On the origin of the extra capacity at low potential in materials for Li batteries reacting through conversion reaction

High-Rate One-Dimensional α-MnO2 Anode for Lithium-Ion Batteries: Impact of Polymorphic and Crystallographic Features on Lithium Storage

Manganese dioxide (MnO2) exists in a variety of polymorphs and crystallographic structures. The electrochemical performance of Li storage can vary depending on the polymorph and the morphology. In this study, we present a new approach to fabricate polymorph- and aspect-ratio-controlled α-MnO2 nanorods. First, δ-MnO2 nanoparticles were synthesized using a solution plasma process assisted by three types of sugars (sucrose, glucose, and fructose) as reducing promoters; this revealed different morphologies depending on the nucleation rate and reaction time from the molecular structure of the sugars. Based on the morphology of δ-MnO2, the polymorphic-transformed three types of α-MnO2 nanorods showed different aspect ratios (c/a), which highly affected the transport of Li ions. Among them, a relatively small aspect ratio (c/a = 5.1) and wide width of α-MnO2-S nanorods (sucrose-assisted) induced facile Li-ion transport in the interior of the particles through an increased Li-ion pathway. Consequently, α-MnO2-S exhibited superior battery performance with a high-rate capability of 673 mAh g-1 at 2 A g-1, and it delivered a high reversible capacity of 1169 mAh g-1 at 0.5 A g-1 after 200 cycles. Our findings demonstrated that polymorphs and crystallographic properties are crucial factors in the electrode design of high-performance Li-ion batteries.

Liquid Processing of Interfacially Grown Iron-Oxide Flowers into 2D-Platelets Yields Lithium-Ion Battery Anodes with Capacities of Twice the Theoretical Value

Iron oxide (Fe₂ O₃ ) is an abundant and potentially low-cost material for fabricating lithium-ion battery anodes. Here, the growth of α-Fe₂ O₃ nano-flowers at an electrified liquid-liquid interface is demonstrated. Sonication is used to convert these flowers into quasi-2D platelets with lateral sizes in the range of hundreds of nanometers and thicknesses in the range of tens of nanometers. These nanoplatelets can be combined with carbon nanotubes to form porous, conductive composites which can be used as electrodes in lithium-ion batteries. Using a standard activation process, these anodes display good cycling stability, reasonable rate performance and low-rate capacities approaching 1500 mAh g^-1 , consistent with the current state-of-the-art for Fe₂ O₃ . However, by using an extended activation process, it is found that the morphology of these composites can be significantly changed, rendering the iron oxide amorphous and significantly increasing the porosity and internal surface area. These morphological changes yield anodes with very good cycling stability and low-rate capacity exceeding 2000 mAh g^-1 , which is competitive with the best anode materials in the literature. However, the data implies that, after activation, the iron oxide displays a reduced solid-state lithium-ion diffusion coefficient resulting in somewhat degraded rate performance.

Unveiling the Genesis and Effectiveness of Negative Fading in Nanostructured Iron Oxide Anode Materials for Lithium-Ion Batteries

Iron oxide anode materials for rechargeable lithium-ion batteries have garnered extensive attention because of their inexpensiveness, safety, and high theoretical capacity. Nanostructured iron oxide anodes often undergo negative fading, that is, unconventional capacity increase, which results in a capacity increasing upon cycling. However, the detailed mechanism of negative fading still remains unclear, and there is no consensus on the provenance. Herein, we comprehensively investigate the negative fading of iron oxide anodes with a highly ordered mesoporous structure by utilizing advanced synchrotron-based analysis. Electrochemical and structural analyses identified that the negative fading originates from an optimization of the electrolyte-derived surface layer, and the thus formed layer significantly contributes to the structural stability of the nanostructured electrode materials, as well as their cycle stability. This work provides an insight into understanding the origin of negative fading and its influence on nanostructured anode materials.

A green low-temperature preparation of iron molybdate with durable electrochemical performances

Herein, nano-Fe2(MoO4)3 (FMO) has been prepared by a green sol–gel route at a low temperature (350 °C), which is far below the conventional value (above 650 °C).

Sub-micro droplet reactors for green synthesis of Li3VO4 anode materials in lithium ion batteries

The conventional solid-state reaction suffers from low diffusivity, high energy consumption, and uncontrolled morphology. These limitations are competed by the presence of water in solution route reaction. Herein, based on concept of combining above methods, we report a facile solid-state reaction conducted in water vapor at low temperature along with calcium doping for modifying lithium vanadate as anode material for lithium-ion batteries. The optimized material, delivers a superior specific capacity of 543.1, 477.1, and 337.2 mAh g-1 after 200 and 1000 cycles at current densities of 100, 1000 and 4000 mA g-1, respectively, which is attributed to the contribution of pseudocapacitance. In this work, we also use experimental and theoretical calculation to demonstrate that the enhancement of doped lithium vanadate is attributed to particles confinement of droplets in water vapor along with the surface and structure variation of calcium doping effect.

Impact of the Transition Metal Dopant in Zinc Oxide Lithium-Ion Anodes on the Solid Electrolyte Interphase Formation

Conversion/alloying materials (CAMs) provide substantially higher specific capacities than graphite, the state-of-the-art lithium-ion battery anode material. The ability to host much more lithium per unit weight and volume is, however, accompanied by significant volume changes, which challenges the realization of a stable solid electrolyte interphase (SEI). Herein, the comprehensive characterization of the composition and evolution of the SEI on transition metal (TM) doped zinc oxide as CAM model compound, is reported, with a particular focus on the impact of the TM dopant (Fe or Co). The results unveil that the presence of iron specifically triggers the electrolyte decomposition. However, this detrimental effect can be avoided by stabilizing the interface with the electrolyte by a carbonaceous coating. These findings provide a great leap forward toward the enhanced understanding of such doped materials and (transition) metal oxide active materials in general.

Real Time Observation of Lithium Insertion into Pre-Cycled Conversion-Type Materials

Conversion-type electrode materials for lithium-ion batteries experience significant structural changes during the first discharge–charge cycle, where a single particle is taken apart into a number of nanoparticles. This structural evolution may affect the following lithium insertion reactions; however, how lithiation occurs in pre-cycled electrode materials is elusive. In this work, in situ transmission electron microscopy was employed to see the lithium-induced structural and chemical evolutions in pre-cycled nickel oxide as a model system. The introduction of lithium ions induced the evolution of metallic nickel, with volume expansion as a result of a conversion reaction. After pre-cycling, the phase evolutions occurred in two separate areas almost at the same time. This is different from the first lithiation, where the phase change takes place successively, with a boundary dividing the reacted and unreacted areas. Structural changes were restricted at the areas having large amount of fluorine, implying the residuals from the decomposition of electrolytes may have hindered the electrochemical reactions. This work provides insights into phase and chemical evolutions in pre-cycled conversion-type materials, which govern electrochemical properties during operation.

Lithium-Ion Capacitors: A Review of Design and Active Materials

Lithium-ion capacitors (LICs) have gained significant attention in recent years for their increased energy density without altering their power density. LICs achieve higher capacitance than traditional supercapacitors due to their hybrid battery electrode and subsequent higher voltage. This is due to the asymmetric action of LICs, which serves as an enhancer of traditional supercapacitors. This culminates in the potential for pollution-free, long-lasting, and efficient energy-storing that is required to realise a renewable energy future. This review article offers an analysis of recent progress in the production of LIC electrode active materials, requirements and performance. In-situ hybridisation and ex-situ recombination of composite materials comprising a wide variety of active constituents is also addressed. The possible challenges and opportunities for future research based on LICs in energy applications are also discussed.

Retracting interphasial stored Li+ ions by transition metal/metal carbide nanoparticles for enhanced Li+ ion storage capacity

Herein, we report the synthesis of metal/metal carbide (Co, Ni, and Fe₃C) nanoparticles (NPs) encapsulated nitrogen-doped carbon nanotubes (NCNT) and its application as the anode materials for lithium-ion battery (LIB). The electron microscopy images confirm the encapsulation of metal NPs inside the carbon nanotubes, which can inhibit the NPs aggregations and offer long cycle life for LIB. The metal/metal carbide encapsulated NCNT as anode material exhibits higher specific capacity than pure NCNT. The cyclic voltammetry studies reveal that Co, Ni, and Fe₃C NPs can oxidize and reduce the solid electrolyte interphase (SEI) layer components of the anode. This offers the extra specific capacity to Fe₃C/NCNT, Co/NCNT, and Ni/NCNT anodes by retracting the interphasial stored Li⁺ ions. Moreover, in this study, the catalytic activity of Co, Ni, and Fe₃C NPs for tailoring the SEI components are compared for the first time, and it shows Fe₃C/NCNT anode has the highest catalytic activity than Co/NCNT and Ni/NCNT. Co/NCNT and Fe₃C/NCNT also exhibit good cycle life up to 1300 cycles at a current density of 1 A g^-1. Overall, this work demonstrates an effective strategy to improve the performance of LIB anode by retracting the interphasial stored Li⁺ ions.

Multi-Step Phase Transitions of Mn3 O4 During Galvanostatic Lithiation: An In Situ Transmission Electron Microscopic Investigation

For study of electrochemical reaction mechanisms at nanoscale, in situ electrochemical transmission electron microscopy (EC-TEM) exceeds many other methods due to its high temporal and spatial resolution. However, the limited amount of active materials used in previous in situ TEM studies prevents the model EC cells to operate in the constant-current (galvanostatic) charge/discharge mode that is required for accurate control of electrochemical processes. Herein, a new in situ EC-TEM technique is developed to investigate multi-step phase transitions of Mn₃ O₄ electrodes under the galvanostatic charge/discharge mode and constant-voltage discharge mode. In galvanostatic mode, the lithiation of Mn₃ O₄ undergoes multi-step phase transitions following a reaction pathway of Mn₃ O₄ + Li⁺ → LiMn₃ O₄ + Li⁺ → MnO + Li₂ O → Mn + Li₂ O. It is also found that lithium ions prefer to enter Mn₃ O₄ along the {101} direction to form LiMn₃ O₄ with the help of transitional boundary phase of Li_x Mn₃ O₄ . These results are in sharp contrast to that obtained under a constant-voltage discharge mode, where only a single-step lithiation process of Mn₃ O₄ + Li⁺ → Mn + Li₂ O is observed.

Scalable Synthesis of Microsized, Nanocrystalline Zn0.9 Fe0.1 O-C Secondary Particles and Their Use in Zn0.9 Fe0.1 O-C/LiNi0.5 Mn1.5 O4 Lithium-Ion Full Cells

Conversion/alloying materials (CAMs) are a potential alternative to graphite as Li-ion anodes, especially for high-power performance. The so far most investigated CAM is carbon-coated Zn0.9 Fe0.1 O, which provides very high specific capacity of more than 900 mAh g-1 and good rate capability. Especially for the latter the optimal particle size is in the nanometer regime. However, this leads to limited electrode packing densities and safety issues in large-scale handling and processing. Herein, a new synthesis route including three spray-drying steps that results in the formation of microsized, spherical secondary particles is reported. The resulting particles with sizes of 10-15 μm are composed of carbon-coated Zn0.9 Fe0.1 O nanocrystals with an average diameter of approximately 30-40 nm. The carbon coating ensures fast electron transport in the secondary particles and, thus, high rate capability of the resulting electrodes. Coupling partially prelithiated, carbon-coated Zn0.9 Fe0.1 O anodes with LiNi0.5 Mn1.5 O4 cathodes results in cobalt-free Li-ion cells delivering a specific energy of up to 284 Wh kg-1 (at 1 C rate) and power of 1105 W kg-1 (at 3 C) with remarkable energy efficiency (>93 % at 1 C and 91.8 % at 3 C).

Mechanistic Insights into the Lithiation and Delithiation of Iron-Doped Zinc Oxide: The Nucleation Site Model

The detailed mechanistic understanding of the electrochemical reactions occurring in lithium-ion battery electrodes is fundamental for their further improvement. Conversion/alloying materials (CAMs), such as Zn_0.9Fe_0.1O, one of the most recent alternatives for classic graphite anodes, offer superior specific capacity and rate capability. However, despite fast kinetics, CAMs suffer from a large voltage hysteresis upon de-/lithiation and improvable Coulombic efficiencies when cycled in a large voltage window. Here, we use isothermal microcalorimetry together with operando X-ray diffraction as well as ex situ ⁷Li NMR and ⁵⁷Fe Mössbauer spectroscopies to investigate the asymmetric reaction mechanism of the lithiation and delithiation of Zn_0.9Fe_0.1O during electrochemical cycling. We demonstrate that the measured heat flow is correlated with compositional changes of the electrode material. This combination of highly complementary techniques allows us to propose a new nucleation site model for the initial lithiation of Zn_0.9Fe_0.1O. Modeling the heat flow provides concrete evidence for the deleterious impact of high anodic cutoff potentials (>2 V), resulting in a continuous quasireversible solid electrolyte interphase formation. The presented methodology is suggested to provide improved insights into the reaction mechanism of conversion- and alloying-type energy-storage materials.

Mechanism Study of Carbon Coating Effects on Conversion-Type Anode Materials in Lithium-Ion Batteries: Case Study of ZnMn2O4 and ZnO-MnO Composites

The carbon coating strategy is intensively used in the modification of conversion-type anode materials to improve their cycling stability and rate capability. Thus, it is necessary to elucidate the modification mechanism induced by carbon coating. For this purpose, bare ZnMn₂O₄, carbon-derivative-coated ZnMn₂O₄, and carbon-coated ZnO-MnO composite materials have been synthesized and investigated in-depth. Herein, high-temperature synchrotron radiation diffraction is used to monitor the phase transition from ZnMn₂O₄ to ZnO-MnO composite during the carbonization process. The electrochemical performance has been evaluated by cyclic voltammetry, galvanostatic cycling, and electrochemical impedance spectroscopy. The carbon- and carbon-derivative-coated samples display well-improved cycling stability in terms of suppressed electrode polarization, a moderate increase in resistance, and slight capacity variation. The influence of carbon coating on the intrinsic conversion process is investigated by ex situ X-ray absorption spectroscopy, which reveals the evolution of Zn and Mn oxidation states. This result confirms that the strong capacity variation of the bare ZnMn₂O₄ is induced not only by the reversible charge storage in the solid electrolyte interphase but also by the phase evolution of active materials. Carbon coating is an effective method to prevent the additional oxidation of MnO to Mn₃O₄, which leads to a stabilization of the main conversion reaction.

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.

A Four-Electron Sulfur Electrode Hosting a Cu2+ /Cu+ Redox Charge Carrier

The elemental sulfur electrode with Cu²⁺ as the charge carrier gives a four-electron sulfur electrode reaction through the sequential conversion of S↔CuS↔Cu₂ S. The Cu-S redox-ion electrode delivers a high specific capacity of 3044 mAh g^-1 based on the sulfur mass or 609 mAh g^-1 based on the mass of Cu₂ S, the completely discharged product, and displays an unprecedently high potential of sulfur/metal sulfide reduction at 0.5 V vs. SHE. The Cu-S electrode also exhibits an extremely low extent of polarization of 0.05 V and an outstanding cycle number of 1200 cycles retaining 72 % of the initial capacity at 12.5 A g^-1 . The remarkable utility of this Cu-S cathode is further demonstrated in a hybrid cell that employs an Zn metal anode and an anion-exchange membrane as the separator, which yields an average cell discharge voltage of 1.15 V, the half-cell specific energy of 547 Wh kg^-1 based on the mass of the Cu₂ S/carbon composite cathode, and stable cycling over 110 cycles.

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.

Asymmetric faradaic assembly of Bi2O3 and MnO2 for a high-performance hybrid electrochemical energy storage device

A hybrid electrochemical energy storage device assembled with faradaic Bi₂O₃ and MnO₂ electrodes exhibits superior electrochemical performance with a high energy density of 79 W h kg⁻¹ at a power density of 702 W kg⁻¹.

Facile Method to Prepare for the Ni2P Nanostructures with Controlled Crystallinity and Morphology as Anode Materials of Lithium-Ion Batteries

Conversion reaction materials (transition metal oxides, sulfides, phosphides, etc.) are attractive in the field of lithium-ion batteries because of their high theoretical capacity and low cost. However, the realization of these materials in lithium-ion batteries is impeded by large voltage hysteresis, high polarization, inferior cycle stability, rate capability, irreversible capacity loss in first cycling, and dramatic volume change during redox reactions. One method to overcome these problems is the introduction of amorphous materials. This work introduces a facile method to synthesize amorphous and crystalline dinickel phosphide (Ni₂P) nanoparticle clusters with identical morphology and presents a direct comparison of the two materials as anode materials for rechargeable lithium-ion batteries. To assess the effect of crystallinity and hierarchical structure of nanomaterials, it is crucial to conserve other factors including size, morphology, and ligand of nanoparticles. Although it is rarely studied about synthetic methods of well-controlled Ni₂P nanomaterials to meet the above criteria, we synthesized amorphous, crystalline Ni₂P, and self-assembled Ni₂P nanoparticle clusters via thermal decomposition of nickel-surfactant complex. Interestingly, simple modulation of the quantity of nickel acetylacetonate produced amorphous, crystalline, and self-assembled Ni₂P nanoparticles. A 0.357 M nickel-trioctylphosphine (TOP) solution leads to a reaction temperature limitation (∼315 °C) by the nickel precursor, and crystalline Ni₂P (c-Ni₂P) nanoparticles clusters are generated. On the contrary, a lower concentration (0.1 M) does not accompany a temperature limitation and hence high reaction temperature (330 °C) can be exploited for the self-assembly of Ni₂P (s-Ni₂P) nanoparticle clusters. Amorphous Ni₂P (a-Ni₂P) nanoparticle clusters are generated with a high concentration (0.714 M) of nickel-TOP solution and a temperature limitation (∼290 °C). The a-Ni₂P nanoparticle cluster electrode exhibits higher capacities and Coulombic efficiency than the electrode based on c-Ni₂P nanoparticle clusters. In addition, the amorphous structure of Ni₂P can reduce irreversible capacity and voltage hysteresis upon cycling. The amorphous morphology of Ni₂P also improves the rate capability, resulting in superior performance to those of c-Ni₂P nanoparticle clusters in terms of electrode performance.

Cobalt Disulfide Nanoparticles Embedded in Porous Carbonaceous Micro-Polyhedrons Interlinked by Carbon Nanotubes for Superior Lithium and Sodium Storage

Transition metal sulfides are appealing electrode materials for lithium and sodium batteries owing to their high theoretical capacity. However, they are commonly characterized by rather poor cycling stability and low rate capability. Herein, we investigate CoS₂, serving as a model compound. We synthesized a porous CoS₂/C micro-polyhedron composite entangled in a carbon-nanotube-based network (CoS₂-C/CNT), starting from zeolitic imidazolate frameworks-67 as a single precursor. Following an efficient two-step synthesis strategy, the obtained CoS₂ nanoparticles are uniformly embedded in porous carbonaceous micro-polyhedrons, interwoven with CNTs to ensure high electronic conductivity. The CoS₂-C/CNT nanocomposite provides excellent bifunctional energy storage performance, delivering 1030 mAh g^-1 after 120 cycles and 403 mAh g^-1 after 200 cycles (at 100 mA g^-1) as electrode for lithium-ion (LIBs) and sodium-ion batteries (SIBs), respectively. In addition to these high capacities, the electrodes show outstanding rate capability and excellent long-term cycling stability with a capacity retention of 80% after 500 cycles for LIBs and 90% after 200 cycles for SIBs. In situ X-ray diffraction reveals a significant contribution of the partially graphitized carbon to the lithium and at least in part also for the sodium storage and the report of a two-step conversion reaction mechanism of CoS₂, eventually forming metallic Co and Li₂S/Na₂S. Particularly the lithium storage capability at elevated (dis-)charge rates, however, appears to be substantially pseudocapacitive, thus benefiting from the highly porous nature of the nanocomposite.

Transition metal cations on the move: simultaneous operando X-ray absorption spectroscopy and X-ray diffraction investigations during Li uptake and release of a NiFe2O4/CNT composite

We report on results of a comprehensive investigation on reaction mechanisms occurring during Li uptake and release of the composite NiFe2O4/CNT. Operando X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) data collected simultaneously using one in situ cell allowed thorough elucidation of structural and electronic alterations happening during Li uptake. From the beginning of Li uptake, the Bragg intensity of the spinel reflections decreases which can be explained by reduction of Fe3+ ions and simultaneous movement of the Fe2+ cations from tetrahedral 8a to empty octahedral 16c sites. The reduction of Fe3+ is clearly evidenced by XAS. The occupation of tetrahedral sites by Li+ can be excluded based on results of density functional theory calculations. Increasing the Li content leads to formation of a new crystalline phase resembling a monoxide with a NaCl-like structure. The appearance of the new phase is accompanied by a steady decrease of the sizes of coherently scattering domains of the spinel and a growth of the domains of the monoxide phase. After uptake of about 2.5 Li per NiFe2O4, all Fe3+ cations are reduced to Fe2+ and the tetrahedral 8a sites are empty (XAS spectra). Careful Rietveld refinements of X-ray powder patterns demonstrate that the tetrahedral 8a site is successively depleted with increasing Li content. Interestingly, the occupancy of the octahedral 16d site is also slightly reduced. Increasing the Li content beyond 2.5 Li/NiFe2O4 leads to successive reduction of the cations to very small metal particles embedded in a Li2O matrix (as evidenced by 7Li MAS NMR investigations). During Li release metallic Ni and Fe are reoxidized to Ni2+ resp. Fe3+. The cycling stability of NiFe2O4/CNT is significantly improved compared to pure NiFe2O4 or a mechanical mixture of NiFe2O4 and CNTs.

Capacity Increase Investigation of Cu2Se Electrode by Using Electrochemical Impedance Spectroscopy

Cu2Se nanoflake arrays supported by Cu foams are synthesized by a facile hydrothermal method in this study. The Cu2Se materials are directly used as an anode for lithium ion batteries, which show superior cycle performance with significant capacity increase. Combining with previous reports and scanning electron microscope images after cycling, the capacity increase caused by the reversible growth of a polymeric film is discussed. Electrochemical impedance spectroscopy is used to test the reversible growth of the polymeric film. By analyzing the three-dimensional Nyquist plots at different potentials during the discharge/charge process, detailed electrochemical reaction information can be obtained, which can further verify the reversible formation of a polymeric film at low potential.

Nanosized CoO Loaded on Copper Foam for High-Performance, Binder-Free Lithium-Ion Batteries

The synthesis of nanosized CoO anodes with unique morphologies via a hydrothermal method is investigated. By adjusting the pH values of reaction solutions, nanoflakes (CoO-NFs) and nanoflowers (CoO-FLs) are successfully located on copper foam. Compared with CoO-FLs, CoO-NFs as anodes for lithium ion batteries present ameliorated lithium storage properties, such as good rate capability, excellent cycling stability, and large reversible capacity. The initial discharge capacity is 1470 mA h g⁻¹, while the reversible capacity is maintained at 1776 m Ah g⁻¹ after 80 cycles at a current density of 100 mA h g⁻¹. The excellent electrochemical performance is ascribed to enough free space and enhanced conductivity, which play crucial roles in facilitating electron transport during repetitive Li⁺ intercalation and extraction reaction as well as buffering the volume expansion.

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.

A Nanocrystalline Fe2O3 Film Anode Prepared by Pulsed Laser Deposition for Lithium-Ion Batteries

Nanocrystalline Fe₂O₃ thin films are deposited directly on the conduct substrates by pulsed laser deposition as anode materials for lithium-ion batteries. We demonstrate the well-designed Fe₂O₃ film electrodes are capable of excellent high-rate performance (510 mAh g^- 1 at high current density of 15,000 mA g^- 1) and superior cycling stability (905 mAh g^- 1 at 100 mA g^- 1 after 200 cycles), which are among the best reported state-of-the-art Fe₂O₃ anode materials. The outstanding lithium storage performances of the as-synthesized nanocrystalline Fe₂O₃ film are attributed to the advanced nanostructured architecture, which not only provides fast kinetics by the shortened lithium-ion diffusion lengths but also prolongs cycling life by preventing nanosized Fe₂O₃ particle agglomeration. The electrochemical performance results suggest that this novel Fe₂O₃ thin film is a promising anode material for all-solid-state thin film batteries.

CoV2O4: a novel anode material for lithium-ion batteries with excellent electrochemical performance

This study reports the electrochemical applications of CoV₂O₄ as a novel anode for lithium-ion batteries.

Capillarity Composited Recycled Paper/Graphene Scaffold for Lithium-Sulfur Batteries with Enhanced Capacity and Extended Lifespan

An effective strategy to tackle the twin crises of global deforestation and fossil fuel depletion is to recycle biomass materials for energy storage devices. This study reports a unique and innovative solution to capitalize on a currently overlooked resource to produce high-performance lithium-sulfur (Li-S) batteries from recycled paper. The recycled paper fibers are creatively composited with graphene oxide sheets via a capillary adsorption method. The recycled paper/graphene oxide hybrid is then converted to activated paper carbon/reduced graphene oxide (APC/graphene) scaffold for sulfur infiltration. The assembled Li-APC/graphene/S battery exhibits a superior lifespan of 620 cycles with an excellent capacity retention rate of 60.5%. An APC interlayer is sandwiched between the Li anode and the separator to suppress the degradation of Li anode by preventing the nonhomogeneous growth of mossy Li whiskers, stretching the battery lifespan up to 1000 cycles with a capacitance retention rate of 52.3%. The capillary adsorption method coupled with the porous carbonaceous anode interlayer configuration creates a new opportunity for the development of batteries derived from porous biomass materials.

Lithiation Mechanism of Tunnel-Structured MnO2 Electrode Investigated by In Situ Transmission Electron Microscopy

Manganese oxide (α-MnO₂ ) has been considered a promising energy material, including as a lithium-based battery electrode candidate, due to its environmental friendliness. Thanks to its unique 1D [2 × 2] tunnel structure, α-MnO₂ can be applied to a cathode by insertion reaction and to an anode by conversion reaction in corresponding voltage ranges, in a lithium-based battery. Numerous reports have attributed its remarkable performance to its unique tunnel structure; however, the precise electrochemical reaction mechanism remains unknown. In this study, finding of the lithiation mechanism of α-MnO₂ nanowire by in situ transmission electron microscopy (TEM) is reported. By elaborately modifying the existing in situ TEM experimental technique, rapid lithium-ion diffusion through the tunnels is verified. Furthermore, by tracing the full lithiation procedure, the evolution of the MnO intermediate phase and the development of the MnO and Li₂ O phases with preferred orientations is demonstrated, which explains how the conversion reaction occurs in α-MnO₂ material. This study provides a comprehensive understanding of the electrochemical lithiation process and mechanism of α-MnO₂ material, in addition to the introduction of an improved in situ TEM biasing technique.

Direct Realization of Complete Conversion and Agglomeration Dynamics of SnO2 Nanoparticles in Liquid Electrolyte

The conversion reaction is important in lithium-ion batteries because it governs the overall battery performance, such as initial Coulombic efficiency, capacity retention, and rate capability. Here, we have demonstrated in situ observation of the complete conversion reaction and agglomeration of nanoparticles (NPs) upon lithiation by using graphene liquid cell transmission electron microscopy. The observation reveals that the Sn NPs are nucleated from the surface of SnO₂, followed by merging with each other. We demonstrate that the agglomeration has a stepwise process, including rotation of a NP, formation of necks, and subsequent merging of individual NPs.

Complex Hollow Nanostructures: Synthesis and Energy-Related Applications

Hollow nanostructures offer promising potential for advanced energy storage and conversion applications. In the past decade, considerable research efforts have been devoted to the design and synthesis of hollow nanostructures with high complexity by manipulating their geometric morphology, chemical composition, and building block and interior architecture to boost their electrochemical performance, fulfilling the increasing global demand for renewable and sustainable energy sources. In this Review, we present a comprehensive overview of the synthesis and energy-related applications of complex hollow nanostructures. After a brief classification, the design and synthesis of complex hollow nanostructures are described in detail, which include hierarchical hollow spheres, hierarchical tubular structures, hollow polyhedra, and multi-shelled hollow structures, as well as their hybrids with nanocarbon materials. Thereafter, we discuss their niche applications as electrode materials for lithium-ion batteries and hybrid supercapacitors, sulfur hosts for lithium-sulfur batteries, and electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions. The potential superiorities of complex hollow nanostructures for these applications are particularly highlighted. Finally, we conclude this Review with urgent challenges and further research directions of complex hollow nanostructures for energy-related applications.