Fading Mechanisms and Voltage Hysteresis in FeF2 -NiF2 Solid Solution Cathodes for Lithium and Lithium-Ion Batteries

Unlocking iron metal as a cathode for sustainable Li-ion batteries by an anion solid solution

Traditional cathode chemistry of Li-ion batteries relies on the transport of Li-ions within the solid structures, with the transition metal ions and anions acting as the static components. Here, we demonstrate that a solid solution of F- and PO43- facilitates the reversible conversion of a fine mixture of iron powder, LiF, and Li3PO4 into iron salts. Notably, in its fully lithiated state, we use commercial iron metal powder in this cathode, departing from electrodes that begin with iron salts, such as FeF3. Our results show that Fe-cations and anions of F- and PO43- act as charge carriers in addition to Li-ions during the conversion from iron metal to a solid solution of iron salts. This composite electrode delivers a reversible capacity of up to 368 mAh/g and a specific energy of 940 Wh/kg. Our study underscores the potential of amorphous composites comprising lithium salts as high-energy battery electrodes.

Boosting the Cycle Performance of Iron Trifluoride Based Solid State Batteries at Elevated Temperatures by Engineering the Cathode Solid Electrolyte Interface

Iron trifluoride (FeF3) is attracting tremendous interest due to its lower cost and the possibility to enable higher energy density in lithium-ion batteries. However, its cycle performance deteriorates rapidly in less than 50 cycles at elevated temperatures due to cracking of the unstable cathode solid electrolyte interface (CEI) followed by active materials dissolution in liquid electrolyte. Herein, by engineering the salt composition, the Fe3O4-type CEI with the doping of boron (B) atoms in a polymer electrolyte at 60 °C is successfully stabilized. The cycle life of the well-designed FeF3-based composite cathode exceeds an unprecedented 1000 cycles and utilizes up to 70% of its theoretical capacities. Advanced electron microscopy combined with density functional theory (DFT) calculations reveal that the B in lithium salt migrates into the cathode and promotes the formation of an elastic and mechanic robust boron-contained CEI (BOR-CEI) during cycling, by which the durability of the CEI to frequent cyclic large volume changes is significantly enhanced. To this end, the notorious active materials dissolution is largely prohibited, resulting in a superior cycle life. The results suggest that engineering the CEI such as tuning its composition is a viable approach to achieving FeF3 cathode-based batteries with enhanced performance.

Monolayer molybdenum diborides containing flat and buckled boride layers as anode materials for lithium-ion batteries

The materials community is interested in discovering new two-dimensional (2D) crystals because of the potential for fascinating features. In this work, by employing a systematic first-principles DFT analysis and MD simulations, we investigated the potential applications of monolayer Mo borides containing flat and buckled boride rings named P6/mmm and R3̄m MoB₂ as anode materials of lithium-ion batteries. Our preliminary investigations show that the MoB₂ monolayers possess significant structural, thermodynamic, mechanical, and dynamical stability. Due to their distinctive crystal structures, the Mo borides exhibit unique electronic properties, as expected. Additionally, we discovered that the highly negative Li adsorption energy achieved can aid in stabilizing the Li's adsorption on the surface of MoB₂ rather than clustering, which ensured its suitability for LIB anode applications. The low computed Li-ion and Li-vacancy migration energy barrier provides robust charge/discharge performance even at a fully lithiated state, signifying their extraordinary possibility of being a suitable anode material for Li batteries. Both the monolayers can hold a maximum of two layers of Li ions on both sides to give a huge specific capacity of 912 mA h g^-1, much higher than graphene and MoS₂-based anodes. The computed in-plane stiffness constants demonstrate that the monolayer pristine and lithiated MoB₂ satisfies Born's criteria, implying its mechanical flexibility. Additionally, its strong mechanical and thermal properties at the pristine and the lithiated state indicate that the 2D MoB₂ can withstand massive volume expansion at a high temperature of 500 K during the lithiation/de-lithiation reaction and is remarkably beneficial for manufacturing flexible anodes. Based on the above findings, these two newly designed classes of monolayers of MoB₂ are anticipated to open a new avenue for the upcoming generation of lithium-ion batteries.

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.

The Emerging Layered Hydroxide Plates with Record Thickness for Enhanced High-Mass-Loading Energy Storage

The past decade has witnessed the development of layered-hydroxide-based self-supporting electrodes, but the low active mass ratio impedes its all-around energy-storage applications. Herein, the intrinsic limit of layered hydroxides is broken by engineering F-substituted β-Ni(OH)₂ (Ni-F-OH) plates with a sub-micrometer thickness (over 700 nm), producing a superhigh mass loading of 29.8 mg cm^-2 on the carbon substrate. Theoretical calculation and X-ray absorption spectroscopy analysis demonstrate that Ni-F-OH shares the β-Ni(OH)₂ -like structure with slightly tuned lattice parameters. More interestingly, the synergy modulation of NH₄ ⁺ and F^- is found to serve as the key enabler to tailor these sub-micrometer-thickness 2D plates thanks to the modification effects on the (001) plane surface energy and local OH^- concentration. Guided by this mechanism, the superstructures of bimetallic hydroxides and their derivatives are further developed, revealing they are a versatile family with great promise. The tailored ultrathick phosphide superstructure achieves a superhigh specific capacity of 7144 mC cm^-2 and a superior rate capability (79% at 50 mA cm^-2 ). This work highlights a multiscale understanding of how exceptional structure modulation happens in low-dimensional layered materials. The as-built unique methodology and mechanisms will boost the development of advanced materials to better meet future energy demands.

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.

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.

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.

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).

Revisiting metal fluorides as lithium-ion battery cathodes

Metal fluorides, promising lithium-ion battery cathode materials, have been classified as conversion materials due to the reconstructive phase transitions widely presumed to occur upon lithiation. We challenge this view by studying FeF₃ using X-ray total scattering and electron diffraction techniques that measure structure over multiple length scales coupled with density functional theory calculations, and by revisiting prior experimental studies of FeF₂ and CuF₂. Metal fluoride lithiation is instead dominated by diffusion-controlled displacement mechanisms, and a clear topological relationship between the metal fluoride F^- sublattices and that of LiF is established. Initial lithiation of FeF₃ forms FeF₂ on the particle's surface, along with a cation-ordered and stacking-disordered phase, A-Li_xFe_yF₃, which is structurally related to α-/β-LiMn²⁺Fe³⁺F₆ and which topotactically transforms to B- and then C-Li_xFe_yF₃, before forming LiF and Fe. Lithiation of FeF₂ and CuF₂ results in a buffer phase between FeF₂/CuF₂ and LiF. The resulting principles will aid future developments of a wider range of isomorphic metal fluorides.

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.

Bismuth iron molybdenum oxide solid solution: a novel and durable electrocatalyst for overall water splitting

The drive for finding active bifunctional electrocatalysts for efficient overall water splitting continues in order to extract energy in the form of hydrogen as a clean fuel. Bismuth iron molybdenum oxide solid solution, composed of orthorhombic Bi2MoO6 as the major component and monoclinic Bi3(FeO4)(MoO4)2 as the minor component, has been identified as a potential electrocatalyst for the first time.

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.

Cycle stability of conversion-type iron fluoride lithium battery cathode at elevated temperatures in polymer electrolyte composites

Metal fluoride conversion cathodes offer a pathway towards developing lower-cost Li-ion batteries. Unfortunately, such cathodes suffer from extremely poor performance at elevated temperatures, which may prevent their use in large-scale energy storage applications. Here we report that replacing commonly used organic electrolytes with solid polymer electrolytes may overcome this hurdle. We demonstrate long-cycle stability for over 300 cycles at 50 °C attained in high-capacity (>450 mAh g^-1) FeF₂ cathodes. The absence of liquid solvents reduced electrolyte decomposition, while mechanical properties of the solid polymer electrolyte enhanced cathode structural stability. Our findings suggest that the formation of an elastic, thin and homogeneous cathode electrolyte interphase layer on active particles is a key for stable performance. The successful operation of metal fluorides at elevated temperatures opens a new avenue for their practical applications and future successful commercialization.

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.

Degradation Mechanism of Conversion-Type Iron Trifluoride: Toward Improvement of Cycle Performance

Conversion-type iron trifluoride (FeF₃) has attracted considerable attention as a positive electrode material for lithium secondary batteries due to its high energy density and low cost. However, the conversion process through which FeF₃ operates leads it to suffer from capacity degradation upon repeated cycling. To improve the cycle performance, in this study we investigated the degradation mechanism of conversion-type FeF₃ electrode material. Bulk analyses of FeF₃ upon cycling reveal incomplete oxidation to Fe³⁺ concomitant with the aggregation of LiF at the charged state. In addition, surface analyses of FeF₃ reveal that a film covered the electrode surface after 10 cycles, which leads to a remarkable increase in resistance. We show that the choice of the electrolyte formulation is crucial in preventing the formation of the film on the electrode surface; thus, FeF₃ shows better performance in an electrolyte comprising LiBF₄ solute in cyclic carbonate solvents than in chain carbonate-containing LiPF₆ as the electrolyte. This study underpins that a careful selection of solvent, rather than solute, is significantly essential to improve the cycle performance of the FeF₃ electrode.

Simplifying the Electrolyte Systems with the Functional Cosolvent

The state-of-the-art electrolytes utilized in lithium-ion batteries are based on liquid carbonates combining a number of additives to fulfill the practical requirements including safety and low temperature. The plenty of components result in the quadruple times of probable radical groups involved into the interfacial reactions, rendering it too difficult to control the surface layer. This work tends to simplify the system with the fluorine-substituted ether as the functional cosolvent to expand the functions of basic electrolytes. The incorporation of this solvent enables the electrolyte to self-extinguish, reduces its freezing point to ∼75 °C lower, and assists in the formation of LiF-rich protective interlayers, resulting in the improvement of the rate capability, cryogenic performance, and cyclic stability for the LiNi_1/3Co_1/3Mn_1/3O₂ cathode. This novel design could significantly diminish the amount of necessary additives and possess the acceptable cost, which provides a probability to revitalize the development of liquid electrolytes.

Precise Surface Engineering of Cathode Materials for Improved Stability of Lithium-Ion Batteries

As lithium-ion batteries continue to climb to even higher energy density, they meanwhile cause serious concerns on their stability and reliability during operation. To make sure the electrode materials, particularly cathode materials, are stable upon extended cycles, surface modification becomes indispensable to minimize the undesirable side reaction at the electrolyte-cathode interface, which is known as a critical factor to jeopardizing the electrode performance. This Review is targeted at a precise surface control of cathode materials with focus on the synthetic strategies suitable for a maximized surface protection ensured by a uniform and conformal surface coating. Detailed discussions are taken on the formation mechanism of the designated surface species achieved by either wet-chemistry routes or instrumental ones, with attention to the optimized electrochemical performance as a result of the surface control, accordingly drawing a clear image to describe the synthesis-structure-performance relationship to facilitate further understanding of functional electrode materials. Finally, perspectives regarding the most promising and/or most urgent developments for the surface control of high-energy cathode materials are provided.