Tuning Transition Metal Oxide–Sulfur Interactions for Long Life Lithium Sulfur Batteries: The “Goldilocks” Principle

The lithium‐sulfur battery is a compelling energy storage system because its high theoretical energy density exceeds Li‐ion batteries at much lower cost, but applications are thwarted by capacity decay caused by the polysulfide shuttle. Here, proof of concept and the critical metrics of a strategy to entrap polysulfides within the sulfur cathode by their reaction to form a surface‐bound active redox mediator are demonstrated. It is shown through a combination of surface spectroscopy and cyclic voltammetry studies that only materials with redox potentials in a targeted window react with polysulfides to form active surface‐bound polythionate species. These species are directly correlated to superior Li‐S cell performance by electrochemical studies of high surface area oxide cathodes with redox potentials below, above, and within this window. Optimized Li‐S cells yield a very low fade rate of 0.048% per cycle. The insight gained into the fundamental surface mechanism and its correlation to the stability of the electrochemical cell provides a bridge between mechanistic understanding and battery performance essential for the design of high performance Li‐S cells.     

A Fluorination Method for Improving Cation‐Disordered Rocksalt Cathode Performance

In Li‐rich cation‐disordered rocksalt oxide cathodes (DRX), partial fluorine substitution in the oxygen anion sublattice can increase the capacity contribution from transition‐metal (TM) redox while reducing that from the less reversible oxygen redox. To date, limited fluorination substitution has been achieved by introducing LiF precursor during the solid‐state synthesis. To take full advantage of the fluorination effect, however, a higher F content is desired. In the present study, the successful use of a fluorinated polymeric precursor is reported to increase the F solubility in DRX and the incorporation of F content up to 10–12.5 at% into the rocksalt lattice of a model Li‐Mn‐Nb‐O (LMNO) system, largely exceeding the 7.5 at% limit achieved with LiF synthesis. Higher F content in the fluorinated‐DRX (F‐DRX) significantly improves electrochemical performance, with a reversible discharge capacity of ≈255 mAh g^?1 achieved at 10 at% of F substitution. After 30 cycles, up to a 40% increase in capacity retention is achieved through the fluorination. The study demonstrates the feasibility of using a new and effective fluorination process to synthesize advanced DRX cathode materials.     

Boosting Polysulfide Redox Kinetics by Graphene‐Supported Ni Nanoparticles with Carbon Coating

Lithium–sulfur batteries have attracted extensive attention because of their high energy density. However, their application is still impeded by the inherent sluggish kinetics and solubility of intermediate products (i.e., polysulfides) of the sulfur cathode. Herein, graphene‐supported Ni nanoparticles with a carbon coating are fabricated by directly carbonizing a metal–organic framework/graphene oxide composite, which is then dispersed on a commercial glass fiber membrane to form a separator with electrocatalytic activity. In situ analysis and electrochemical investigation demonstrate that this modified separator can effectively suppress the shuttle effect and regulate the catalytic conversion of intercepted polysulfides, which is also confirmed by density functional theory calculations. It is found that Ni–C sites can chemically interact with polysulfides and stabilize the radical S₃^?? through Ni? S bonds to enable fast dynamic equilibrium with S₆^2?, while Ni nanoparticles reduce the oxidation barrier of Li₂S and accelerate ion/electron transport. As a result, the corresponding lithium–sulfur battery shows a high cycle stability (88% capacity retention over 100 cycles) even with a high sulfur mass loading of 8 mg cm^?2 and lean electrolyte (6.25 µ L mg^?1). Surprisingly, benefitting from the improved kinetics, the battery can work well at ?50 °C, which is rarely achieved by conventional Li–S batteries.     

High‐Fluorinated Electrolytes for Li–S Batteries

Rechargeable Li–S batteries are regarded as one of the most promising next‐generation energy‐storage systems. However, the inevitable formation of Li dendrites and the shuttle effect of lithium polysulfides significantly weakens electrochemical performance, preventing its practical application. Herein, a new class of localized high‐concentration electrolyte (LHCE) enabled by adding inert fluoroalkyl ether of 1H,1H,5H‐octafluoropentyl‐1,1,2,2‐tetrafluoroethyl ether into highly‐concentrated electrolytes (HCE) lithium bis(fluorosulfonyl) imide/dimethoxyether (DME) system is reported to suppress Li dendrite formation and minimize the solubility of the high‐order polysulfides in electrolytes, thus reducing the amount of electrolyte in cells. Such a unique LHCE can achieve a high coulombic efficiency of Li plating/stripping up to 99.3% and completely suppressing the shuttling effect, thus maintaining a S cathode capacity of 775 mAh g^?1 for 150 cycles with a lean electrolyte of 4.56 g A^?1 h^?1. The LHCE reduces the solubility of lithium polysulfides, allowing the Li/S cell to achieve super performance in a lean electrolyte. This conception of using inert diluents in a highly concentrated electrolyte can accelerate commercialization of Li–S battery technology.     

Elastic Sandwich‐Type rGO–VS2/S Composites with High Tap Density: Structural and Chemical Cooperativity Enabling Lithium–Sulfur Batteries with High Energy Density

Driven by increasing demand for high‐energy‐density batteries for consumer electronics and electric vehicles, substantial progress is achieved in the development of long‐life lithium–sulfur (Li–S) batteries. Less attention is given to Li–S batteries with high volume energy density, which is crucial for applications in compact space. Here, a series of elastic sandwich‐structured cathode materials consisting of alternating VS₂‐attached reduced graphene oxide (rGO) sheets and active sulfur layers are reported. Due to the high polarity and conductivity of VS₂, a small amount of VS₂ can suppress the shuttle effect of polysulfides and improve the redox kinetics of sulfur species in the whole sulfur layer. Sandwich‐structured rGO–VS₂/S composites exhibit significantly improved electrochemical performance, with high discharge capacities, low polarization, and excellent cycling stability compared with their bare rGO/S counterparts. Impressively, the tap density of rGO–VS₂/S with 89 wt% sulfur loading is 1.84 g cm^?3, which is almost three times higher than that of rGO/S with the same sulfur content (0.63 g cm^?3), and the volumetric specific capacity of the whole cell is as high as 1182.1 mA h cm^?3, comparable with the state‐of‐the‐art reported for energy storage devices, demonstrating the potential for application of these composites in long‐life and high‐energy‐density Li–S batteries.     

Catalytic Polysulfide Conversion and Physiochemical Confinement for Lithium–Sulfur Batteries

The lithium–sulfur (Li–S) battery is widely regarded as a promising energy storage device due to its low price and the high earth‐abundance of the materials employed. However, the shuttle effect of lithium polysulfides (LiPSs) and sluggish redox conversion result in inefficient sulfur utilization, low power density, and rapid electrode deterioration. Herein, these challenges are addressed with two strategies 1) increasing LiPS conversion kinetics through catalysis, and 2) alleviating the shuttle effect by enhanced trapping and adsorption of LiPSs. These improvements are achieved by constructing double‐shelled hollow nanocages decorated with a cobalt nitride catalyst. The N‐doped hollow inner carbon shell not only serves as a physiochemical absorber for LiPSs, but also improves the electrical conductivity of the electrode; significantly suppressing shuttle effect. Cobalt nitride (Co₄N) nanoparticles, embedded in nitrogen‐doped carbon in the outer shell, catalyze the conversion of LiPSs, leading to decreased polarization and fast kinetics during cycling. Theoretical study of the Li intercalation energetics confirms the improved catalytic activity of the Co₄N compared to metallic Co catalyst. Altogether, the electrode shows large reversible capacity (1242 mAh g^?1 at 0.1 C), robust stability (capacity retention of 658 mAh g^?1 at 5 C after 400 cycles), and superior cycling stability at high sulfur loading (4.5 mg cm^?2).     

Adsorption‐Catalysis Design in the Lithium‐Sulfur Battery

Lithium‐sulfur (Li‐S) batteries are one of the most promising next‐generation energy‐storage systems. Nevertheless, the sluggish sulfur redox and shuttle effect in Li‐S batteries are the major obstacles to their commercial application. Previous investigations on adsorption for LiPSs have made great progress but cannot restrain the shuttle effect. Catalysts can enhance the reaction kinetics, and then alleviate the shuttle effect. The synergistic relationship between adsorption and catalysis has become the hotspot for research into suppressing the shuttle effect and improving battery performance. Herein, the adsorption‐catalysis synergy in Li‐S batteries is reviewed, the adsorption‐catalysis designs are divided into four categories: adsorption‐catalysis for LiPSs aggregation, polythionate or thiosulfate generation, and sulfur radical formation, as well as other adsorption‐catalysis. Then advanced strategies, future perspectives, and challenges are proposed to aim at long‐life and high‐efficiency Li‐S batteries.     

Understanding Performance Degradation in Cation‐Disordered Rock‐Salt Oxide Cathodes

The collective redox activities of transition‐metal (TM) cations and oxygen anions have been shown to increase charge storage capacity in both Li‐rich layered and cation‐disordered rock‐salt cathodes. Repeated cycling involving anionic redox is known to trigger TM migration and phase transformation in layered Li‐ and Mn‐rich (LMR) oxides, however, detailed mechanistic understanding on the recently discovered Li‐rich rock‐salt cathodes is largely missing. The present study systematically investigates the effect of oxygen redox on a Li_1.3Nb_0.3Mn_0.4O₂ cathode and demonstrates that performance deterioration is directly correlated to the extent of oxygen redox. It is shown that voltage fade and hysteresis begin only after initiating anionic redox at high voltages, which grows progressively with either deeper oxidation of oxygen at higher potential or extended cycling. In contrast to what is reported on layered LMR oxides, extensive TM reduction is observed but phase transition is not detected in the cycled oxide. A densification/degradation mechanism is proposed accordingly which elucidates how a unique combination of extensive chemical reduction of TM and reduced quality of the Li percolation network in cation‐disordered rock‐salts can lead to performance degradation in these newer cathodes with 3D Li migration pathways. Design strategies to achieve balanced capacity and stability are also discussed.     

Stabilization of Li Metal Anode in DMSO‐Based Electrolytes via Optimization of Salt–Solvent Coordination for Li–O2 Batteries

The conventional electrolyte of 1 m lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in dimethyl sulfoxide (DMSO) is unstable against the Li metal anode and therefore cannot be used directly in practical Li–O₂ batteries. Here, we demonstrate that a highly concentrated electrolyte based on LiTFSI in DMSO (with a molar ratio of 1:3) can greatly improve the stability of the Li metal anode against DMSO and significantly improve the cycling stability of Li–O₂ batteries. This highly concentrated electrolyte contains no free DMSO solvent molecules, but only complexes of (TFSI^?)_a ? Li⁺? (DMSO)_b (where a + b = 4), and thus enhances their stability with Li metal anodes. In addition, such salt–solvent complexes have higher Gibbs activation energy barriers than the free DMSO solvent molecules, indicating improved stability of the electrolyte against the attack of superoxide radical anions. Therefore, the stability of this highly concentrated electrolyte at both Li metal anodes and carbon‐based air electrodes has been greatly enhanced, resulting in improved cycling performance of Li–O₂ batteries. The fundamental stability of the electrolyte in the absence of free‐solvent against the chemical and electrochemical reactions can also be used to enhance the stability of other electrochemical systems.     

Synthesis and Electrochemical Activity of Some Na(Li)‐Rich Ruthenium Oxides with the Feasibility to Stabilize Ru6+

The capacity of Li‐ion cathode materials has recently been greatly improved by the feasibility to trigger both cationic and anionic redox reactions within the same material. This concept has rapidly been implemented to Na‐ion batteries to boost their energy density. The electrochemical properties of Na₃RuO₄ with Li₃RuO₄ are reported and compared herein. Strikingly, it is found that 3 Na can be extracted from Na₃RuO₄ with the charge compensation mechanism enlisting first the oxidation of Ru⁵⁺ to Ru⁶⁺, leading to Na₂RuO₄, and then the oxidation of oxygen during the rest of the charge. This drastically contrasts with the behavior of the Li counterpart since Ru never reaches that high oxidation state during lithium removal. By comparing the phase diagrams of A_xRuO₄ (A being Li, Na, or K) together with density functional theory calculations, this finding is rationalized and it is demonstrated that this difference is mainly rooted in the size of the alkali cation. The bigger the alkali, the lower the coordination of Ru will be, stabilized by the same higher oxidation states. This work highlights the difference between Li and Na materials toward anionic redox and suggests the unusual coexistence of Ru⁶⁺ and (O₂)^n?, hence setting new challenges to theoreticians and opening opportunities for materials design.     

Unraveling the Rapid Redox Behavior of Li‐Excess 3d‐Transition Metal Oxides for High Rate Capability

Li‐excess 3d‐transition metal layered oxides are promising candidates in high‐energy‐density cathode materials for improving the mileage of electric vehicles. However, their low rate capability has hindered their practical application. The lack of understanding about the redox reactions and migration behavior at high C‐rates make it difficult to design Li‐excess materials with high rate capability. In this study, the characteristics of the atomic behavior that is predominant at fast charge/discharge are investigated by comparing cation‐ordered and cation‐disordered materials using X‐ray absorption spectroscopy (XAS). The difference in the atomic arrangement determines the dominance of the transition metal/oxygen redox reaction and the variations in transition metal–oxygen hybridization. In‐depth electrochemical analysis is combined with operando XAS analysis to reveal electronically and structurally preferred atomic behavior when a redox reaction occurs between oxygen and each transition metal under fast charge/discharge conditions. This provides a fundamental insight into the improvement of rate capability. Furthermore, this work provides guidance for identifying high‐energy‐density materials with complex structural properties.     

Annulated Dialkoxybenzenes as Catholyte Materials for Non‐aqueous Redox Flow Batteries: Achieving High Chemical Stability through Bicyclic Substitution

1,4‐Dimethoxybenzene derivatives are materials of choice for use as catholytes in non‐aqueous redox flow batteries, as they exhibit high open‐circuit potentials and excellent electrochemical reversibility. However, chemical stability of these materials in their oxidized form needs to be improved. Disubstitution in the arene ring is used to suppress parasitic reactions of their radical cations, but this does not fully prevent ring‐addition reactions. By incorporating bicyclic substitutions and ether chains into the dialkoxybenzenes, a novel catholyte molecule, 9,10‐bis(2‐methoxyethoxy)‐1,2,3,4,5,6,7,8‐octahydro‐1,4:5,8‐dimethanenoanthracene (BODMA), is obtained and exhibits greater solubility and superior chemical stability in the charged state. A hybrid flow cell containing BODMA is operated for 150 charge–discharge cycles with a minimal loss of capacity.     

Long‐Life,High‐Rate Lithium–Sulfur Cells with a Carbon‐Free VN Host as an Efficient Polysulfide Adsorbent and Lithium Dendrite Inhibitor

Lithium–sulfur (Li‐S) batteries are a promising next‐generation energy‐storage system, but the polysulfide shuttle and dendritic Li growth seriously hinder their commercial viability. Most of the previous studies have focused on only one of these two issues at a time. To address both the issues simultaneously, presented here is a highly conductive, noncarbon, 3D vanadium nitride (VN) nanowire array as an efficient host for both sulfur cathodes and lithium‐metal anodes. With fast electron and ion transport and high porosity and surface area, VN traps the soluble polysulfides, promotes the redox kinetics of sulfur cathodes, facilitates uniform nucleation/growth of lithium metal, and inhibits lithium dendrite growth at an unprecedented high current density of 10 mA cm^?2 over 200 h of repeated plating/stripping. As a result, VN‐Li||VN‐S full cells constructed with VN as both an anode and cathode host with a negative to positive electrode capacity ratio of only ≈2 deliver remarkable electrochemical performance with a high Coulombic efficiency of ≈99.6% over 850 cycles at a high 4 C rate and a high areal capacity of 4.6 mA h cm^?2. The strategy presented here offers a viable approach to realize high‐energy‐density, safe Li‐metal‐based batteries.     

Highly Stable Lithium–Sulfur Batteries Based on Laponite Nanosheet‐Coated Celgard Separators

Lithium–sulfur (Li–S) batteries are of great interest due to their high theoretical energy density. However, one of the key issues hindering their real world applications is polysulfide shuttle, which results in severe capacity decay and self‐discharge. Here, a laponite nanosheets/carbon black coated Celgard (LNS/CB‐Celgard) separator to inhibit polysulfide shuttle and to enhance the Li⁺ conductivity simultaneously is reported. The polysulfide shuttle is efficiently inhibited through strong interactions between the O active sites of the LNS and polysulfides by forming the Li···O and O? S bonds. Moreover, the separator features high Li⁺ conductivity, fast Li⁺ diffusion, excellent electrolyte wettability, and high thermal stability. Consequently, the Li–S batteries with the LNS/CB‐Celgard separator and the pure S cathode show a high initial reversible capacity of 1387 mA h g^?1 at 0.1 C, high rate performance, superior cycling stability (with a capacity decay rate of 0.06% cycle^?1 at 0.2 C and 0.028% cycle^?1 at 1.0 C over 500 cycles), and ultralow self‐discharge. The separator could also enhance the performance of other batteries such as the LiFePO₄/separator/Li battery. This work sheds a new light on the design and preparation of novel separators for highly stable Li–S batteries via a “green” and cost‐effective approach.     

A Conductive Molecular Framework Derived Li2S/N,P‐Codoped Carbon Cathode for Advanced Lithium–Sulfur Batteries

Li₂S is one of the most promising cathode materials for Li‐ion batteries because of its high theoretical capacity and compatibility with Li‐metal‐free anode materials. However, the poor conductivity and electrochemical reactivity lead to low initial capacity and severe capacity decay. In this communication, a nitrogen and phosphorus codoped carbon (N,P–C) framework derived from phytic acid doped polyaniline hydrogel is designed to support Li₂S nanoparticles as a binder‐free cathode for Li–S battery. The porous 3D architecture of N and P codoped carbon provides continuous electron pathways and hierarchically porous channels for Li ion transport. Phosphorus doping can also suppress the shuttle effect through strong interaction between sulfur and the carbon framework, resulting in high Coulombic efficiency. Meanwhile, P doping in the carbon framework plays an important role in improving the reaction kinetics, as it may help catalyze the redox reactions of sulfur species to reduce electrochemical polarization, and enhance the ionic conductivity of Li₂S. As a result, the Li₂S/N,P–C composite electrode delivers a stable capacity of 700 mA h g^?1 with average Coulombic efficiency of 99.4% over 100 cycles at 0.1C and an areal capacity as high as 2 mA h cm^?2 at 0.5C.     

Structural Changes in Li2MnO3 Cathode Material for Li‐Ion Batteries

Structural changes in Li₂MnO₃ cathode material for rechargeable Li‐ion batteries are investigated during the first and 33^rd cycles. It is found that both the participation of oxygen anions in redox processes and Li⁺‐H⁺ exchange play an important role in the electrochemistry of Li₂MnO₃. During activation, oxygen removal from the material along with Li gives rise to the formation of a layered MnO₂‐type structure, while the presence of protons in the interslab region, as a result of electrolyte oxidation and Li⁺‐H⁺ exchange, alters the stacking sequence of oxygen layers. Li re‐insertion by exchanging already present protons reverts the stacking sequence of oxygen layers. The re‐lithiated structure closely resembles the parent Li₂MnO₃, except that it contains less Li and O. Mn⁴⁺ ions remain electrochemically inactive at all times. Irreversible oxygen release occurs only during activation of the material in the first cycle. During subsequent cycles, electrochemical processes seem to involve unusual redox processes of oxygen anions of active material along with the repetitive, irreversible oxidation of electrolyte species. The deteriorating electrochemical performance of Li₂MnO₃ upon cycling is attributed to the structural degradation caused by repetitive shearing of oxygen layers.     

MoN Supported on Graphene as a Bifunctional Interlayer for Advanced Li‐S Batteries

Rational design of effective polysulfide barriers is highly important for high‐performance lithium‐sulfur (Li‐S) batteries. A variety of adsorbents have been applied as interlayers to alleviate the shuttle effect. Nevertheless, the unsuccessful oxidation of Li₂S on interlayers leads to loss of active materials and blocks Li ion transport. In this work, a MoN‐based interlayer sandwiched between the C‐S cathode and the separator is developed. Such an interlayer not only strongly binds lithium polysulfides via Mo‐S bonding but also efficiently accelerates the decomposition of Li₂S. The acceleration mechanism toward Li₂S decomposition is determined to be a combination of contributions of catalytic cleavage of Li‐S bond in Li₂S based on the proposed covalence‐activation mechanism and rapid migration of the produced Li ions. As a result, the C–S cathodes with the as‐developed interlayer manifest a negligible charging potential barrier and outstanding cycling stability with a very low capacity fading rate of 0.023% per cycle during 1500 cycles at 1 C. High areal capacity of 6.02 mAh cm^?2 is achieved for high sulfur loading of 7.0 mg cm^?2 after cycling at 0.1 C. The material and strategy demonstrated in this work can open the door toward developing shuttle suppression interlayers without impairing cathode performance.     

Stable Li–Organic Batteries with Nafion‐Based Sandwich‐Type Separators

Similar to Li–S batteries, Li–organic batteries have also been plagued by the dissolution of active materials and the resulting shuttle effect for many years. An effective strategy to eliminate the shuttle effect is adopting solid electrolytes or Li–ion permselective separators to prohibit the dissolved electroactive species from migrating to the Li anode. A polypropylene/Nafion/polypropylene (PNP) sandwich‐type separator is reported with many advantages in comparison with previously reported LISICON, polymer electrolyte, and other Nafion utilization forms. The physical and chemical properties of PNP separators are studied in detail by cross‐section scanning electron microscopy (SEM), infrared spectroscopy (IR), and electrochemical impedance spectroscopy. 1,1′‐Iminodianthraquinone (IDAQ), a novel organic cathode, is taken as an example to quantitatively investigate the function of PNP separators. In the presence of PNP5 with the most appropriate Nafion loading of 0.5 mg cm^–2, IDAQ is able to achieve dramatically improved cycling stability with capacity retention of 76% after 400 cycles and Coulombic efficiency above 99.6%, which reaches the highest level for reported soluble organic electrode materials. Besides Li–organic batteries, such kind of Nafion‐based sandwich‐type separators are also promising for Li–S batteries and other new battery designs involving dissolved electroactive species.     

Polymerization of monolignols by redox shuttle-mediated enzymatic oxidation: a new model in lignin biosynthesis I

Lignin is one of the most abundant biopolymers, and it has a complex racemic structure. It may be formed by a radical polymerization initiated by redox enzymes, but much remains unknown about the process, such as how molecules as large as enzymes can generate the compact structure of the lignified plant cell wall. We have synthesized lignin oligomers according to a new concept, in which peroxidase is never in direct contact with the lignin monomers coniferaldehyde and coniferyl alcohol. Instead, manganese oxalate worked as a diffusible redox shuttle, first being oxidized from Mn(II) to Mn(III) by a peroxidase and then being reduced to Mn(II) by a simultaneous oxidation of the lignin monomers to radicals that formed covalent linkages of the lignin type. Furthermore, a high molecular mass polymer was generated by oxidation of coniferyl alcohol by Mn(III) acetate in a dioxane and water mixture. This polymer was very similar to natural spruce wood lignin, according to its NMR spectrum. The possible involvement of a redox shuttle/peroxidase system in lignin biosynthesis is discussed.     

High‐Performance Lithium‐Iodine Flow Battery

A cathode‐flow lithium‐iodine (Li–I) battery is proposed operating by the triiodide/iodide (I₃^?/I^?) redox couple in aqueous solution. The aqueous Li–I battery has noticeably high energy density (≈0.28 kWh kg^?1_cell) because of the considerable solubility of LiI in aqueous solution (≈8.2 m ) and reasonably high power density (≈130 mW cm^?2 at a current rate of 60 mA cm^?2, 328 K). In the operation of cathode‐flow mode, the Li–I battery attains high storage capacity (≈90% of the theoretical capacity), Coulombic efficiency (100% ± 1% in 2–20 cycles) and cyclic performance (>99% capacity retention for 20 cycles) up to total capacity of 100 mAh.