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

Molybdenum Nitride Nanocrystals Anchored on Phosphorus-Incorporated Carbon Fabric as a Negative Electrode for High-Performance Asymmetric Pseudocapacitor

Pseudocapacitors hold great promise to provide high energy-storing capacity; however, their capacitances are still far below their theoretical values and they deliver much lower power than the traditional electric double-layer capacitors due to poor ionic accessibility. Here, we have engineered MoN nanoparticles as pseudocapacitive material on phosphorus-incorporated carbon fabric with enhanced ionic affinity and thermodynamic stability. This nanocomposite boosts surface redox kinetics, leading to pseudocapacitance of 400 mF/cm² (2-fold higher than that of molybdenum nitride-based electrodes) with rapid charge-discharge rates. Density functional theory simulations are used to explain the origin of the good performance of MoN@P-CF in proton-based aqueous electrolytes. Finally, an all-pseudocapacitive solid-state asymmetric cell was assembled using MoN@P-CF and RuO₂ (RuO₂@CF) as negative and positive electrodes, respectively, which delivered good energy density with low relaxation time constant (τ₀) of 13 ms (significantly lower than that of carbon-based supercapacitors).

•

MoN nanocrystals coated on P-doped CF are used as high-performance pseudocapacitors

•

DFT simulations explain the origin of the good performance of MoN@P-CF electrode

•

MoN@P-CF and RuO₂@CF serve as negative and positive electrodes, in asymmetric SC

•

All-pseudocapacitive ASC delivers high power density with long cycling stability

Atomic-scale combination of germanium-zinc nanofibers for structural and electrochemical evolution

Alloys are recently receiving considerable attention in the community of rechargeable batteries as possible alternatives to carbonaceous negative electrodes; however, challenges remain for the practical utilization of these materials. Herein, we report the synthesis of germanium-zinc alloy nanofibers through electrospinning and a subsequent calcination step. Evidenced by in situ transmission electron microscopy and electrochemical impedance spectroscopy characterizations, this one-dimensional design possesses unique structures. Both germanium and zinc atoms are homogenously distributed allowing for outstanding electronic conductivity and high available capacity for lithium storage. The as-prepared materials present high rate capability (capacity of ~ 50% at 20 C compared to that at 0.2 C-rate) and cycle retention (73% at 3.0 C-rate) with a retaining capacity of 546 mAh g⁻¹ even after 1000 cycles. When assembled in a full cell, high energy density can be maintained during 400 cycles, which indicates that the current material has the potential to be used in a large-scale energy storage system.

Alloy anode materials are receiving renewed interest. Here the authors show the design of Ge-Zn nanofibers for lithium ion batteries. Featured by a homogeneous composition at the atomic level and other favorable structural attributes, the materials allow for impressive electrochemical performance.

In Situ TEM of Phosphorus-Dopant-Induced Nanopore Formation in Delithiated Silicon Nanowires

Through in situ transmission electron microscopy (TEM) observation, we report the behaviors of phosphorus (P)-doped silicon nanowires (SiNWs) during electrochemical lithiation/delithiation cycling. Upon lithiation, lithium (Li) insertion causes volume expansion and formation of the crystalline Li₁₅Si₄ phase in the P-doped SiNWs. During delithiation, vacancies induced by Li extraction aggregate gradually, leading to the generation of nanopores. The as-formed nanopores can get annihilated with Li reinsertion during the following electrochemical cycle. As demonstrated by our phase-field simulations, such first-time-observed reversible nanopore formation can be attributed to the promoted lithiation/delithiation rate by the P dopant in the SiNWs. Our phase-field simulations further reveal that the delithiation-induced nanoporous structures can be controlled by tuning the electrochemical reaction rate in the SiNWs. The findings of this study shed light on the rational design of high-power performance Si-based anodes.

Polypyrrole-Coated Sodium Manganate Hollow Microspheres as a Superior Cathode for Sodium Ion Batteries

Advanced electrode materials play a very important role in the development of large-scale production of sodium-ion batteries. Herein, Na_0.7MnO_2.05 hollow microspheres with diameters of 2 μm and a shell thickness of 200 nm are prepared and then modified by polypyrrole (PPy) coating. As cathodes for sodium-ion batteries, the designed PPy-coated sodium manganate hollow microspheres demonstrate enhanced electrochemical performances, with an initial capacity of 165.1 mAh g^-1, capacity retention of 88.6% at 0.1 A g^-1 after 100 cycles, and improved rate capability. The excellent electrochemical properties are attributed to the improved electroconductivity and the high stability of hollow spherical structure of sodium manganate oxide particles due to the introduction of conductive polymer coating.

Electrochemically primed functional redox mediator generator from the decomposition of solid state electrolyte

Recent works into sulfide-type solid electrolyte materials have attracted much attention among the battery community. Specifically, the oxidative decomposition of phosphorus and sulfur based solid state electrolyte has been considered one of the main hurdles towards practical application. Here we demonstrate that this phenomenon can be leveraged when lithium thiophosphate is applied as an electrochemically “switched-on” functional redox mediator-generator for the activation of commercial bulk lithium sulfide at up to 70 wt.% lithium sulfide electrode content. X-ray adsorption near-edge spectroscopy coupled with electrochemical impedance spectroscopy and Raman indicate a catalytic effect of generated redox mediators on the first charge of lithium sulfide. In contrast to pre-solvated redox mediator species, this design decouples the lithium sulfide activation process from the constraints of low electrolyte content cell operation stemming from pre-solvated redox mediators. Reasonable performance is demonstrated at strict testing conditions.

The decomposition of solid state electrolyte material has been well-known in the literature. Here the authors report that the same decomposition process can be leveraged to act as a source of redox mediator that is only activated at certain voltages for application in Li₂S based cathodes.

Metal-organic framework derived 3D graphene decorated NaTi2(PO4)3 for fast Na-ion storage

NASCION-type materials featuring super ionic conductivity are of considerable interest for energy storage in sodium ion batteries. However, the issue of inherent poor electronic conductivity of these materials represents a fundamental limitation in their utilization as battery electrodes. Here, for the first time, we develop a facile strategy for the synthesis of NASICON-type NaTi2(PO4)3/reduced graphene oxide (NTP-rGO) Na-ion anode materials from three-dimensional (3D) metal-organic frameworks (MOFs). The selected MOF serves as an in situ etching template for the titanium resource, and importantly, endows the materials with structure-directing properties for the self-assembly of graphene oxide (GO) through a one-step solvothermal process. Through the subsequent carbonization, an rGO decorated NTP architecture is obtained, which offers fast electron transfer and improved Na+ ion accessibility to active sites. Benefiting from its unique structural merits, the NTP-rGO exhibits improved sodium storage properties in terms of high capacity, excellent rate performance and good cycling life. We believe that the findings of this work provide new opportunities to design high performance NASICON-type materials for energy storage.

Fabrication of SnS2/Mn2SnS4/Carbon Heterostructures for Sodium-Ion Batteries with High Initial Coulombic Efficiency and Cycling Stability

SnS₂ has been extensive studied as an anode material for sodium storage owing to its high theoretical specific capacity, whereas the unsatisfied initial Coulombic efficiency (ICE) caused by the partial irreversible conversion reaction during the charge/discharge process is one of the critical issues that hamper its practical applications. Hence, heterostructured SnS₂/Mn₂SnS₄/carbon nanoboxes (SMS/C NBs) have been developed by a facial wet-chemical method and utilized as the anode material of sodium ion batteries. SMS/C NBs can deliver an initial capacity of 841.2 mAh g^-1 with high ICE of 90.8%, excellent rate capability (752.3, 604.7, 570.1, 546.9, 519.7, and 488.7 mAh g^-1 at the current rate of 0.1, 0.5, 1.0, 2.0, 5.0, and 10.0 A g^-1, respectively), and long cycling stability (522.5 mAh g^-1 at 5.0 A g^-1 after 500 cycles). The existence of SnS₂/Mn₂SnS₄ heterojunctions can effectively stabilize the reaction products Sn and Na₂S, greatly prevent the coarsening of nanosized Sn⁰, and enhance reversible conversion--alloying reaction, which play a key role in improving the ICE and extending the cycling performance. Moreover, the heterostructured SMS coupled with the interacting carbon network provides efficient channels for electrons and Na⁺ diffusion, resulting in an excellent rate performance.

Bridging the academic and industrial metrics for next-generation practical batteries

Batteries have shaped much of our modern world. This success is the result of intense collaboration between academia and industry over the past several decades, culminating with the advent of and improvements in rechargeable lithium-ion batteries. As applications become more demanding, there is the risk that stunted growth in the performance of commercial batteries will slow the adoption of important technologies such as electric vehicles. Yet the scientific literature includes many reports describing material designs with allegedly superior performance. A considerable gap needs to be filled if we wish these laboratory-based achievements to reach commercialization. In this Perspective, we discuss some of the most relevant testing parameters that are often overlooked in academic literature but are critical for practical applicability outside the laboratory. We explain metrics such as anode energy density, voltage hysteresis, mass of non-active cell components and anode/cathode mass ratio, and we make recommendations for future reporting. We hope that this Perspective, together with other similar guiding principles that have recently started to emerge, will aid the transition from lab-scale research to next-generation practical batteries.

The Absence and Importance of Operando Techniques for Metal-Free Catalysts

Operando characterization techniques have played a crucial role in modern technological developments. In contrast to the experimental uncertainties introduced by ex situ techniques, the simultaneous measurement of desired sample characteristics and near-realistic electrochemical testing provides a representative picture of the underlying physics. From Li-ion batteries to metal-based electrocatalysts, the insights offered by real-time characterization data have enabled more efficient research programs. As an emerging class of catalyst, much of the mechanistic understanding of metal-free electrocatalysts continues to be elusive in comparison to their metal-based counterparts. However, there is a clear absence of operando characterization performed on metal-free catalysts. Through the proper execution of operando techniques, it can be expected that metal-free catalysts can achieve exceptional technological progress. Here, the motivation of using operando characterization techniques for metal-free carbon-based catalyst system is considered, followed by a discussion of the possibilities, difficulties and benefits of their applications.

Strategies for Building Robust Traffic Networks in Advanced Energy Storage Devices: A Focus on Composite Electrodes

The charge transport system in an energy storage device (ESD) fundamentally controls the electrochemical performance and device safety. As the skeleton of the charge transport system, the "traffic" networks connecting the active materials are primary structural factors controlling the transport of ions/electrons. However, with the development of ESDs, it becomes very critical but challenging to build traffic networks with rational structures and mechanical robustness, which can support high energy density, fast charging and discharging capability, cycle stability, safety, and even device flexibility. This is especially true for ESDs with high-capacity active materials (e.g., sulfur and silicon), which show notable volume change during cycling. Therefore, there is an urgent need for cost-effective strategies to realize robust transport networks, and an in-depth understanding of the roles of their structures and properties in device performance. To address this urgent need, the primary strategies reported recently are summarized here into three categories according to their controllability over ion-transport networks, electron-transport networks, or both of them. More specifically, the significant studies on active materials, binders, electrode designs based on various templates, pore additives, etc., are introduced accordingly. Finally, significant challenges and opportunities for building robust charge transport system in next-generation energy storage devices are discussed.

Size-dependent kinetics during non-equilibrium lithiation of nano-sized zinc ferrite

Spinel transition metal oxides (TMOs) have emerged as promising anode materials for lithium-ion batteries. It has been shown that reducing their particle size to nanoscale dimensions benefits overall electrochemical performance. Here, we use in situ transmission electron microscopy to probe the lithiation behavior of spinel ZnFe₂O₄ as a function of particle size. We have found that ZnFe₂O₄ undergoes an intercalation-to-conversion reaction sequence, with the initial intercalation process being size dependent. Larger ZnFe₂O₄ particles (40 nm) follow a two-phase intercalation reaction. In contrast, a solid-solution transformation dominates the early stages of discharge when the particle size is about 6–9 nm. Using a thermodynamic analysis, we find that the size-dependent kinetics originate from the interfacial energy between the two phases. Furthermore, the conversion reaction in both large and small particles favors {111} planes and follows a core-shell reaction mode. These results elucidate the intrinsic mechanism that permits fast reaction kinetics in smaller nanoparticles.

Reducing particle size of electrode materials to nanoscale dimensions is believed responsible for their enhanced reaction kinetics and electrochemical performance. Here, the authors use in situ transmission electron microscopy to study the dynamic process of the spinel zinc ferrite nanoparticles as a function of size, finding that the intercalation reaction pathway changes below a critical particle size.

Enhanced Lithium Transport by Control of Crystal Orientation in Spinel LiMn2O4 Thin Film Cathodes

A promising cathode material for rechargeable batteries is LiMn2O4, which exhibits higher operating voltage, reduced toxicity and lower costs as compared to commonly used LiCoO2 cathodes. However, LiMn2O4 suffers from limited cycle life, as excessive capacity fading occurs during battery cycling due to dissolution of Mn into the acidic electrolyte. Here, we show that by structural engineering of stable, epitaxial LiMn2O4 thin films the electrochemical properties can be enhanced as compared to polycrystalline samples. Control of the specific crystal orientation of the LiMn2O4 thin films resulted in dramatic differences in surface morphology with pyramidal, rooftop or flat features for respectively (100), (110), and (111) orientations. All three types of LiMn2O4 films expose predominantly  ⟨111⟩ crystal facets, which is the lowest energy state surface for this spinel structure. The (100)-oriented LiMn2O4 films exhibited the highest capacities and (dis)charging rates up to 33C, and good cyclability over a thousand cycles, demonstrating enhanced cycle life without excessive capacity fading as compared to previous polycrystalline studies.

Direct Visualization of Li Dendrite Effect on LiCoO2 Cathode by In Situ TEM

Nonuniform and highly localized Li dendrites are known to cause deleterious and, in many cases, catastrophic effects on the performance of rechargeable Li batteries. However, the mechanisms of cathode failures upon contact with Li metal are far from clear. In this study, using in situ transmission electron microscopy, the interaction of Li metal with well-defined, epitaxial thin films of LiCoO₂ , the most widely used cathode material, is directly visualized at an atomic scale. It is shown that a spontaneous and prompt chemical reaction is triggered once Li contact is made, leading to expansion and pulverization of LiCoO₂ and ending with the final reaction products of Li₂ O and Co metal. A topotactic phase transition is identified close to the reaction front, resulting in the formation of CoO as a metastable intermediate. Dynamic structural and chemical imaging, in combination with ab initio simulations, reveal that a high density of grain and antiphase boundaries is formed at the reaction front, which are critical for enabling the short-range topotactic reactions and long-range Li propagation. The fundamental insights are of general importance in mitigating Li dendrites related issues and guiding the design principle for more robust energy materials.

In Situ Electron Diffraction Tomography Using a Liquid-Electrochemical Transmission Electron Microscopy Cell for Crystal Structure Determination of Cathode Materials for Li-Ion batteries

We demonstrate that changes in the unit cell structure of lithium battery cathode materials during electrochemical cycling in liquid electrolyte can be determined for particles of just a few hundred nanometers in size using in situ transmission electron microscopy (TEM). The atomic coordinates, site occupancies (including lithium occupancy), and cell parameters of the materials can all be reliably quantified. This was achieved using electron diffraction tomography (EDT) in a sealed electrochemical cell with conventional liquid electrolyte (LP30) and LiFePO₄ crystals, which have a well-documented charged structure to use as reference. In situ EDT in a liquid environment cell provides a viable alternative to in situ X-ray and neutron diffraction experiments due to the more local character of TEM, allowing for single crystal diffraction data to be obtained from multiphased powder samples and from submicrometer- to nanometer-sized particles. EDT is the first in situ TEM technique to provide information at the unit cell level in the liquid environment of a commercial TEM electrochemical cell. Its application to a wide range of electrochemical experiments in liquid environment cells and diverse types of crystalline materials can be envisaged.

The Design and Synthesis of Hollow Micro-/Nanostructures: Present and Future Trends

Hollow micro-/nanostructures have attracted tremendous interest owing to their intriguing structure-induced physicochemical properties and great potential for widespread applications. With the development of modern synthetic methodology and analytical instruments, a rapid structural/compositional evolution of hollow structures from simple to complex has occurred in recent decades. Here, an updated overview of research progress made in the synthesis of hollow structures is provided. After an introduction of definition and classification, achievements in synthetic approaches for these delicate hollow architectures are presented in detail. According to formation mechanisms, these strategies can be categorized into four different types, including hard-templating, soft-templating, self-templated, and template-free methods. In particular, the rationales and emerging innovations in conventional templating syntheses are in focus. The development of burgeoning self-templating strategies based on controlled etching, outward diffusion, and heterogeneous contraction is also summarized. In addition, a brief overview of template-free methods and recent advances on combined mechanisms is provided. Notably, the strengths and weaknesses of each category are discussed in detail. In conclusion, a perspective on future trends in the research of hollow micro-/nanostructures is given.

Developing High-Performance Lithium Metal Anode in Liquid Electrolytes: Challenges and Progress

Lithium metal anodes are potentially key for next-generation energy-dense batteries because of the extremely high capacity and the ultralow redox potential. However, notorious safety concerns of Li metal in liquid electrolytes have significantly retarded its commercialization: on one hand, lithium metal morphological instabilities (LMI) can cause cell shorting and even explosion; on the other hand, breaking of the grown Li arms induces the so-called "dead Li"; furthermore, the continuous consumption of the liquid electrolyte and cycleable lithium also shortens cell life. The research community has been seeking new strategies to protect Li metal anodes and significant progress has been made in the last decade. Here, an overview of the fundamental understandings of solid electrolyte interphase (SEI) formation, conceptual models, and advanced real-time characterizations of LMI are presented. Instructed by the conceptual models, strategies including increasing the donatable fluorine concentration (DFC) in liquid to enrich LiF component in SEI, increasing salt concentration (ionic strength) and sacrificial electrolyte additives, building artificial SEI to boost self-healing of natural SEI, and 3D electrode frameworks to reduce current density and delay Sand's extinction are summarized. Practical challenges in competing with graphite and silicon anodes are outlined.

Origin of Fracture-Resistance to Large Volume Change in Cu-Substituted Co3 O4 Electrodes

The electrode materials conducive to conversion reactions undergo large volume change in cycles which restrict their further development. It has been demonstrated that incorporation of a third element into metal oxides can improve the cycling stability while the mechanism remains unknown. Here, an in situ and ex situ electron microscopy investigation of structural evolutions of Cu-substituted Co₃ O₄ supplemented by first-principles calculations is reported to reveal the mechanism. An interconnected framework of ultrathin metallic copper formed provides a high conductivity backbone and cohesive support to accommodate the volume change and has a cube-on-cube orientation relationship with Li₂ O. In charge, a portion of Cu metal is oxidized to CuO, which maintains a cube-on-cube orientation relationship with Cu. The Co metal and oxides remain as nanoclusters (less than 5 nm) thus active in subsequent cycles. This adaptive architecture accommodates the formation of Li₂ O in the discharge cycle and underpins the catalytic activity of Li₂ O decomposition in the charge cycle.