Metal–Sulfur Battery Cathodes Based on PAN–Sulfur Composites

Sulfurized Polyacrylonitrile as a High-Performance and Low-Volume Change Anode for Robust Potassium Storage

Potassium-ion batteries (KIBs) are considered as low-cost electrochemical energy storage technologies because of the abundant potassium resources. However, the practical applications of KIBs are mainly hampered by the unsatisfactory electrochemical performance of anode materials which often undergo large volume variations during potassiation-depotassiation, limiting their cycling life. Here, low-cost sulfurized polyacrylonitrile (S-PAN) is reported as an attractive anode candidate for KIBs. It provides a high potassium storage capacity of 569 mAh g_(S-PAN)^-1 with decent rate capability and cycling stability (no capacity loss after 1500 cycles, running time ∼188 days). Detailed ex situ spectroscopic and in situ microscopic characterizations reveal that the distinguished electrochemical performance of S-PAN is attributed to the high reversibility of its covalent C-S and S-S bonds which undergo repeated cleavage-redimerization during potassiation-depotassiation concomitant with relatively small volume variation (less than 24.2%). Subsequently, a full-cell constructed by pairing high-voltage K₂MnFe(CN)₆ cathode with high-capacity S-PAN anode demonstrates an attractive energy density (290.9 Wh kg^-1) and long-term cycling stability (1200 cycles with 95.4% capacity retention). Given the high performance and low cost of both anode and cathode materials, it is believed that the present full-cell promises it as a competitive energy storage system for the cost-sensitive grid-scale applications.

Inverse Vulcanization of a Natural Monoene with Sulfur as Sustainable Electrochemically Active Materials for Lithium-Sulfur Batteries

A novel soluble copolymer poly(S-MVT) was synthesized using a relatively quick one-pot solvent-free method, inverse vulcanization. Both of the two raw materials are sustainable, i.e., elemental sulfur is a by-product of the petroleum industry and 4-Methyl-5-vinylthiazole (MVT) is a natural monoene compound. The microstructure of poly(S-MVT) was characterized by FT-IR, 1H NMR, XPS spectroscopy, XRD, DSC SEM, and TEM. Test results indicated that the copolymers possess protonated thiazole nitrogen atoms, meso/macroporous structure, and solubility in tetrahydrofuran and chloroform. Moreover, the improved electronic properties of poly(S-MVT) relative to elemental sulfur have also been investigated by density functional theory (DFT) calculations. The copolymers are utilized successfully as the cathode active material in Li-S batteries. Upon employment, the copolymer with 15% MVT content provided good cycling stability at a capacity of ∼514 mA h g-1 (based on the mass of copolymer) and high Coulombic efficiencies (∼100%) over 100 cycles, as well as great rate performance.

Feasible Catalytic-Insoluble Strategy Enabled by Sulfurized Polyacrylonitrile with In Situ Built Electrocatalysts for Ultrastable Lithium-Sulfur Batteries

To date, elemental sulfur has been considered as a prospective cathode material for exploring high-energy power systems with low cost and sustainability. However, its practical commercialization has been impeded by inherent drawbacks of notorious capacity decay, unsatisfied insulating nature, and sluggish conversion chemistry. To address these issues, for the first time, freestanding nanofibrous networks with hierarchical nanostructures are facilely constructed by inlaying electrocatalytic bimetallic chalcogenides (Fe_xMn_1-xS nanoparticles) into conductive graphene nanosheet (GN)-doped sulfurized polyacrylonitrile (SPAN) fiber matrices. Covalent-bonded SPAN featuring an insoluble mechanism serves as a reliable cathode substrate with enhanced electrostability and high sulfur utilization, while high-surface-area GN dopants promote conductivity improvement and rapid electron transfer. Meanwhile, the results prove that sulfiphilic Fe_xMn_1-xS nanoparticles with abundant electrochemically active sites facilitate construction of uniform deposition interfaces and efficient electrocatalysis conversion toward lithium polysufides. This feasible catalytic-insoluble cathode strategy drives the Li-S battery, which exhibits excellent electrochemical performances with a remarkable reversible discharge capacity of 967 mA h g^-1 and a capacity retention of 623 mA h g^-1 after 500 cycles. Moreover, the corresponding lithiation/delithiation mechanisms are systematically investigated through complementary morphological and spectral analyses, providing valuable insights into advanced metal-sulfur batteries.

Multiscale Understanding of Covalently Fixed Sulfur-Polyacrylonitrile Composite as Advanced Cathode for Metal-Sulfur Batteries

Metal-sulfur batteries (MSBs) provide high specific capacity due to the reversible redox mechanism based on conversion reaction that makes this battery a more promising candidate for next-generation energy storage systems. Recently, along with elemental sulfur (S8 ), sulfurized polyacrylonitrile (SPAN), in which active sulfur moieties are covalently bounded to carbon backbone, has received significant attention as an electrode material. Importantly, SPAN can serve as a universal cathode with minimized metal-polysulfide dissolution because sulfur is immobilized through covalent bonding at the carbon backbone. Considering these unique structural features, SPAN represents a new approach beyond elemental S8 for MSBs. However, the development of SPAN electrodes is in its infancy stage compared to conventional S8 cathodes because several issues such as chemical structure, attached sulfur chain lengths, and over-capacity in the first cycle remain unresolved. In addition, physical, chemical, or specific treatments are required for tuning intrinsic properties such as sulfur loading, porosity, and conductivity, which have a pivotal role in improving battery performance. This review discusses the fundamental and technological discussions on SPAN synthesis, physicochemical properties, and electrochemical performance in MSBs. Further, the essential guidance will provide research directions on SPAN electrodes for potential and industrial applications of MSBs.

Influence of microstructure and crystalline phases on impedance spectra of sodium conducting glass ceramics produced from glass powder

Crystallization of highly ionic conductive N5 (Na5YSi4O12) phase from melted Na3+3x-1Y1-xPySi3-yO9 parent glass provides an attractive pathway for cost-effective manufacturing of Na-ion conducting thin electrolyte substrates. The temperature-dependent crystallization of parent glass results in several crystalline phases in the microstructure (N3 (Na3YSi2O7), N5 and N8 (Na8.1Y Si6O18) phases) as well as in rest glass phase with temperature dependent viscosity. The electrical properties of dense parent glass and of compositions densified and crystallized at 700 °C, 800 °C, 900 °C, 1000 °C, and 1100 °C are investigated by impedance spectroscopy and linked to their microstructure and crystalline phase content determined by Rietveld refinement. The parent glass has high isolation resistance and predominantly electrons as charge carriers. For sintering at ≥ 900 °C, sufficient N5 phase content is formed to exceed the percolation limit and form ion-conducting pathways. At the same time, the highest content of crystalline phase and the lowest grain boundary resistance are observed. Further increase of the sintering temperature leads to a decrease of the grain resistance and an increase of grain boundary resistance. The grain boundary resistance increases remarkably for samples sintered at 1100 °C due to softening of the residual glass phase and wetting of the grain boundaries. The conductivity of fully crystallized N5 phase (grain conductivity) is calculated from thorough impedance spectra analysis using its volume content estimated from Rietveld analysis, density measurements and assuming reasonable tortuosity to 2.8 10−3 S cm−1 at room temperature. The excellent conductivity and easy processing demonstrate the great potential for the use of this phase in the preparation of solid-state sodium electrolytes.

On the crystallography and reversibility of lithium electrodeposits at ultrahigh capacity

Lithium metal is a promising anode for energy-dense batteries but is hindered by poor reversibility caused by continuous chemical and electrochemical degradation. Here we find that by increasing the Li plating capacity to high values (e.g., 10–50 mAh cm⁻²), Li deposits undergo a morphological transition to produce dense structures, composed of large grains with dominantly (110)_Li crystallographic facets. The resultant Li metal electrodes manifest fast kinetics for lithium stripping/plating processes with higher exchange current density, but simultaneously exhibit elevated electrochemical stability towards the electrolyte. Detailed analysis of these findings reveal that parasitic electrochemical reactions are the major reason for poor Li reversibility, and that the degradation rate from parasitic electroreduction of electrolyte components is about an order of magnitude faster than from chemical reactions. The high-capacity Li electrodes provide a straightforward strategy for interrogating the solid electrolyte interphase (SEI) on Li —with unprecedented, high signal to noise. We find that an inorganic rich SEI is formed and is primarily concentrated around the edges of lithium particles. Our findings provide straightforward, but powerful approaches for enhancing the reversibility of Li and for fundamental studies of the interphases formed in liquid and solid-state electrolytes using readily accessible analytical tools.

Lithium metal batteries offer high-capacity electrical energy storage but suffer from poor reversibility of the metal anode. Here, the authors report that at very high capacities, lithium deposits as dense structures with a preferred crystallite orientation, yielding highly reversible lithium anodes.

High-performance lithium-sulfur batteries enabled by regulating Li2S deposition

Lithium-sulfur batteries (LSBs) have received intensive attention in recent years due to their high theoretical energy density derived from the lithiation of sulfur. In the discharge process, sulfur transforms into lithium polysulfides (LiPSs) that dissolve in liquid electrolytes and then into insoluble Li₂S precipitated on the electrode surface. The electronically and ionically insulating Li₂S leads to two critical issues, including the sluggish reaction kinetics from LiPSs to Li₂S and the passivation of the electrode. In this regard, controlling the Li₂S deposition is significant for improving the performance of LSBs. In this perspective, we have summarized the recent achievements in regulating the Li₂S deposition to enhance the performance of LSBs, including the solution-mediated growth of Li₂S, sulfur host enhanced nucleation and catalysis induced kinetic improvement. Moreover, the challenges and possibilities for future research studies are discussed, highlighting the significance of regulating the Li₂S deposition to realize the high electrochemical performance and promote the practical uses of LSBs.

Graphene-Based Materials for Flexible Lithium-Sulfur Batteries

The increasing demand for wearable electronic devices necessitates flexible batteries with high stability and desirable energy density. Flexible lithium-sulfur batteries (FLSBs) have been increasingly studied due to their high theoretical energy density through the multielectron chemistry of low-cost sulfur. However, the implementation of FLSBs is challenged by several obstacles, including their low practical energy density, short life, and poor flexibility. Various graphene-based materials have been applied to address these issues. Graphene, with good conductivity and flexibility, exhibits synergistic effects with other active/catalytic/flexible materials to form multifunctional graphene-based materials, which play a pivotal role in FLSBs. This review summarizes the recent progress of graphene-based materials that have been used as various FLSB components, including cathodes, interlayers, and anodes. Particular attention is focused on the precise nanostructures, graphene efficacy, interfacial effects, and battery layout for realizing FLSBs with good flexibility, energy density, and cycling stability.

Hybrid nanostructures for electrochemical potassium storage

The wide availability and low cost of potassium resources have made electrochemical potassium storage a promising energy storage solution for sustainable decarbonisation. Research activities have been rapidly increasing in the last few years to investigate various potassium batteries such as K-ion batteries (KIBs), K-S batteries and K-Se batteries. The electrode materials of these battery technologies are being extensively studied to examine their suitability and performance, and the utilisation of hybrid nanostructures has undoubtedly contributed to the advancement of the performance. This review presents a timely summary of utilising hybrid nanostructures as battery electrodes to address the issues currently existing in potassium batteries via taking advantage of the compositional and structural diversity of hybrid nanostructures. The complex challenges in KIBs and K-S and K-Se batteries are outlined and the role of hybrid nanostructures is discussed in detail regarding the characteristics of intercalation, conversion and alloying reactions that take place to electrochemically store K in hybrid nanostructures, highlighting their multifunctionality in addressing the challenges. Finally, outlooks are given to stimulate new ideas and insights into the future development of hybrid nanostructures for electrochemical potassium storage.

Manipulating the Electronic Structure of Nickel via Alloying with Iron: Toward High-Kinetics Sulfur Cathode for Na-S Batteries

The sluggish conversion kinetics and severe shuttle effect in room-temperature Na-S (RT Na-S) batteries cause knotty issues, such as poor rate performance, fast capacity decay as well as low Coulombic efficiency, which seriously impede their practical application. Therefore, exploiting cost-effective and efficient electrocatalysts for absorbing soluble long-chain sodium polysulfides (NaPSs) and expediting NaPSs conversion is of paramount importance. Herein, catalyst mining is first conducted by density functional theory calculations, which reveal that the alloying of Fe into Ni can tailor the electronic structure, leading to lower reaction energy barrier for polysulfide conversion. Based on this, FeNi₃@hollow porous carbon spheres (FeNi₃@HC) as a promising sulfur host for RT Na-S batteries are rationally designed and fabricated. As expected, the S@FeNi₃@HC cathode exhibits an excellent cycling stability (591 mAh g^-1 after 500 cycles at 2 A g^-1) and outstanding rate performance (383 mAh g^-1 at 5 A g^-1). Our work demonstrates an effective strategy (i.e., alloying strategy with a rich electron state) to design superior electrocatalysts for RT Na-S batteries.

An organodiselenide containing electrolyte enables sulfurized polyacrylonitrile cathodes with fast redox kinetics in Li-S batteries

An organoselenide compound, phenyl diselenide (PDSe), is employed as a soluble electrolyte additive to enhance the kinetics of a sulfurized polyacrylonitrile cathode, in which radical exchange in the solid-liquid interface forms dynamic S-Se bonds. Consequently, the PDSe assisted cathode exhibits enhanced battery performance in both ether and carbonate electrolytes.

Mesoporous Yolk-Shell Structured Organosulfur Nanotubes with Abundant Internal Joints for High-Performance Lithium-Sulfur Batteries by Kinetics Acceleration

Although organosulfur compounds can protect lithium anodes, participate in the redox reaction, and suppress the shuttle effect, the sluggish electrochemical dynamics of their bulk structure and the notorious shuttle effect of covalent long-chain sulfurs largely impede their actual applications. Herein, sulfurized carbon nanotube@aminophenol-formaldehyde (SC@A) with covalently linked short-chain sulfurs is firstly synthesized by in situ polymerization of aminophenol-formaldehyde (AF) on the surface of carbon nanotubes (CNTs) followed by acetone etching and inverse sulfurization processes, forming mesoporous yolk-shell organosulfur nanotubes with abundant internal joints between the yolk of CNTs and the shell of sulfurized AF for the first time. In situ Raman spectra, in situ XRD patterns, and ex situ XPS spectra verify that the covalent short-chain sulfurs bring about a reversible solid-solid conversion process of sulfur, thoroughly avoiding the shuttle effect. The mesoporous yolk-shell structure with abundant internal joints can effectively accommodate the volume change, fully expose active sites and efficiently improve the transport of electrons and lithium ions, thus highly promoting the solid-solid electrochemical reaction kinetics. Therefore, the SC@A cathode exhibits a superior specific capacity of 841 mAh g^-1 and a capacity decay of 0.06% per cycle within 500 cycles at a large current density of 5.0 C.

Strategies for Polysulfide Immobilization in Sulfur Cathodes for Room-Temperature Sodium-Sulfur Batteries

Room-temperature sodium-sulfur batteries are one of the most attractive energy storage systems due to their low cost and ultrahigh energy density (2600 W h kg^-1 ). During the charge/discharge process, the sulfur can react with sodium via a multistep redox reaction to obtain a high specific capacity (1675 mA h g^-1 ). However, these batteries face the difficult challenge of the "shuttle effect," which hinders their practical application. Many strategies have been employed to address this issue on sulfur electrodes, such as intact physical confinement, chemical inhibition, and electrocatalysis. In this review, the mechanisms of the abovementioned strategies are summarized, the remaining issues are clarified, and research directions are proposed for developing advanced sodium-sulfur batteries.

Cationic polymer-grafted graphene oxide/CNT cathode-coating material for lithium-sulfur batteries

A cathode-coating material composed of cationic polymer-grafted graphene oxide (CPGO) and carbon nanotube (CNT) was prepared, where the CPGO was synthesized by grafting quaternized 2-(dimethylamino)ethyl methacrylate (QDMAEMA) onto graphene oxide (GO) via atom transfer radical polymerization (ATRP). GO has good compatibility with carbon black, the main component of the cathode in lithium–sulfur (Li–S) batteries. Here, the cationic polymer having the QDMAEMA unit was intentionally grafted onto GO to decrease the shuttle effect by increasing the chemical adsorption of polysulfide (PS). In addition, when CNT was mixed with CPGO, the compatibility with carbon black was found to be further increased. The lithium–sulfur (Li–S) battery with a sulfur-deposited Super P® carbon black (S/C) cathode coated with a mixture of CPGO and CNT was found to have much improved cell performance compared to those coated without any coating material, with only CPGO, with the mixture of GO and CNT, and with the mixture of PQDMAEMA and CNT. For example, the Li–S battery with the cathode coated using the mixture of CPGO and CNT retained a discharge capacity of 744 mA h g⁻¹ after 50 cycles at 0.2C-rate, while those of the Li–S batteries with bare S/C and CPGO-S/C cathodes were found to be much smaller, i.e., 488 mA h g⁻¹ and 641 mA h g⁻¹, respectively, under the same conditions. Therefore, the mixture of CPGO with CNT as the cathode-coating material showed a synergetic effect to enhance the cell performance of the Li–S battery system.

A cathode-coating material composed of cationic polymer-grafted graphene oxide (CPGO) and carbon nanotube (CNT) was prepared and used as a cathode-coating material for lithium sulfur batteries.

Understanding the Roles of the Electrode/Electrolyte Interface for Enabling Stable Li∥Sulfurized Polyacrylonitrile Batteries

Sulfurized polyacrylonitrile (SPAN) is a promising high-capacity cathode material. In this work, we use spatially resolved X-ray absorption spectroscopy combined with X-ray fluorescence (XRF) microscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy to examine the structural transformation of SPAN and the critical role of a robust cathode-electrolyte interface (CEI) on the electrode. LiS_x species forms during the cycling of SPAN. However, in carbonate-based electrolytes and ether-based electrolytes with LiNO₃ additives, these species are well protected by the CEI and do not dissolve into the electrolytes. In contrast, in an ether-based electrolyte without the LiNO₃ additive, LiS_x species dissolve into the electrolyte, resulting in the shuttle effect and capacity loss. Examination of the Li anode by XRF and SEM reveals dense spherical Li morphology in ether-based electrolytes, but sulfur is present in the absence of the LiNO₃ additive. In contrast, porous dendritic Li is found in the carbonate electrolyte. These analyses established that an ether-based electrolyte with LiNO₃ is a superior choice that enables stable cycling of both electrodes. Based on these insights, we successfully demonstrate the stable cycling of high areal loading SPAN cathode (>6.5 mA h cm^-2) with lean electrolyte amounts, showing promising Li∥SPAN cell performance under practical conditions.

Advances in Lithium-Sulfur Batteries: From Academic Research to Commercial Viability

Lithium-ion batteries, which have revolutionized portable electronics over the past three decades, were eventually recognized with the 2019 Nobel Prize in chemistry. As the energy density of current lithium-ion batteries is approaching its limit, developing new battery technologies beyond lithium-ion chemistry is significant for next-generation high energy storage. Lithium-sulfur (Li-S) batteries, which rely on the reversible redox reactions between lithium and sulfur, appears to be a promising energy storage system to take over from the conventional lithium-ion batteries for next-generation energy storage owing to their overwhelming energy density compared to the existing lithium-ion batteries today. Over the past 60 years, especially the past decade, significant academic and commercial progress has been made on Li-S batteries. From the concept of the sulfur cathode first proposed in the 1960s to the current commercial Li-S batteries used in unmanned aircraft, the story of Li-S batteries is full of breakthroughs and back tracing steps. Herein, the development and advancement of Li-S batteries in terms of sulfur-based composite cathode design, separator modification, binder improvement, electrolyte optimization, and lithium metal protection is summarized. An outlook on the future directions and prospects for Li-S batteries is also offered.

Two Competing Reactions of Sulfurized Polyacrylonitrile Produce High-Performance Lithium-Sulfur Batteries

Sulfurized polyacrylonitrile (SPAN) is an attractive cathode candidate for the advanced lithium-sulfur (Li-S) batteries owing to its outstanding cyclic stability. Nevertheless, SPAN suffers from inadequate initial Coulombic efficiency (CE) induced by the sluggish reaction kinetics, which is primarily ascribed to the low Li-ion diffusivity and high electronic resistivity of the Li₂S product. In this work, an optimal trace amount of soluble lithium polysulfide of Li₂S₈ is introduced as a redox mediator for a freestanding fibrous SPAN cathode to enhance the reversible oxidation efficiency of Li₂S. During the delithiation process, the chemical interactions between Li₂S and Li₂S₈ additive facilitate the electrochemical oxidation of Li₂S, resulting in the transformation of not only C-S/S-S bonds in SPAN but also elemental sulfur. Benefiting from the synergistic effect of the two competing reactions, a high initial CE of 82.9% could be achieved at a current density of 200 mA g^-1. Moreover, a superior capacity retention along with a high capacity of 1170 mAh g^-1 up to the 400th cycle is available at 1000 mA g^-1. The study offers a feasible approach for Li-S batteries toward the practical applications of SPAN.

Understanding Sulfur Redox Mechanisms in Different Electrolytes for Room-Temperature Na-S Batteries

This work reports influence of two different electrolytes, carbonate ester and ether electrolytes, on the sulfur redox reactions in room-temperature Na-S batteries. Two sulfur cathodes with different S loading ratio and status are investigated. A sulfur-rich composite with most sulfur dispersed on the surface of a carbon host can realize a high loading ratio (72% S). In contrast, a confined sulfur sample can encapsulate S into the pores of the carbon host with a low loading ratio (44% S). In carbonate ester electrolyte, only the sulfur trapped in porous structures is active via 'solid-solid' behavior during cycling. The S cathode with high surface sulfur shows poor reversible capacity because of the severe side reactions between the surface polysulfides and the carbonate ester solvents. To improve the capacity of the sulfur-rich cathode, ether electrolyte with NaNO₃ additive is explored to realize a 'solid-liquid' sulfur redox process and confine the shuttle effect of the dissolved polysulfides. As a result, the sulfur-rich cathode achieved high reversible capacity (483 mAh g^-1), corresponding to a specific energy of 362 Wh kg^-1 after 200 cycles, shedding light on the use of ether electrolyte for high-loading sulfur cathode.

Density Functional Theory Studies on Sulfur-Polyacrylonitrile as a Cathode Host Material for Lithium-Sulfur Batteries

Cyclized polyacrylonitrile, which can be obtained by vulcanization of polyacrylonitrile with sulfur, is an electron-conductive polymer that can be used as a host material in lithium-sulfur batteries. Using density functional theory, we investigated the interaction between a surrounding electrolyte and the polymeric sulfur-polyacrylonitrile (SPAN) electrode. In particular, we focused on different configurations, where the system contains 1,3-dioxane as a solvent and can have (i) polysulfide (PS) solvated in the electrolyte, (ii) a PS attached to the polymer backbone, (iii) lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) as a salt dissolved in the electrolyte, and (iv) both PS and LiTFSI dissolved in the electrolyte. We found that the polymer, when having a hydrogen vacancy at a carbon atom (undercoordinated carbon) of the polymer backbone, is able to not only capture a PS from the electrolyte but also decompose and bind to the solvent and/or remove lithium from the PS. During this capturing process, the polysulfide might undergo S-S bond cleavage and recombination, accompanied by a charge transfer between the polysulfide and polymer. Thus, cyclized polyacrylonitrile not only is an interesting host material but also acts as an active material, together with sulfur, by capturing Li from the polysulfide.

New Insights into the N-S Bond Formation of a Sulfurized-Polyacrylonitrile Cathode Material for Lithium-Sulfur Batteries

Sulfurized polyacrylonitrile (S-cPAN) has been recognized as a particularly promising cathode material for lithium-sulfur (Li-S) batteries due to its ultra-stable cycling performance and high degree of sulfur utilization. Though the synthetic conditions and routes for modification of S-cPAN have been extensively studied, details of the molecular structure of S-cPAN remain yet unclear. Herein, a more reasonable molecular structure consisting of pyridinic/pyrrolic nitrogen (N_PD/N_PL) is proposed, based on the analysis of combined X-ray photoelectron spectroscopy, ¹³C/¹⁵N solid-state nuclear magnetic resonance, and density functional theory data. The coexistence of vicinal N_PD/N_PL entities plays a vital role in attracting S₂ molecules and facilitating N-S bond formation apart from the generally accepted C-S bond in S-cPAN, which could explain the extraordinary electrochemical features of S-cPAN among various nitrogen-containing sulfurized polymers. This study provides new insights and a better understanding of structural details and relevant bond formation mechanisms in S-cPAN, providing a foundation for the design of new types of sulfurized cathode materials suitable for application in next-generation high-performance Li-S batteries.

A Scalable Cathode Chemical Prelithiation Strategy for Advanced Silicon-Based Lithium Ion Full Batteries

A silicon anode with ultra-high specific capacity has motivated tremendous exploration for high-energy-density lithium ion batteries while it still faces serious issues of irreversible lithium loss, unstable electrode electrolyte interface (SEI), and huge volume expansion. Prelithiation is a crucial technology to alleviate the harm of active lithium loss of silicon-based full-cell systems. Herein, we reported a cathode prelithiation method using Li₂S-PAN as a lithium "donor", which was synthesized via chemical reaction between sulfurized polyacrylonitrile and Li-biphenyl complex. The Li₂S-PAN with an initial charging capacity of 668 mAh g^-1 (2.5-4.0 V) is loaded on the LiFePO₄ electrode, and the LiFePO₄/Li₂S-PAN composite electrode displays a high initial charge capacity of 206 mAh g^-1, which is 22.3% higher than the pristine LiFePO₄. With a silicon/graphite/carbon (Si/G/C) composite anode, the Si/G/C||LiFePO₄/Li₂S-PAN full cell exhibits a reversible capacity of 123 and 107 mAh g^-1 in the 1st and 10th cycle, which is 15.5 and 24.5% higher than the Si/G/C||LiFePO₄ battery, respectively. The SEI layer of the silicon anode in the Si/G/C||LiFePO₄/Li₂S-PAN cell contains abundant conductive LiF species, which can enhance the interfacial stability and reaction kinetics of the cells. The proposed cathode prelithiation process is compatible with the industrial roll-to-roll electrode preparation process, exhibiting a promising application prospect.

A new strategy for achieving high K+ storage capacity with fast kinetics: realizing covalent sulfur-rich carbon by phosphorous doping

Designing carbon anodes with rich heteroatoms and dilated graphitic interlayer spacing via a one-step synthesis process plays a vital role in accelerating the practical application of potassium ion batteries, but it is still a big challenge. Herein, P-doped S-rich mesoporous carbon (PSMC) is prepared by direct phosphate-assisted carbonization of carrageenan, and it exhibits excellent potassium storage capacity (449 mA h g-1 at 0.1 A g-1), superior rate performance (233 mA h g-1 at 2 A g-1) and long-term stability (97.3% capacity retention after 1000 cycles), due to the high sulfur doping (16.48 wt%) and the coexistence of ordered and disordered regions in the structure. Ex situ characterization, GITT and theoretical calculations reveal that the promotion of covalent sulfur can effectively increase the adsorption of K+ and enhance the K+ reaction kinetics. The proposed one-step synthesis strategy demonstrates the precise use of the composition in biomass, enabling large-scale production of high-performance anodes for K+ storage.

Material design strategies to improve the performance of rechargeable magnesium-sulfur batteries

Beyond current lithium-ion technologies, magnesium-sulfur (Mg-S) batteries represent one of the most attractive battery chemistries that utilize low cost, sustainable, and high capacity materials. In addition to high gravimetric and volumetric energy densities, Mg-S batteries also enable safer operation due to the lower propensity for magnesium dendrite growth compared to lithium. However, the development of practical Mg-S batteries remains challenging. Major problems such as self-discharge, rapid capacity loss, magnesium anode passivation, and low sulfur cathode utilization still plague these batteries, necessitating advanced material design strategies for the cathode, anode, and electrolyte. This review critically appraises the latest research and design principles to address specific issues in state-of-the-art Mg-S batteries. In the process, we point out current limitations and open-ended questions, and propose future research directions for practical realization of Mg-S batteries and beyond.

Tailoring Electrolyte Solvation for Li Metal Batteries Cycled at Ultra-Low Temperature

Lithium metal batteries (LMBs) hold the promise to pushing cell level energy densities beyond 300 Wh kg-1 while operating at ultra-low temperatures (< -30°C). Batteries capable of both charging and discharging at these temperature extremes are highly desirable due to their inherent reduction of external warming requirements. Here we demonstrate that the local solvation structure of the electrolyte defines the charge-transfer behavior at ultra-low temperature, which is crucial for achieving high Li metal coulombic efficiency (CE) and avoiding dendritic growth. These insights were applied to Li metal full cells, where a high-loading 3.5 mAh cm-2 sulfurized polyacrylonitrile (SPAN) cathode was paired with a one-fold excess Li metal anode. The cell retained 84 % and 76 % of its room temperature capacity when cycled at -40 and -60 °C, respectively, which presented stable performance over 50 cycles. This work provides design criteria for ultra-low temperature LMB electrolytes, and represents a defining step for the performance of low-temperature batteries.

Advanced High-Performance Potassium-Chalcogen (S, Se, Te) Batteries

Current great progress on potassium-chalcogen (S, Se, Te) batteries within much potential to become promising energy storage systems opens a new avenue for the rapid development of potassium batteries as a complementary option to lithium ion batteries. The discussion mainly concentrates on recent research advances of potassium-chalcogen (S, Se, Te) batteries and their corresponding cathode materials in this review. Initially, the development of cathode materials for four types of batteries is introduced, including: potassium-sulfur (K-S), potassium-selenium (K-Se), potassium-selenium sulfide (K-Se_x S_y ), and potassium-tellurium (K-Te) batteries. Next, critical challenges for chalcogen-based electrodes in devices operation are summarized. In addition, some pragmatic strategies are proposed as well to relieve the low electronic conductivity, large volumetric expansion, shuttle effect, and potassium dendrite growth. At last, the perspectives on designing advanced cathode materials for potassium-chalcogen batteries are provided with the goal of developing high-performance potassium storage devices.

Sulfurized Polyacrylonitrile for High-Performance Lithium-Sulfur Batteries: In-Depth Computational Approach Revealing Multiple Sulfur's Reduction Pathways and Hidden Li+ Storage Mechanisms for Extra Discharge Capacity

Like no other sulfur host material, polyacrylonitrile-derived sulfurized carbon (SPAN) promises improved electrochemical performance for lithium-sulfur batteries, based on its compatibility with carbonate solvents and ability to prevent self-discharge and shuttle effect. However, a complete understanding of the SPAN's lithiation mechanism is still missing because its structural features vary widely with synthesis conditions, and its electrochemical performance deviates from elemental sulfur. This study continues our research on the elucidation of the SPAN's structural characteristics and lithiation mechanisms via computational approaches. Our models reproduce most experimental data regarding carbon's graphitization level and conjugated ordering, sulfur-carbon covalent bonding, sulfur loading, and elemental composition. Our simulations emulate the discharge voltage observed in experiments for the first discharge, which reveals that sulfur follows multiple reduction pathways based on its interaction with the carbon backbone. Sulfur reduction takes place above 1.0 V vs Li/Li⁺ mostly in the SPAN-like material, with no long-chain lithium polysulfide formation. Below 1.0 V vs Li/Li⁺, the backbone's electrochemical activity occurs via multiple C-Li and N-Li interactions, mostly with edge carbon atoms and pyridinic nitrogen. Moreover, we identify Li⁺ binding sites throughout the graphitized backbone that might lead to prohibited energy costs for Li⁺ deintercalation, which may explain the irreversible capacity loss between the first and second discharges. This work improves understanding of lithiation mechanisms in sulfurized carbon, which is useful for rationally designing SPAN synthesis pathways tailored to increase sulfur loading and enhanced electrochemical performance.

Theoretical investigation on lithium polysulfide adsorption and conversion for high-performance Li-S batteries

Lithium-sulfur (Li-S) batteries have shown great application prospects as next-generation energy storage systems due to their high theoretical capacity and high energy density. However, the practical application of Li-S batteries is still hindered by several challenges, such as their sluggish sulfur redox kinetics and shuttle effect of lithium polysulfides (LiPSs). To date, significant research has been focused on the confinement adsorption and catalytic conversion of LiPSs using theoretical or/and experimental methods. Among them, theoretical calculations are highly attractive to observe complex LiPS conversion reactions, which facilitate the rational design of S mediators for high-performance Li-S batteries. In this review, we summarize and discuss the recent advances in the adsorption and conversion of LiPSs from the viewpoint of theoretical calculations. Moreover, a set of theoretical principles to guide the screening of suitable host materials for Li-S batteries is presented and discussed. Finally, some personal insights about the future challenges and the focus of research in this field are presented, which will push a milestone step toward high-efficiency and long-life Li-S batteries.

Artificial Lithium Isopropyl-Sulfide Macromolecules as an Ion-Selective Interface for Long-Life Lithium-Sulfur Batteries

The persistent reduction reactions between the hyperactive lithium metal (Li) and dissolved polysulfides would passivate the Li metal and rapidly decrease the cathodic active materials, thus leading to low Coulombic efficiency and a short cycle life of lithium-sulfur (Li-S) batteries. Herein, we construct artificial lithium isopropyl-sulfide macromolecules as an ion-selective interface on the Li metal (IS-Li) by a facile electrochemical polymerization method, in which the polymer network improves the elasticity and toughness to accommodate the volume change of the Li anode and the formed lithium-organosulfides provide great mechanical strength to resist the destruction of Li dendrites. Importantly, this interfacial layer is proved to be sufficient in damping polysulfide anion diffusion and stopping irreversible reduction between polysulfides and metallic Li, which greatly contribute to the performance improvement of Li-S batteries. The resulting Li-S batteries exhibit long-term stability with high capacity retention and Coulombic efficiency. This effective strategy sets a new approach for regulating the interfacial chemistry of Li metal anodes, which is significant for highly stable Li-S batteries.

Controllable Substitution of S Radicals on Triazine Covalent Framework to Expedite Degradation of Polysulfides

Lithium-sulfur (Li-S) batteries are facing a significant barrier due to the diffusion of intermediate redox species. Although some S doped covalent framework cathodes have been reported with outstanding reversibility, the low content of sulfur (less than 30%) limits the practical applications. To overcome the issue, the sulfur and nitrogen co-doped covalent compounds (S-NC) as a host-type cathode have been developed through the radical transfer process during thermal cracking amino groups on the precursor, and then plentiful positively charged sulfur radicals can be controllably introduced. The experimental characterization and DFT theoretical calculation certificate that the sulfur radicals in S-NC/S can expedite redox reactions of intermediate polysulfides to impede their dissolution. Moreover, the energy barriers during ions transfer also obviously decreased after introducing S radicals, which lead to improved rate performance.

Reconfiguring Organosulfur Cathode by Over-Lithiation to Enable Ultrathick Lithium Metal Anode toward Practical Lithium-Sulfur Batteries

An ultrathick lithium metal anode (LMA) is a prerequisite for developing practical lithium-sulfur (Li-S) batteries that simultaneously meet the requirements of high areal capacity, lean electrolyte, and limited excess Li. Inspired by the electrochemical process for an organosulfur cathode, herein, we reconfigure such a sulfur cathode by using an overlithiation strategy to enable the formation of a high performance LMA. Specifically, an applicable ultrathick LMA is successfully constructed by overlithiating a well-known organosulfur cathode material, sulfurized polyacrylonitrile (SPAN). SPAN contains a polymeric pyridine structure with an outstanding lithium-ion affinity, so that it can act as a lithiophilic matrix. More importantly, a Li₂S-rich solid electrolyte interphase (SEI) can be generated on the surface of SPAN during the overlithiation process. The synergistic effect of the lithiophilic matrix and a robust SEI leads to a dense deposition of lithium, which enables one to form an ultrathick LMA (159 μm, 30 mAh cm^-2) with high Coulombic efficiency (99.7%). Such an LMA paired with a sulfur cathode of high areal capacity (up to 16 mAh cm^-2) shows stable cycling under practical conditions of a lean electrolyte (2.2 μL mg_S^-1) and a negative-to-positive capacity (N/P) ratio as low as 1.3. The applicability of the ultrathick LMA was further verified with Li-S pouch cells, indicating a highly prospective route toward realization of practical Li-S batteries.

Metal-organic framework based bifunctional oxygen electrocatalysts for rechargeable zinc-air batteries: current progress and prospects

Zinc-air batteries (ZABs) are regarded as ideal candidates for next-generation energy storage equipment due to their high energy density, non-toxicity, high safety, and environmental friendliness. However, the slow oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics on the air cathode limit their efficiency and the development of highly efficient, low cost and stable bifunctional electrocatalysts is still challenging. Metal-Organic Framework (MOF) based bifunctional oxygen electrocatalysts have been demonstrated as promising alternative catalysts due to the regular structure, tunable chemistry, high specific surface area, and simple and easy preparation of MOFs, and great progress has been made in this area. Herein, we summarize the latest research progress of MOF-based bifunctional oxygen electrocatalysts for ZABs, including pristine MOFs, derivatives of MOFs and MOF composites. The effects of the catalysts' composites, morphologies, specific surface areas and active sites on catalytic performances are specifically addressed to reveal the underlying mechanisms for different catalytic activity of MOF based catalysts. Finally, the main challenges and prospects for developing advanced MOF-based bifunctional electrocatalysts are proposed.

Multiregion Janus-Featured Cobalt Phosphide-Cobalt Composite for Highly Reversible Room-Temperature Sodium-Sulfur Batteries

Electrode materials with high conductivity, strong chemisorption, and catalysis toward polysulfides are recognized as key factors for metal-sulfur batteries. Nevertheless, the construction of such functional material is a challenge for room-temperature sodium-sulfur (RT-Na/S) batteries. Herein, a multiregion Janus-featured CoP-Co structure obtained via sequential carbonization-oxidation-phosphidation of heteroseed zeolitic imidazolate frameworks is introduced. The structural virtues include a heterostructure existing in a CoP-Co structure and a conductive network of N-doped porous carbon nanotube hollow cages (NCNHCs), endowing it with superior conductivity in both the short- and long-range and strong polarity toward polysulfides. Thus, the S@CoP-Co/NCNHC cathode exhibits superior electrochemical performance (448 mAh g^-1 remained for 700 times cycling under 1 A g^-1) and an optimized redox mechanism in polysulfides conversion. Density functional theory calculations present that the CoP-Co structure optimizes bond structure and bandwidth, whereas the pure CoP is lower than the corresponding Fermi level, which could essentially benefit the adsorptive capability and charge transfer from the CoP-Co surface to Na₂S_x and therefore improve its affinity to polysulfides.

Siloxane based copolymer sulfur as binder-free cathode for advances lithium-sulfur batteries

One of the reasons why lithium-sulfur batteries have not yet been commercialized is a great attenuation of the capacity, although their theoretical capacity is ultrahigh. To alleviate this problem, we developed multifunctional organic matter, 3-aminopropyltriethoxysilane (APS), for preparing a uniform hybrid consists of silica 3D network skeleton and copolymerized sulfur nanoparticles (cpS). In this hybrid, abundant amidogen in APS induced polymerization with sulfur, which formed uniform copolymerized sulfur nanoparticles (nano sulfur), restraining the shuttle effect of polysulfides. Meanwhile, polycondensation APS not only acted as an efficiently binder, affording compact cathodes, but also formed a stable 3D silica structure to encapsulate sulfur nanoparticles, relieving the volume change of sulfur. Consequently, this binder-free cpS composite exhibited much enhanced electrochemical performances, including a high capacity of 1187.8 mAh·g^-1 at 0.1 C and a low capacity fading of 0.0654% per cycle for 500 cycles at 1 C. This study paved the way for high-energy-density batteries by simply applying a low-cost, environmentally-friendly, powerful network polymer cathode material.

Solid Electrolytes for Li-S Batteries: Solid Solutions of Poly(ethylene oxide) with LixPON- and LixSiPON-Based Polymers

We report here efforts to synthesize free-standing, dry polymer electrolytes that exhibit superior ionic conductivities at ambient for Li-S batteries. Co-dissolution of poly(ethylene oxide) (PEO) (M_n 900k) with Li_xPON and Li_xSiPON polymer systems at a ratio of approximately 3:2 followed by casting provides transparent, solid-solution films 25-50 μm thick, lowering PEO crystallinity, and providing measured impedance values of 0.1-2.8 × 10^-3 S/cm at ambient. These values are much higher than simple PEO/Li⁺ salt systems. These solid-solution polymer electrolytes (PEs) are (1) thermally stable to 100 °C; (2) offer activation energies of 0.2-0.5 eV; (3) suppress dendrite formation; and (4) enable the use of lithium anodes at current densities as high as 3.5 mAh/cm². Galvanostatic cycling of SPAN/PEs/Li cell (SPAN = sulfurized, carbonized polyacrylonitrile) shows discharge capacities of 1000 mAh/g_sulfur at 0.25C and 800 mAh/g_sulfur at 1C with high coulumbic efficiency over 100 cycles.

Engineering Bifunctional Host Materials of Sulfur and Lithium-Metal Based on Nitrogen-Enriched Polyacrylonitrile for Li-S Batteries

Lithium-sulfur batteries (LSBs) still suffer from the shuttle effect on the cathode and the lithium dendrite on the anode. Herein, polyacrylonitrile (PAN) is developed into a bifunctional host material to simultaneously address the challenges faced on both the sulfur cathode and lithium anode in LSBs. For the sulfur cathode, PAN is bonded with sulfur to produce sulfurized PAN (SPAN) to avoid the shuttle effect. The SPAN is accommodated into a conductive 3D CNTs-wrapped carbon foam to prepare a self-supporting cathode, which improves the electronic and ionic conductivity, and buffers the volume expansion. Thereby, it delivers reversible capacity, superb rate capability, and outstanding cycling stability. For the Li-metal anode, PAN aerogel is carbonized to give macroporous N-doped cross-linked carbon nanofiber that behaves as a lithiophilic host to regulate Li plating and suppress the growth of Li dendrite. Combining the improvements for both the cathode and anode realizes a remarkable long-term cyclability (765 mAh g^-1 after 300 cycles) in a full cell. It provides new opportunity to propel the practical application of advanced LSBs.

A chemically stabilized sulfur cathode for lean electrolyte lithium sulfur batteries

Lithium sulfur batteries (LSBs) are promising next-generation rechargeable batteries due to the high gravimetric energy, low cost, abundance, nontoxicity, and high sustainability of sulfur. However, the dissolution of high-order polysulfide in electrolytes and low Coulombic efficiency of Li anode require excess electrolytes and Li metal, which significantly reduce the energy density of LSBs. Quasi-solid-state LSBs, where sulfur is encapsulated in the micropores of carbon matrix and sealed by solid electrolyte interphase, can operate under lean electrolyte conditions, but a low sulfur loading in carbon matrix (<40 wt %) and low sulfur unitization (<70%) still limit the energy density in a cell level. Here, we significantly increase the sulfur loading in carbon to 60 wt % and sulfur utilization to ∼87% by dispersing sulfur in an oxygen-rich dense carbon host at a molecular level through strong chemical interactions of C-S and O-S. In an all-fluorinated organic lean electrolyte, the C/S cathode experiences a solid-state lithiation/delithiation reaction after the formation of solid electrolyte interphase in the first deep lithiation, completely avoiding the shuttle reaction. The chemically stabilized C/S composite retains a high reversible capacity of 541 mAh⋅g^-1 (based on the total weight of the C/S composite) for 200 cycles under lean electrolyte conditions, corresponding to a high energy density of 974 Wh⋅kg^-1 The superior electrochemical performance of the chemical bonding-stabilized C/S composite renders it a promising cathode material for high-energy and long-cycle-life LSBs.

Template method for fabricating Co and Ni nanoparticles/porous channels carbon for solid-state sodium-sulfur battery

Room-temperature sodium/sulfur battery has raised concern due to the superiority of high theoretical capacity and low cost that promise for large-scale application. However, the sluggish electrochemical activity and "shuttle effect" limits the progress of practical application. This work designs a template method for constructing metal/carbon sulfur host, which possesses metal (Co, Ni) nanoparticles highly distributed in large amounts of porous channels in carbon sphere. The metal nanoparticles assist in sulfur immobilization, electric conductivity and catalyze reaction kinetics, meanwhile the hollow channels can buffer the volume change of sulfur. When testing as the liquid/solid-state room-temperature Na/S batteries, the S@Co/C and S@Ni/C electrodes deliver high capacities and rate capability. This template method possesses utility potential in developing high-powered RT Na/S batteries, which provides possibility to for the preparation of various electrode materials in battery technology.

Carbonization of single polyacrylonitrile chains in coordination nanospaces

It has been over half a century since polyacrylonitrile (PAN)-based carbon fibers were first developed. However, the mechanism of the carbonization reaction remains largely unknown. Structural evolution of PAN during the preoxidation reaction, a stabilization reaction, is one of the most complicated stages because many chemical reactions, including cyclization, dehydration, and cross-linking reactions, simultaneously take place. Here, we report the stabilization reaction of single PAN chains within the one-dimensional nanochannels of metal–organic frameworks (MOFs) to study an effect of interchain interactions on the stabilization process as well as the structure of the resulting ladder polymer (LP). The stabilization reaction of PAN within the MOFs could suppress the rapid generation of heat that initiates the self-catalyzed reaction and inevitably provokes many side-reactions and scission of PAN chains in the bulk state. Consequently, LP prepared within the MOFs had a more extended conjugated backbone than the bulk condition.

Accommodation of polyacrylonitrile in MOFs facilitated and regulated the transformation to ladder polymer in the carbonization process.

Synergistic Effect of Salinized Quinone for Entrapment of Polysulfides for High-Performance Li-S Batteries

Lithium-sulfur (Li-S) batteries have attracted considerable attention in the energy storage field due to their high theoretical energy density and low price. However, the dissolution of polysulfides and the "shuttle effect" lead to serious capacity degradation, which greatly hinders the industrial application of Li-S batteries. Herein, we propose a bifunctional quinone-type salt to anchor polysulfides and suppress their dissolution for use in high-performance Li-S batteries. We find that the tetrahydroxy-1,4-benzoquinone disodium salt dimer (TBS-dimer) does not dissolve in organic electrolytes and can be generated at 400 °C. The abundant reactive keto groups and double bonds result in the TBS-dimers having numerous "hot spots" for capturing sulfur (TBS/S-400) in the three-dimensional space of the molecule. The insolubility and abundant active sites of the organic salt remarkably suppress the dissolution of lithium polysulfides. As a result, the TBS/S-400 composite delivers a capacity decay rate of only 0.023% per cycle over 600 cycles at 2 C. The use of organic salts to effectively suppress the dissolution of lithium polysulfides opens a new avenue for the practical applications of high-performance Li-S batteries.

Stable Lithium Metal Anode Enabled by a Lithiophilic and Electron/Ion Conductive Framework

Li metal anode has been considered as the ideal anode for next-generation batteries due to its ultrahigh capacity and lowest electrochemical potential. However, its practical application is still impeded by low Coulombic efficiency, huge volume change, and safety hazards arising from Li dendrite growth. In this work, a three-dimensional (3D) structured highly stable Li metal anode is designed and easily preapred. Benefiting from the in situ reaction between Li metal and AlN, highly Li⁺ conductive Li₃N and lithiophilic LiAl alloy have been simultaneously formed and homogeneously distributed in the framework, in which Li metal is finely dispersed and embedded. The outstanding electron/ion mixed conductivity of Li₃N/LiAl and 3D composite structure with enhanced interfacial area significantly improve the electrode kinetics and suppress the volume change on cycling, while a lithiophilic effect of LiAl alloy and uniform distribution of Li ion flux inside the electrode avoid dendritic Li deposition. As a result, the proposed Li metal electrode exhibits exceptional electrochemical reversibility in both carbonate and ether-based electrolytes. Paired with LiFePO₄ and sulfurized polyacrylonitrile (S@pPAN) cathodes, the full cells deliver highly stable and long-term cycling performance. Therefore, the proposed strategy to fabricate Li metal anodes could promote the practical application of Li metal batteries.