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

A gel polymer electrolyte functionalized separator for high-performance lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries, featuring ultrahigh specific theoretical energy density with low-cost raw materials, have been deemed one of the most promising candidates for next-generation energy storage and conversion devices. However, the shuttle effect of soluble Li polysulfides (LiPSs) has seriously hindered their practical deployment. Herein, we report that tris(pentafluorophenyl)borane (TPFPB) is used to modify the separator (TPFPB/Al2O3) for suppressing the shuttle effect of LiPSs. In detail, the introduction of TPFPB induces 1,3-dioxolane solvent ring-opening polymerization to form a gel layer between the S cathode and separator for suppressing the shuttle effect of Li polysulfides, effectively improving the electrochemical performance of Li-S batteries. The Li-S batteries using the TPFPB/Al2O3 separator demonstrate outstanding cycling stability and high capacity retention rates. This work provides a useful guideline for separator modification using a functional interface layer to design high-performance Li-S batteries.

Fluorine-Modulated MXene-Derived Catalysts for Multiphase Sulfur Conversion in Lithium-Sulfur Battery

Fluorine owing to its inherently high electronegativity exhibits charge delocalization and ion dissociation capabilities; as a result, there has been an influx of research studies focused on the utilization of fluorides to optimize solid electrolyte interfaces and provide dynamic protection of electrodes to regulate the reaction and function performance of batteries. Nonetheless, the shuttle effect and the sluggish redox reaction kinetics emphasize the potential bottlenecks of lithium-sulfur batteries. Whether fluorine modulation regulate the reaction process of Li-S chemistry? Here, the TiOF/Ti3C2 MXene nanoribbons with a tailored F distribution were constructed via an NH4F fluorinated method. Relying on in situ characterizations and electrochemical analysis, the F activates the catalysis function of Ti metal atoms in the consecutive redox reaction. The positive charge of Ti metal sites is increased due to the formation of O-Ti-F bonds based on the Lewis acid-base mechanism, which contributes to the adsorption of polysulfides, provides more nucleation sites and promotes the cleavage of S-S bonds. This facilitates the deposition of Li2S at lower overpotentials. Additionally, fluorine has the capacity to capture electrons originating from Li2S dissolution due to charge compensation mechanisms. The fluorine modulation strategy holds the promise of guiding the construction of fluorine-based catalysts and facilitating the seamless integration of multiple consecutive heterogeneous catalytic processes.

Boosting the Electrochemical Performance of Primary and Secondary Lithium Batteries with Mn-Doped All-Fluoride Cathodes

Transition metal fluorides are potentially high specific energy cathode materials of next-generation lithium batteries, and strategies to address their low conductivity typically involve a large amount of carbon coating, which reduces the specific energy of the electrode. In this study, MnyFe1-yF3@CFx was generated by the all-fluoride strategy, converting most of the carbon in MnyFe1-yF3@C into electrochemical active CFx through a controllable NF3 gas phase fluorination method, while still retaining a tightly bound graphite layer to provide initial conductivity, which greatly improved the energy density of the composite. This synergistic effect of nonfluorinated residual carbon (∼11%) and Mn doping ensures the electrochemical kinetics of the composite. The loading mass of the active substance had been increased to 86%. The theoretical and actual discharge capacity of MnyFe1-yF3@CFx composite was up to 765 mAh g-1 (pure FeF3 is 712 mAh g-1) and 728 mAh g-1, respectively. The discharge capacity at the high-voltage (3.0 V) platform was more than three times higher than that of the non-Mn-doped composite (FeF3@CFx).

Pillaring Electronic Nano-Wires to Slice T-Nb2O5 Laminated Particles for Durable Lithium-Ion Batteries

T-Nb2O5 characterized by the pronounced intercalation pseudocapacitance effect, is regarded as a promising and alternative anode for fast-charging Li-ion batteries. However, its electrochemical kinetics are still hindered by the absence of sufficient and homogenous conductive wiring inside active microparticles. Herein, an in situ pillaring strategy of electronic nano-wires is proposed to slice T-Nb2O5 laminated particles for the development of durable and fast-charging anodes for Li-ion batteries. A micro-level layered structure consisting of nano-carbon-inserted T-Nb2O5 composite flakes is designed and enabled by successive ion exchange, slice exfoliation, in situ polymerization, and carbonization processes. The pillared carbon interlayer (derived from polyaniline) can serve as in-built conductive wires to promote and homogenize electron transfer inside the micro-level particles. The porous structure (formed by the self-assembly of exfoliated flakes) contributes to the improved electrolyte immersion and enhanced lithium migration. Benefitting from the kinetically favorable effects, the modified T-Nb2O5 anode achieves the high-rate capability (108.4 mAh g-1 at 10 A g-1) and ultralong cycling durability (138 mAh g-1 at 1.0 A g-1 after 8000 cycles, with an average capacity decaying rate as small as 0.043‰). This work provides an effective strategy of electron wire pillaring with the slicing effect for laminated electrode materials with high tap density.

Dual strategies of mild C-F scissoring fluorination and local high-concentration electrolyte to enable reversible Li-Fe-F conversion batteries

Batteries taking conversion-type iron fluorides as energy-dense cathodes provide the possibility for the power electrification of the transportation and aviation industries. However, a safe and low-toxicity synthesis method for fluorides and the design of a compatible electrolyte formula are still challenging. Here, we propose a dual strategy of mild C-F scissoring fluorination and a local high-concentration electrolyte (LHCE) to enable highly reversible Li-Fe-F conversion batteries. A facile and safe scissoring strategy at a low temperature (95 °C) enables the preparation of a carbon-iron fluoride composite with a porous cubic cage-like structure. CFx plays a double role as a solid fluorination agent and an in situ conductive network after defluorination. The as-prepared fluoride cathode delivers a reversible capacity as high as 300 mA h g-1 over 100 cycles. The further LHCE strategy not only enhances the oxidation stable voltage of the electrolyte (>5 V) and the transference number of Li+ (0.74), but also realizes dual protection of the fluoride cathode and Li metal anode by facilitating the construction of robust cathode- and anode-electrolyte interfaces, respectively. The LHCE-assisted fluoride battery releases a higher reversible capacity of 335 mA h g-1 after 130 cycles. This work provides a solution to high-performance carbon-fluoride conversion cathodes by a synergetic effect of tailored synthesis, electroactive particle texture and electrolyte formula.

Liquid Metal Mediated Heterostructure Fluoride Solid Electrolytes of High Conductivity and Air Stability for Sustainable Na Metal Batteries

Fluoride-based solid electrolytes (SEs) have emerged as a promising component for high-energy-density rechargeable solid-state batteries (SSBs) in view of their wide electrochemical window, high air stability, and interface compatibility, but they still face the challenge of low ion conductivity and the lack of a desired structure for sodium metal SSBs. Here, we report a sodium-rich heterostructure fluoride SE, Na3GaF6-Ga2O3-NaCl (NGFOC-G), synthesized via in situ oxidation of liquid metal gallium and in situ chlorination using low-melting GaCl3. The distinctive features of NGFOC-G include single-crystal Na3GaF6 domains within an open-framework structure, composite interface decoration of Ga2O3 and NaCl with a concentration gradient, exceptional air stability, and high electrochemical oxidation stability. By leveraging the penetration of gallium at NaF grain boundaries and the in situ self-oxidation to form Ga2O3 nanodomains, the solid-phase reaction kinetics of NaF and GaF3 is activated for facilitating the synthesis of main component Na3GaF6. The introduction of a small amount of a chlorine source during synthesis further softens and modifies the boundaries of Na3GaF6 along with Ga2O3. Benefiting from the enhanced interface ion transport, the optimized NGFOC-G exhibits an ionic conductivity up to 10-4 S/cm at 40 °C, which is the highest level reported among fluoride-based sodium-ion SEs. This SE demonstrates a "self-protection" mechanism, where the formation of a high Young's modulus transition layer rich in NaF and Na2O under electrochemical driving prevents the dendrite growth of sodium metal. The corresponding Na/Na symmetric cells show minimal voltage hysteresis and stable cycling performance for at least 1000 h. The Na/NGFOC-G/Na3V2(PO4)3 cell demonstrates stable capacity release around 100 mAh/g at room temperature. The Na/NGFOC-G/FeF3 cell delivers a high capacity of 461 mAh/g with an excellent stability of conversion reaction cycling.

Room-temperature reversible F-ion batteries based on sulfone electrolytes with a mild anion acceptor additive

Rechargeable fluoride ion batteries (FIBs) as an emerging anion shuttle system are attracting much attention due to their potential advantages in terms of energy density, cost and safety. A liquid electrolyte system enables the FIB operation at low or room temperature due to its higher ionic conductivity than that of a solid F-ion electrolyte. However, the insolubility of fluoride salts in aprotic solvents limits the development of liquid F-ion electrolytes. Although the boron-based anion acceptors (AAs) can facilitate the dissolution of F-ion salts, they are prone to lead to a tough desolvation process for F- due to strong Lewis acidity and therefore an inferior electrochemical performance. Here, a new non-boron AA (6-thioguanine) with moderate Lewis acidity is proposed to dissolve F- in the sulfone solvent. The ionic conductivity of the corresponding electrolytes reaches a level of mS cm-1 at room temperature. A model FIB coin cell is successfully operated with high conversion reaction reversibility based on the coupled defluorination/fluorination mechanism of electrodes, enabling a low overpotential of 0.36 V and a reversible capacity of 126 mA h g-1 after 40 cycles.

Advancing Fluoride-Ion Batteries with a Pb-PbF2 Counter Electrode and a Diluted Liquid Electrolyte

Fluoride ion batteries (FIB) are a promising post lithium-ion technology thanks to their high theoretical energy densities and Earth-abundant materials. However, the flooded cells commonly used to test liquid electrolyte FIBs severely affect the overall performance and impede comparability across different studies, hindering FIB progress. Here, we report a reliable Pb-PbF2 counter electrode that enables the use of two-electrode coin cells. To test this setup, we first introduce a liquid electrolyte that combines the advantages of a highly concentrated electrolyte (tetramethylammonium fluoride in methanol) while addressing its transport and high-cost shortcomings by introducing a diluent (propionitrile). We then demonstrate the viability of this system by reporting a BiF3-Pb-PbF2 cell with the highest capacity retention to date.

A review on nickel-rich nickel-cobalt-manganese ternary cathode materials LiNi0.6Co0.2Mn0.2O2 for lithium-ion batteries: performance enhancement by modification

The new energy era has put forward higher requirements for lithium-ion batteries, and the cathode material plays a major role in the determination of electrochemical performance. Due to the advantages of low cost, environmental friendliness, and reversible capacity, high-nickel ternary materials are considered to be one of ideal candidates for power batteries now and in the future. At present, the main design idea of ternary materials is to fully consider the structural stability and safety performance of batteries while maintaining high energy density. Ternary materials currently face problems such as low lithium-ion diffusion rate and irreversible collapse of the structure, although the battery performance can be improved utilizing coating, ion doping, etc., the actual demand requires a more effective modification method based on the intrinsic properties of the material. Based on the summary of the current research status of the ternary material LiNi0.6Co0.2Mn0.2O2 (NCM622), a comparative study of the modification paths of the material was conducted from the level of molecular action mechanism. Finally, the major problems of ternary cathode materials and the future development direction are pointed out to stimulate more innovative insights and facilitate their practical applications.

A New Class of High-Capacity Fe-Based Cation-Disordered Oxide for Li-Ion Batteries: Li-Fe-Ti-Mo Oxide

Low-cost Fe can be used for forming cation-disordered rocksalt Li-excess (DRX) materials instead of high-cost d⁰ -species and then the Fe-based DRX can be promising electrode materials because they can theoretically achieve high capacity, resulting from additional oxygen redox reaction and stable cation-disordered structure. However, Fe-based DRX materials suffer from large voltage hysteresis, low electrochemical activity, and poor cyclability, so it is highly challenging to utilize them as practical electrode materials for a cell. Here, novel high-capacity Li-Fe-Ti-Mo electrode materials (LFTMO) with high average discharge voltage and reasonable stability are reported. The effect of Ti/Mo on electrochemical reactions in Fe-based DRX materials (LFTMO) is studied by controlling its composition ratio and using techniques for analyzing the local environment to find the key factors that improve its activity. It is found out that the introduction of appropriate quantity of redox-active Mo^4+/5+ to Fe-based DRX materials can help stabilize the oxygen redox reaction via changing a local structure and can suppress a Fe redox reaction, which can cause poor performance. The understandings will help develop high capacity and long cyclability Fe-based DRX electrode materials.

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.

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.