Beyond intercalation-based Li-ion batteries: the state of the art and challenges of electrode materials reacting through conversion reactions

Origins of Severe Structural Changes during Alloying-Dealloying Reactions in Black Phosphorus

Li-alloying reactions facilitate the incorporation of a large number of Li atoms into the crystalline structures of electrodes, such as black phosphorus (BP). However, the reactions inevitably induce multistep phase transitions characterized by drastic atomic rearrangements and lattice collapse. Despite many theoretical and experimental studies on alloying mechanisms, long-term debates persist regarding the structures of the intermediate phases, the accurate pathways of phase transitions, the formation of specific configurations, and alloying/dealloying reversibility. Here, through a combination of operando electron diffraction measurements and ab initio simulations at the atomic and electronic scales, we identify key factors that govern the severe structural changes during alloying-dealloying reactions in BP. P-P bonds of three-bond P atoms are continuously broken during lithiation, generating two-bond P atoms with a high ability to accept inserted electrons and Li ions. Consequently, the pristine layered structure in BP is transformed to P7 cages in Li3P7, which then evolve to chain configurations in LiP and finally to isolated P atoms in Li3P. Specifically, the preferential formation of the P7 cage results from its lowest binding energy with three Li ions compared to other cage isomers. Furthermore, only LiP can be reversibly transformed to the crystalline structure of Li3P7 during charge, but it is thermodynamically favorable for Li3P7 and Li3P intermediates to be delithiated to amorphous structures. Our findings offer unique insights into the alloying mechanisms and deepen the fundamental understanding of alloying anode systems.

Strategies for improving cathode electrolyte interphase in high-performance dual-ion batteries

Dual-ion batteries (DIBs) offer high energy density due to the ability to intercalate both anions and cations, thereby increasing the cutoff voltage and battery capacity. Graphite, with its ordered layered structure and cost-effectiveness, is commonly employed as the cathode material for DIBs. However, the discharge capacity of graphite cathodes is relatively low, and their cycling stability is poor, limiting the practical applications of DIBs. The formation of cathode electrolyte interphase (CEI) on the graphite cathode surface is closely related to anion behavior. Constructing a stable cathode electrolyte interface is crucial for improving the stability of anion storage. Therefore, we introduce a series of strategies to enhance the quality of the CEI layer, including additives, binders, main salts or solvents, high-concentration electrolytes, doping elements, artificial CEI, and graphite surface modifications. These strategies improve the CEI by enhancing anion transport rates, increasing anion solvation capabilities, and improving the structural stability of graphite cathodes, which is of profound significance for increasing the capacity and stability of DIBs. This review provides inspiration for future CEI research, encouraging further exploration of resources of CEI components and improvement strategies to further promote the development of DIBs technology.

Recent advances in zinc-ion dehydration strategies for optimized Zn-metal batteries

Aqueous Zn-metal batteries have attracted increasing interest for large-scale energy storage owing to their outstanding merits in terms of safety, cost and production. However, they constantly suffer from inadequate energy density and poor cycling stability due to the presence of zinc ions in the fully hydrated solvation state. Thus, designing the dehydrated solvation structure of zinc ions can effectively address the current drawbacks of aqueous Zn-metal batteries. In this case, considering the lack of studies focused on strategies for the dehydration of zinc ions, herein, we present a systematic and comprehensive review to deepen the understanding of zinc-ion solvation regulation. Two fundamental design principles of component regulation and pre-desolvation are summarized in terms of solvation environment formation and interfacial desolvation behavior. Subsequently, specific strategy based distinct principles are carefully discussed, including preparation methods, working mechanisms, analysis approaches and performance improvements. Finally, we present a general summary of the issues addressed using zinc-ion dehydration strategies, and four critical aspects to promote zinc-ion solvation regulation are presented as an outlook, involving updating (de)solvation theories, revealing interfacial evolution, enhancing analysis techniques and developing functional materials. We believe that this review will not only stimulate more creativity in optimizing aqueous electrolytes but also provide valuable insights into designing other battery systems.

Hexagonal MoO3 Anode with Extremely High Capacity and Cyclability for Lithium-Ion Battery: A Combined Theoretical and Experimental Study

It is essential and still remains a big challenge to obtain fast-charge anodes with large capacities and long lifespans for Li-ion batteries (LIBs). Among all of the alternative materials, molybdenum trioxide shows the advantages of large theoretical specific capacity, distinct tunnel framework, and low cost. However, there are also some key shortcomings, such as fast capacity decaying due to structural instability during Li insertion and poor rate performance due to low intrinsic electron conductivity and ion diffusion capability, dying to be overcome. A unique strategy is proposed to prepare Ti-h-MoO3-x@TiO2 nanosheets by a one-step hydrothermal approach with NiTi alloy as a control reagent. The density functional theory (DFT) calculations indicate that the doping of Ti element can make the hexagonal h-MoO3-x material show the best electronic structure and it is favor to be synthesized. Furthermore, the hexagonal Ti-h-MoO3-x material has better lithium storage capacity and lithium diffusion capacity than the orthogonal α-MoO3 material, and its theoretical capacity is more than 50% higher than that of the orthogonal α-MoO3 material. Additionally, it is found that Ti-h-MoO3-x@TiO2 as an anode displays extremely high reversible discharge/charge capacities of 1326.8/1321.3 mAh g-1 at 1 A g-1 for 800 cycles and 611.2/606.6 mAh g-1 at 5 A g-1 for 2000 cycles. Thus, Ti-h-MoO3-x@TiO2 can be considered a high-power-density and high-energy-density anode material with excellent stability for LIBs.

Achievements, challenges, and perspectives in the design of polymer binders for advanced lithium-ion batteries

Energy storage devices with high power and energy density are in demand owing to the rapidly growing population, and lithium-ion batteries (LIBs) are promising rechargeable energy storage devices. However, there are many issues associated with the development of electrode materials with a high theoretical capacity, which need to be addressed before their commercialization. Extensive research has focused on the modification and structural design of electrode materials, which are usually expensive and sophisticated. Besides, polymer binders are pivotal components for maintaining the structural integrity and stability of electrodes in LIBs. Polyvinylidene difluoride (PVDF) is a commercial binder with superior electrochemical stability, but its poor adhesion, insufficient mechanical properties, and low electronic and ionic conductivity hinder its wide application as a high-capacity electrode material. In this review, we highlight the recent progress in developing different polymeric materials (based on natural polymers and synthetic non-conductive and electronically conductive polymers) as binders for the anodes and cathodes in LIBs. The influence of the mechanical, adhesion, and self-healing properties as well as electronic and ionic conductivity of polymers on the capacity, capacity retention, rate performance and cycling life of batteries is discussed. Firstly, we analyze the failure mechanisms of binders based on the operation principle of lithium-ion batteries, introducing two models of "interface failure" and "degradation failure". More importantly, we propose several binder parameters applicable to most lithium-ion batteries and systematically consider and summarize the relationships between the chemical structure and properties of the binder at the molecular level. Subsequently, we select silicon and sulfur active electrode materials as examples to discuss the design principles of the binder from a molecular structure point of view. Finally, we present our perspectives on the development directions of binders for next-generation high-energy-density lithium-ion batteries. We hope that this review will guide researchers in the further design of novel efficient binders for lithium-ion batteries at the molecular level, especially for high energy density electrode materials.

Revealing the Dynamic Lithiation Process of Copper Disulfide by in Situ TEM

Transition metal oxides, fluorides, and sulfides are extensively studied as candidate electrode materials for lithium-ion batteries driven by the urgency of developing next-generation higher energy density lithium batteries. These conversion-type electrode materials often require nanosized active materials to enable a "smooth" lithiation and de-lithiation process during charge/discharge cycles, determined by their size, structure, and phase. Herein, the structural and chemical changes of Copper Disulfide (CuS2) hollow nanoparticles during the lithiation process through an in situ transmission electron microscopy (TEM) method are investigated. The study finds the hollow structure of CuS2 facilitates the quick formation of fluidic Li2S "drops," accompanied by a de-sulfurization to the Cu7S4 phase. Meanwhile, the metallic Cu phase emerges as fine nanoparticles and grows into nano-strips, which are embedded in the Li2S/Cu7S4 matrix. These complex nanostructured phases and their spatial distribution can lead to a low de-lithiation barrier, enabling fast reaction kinetics.

High-Entropy Spinel Oxide Ferrites for Battery Applications

Four different high-entropy spinel oxide ferrite (HESO) electrode materials containing 5-6 distinct metals were synthesized by a simple, rapid combustion synthesis process and evaluated as conversion anode materials in lithium half-cells. All showed markedly superior electrochemical performance compared to conventional spinel ferrites such as Fe3O4 and MgFe2O4, having capacities that could be maintained above 600 mAh g-1 for 150 cycles, in most cases. X-ray absorption spectroscopy (XAS) results on pristine, discharged, and charged electrodes show that Fe, Co, Ni, and Cu are reduced to the elemental state during the first discharge (lithiation), while Mn is only slightly reduced. Upon recharge (delithiation), Fe is reoxidized to an average oxidation state of about 2.6+, while Co, Ni, and Cu are not reoxidized. The ability of Fe to be oxidized past 2+ accounts for the high capacities observed in these materials, while the presence of metallic elements after the initial lithiation provides an electronically conductive network that aids in charge transfer.

Reinterpreting the Intercalation-Conversion Mechanism of FeP Anodes in Lithium/Sodium-Ion Batteries from Evolution of the Magnetic Phase

Batteries with intercalation-conversion-type electrodes tend to achieve high-capacity storage, but the complicated reaction process often suffers from confusing electrochemical mechanisms. Here, we reinterpreted the essential issue about the potential of the conversion reaction and whether there is an intercalation reaction in a lithium/sodium-ion battery (LIB/SIB) with the FeP anode based on the evolution of the magnetic phase. Especially, the ever-present intercalation process in a large voltage range followed by the conversion reaction with extremely low potential was confirmed in FeP LIB, while it is mainly the conversion reaction for the sodium storage mechanism in FeP SIB. The insufficient conversion reaction profoundly limits the actual capacity to the expectedly respectable value. Accordingly, a graphene oxide modification strategy was proposed to increase the reversible capacity of FeP LIB/SIB by 99% and 132%, respectively. The results facilitate the development of anode materials with a high capacity and low operating potential.

Lightweight Carbon-Metal-Based Fabric Anode for Lithium-Ion Batteries

Lithium-ion battery electrodes are typically manufactured via slurry casting, which involves mixing active material particles, conductive carbon, and a polymeric binder in a solvent, followed by casting and drying the coating on current collectors (Al or Cu). These electrodes are functional but still limited in terms of pore network percolation, electronic connectivity, and mechanical stability, leading to poor electron/ion conductivities and mechanical integrity upon cycling, which result in battery degradation. To address this, we fabricate trichome-like carbon-iron fabrics via a combination of electrospinning and pyrolysis. Compared with slurry cast Fe2O3 and graphite-based electrodes, the carbon-iron fabric (CMF) electrode provides enhanced high-rate capacity (10C and above) and stability, for both half cell and full cell testing (the latter with a standard lithium nickel manganese oxide (LNMO) cathode). Further, the CMFs are free-standing and lightweight; therefore, future investigation may include scaling this as an anode material for pouch cells and 18,650 cylindrical batteries.

Insertion of fluorine into a LiFePO4 electrode material by gas-solid fluorination

In this study, the insertion of fluorine into LiFePO4 was carried out under molecular fluorine F2 at different temperatures. The reactivity strongly depends on the applied fluorination temperature, leading to very different products: core delithiation of the material is observed at low temperatures with the formation of a LiF shell around particles, while the material decomposes to gradually form a mixture of α-FeF3 and α-Li3FeF6 iron fluorides at higher temperatures. A second thermal treatment under N2 leads to the formation of LiFePO4F in a new way that has not been reported before. Supported by X-ray diffraction, Raman, infrared, Mössbauer spectroscopies, 7Li nuclear magnetic resonance and electrochemical characterization of the different materials, this report demonstrates various fluorination mechanisms for LiFePO4, from chemical delithiation to the stable pure fluorinated form LiFePO4F and illustrates an innovative method that can be extended to obtain the triphylite form of NaFePO4.

Metal chloride cathodes for next-generation rechargeable lithium batteries

Rechargeable lithium-ion batteries (LIBs) have prospered a rechargeable world, predominantly relying on various metal oxide cathode materials for their abilities to reversibly de-/intercalate lithium-ion, while also serving as lithium sources for batteries. Despite the success of metal oxide, issues including low energy density have raised doubts about their suitability for next-generation lithium batteries. This has sparked interest in metal chlorides, a neglected cathode material family. Metal chlorides show promise with factors like energy density, diffusion coefficient, and compressibility. Unfortunately, challenges like high solubility hamper their utilization. In this review, we highlight the opportunities for metal chlorides in the post-lithium-ion era. Subsequently, we summarize their dissolution challenges. Furthermore, we discuss recent advancements, encompassing liquid-state electrolyte engineering, solid-state electrolytes (SSEs) cooperation, and LiCl-based cathodes. Finally, we provide an outlook on future research directions of metal chlorides, emphasizing electrode fabrication, electrolyte design, the application of SSEs, and the exploration of conversion reactions.

Thermodynamics, Adhesion, and Wetting at Li/Cu(-Oxide) Interfaces: Relevance for Anode-Free Lithium-Metal Batteries

A rechargeable battery that employs a Li metal anode requires that Li be plated in a uniform fashion during charging. In "anode-free" configurations, this plating will occur on the surface of the Cu current collector (CC) during the initial cycle and in any subsequent cycle where the capacity of the cell is fully accessed. Experimental measurements have shown that the plating of Li on Cu can be inhomogeneous, which can lower the efficiency of plating and foster the formation of Li dendrites. The present study employs a combination of first-principles calculations and sessile drop experiments to characterize the thermodynamics and adhesive (i.e., wetting) properties of interfaces involving Li and other phases present on or near the CC. Interfaces between Li and Cu, Cu2O, and Li2O are considered. The calculations predict that both Cu and Cu2O surfaces are lithiophilic. However, sessile drop measurements reveal that Li wetting occurs readily only on pristine Cu. This apparent discrepancy is explained by the occurrence of a spontaneous conversion reaction, 2 Li + Cu2O → Li2O + 2 Cu, that generates Li2O as one of its products. Calculations and sessile drop measurements show that Li does not wet (newly formed) Li2O. Hence, Li that is deposited on a Cu CC where surface oxide species are present will encounter a compositionally heterogeneous substrate comprising lithiophillic (Cu) and lithiophobic (Li2O) regions. These initial heterogeneities have the potential to influence the longer-term behavior of the anode under cycling. In sum, the present study provides insights into the early stage processes associated with Li plating in anode-free batteries and describes mechanisms that contribute to inefficiencies in their operation.

HsGDY 3D Framework-Encapsulated Cu2O Quantum Dots for High-Efficiency Energy Storage

Nanostructured electrode materials become a vital component for future electrode materials because of their short electron and ion transport distances for fast charge and discharge processes and sufficient space between particles for volume expansion. So, achieving a smaller size of the nanomaterial with stable structure and high electrode performance is always the pursuit. Herein, the hybrid electrode material system hydrogen-substituted graphdiyne (HsGDY)/Cu2O-quantum dots (QDs) composed of an active carbon substrate and vibrant metal oxide QD load was established by HsGDY and cuprous oxide. The HsGDY frame with conjugated structure not only delivers impressive capacity by a self-exchange mechanism but also characterizes a matrix to forge strong connections with numerous active Cu2O-QDs for the prevention of aggregation, leading to a homogeneous storage and transport of charge in a bulk material of crisscross structural pores. QD-based electrode materials would exhibit desired capacities by their large surface area, abundant active surface atoms, and the short diffusion pathway. The hybrid system of HsGDY/Cu2O-QDs delivers an ultrahigh capacity of 1230 mA h g-1 with loading density reaching up to 1 mg cm-2. In the meantime, the electrode exhibits a long cycle stability of over 8000 cycles. The synergistic effect endows the hybrid system electrode with an approximately theoretical energy density, suggesting the great potential of such carbon/QD hybrid material system applied for high-performance batteries.

Hollow Bowl NiS2 @polyaniline Conductive Linker/Graphene Conductive Network: A Triple Composite for High-Performance Supercapacitor Applications

The achievement of the outstanding theoretical capacitance of nickel sulfide (NiS2 ) is challenging due to its low conductivity, slow electrochemical kinetics, and poor structural stability. In this study, we utilize polyaniline (PANI) as a linker to anchor the NiS2 with a hollow bowl-like structure, uniformly dispersed at the surface of graphene oxide (GO)(NiS2 @15PG). The presence of PANI provides growth sites, resulting in a uniform and dense arrangement of NiS2 . This morphological modulation of NiS2 increases the contact area between the active material to electrolyte. Additionally, PANI effectively connects NiS2 with the conductive network of GO, which advances the electrical conductivity and ion diffusion properties. As a result, the Rct (charge transfer resistance) and Zw (Warburg impedance) of NiS2 @15PG decrease by 82.61 % and 66.76 % respectively. This unique structure confers NiS2 @15PG with high specific capacitance (536.13 C g-1 at 1 A g-1 ) and excellent multiplicative property of 60.93 % at 20 A g-1 . The assembled NiS2 @15PG//YP-50 supercapacitors (HSC) demonstrates an energy density (13.09 Wh kg-1 ) at a high-power density (16 kW kg-1 ). The capacity retention after 10,000 cycles at 5 A g-1 is 86.59 %, indicating its significant potential for practical applications.

Li Salt Assisted Highly Flexible Carbonaceous Ni3N@polyimide Electrode for an Efficient Asymmetric Supercapacitor

This work used a highly flexible, sustainable polyimide tape as a substrate to deposit ductile-natured carbonaceous Ni3N (C/Ni3N@polyimide) material for supercapacitor application. C/Ni3N was prepared using a co-sputtering technique, and this method also provided better adhesion of the electrode material over the substrate, which is helpful in improving bending performance. The ductile behavior of the sputter-grown electrode and the high flexibility of the polyimide tape provide ultimate flexibility to the C/Ni3N@polyimide-based supercapacitor. To achieve optimum electrochemical performance, a series of electrochemical tests were done in the presence of various electrolytes. Further, a flexible asymmetric supercapacitor (NC-FSC) (C/Ni3N//carbon@polyimide) was assembled by using C/Ni3N as a cathode and a carbon thin film as an anode, separated by a GF/C-glass microfiber soaked in optimized 1 M Li2SO4 aqueous electrolyte. The NC-FSC offers a capacitance of 324 mF cm-2 with a high areal energy density of 115.26 μWh cm-2 and a power density of 811 μW cm-2, with ideal bending performance.

Carbon-coated iron selenide derived from double-framework as an advance anode for Na-ion battery

Owing to the desirable nano-morphology, controllable structure, and ease of preparation, metal-organic frameworks (MOFs) are widely used as the precursors for electrodes in Na-ion battery (NIB). However, MOF structures are prone to fracture and collapse during the reactions. Additionally, MOF-derived electrodes often exhibit a high expansion rate, which negatively impacts the long cyclic capability of NIBs. Herein, we employed a stable covalent-organic framework (COF) as a protective coating for the first time to preserve the MOF structure. A shuttle-like iron selenide (Fe3Se4) coated with N-doped carbon (NC) was synthesized using a simple hydrothermal method, surface coating, and subsequent selenizing process. Due to its large specific surface area and well-developed porosity, the double-framework derived Fe3Se4/NC electrode provides abundant active sites for Na+ storage. The COF and COF-derived NC protect the structure of Fe3Se4/NC during synthesis and cyclic process, respectively. The high conductivity of the NC coating enhances the electron/ion conductivity of Fe3Se4/NC, thereby beneficial the rate performance. As the material anode for NIB, the Fe3Se4/NC electrode exhibits a high initial charging/discharging capacity (425.7/478.4 mAh·g-1 with an initial Coulombic efficiency of 89.0 %), excellent rate performance (333.5 mAh·g-1 at 12 A·g-1), long-durable cycle capability (290.8 mAh·g-1 after 1000 cycles at 8 A·g-1) and fast charging ability (143 s). This work provides a novel strategy of "COF on MOF" to prepare high-performance electrode materials for NIB.

Electrochemical interfacial catalysis in Co-based battery electrodes involving spin-polarized electron transfer

Interfacial catalysis occurs ubiquitously in electrochemical systems, such as batteries, fuel cells, and photocatalytic devices. Frequently, in such a system, the electrode material evolves dynamically at different operating voltages, and this electrochemically driven transformation usually dictates the catalytic reactivity of the material and ultimately the electrochemical performance of the device. Despite the importance of the process, comprehension of the underlying structural and compositional evolutions of the electrode material with direct visualization and quantification is still a significant challenge. In this work, we demonstrate a protocol for studying the dynamic evolution of the electrode material under electrochemical processes by integrating microscopic and spectroscopic analyses, operando magnetometry techniques, and density functional theory calculations. The presented methodology provides a real-time picture of the chemical, physical, and electronic structures of the material and its link to the electrochemical performance. Using Co(OH)2 as a prototype battery electrode and by monitoring the Co metal center under different applied voltages, we show that before a well-known catalytic reaction proceeds, an interfacial storage process occurs at the metallic Co nanoparticles/LiOH interface due to injection of spin-polarized electrons. Subsequently, the metallic Co nanoparticles act as catalytic activation centers and promote LiOH decomposition by transferring these interfacially residing electrons. Most intriguingly, at the LiOH decomposition potential, electronic structure of the metallic Co nanoparticles involving spin-polarized electrons transfer has been shown to exhibit a dynamic variation. This work illustrates a viable approach to access key information inside interfacial catalytic processes and provides useful insights in controlling complex interfaces for wide-ranging electrochemical systems.

Lithiation of Anodic Magnetite-Hematite Nanotubes Formed on Iron

Electrochemically active iron oxide nanotubes formed by anodization are of high interest as battery components in various battery systems due to their 1D geometry, offering high volume expansion tolerance and applications without the use of binders and conductive additives. This work takes a step forward toward understanding lithium-ion storage in 1D nanotubes through the analysis of differential capacity plots d(Q - Q0)·dE-1 supported by in situ Raman spectroscopy observations. The iron oxide nanotubes were synthesized by anodizing polycrystalline iron and subsequently modified by thermal treatment in order to control the degree of crystallinity and the ratio of hematite (Fe2O3) to magnetite (Fe3O4). The electrochemical fingerprints revealed a quasi-reversible lithiation/delithiation process through Li2O formation. Significant improvement in electrochemical performance was found to be related to the high degree of crystallinity and the increase of the hematite (Fe2O3) to magnetite (Fe3O4) ratio. In situ mechanistic studies revealed a reversible reduction of iron oxide to metallic iron simultaneously with Li2O formation.

Tailored synthesis of NdMnxFe1-xO3 perovskite nanoparticles with oxygen-vacancy defects for lithium-ion battery anodes

In this study, we synthesize nanostructured NdMnxFe1-xO3 perovskites using a facile method to produce materials for the high-working-efficiency anodes of Li-ion batteries. A series of characterization assessments (e.g., X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and electron microscopy) were conducted, and the results confirmed the efficacious partial replacement of Fe ions with Mn ions in the NdFeO3 perovskite structure, occurrence of both amorphous and crystalline structures, presence of oxygen vacancies (VO), and interconnection between nanoparticles. The possibility of Mn ion replacement significantly affects the size, amount of VO, and ratio of amorphous phase in NdMnxFe1-xO3 perovskites. The NdMnxFe1-xO3 perovskite with x = 0.3 presents a notable electrochemical performance, including low charge transfer resistance, durable Coulombic efficiency, first-rate capacity reservation, high pseudo-behavior, and elongated 150-cycle service life, whereas no discernible capacity deterioration is observed. The reversible capacity of the anode after the 150th-cylcle was 713 mAh g-1, which represents a high-capacity value. The outstanding electrochemical efficiency resulted from the optimum presence of VO, interconnection between the nanoparticles, and distinctive properties of the NdFeO3 perovskite. The interconnection between nanoparticles was advantageous for forming a large electrolyte-electrode contact area, improving Li-ion diffusion rates, and enhancing pseudocapacitive effect. The attributes of perovskite crystals, coexistence of Mn and Fe throughout the charge/discharge process, and optimum VO precluded the electrode devastation that caused the Li2O-phase decomposition catalysis, enabling favorable reversible Li storage.

A triple-synergistic small-molecule sulfur cathode promises energetic Cu-S electrochemistry

Metal-sulfur batteries have received great attention for electrochemical energy storage due to high theoretical capacity and low cost, but their further development is impeded by low sulfur utilization, poor electrochemical kinetics, and serious shuttle effect of the sulfur cathode. To avoid these problems, herein, a triple-synergistic small-molecule sulfur cathode is designed by employing N, S co-doped hierarchical porous bamboo charcoal as a sulfur host in an aqueous Cu-S battery. Expect the enhanced conductivity and chemisorption induced by N, S synergistic co-doping, the intrinsic synergy of macro-/meso-/microporous triple structure also ensures space-confined small-molecule sulfur as high utilization reactant and effectively alleviates the volume expansion during conversion reaction. Under a further joint synergy between hierarchical structure and heteroatom doping, the resulting sulfur cathode endows the Cu-S battery with outstanding electrochemical performance. Cycled at 5 A g-1, it can deliver a high reversible capacity of 2,509.8 mAh g-1 with a good capacity retention of 97.9% after 800 cycles. In addition, a flexible hybrid pouch cell built by a small-molecule sulfur cathode, Zn anode, and gel electrolytes can firmly deliver high average operating voltage of about 1.3 V with a reversible capacity of over 2,500 mAh g-1 under various destructive conditions, suggesting that the triple-synergistic small-molecule sulfur cathode promises energetic metal-sulfur batteries.

A Minireview of the Solid-State Electrolytes for Zinc Batteries

Aqueous zinc-ion batteries (ZIBs) have gained significant recognition as highly promising rechargeable batteries for the future due to their exceptional safety, low operating costs, and environmental advantages. Nevertheless, the widespread utilization of ZIBs for energy storage has been hindered by inherent challenges associated with aqueous electrolytes, including water decomposition reactions, evaporation, and liquid leakage. Fortunately, recent advances in solid-state electrolyte research have demonstrated great potential in resolving these challenges. Moreover, the flexibility and new chemistry of solid-state electrolytes offer further opportunities for their applications in wearable electronic devices and multifunctional settings. Nonetheless, despite the growing popularity of solid-state electrolyte-based-ZIBs in recent years, the development of solid-state electrolytes is still in its early stages. Bridging the substantial gap that exists is crucial before solid-state ZIBs become a practical reality. This review presents the advancements in various types of solid-state electrolytes for ZIBs, including film separators, inorganic additives, and organic polymers. Furthermore, it discusses the performance and impact of solid-state electrolytes. Finally, it outlines future directions for the development of solid-state ZIBs.

Metal-Redox Bicatalysis Batteries for Energy Storage and Chemical Production

New types of electrochemical energy conversion and storage devices based on redox electrocatalytic reactions possess great potential in renewable energy to maximize energy utilization and balance environmental issues. The typical device is the metal-redox bicatalysis battery, where the cathode is redox bifunctional catalyst (named as redox bicatalyst) with gas, solid, liquid as active reactants while anode is metal, driven by cathodic redox electrocatalytic reactions during charge/discharge processes, which promotes the energy storage and chemical production. In this system, the metal anode, redox-bicatalyst cathode, electrolytes, and the redox electrochemical reactions can be modified and adjusted to achieve the optimal energy conversion and utilization. Therefore, the deep understanding of the electrochemical system is conducive to designing new devices to meet the demand among various applications, including energy storage and conversion. In this review, the authors clarify the fundamentals and design principles of the rechargeable/reversible metal-redox bicatalysis batteries and how each part affects the devices in energy conversion and chemical production. The authors summarize the electrocatalytic reduction/oxidation reactions, the reported systems relied on redox reactions, and the corresponding redox bicatalysts. Finally, a perspective of the key challenges and the possible new types of metal-redox bicatalysis batteries for efficient energy utilization and chemical production are given.

Does Water-in-Salt Electrolyte Subdue Issues of Zn Batteries?

Zn-metal batteries (ZnBs) are safe and sustainable because of their operability in aqueous electrolytes, abundance of Zn, and recyclability. However, the thermodynamic instability of Zn metal in aqueous electrolytes is a major bottleneck for its commercialization. As such, Zn deposition (Zn2+ → Zn(s)) is continuously accompanied by the hydrogen evolution reaction (HER) (2H+ → H2 ) and dendritic growth that further accentuate the HER. Consequently, the local pH around the Zn electrode increases and promotes the formation of inactive and/or poorly conductive Zn passivation species (Zn + 2H2 O → Zn(OH)2 + H2 ) on the Zn. This aggravates the consumption of Zn and electrolyte and degrades the performance of ZnB. To propel HER beyond its thermodynamic potential (0 V vs standard hydrogen electrode (SHE) at pH 0), the concept of water-in-salt-electrolyte (WISE) has been employed in ZnBs. Since the publication of the first article on WISE for ZnB in 2016, this research area has progressed continuously. Here, an overview and discussion on this promising research direction for accelerating the maturity of ZnBs is provided. The review briefly describes the current issues with conventional aqueous electrolyte in ZnBs, including a historic overview and basic understanding of WISE. Furthermore, the application scenarios of WISE in ZnBs are detailed, with the description of various key mechanisms (e.g., side reactions, Zn electrodeposition, anions or cations intercalation in metal oxide or graphite, and ion transport at low temperature).

Highly durable and sustainable copper-iron-tin-sulphide (Cu2FeSnS4) anode for Li-ion batteries: effect of operating temperatures

Operating temperatures considerably influence the energy storage mechanism of the anode of Li-ion batteries (LiBs). This effect must be comprehensively studied to facilitate the effective integration of LiBs in practical applications and battery management. In this study, we fabricated a novel anode material, i.e., copper-iron-tin-sulphide (Cu2FeSnS4, CFTS), and investigated the corresponding LiB performance at operating temperatures ranging from 10 °C to 55 °C. The CFTS anode exhibited a discharge capacity of 283.1 mA h g-1 at room temperature (25 °C), which stabilized to 174.0 mA h g-1 in repeated cycles tested at a current density of 0.1 A g-1. The discharge capacity at higher operating temperatures, such as 40 °C and 55 °C, is found to be 209.3 and 230.0 mA h g-1 respectively. In contrast, the discharge capacity decreased to 36.2 mA h g-1 when the temperature decreased to 10 °C. Electrothermal impedance spectroscopy was performed to determine the rate of chemical reactions, mobility of active species, and change in internal resistance at different operating temperatures. In terms of the cycle life, CFTS exhibited outstanding cycling stability for more than 500 charge/discharge cycles, with a 146% capacity retention and more than 80% coulombic efficiency. The electrochemical investigation revealed that the charge storage in the CFTS anode is attributable to capacitive-type and diffusion-controlled mechanisms.

Covalent netting restrains dissolution enabling stable high-loading and high-rate iron difluoride cathodes

Metal fluoride conversion cathodes are promising for the production of cheap, sustainable, and high-energy lithium-ion batteries. Yet, such systems are plagued by active material dissolution that causes capacity fade and hinders commercialization. Here, a covalent netting strategy is proposed to overcome this hurdle. In a proof-of-concept design, polydopamine derived carbon-mediated covalent binding inhibited the dissolution, while the pyrolyzed bacterial cellulose netting structure furnished fast electronic and ionic transport pathways. We demonstrate high-capacity, high-rate and long-lasting stability attained at practical loading levels. Our investigations suggest that the covalent netting-enabled formation of a robust and efficient blocking layer, highly competent in suppressing the leaching, is key for a stable performance. The successful stabilization of metal difluorides in the absence of electrolyte engineering opens an avenue for their practical deployment in future higher-level but lower-cost batteries, and provides a solution to similar challenges encountered by other dissolving energy electrode materials.

Decoupling of the anode and cathode ultrasonic responses to the state of charge of a lithium-ion battery

An ultrasonic method for lithium-ion battery (LIB) state of charge (SoC) estimation is a promising emerging technology which may largely improve the SoC estimation accuracy. Previously, it was unknown whether the SoC change induced ultrasonic signal change originated from the anode or the cathode, because the thicknesses of cathodes, anodes and separators are much smaller than the ultrasonic wavelength, which makes it impossible to decouple the anodic and cathodic influence. To quantitatively solve the above problem, we have designed a special half-cell architecture with an extra-thick separator (675 μm) to study the reflected ultrasonic signal. The thickened separator would significantly delay the reflection of ultrasonic waves from the counter-electrode (Li), so that the influence of the working electrode (LiFePO4 or graphite) on the ultrasonic wave can be studied separately. As a result, in the Gr anode, the time of flight (ToF) of the ultrasonic wave decreases with SoC, the changing rate coefficient of which is in the range of -110 to -70 ps μmGr thickness-1, depending on the compact density. A lower compact density electrode leads to a more significant ultrasonic ToF decrease and intensity increase while in the LFP cathode, the ToF increases with SoC, the changing rate coefficient of which is in the range of 15-43 ps μmLFP thickness-1. The ToF change of the transmitted ultrasound through multilayered LIB matches very well with the sum of the ToF change in each electrode measured with our half-cells.

Recent Configurational Advances for Solid-State Lithium Batteries Featuring Conversion-Type Cathodes

Solid-state lithium metal batteries offer superior energy density, longer lifespan, and enhanced safety compared to traditional liquid-electrolyte batteries. Their development has the potential to revolutionize battery technology, including the creation of electric vehicles with extended ranges and smaller more efficient portable devices. The employment of metallic lithium as the negative electrode allows the use of Li-free positive electrode materials, expanding the range of cathode choices and increasing the diversity of solid-state battery design options. In this review, we present recent developments in the configuration of solid-state lithium batteries with conversion-type cathodes, which cannot be paired with conventional graphite or advanced silicon anodes due to the lack of active lithium. Recent advancements in electrode and cell configuration have resulted in significant improvements in solid-state batteries with chalcogen, chalcogenide, and halide cathodes, including improved energy density, better rate capability, longer cycle life, and other notable benefits. To fully leverage the benefits of lithium metal anodes in solid-state batteries, high-capacity conversion-type cathodes are necessary. While challenges remain in optimizing the interface between solid-state electrolytes and conversion-type cathodes, this area of research presents significant opportunities for the development of improved battery systems and will require continued efforts to overcome these challenges.

Magnesium: properties and rich chemistry for new material synthesis and energy applications

Magnesium (Mg) has many unique properties suitable for applications in the fields of energy conversion and storage. These fields presently rely on noble metals for efficient performance. However, among other challenges, noble metals have low natural abundance, which undermines their sustainability. Mg has a high negative standard reduction potential and a unique crystal structure, and its low melting point at 650 °C makes it a good candidate to replace or supplement numerous other metals in various energy applications. These attractive features are particularly helpful for improving the properties and limits of materials in energy systems. However, knowledge of Mg and its practical uses is still limited, despite recent studies which have reported Mg's key roles in synthesizing new structures and modifying the chemical properties of materials. At present, information about Mg chemistry has been rather scattered without any organized report. The present review highlights the chemistry of Mg and its uses in energy applications such as electrocatalysis, photocatalysis, and secondary batteries, among others. Future perspectives on the development of Mg-based materials are further discussed to identify the challenges that need to be addressed.

Lignin-based nitrogen/sulfur dual-doped nanosheets decorated with Co1-xS nanoparticles as efficient bifunctional oxygen electrocatalysts

The development of efficient, cost-effective, bifunctional cathode catalyst materials to replace precious metals is highly attractive for the fabrication of Zn-air battery. Here, the three-dimensional N and S co-doped carbon nanosheets loaded with cobalt sulfide nanoparticles (Co_1-xS@SNFC) for bifunctional oxygen electrocatalysis were synthesized with Co(NO₃)₂·6H₂O as the Co source, lignin as the carbon source, thiourea as the nitrogen/ sulfur source, and MgO as the template. The synergistic effect of multiple active sites gives the Co_1-xS@SNFC fast electrochemical kinetic properties and excellent stability to oxygen reduction reactions (ORR) and oxygen evolution reactions (OER). The half-wave potential and overpotential of Co_1-xS@SNFC were 0.84 mV and 306 mV, respectively, which is closed to commercial noble metal catalysts. In addition, Co_1-xS@SNFC exhibited four-electron transfer characteristics and ultra-low tafel slope. Compared with commercial Pt/C, the Zn-air battery assembled from Co_1-xS@SNFC exhibited a low voltage gap of polarization curve (0.75 V) between charging and discharge and high power density (207 mWcm^-2) in alkaline electrolyte. This work developed a green and novel fabrication approach for the synthesis of bifunctional electrocatalyst and provides a new idea for high-value utilization of biomass.

Initiating Reversible Aqueous Copper-Tellurium Conversion Reaction with High Volumetric Capacity through Electrolyte Engineering

Pursuing conversion-type cathodes with high volumetric capacity that can be used in aqueous environments remains rewarding and challenging. Tellurium (Te) is a promising alternative electrode due to its intrinsic attractive electronic conductivity and high theoretical volumetric capacity yet still to be explored. Herein, the kinetically/thermodynamically co-dominat copper-tellurium (Cu-Te) alloying phase-conversion process and corresponding oxidation failure mechanism of tellurium are investigated using in situ synchrotron X-ray diffraction and comprehensive ex situ characterization techniques. By virtue of the fundamental insights into the tellurium electrode, facile and precise electrolyte engineering (solvated structure modulation or reductive antioxidant addition) is implemented to essentially tackle the dramatic capacity loss in tellurium, affording reversible aqueous Cu-Te conversion reaction with an unprecedented ultrahigh volumetric capacity of up to 3927 mAh cm^-3 , a flat long discharge plateau (capacity proportion of ≈81%), and an extraordinary level of capacity retention of 80.4% over 2000 cycles at 20 A g^-1 of which lifespan thousand-fold longer than Cu-Te conversion using CuSO₄ -H₂ O electrolyte. This work paves a significant avenue for expanding high-performance conversion-type cathodes toward energetic aqueous multivalent-ion batteries.

Uncovering the origin of the anomalously high capacity of a 3d anode via in situ magnetometry

Transition metals can deliver high lithium storage capacity, but the reason behind this remains elusive. Herein, the origin of this anomalous phenomenon is uncovered by in situ magnetometry taking metallic Co as a model system. It is revealed that the lithium storage in metallic Co undergoes a two-stage mechanism involving a spin-polarized electron injection to the 3d orbital of Co and subsequent electron transfer to the surrounding solid electrolyte interphase (SEI) at lower potentials. These effects create space charge zones for fast lithium storage on the electrode interface and boundaries with capacitive behavior. Therefore, the transition metal anode can enhance common intercalation or pseudocapacitive electrodes at high capacity while showing superior stability to existing conversion-type or alloying anodes. These findings pave the way for not only understanding the unusual lithium storage behavior of transition metals but also for engineering high-performance anodes with overall enhancement in capacity and long-term durability.

Achieving ultrastability and efficient lithium storage capacity with high-energy iron(II) oxalate anode materials by compositing Ge nano-conductive sites

Transition metal oxalates (TMOxs, represented by iron oxalate) have attracted considerable interest in anode materials due to their excellent lithium storage properties and consistent cyclic performance. Although investigations into their electrochemical capabilities and lithium storage mechanisms are gradually deepening, the complex and varied electrochemical reactions in the initial cycle, poor inherent conductivity, and high irreversible capacity constrain their further development. Herein, to solve the above-mentioned problems, we controlled the hydrothermal synthesis conditions of iron oxalate with the assistance of organic solvents, which induced the growth of iron oxalate crystals with nano Ge metal as the core. The metal Ge space sites compounded to the stacked iron oxalate particles act as conductive nodes and metal frames, which enhances both the strength of iron oxalate samples and electronic conductivity and lithium-ion diffusion inside the electrode materials. This special structure enhances the electrochemical activity of iron oxalates and improves their lithium storage capability. The iron oxalate @ nano Ge metal composite (FCO@Ge-1) exhibits an excellent cycling performance and an appreciable reversible specific capacity (1090 mA h g^-1 after 200 cycles at 1 A g^-1). The obvious polarization and variation of the electrochemical reaction in the initial cycle of iron oxalate are reduced by compositing nano Ge metal. It is demonstrated that nano Ge metal can promote reversible capacity retention from 67.72% to 80.69% in the early cycles. The distinctive structure of iron oxalate @ nano Ge metal composite provides a fresh pathway to enhance oxalate electrochemical reversible lithium storage activity and develop high-energy electrode material by constructing composite space conductive sites.

Electrospun Nanofiber Electrodes for Lithium-Ion Batteries

Electrospun nanofiber materials have the advantages of good continuity, large specific surface areas, and high structural tunability, which provide many desirable characteristics for lithium-ion battery electrodes. Here, the principles and advantages of electrospinning technology are first elaborated, then the previous studies on high-performance nanofibrous electrode materials prepared by electrospinning technology are comprehensively summarized, and the correlation between 1D nanostructured materials and electrode performances is discussed. Finally, the remaining challenges of nanofibrous electrodes are proposed and some future study directions of this particular area are pointed out. This review provides new enlightenment for the design of nanofibrous electrodes toward high-performance lithium-ion batteries.

Metal Fluoride Cathode Materials for Lithium Rechargeable Batteries: Focus on Iron Fluorides

Exploring prospective rechargeable batteries with high energy densities is urgently needed on a worldwide scale to address the needs of the large-scale electric vehicle market. Conversion-type metal fluorides (MFs) are attractive cathodes for next-generation rechargeable batteries because of their high theoretical potential and capacities and provide new perspectives for developing novel battery systems that satisfy energy density requirements. However, some critical issues, such as high voltage hysteresis and poor cycling stability must be solved to further enhance MF cathode materials. In this review, the recent advances in mechanisms focused on FeF₃ cathodes under lithiation/delithiation processes are discussed in detail. Then, the classifications and advantages of various synthesis methods to prepare MF-based materials are first minutely discussed. Moreover, the performance attenuation mechanisms of MFs and the effort in the development of mitigation strategies are comprehensively reviewed. Finally, prospects for the current obstacles and possible research directions, with the aim to provide some inspiration for the development of MF cathode-based batteries are presented.

Synthesis of TiO2 Nanobelt Bundles Decorated with TiO2 Nanoparticles and Aggregates and Their Use as Anode Materials for Lithium-Ion Batteries

TiO₂ nanobelt bundles decorated with TiO₂ aggregates were prepared using an easy and scalable hydrothermal method at various temperatures (170, 190, 210, and 230 °C). It was demonstrated that the synthesis temperature is a key parameter to tune the number of aggregates on the nanobelt surface. Prepared TiO₂ aggregates and nanobelt bundles were used to design anode materials in which the aggregates regulated the pore size and connectivity of the interconnected nanobelt bundle structure. A galvanostatic technique was employed for the electrochemical characterization of TiO₂ samples. Using TiO₂ as a model material due to its small volume change during the cycling of lithium-ion batteries (LIBs), the relationship between the morphology of the anode materials and the capacity retention of the LIBs on cycling is discussed. It was clearly found that the size and connectivity of the pores and the specific surface area had a striking impact on the Li insertion behavior, lithium storage capability, and cycling performance of the batteries. The initial irreversible capacity was shown to increase as the specific surface area increased. As the pore size increased, the ability of the mesoporous anatase to release strain was stronger, resulting in better cycling stability. The TiO₂ powder prepared at a temperature of 230 °C displayed the highest discharge and charge capacities (203.3 mAh/g and 140.8 mAh/g) and good cycling stability.

Reversible flowering of CuO nanoclusters via conversion reaction for dual-ion Li metal batteries

Dual-ion Li metal batteries based on non-flammable SO₂-in-salt inorganic electrolytes ( Li-SO₂ batteries) offer high safety and energy density. The use of cupric oxide (CuO) as a self-activating cathode material achieves a high specific capacity with cost-effective manufacturing in Li-SO₂ batteries, but its cycle retention performance deteriorates owing to the significant morphological changes of the cathode active materials. Herein, we report the catalytic effect of carbonaceous materials used in the cathode material of Li-SO₂ batteries, which act as templates to help recrystallize the active materials in the activation and conversion reactions. We found that the combination of oxidative-cyclized polyacrylonitrile (PAN) with N-doped carbonaceous materials and multi-yolk-shell CuO (MYS-CuO) nanoclusters as cathode active materials can significantly increase the specific capacity to 315.9 mAh g^- 1 (93.8% of the theoretical value) at 0.2 C, which corresponds to an energy density of 1295 Wh kg_CuO^-1, with a capacity retention of 84.46% at the 200th cycle, and the cathode exhibited an atypical blossom-like morphological change.

Effect of Calcination Temperature on the Physicochemical Properties and Electrochemical Performance of FeVO4 as an Anode for Lithium-Ion Batteries

Several electrode materials have been developed to provide high energy density and a long calendar life at a low cost for lithium-ion batteries (LIBs). Iron (III) vanadate (FeVO₄), a semiconductor material that follows insertion/extraction chemistry with a redox reaction and provides high theoretical capacity, is an auspicious choice of anode material for LIBs. The correlation is investigated between calcination temperatures, morphology, particle size, physicochemical properties, and their effect on the electrochemical performance of FeVO₄ under different binders. The crystallite size, particle size, and tap density increase while the specific surface area (S_BET) decreases upon increasing the calcination temperature (500 °C, 600 °C, and 700 °C). The specific capacities are reduced by increasing the calcination temperature and particle size. Furthermore, FeVO₄ fabricated with different binders (35 wt.% PAA and 5 wt.% PVDF) and their electrochemical performance for LIBs was explored regarding the effectiveness of the PAA binder. FV500 (PAA and PVDF) initially delivered higher discharge/charge capacities of 1046.23/771.692 mAhg^-1 and 1051.21/661.849 mAhg^-1 compared to FV600 and FV700 at the current densities of 100 mAg^-1, respectively. The intrinsic defects and presence of oxygen vacancy along with high surface area and smaller particle sizes efficiently enhanced the ionic and electronic conductivities and delivered high discharge/charge capacities for FeVO₄ as an anode for LIBs.

Dual fluorination of polymer electrolyte and conversion-type cathode for high-capacity all-solid-state lithium metal batteries

All-solid-state batteries are appealing electrochemical energy storage devices because of their high energy content and safety. However, their practical development is hindered by inadequate cycling performances due to poor reaction reversibility, electrolyte thickening and electrode passivation. Here, to circumvent these issues, we propose a fluorination strategy for the positive electrode and solid polymeric electrolyte. We develop thin laminated all-solid-state Li||FeF3 lab-scale cells capable of delivering an initial specific discharge capacity of about 600 mAh/g at 700 mA/g and a final capacity of about 200 mAh/g after 900 cycles at 60 °C. We demonstrate that the polymer electrolyte containing AlF3 particles enables a Li-ion transference number of 0.67 at 60 °C. The fluorinated polymeric solid electrolyte favours the formation of ionically conductive components in the Li metal electrode's solid electrolyte interphase, also hindering dendritic growth. Furthermore, the F-rich solid electrolyte facilitates the Li-ion storage reversibility of the FeF3-based positive electrode and decreases the interfacial resistances and polarizations at both electrodes.

Nano-Confined Tin Oxide in Carbon Nanotube Electrodes via Electrostatic Spray Deposition for Lithium-Ion Batteries

The development of novel materials is essential for the next generation of electric vehicles and portable devices. Tin oxide (SnO2), with its relatively high theoretical capacity, has been considered as a promising anode material for applications in energy storage devices. However, the SnO2 anode material suffers from poor conductivity and huge volume expansion during charge/discharge cycles. In this study, we evaluated an approach to control the conductivity and volume change of SnO2 through a controllable and effective method by confining different percentages of SnO2 nanoparticles into carbon nanotubes (CNTs). The binder-free confined SnO2 in CNT composite was deposited via an electrostatic spray deposition technique. The morphology of the synthesized and deposited composite was evaluated by scanning electron microscopy and high-resolution transmission electron spectroscopy. The binder-free 20% confined SnO2 in CNT anode delivered a high reversible capacity of 770.6 mAh g-1. The specific capacity of the anode increased to 1069.7 mAh g-1 after 200 cycles, owing to the electrochemical milling effect. The delivered specific capacity after 200 cycles shows that developed novel anode material is suitable for lithium-ion batteries (LIBs).

New Crosslinked Single-Ion Silica-PEO Hybrid Electrolytes

New single-ion hybrid electrolytes have been synthetized via an original and simple synthetic approach combining Michael addition, epoxidation, and sol-gel polycondensation. We designed an organic PEO network as a matrix for the lithium transport, mechanically reinforced thanks to crosslinking inorganic (SiO_1.5) sites, while highly delocalized anions based on lithium vinyl sulfonyl(trifluoromethane sulfonyl)imide (VSTFSILi) were grafted onto the inorganic sites to produce single-ion hybrid electrolytes (HySI). The influence of the electrolyte composition in terms of the inorganic/organic ratio and the grafted VSTFSILi content on the local structural organization, the thermal, mechanical, and ionic transport properties (ionic conductivity, transference number) are studied by a variety of techniques including SAXS, DSC, rheometry, and electrochemical impedance spectroscopy. SAXS measurements at 25 °C and 60 °C reveal that HySI electrolyte films display locally a spatial phase separation with domains composed of PEO rich phase and silica/VSTFSILi clusters. The size of these clusters increases with the silica and VSTFSILi content. A maximum ionic conductivity of 2.1 × 10^-5 S·cm^-1 at 80 °C has been obtained with HySI having an EO/Li ratio of 20. The Li⁺ ion transfer number of HySI electrolytes is high, as expected for a single-ion electrolyte, and comprises between 0.80 and 0.92.

Latticed-Confined Conversion Chemistry of Battery Electrode

The electrochemical conversion reaction, usually featured by multiple redox processes and high specific capacity, holds great promise in developing high-energy rechargeable battery technologies. However, the complete structural change accompanied by spontaneous atomic migration and volume variation during the charge/discharge cycle leads to electrode disintegration and performance degradation, therefore severely restricting the application of conventional conversion-type electrodes. Herein, latticed-confined conversion chemistry is proposed, where the "intercalation-like" redox behavior is realized on the electrode with a "conversion-like" high capacity. By delicately formulating the high-entropy compounds, the pristine crystal structure can be preserved by the inert lattice framework, thus enabling an ultra-high initial Coulombic efficiency of 92.5% and a long cycling lifespan over a thousand cycles after the quasistatic charge-discharge cycle. This lattice-confined conversion chemistry unfolds a ubiquitous insight into the localized redox reaction and sheds light on developing high-performance electrodes toward next-generation high-energy rechargeable batteries.

Electrochemical Sodiation Mechanism in Magnetite Nanoparticle-Based Anodes: Understanding of Nanoionics-Based Sodium Ion Storage Behavior of Fe3O4

Electrochemical ion storage behaviors of Fe₃O₄ nanoparticles, as a representative transition metal oxide for an environmentally benign and low-cost anode for a sodium-ion battery, are thoroughly investigated through a combination of electrochemical analysis and diagnostics of Fe₃O₄ electrode cells, X-ray-based and spectroscopic analysis of material structure evolution as functions of depth of discharge (DoD) and state of charge (SoC), and first principle modeling. The gravimetric capacity is found to be 50 mA h/g for bulk Fe₃O₄ (50 nm average crystallite size) and 100 mA h/g─about a tenth of the theoretical prediction for complete conversion─for Fe₃O₄ nanoparticles (8.7 nm average particle size), respectively. A fundamental and mechanistic study of material evolution as functions of DoD and SoC shows that Fe₃O₄ does not allow electrochemical incorporation of Na⁺ ions into the empty cation positions of the inverse spinel structure, leading to our assertion that electrochemical intercalation of Na⁺ ions to conversion of the Fe₃O₄ anode in sodium-ion batteries is nonviable. A density functional theory investigation points to the impracticality of the intercalation of Na⁺ ions into Fe₃O₄ and further validates our experimental findings. We propose several possible mechanisms corresponding to the observed low capacity, including formation of solid electrolyte interphases with unfavorable properties and adsorption of Na⁺ ions onto surfaces of nanoparticles and/or at heterointerfaces in Fe₃O₄ composite electrodes in a NaPF₆-based electrolyte system.

Robust Electrocatalytic Li2 S Redox of Li-S Batteries Facilitated by Rationally Fabricated Dual-Defects

The commercialization of lithium-sulfur batteries with ultra-high theoretical energy density is restricted mainly by the notorious polysulfides "shuttle effect" and slow Li₂ S redox reaction kinetics. A sulfur host material with high catalytic activity and high conductivity is greatly desired to improve its electrochemical performance. Herein, a sulfur host material, etched cotton@petroleum asphalt carbon (eCPAC), with high specific surface area and excellent catalytic activity, is demonstrated based on a synergistic strategy of introducing intrinsic lattice defects and composite carbon structure. Benefiting from in situ coupling of amorphous and crystalline materials, eCPAC exhibits high conductivity and high sulfur adsorbability. Furthermore, eCPAC containing dual intrinsic defect sites can catalyze the bidirectional sulfur chemistry of Li₂ S and capture polysulfides, which is also demonstrated by systematic density functional theory calculations and the potential intermittent titration technique. S@eCPAC/Li cells exhibit excellent cycling stability and rate performance, with an average capacity decay rate of only 0.05% over 1000 cycles at 0.5 C and even 0.03% over 600 cycles at 5 C. Meanwhile, the practicality of eCPAC is proven in high-load batteries and pouch batteries. eCPAC provides a reliable strategy for achieving a win-win situation of capturing polysulfides and accelerating Li₂ S redox kinetics.