High energy-density and reversibility of iron fluoride cathode enabled via an intercalation-extrusion reaction

The Effect of Ni Doping on FeOF Cathode Material for High-Performance Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have emerged as a promising alternative to lithium-ion batteries for large-scale energy storage systems due to the abundance and low price of sodium. Until recently, the low theoretical capacities of intercalation-type cathodes less than 250 mAh g-1 have limited the energy density of SIBs. On the other hand, iron oxyfluoride (FeOF) has a high theoretical capacity of ≈885 mAh g-1 as a conversion-type cathode material for SIBs. However, FeOF suffers from poor cycling stability, rate capability, and low initial Coulombic efficiency caused by its low electrical conductivity and slow ionic diffusion kinetics. To solve these problems, doping aliovalent Ni2+ on FeOF electrodes is attempted to improve the electronic conductivity without using a carbon matrix. The ionic conductivity of FeOF is also enhanced due to the formation of oxygen defects in the FeOF crystal structure. The FeOF-Ni1 electrode shows an excellent cycling performance with a reversible discharge capacity of 450.4 mAh g-1 at 100 mAh g-1 after 100 cycles with a fading rate of 0.20% per cycle. In addition, the FeOF-Ni1//hard carbon full cell exhibited a high energy density of 876.9 Wh kg-1 cathode with a good cycling stability.

Li3PO4-Enriched SEI on Graphite Anode Boosts Li+ De-Solvation Enabling Fast-Charging and Low-Temperature Lithium-Ion Batteries

Li+ de-solvation at solid-electrolyte interphase (SEI)-electrolyte interface stands as a pivotal step that imposes limitations on the fast-charging capability and low-temperature performance of lithium-ion batteries (LIBs). Unraveling the contributions of key constituents in the SEI that facilitate Li+ de-solvation and deciphering their mechanisms, as a design principle for the interfacial structure of anode materials, is still a challenge. Herein, we conducted a systematic exploration of the influence exerted by various inorganic components (Li2CO3, LiF, Li3PO4) found in the SEI on their role in promoting the Li+ de-solvation. The findings highlight that Li3PO4-enriched SEI effectively reduces the de-solvation energy due to its ability to attenuate the Li+-solvent interaction, thereby expediting the de-solvation process. Building on this, we engineer Li3PO4 interphase on graphite (LPO-Gr) anode via a simple solid-phase coating, facilitating the Li+ de-solvation and building an inorganic-rich SEI, resulting in accelerated Li+ transport crossing the electrode interfaces and interphases. Full cells using the LPO-Gr anode can replenish its 80 % capacity in 6.5 minutes, while still retaining 70 % of the room temperature capacity even at -20 °C. Our strategy establishes connection between the de-solvation characteristics of the SEI components and the interfacial structure design of anode materials for high performance LIBs.

New Emerging Fast Charging Microscale Electrode Materials

Fast charging lithium (Li)-ion batteries are intensively pursued for next-generation energy storage devices, whose electrochemical performance is largely determined by their constituent electrode materials. While nanosizing of electrode materials enhances high-rate capability in academic research, it presents practical limitations like volumetric packing density and high synthetic cost. As an alternative to nanosizing, microscale electrode materials cannot only effectively overcome the limitations of the nanosizing strategy but also satisfy the requirement of fast-charging batteries. Therefore, this review summarizes the new emerging microscale electrode materials for fast charging from the commercialization perspective. First, the fundamental theory of electronic/ionic motion in both individual active particles and the whole electrode is proposed. Then, based on these theories, the corresponding optimization strategies are summarized toward fast-charging microscale electrode materials. In addition, advanced functional design to tackle the mechanical degradation problems related to next generation high capacity alloy- and conversion-type electrode materials (Li, S, Si et al.) for achieving fast charging and stable cycling batteries. Finally, general conclusions and the future perspective on the potential research directions of microscale electrode materials are proposed. It is anticipated that this review will provide the basic guidelines for both fundamental research and practical applications of fast-charging batteries.

Fluorine Chemistry in Rechargeable Batteries: Challenges, Progress, and Perspectives

The renewable energy industry demands rechargeable batteries that can be manufactured at low cost using abundant resources while offering high energy density, good safety, wide operating temperature windows, and long lifespans. Utilizing fluorine chemistry to redesign battery configurations/components is considered a critical strategy to fulfill these requirements due to the natural abundance, robust bond strength, and extraordinary electronegativity of fluorine and the high free energy of fluoride formation, which enables the fluorinated components with cost effectiveness, nonflammability, and intrinsic stability. In particular, fluorinated materials and electrode|electrolyte interphases have been demonstrated to significantly affect reaction reversibility/kinetics, safety, and temperature tolerance of rechargeable batteries. However, the underlining principles governing material design and the mechanistic insights of interphases at the atomic level have been largely overlooked. This review covers a wide range of topics from the exploration of fluorine-containing electrodes, fluorinated electrolyte constituents, and other fluorinated battery components for metal-ion shuttle batteries to constructing fluoride-ion batteries, dual-ion batteries, and other new chemistries. In doing so, this review aims to provide a comprehensive understanding of the structure-property interactions, the features of fluorinated interphases, and cutting-edge techniques for elucidating the role of fluorine chemistry in rechargeable batteries. Further, we present current challenges and promising strategies for employing fluorine chemistry, aiming to advance the electrochemical performance, wide temperature operation, and safety attributes of rechargeable batteries.

In Situ Transmission Electron Microscopy Methods for Lithium-Ion Batteries

In situ Transmission Electron Microscopy (TEM) stands as an invaluable instrument for the real-time examination of the structural changes in materials. It features ultrahigh spatial resolution and powerful analytical capability, making it significantly versatile across diverse fields. Particularly in the realm of Lithium-Ion Batteries (LIBs), in situ TEM is extensively utilized for real-time analysis of phase transitions, degradation mechanisms, and the lithiation process during charging and discharging. This review aims to provide an overview of the latest advancements in in situ TEM applications for LIBs. Additionally, it compares the suitability and effectiveness of two techniques: the open cell technique and the liquid cell technique. The technical aspects of both the open cell and liquid cell techniques are introduced, followed by a comparison of their applications in cathodes, anodes, solid electrolyte interphase (SEI) formation, and lithium dendrite growth in LIBs. Lastly, the review concludes by stimulating discussions on possible future research trajectories that hold potential to expedite the progression of battery technology.

Unravelling Twin Topotactic/Nontopotactic Reactive TiSe2 Cathodes for Aqueous Batteries

Titanium selenide (TiSe2 ), a model transition metal chalcogenide material, typically relies on topotactic ion intercalation/deintercalation to achieve stable ion storage with minimal disruption of the transport pathways but has restricted capacity (<130 mAh g-1 ). Developing novel energy storage mechanisms beyond conventional intercalation to break capacity limits in TiSe2 cathodes is essential yet challenging. Herein, the ion storage properties of TiSe2 are revisited and an unusual thermodynamically stable twin topotactic/nontopotactic Cu2+ accommodation mechanism for aqueous batteries is unraveled. In situ synchrotron X-ray diffraction and ex situ microscopy jointly demonstrated that topotactic intercalation sustained the ion transport framework, nontopotactic conversion involved localized multielectron reactions, and these two parallel reactions are miraculously intertwined in nanoscale space. Comprehensive experimental and theoretical results suggested that the twin-reaction mechanism significantly improved the electron transfer ability, and the reserved intercalated TiSe2 structure anchored the reduced titanium monomers with high affinity and promoted efficient charge transfer to synergistically enhance the capacity and reversibility. Consequently, TiSe2 nanoflake cathodes delivered a never-before-achieved capacity of 275.9 mAh g-1 at 0.1 A g-1 , 93.5% capacity retention over 1000 cycles, and endow hybrid batteries (TiSe2 -Cu||Zn) with a stable energy supply of 181.34 Wh kg-1 at 2339.81 W kg-1 , offering a promising model for aqueous ion storage.

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.

Impact of Structural Flexibility of Amine Moieties as Bridges for Redox-Active Sites on Secondary Battery Performance

Although environmentally benign organic cathode materials for secondary batteries are in demand, their high solubility in electrolyte solvents hinders broad applicability. In this study, a bridging fragment to link redox-active sites is incorporated into organic complexes with the aim of preventing dissolution in electrolyte systems with no significant performance loss. Evaluation of these complexes using an advanced computational approach reveals that the type of redox-active site (i. e., dicyanide, quinone, or dithione) is a key parameter for determining the intrinsic redox activity of the complexes, with the redox activity decreasing in the order of dithione>quinone>dicyanide. In contrast, the structural integrity is strongly reliant on the bridging style (i. e., amine-based single linkage or diamine-based double linkage). In particular, owing to their rigid anchoring effect, diamine-based double linkages incorporated at dithione sites allow structural integrity to be maintained with no significant decrease in the high thermodynamic performance of dithione sites. These findings provide insights into design directions for insoluble organic cathode materials that can sustain high performance and structural durability during repeated cycling.

Reversible Iron Oxyfluoride (FeOF)-Graphene Composites as Sustainable Cathodes for High Energy Density Lithium Batteries

Two large barriers are impeding the wide implementation of electric vehicles, namely driving-range and cost, primarily due to the low specific energy and high cost of mono-valence cathodes used in lithium-ion batteries. Iron is the ideal element for cathode materials considering its abundance, low cost and toxicity. However, the poor reversibility of (de)lithiation and low electronic conductivity prevent iron-based high specific energy multi-valence conversion cathodes from practical applications. In this work, a sustainable FeOF nanocomposite is developed with extraordinary performance. The specific capacity and energy reach 621 mAh g^-1 and 1124 Wh kg^-1 with more than 100 cycles, which triples the specific capacity, and doubles the specific energy of current mono-valence intercalation LiCoO₂ . This is the result of an effective approach, combing the nanostructured FeOF with graphene, realized by making the (de)lithiation reversible by immobilizing FeOF nanoparticles and the discharge products over the graphene surface and providing the interparticle electric conduction. Importantly, it demonstrates that introducing small amount of graphene can create new materials with desired properties, opening a new avenue for altering the (de)lithiation process. Such extraordinary performance represents a significant breakthrough in developing sustainable conversion materials, eventually overcoming the driving range and cost barriers.

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.

Room-Temperature Pseudo-Solid-State Iron Fluoride Conversion Battery with High Ionic Conductivity

Li-metal batteries (LMBs) employing conversion cathode materials (e.g., FeF₃) are a promising way to prepare inexpensive, environmentally friendly batteries with high energy density. Pseudo-solid-state ionogel separators harness the energy density and safety advantages of solid-state LMBs, while alleviating key drawbacks (e.g., poor ionic conductivity and high interfacial resistance). In this work, a pseudo-solid-state conversion battery (Li-FeF₃) is presented that achieves stable, high rate (1.0 mA cm^-2) cycling at room temperature. The batteries described herein contain gel-infiltrated FeF₃ cathodes prepared by exchanging the ionic liquid in a polymer ionogel with a localized high-concentration electrolyte (LHCE). The LHCE gel merges the benefits of a flexible separator (e.g., adaptation to conversion-related volume changes) with the excellent chemical stability and high ionic conductivity (∼2 mS cm^-1 at 25 °C) of an LHCE. The latter property is in contrast to previous solid-state iron fluoride batteries, where poor ionic conductivities necessitated elevated temperatures to realize practical power levels. The stable, room-temperature Li-FeF₃ cycling performance obtained with the LHCE gel at high current densities paves the way for exploring a range of architectures including flexible, three-dimensional, and custom shape batteries.

Designing better electrolytes

Electrolytes and the associated interphases constitute the critical components to support the emerging battery chemistries that promise tantalizing energy but involve drastic phase and structure complications. Designing better electrolytes and interphases holds the key to the success of these batteries. As the only component that interfaces with every other component in the device, an electrolyte must satisfy multiple criteria simultaneously. These include transporting ions while insulating electrons between the electrodes and maintaining stability against electrodes of extreme chemical natures: the strongly oxidative cathode and the strongly reductive anode. In most advanced batteries, the two electrodes operate at potentials far beyond the thermodynamic stability limits of electrolytes, so the stability therein has to be realized kinetically through an interphase formed from the sacrificial reactions between electrolyte and electrodes.

Pseudocapacitance-Enhanced Storage Kinetics of 3D Anhydrous Iron (III) Fluoride as a Cathode for Li/Na-Ion Batteries

Transition metal fluoride (TMF) conversion cathodes, with high energy density, are recognized as promising candidates for next-generation high-energy Li/Na-ion batteries (LIBs/SIBs). Unfortunately, the poor electronic conductivity and detrimental active material dissolution of TMFs seriously limit the performance of TMF-LIBs/SIBs. A variety of FeF₃-based composites are designed to improve their electrochemical characteristics. However, the storage mechanism of the conversion-type cathode for Li⁺ and Na⁺ co-storage is still unclear. Here, the storage mechanism of honeycomb iron (III) fluoride and carbon (FeF₃@C) as a general cathode for LIBs/SIBs is analyzed by kinetics. In addition, the FeF₃@C cathode shows high electrochemical performance in a full-cell system. The results show that the honeycomb FeF₃@C shows excellent long-term cycle stability in LIBs (208.3 mA h g^-1 at 1.0 C after 100 cycles with a capacity retention of 98.1%). As a cathode of SIBs, the rate performance is unexpectedly stable. The kinetic analysis reveals that the FeF₃@C cathode exhibit distinct ion-dependent charge storage mechanisms and exceptional long-durability cyclic performance in the storage of Li⁺/Na⁺, benefiting from the synergistic contribution of pseudocapacitive and reversible redox behavior. The work deepens the understanding of the conversion-type cathode in Li⁺/Na⁺ storage.

50C Fast-Charge Li-Ion Batteries using a Graphite Anode

Li-ion batteries have made inroads into the electric vehicle market with high energy densities, yet they still suffer from slow kinetics limited by the graphite anode. Here, electrolytes enabling extreme fast charging (XFC) of a microsized graphite anode without Li plating are designed. Comprehensive characterization and simulations on the diffusion of Li⁺ in the bulk electrolyte, charge-transfer process, and the solid electrolyte interphase (SEI) demonstrate that high ionic conductivity, low desolvation energy of Li⁺ , and protective SEI are essential for XFC. Based on the criterion, two fast-charging electrolytes are designed: low-voltage 1.8 m LiFSI in 1,3-dioxolane (for LiFePO₄ ||graphite cells) and high-voltage 1.0 m LiPF₆ in a mixture of 4-fluoroethylene carbonate and acetonitrile (7:3 by vol) (for LiNi_0.8 Co_0.1 Mn_0.1 O₂ ||graphite cells). The former electrolyte enables the graphite electrode to achieve 180 mAh g^-1 at 50C (1C = 370 mAh g^-1 ), which is 10 times higher than that of a conventional electrolyte. The latter electrolyte enables LiNi_0.8 Co_0.1 Mn_0.1 O₂ ||graphite cells (2 mAh cm^-2 , N/P ratio = 1) to provide a record-breaking reversible capacity of 170 mAh g^-1 at 4C charge and 0.3C discharge. This work unveils the key mechanisms for XFC and provides instructive electrolyte design principles for practical fast-charging LIBs with graphite anodes.

Super-Reversible CuF2 Cathodes Enabled by Cu2+ -Coordinated Alginate

Copper fluoride (CuF₂ ) has the highest energy density among all metal fluoride cathodes owing to its high theoretical potential (3.55 V) and high capacity (528 mAh g^-1 ). However, CuF₂  can only survive for less than five cycles, mainly due to serious Cu-ion dissolution during charge/discharge cycles. Herein, copper dissolution is successfully suppressed by forming Cu²⁺ -coordinated sodium alginate (Cu-SA) on the surface of CuF₂  particles during the electrode fabrication process, by using water as a slurry solvent and sodium alginate (SA) as a binder. The trace dissolved Cu²⁺ in water from CuF₂  can in situ cross-link with SA binder forming a conformal Cu-SA layer on CuF₂  surface. After water evaporation during the electrode dry process, the Cu-SA layer is Li-ion conductor but Cu²⁺ insulator, which can effectively suppress the dissolution of Cu-ions in the organic 4 m LiClO₄ /ethylene carbonate/propylene carbonate electrolyte, enhancing the reversibility of CuF₂ . CuF₂  electrode with SA binder delivers a reversible capacity of 420.4 mAh g^-1  after 50 cycles at 0.05 C, reaching an energy density of 1009.1 Wh kg^-1 . Cu²⁺ cross-link polymer coating on CuF₂  opens the door for stabilizing the high-energy and low-cost CuF₂  cathode for next-generation Li-ion batteries.

Fluorinated Materials as Positive Electrodes for Li- and Na-Ion Batteries

Fluorine is known to be a key element for various components of batteries since current electrolytes rely on Li-ion salts having fluorinated ions and electrode binders are mainly based on fluorinated polymers. Metal fluorides or mixed anion metal fluorides (mainly oxyfluorides) have also gained a substantial interest as active materials for the electrode redox reactions. In this review, metal fluorides for cathodes are considered; they are listed according to the dimensionality of the metal fluoride subnetwork. The synthesis conditions and the crystal structures are described; the electrochemical properties are briefly indicated, and the nature of the electron transport agent is noted. We stress the crucial importance of the elaboration processes to induce the presence of cation disorders, of anion substitutions (mainly F-/O2- or F-/OH-) or vacancies. Finally, we show that an accurate structural characterization is a key step to enable enhanced material performances to overcome several lasting roadblocks, namely the large irreversible capacity and poor energy efficiency that are frequently encountered.

Sodium storage mechanism of a GeP5/C composite as a high capacity anode material for sodium-ion batteries

The sodium storage mechanism of a GeP₅/C composite electrode was revealed. Metallic Ge formed during discharge enhances the electronic conductivity of the electrode, while Na_xP mitigates the agglomeration and volume change of Ge in the alloying process. The GeP₅ phase is regenerated after recharge along with elemental Ge and P, implying a reversible phase transition of GeP₅ during cycling.

Li/Na Ion Storage Performance of a FeOF Nano Rod with Controllable Morphology

Although the conversion material iron oxyfluoride (FeOF) possesses a high theoretical specific capacity as a cathode material for Li/Na ion batteries, its poor rate and cycling performances, caused mainly by sluggish (Li+/Na+) reaction kinetics, restrict its practical application. Herein, FeOF with high purity, a fusiform nanorod shape and high crystallinity is prepared through a facile chemical solution reaction. The electrochemical measurements show that the present FeOF exhibits high capacity and good cycling stability as a cathode material for Li-ion batteries. Capacities of 301, 274, 249, 222, and 194 mAh/g at stepwise current densities of 20, 50, 100, 200, and 400 mA/g are achieved, respectively. Additionally, the capacity at 100 mA/g retains 123 mAh/g after 140 cycles. Meanwhile, as a cathode material for Na ion battery, it delivers discharge capacities of 185, 167, 151, 134 and 115 mAh/g at stepwise current densities of 20, 50, 100, 200, and 400 mA/g, respectively. A discharge capacity of 83 mAh/g at 100 mA/g is achieved after 140 cycles. The excellent lithium/sodium-storage performance of the present FeOF material is ascribed to its unique nanostructure.

Enabling Long Cycle Life and High Rate Iron Difluoride Based Lithium Batteries by In Situ Cathode Surface Modification

Metals fluorides (MFs) are potential conversion cathodes to replace commercial intercalation cathodes. However, the application of MFs is impeded by their poor electronic/ionic conductivity and severe decomposition of electrolyte. Here, a composite cathode of FeF₂ and polymer-derived carbon (FeF₂ @PDC) with excellent cycling performance is reported. The composite cathode is composed of nanorod-shaped FeF₂ embedded in PDC matrix with excellent mechanical strength and electronic/ionic conductivity. The FeF₂ @PDC enables a reversible capacity of 500 mAh g^-1 with a record long cycle lifetime of 1900 cycles. Remarkably, the FeF₂ @PDC can be cycled at a record rate of 60 C with a reversible capacity of 107 mAh g^-1 after 500 cycles. Advanced electron microscopy reveals that the in situ formation of stable Fe₃ O₄ layers on the surface of FeF₂ prevents the electrolyte decomposition and leaching of iron (Fe), thus enhancing the cyclability. The results provide a new understanding to FeF₂ electrochemistry, and a strategy to radically improve the electrochemical performance of FeF₂ cathode for lithium-ion battery applications.

High-Energy Batteries: Beyond Lithium-Ion and Their Long Road to Commercialisation

Rechargeable batteries of high energy density and overall performance are becoming a critically important technology in the rapidly changing society of the twenty-first century. While lithium-ion batteries have so far been the dominant choice, numerous emerging applications call for higher capacity, better safety and lower costs while maintaining sufficient cyclability. The design space for potentially better alternatives is extremely large, with numerous new chemistries and architectures being simultaneously explored. These include other insertion ions (e.g. sodium and numerous multivalent ions), conversion electrode materials (e.g. silicon, metallic anodes, halides and chalcogens) and aqueous and solid electrolytes. However, each of these potential "beyond lithium-ion" alternatives faces numerous challenges that often lead to very poor cyclability, especially at the commercial cell level, while lithium-ion batteries continue to improve in performance and decrease in cost. This review examines fundamental principles to rationalise these numerous developments, and in each case, a brief overview is given on the advantages, advances, remaining challenges preventing cell-level implementation and the state-of-the-art of the solutions to these challenges. Finally, research and development results obtained in academia are compared to emerging commercial examples, as a commentary on the current and near-future viability of these "beyond lithium-ion" alternatives.

Unveiling the Role and Mechanism of Nb Doping and In Situ Carbon Coating on Improving Lithium-Ion Storage Characteristics of Rod-Like Morphology FeF3 ·0.33H2 O

Given the inherent characteristics of transition metal fluorides and open tunnel-type frameworks, intercalation-conversion-type FeF₃ ·0.33H₂ O has attracted widespread attention as a promising lithium-ion battery cathode material with high operating voltage and high energy density. However, its low electronic conductivity and poor structural stability impede its practical application in high-rate capacity and long-lifetime batteries. Herein, rod-like Nb-substituted FeF₃ ·0.33H₂ O (Nb-FeF₃ ·0.33H₂ O@C) nanocrystals with a carbon coating derived from in situ carbonization in an ionic liquid are deliberately designed and prepared. Based on first-principles calculations and electrochemical analysis, it is shown that substitution of Nb into a proportion of Fe sites can dramatically reduce the total energy of the system and the bandgap, thus boosting the structural stability and electronic conductivity of FeF₃ ·0.33H₂ O. Simultaneously, the combination of a surface conductive carbon coating and assembly of the nanoparticles into a rod-like mesoporous architecture can produce an omni-directional ion/electron transmission network and a robust 3D composite structure. The Nb-FeF₃ ·0.33H₂ O@C composite with 3% Nb-doping displays high capacity (583.2 mAh g^-1 at 0.2 C), good rate capacity (187.8 mAh g^-1 at a high rate of 5.0 C), and excellent long-term cycle stability (160.4 mAh g^-1 after 300 long cycles).

Construction of solid-liquid fluorine transport channel to enable highly reversible conversion cathodes

Conversion-type iron fluoride is a promising alternative cathode to intercalation oxides because of its higher energy density. However, its intrinsic solid-solid conversion is sluggish during repeated splitting and rebonding of metal-fluorine moieties. Here, we propose a solid-liquid conversion mechanism to activate the fluorine transport kinetics of iron oxyfluorides enabled by fluoride anion receptor of tris(pentafluorophenyl)borane (TPFPB). TPFPB promotes the dissociation of inert lithium fluoride and provides a facile fluorine transport channel at multiphase interfaces via the formation of solvated F⁻ intermediate therein. The construction of solid-liquid channel with fluorinated cathode electrolyte interface is the key for the achievement of FeO_0.3F_1.7 and FeO_0.7F_1.3 in terms of sustaining conversion reaction (with an energy efficiency approaching 80%) and high-rate performance (with reversible capacity of 320 mAh/g at 2 A/g). The cathode energy densities can reach 1100 Wh/kg for FeO_0.3F_1.7 and 700 Wh/kg for FeO_0.7F_1.3 under the power densities of 220 and 4300 W/kg, respectively.

In Situ, Atomic-Resolution Observation of Lithiation and Sodiation of WS2 Nanoflakes: Implications for Lithium-Ion and Sodium-Ion Batteries

WS₂ nanoflakes have great potential as electrode materials of lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) because of their unique 2D structure, which facilitates the reversible intercalation and extraction of alkali metal ions. However, a fundamental understanding of the electrochemical lithiation/sodiation dynamics of WS₂ nanoflakes especially at the nanoscale level, remains elusive. Here, by combining battery electrochemical measurements, density functional theory calculations, and in situ transmission electron microscopy, the electrochemical-reaction kinetics and mechanism for both lithiation and sodiation of WS₂ nanoflakes are investigated at the atomic scale. It is found that compared to LIBs, SIBs exhibit a higher reversible sodium (Na) storage capacity and superior cyclability. For sodiation, the volume change due to ion intercalation is smaller than that in lithiation. Also, sodiated WS₂ maintains its layered structure after the intercalation process, and the reduced metal nanoparticles after conversion in sodiation are well-dispersed and aligned forming a pattern similar to the layered structure. Overall, this work shows a direct interconnection between the reaction dynamics of lithiated/sodiated WS₂ nanoflakes and their electrochemical performance, which sheds light on the rational optimization and development of advanced WS₂ -based electrodes.

In Situ TEM Study on Conversion-Type Electrodes for Rechargeable Ion Batteries

Conversion-type materials have been considered as potentially high-energy-density alternatives to commercially dominant intercalation-based electrodes for rechargeable ion batteries and have attracted tremendous research effort to meet the performance for viable energy-storage technologies. In situ transmission electron microscopy (TEM) has been extensively employed to provide mechanistic insights into understanding the behavior of battery materials. Noticeably, a great portion of previous in situ TEM studies has been focused on conversion-type materials, but a dedicated review for this group of materials is missing in the literature. Herein, recent developments of in situ TEM techniques for investigation of dynamic phase transformation and associated structural, morphological, and chemical evolutions during conversion reactions with alkali ions in secondary batteries are comprehensively summarized. The materials of interest broadly cover metal oxides, chalcogenides, fluorides, phosphides, nitrides, and silicates with specific emphasis on spinel metal oxides and recently emerged 2D metal chalcogenides. Special focus is placed on the scientific findings that are uniquely obtained by in situ TEM to address fundamental questions and practical issues regarding phase transformation, structural evolution, electrochemical redox, reaction mechanism, kinetics, and degradation. Critical challenges and perspectives are discussed for advancing new knowledge that can bridge the gap between prototype materials and real-world applications.

Metal-Organic Framework-Derived Nanoconfinements of CoF2 and Mixed-Conducting Wiring for High-Performance Metal Fluoride-Lithium Battery

Metal fluoride (MF) conversion cathodes theoretically show higher gravimetric and volumetric capacities than Ni- or Co-based intercalation oxide cathodes, which makes metal fluoride-lithium batteries promising candidates for next-generation high-energy-density batteries. However, their high-energy characteristics are clouded by low-capacity utilization, large voltage hysteresis, and poor cycling stability of transition MF cathodes. A variety of reasons is responsible for this: poor reaction kinetics, low conductivities, unstable MF/electrolyte interfaces and dissolution of active species upon cycling. Herein, we combine the synthesis of the metal-organic-framework (MOF) with the low-temperature fluorination to prepare MOF-shaped CoF₂@C nanocomposites that exhibit confinement of the CoF₂ nanoparticles and efficient mixed-conducting wiring in the produced architecture. The ultrasmall CoF₂ nanoparticles (5-20 nm on average) are uniformly covered by graphitic carbon walls and embedded in the porous carbon framework. Within the CoF₂@C nanocomposite, the cross-linked carbon wall and interconnected nanopores serve as electron- and ion-conducting pathways, respectively, enabling a highly reversible conversion reaction of CoF₂. As a result, the produced CoF₂@C composite cathodes successfully restrain the above-mentioned challenges and demonstrate high-capacity utilization of ∼500 mAh g^-1 at 0.2C, good rate capability (up to 2C), and long-term cycle stability over 400 cycles. Overall, the presented study not only reports on a simple composite design to achieve high-energy characteristics in CoF₂-Li batteries but also may provide a general solution for many other metal fluoride-lithium batteries.

Bamboo-structured N-doped CNTs/FeF3·0.33H2O derived from melamine as a high-performance cathode for Li-ion batteries

The N-doped CNTs/FeF3·0.33H2O composite with a bamboo-like morphology was prepared via catalyzed pyrolysis and fluorination–annealing and used as a high-performance cathode for Li-ion batteries.

Reaction Mechanism and Structural Evolution of Fluorographite Cathodes in Solid-State K/Na/Li Batteries

Fluorographites (CF_x ) are ultrahigh-energy-density cathode materials for alkaline-metal primary batteries. However, they are generally not rechargeable. To elucidate the reaction mechanism of CF_x cathodes, in situ transmission electron microscopy characterizations and ab initio calculations are employed. It is found that it is a two-phase mechanism upon K/Na/Li ion insertion; crystalline KF (crystalline NaF nanoparticles and amorphous LiF) is generated uniformly within the amorphous carbon matrix, retaining an unchanged volume during the discharge process. The diffusivity for K/Na/Li ion migration within the CF_x is ≈2.2-2.5 × 10^-12 , 3.4-5.3 × 10^-12 , and 1.8-2.5 × 10^-11 cm² s^-1 , respectively, which is comparable to the diffusivity of K/Na/Li ions in liquid-state cells. Encouraged by the in situ transmission electron microscopy (TEM) results, a new rechargeable all-solid-state Li/CF_x battery is further designed that shows a part of the reversible specific discharge capacity at the 2nd cycle. These findings demonstrate that a solid-state electrolyte provides a different reaction process compared with a conventional liquid electrolyte, and enables CF_x to be partly rechargeable in solid-state Li batteries.

Revealing Reaction Pathways of Collective Substituted Iron Fluoride Electrode for Lithium Ion Batteries

Metal fluorides present a high redox potential among the conversion-type compounds, which make them specially work as cathode materials of lithium ion batteries. To mitigate the notorious cycling instability of conversion-type materials, substitutions of anion and cation have been proposed but the role of foreign elements in reaction pathway is not fully assessed. In this work, we explored the lithiation pathway of a rutile-Fe_0.9Co_0.1OF cathode with multimodal analysis, including ex situ and in situ transmission electron microscopy and synchrotron X-ray techniques. Our work revealed a prolonged intercalation-extrusion-cation disordering process during phase transformations from the rutile phase to rocksalt phase, which microscopically corresponds to topotactic rearrangement of Fe/Co-O/F octahedra. During this process, the diffusion channels of lithium transformed from 3D to 2D while the corner-sharing octahedron changed to edge-sharing octahedron. DFT calculations indicate that the Co and O cosubstitution of the Fe_0.9Co_0.1OF cathode can improve its structural stability by stabilizing the thermodynamic semistable phases and reducing the thermodynamic potentials. We anticipate that our study will inspire further explorations on untraditional intercalation systems for secondary battery applications.

Insights on Redox Properties of Sumanene Derivatives for High-Performance Organic Cathodes

Despite the potential of large organic molecules for insoluble cathode materials in lithium-ion batteries, they have attracted less attention owing to the penalty in the molecular weight. Herein, an advanced computational modeling approach is employed to comprehensively explore the electrochemical characteristics and theoretical charge/energy storage capability for a series of sumanene derivatives. It is highlighted from this investigation that the carbonyl moiety is generally beneficial to the improvement of the redox properties for the sumanenes. The sumanene with hexagon rings fully functionalized by six carbonyls particularly exhibits both the remarkably high redox potential (3.53 V vs Li/Li⁺) and performance parameters (454 mAh/g and 1129 mWh/g), implying its candidacy as high-potential organic cathodes. It is further demonstrated from a universal relationship of redox potential-electronic property-solvation property that a sumanene derivative would experience a two-stage discharging behavior. This indicates that the sumanene derivative would be cathodically inactive due to a sudden increase of solvation energy.

Growth behavior and influence factors of three-dimensional hierarchical flower-like FeF3·0.33H2O

Anion-assisted hydrothermal synthesis of a three-dimensional graded flower-like FeF₃·0.33H₂O material with adjustable morphology.

Guidelines and trends for next-generation rechargeable lithium and lithium-ion batteries

This review article summarizes the current trends and provides guidelines towards next-generation rechargeable lithium and lithium-ion battery chemistries.

Pyrenetetrone Derivatives Tailored by Nitrogen Dopants for High-Potential Cathodes in Lithium-Ion Batteries

To overcome limited information on organic cathode materials for lithium-ion batteries, we studied the electrochemical redox properties of pyrenetetrone and its nitrogen-doped derivatives. Three primary conclusions are highlighted from this study. First, the redox potential increases as the number of electron-withdrawing nitrogen dopants increases. Second, the redox potentials of pyrenetetrone derivatives continuously decrease with the number of bound Li atoms during the discharging process owing to the decrease in the reductive ability until the compounds become cathodically deactivated exhibiting negative redox potentials. Notably, pyrenetetrone with four nitrogen dopants loses its cathodic activity after the binding of five Li atoms, indicating remarkably high performance (496 mAh/g and 913 mWh/g). Last, the redox potential is strongly correlated not only with electronic properties but also with solvation energy. This highlights that pyrenetetrone derivatives would follow two-stage transition behaviors during the discharging process, implying a crucial contribution of solvation energy to their cathodic deactivation.

3D Honeycomb Architecture Enables a High-Rate and Long-Life Iron (III) Fluoride-Lithium Battery

Metal fluoride-lithium batteries with potentially high energy densities, even higher than lithium-sulfur batteries, are viewed as very promising candidates for next-generation lightweight and low-cost rechargeable batteries. However, so far, metal fluoride cathodes have suffered from poor electronic conductivity, sluggish reaction kinetics and side reactions causing high voltage hysteresis, poor rate capability, and rapid capacity degradation upon cycling. Herein, it is reported that an FeF₃ @C composite having a 3D honeycomb architecture synthesized by a simple method may overcome these issues. The FeF₃ nanoparticles (10-50 nm) are uniformly embedded in the 3D honeycomb carbon framework where the honeycomb walls and hexagonal-like channels provide sufficient pathways for the fast electron and Li-ion diffusion, respectively. As a result, the as-produced 3D honeycomb FeF₃ @C composite cathodes even with high areal FeF₃ loadings of 2.2 and 5.3 mg cm^-2 offer unprecedented rate capability up to 100 C and remarkable cycle stability within 1000 cycles, displaying capacity retentions of 95%-100% within 200 cycles at various C rates, and ≈85% at 2C within 1000 cycles. The reported results demonstrate that the 3D honeycomb architecture is a powerful composite design for conversion-type metal fluorides to achieve excellent electrochemical performance in metal fluoride-lithium batteries.

High-Energy-Density Rechargeable Mg Battery Enabled by a Displacement Reaction

Because of its high theoretical volumetric capacity and dendrite-free stripping/plating of Mg, rechargeable magnesium batteries (RMBs) hold great promise for high energy density in consumer electronics. However, the lack of high-energy-density cathodes severely constrains their practical applications. Herein, for the first time, we report that a CuS cathode can fully reversibly work through a displacement reaction in CuS/Mg pouch cells at room temperature and provide a high capacity of ∼400 mA h/g in a MACC electrolyte, corresponding to the gravimetric and volumetric energy density of 608 W h/kg and1042 W h/L, respectively. Even after 80 cycles, CuS/Mg pouch cells can maintain a high capacity of 335 mA h/g. Detailed mechanistic studies reveal that CuS undergoes a displacement reaction route rather than a typical conversion mechanism. This work will provide a guide for more discovery of high-performance cathode candidates for RMBs.

In Situ Electron Microscopy Investigation of Sodiation of Titanium Disulfide Nanoflakes

Two-dimensional (2D) metal sulfides show great promise for their potential applications as electrode materials of sodium ion-batteries because of the weak interlayer van der Waals interactions, which allow the reversible accommodation and extraction of sodium ions. The sodiation of metal sulfides can undergo a distinct process compared to that of lithiation, which is determined by their metal and structural types. However, the structural and morphological evolution during their electrochemical sodiation is still unclear. Here, we studied the sodiation reaction dynamics of TiS₂ by employing in situ transmission electron microscopy and first-principles calculations. During the sodium-ion intercalation process, we observed multiple intermediate phases (phase II, phase Ib, and phase Ia), different from its lithiation counterpart, with varied sodium occupation sites and interlayer stacking sequences. Further insertion of Na ions prompted a multistep extrusion reaction, which led to the phase separation of Ti metal from the Na₂S matrix, with its 2D morphology expanded to a 3D morphology. In contrast to regular conversion electrodes, TiS₂ still maintained a compact structure after a full sodiation. First-principles calculations reveal that the as-identified phases are thermodynamically preferred at corresponding intercalation/extrusion stages compared to other possible phases. The present work provides the fundamental mechanistic understanding of the sodiation process of 2D transition metal sulfides.

Phase evolution of conversion-type electrode for lithium ion batteries

Batteries with conversion-type electrodes exhibit higher energy storage density but suffer much severer capacity fading than those with the intercalation-type electrodes. The capacity fading has been considered as the result of contact failure between the active material and the current collector, or the breakdown of solid electrolyte interphase layer. Here, using a combination of synchrotron X-ray absorption spectroscopy and in situ transmission electron microscopy, we investigate the capacity fading issue of conversion-type materials by studying phase evolution of iron oxide composited structure during later-stage cycles, which is found completely different from its initial lithiation. The accumulative internal passivation phase and the surface layer over cycling enforce a rate−limiting diffusion barrier for the electron transport, which is responsible for the capacity degradation and poor rate capability. This work directly links the performance with the microscopic phase evolution in cycled electrode materials and provides insights into designing conversion-type electrode materials for applications.

Conversion electrodes possess high energy density but suffer a rapid capacity loss over cycling compared to their intercalation equivalents. Here the authors reveal the microscopic origin of the fading behavior, showing that the formation and augmentation of passivation layers are responsible.

3D Starfish-Like FeOF on Graphene Sheets: Engineered Synthesis and Lithium Storage Performance

To address the problems associated with poor conductivity and large volume variation in practical applications as a conversion cathode, engineering of hierarchical nanostructured FeOF coupled with conductive decoration is highly desired, yet rarely reported. Herein, 3D starfish-like FeOF on reduced graphene oxide sheets (FeOF/rGO) is successfully prepared, for the first time, through a combination of solvothermal reaction, self-assembly, and thermal reduction. Integrating the structural features of the 3D hierarchical nanostructure, which favorably shorten the path for electron/ion transport and alleviate volumetric changes, with those of graphene wrapping, which can further enhance the electrical conductivity and maintain the structural stability of the electrode, the as-prepared FeOF/rGO composite exhibits a superior lithium-storage performance, including a high reversible capacity (424.5 mA h^-1  g^-1 at 50 mA g^-1 ), excellent stability (0.016 % capacity decay per cycle during 180 cycles), and remarkable rate capability (275.8 mA h^-1  g^-1 at 2000 mA g^-1 ).

Mn-Doped Fe1- xMn xF3·0.33H2O/C Cathodes for Li-Ion Batteries: First-Principles Calculations and Experimental Study

Increasing attention has been paid on iron fluoride as an alternative cathode material for Li-ion batteries (LIBs) owing to its high energy density and low cost. However, the poor electric conductivity and low diffusivity for Li-ions set great challenges for iron fluoride to be used in practical LIBs. Here, we employ first-principles calculations to probe the influence of Mn-doping on the crystal structure and electronic structure of FeF₃·0.33H₂O. The calculated results suggest that Mn-doping can enlarge the hexagonal cavity and reduce the band gap of FeF₃·0.33H₂O as well as improve its intrinsic conductivity. Furthermore, Fe_{1- x}Mn _xF₃·0.33H₂O/C ( x = 0, 0.06, 0.08, and 0.10) nanocomposites were successfully fabricated by a hydrothermal method and ball-milling. Owing to the Mn-doping effect combined with highly conductive acetylene black (AB) modification, the typical Fe_0.92Mn_0.08F₃·0.33H₂O/C composite exhibits a high discharge capacity of 180 mA h g^-1 at 50 mA g^-1 after 100 cycles and delivers excellent cycling stability as well as good rate capability.

Cu-Substituted NiF2 as a Cathode Material for Li-Ion Batteries

Metal fluorides usually have a large electronegativity and are promising electrode materials for high-power lithium-ion batteries. However, like other conversion-reaction-based materials, large volumetric expansions and large capacity losses in cycling are the major issues for metal fluorides. Here, we explore substitution of Ni with Cu for binary NiF₂ and its effects on the electrochemical properties. By in situ transmission electron microscopy, the structural evolutions of several ternary metal fluorides with different Cu/Ni ratios are observed and correlated with their electrochemical properties. With increased Cu substitution from 0 to 25 wt %, the areal expansion during the first lithiation is reduced. Meanwhile, the fluorine loss (due to reaction irreversibility) in the delithiation cycle is also reduced. This provides an explanation for the advantage of Cu substitution for improved cycling stability and capacity. We believe that our observations provide insight into the development of better ternary metal fluorides as cathodes for high power density lithium-ion batteries.