A critical review-promises and barriers of conversion electrodes for Li-ion batteries

Designer Anions for Better Rechargeable Lithium Batteries and Beyond

Non-aqueous electrolytes, generally consisting of metal salts and solvating media, are indispensable elements for building rechargeable batteries. As the major sources of ionic charges, the intrinsic characters of salt anions are of particular importance in determining the fundamental properties of bulk electrolyte, as well as the features of the resulting electrode-electrolyte interphases/interfaces. To cope with the increasing demand for better rechargeable batteries requested by emerging application domains, the structural design and modifications of salt anions are highly desired. Here, salt anions for lithium and other monovalent (e.g., sodium and potassium) and multivalent (e.g., magnesium, calcium, zinc, and aluminum) rechargeable batteries are outlined. Fundamental considerations on the design of salt anions are provided, particularly involving specific requirements imposed by different cell chemistries. Historical evolution and possible synthetic methodologies for metal salts with representative salt anions are reviewed. Recent advances in tailoring the anionic structures for rechargeable batteries are scrutinized, and due attention is paid to the paradigm shift from liquid to solid electrolytes, from intercalation to conversion/alloying-type electrodes, from lithium to other kinds of rechargeable batteries. The remaining challenges and key research directions in the development of robust salt anions are also discussed.

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

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

Construction of ion-electron conduction network on FeS2 as high-performance cathodes enables all-solid-state lithium batteries

The all-solid-state lithium batteries (ASSLBs) with high energy density are considered to be one of the most promising candidates for next-generation lithium battery systems. Nevertheless, the low ionic and electronic conduction inside the cathode and the poor interfacial contact of the cathode/electrolyte seriously impede the large-scale application of ASSLBs. In this work, a novel multiple ion-electron conductive network is constructed on the FeS2 cathode to realize a high-energy all-solid-state battery. The internal disordered carbon matrix acts as electronic network to accelerate the electronic transmission. Meanwhile, reduced graphene oxide (rGO) tightly wrapping FeS2/C microspheres' surface serves as external electronic pathway. Moreover, the in-situ formed Li7P3S11 electrolyte infiltrates into the nanoparticles to improve lithium-ion transport kinetics. Therefore, the dual-carbon framework and Li7P3S11 coating layer strategies significantly enhance ion-electron transport kinetics and improve interfacial contact during cycling. As expected, the FeS2@C/rGO@Li7P3S11 cathode exhibits excellent rate capability and cycling stability, showing a reversible discharge capacity of 350.3 mAh/g at 0.5C after 200 cycles. More importantly, ex-situ XPS and dQ/dV results reveal that the synergistic effect of dual-carbon frameworks and Li7P3S11 coating layer not only provides fast electron-ion transfer channels, but also wraps the reaction products with poor electrochemical activity such as Fe0, FeSy, and S to accelerate the reaction kinetics and strengthens the reaction reversibility. This work provides valuable insights for improving the electrochemical performance and understanding the reaction mechanism of the conversion-type metal sulfide cathodes in ASSLBs.

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-Use of Seawater Batteries for Energy Storage and Water Desalination

Seawater batteries are unique energy storage systems for sustainable renewable energy storage by directly utilizing seawater as a source for converting electrical energy and chemical energy. This technology is a sustainable and cost-effective alternative to lithium-ion batteries, benefitting from seawater-abundant sodium as the charge-transfer ions. Research has significantly improved and revised the performance of this type of battery over the last few years. However, fundamental limitations of the technology remain to be overcome in future studies to make this method even more viable. Disadvantages include degradation of the anode materials or limited membrane stability in aqueous saltwater resulting in low electrochemical performance and low Coulombic efficiency. The use of seawater batteries exceeds the application for energy storage. The electrochemical immobilization of ions intrinsic to the operation of seawater batteries is also an effective mechanism for direct seawater desalination. The high charge/discharge efficiency and energy recovery make seawater batteries an attractive water remediation technology. Here, the seawater battery components and the parameters used to evaluate their energy storage and water desalination performances are reviewed. Approaches to overcoming stability issues and low voltage efficiency are also introduced. Finally, an overview of potential applications, particularly in desalination technology, is provided.

Exploiting the Iron Difluoride Electrochemistry by Constructing Hierarchical Electron Pathways and Cathode Electrolyte Interface

Conversion-type cathodes such as metal fluorides, especially FeF₂ and FeF₃ , are potential candidates to replace intercalation cathodes for the next generation of lithium ion batteries. However, the application of iron fluorides is impeded by their poor electronic conductivity, iron/fluorine dissolution, and unstable cathode electrolyte interfaces (CEIs). A facile route to fabricate a mechanical strong electrode with hierarchical electron pathways for FeF₂ nanoparticles is reported here. The FeF₂ /Li cell demonstrates remarkable cycle performances with a capacity of 300 mAh g^-1 after a record long 4500 cycles at 1C. Meanwhile, a record stable high area capacity of over 6 mAh cm^-2 is achieved. Furthermore, ultra-high rate capabilities at 20C and 6C for electrodes with low and high mass loading, respectively, are attained. Advanced electron microscopy reveals the formation of stable CEIs. The results demonstrate that the construction of viable electronic connections and favorable CEIs are the key to boost the electrochemical performances of FeF₂ cathode.

Thermal synthesis of conversion-type bismuth fluoride cathodes for high-energy-density Li-ion batteries

Towards enhancement of the energy density of Li-ion batteries, BiF₃ has recently attracted considerable attention as a compelling conversion-type cathode material due to its high theoretical capacity of 302 mAh g^-1, average discharge voltage of ca. 3.0 V vs. Li⁺/Li, the low theoretical volume change of ca. 1.7% upon lithiation, and an intrinsically high oxidative stability. Here we report a facile and scalable synthesis of phase-pure and highly crystalline orthorhombic BiF₃ via thermal decomposition of bismuth(III) trifluoroacetate at T = 300 °C under inert atmosphere. The electrochemical measurements of BiF₃ in both carbonate (LiPF₆-EC/DMC)- and ionic liquid-based (LiFSI-Pyr_1,4TFSI) Li-ion electrolytes demonstrated that ionic liquids improve the cyclic stability of BiF₃. In particular, BiF₃ in 4.3 M LiFSI-Pyr_1,4TFSI shows a high initial capacity of 208 mA g^-1 and capacity retention of ca. 50% over at least 80 cycles at a current density of 30 mA g^-1.

Influence of Iron Sulfide Nanoparticle Sizes in Solid-State Batteries*

Given the inherent performance limitations of intercalation‐based lithium‐ion batteries, solid‐state conversion batteries are promising systems for future energy storage. A high specific capacity and natural abundancy make iron disulfide (FeS₂) a promising cathode‐active material. In this work, FeS₂ nanoparticles were prepared solvothermally. By adjusting the synthesis conditions, samples with average particle diameters between 10 nm and 35 nm were synthesized. The electrochemical performance was evaluated in solid‐state cells with a Li‐argyrodite solid electrolyte. While the reduction of FeS₂ was found to be irreversible in the initial discharge, a stable cycling of the reduced species was observed subsequently. A positive effect of smaller particle dimensions on FeS₂ utilization was identified, which can be attributed to a higher interfacial contact area and shortened diffusion pathways inside the FeS₂ particles. These results highlight the general importance of morphological design to exploit the promising theoretical capacity of conversion electrodes in solid‐state batteries.

Unifying the Clustering Kinetics of Lithium Polysulfides with the Nucleation Behavior of Li2S in Lithium-Sulfur Batteries

Within the lithium-sulfur (Li-S) battery, a wide variety of soluble lithium polysulfide intermediates form during operation. Under lean-electrolyte or low-temperature conditions, the solution coordination of polysulfides dynamically shifts to highly clustered states, which is subsequently accompanied by inhibited electrochemical kinetics. In fact, it has been shown that the tendency for polysulfides to strongly aggregate is one of the dominant kinetically limiting obstacles towards achieving adequate utilization of active material under such conditions. While this association has been noted before, it is not explicitly understood what mechanism intrinsic to polysulfide clustering curtails the electrochemical utilization of active material, particularly during the conversion to insoluble Li2S. Here, we perform a series of investigations to unify and link the kinetic constraints that arise from polysulfide clustering to the nucleation and growth behavior of Li2S. We find that there is a drastic decrease in polysulfide diffusion coefficient arising from the advent of clustering, and that this decline functionally matches that seen for the nucleation and growth rate constants for Li2S deposition. Additionally, it is found that there is a less favorable minimization of energy during Li2S nucleation, arising from the altered solvation stability of polysulfide clusters. This knowledge expands our understanding of the Li-S materials chemistry and the primary factors dictating the electrochemical behavior.

Cobalt Chalcogenides/Cobalt Phosphides/Cobaltates with Hierarchical Nanostructures for Anode Materials of Lithium-Ion Batteries: Improving the Lithiation Environment

Lithium-ion batteries (LIBs) are widely used in electric vehicles and portable electronic devices due to their high energy density, long cycle life, environmental friendliness, and negligible memory effect, though they also suffer from low power density, safety issues, and an aging effect. Cobalt chalcogenides/phosphides as promising anode materials have attracted intensive interests due to their high theoretical capacity based on the conversion mechanism. Cobaltates (XCo₂ O₄ , X = the other metal) have attracted attention because the X element can partially replace the high cost and toxic cobalt element. The serious volume variation during the cycling process has an impact, however, on the lithiation environment of above materials. Hierarchical construction can provide more active sites and shorten the diffusion pathways of Li ions as well as accommodating the volume expansion during lithiation processes. Herein, the research progress on the synthesis methods, structural characteristics, and electrochemical performances of cobalt chalcogenides/cobalt phosphides/cobaltates with hierarchical nanostructures for LIBs is presented. The concluding remarks highlight the research challenges and possible development directions of cobalt chalcogenides/cobalt phosphides/cobaltates with tailored hierarchical nanostructures for LIBs.

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.

Flowerlike Ti-Doped MoO3 Conductive Anode Fabricated by a Novel NiTi Dealloying Method: Greatly Enhanced Reversibility of the Conversion and Intercalation Reaction

Anodes made of molybdenum trioxide (MoO₃) suffer from insufficient conductivity and low catalytic reactivity. Here, we demonstrate that by using a dealloying method, we were able to fabricate anode of Ti-doped MoO₃ (Ti-MoO₃), which exhibits high catalytic reactivity, along with enhanced rate performance and cycling stability. We found that after doping, interestingly, the Ti-MoO₃ forms nanosheets and assembles into a micrometer-sized flowerlike morphology with enhanced interlayer distance. The density functional theory result has further concluded that the band gap of the Ti-doped anode has been reduced significantly, thus greatly enhancing the electronic conductivity. As a result, the structure maintains stability during the Li⁺ intercalation/deintercalation processes, which enhances the cycling stability and rate capability. This engineering strategy and one-step synthesis route opens up a new pathway in the design of anode materials.

Heteroanionic Materials by Design: Progress Toward Targeted Properties

The burgeoning field of anion engineering in oxide-based compounds aims to tune physical properties by incorporating additional anions of different size, electronegativity, and charge. For example, oxychalcogenides, oxynitrides, oxypnictides, and oxyhalides may display new or enhanced responses not readily predicted from or even absent in the simpler homoanionic (oxide) compounds because of their proximity to the ionocovalent-bonding boundary provided by contrasting polarizabilities of the anions. In addition, multiple anions allow heteroanionic materials to span a more complex atomic structure design palette and interaction space than the homoanionic oxide-only analogs. Here, established atomic and electronic principles for the rational design of properties in heteroanionic materials are contextualized. Also described are synergistic quantum mechanical methods and laboratory experiments guided by these principles to achieve superior properties. Lastly, open challenges in both the synthesis and the understanding and prediction of the electronic, optical, and magnetic properties afforded by anion-engineering principles in heteroanionic materials are reviewed.

High-Tap-Density Fe-Doped Nickel Hydroxide with Enhanced Lithium Storage Performance

Nickel hydroxide has attracted much attention as an anode material for lithium-ion batteries (LIBs) due to its high specific capacity, low cost, and easy preparation. However, the poor cycling stability greatly hampers its application. Herein, Fe-doped nickel hydroxide powders with a high tap density (2.16 g cm^-3) are synthesized by a simple chemical co-precipitation method. Compared to undoped nickel hydroxide, this Fe-doped nickel hydroxide exhibits better lithium storage activity, enhanced cycling stability and rate capability, and improved electrochemical reaction kinetics. As an anode material for LIBs, the Fe-doped nickel hydroxide delivers a specific discharge capacity of 1080 mA h g^-1 at 200 mA g^-1 after 30 cycles, which is almost twice that (519 mA h g^-1) of undoped nickel hydroxide; at a high current density of 2000 mA g^-1, Fe-doped nickel hydroxide shows a specific capacity of 661 mA h g^-1, significantly higher than that (182 mA h g^-1) of undoped nickel hydroxide. Kinetic analysis reveals that Fe doping decreases the electrochemical reaction resistance and improves the lithium ion diffusivity in a nickel hydroxide electrode.

Multi-electron transfer enabled by topotactic reaction in magnetite

A bottleneck for the large-scale application of today’s batteries is low lithium storage capacity, largely due to the use of intercalation-type electrodes that allow one or less electron transfer per redox center. An appealing alternative is multi-electron transfer electrodes, offering excess capacity, which, however, involves conversion reaction; according to conventional wisdom, the host would collapse during the process, causing cycling instability. Here, we report real-time observation of topotactic reaction throughout the multi-electron transfer process in magnetite, unveiled by in situ single-crystal crystallography with corroboration of first principles calculations. Contradicting the traditional belief of causing structural breakdown, conversion in magnetite resembles an intercalation process—proceeding via topotactic reaction with the cubic close packed oxygen-anion framework retained. The findings from this study, with unique insights into enabling  multi-electron transfer via topotactic reaction, and its implications to the cyclability and rate capability, shed light on designing viable multi-electron transfer electrodes for high energy batteries.

In contrast to the conventional wisdom on conversion-driven structural collapse of the host, this work shows that lithium conversion in magnetite resembles the intercalation process, going via topotactic reactions, thereby enabling multi-electron transfer and high reversible capacity.

Isothermal Microcalorimetry: Insight into the Impact of Crystallite Size and Agglomeration on the Lithiation of Magnetite, Fe3O4

Magnetite, Fe₃O₄, holds significant interest as a Li-ion anode material because of its high theoretical capacity (926 mAh/g) associated with multiple electron transfers per cation center. Notably, both crystallite size and agglomeration influence ion transport. This report probes the effects of crystallite size (12 and 29 nm) and agglomeration on the reactions involved with the formation of the surface electrolyte interphase on Fe₃O₄. Isothermal microcalorimetry (IMC) was used to determine the parasitic heat evolved during lithiation by considering the total heat measured, cell polarization, and entropic contributions. Interestingly, the 29 nm Fe₃O₄-based electrodes produced more parasitic heat than the 12 nm samples (1346 vs 1155 J/g). This observation was explored using scanning electron microscopy (SEM) and X-ray fluorescence (XRF) mapping in conjunction with spatially resolved X-ray absorption spectroscopy (XAS). SEM imaging of the electrodes revealed more agglomerates for the 12 nm material, affirmed by XRF maps. Further, XAS results suggest that Li⁺ transport is more restricted for the smaller crystallite size (12 nm) material, attributed to its greater degree of agglomeration. These results rationalize the IMC data, where agglomerates of the 12 nm material limit solid electrolyte interphase formation and parasitic heat generation during lithiation of Fe₃O₄.

Structural analysis of the initial lithiation of NiO thin film electrodes

Our results reveal that conversion reactions and structural changes in NiO thin film electrodes begin near the theoretical lithiation potential.

Feasible Defect Engineering by Employing Metal Organic Framework Templates into One-Dimensional Metal Oxides for Battery Applications

Facile synthesis of rationally designed nanostructured electrode materials with high reversible capacity is highly critical to meet ever-increasing demands for lithium-ion batteries. In this work, we employed defect engineering by incorporating metal organic framework (MOF) templates into one-dimensional nanostructures by simple electrospinning and subsequent calcination. The introduction of Co-based zeolite imidazole frameworks (ZIF-67) resulted in abundant oxygen vacancies, which induce not only more active sites for Li storage but also enhanced electrical conductivity. Moreover, abundant mesoporous sites are formed by the decomposition of ZIF-67, which are present both inside and outside the resultant SnO₂-Co₃O₄ nanofibers (NFs). Attributed to the creation of vacancy sites along with the synergistic effects of SnO₂ and Co₃O₄, SnO₂-Co₃O₄ NFs exhibit an excellent reversible capacity for 300 cycles (1287 mA h g^-1 at a current density of 500 mA g^-1) along with superior rate capabilities and improved initial Coulombic efficiency compared with pristine SnO₂ NFs. This is an early report on utilizing MOF structures as the defect formation platform into one-dimensional nanostructures, which is expected to result in superior electrochemical performances required for advanced electrodes.

Hierarchically Nanostructured Transition Metal Oxides for Lithium-Ion Batteries

Lithium-ion batteries (LIBs) have been widely used in the field of portable electric devices because of their high energy density and long cycling life. To further improve the performance of LIBs, it is of great importance to develop new electrode materials. Various transition metal oxides (TMOs) have been extensively investigated as electrode materials for LIBs. According to the reaction mechanism, there are mainly two kinds of TMOs, one is based on conversion reaction and the other is based on intercalation/deintercalation reaction. Recently, hierarchically nanostructured TMOs have become a hot research area in the field of LIBs. Hierarchical architecture can provide numerous accessible electroactive sites for redox reactions, shorten the diffusion distance of Li-ion during the reaction, and accommodate volume expansion during cycling. With rapid research progress in this field, a timely account of this advanced technology is highly necessary. Here, the research progress on the synthesis methods, morphological characteristics, and electrochemical performances of hierarchically nanostructured TMOs for LIBs is summarized and discussed. Some relevant prospects are also proposed.