐doped conjugated polymer with reversible redox behavior can be employed as anode material for lithium‐ion batteries (LIBs). However, the low redox activity and poor rate performance for the poly(thiophene)‐based anodes limit its further development. Herein, a structure‐design strategy is reported for thiophene‐containing conjugated microporous polymers (CMPs) with extraordinary electrochemical performance as anode materials in LIBs. The comparative study on the electrochemical performance of the structure‐designed thiophene‐containing CMPs reveals that high redox‐active thiophene content, highly crosslinked porous structure, and improved surface area play significant roles for enhancing electrochemical performances of the resulting CMPs. The all‐thiophene‐based polymer of poly(3,3′‐bithiophene) with crosslinked structure and a high surface area of 696 m² g^?1 exhibits a discharge capacity of as high as 1215 mAh g^?1 at 45 mA g^?1, excellent rate capability, and outstanding cycling stability with a capacity retention of 663 mAh g^?1 at 500 mA g^?1 after 1000 cycles. The structure–performance relationships revealed in this work offer a fundamental understanding in the rational design of CMPs anode materials for high performance LIBs.     

Activation of Oxygen‐Stabilized Sulfur for Li and Na Batteries

The sulfur‐based cathode materials suffer severely from poor cycling stability and low utilization, incurred by their stepwise reaction mechanism that generates polysulfide intermediates and the subsequent irreversible losses. In this work, those issues are significantly relieved by entrapping sulfur species in carbon host rich in oxygen functionalities. Sulfur species in such C/S composite are highly stabilized by their interaction with oxygen, and can deliver a reversible capacity of 508 mAh/(g of S) for 2000 cycles when coupled with Li, representing the best cycling stability up to date. More interestingly, extra capacity can be accessed by simply prelithiating the oxygen‐stabilized C/S composites down to 0.6 V for a few cycles, which enables a high capacity of 1621 mAh/(g of S) that eventually stabilizes at 820 mAh/(g of S) for 600 cycles. The mechanism for this electrochemical activation process is investigated with both spectroscopic and electrochemical techniques, which reveal that the inactive sulfur bonded to oxygen is liberated in the initial deep lithiation precycles and becomes electrochemically active. The oxygen‐stabilized sulfur can also be coupled with Na anode to form Na/S cell, confirming that the formation of S?O interaction in C/S composite generates promising sulfur‐based cathode materials for Li–S and Na–S batteries.