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

Recent Development of Mg Ion Solid Electrolyte

Although the successful deployment of lithium-ion batteries (LIBs) in various fields such as consumer electronics, electric vehicles and electric grid, the efforts are still ongoing to pursue the next-generation battery systems with higher energy densities. Interest has been increasing in the batteries relying on the multivalent-ions such as Mg²⁺, Zn²⁺, and Al³⁺, because of the higher volumetric energy densities than those of monovalent-ion batteries including LIBs and Na-ion batteries. Among them, magnesium batteries have attracted much attention due to the promising characteristics of Mg anode: a low redox potential (−2.356 V vs. SHE), a high volumetric energy density (3,833 mAh cm⁻³), atmospheric stability and the earth-abundance. However, the development of Mg batteries has progressed little since the first Mg-ion rechargeable battery was reported in 2000. A severe technological bottleneck concerns the organic electrolytes, which have limited compatibility with Mg anode and form an Mg-ion insulating passivation layer on the anode surface. Consequently, beneficial to the good chemical and mechanical stability, Mg-ion solid electrolyte should be a promising alternative to the liquid electrolyte. Herein, a mini review is presented to focus on the recent development of Mg-ion solid conductor. The performances and the limitations were also discussed in the review. We hope that the mini review could provide a quick grasp of the challenges in the area and inspire researchers to develop applicable solid electrolyte candidates for Mg batteries.

Quantum-Chemical Study of the FeNCN Conversion-Reaction Mechanism in Lithium- and Sodium-Ion Batteries

We report a computational study on 3d transition-metal (Cr, Mn, Fe, and Co) carbodiimides in Li- and Na-ion batteries. The obtained cell voltages semi-quantitatively fit the experiments, highlighting the practicality of PBE+U as an approach for modeling the conversion-reaction mechanism of the FeNCN archetype with lithium and sodium. Also, the calculated voltage profiles agree satisfactorily with experiment both for full (Li-ion battery) and partial (Na-ion battery) discharge, even though experimental atomistic knowledge is missing up to now. Moreover, we rationalize the structural preference of intermediate ternaries and their characteristic lowering in the voltage profile using chemical-bonding and Mulliken-charge analysis. The formation of such ternary intermediates for the lithiation of FeNCN and the contribution of at least one ternary intermediate is also confirmed experimentally. This theoretical approach, aided by experimental findings, supports the atomistic exploration of electrode materials governed by conversion reactions.

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.

Sulfur-Deficient Porous SnS2-x Microflowers as Superior Anode for Alkaline Ion Batteries

SnS₂ as a high energy anode material has attracted extensive research interest recently. However, the fast capacity decay and low rate performance in alkaline-ion batteries associated with repeated volume variation and low electrical conductivity plague them from practical application. Herein, we propose a facile method to solve this problem by synthesizing porous SnS₂ microflowers with in-situ formed sulfur vacancies. The flexible porous nanosheets in the three-dimensional flower-like nanostructure provide facile strain relaxation to avoid stress concentration during the volume changes. Rich sulfur vacancies and porous structure enable the fast and efficient electron transport. The porous SnS_2-x microflowers exhibit outstanding performance for lithium ion battery in terms of high capacity (1375 mAh g^-1 at 100 mA g^-1) and outstanding rate capability (827 mA h g^-1 at high rate of 2 A g^-1). For sodium ion battery, a high capacity (~522 mAh g^-1) can be achieved at 5 A g^-1 after 200 cycles for SnS_2-x microflowers. The rational design in nanostructures, as well as the chemical compositions, might create new opportunities in designing the new architecture for highly efficient energy storage devices.

Two-Phase Reaction Mechanism for Fluorination and Defluorination in Fluoride-Shuttle Batteries: A First-Principles Study

Fluoride-shuttle batteries (FSBs), which are based on fluoride-ion transfer, have attracted attention because of their high theoretical energy densities. The fluorination and defluorination reactions at the electrodes are the possible rate-determining steps in FSBs, and understanding the mechanism is important to achieve smooth charge/discharge. In this study, we discuss the thermodynamically favored pathways for the fluorination and defluorination reactions and compare the reactions through the solid-solution and two-phase-coexistent states by density functional theory (DFT) calculations. The free energies of the solid-solution and two-phase states approximate the energies calculated by DFT, and their accuracy was validated by comparison with experimental formation enthalpies and free energies. The relative formation enthalpies of typical, transition, and relativistic metal (Tl, Pb, and Bi) fluorides are well reproduced by DFT calculations within 0.1, 0.2, and 0.4 eV, respectively. We also show that the reaction pathway can be determined by comparing the formation enthalpies of the metal fluoride H, a fluorine vacancy H_V, and an interstitial fluorine defect H_I from the simple selection rule. The enthalpy relation of H_I > H > -H_V observed in all the calculations strongly suggests that fluorination and defluorination in FSB electrodes occur by a two-phase reaction. This fluorination and defluorination mechanism will be useful to clarify the rate-determining step in FSBs.

Nanostructured diatom earth SiO2 negative electrodes with superior electrochemical performance for lithium ion batteries

Diatomaceous earth (DE) is a naturally occurring silica source constituted by fossilized remains of diatoms, a type of hard-shelled algae, which exhibits a complex hierarchically nanostructured porous silica network. In this work, we analyze the positive effects of reducing DE SiO₂ particles to the sub-micrometer level and implementing an optimized carbon coating treatment to obtain DE SiO₂ anodes with superior electrochemical performance for Li-ion batteries. Pristine DE with an average particle size of 17 μm is able to deliver a specific capacity of 575 mA h g⁻¹ after 100 cycles at a constant current of 100 mA g⁻¹, and reducing the particle size to 470 nm enhanced the reversible specific capacity to 740 mA h g⁻¹. Ball-milled DE particles were later subjected to a carbon coating treatment involving the thermal decomposition of a carbohydrate precursor at the surface of the particles. Coated ball-milled silica particles reached stable specific capacities of 840 mA h g⁻¹ after 100 cycles and displayed significantly improved rate capability, with discharge specific capacities increasing from 220 mA h g⁻¹ (uncoated ball-milled SiO₂) to 450 mA h g⁻¹ (carbon coated ball-milled SiO₂) at 2 A g⁻¹. In order to trigger SiO₂ reactivity towards lithium, all samples were subjected to an electrochemical activation procedure prior to electrochemical testing. XRD measurements on the activated electrodes revealed that the initial crystalline silica was completely converted to amorphous phases with short range ordering, therefore evidencing the effective role of the activation procedure.

Diatomaceous earth SiO₂ anodes with superior electrochemical performance are obtained by ball milling, carbon coating and electrochemical activation of SiO₂ particles.

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.

MOF-derived 1D hollow bimetallic iron(iii) oxide nanorods: effects of metal-addition on phase transition, morphology and magnetic properties

It is demonstrated that 1D hollow bimetallic iron oxide nanorods containing Mn, Ru, Ni, La and Ag ions can be obtained regardless of the different values of ionic radius and hardness of metal dopants from NH₄OH-etched MIL-88A MOF particles.

Fibrous Network of C@MoS2 Nanocapsule-Decorated Cotton Linters Interconnected by Bacterial Cellulose for Lithium- and Sodium-Ion Batteries

To protect the structure of MoS₂ from collapse, a strong skeleton is expected to help maintain the integrity. In this study, cotton linters burdened with hollow C@MoS₂ nanocapsules are added into nutrient medium for the growth of a bacterial cellulose membrane. Benefitting from good conductivity and structural integrity, the resultant fibrous membrane anode gives reversible capacities of 559 and 155 mAh g^-1 for Li-ion batteries and Na-ion batteries after 100 cycles, respectively. The structural transformation and component evolution in lithiation-delithiation and sodiation-desodiation was elucidated by in situ Raman spectroscopy. After sodiation, the Na₂ S did not transform back into MoS₂ but was more likely converted into elemental sulfur during the conversion reaction. Layered semiconducting transition metal chalcogenides, such as molybdenum disulfide (MoS₂ ), feature open 2 D ion-transport channels amenable to receive various guest ions with high theoretical capacities.^[2] One serious challenge curtailing the applicability of such materials is their volume changes during discharge-charge processes.^{[3, 4]} However, particular morphologies of MoS₂ are proposed to improve the specific capacity.^[5,6,7] Many works have focused on core-shell and hollow MoS₂ micro- and nanostructures, and the results validate the advantages of shortening the lithium-ion diffusion distance and enhancing specific capacity.^[8,9] Unfortunately, the issue of inferior capacity stability is not resolved, because the structure is not effectively protected and is prone to collapse.

Nanoscale Phenomena in Lithium-Ion Batteries

The electrochemical properties and performances of lithium-ion batteries are primarily governed by their constituent electrode materials, whose intrinsic thermodynamic and kinetic properties are understood as the determining factor. As a part of complementing the intrinsic material properties, the strategy of nanosizing has been widely applied to electrodes to improve battery performance. It has been revealed that this not only improves the kinetics of the electrode materials but is also capable of regulating their thermodynamic properties, taking advantage of nanoscale phenomena regarding the changes in redox potential, solid-state solubility of the intercalation compounds, and reaction paths. In addition, the nanosizing of materials has recently enabled the discovery of new energy storage mechanisms, through which unexplored classes of electrodes could be introduced. Herein, we review the nanoscale phenomena discovered or exploited in lithium-ion battery chemistry thus far and discuss their potential implications, providing opportunities to further unveil uncharted electrode materials and chemistries. Finally, we discuss the limitations of the nanoscale phenomena presently employed in battery applications and suggest strategies to overcome these limitations.

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.

Interpenetrating graphene network bct-C40: a promising anode material for Li ion batteries

The stable sp²-C atoms in graphite enable its excellent structural and electrochemical stability as an anode material for Li-ion battery applications, while the limited Li-storage capacity of graphite also originates from the sp² hybridization. Herein, from first-principles calculations, we show that a synergistic effect of sp² and sp³ hybridized C atoms can substantially enhance the Li-storage performance in carbon-based anodes, using bct-C₄₀ as an example, which is constructed with interconnected graphene layers (sp² hybridized C atoms) and the connecting points are composed of sp³-C atoms. Charge transfer from sp²-C atoms to sp³-C atoms has been found, leading to unoccupied electronic states forming around the Fermi level. Furthermore, we found that the unoccupied electronic states are contributed by the p_z orbital of the sp²-C atoms, resulting in stronger interactions between C atoms and intercalated Li atoms. As a result, the Li intercalation concentration in bct-C₄₀ can reach as high as LiC_2.5 (corresponding to a capacity of 893 mA h g^-1), much higher than that of LiC₆ in graphite (372 mA h g^-1). Furthermore, bct-C₄₀ inherits good structural and electrochemical stability, a metallic electronic structure, and low Li-ion migration energy barriers (0.067-0.112 eV) from the sp² hybridized graphene structures, therefore very good Li-storage performance is expected, indicating that bct-C₄₀ can be used as a high-performance anode material for lithium ion batteries. Our study provides new insights into the functionality of sp²- and sp³-C atoms in carbon-based anode materials and is helpful for the designing of new carbon-based anodes.

MOF-Derived CuS@Cu-BTC Composites as High-Performance Anodes for Lithium-Ion Batteries

The CuS(x wt%)@Cu-BTC (BTC = 1,3,5-benzenetricarboxylate; x = 3, 10, 33, 58, 70, 99.9) materials are synthesized by a facile sulfidation reaction. The composites are composed of octahedral Cu₃ (BTC)₂ ·(H₂ O)₃ (Cu-BTC) with a large specific surface area and CuS with a high conductivity. The as-prepared CuS@Cu-BTC products are first applied as the anodes of lithium-ion batteries (LIBs). The synergistic effect between Cu-BTC and CuS components can not only accommodate the volume change and stress relaxation of electrodes but also facilitate the fast transport of Li ions. Thus, it can greatly suppress the transformation process from Li₂ S to polysulfides by improving the reversibility of the conversion reaction. Benefiting from the unique structural features, the optimal CuS(70 wt%)@Cu-BTC sample exhibits a remarkably improved electrochemical performance, showing an over-theoretical capacity up to 1609 mAh g^-1 after 200 cycles (100 mA g^-1 ) with an excellent rate-capability of ≈490 mAh g^-1 at 1000 mA g^-1 . The outstanding LIB properties indicate that the CuS(70 wt%)@Cu-BTC sample is a highly desirable electrode material candidate for high-performance LIBs.

Heterocarbides Reinforced Electrochemical Energy Storage

The feasibility of transition metal carbides (TMCs) as promising high-rate electrodes is still hindered by low specific capacity and sluggish charge transfer kinetics. Improving charge transport kinetics motivates research toward directions that would rely on heterostructures. In particular, heterocomposing with carbon-rich TMCs is highly promising for enhancing Li storage. However, due to limited synthesis methods to prepare carbon-rich TMCs, understanding the interfacial interaction effect on the high-rate performance of TMCs is often neglected. In this work, a novel strategy is proposed to construct a binary carbide heteroelectrode, i.e. incorporating the carbon-rich TMC (M=Mo) with its metal-rich TMC nanowires (nws) via an ingenious in situ disproportionation reaction. Results show that the as-prepared MoC-Mo₂ C-heteronanowires (hnws) electrode could fully recover its capacity after high-rates testing, and also possesses better lithium accommodation performance. Kinetic analysis verified that both electron and ion transfer in MoC-Mo₂ C-hnws are superior to those of its singular counterparts. Such improvements suggest that by taking utilization of the interfacial component interactions of stoichiometry tunable heterocarbides, the electrochemical performance, especially high-rate capability of carbides, could be significantly enhanced.

Cu4SnS4-Rich Nanomaterials for Thin-Film Lithium Batteries with Enhanced Conversion Reaction

Through a simple gelation-solvothermal method with graphene oxide as the additive, a Cu₄SnS₄-rich composite of nanoparticles and nanotubes is synthesized and applied for thin and flexible Li-metal batteries. Unlike the Cu₂SnS₃-rich electrode, the Cu₄SnS₄-rich electrode cycles stably with an enhanced conversion capacity of ∼416 mAh g^-1 (∼52% of total capacity) after 200 cycles. The lithiation/delithiation mechanisms of Cu-Sn-S electrodes and the voltage ranges of conversion and alloying reactions are informed by in situ X-ray diffraction tests. The conversion process of three Cu-Sn-S compounds is compared by density functional theory (DFT) calculations based on three algorithms, elucidating the enhanced conversion stability and superior diffusion kinetics of Cu₄SnS₄ electrodes. The reaction pathway of Cu-Sn-S electrodes and the root cause for the unstable capacity are revealed by in situ/ex situ characterizations, DFT calculations, and various electrochemical tests. This work provides insight into developing energy materials and power devices based on multiple lithiation mechanisms.

Heightened Integration of POM-based Metal-Organic Frameworks with Functionalized Single-Walled Carbon Nanotubes for Superior Energy Storage

To increase the conductivity of polyoxometalate-based metal-organic frameworks (POMOFs) and promote their applications in the field of energy storage, herein, a simple approach was employed to improve their overall electrochemical performances by introducing a functionalized single-walled carbon nanotubes (SWNT-COOH). A new POMOF compound, [Cu₁₈ (trz)₁₂ Cl₃ (H₂ O)₂ ][PW₁₂ O₄₀ ] (CuPW), was successfully synthesized, then the size-matched functionalized SWNT-COOH was introduced to fabricate CuPW/SWNT-COOH composite (PMNT-COOH) by employing a simple sonication-driven periodic functionalization strategy. When the PMNT-COOH nanocomposite was used as the anode material for Lithium-ion batteries (LIBs), PMNT-COOH(3) (CuPWNC:SWNT-COOH=3:1) showed superior behavior of energy storage, a high reversible capacity of 885 mA h g^-1 up to a cycle life of 170 cycles. The electrochemical results indicate that the uniform packing of SWNT-COOH provided a favored contact between the electrolyte and the electrode, resulting in enhanced specific capacity during lithium insertion/extraction process. This fabrication of PMNT-COOH nanocomposite opens new avenues for the design and synthesis of new generation electrode materials for LIBs.

Iodine encapsulated in mesoporous carbon enabling high-efficiency capacitive potassium-Ion storage

The development of potassium-ion batteries (KIBs) are hampered by the lack of appropriate electrode materials allowing for the reversible insertion/de-insertion of the large K-ion. Iodine, as a conversion-type cathode for rechargeable batteries, has high theoretical capacity and excellent electrochemical reversibility, making it a potential cathode material for KIBs. However, due to the defects of iodine with the poor electronic conductivity and easy dissolution in the electrolyte, an intensive quest for iodine-based KIBs enabling high-performance potassium-ion storage is still underway. In this work, a high-efficiency capacitive K-I₂ battery has been successfully achieved by constructing a nanocomposite of iodine encapsulated in mesoporous carbon (CMK-3). The as-prepared CMK-3/iodine nanocomposite exhibites excellent rate performance (89.3 mA h g^-1 at 0.5 A g^-1) and superior cycling stability, which remarkably exceeds most of reported KIBs cathode materials. Such a excellent electrochemical performance can be ascribed to the engineered structure of CMK-3/iodine hybridized electrode which can alleviate the impact of the shuttle phenomenon, improve electronic conductivity and facilitate ion diffusion. As a consequence, iodine within the conductive protecting CMK-3 can afford an extraordinary pseudo-capacitive potassium-ion storage, which sheds light on the development prospect of conversion-type electrode materials to meet urgent demand for advanced KIBs.

Aggregation-Morphology-Dependent Electrochemical Performance of Co3O4 Anode Materials for Lithium-Ion Batteries

The aggregation morphology of anode materials plays a vital role in achieving high performance lithium-ion batteries. Herein, Co₃O₄ anode materials with different aggregation morphologies were successfully prepared by modulating the morphology of precursors with different cobalt sources by the mild coprecipitation method. The fabricated Co₃O₄ can be flower-like, spherical, irregular, and urchin-like. Detailed investigation on the electrochemical performance demonstrated that flower-like Co₃O₄ consisting of nanorods exhibited superior performance. The reversible capacity maintained 910.7 mAh·g⁻¹ at 500 mA·g⁻¹ and 717 mAh·g⁻¹ at 1000 mA·g⁻¹ after 500 cycles. The cyclic stability was greatly enhanced, with a capacity retention rate of 92.7% at 500 mA·g⁻¹ and 78.27% at 1000 mA·g⁻¹ after 500 cycles. Electrochemical performance in long-term storage and high temperature conditions was still excellent. The unique aggregation morphology of flower-like Co₃O₄ yielded a reduction of charge-transfer resistance and stabilization of electrode structure compared with other aggregation morphologies.

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.

X-ray Nano-computed Tomography of Electrochemical Conversion in Lithium-ion Battery

Herein, a nanometric CuO anode for lithium-ion batteries was investigated by combining electrochemical measurements and ex situ X-ray computed tomography (CT) at the nanoscale. The electrode reacted by conversion at about 1.2 and 2.4 V versus Li⁺ /Li during discharge and charge, respectively, to deliver a capacity ranging from 500 mAh g^-1 to over 600 mAh g^-1 . Three-dimensional nano-CT imaging revealed substantial reorganization of the CuO particles and precipitation of a Li⁺ -conducting film suitable for a possible application in the battery. A lithium-ion cell, exploiting the high capacity of the conversion process, was assembled by using a high-performance LiNi_0.33 Co_0.33 Mn_0.33 O₂ cathode reacting at 3.9 V versus Li⁺ /Li. The cell was proposed as an energy-storage system with an average working voltage of about 2.5 V, specific capacity of 170 mAh g_cathode ^-1 , and efficiency exceeding 99 % with a very stable cycling.

Understanding the Role of Overpotentials in Lithium Ion Conversion Reactions: Visualizing the Interface

Oxide conversion reactions are known to have substantially higher specific capacities than intercalation materials used in Li-ion batteries, but universally suffer from large overpotentials associated with the formation of interfaces between the resulting nanoscale metal and Li₂O products. Here we use the interfacial sensitivity of operando X-ray reflectivity to visualize the structural evolution of ultrathin NiO electrodes and their interfaces during conversion. We observe two additional reactions prior to the well-known bulk, three-dimensional conversion occurring at 0.6 V: an accumulation of lithium at the buried metal/oxide interface (at 2.2 V) followed by interfacial lithiation of the buried NiO/Ni interface at the theoretical potential for conversion (at 1.9 V). To understand the mechanisms for bulk and interfacial lithiation, we calculate interfacial energies using density functional theory to build a potential-dependent nucleation model for conversion. These calculations show that the additional space charge layer of lithium is a crucial component for reducing energy barriers for conversion in NiO.

Vanadium Organometallics as an Interfacial Stabilizer for Ca xV2O5/Vanadyl Acetylacetonate Hybrid Nanocomposite with Enhanced Energy Density and Power Rate for Full Lithium-Ion Batteries

Vanadium pentoxide (V₂O₅) offers high capacity and energy density as a cathode candidate for lithium-ion batteries (LIBs). Unfortunately, its practical utilization is intrinsically handicapped by the low conductivity, poor electrode kinetics, and lattice instability. In this study, the synergistical optimization protocol has been proposed in the conjunction of interstitial Ca incorporation and organic vanadate surface protection. It is revealed that regulating Ca occupation in the body phase at a relatively low concentration can effectively expand the layer distance of α-V₂O₅, which facilitates the intercalation access for Li-ion insertion. On the other hand, organometallics are first applied as the protective layer to stabilize the electrode interface during cycling. The optimized coating layer, vanadium oxy-acetylacetonate (VO(acac)₂), plays an important role to generate a more inorganic component (LiF) within the solid electrolyte interface, contributing to the protection of the Ca-incorporated V₂O₅ electrode. As a result, the optimized Ca_0.05V₂O₅/VO(acac)₂ hybrid electrode exhibits much improved capacity utilization, rate capability, and cycling stability, delivering capacity as high as 297 mAh g^-1 for full LIBs. The first-principle computations reveal the lattice change caused by the Ca incorporation, further confirming the lattice advantage of Ca_0.05V₂O₅/VO(acac)₂ with respect to Li-ion intercalation.

Highly Reversible Conversion Anodes Composed of Ultralarge Monolithic Grains with Seamless Intragranular Binder and Wiring Network

Conversion anodes enable a high capacity for lithium-ion batteries due to more than one electron transfer. However, the collapse of the host structure during cycling would cause huge volume expansion and phase separation, leading to the degradation and disconnection of the mixed conductive network of the electrode. The initial nanostructuring and loose spatial distribution of active species are often resorted to in order to alleviate the evolution of the electrode morphology, but at the cost of the decrease of grain packing density. The utilization of ultralarge microsized grains of high density as the conversion anode is still highly challenging. Here, a proof-of-concept grain architecture characterized by endogenetic binder matrix and wiring network is proposed to guarantee the structural integrity of monolithic grains as large as 50-100 μm during deep conversion reaction. Such big grains were fabricated by self-assembly and pyrolysis of a Keggin-type polyoxometalate-based complex with protonated tris[2-(2-methoxyethoxy)-ethyl]amine (TDA-1-H⁺). The metal-organic precursor can guarantee the firm adherence of numerous Mo-O clusters and nuclei into a highly elastic monolithic structure without evident grain boundaries and intergranular voids. The pyrolyzed TDA-1-H⁺ not only serves as in situ binder and conductive wire to glue adjacent Mo-O moieties but also acts as a Li-ion pathway to promote sufficient lithiation on surrounding Mo-O. Such a monolithic electrode design leads to an unusual high-conversion-capacity performance (1000 mAh/g) with satisfactory reversibility (reaching at least 750 cycles at 1 A/g). These cycled grains are not disassembled even after undergoing long-term cycling. The conception of the intragranular binder is further confirmed by consolidating the MoO₂ porous network after in situ stuffing of MoS₂ nanobinders.

Difference in Electrochemical Mechanism of SnO2 Conversion in Lithium-Ion and Sodium-Ion Batteries: Combined in Operando and Ex Situ XAS Investigations

Conversion and alloying type negative electrodes attracted huge attention in the present research on lithium/sodium-ion batteries (LIBs/SIBs) due to the high capacity delivered. Among these, SnO₂ is investigated intensively in LIBs due to high cyclability, low reaction potential, cost-effectiveness, and environmental friendliness. Most of the LIB electrodes are explored in SIBs too due to expected similar electrochemical performance. Though several LIB negative electrode materials successfully worked in SIBs, bare SnO₂ shows very poor electrochemical performance in SIB. The reason for this difference is investigated here through combined in operando and ex situ X-ray absorption spectroscopy (XAS). For this, the electrodes of SnO₂ (space group P4₂/mnm synthesized via one-pot hydrothermal method) were cycled in Na-ion and Li-ion half-cells. The Na/SnO₂ half-cell delivered a much lower discharge capacity than the Li/SnO₂ half-cell. In addition, higher irreversibility was observed for Na/SnO₂ half-cell during electrochemical investigations compared to that for Li/SnO₂ half-cell. In operando XAS investigations on the Na/SnO₂ half-cell confirms incomplete conversion and alloying reactions in the Na/SnO₂ half-cell, resulting in poor electrochemical performance. The difference in the lithiation and sodiation mechanisms of SnO₂ is discussed in detail.

Controllable two-dimensional movement and redistribution of lithium ions in metal oxides

Rechargeable lithium batteries are the most practical and widely used power sources for portable and mobile devices in modern society. Manipulation of the electronic and ionic charge transport and accumulation in solid materials has always been crucial for rechargeable lithium batteries. The transport and accumulation of lithium ions in electrode materials, which is a diffusion process, is determined by the concentration distribution of lithium ions and the intrinsic structure of the electrode material and thus far has not been manipulated by an external force. Here, we report the realization of controllable two-dimensional movement and redistribution of lithium ions in metal oxides. This achievement is one kind of centimeter-scale control and is achieved by a magnetic field based on the 'current-driving model'. This work provides additional insight for building safe and high-capacity rechargeable lithium batteries.

A novel Zr-MOF-based and polyaniline-coated UIO-67@Se@PANI composite cathode for lithium-selenium batteries

In this work, we synthesized a novel UIO-67@Se@PANI composite cathode material for Li-Se battery applications. Zr-MOFs (metal organic frameworks) were used as a support and a PANI (polyaniline) layer was employed as the coating. UIO-67@Se@PANI was expected to be one of the candidates for Li-Se batteries, with a high specific capacity of 248.3 mA h g^-1 at 1C (1C = 675 mA g^-1) after 100 cycles. In particular, stable capacities of 203.1 and 167.6 mA h g^-1 were received after 100 cycles at high rates of 2C and 5C, respectively. To explain such a good electrochemistry performance of the composite cathode material, multiple characterization methods were carried out. And that can be attributed to the sandwich-like structure of the UIO-67@Se@PANI composite and the fact that UIO-67 can provide unsaturated sites to tether the selenium effectively and the PANI layer can improve the electronic conductivity of the whole electrode significantly.

Interconnected Vertically Stacked 2D-MoS2 for Ultrastable Cycling of Rechargeable Li-Ion Battery

A two-dimensional (2D) layer-structured material is often a high-capacity ionic storage material with fast ionic transport within the layers. This appears to be the case for nonconversion layer structure, such as graphite. However, this is not the case for conversion-type layered structure such as transition-metal sulfide, in which localized congestion of ionic species adjacent to the surface will induce localized conversion, leading to the blocking of the fast diffusion channels and fast capacity fading, which therefore constitutes one of the critical barriers for the application of transition-metal sulfide layered structure. In this work, we report the tackling of this critical barrier through nanoscale engineering. We discover that interconnected vertically stacked two-dimensional-molybdenum disulfide can dramatically enhance the cycling stability. Atomic-level in situ transmission electron microscopy observation reveals that the molybdenum disulfide (MoS₂) nanocakes assembled with tangling {100}-terminated nanosheets offer abundant open channels for Li⁺ insertion through the {100} surface, featuring an exceptional cyclability performance for over 200 cycles with a capacity retention of 90%. In contrast, (002)-terminated MoS₂ nanoflowers only retain 10% of original capacity after 50 cycles. The present work demonstrates a general principle and opens a new route of crystallographic design to enhance electrochemical performance for assembling 2D materials for energy storage.

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.

The dealloying-lithiation/delithiation-realloying mechanism of a breithauptite (NiSb) nanocrystal embedded nanofabric anode for flexible Li-ion batteries

Antimony (Sb) based anodes with high conductivity and capability have shown great promise for applications in lithium ion batteries (LIBs). However, they often suffer from poor cycling stability because of the drastic volume variation and structural degradation on undergoing lithiation-delithiation processes. Here we demonstrate a novel Sb-based anode with a free-standing structure realized by uniformly implanting intermetallic compound breithauptite (nickel antimonide, NiSb) nanocrystals into nitrogen-doped carbon nanofibers (NiSb@NCNFs). The discharge/charge behavior of NiSb@NCNFs was systematically investigated by ex situ characterization, which revealed a special "dealloying-lithiation/delithiation-realloying" cycling mechanism. The NiSb nanocrystals possess high lithium storage capacity, and the interconnected network of NCNFs can accommodate the volume variation of encapsulated NiSb nanoparticles, while also providing smooth pathways for charge transport. Compared to other Sb-based anodes, the NiSb@NCNF anode presents exceptional reversible capacity (720 mA h g-1 at a current density of 100 mA g-1) and greatly enhanced cycling life at high rates (510 mA h g-1 after 2000 cycles at 2000 mA g-1). Furthermore, the free-standing NiSb@NCNF anode is free of binders, conductive additives and metal current collectors, exhibiting high flexibility and remarkable performances for the construction of flexible and bendable soft-packed full Li-ion pouch cells.

ZIF-67-Derived N-Doped Co/C Nanocubes as High-Performance Anode Materials for Lithium-Ion Batteries

Co nanoparticles embedded in nitrogen-doped carbon nanocubes (Co/NCs) for applications as anode materials in rechargeable lithium-ion batteries were synthesized by calcining Co-based metal-organic framework. Sizes of Co nanoparticles were ∼15 nm according to X-ray diffraction (XRD) and transmission electron microscopy. Electrochemical performances of the as-prepared anode nanocube composite at 700 °C showed a high initial capacity of 1375.1 mAh g^-1 in the voltage range of 0.01-3.0 V at the current rate of 0.1 A g^-1. After 100 cycles, capacity remained at 688.6 mAh g^-1. Thereinto, the role of Co nanoparticles in electrochemical reaction was also elucidated by in situ XRD experiment. Capacity increase of Co/NCs at the high currents was observed, which are potentially caused by the activation of electrode and pseudocapacitance during cycling. High surface area and abundant mesopores contributed to the improved electrochemical performances of the anode, providing numerous pathways and sites for Li⁺ transfer and storage and accordingly contributing to pseudocapacitance capacity.

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.

Bi2MoO6 Microsphere with Double-Polyaniline Layers toward Ultrastable Lithium Energy Storage by Reinforced Structure

Given its competitive theoretical capacity, Bi₂MoO₆ is deemed as a promising anode material for the realization of efficient Li storage. Considering the severe capacity attenuation caused by the lithiation-induced expansion, it is essential to introduce effective modification. Remarkably, in this work, Bi₂MoO₆ microsphere with double-layered spherical shells are successfully prepared, and the polyaniline are coated on both inner and outer surfaces of double-layered spherical shells, working as buffer layers to strain the volume expansion during electrochemical cycling. Inspiringly, when utilized as anode in LIBs, the specific capacity of Bi₂MoO₆@PANI is maintained at 656.3 mAh g^-1 after 200 cycles at 100 mA g^-1, corresponding to a high capacity of 82%. However, the counterpart of individual Bi₂MoO₆ is only 36%. This result confirms that the polyaniline layer can dramatically promote stable cycling performances. Supported by in situ EIS and ex situ technologies followed by detailed analysis, the enhanced pseudocapacitance-dominated contributions and electron/ion transfer rate, benefiting from the combination with polyaniline, are further proved. This work confirms the significant effect of polyaniline on the ultrastable energy storage, further providing an in-depth sight on the impacts of polyaniline coating to the electrical conductivity as well as the resistances of electron/ion transport.

Constructing hyperbranched polymers as a stable elastic framework for copper sulfide nanoplates for enhancing sodium-storage performance

Electrochemical conversion reactions offer a new avenue to build high-capacity anodes for sodium-ion batteries. However, poor long-term cyclability and low coulombic efficiency at the first cycle remain a significant challenge for practical Na-ion battery applications. Herein, a novel hyperbranched polymer is used as a template and electrode additive to construct unique hierarchical Cu9S5 nanoplates. With an internal uniform distribution, the additive could regulate the morphology and microstructure of Cu9S5 and offer a buffering matrix to alleviate nanoparticle aggregation and enhance solid-state Na+ ion diffusion. This Cu9S5 composite anode exhibits a high reversible capacity of 429 mA h g-1 at 100 mA g-1, a high coulombic efficiency of 94.3% at the first cycle, a superior rate capability of 300 mA h g-1 at 20 A g-1, and an outstanding cyclability with 82.2% capacity retention after 1000 cycles. The kinetic study reveals that Cu9S5-AHP nanoplates show a low charge transfer resistance and high Na+ diffusion coefficient (∼10-9 cm2 s-1). The present work suggests a potentially feasible anode material for sodium-ion batteries and, more significantly, demonstrates a novel strategy for the construction of high-performance conversion materials for sodium-ion batteries.

Development of Divide-and-Conquer Density-Functional Tight-Binding Method for Theoretical Research on Li-Ion Battery

The density-functional tight-binding (DFTB) method is one of the useful quantum chemical methods, which provides a good balance between accuracy and computational efficiency. In this account, we reviewed the basis of the DFTB method, the linear-scaling divide-and-conquer (DC) technique, as well as the parameterization process. We also provide some refinement, modifications, and extension of the existing parameters that can be applicable for lithium-ion battery systems. The diffusion constants of common electrolyte molecules and LiTFSA salt in solution have been estimated using DC-DFTB molecular dynamics simulation with our new parameters. The resulting diffusion constants have good agreement to the experimental diffusion constants.