Applications of electron microscopic observations to electrochemistry in liquid electrolytes for batteries

Herein, we review notable points from observations of electrochemical reactions in a liquid electrolyte by liquid-phase electron microscopy. In situ microscopic observations of electrochemical reactions are urgently required, particularly to solve various battery issues. Battery performance is evaluated by various electrochemical measurements of bulk samples. However, it is necessary to understand the physical/chemical phenomena occurring in batteries to elucidate the reaction mechanisms. Thus, in situ microscopic observation is effective for understanding the reactions that occur in batteries. Herein, we focus on two methods, of the liquid phase (scanning) transmission electron microscopy and liquid phase scanning electron microscopy, and summarize the advantages and disadvantages of both methods.

A universal electrochemical lithiation-delithiation method to prepare low-crystalline metal oxides for high-performance hybrid supercapacitors

The electrochemical performance of transition metal oxides (TMOs) for hybrid supercapacitors has been optimized through various methods in previous reports. However, most previous research was mainly focused on well-crystalline TMOs. Herein, the electrochemical lithiation–delithiation method was performed to synthesise low-crystallinity TMOs for hybrid supercapacitors. It was found that the lithiation–delithiation process can significantly improve the electrochemical performance of “conversion-type” TMOs, such as CoO, NiO, etc. The as-prepared low-crystallinity CoO exhibits high specific capacitance of 2154.1 F g⁻¹ (299.2 mA h g⁻¹) at 0.8 A g⁻¹, outstanding rate capacitance retention of 63.9% even at 22.4 A g⁻¹ and excellent cycling stability with 90.5% retention even after 10 000 cycles. When assembled as hybrid supercapacitors using active carbon (AC) as the active material of the negative electrode, the devices show a high energy density of 50.9 W h kg⁻¹ at 0.73 kW kg⁻¹. Another low-crystallinity NiO prepared by the same method also possesses a much higher specific capacitance of 2317.6 F g⁻¹ (302.6 mA h g⁻¹) compared to that for pristine commercial NiO of 497.2 F g⁻¹ at 1 A g⁻¹. The improved energy storage performance of the low-crystallinity metal oxides can be ascribed to the disorder of as-prepared low-crystallinity metal oxides and interior 3D-connected channels originating from the lithiation–delithiation process. This method may open new opportunities for scalable and facile synthesis of low-crystallinity metal oxides for high-performance hybrid supercapacitors.

The electrochemical performance of transition metal oxides (TMOs) for hybrid supercapacitors has been optimized through various methods in previous reports.

A Cellulose-Derived Nanofibrous MnO2-TiO2-Carbon Composite as Anodic Material for Lithium-Ion Batteries

A bio-inspired nanofibrous MnO₂-TiO₂-carbon composite was prepared by utilizing natural cellulosic substances (e.g., ordinary quantitative ashless filter paper) as both the carbon source and structural matrix. Mesoporous MnO₂ nanosheets were densely immobilized on an ultrathin titania film precoated with cellulose-derived carbon nanofibers, which gave a hierarchical MnO₂-TiO₂-carbon nanoarchitecture and exhibited excellent electrochemical performances when used as an anodic material for lithium-ion batteries. The MnO₂-TiO₂-carbon composite with a MnO₂ content of 47.28 wt % exhibited a specific discharge capacity of 677 mAh g⁻¹ after 130 repeated charge/discharge cycles at a current rate of 100 mA g⁻¹. The contribution percentage of MnO₂ in the composite material is equivalent to 95.1% of the theoretical capacity of MnO₂ (1230 mAh g⁻¹). The ultrathin TiO₂ precoating layer with a thickness ca. 2 nm acts as a crucial interlayer that facilitates the growth of well-organized MnO₂ nanosheets onto the surface of the titania-carbon nanofibers. Due to the interweaved network structures of the carbon nanofibers and the increased content of the immobilized MnO₂, the exfoliation and aggregation, as well as the large volume change of the MnO₂ nanosheets, are significantly inhibited; thus, the MnO₂-TiO₂-carbon electrodes displayed outstanding cycling performance and a reversible rate capability during the Li⁺ insertion/extraction processes.

In Situ TEM Study on Conversion-Type Electrodes for Rechargeable Ion Batteries

Conversion-type materials have been considered as potentially high-energy-density alternatives to commercially dominant intercalation-based electrodes for rechargeable ion batteries and have attracted tremendous research effort to meet the performance for viable energy-storage technologies. In situ transmission electron microscopy (TEM) has been extensively employed to provide mechanistic insights into understanding the behavior of battery materials. Noticeably, a great portion of previous in situ TEM studies has been focused on conversion-type materials, but a dedicated review for this group of materials is missing in the literature. Herein, recent developments of in situ TEM techniques for investigation of dynamic phase transformation and associated structural, morphological, and chemical evolutions during conversion reactions with alkali ions in secondary batteries are comprehensively summarized. The materials of interest broadly cover metal oxides, chalcogenides, fluorides, phosphides, nitrides, and silicates with specific emphasis on spinel metal oxides and recently emerged 2D metal chalcogenides. Special focus is placed on the scientific findings that are uniquely obtained by in situ TEM to address fundamental questions and practical issues regarding phase transformation, structural evolution, electrochemical redox, reaction mechanism, kinetics, and degradation. Critical challenges and perspectives are discussed for advancing new knowledge that can bridge the gap between prototype materials and real-world applications.

Utilizing Cyclic Voltammetry to Understand the Energy Storage Mechanisms for Copper Oxide and its Graphene Oxide Hybrids as Lithium-Ion Battery Anodes

Graphene-based materials have been extensively researched as a means improve the electrochemical performance of transition metal oxides in Li-ion battery applications, however an understanding of the effect of the different synthesis routes, and the factors underlying the oft-stated better performance of the hybrid materials (compared to the pure metal oxides) is not always demonstrated. For the first time, we report a range of synthetic routes to produce graphene oxide (GO)-coated CuO, micro-particle/GO "bundles" as well as nano-particulates decorated on GO sheets to enable a comparison with CuO and its carbon-coated analogue, as confirmed using scanning electron microscopy (SEM) imaging and Raman spectroscopy. Cyclic voltammetry was utilized to probe the lithiation/delithiation mechanism of CuO by scanning at successively decreasing vertex potentials, uncovering the importance of a full reduction to Cu metal on the reduction step. The GO hybrid materials clearly show enhanced specific capacities and cycling stabilities comparative to the CuO, with the most promising material achieving a capacity of 746 mAh g-1 and capacity retention of 92 % after 30 cycles, which is the highest stable capacity quoted in literature for CuO. The simple cyclic voltammetry technique used in this work could be implemented to help further understand any conversion-type anode materials, in turn accelerating the research and industrial development of conversion anodes.

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.

Review of Recent Development of In Situ/Operando Characterization Techniques for Lithium Battery Research

The increasing demands of energy storage require the significant improvement of current Li-ion battery electrode materials and the development of advanced electrode materials. Thus, it is necessary to gain an in-depth understanding of the reaction processes, degradation mechanism, and thermal decomposition mechanisms under realistic operation conditions. This understanding can be obtained by in situ/operando characterization techniques, which provide information on the structure evolution, redox mechanism, solid-electrolyte interphase (SEI) formation, side reactions, and Li-ion transport properties under operating conditions. Here, the recent developments in the in situ/operando techniques employed for the investigation of the structural stability, dynamic properties, chemical environment changes, and morphological evolution are described and summarized. The experimental approaches reviewed here include X-ray, electron, neutron, optical, and scanning probes. The experimental methods and operating principles, especially the in situ cell designs, are described in detail. Representative studies of the in situ/operando techniques are summarized, and finally the major current challenges and future opportunities are discussed. Several important battery challenges are likely to benefit from these in situ/operando techniques, including the inhomogeneous reactions of high-energy-density cathodes, the development of safe and reversible Li metal plating, and the development of stable SEI.

Mechanistic Origin of the High Performance of Yolk@Shell Bi2S3@N-Doped Carbon Nanowire Electrodes

High-performance lithium-ion batteries are commonly built with heterogeneous composite electrodes that combine multiple active components for serving various electrochemical and structural functions. Engineering these heterogeneous composite electrodes toward drastically improved battery performance is hinged on a fundamental understanding of the mechanisms of multiple active components and their synergy or trade-off effects. Herein, we report a rational design, fabrication, and understanding of yolk@shell Bi₂S₃@N-doped mesoporous carbon (C) composite anode, consisting of a Bi₂S₃ nanowire (NW) core within a hollow space surrounded by a thin shell of N-doped mesoporous C. This composite anode exhibits desirable rate performance and long cycle stability (700 cycles, 501 mAhg^-1 at 1.0 Ag^-1, 85% capacity retention). By in situ transmission electron microscopy (TEM), X-ray diffraction, and NMR experiments and computational modeling, we elucidate the dominant mechanisms of the phase transformation, structural evolution, and lithiation kinetics of the Bi₂S₃ NWs anode. Our combined in situ TEM experiments and finite element simulations reveal that the hollow space between the Bi₂S₃ NWs core and carbon shell can effectively accommodate the lithiation-induced expansion of Bi₂S₃ NWs without cracking C shells. This work demonstrates an effective strategy of engineering the yolk@shell-architectured anodes and also sheds light onto harnessing the complex multistep reactions in metal sulfides to enable high-performance lithium-ion batteries.

Freestanding Three-Dimensional CuO/NiO Core-Shell Nanowire Arrays as High-Performance Lithium-Ion Battery Anode

We demonstrate significant improvement of CuO nanowire arrays as anode materials for lithium ion batteries by coating with thin NiO nanosheets conformally. The NiO nanosheets were designed two kinds of morphologies, which are porous and non-porous. By the NiO nanosheets coating, the major active CuO nanowires were protected from direct contact with the electrolyte to improve the surface chemical stability. Simultaneously, through the observation and comparison of TEM results of crystalline non-porous NiO nanosheets, before and after lithiation process, we clearly prove the effect of expected protection of CuO, and clarify the differences of phase transition, crystallinity change, ionic conduction and the mechanisms of the capacity decay further. Subsequently, the electrochemical performances exhibit lithiation and delithiation differences of the porous and non-porous NiO nanosheets, and confirm that the presence of the non-porous NiO coating can still effectively assist the diffusion of Li+ ions into the CuO nanowires, maintaining the advantage of high surface area, and improves the cycle performance of CuO nanowires, leading to enhanced battery capacity. Optimally, the best structure is validated to be non-porous NiO nanosheets, in contrary to the anticipated porous NiO nanosheets. In addition, considering the low cost and facile fabrication process can be realized further for practical applications.

Fast-Charging and High Volumetric Capacity Anode Based on Co3 O4 /CuO@TiO2 Composites for Lithium-Ion Batteries

This paper presents an investigation of anodic TiO₂ nanotube arrays (TNAs), with a Co₃ O₄ /CuO coating, for lithium-ion batteries (LIBs). The coated TNAs are investigated using various analytical techniques, with the results clearly suggesting that the molar ratio of Co₃ O₄ /CuO in the TiO₂ nanotubes substantially influences its battery performance. In particular, a cobalt/copper molar ratio of 2:1 on the TNAs (Co₂ Cu₁ @TNAs) features the best LIBs anode performance, exhibiting high reversible capacity and enhanced cycling stability. Noticeably, Co₂ Cu₁ @TNAs achieve excellent rate capability even after quite a high current density of 20.0 A g^-1 (≈25 C, where C corresponds to complete discharge in 1 h) and superior volumetric reversible capacity of ≈3330 mA h^-1  cm^-3 . This value is approximately seven times higher than those of a graphite-based anode. This outstanding performance is attributed to the synergistic effects of Co₂ Cu₁ @TNAs: 1) the structural advantage of TNAs, with their large amount of free space to accommodate the large volume expansion during Li⁺ insertion/extraction and 2) the optimized ratio of Co₃ O₄ and CuO in the composite for improved capacity. In addition, no binder or conductive agent is used, which is partly responsible for the overall improved volumetric capacity and electrochemical performance.

Electrochemical solid-state amorphization in the immiscible Cu-Li system

As a typical immiscible binary system, copper (Cu) and lithium (Li) show no alloying and chemical intermixing under normal circumstances. Here we show that, when decreasing Cu nanoparticle sizes into ultrasmall range, the nanoscale size effect can play a subtle yet critical role in mediating the chemical activity of Cu and therefore its miscibility with Li, such that the electrochemical alloying and solid-state amorphization will occur in such an immiscible system. This unusual observation was accomplished by performing in-situ studies of the electrochemical lithiation processes of individual CuO nanowires inside a transmission electron microscopy (TEM). Upon lithiation, CuO nanowires are first electrochemically reduced to form discrete ultrasmall Cu nanocrystals that, unexpectedly, can in turn undergo further electrochemical lithiation to form amorphous CuLi_x nanoalloys. Real-time TEM imaging unveils that there is a critical grain size (ca. 6 nm), below which the nanocrystalline Cu particles can be continuously lithiated and amorphized. The possibility that the observed solid-state amorphization of Cu-Li might be induced by electron beam irradiation effect can be explicitly ruled out; on the contrary, it was found that electron beam irradiation will lead to the dealloying of as-formed amorphous CuLi_x nanoalloys.

Air-Stable Lithium Spheres Produced by Electrochemical Plating

Lithium metal is an ideal anode for next-generation lithium batteries owing to its very high theoretical specific capacity of 3860 mAh g^-1 but very reactive upon exposure to ambient air, rendering it difficult to handle and transport. Air-stable lithium spheres (ASLSs) were produced by electrochemical plating under CO₂ atmosphere inside an advanced aberration-corrected environmental transmission electron microscope. The ASLSs exhibit a core-shell structure with a Li core and a Li₂ CO₃ shell. In ambient air, the ASLSs do not react with moisture and maintain their core-shell structures. Furthermore, the ASLSs can be used as anodes in lithium-ion batteries, and they exhibit similar electrochemical behavior to metallic Li, indicating that the surface Li₂ CO₃ layer is a good Li⁺ ion conductor. The air stability of the ASLSs is attributed to the surface Li₂ CO₃ layer, which is barely soluble in water and does not react with oxygen and nitrogen in air at room temperature, thus passivating the Li core.

Recent Advances in Designing High-Capacity Anode Nanomaterials for Li-Ion Batteries and Their Atomic-Scale Storage Mechanism Studies

Lithium-ion batteries (LIBs) have been widely applied in portable electronics (laptops, mobile phones, etc.) as one of the most popular energy storage devices. Currently, much effort has been devoted to exploring alternative high-capacity anode materials and thus potentially constructing high-performance LIBs with higher energy/power density. Here, high-capacity anode nanomaterials based on the diverse types of mechanisms, intercalation/deintercalation mechanism, alloying/dealloying reactions, conversion reaction, and Li metal reaction, are reviewed. Moreover, recent studies in atomic-scale storage mechanism by utilizing advanced microscopic techniques, such as in situ high-resolution transmission electron microscopy and other techniques (e.g., spherical aberration-corrected scanning transmission electron microscopy, cryoelectron microscopy, and 3D imaging techniques), are highlighted. With the in-depth understanding on the atomic-scale ion storage/release mechanisms, more guidance is given to researchers for further design and optimization of anode nanomaterials. Finally, some possible challenges and promising future directions for enhancing LIBs' capacity are provided along with the authors personal viewpoints in this research field.

Hierarchically Nanostructured CuO⁻Cu Current Collector Fabricated by Hybrid Methods for Developed Li-Ion Batteries

We present a simple method of fabricating a hierarchically nanostructured CuO–Cu current collector by using laser ablation and metal mold imprinting to maximize the surface area. The laser ablation of the Cu current collector created the CuO nanostructure on the Cu-collector surface. The microstructure was transferred by subsequent imprinting of the microstructure metal mold on the Cu collector. Then, the laser-ablation nanostructure was formed. Consequently, a hierarchical structure is generated. The laser-ablated hierarchical CuO–Cu current collector exhibited an improved capacity while maintaining a cyclability that is similar to those of conventional graphite batteries.

Cobalt Oxide Porous Nanofibers Directly Grown on Conductive Substrate as a Binder/Additive-Free Lithium-Ion Battery Anode with High Capacity

In order to reduce the amount of inactive materials, such as binders and carbon additives in battery electrode, porous cobalt monoxide nanofibers were directly grown on conductive substrate as a binder/additive-free lithium-ion battery anode. This electrode exhibited very high specific discharging/charging capacities at various rates and good cycling stability. It was promising as high capacity anode materials for lithium-ion battery.

A Convenient Synthesis Route for Co3O4 Hollow Microspheres and Their Application as a High Performing Anode in Li-Ion Batteries

A new, rapid, and cost-effective method for synthesizing hollow microspheres (HMSs) of cobalt oxide (Co₃O₄) using the phloroglucinol-formaldehyde gel route is reported here. Further, the synthesized hollow Co₃O₄ microspheres were investigated as an anode material for Li-ion batteries. The Co₃O₄ hollow spheres exhibited excellent electrochemical performance and cycling stability, for example, a capacity of 915 mA h g^-1 was obtained at 1 C rate over 350 cycles. The material also exhibited good performance at high rates, viz., capacities of 500, 350, and 250 mA h g^-1 at 10 C, 25 C, and 50 C, respectively, with good capacity retention over 500 cycles. The excellent electrochemical performance of Co₃O₄ can be ascribed to the porous nanoarchitecture that provides a short diffusion length for Li⁺ ions and high electrolyte percolation in the porous structure. Additionally, the thin porous wall of the nanocages provides an effective way to overcome the issues associated with the volume change occurring during Li charge/discharge. The conversion of Co₃O₄ into Co upon discharge was also probed by measuring the magnetic properties.

Understanding materials challenges for rechargeable ion batteries with in situ transmission electron microscopy

An in-depth understanding of material behaviours under complex electrochemical environment is critical for the development of advanced materials for the next-generation rechargeable ion batteries. The dynamic conditions inside a working battery had not been intensively explored until the advent of various in situ characterization techniques. Real-time transmission electron microscopy of electrochemical reactions is one of the most significant breakthroughs poised to enable radical shift in our knowledge on how materials behave in the electrochemical environment. This review, therefore, summarizes the scientific discoveries enabled by in situ transmission electron microscopy, and specifically emphasizes the applicability of this technique to address the critical challenges in the rechargeable ion battery electrodes, electrolyte and their interfaces. New electrochemical systems such as lithium–oxygen, lithium–sulfur and sodium ion batteries are included, considering the rapidly increasing application of in situ transmission electron microscopy in these areas. A systematic comparison between lithium ion-based electrochemistry and sodium ion-based electrochemistry is also given in terms of their thermodynamic and kinetic differences. The effect of the electron beam on the validity of in situ observation is also covered. This review concludes by providing a renewed perspective for the future directions of in situ transmission electron microscopy in rechargeable ion batteries.

In situ TEM is a powerful tool that helps to understand energy storage behaviors of various materials. This review summarizes the critical discoveries, enabled by in situ TEM, in rechargeable ion batteries, and foresees its bright future for extensive applications.

A New CuO-Fe2 O3 -Mesocarbon Microbeads Conversion Anode in a High-Performance Lithium-Ion Battery with a Li1.35 Ni0.48 Fe0.1 Mn1.72 O4 Spinel Cathode

A ternary CuO-Fe₂ O₃ -mesocarbon microbeads (MCMB) conversion anode was characterized and combined with a high-voltage Li_1.35 Ni_0.48 Fe_0.1 Mn_1.72 O₄ spinel cathode in a lithium-ion battery of relevant performance in terms of cycling stability and rate capability. The CuO-Fe₂ O₃ -MCMB composite was prepared by using high-energy milling, a low-cost pathway that leads to a crystalline structure and homogeneous submicrometrical morphology as revealed by XRD and electron microscopy. The anode reversibly exchanges lithium ions through the conversion reactions of CuO and Fe₂ O₃ and by insertion into the MCMB carbon. Electrochemical tests, including impedance spectroscopy, revealed a conductive electrode/electrolyte interface that enabled the anode to achieve a reversible capacity value higher than 500 mAh g^-1 when cycled at a current of 120 mA g^-1 . The remarkable stability of the CuO-Fe₂ O₃ -MCMB electrode and the suitable characteristics in terms of delivered capacity and voltage-profile retention allowed its use in an efficient full lithium-ion cell with a high-voltage Li_1.35 Ni_0.48 Fe_0.1 Mn_1.72 O₄ cathode. The cell had a working voltage of 3.6 V and delivered a capacity of 110 mAh g_cathode^-1 with a Coulombic efficiency above 99 % after 100 cycles at 148 mA g_cathode^-1 . This relevant performances, rarely achieved by lithium-ion systems that use the conversion reaction, are the result of an excellent cell balance in terms of negative-to-positive ratio, favored by the anode composition and electrochemical features.

Recent advances in real-time and in situ analysis of an electrode–electrolyte interface by mass spectrometry

Novelty: Recent advances in real-time and in situ monitoring of an electrode–electrolyte interface by mass spectrometry are reviewed.

One-Step Catalytic Synthesis of CuO/Cu2O in a Graphitized Porous C Matrix Derived from the Cu-Based Metal-Organic Framework for Li- and Na-Ion Batteries

The hybrid composite electrode comprising CuO and Cu2O micronanoparticles in a highly graphitized porous C matrix (CuO/Cu2O-GPC) has a rational design and is a favorable approach to increasing the rate capability and reversible capacity of metal oxide negative materials for Li- and Na-ion batteries. CuO/Cu2O-GPC is synthesized through a Cu-based metal-organic framework via a one-step thermal transformation process. The electrochemical performances of the CuO/Cu2O-GPC negative electrode in Li- and Na-ion batteries are systematically studied and exhibit excellent capacities of 887.3 mAh g(-1) at 60 mA g(-1) after 200 cycles in a Li-ion battery and 302.9 mAh g(-1) at 50 mA g(-1) after 200 cycles in a Na-ion battery. The high electrochemical stability was obtained via the rational strategy, mainly owing to the synergy effect of the CuO and Cu2O micronanoparticles and highly graphitized porous C formed by catalytic graphitization of Cu nanoparticles. Owing to the simple one-step thermal transformation process and resulting high electrochemical performance, CuO/Cu2O-GPC is one of the prospective negative active materials for rechargeable Li- and Na-ion batteries.

Amorphous Phosphorus/Nitrogen-Doped Graphene Paper for Ultrastable Sodium-Ion Batteries

As the most promising anode material for sodium-ion batteries (SIBs), elemental phosphorus (P) has recently gained a lot of interest due to its extraordinary theoretical capacity of 2596 mAh/g. The main drawback of a P anode is its low conductivity and rapid structural degradation caused by the enormous volume expansion (>490%) during cycling. Here, we redesigned the anode structure by using an innovative methodology to fabricate flexible paper made of nitrogen-doped graphene and amorphous phosphorus that effectively tackles this problem. The restructured anode exhibits an ultrastable cyclic performance and excellent rate capability (809 mAh/g at 1500 mA/g). The excellent structural integrity of the novel anode was further visualized during cycling by using in situ experiments inside a high-resolution transmission electron microscope (HRTEM), and the associated sodiation/desodiation mechanism was also thoroughly investigated. Finally, density functional theory (DFT) calculations confirmed that the N-doped graphene not only contributes to an increase in capacity for sodium storage but also is beneficial in regards to improved rate performance of the anode.

Twin structures in CuO nanowires

The structural characteristics of monoclinic CuO nanowires (NWs) fabricated by heating pure Cu in ambient air were investigated by electron microscopy. Besides the single-crystalline NW, four different twinned NWs with twinning planes of (11\bar 1), (002), (110) and {{(20\bar 2)}} have been found. The twin boundaries are generally in parallel with the NW axial direction. To the best of the authors' knowledge, the {{(11\bar 1)}} and (110) twins are reported for the first time in CuO. Moreover, the prevailing existence of {{(11\bar 1)}} and (002) twinned NWs could be closely related to the NW growth as well as the oxidation processes of Cu. The presented results provide a systematic investigation on the twin structures of CuO NWs, which may open up a pathway to explore new potential applications of CuO nanostructures.

In situ transmission electron microscopy study of the electrochemical sodiation process for a single CuO nanowire electrode

The electrochemically driven sodiation and desodiation process in a CuO nanowire was characterized by using in situ TEM. This work contributes to understanding of the electrochemical conversion mechanism of CuO nanowire anodes applied in NIBs.

Hybrid two-dimensional materials in rechargeable battery applications and their microscopic mechanisms

Advances in two-dimensional (2D) hybrid nanomaterials in electrochemical energy storage and their microscopic mechanisms are summarized and reviewed.

In Situ Transmission Electron Microscopy Observation of the Lithiation-Delithiation Conversion Behavior of CuO/Graphene Anode

The electrochemical conversion behavior of metal oxides as well as its influence on the lithium-storage performance remains unclear. In this paper, we studied the dynamic electrochemical conversion process of CuO/graphene as anode by in situ transmission electron microscopy. The microscopic conversion behavior of the electrode was further correlated with its macroscopic lithium-storage properties. During the first lithiation, the porous CuO nanoparticles transformed to numerous Cu nanograins (2-3 nm) embedded in Li2O matrix. The porous spaces were found to be favorable for accommodating the volume expansion during lithium insertion. Two types of irreversible processes were revealed during the lithiation-delithiation cycles. First, the nature of the charge-discharge process of CuO anode is a reversible phase conversion between Cu2O and Cu nanograins. The delithiation reaction cannot recover the electrode to its pristine structure (CuO), which is responsible for about ∼55% of the capacity fading in the first cycle. Second, there is a severe nanograin aggregation during the initial conversion cycles, which leads to low Coulombic efficiency. This finding could also account for the electrochemical behaviors of other transition metal oxide anodes that operate with similar electrochemical conversion mechanism.

Asynchronous Crystal Cell Expansion during Lithiation of K(+)-Stabilized α-MnO2

α-MnO2 is a promising material for Li-ion batteries and has unique tunneled structure that facilitates the diffusion of Li(+). The overall electrochemical performance of α-MnO2 is determined by the tunneled structure stability during its interaction with Li(+), the mechanism of which is, however, poorly understood. In this paper, a novel tetragonal-orthorhombic-tetragonal symmetric transition during lithiation of K(+)-stabilized α-MnO2 is observed using in situ transmission electron microscopy. Atomic resolution imaging indicated that 1 × 1 and 2 × 2 tunnels exist along c ([001]) direction of the nanowire. The morphology of a partially lithiated nanowire observed in the ⟨100⟩ projection is largely dependent on crystallographic orientation ([100] or [010]), indicating the existence of asynchronous expansion of α-MnO2's tetragonal unit cell along a and b lattice directions, which results in a tetragonal-orthorhombic-tetragonal (TOT) symmetric transition upon lithiation. Such a TOT transition is confirmed by diffraction analysis and Mn valence quantification. Density functional theory (DFT) confirms that Wyckoff 8h sites inside 2 × 2 tunnels are the preferred sites for Li(+) occupancy. The sequential Li(+) filling at 8h sites leads to asynchronous expansion and symmetry degradation of the host lattice as well as tunnel instability upon lithiation. These findings provide fundamental understanding for appearance of stepwise potential variation during the discharge of Li/α-MnO2 batteries as well as the origin for low practical capacity and fast capacity fading of α-MnO2 as an intercalated electrode.

Hierarchically mesoporous CuO/carbon nanofiber coaxial shell-core nanowires for lithium ion batteries

Hierarchically mesoporous CuO/carbon nanofiber coaxial shell-core nanowires (CuO/CNF) as anodes for lithium ion batteries were prepared by coating the Cu₂(NO₃)(OH)₃ on the surface of conductive and elastic CNF via electrophoretic deposition (EPD), followed by thermal treatment in air. The CuO shell stacked with nanoparticles grows radially toward the CNF core, which forms hierarchically mesoporous three-dimensional (3D) coaxial shell-core structure with abundant inner spaces in nanoparticle-stacked CuO shell. The CuO shells with abundant inner spaces on the surface of CNF and high conductivity of 1D CNF increase mainly electrochemical rate capability. The CNF core with elasticity plays an important role in strongly suppressing radial volume expansion by inelastic CuO shell by offering the buffering effect. The CuO/CNF nanowires deliver an initial capacity of 1150 mAh g⁻¹ at 100 mA g⁻¹ and maintain a high reversible capacity of 772 mAh g⁻¹ without showing obvious decay after 50 cycles.

Ex situ identification of the Cu+ long-range diffusion path of a Cu-based anode for lithium ion batteries

An ex situ observation was made by XRD patterns, SEM and TEM images, as well as cyclic voltammogram curves of CuO/Cu integrated anodes for lithium ion batteries. For the first time, the existence of a Cu(+) ion long-range transfer path was identified at the potential widow of 1.30-1.60 V during both charging and discharging processes. Both SEM and TEM images show that these nanowires networks hanging CuO nanoparticles provide a Cu(+) diffusion path within our designed CuO/Cu integrated anode. This work provides new insights into the conversion reaction of inorganic anode materials, and can favor the development of high-performance conversion anodes for lithium-ion batteries.

Micro-nanostructured CuO/C spheres as high-performance anode materials for Na-ion batteries

In this paper, we report on the synthesis of micro-nanostructured CuO/C spheres by aerosol spray pyrolysis and their application as high-performance anodes in sodium-ion batteries. Micro-nanostructured CuO/C spheres with different CuO contents were synthesized through aerosol spray pyrolysis by adjusting the ratio of reactants and heat-treated by an oxidation process. The as-prepared CuO/C spheres show uniformly spherical morphology, in which CuO nanoparticles (∼10 nm) are homogeneously embedded in the carbon matrix (denoted as 10-CuO/C). The electrochemical performance of 10-CuO/C with a carbon weight of 44% was evaluated as the anode material for Na-ion batteries. It can deliver a capacity of 402 mA h g(-1) after 600 cycles at a current density of 200 mA g(-1). Furthermore, a capacity of 304 mA h g(-1) was obtained at a high current density of 2000 mA g(-1). The superior electrochemical performance of the micro-nanostructured CuO/C spheres leads to the enhancement of the electronic conductivity of the nanocomposite and the accommodation of the volume variation of CuO/C during charge/discharge cycling.

In situ observation of the sodiation process in CuO nanowires

We observed the dynamic evolution of the morphology and phase transformations of CuO nanowires during sodiation using in situ transmission electron microscopy. These results will facilitate our fundamental understanding of the sodiation mechanism of CuO nanostructures used as electrode materials in sodium ion batteries.

Synthesis of different CuO nanostructures from Cu(OH)2 nanorods through changing drying medium for lithium-ion battery anodes

Schematic illustration of synthesis processes of different CuO nanostructures by changing the drying medium.

Carbonate-assisted hydrothermal synthesis of porous, hierarchical CuO microspheres and CuO/GO for high-performance lithium-ion battery anodes

Porous, hierarchical CuO microspheres were synthesized by a facile carbonate-assisted hydrothermal method and encapsulated with GO sheets through engineering the ionic strength in NaCl solution.

Morphology engineering of high performance binary oxide electrodes

Advances in materials have preceded almost every major technological leap since the beginning of civilization. On the nanoscale and microscale, mastery over the morphology, size, and structure of a material enables control of its properties and enhancement of its usefulness for a given application, such as energy storage. In this review paper, our aim is to present a review of morphology engineering of high performance oxide electrode materials for electrochemical energy storage. We begin with the chemical bonding theory of single crystal growth to direct the growth of morphology-controllable materials. We then focus on the growth of various morphologies of binary oxides and their electrochemical performances for lithium ion batteries and supercapacitors. The morphology-performance relationships are elaborated by selecting examples in which there is already reasonable understanding for this relationship. Based on these comprehensive analyses, we proposed colloidal supercapacitor systems beyond morphology control on the basis of system- and ion-level design. We conclude this article with personal perspectives on the directions toward which future research in this field might take.