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

Microelectrodes for Battery Materials

The ability to measure current and voltage is core to both fundamental study and engineering of electrochemical systems, including batteries for energy storage. Electrochemical measurements have traditionally been conducted on macroscopic electrodes on the order of 1 cm or larger. In this Perspective, we review recent developments in using microscopic electrodes (<100 μm) for the study of battery materials. Microelectrodes allow us to explore spatiotemporal regimes that are not accessible with macroscopic electrodes. Temporally, microelectrodes can generate ultrahigh current densities, enabling the distinction between solid electrolyte interphase (SEI) kinetics and metal deposition kinetics. Spatially, they confine electrochemistry to single particles, allowing us to study their intrinsic properties. We outline future opportunities for the use of microelectrodes for future studies of battery systems. We propose their use for analyzing the electrochemistry of other reactive metals and exploring the potential of combining them with in situ imaging techniques.

Interfacial Engineering of Polymer Solid-State Lithium Battery Electrolytes and Li-Metal Anode: Current Status and Future Directions

A combination of material innovations, advanced manufacturing, battery management systems, and regulatory standards is necessary to improve the energy density and safety of lithium (Li) batteries. High-energy-density solid-state Li-batteries have the potential to revolutionize industries and technologies, making them a research priority. The combination of improved safety and compatibility with high-capacity electrode materials makes solid-stateLi-batteries with polymer solid-electrolytes an attractive option for applications where energy density and safety are critical. While polymer-based solid-state Li-batteries hold enormous promise, there are still several challenges that must be addressed, particularly regarding interface between polymer solid-electrolyte and Lianode. There are significant advancements in improving the performance of solid-state Li batteries, and researchers continue to explore new methods to address these challenges. These improvements are critical for enabling the widespread adoption of solid-state Li-batteries invariety of applications, from electrical vehicles to portable electronics. Here, common polymer solid-electrolyte and its interface challenges with Lianode are first introduced, highlighting the trend in polymer solid-state-electrolyte research toward enhancing stability, safety, and performance of solid-state Li-batteries. This includes developing novel polymer materials with improved properties, exploring advanced fabrication techniques, and integrating these electrolytes into battery designs that optimize both safety and energy density.

A Star-Structured Polymer Electrolyte for Low-Temperature Solid-State Lithium Batteries

Solid-state polymer lithium metal batteries (SSLMBs) have attracted considerable attention because of their excellent safety and high energy density. However, the application of SSLMBs is significantly impeded by uneven Li deposition at the interface between solid-state electrolytes and lithium metal anode, especially at a low temperature. Herein, this issue is addressed by designing an agarose-based solid polymer electrolyte containing branched structure. The star-structured polymer is synthesized by grafting poly (ethylene glycol) monomethyl-ether methacrylate and lithium 2-acrylamido-2-methylpropanesulfonate onto tannic acid. The star structure regulates Li-ion flux in the bulk of the electrolyte and at the electrolyte/electrode interfaces. This unique omnidirectional Li-ion transportation effectively improves ionic conductivity, facilitates a uniform Li-ion flux, inhibits Li dendrite growth, and alleviates polarization. As a result, a solid-state LiFePO4||Li battery with the electrolyte exhibits outstanding cyclability with a specific capacity of 134 mAh g-1 at 0.5C after 800 cycles. The battery shows a high discharge capacity of 145 mAh g-1 at 0.1 C after 200 cycles, even at 0 °C. The study offers a promising strategy to address the uneven Li deposition at the solid-state electrolyte/electrode interface, which has potential applications in long-life solid-state lithium metal batteries at a low temperature.

Coupling Liquid Electrochemical TEM and Mass-Spectrometry to Investigate Electrochemical Reactions Occurring in a Na-Ion Battery Anode

A novel approach for investigating the formation of solid electrolyte interphase (SEI) in Na-ion batteries (NIB) through the coupling of in situ liquid electrochemical transmission electron microscopy (ec-TEM) and gas-chromatography mass-spectrometry (GC/MS) is proposed. To optimize this coupling, experiments are conducted on the sodiation of hard carbon materials (HC) using two setups: in situ ec-TEM holder and ex situ setup. Electrolyte (NP30) is intentionally degraded using cyclic voltammetry (CV), and the recovered liquid product is analyzed using GC/MS. Solid product (µ-chip) is analyzed using TEM techniques in a post-mortem analysis. The ex situ experiments served as a reference to for insertion of Na+ ions in the HC, SEI size (389 nm), SEI composition (P, Na, F, and O), and Na plating. The in situ TEM analysis reveals a cyclability limitation, this issue appears to be caused by the plating of Na in the form of a "foam" structure, resulting from the gas release during the reaction of Na with DMC/EC electrolyte. The foam structure, subsequently transformes into a second SEI, is electrochemically inactive and reduces the cyclability of the battery. Overall, the results demonstrate the powerful synergy achieved by coupling in situ ec-TEM and GC/MS techniques.

Advanced electrode design enables homogeneous electric field distribution for metal deposition studies via in situ liquid cell TEM

In situ liquid-phase electrochemical transmission electron microscopy (ec-TEM) as a valuable technique has been widely used in studying metal deposition in battery materials. While real-time observations of metallic nucleation, growth, and dendrite formation using microscale ec-TEM liquid cells are investigated, existing cells exhibit nonuniform electric field distribution along electrodes, limiting measurement reliability and quantitative analysis. Here, we introduce an advanced electrode design for ec-TEM chips, ensuring a uniform electric field for precise characterization of early-stage metal deposition closer to practical battery conditions. Both simulation and experimental investigations demonstrate that these specially designed ec-TEM chips facilitate quantitative electrochemical characterization combined with the in situ TEM technique in comparison with commercially available chips. We thus provide a significant progression toward optimizing the performance and reliability of quantitative in situ liquid-phase TEM measurements, essential for understanding and improving electrochemical systems.

Revealing Asymmetric Phase Transformation of the BiVO4 Anodes for Potassium-Ion Batteries by In Situ Transmission Electron Microscopy

Potassium-ion batteries (PIBs) have emerged as a promising alternative to lithium-ion batteries (LIBs), thanks to the cost-effectiveness of potassium resources and a favorable redox potential of approximately -2.936 V. The monoclinic BiVO4, known for its layered structure, shows noteworthy electrochemical properties when utilized as an anode material for both LIBs and sodium-ion batteries. However, the fundamental electrochemical reaction mechanisms of the BiVO4 anode during the potassium insertion/extraction processes remain unclear. Here, we constructed a BiVO4 anode PIB inside the transmission electron microscope (TEM) to explore the real-time potassiation/depotassiation behaviors of BiVO4 during electrochemical cycling. Utilizing the state-of-art in situ TEM technique, the BiVO4 nanorods are found to undergo an asymmetric phase transformation for the first time, where the pristine BiVO4 material is transformed into an amorphous KxBiVO4 phase after the first cycle. More interestingly, the anode materials near and far from the potassium source exhibit opposite volume-changing trends under the same voltage potential. Also, this phenomenon should be attributed to the mass flow of the unstable K-Bi alloy under the electric field. Our findings provide significant insights into the electrochemical mechanism of BiVO4 nanorods during the potassiation/depotassiation process, with the hope of assistance in designing anodes for high-performance PIBs.

In-Depth Understanding of Interfacial Na+ Behaviors in Sodium Metal Anode: Migration, Desolvation, and Deposition

Interfacial Na+ behaviors of sodium (Na) anode severely threaten the stability of sodium-metal batteries (SMBs). This review systematically and in-depth discusses the current fundamental understanding of interfacial Na+ behaviors in SMBs including Na+ migration, desolvation, diffusion, nucleation, and deposition. The key influencing factors and optimization strategies of these behaviors are further summarized and discussed. More importantly, the high-energy-density anode-free sodium metal batteries (AFSMBs) are highlighted by addressing key issues in the areas of limited Na sources and irreversible Na loss. Simultaneously, recent advanced characterization techniques for deeper insights into interfacial Na+ deposition behavior and composition information of SEI film are spotlighted to provide guidance for the advancement of SMBs and AFSMBs. Finally, the prominent perspectives are presented to guide and promote the development of SMBs and AFSMBs.

In Situ Transmission Electron Microscopy Methods for Lithium-Ion Batteries

In situ Transmission Electron Microscopy (TEM) stands as an invaluable instrument for the real-time examination of the structural changes in materials. It features ultrahigh spatial resolution and powerful analytical capability, making it significantly versatile across diverse fields. Particularly in the realm of Lithium-Ion Batteries (LIBs), in situ TEM is extensively utilized for real-time analysis of phase transitions, degradation mechanisms, and the lithiation process during charging and discharging. This review aims to provide an overview of the latest advancements in in situ TEM applications for LIBs. Additionally, it compares the suitability and effectiveness of two techniques: the open cell technique and the liquid cell technique. The technical aspects of both the open cell and liquid cell techniques are introduced, followed by a comparison of their applications in cathodes, anodes, solid electrolyte interphase (SEI) formation, and lithium dendrite growth in LIBs. Lastly, the review concludes by stimulating discussions on possible future research trajectories that hold potential to expedite the progression of battery technology.

In Situ Characterization Techniques for Electrochemical Nitrogen Reduction Reaction

The electrochemical reduction of nitrogen to produce ammonia is pivotal in modern society due to its environmental friendliness and the substantial influence that ammonia has on food, chemicals, and energy. However, the current electrochemical nitrogen reduction reaction (NRR) mechanism is still imperfect, which seriously impedes the development of NRR. In situ characterization techniques offer insight into the alterations taking place at the electrode/electrolyte interface throughout the NRR process, thereby helping us to explore the NRR mechanism in-depth and ultimately promote the development of efficient catalytic systems for NRR. Herein, we introduce the popular theories and mechanisms of the electrochemical NRR and provide an extensive overview on the application of various in situ characterization approaches for on-site detection of reaction intermediates and catalyst transformations during electrocatalytic NRR processes, including different optical techniques, X-ray-based techniques, electron microscopy, and scanning probe microscopy. Finally, some major challenges and future directions of these in situ techniques are proposed.

Advances in acoustic techniques for evaluating defects and properties in lithium-ion batteries: A review

With the rapid demand for high-performance energy storage systems, lithium-ion batteries (LiBs) have emerged as the predominant technology in various applications. However, ensuring the safety and reliability of these batteries remains a critical challenge. Ultrasound-based detection, as a non-destructive and effective method for monitoring the internal state of LiBs, has gradually emerged as a valuable tool to enhance battery safety, reliability, and performance. This paper provides a review of recent advancements in the field of acoustic detection for LiBs, delving into the fundamental principles and mechanisms governing the propagation of acoustic signals within these batteries. This paper reviews the correlation between these acoustic signals and the operational status of the battery, as well as the association with internal side reactions during abnormal conditions. The strengths and limitations of current ultrasound-based detection methods are emphasized, offering insights to guide researchers, engineers, and industry professionals in advancing the field. The review aims to foster the development of robust ultrasound-based detection solutions for the next generation of energy storage systems.

Morphology characterization of dendrites on lithium metal electrodes by NMR spectroscopy

Despite the considerable potential offered by lithium metal's high capacity for rechargeable batteries, challenges such as dendrite formation and safety concerns persist. As strategies continue to advance in dendrite management, the demand for efficient monitoring tools becomes increasingly pronounced. In this study, we delve into the characterization of dendrites, elucidating the influence of microstructure morphology on the NMR spectrum using a combination of simulations and experiments. Systematic variations in various geometrical parameters highlight dendrite density as a pivotal distinguishing feature. Furthermore, the investigation explores the effectiveness of a pulse sequence in selectively exciting microstructures over the bulk, providing valuable insights into mitigating dendrite-related challenges in lithium-metal batteries.

In situ observation of thermal-driven structural transitions of a β-NaYF4 single nanoparticle aided with correlative cathodoluminescence electron microscopy

NaYF4 systems have been widely studied as up-conversion host matrices, and their phase transitions are flexible and worth investigating in great detail. Herein, the evolution of morphology and crystal structure of a Eu3+-doped β-NaYF4 single nanoparticle heated in an air atmosphere was investigated using in situ transmission electron microscopy (TEM). The annealing process revealed that the hexagonal β-NaYF4 phase undergoes sequential transformations into high-temperature cubic phases at both 350 °C and 500 °C. The emission characteristics of Eu3+ in the single nanoparticle after heating treatment were also analyzed using Correlative Cathodoluminescence Electron Microscopy (CCLEM). The results of CCLEM suggest a gradual decrease followed by a subsequent increase in structural symmetry. A comprehensive spectroscopic and structural analysis encapsulates the entire transformation process as NaYF4 → YOF → Y2O3. In situ energy dispersive spectroscopy analyses (EDS) support this reaction process. The aforementioned technique yields correlative lattice-resolved TEM images and nanoscale spectroscopic information, which can be employed to assess the structure-function relationships on the nanoscale.

Signal2signal: Pushing the Spatiotemporal Resolution to the Limit by Single Chemical Hyperspectral Imaging

There is growing interest in developing a high-performance self-supervised denoising algorithm for real-time chemical hyperspectral imaging. With a good understanding of the working function of the zero-shot Noise2Noise-based denoising algorithm, we developed a self-supervised Signal2Signal (S2S) algorithm for real-time denoising with a single chemical hyperspectral image. Owing to the accurate distinction and capture of the weak signal from the random fluctuating noise, S2S displays excellent denoising performance, even for the hyperspectral image with a spectral signal-to-noise ratio (SNR) as low as 1.12. Under this condition, both the image clarity and the spatial resolution could be significantly improved and present an almost identical pattern with a spectral SNR of 7.87. The feasibility of real-time denoising during imaging was well demonstrated, and S2S was applied to monitor the photoinduced exfoliation of transition metal dichalcogenide, which is hard to accomplish by confocal Raman spectroscopy. In general, the real-time denoising capability of S2S offers an easy way toward in situ/in vivo/operando research with much improved spatial and temporal resolution. S2S is open-source at https://github.com/3331822w/Signal2signal and will be accessible online at https://ramancloud.xmu.edu.cn/tutorial.

Structure-Property Interplay Within Microporous Manganese Dioxide Tunnels For Sustainable Energy Storage

Tunnel-structured manganese dioxides (MnO2 ), also known as octahedral molecule sieves (OMS), are widely studied in geochemistry, deionization, energy storage and (electro)catalysis. These functionalities originate from their characteristic sub-nanoscale tunnel framework, which, with a high degree of structural polymorphism and rich surface chemistry, can reversibly absorb and transport various ions. An intensive understanding of their structure-property relationship is prerequisite for functionality optimization, which has been recently approached by implementation of advanced (in situ) characterizations providing significant atomistic sciences. This review will thus timely cover recent advancements related to OMS and their energy storage applications, with a focus on the atomistic insights pioneered by researchers including our group: the origins of structural polymorphism and heterogeneity, the evolution of faceted OMS crystals and its effect on electrocatalysis, the ion transport/storage properties and their implication for processing OMS. These studies represent a clear rational behind recent endeavors investigating the historically applied OMS materials, the summary of which is expected to deepen the scientific understandings and guide material engineering for functionality control.

Replica exchange molecular dynamics for Li-intercalation in graphite: a new solution for an old problem

Li intercalation and graphite stacking have been extensively studied because of the importance of graphite in commercial Li-ion batteries. Despite this attention, there are still questions about the atomistic structures of the intermediate states that exist during lithiation, especially when phase dynamics cause a disordered Li distribution. The Li migration event (diffusion coefficient of 10-5 nm2 ns-1) makes it difficult to explore the various Li-intercalation configurations in conventional molecular dynamics (MD) simulations with an affordable simulation timescale. To overcome these limitations, we conducted a comprehensive study using replica-exchange molecular dynamics (REMD) in combination with the ReaxFF force field. This approach allowed us to study the behavior of Li-intercalated graphite from any starting arrangement of Li at any value of x in LixC6. Our focus was on analyzing the energetic favorability differences between the relaxed structures. We rationalized the trends in formation energy on the basis of observed structural features, identifying three main structural features that cooperatively control Li rearrangement in graphite: Li distribution, graphite stacking mode and gallery height (graphene layer spacing). We also observed a tendency for clustering of Li, which could lead to dynamic local structures that approximate the staging models used to explain intercalation into graphite.

Thioacetamide Additive Homogenizing Zn Deposition Revealed by In Situ Digital Holography for Advanced Zn Ion Batteries

Zinc ion batteries are considered as potential energy storage devices due to their advantages of low-cost, high-safety, and high theoretical capacity. However, dendrite growth and chemical corrosion occurring on Zn anode limit their commercialization. These problems can be tackled through the optimization of the electrolyte. However, the screening of electrolyte additives using normal electrochemical methods is time-consuming and labor-intensive. Herein, a fast and simple method based on the digital holography is developed. It can realize the in situ monitoring of electrode/electrolyte interface and provide direct information concerning ion concentration evolution of the diffusion layer. It is effective and time-saving in estimating the homogeneity of the deposition layer and predicting the tendency of dendrite growth, thus able to value the applicability of electrolyte additives. The feasibility of this method is further validated by the forecast and evaluation of thioacetamide additive. Based on systematic characterization, it is proved that the introduction of thioacetamide can not only regulate the interficial ion flux to induce dendrite-free Zn deposition, but also construct adsorption molecule layers to inhibit side reactions of Zn anode. Being easy to operate, capable of in situ observation, and able to endure harsh conditions, digital holography method will be a promising approach for the interfacial investigation of other battery systems.

Real-time tracking of electron transfer at catalytically active interfaces in lithium-ion batteries

Transition metals and related compounds are known to exhibit high catalytic activities in various electrochemical reactions thanks to their intriguing electronic structures. What is lesser known is their unique role in storing and transferring electrons in battery electrodes which undergo additional solid-state conversion reactions and exhibit substantially large extra capacities. Here, a full dynamic picture depicting the generation and evolution of electrochemical interfaces in the presence of metallic nanoparticles is revealed in a model CoCO3/Li battery via an in situ magnetometry technique. Beyond the conventional reduction to a Li2CO3/Co mixture under battery operation, further decomposition of Li2CO3 is realized by releasing interfacially stored electrons from its adjacent Co nanoparticles, whose subtle variation in the electronic structure during this charge transfer process has been monitored in real time. The findings in this work may not only inspire future development of advanced electrode materials for next-generation energy storage devices but also open up opportunities in achieving in situ monitoring of important electrocatalytic processes in many energy conversion and storage systems.

Controlled Growth of Pd Nanocrystals by Interface Interaction on Monolayer MoS2: An Atom-Resolved in Situ Study

The crystal growth kinetics is crucial for the controllable preparation and performance modulation of metal nanocrystals (NCs). However, the study of growth mechanisms is significantly limited by characterization techniques, and it is still challenging to in situ capture the growth process. Real-time and real-space imaging techniques at the atomic scale can promote the understanding of microdynamics for NC growth. Herein, the growth of Pd NCs on monolayer MoS2 under different atmospheres was in situ studied by environmental transmission electron microscopy. Introducing carbon monoxide can modulate the diffusion of Pd monomers, resulting in the epitaxial growth of Pd NCs with a uniform orientation. The electron energy loss spectroscopy and theoretical calculations showed that the CO adsorption assured the specific exposed facets and good uniformity of Pd NCs. The insight into the gas-solid interface interaction and the microscopic growth mechanism of NCs may shed light on the precise synthesis of NCs on two-dimensional (2D) materials.

Application of neutron imaging in observing various states of matter inside lithium batteries

Lithium batteries have been essential technologies and become an integral part of our daily lives, powering a range of devices from phones to electric vehicles. To fully understand and optimize the performance of lithium batteries, it is necessary to investigate their internal states and processes through various characterization methods. Neutron imaging has been an indispensable complementary characterization technique to X-ray imaging or electron microscopy because of the unique interaction principle between neutrons and matter. It provides particular insights into the various states of matter inside lithium batteries, including the Li+ concentration in solid electrodes, the Li plating/stripping behavior of Li-metal anodes, the Li+ diffusion in solid ionic conductors, the distribution of liquid electrolytes and the generation of gases. This review aims to highlight the capabilities and advantages of neutron imaging in characterizing lithium batteries, as well as its current state of application in this field. Additionally, we discuss the potential of neutron imaging to contribute to the ongoing development of advanced batteries through its ability to visualize internal evolution.

In Situ Transmission Electron Microscopy for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have attracted tremendous attentions in recent years due to the abundance and wide distribution of Na resource on the earth. However, SIBs still face the critical issues of low energy density and unsatisfactory cyclic stability at present. The enhancement of electrochemical performance of SIBs depends on comprehensive and precise understanding of the underlying sodium storage mechanism. Although extensive transmission electron microscopy (TEM) investigations have been performed to reveal the sodium storage property and mechanism of SIBs, a dedicated review on the in situ TEM investigations of SIBs has not been reported. In this review, recent progress in the in situ TEM investigations on the morphological, structural, and chemical evolutions of cathode materials, anode materials, and solid-electrolyte interface during the sodium storage of SIBs is comprehensively summarized. The detailed relationship between structure/composition of electrode materials and electrochemical performance of SIBs has been clarified. This review aims to provide insights into the effective selection and rational design of advanced electrode materials for high-performance SIBs.

Goethite Mineral Dissolution to Probe the Chemistry of Radiolytic Water in Liquid-Phase Transmission Electron Microscopy

Liquid-Phase Transmission Electron Microscopy (LP-TEM) enables in situ observations of the dynamic behavior of materials in liquids at high spatial and temporal resolution. During LP-TEM, incident electrons decompose water molecules into highly reactive species. Consequently, the chemistry of the irradiated aqueous solution is strongly altered, impacting the reactions to be observed. However, the short lifetime of these reactive species prevent their direct study. Here, the morphological changes of goethite during its dissolution are used as a marker system to evaluate the influence of radiation on the changes in solution chemistry. At low electron flux density, the morphological changes are equivalent to those observed under bulk acidic conditions, but the rate of dissolution is higher. On the contrary, at higher electron fluxes, the morphological evolution does not correspond to a unique acidic dissolution process. Combined with kinetic simulations of the steady state concentrations of generated reactive species in the aqueous medium, the results provide a unique insight into the redox and acidity interplay during radiation induced chemical changes in LP-TEM. The results not only reveal beam-induced radiation chemistry via a nanoparticle indicator, but also open up new perspectives in the study of the dissolution process in industrial or natural settings.

Operando characterization and regulation of metal dissolution and redeposition dynamics near battery electrode surface

Mn dissolution has been a long-standing, ubiquitous issue that negatively impacts the performance of Mn-based battery materials. Mn dissolution involves complex chemical and structural transformations at the electrode-electrolyte interface. The continuously evolving electrode-electrolyte interface has posed great challenges for characterizing the dynamic interfacial process and quantitatively establishing the correlation with battery performance. In this study, we visualize and quantify the temporally and spatially resolved Mn dissolution/redeposition (D/R) dynamics of electrochemically operating Mn-containing cathodes. The particle-level and electrode-level analyses reveal that the D/R dynamics is associated with distinct interfacial degradation mechanisms at different states of charge. Our results statistically differentiate the contributions of surface reconstruction and Jahn-Teller distortion to the Mn dissolution at different operating voltages. Introducing sulfonated polymers (Nafion) into composite electrodes can modulate the D/R dynamics by trapping the dissolved Mn species and rapidly establishing local Mn D/R equilibrium. This work represents an inaugural effort to pinpoint the chemical and structural transformations responsible for Mn dissolution via an operando synchrotron study and develops an effective method to regulate Mn interfacial dynamics for improving battery performance.

Understanding and Engineering Interfacial Adhesion in Solid-State Batteries with Metallic Anodes

High performance alkali metal anode solid-state batteries require solid/solid interfaces with fast ion transfer that are morphologically and chemically stable upon electrochemical cycling. Void formation at the alkali metal/solid-state electrolyte interface during alkali metal stripping is responsible for constriction resistances and hotspots that can facilitate dendrite propagation and failure. Both externally applied pressures (35-400 MPa) and temperatures above the melting point of the alkali metal have been shown to improve the interfacial contact with the solid electrolyte, preventing the formation of voids. However, the extreme pressure and temperature conditions required can be difficult to meet for commercial solid-state battery applications. In this review, we highlight the importance of interfacial adhesion or 'wetting' at alkali metal/solid electrolyte interfaces for achieving solid-state batteries that can withstand high current densities without cell failure. The intrinsically poor adhesion at metal/ceramic interfaces poses fundamental limitations on many inorganics solid-state electrolyte systems in the absence of applied pressure. Suppression of alkali metal voids can only be achieved for systems with high interfacial adhesion (i. e. 'perfect wetting') where the contact angle between the alkali metal and the solid-state electrolyte surface goes to θ=0°. We identify key strategies to improve interfacial adhesion and suppress void formation including the adoption of interlayers, alloy anodes and 3D scaffolds. Computational modeling techniques have been invaluable for understanding the structure, stability and adhesion of solid-state battery interfaces and we provide an overview of key techniques. Although focused on alkali metal solid-state batteries, the fundamental understanding of interfacial adhesion discussed in this review has broader applications across the field of chemistry and material science from corrosion to biomaterials development.

Unraveling Li growth kinetics in solid electrolytes due to electron beam charging

Revealing the local structure of solid electrolytes (SEs) with electron microscopy is critical for the fundamental understanding of the performance of solid-state batteries (SSBs). However, the intrinsic structural information in the SSB can be misleading if the sample's interactions with the electron beams are not fully understood. In this work, we systematically investigate the effect of electron beams on Al-doped lithium lanthanum zirconium oxide (LLZO) under different imaging conditions. Li metal is observed to grow directly on the clean surface of LLZO. The Li metal growth kinetics and the morphology obtained are found to be heavily influenced by the temperature, accelerating voltage, and electron beam intensity. We prove that the lithium growth is due to the LLZO delithiation activated by a positive charging effect under electron beam emission. Our results deepen the understanding of the electron beam impact on SEs and provide guidance for battery material characterization using electron microscopy.

Imaging the Surface/Interface Morphologies Evolution of Silicon Anodes Using in Situ/Operando Electron Microscopy

Si-based rechargeable lithium-ion batteries (LIBs) have generated interest as silicon has remarkably high theoretical specific capacity. It is projected that LIBs will meet the increasing need for extensive energy storage systems, electric vehicles, and portable electronics with high energy densities. However, the Si-based LIB has a substantial problem due to the volume cycle variations brought on by Si, which result in severe capacity loss. Making Si-based anodes-enabled high-performance LIBs that are easy to utilize requires an understanding of the fading mechanism. Due to its distinct advantage in morphological changes from microscale to nanoscale, even approaching atomic resolution, electron microscopy is one of the most popular methods. Based on operando electron microscopy characterization, the general comprehension of the fading mechanism and the morphology evolution of Si-based LIBs are debated in this review. The current advancements in compositional and structural interpretation for Si-based LIBs using advanced electron microscopy characterization methods are outlined. The future development trends in pertinent silicon materials characterization methods are also highlighted, along with numerous potential research avenues for Si-based LIBs design and characterization.

Direct Visualization of the Self-Alignment Process for Nanostructured Block Copolymer Thin Films by Transmission Electron Microscopy

Herein, this work aims to directly visualize the morphological evolution of the controlled self-assembly of star-block polystyrene-block-polydimethylsiloxane (PS-b-PDMS) thin films via in situ transmission electron microscopy (TEM) observations. With an environmental chip, possessing a built-in metal wire-based microheater fabricated by the microelectromechanical system (MEMS) technique, in situ TEM observations can be conducted under low-dose conditions to investigate the development of film-spanning perpendicular cylinders in the block copolymer (BCP) thin films via a self-alignment process. Owing to the free-standing condition, a symmetric condition of the BCP thin films can be formed for thermal annealing under vacuum with neutral air surface, whereas an asymmetric condition can be formed by an air plasma treatment on one side of the thin film that creates an end-capped neutral layer. A systematic comparison of the time-resolved self-alignment process in the symmetric and asymmetric conditions can be carried out, giving comprehensive insights for the self-alignment process via the nucleation and growth mechanism.

Na+ Migration Mediated Phase Transitions Induced by Electric Field in the Framework Structured Tungsten Bronze

Framework structured tungsten bronzes serve as promising candidates for electrode materials in sodium-ion batteries (SIBs). However, the tungsten bronze framework structure changes drastically as mediated by the sodium ion concentration at high temperatures. While the three-dimensional ion channels facilitate fast ion storage and transport capabilities, the structural instability induced by Na⁺ migration is a big concern regarding the battery performance and safety, which unfortunately remains elusive. Here, we show the real-time experimental evidence of the phase transitions in framework structured Na_0.36WO_3.14 (triclinic phase) by applying different external voltages. The Na⁺-rich (Na_0.48WO₃, tetragonal phase) or -deficient (Na_xWO₃, x < 0.36, hexagonal phase) phase nucleates under the positive or negative bias, respectively. Combined with the theoretical calculations, the atomistic phase transition mechanisms associated with the Na⁺ migration are directly uncovered. Our work sheds light on the phase instability in sodium tungsten bronzes and paves the way for designing advanced SIBs with high-stability.

Electron and Ion Transport in Lithium and Lithium-Ion Battery Negative and Positive Composite Electrodes

Electrochemical energy storage systems, specifically lithium and lithium-ion batteries, are ubiquitous in contemporary society with the widespread deployment of portable electronic devices. Emerging storage applications such as integration of renewable energy generation and expanded adoption of electric vehicles present an array of functional demands. Critical to battery function are electron and ion transport as they determine the energy output of the battery under application conditions and what portion of the total energy contained in the battery can be utilized. This review considers electron and ion transport processes for active materials as well as positive and negative composite electrodes. Length and time scales over many orders of magnitude are relevant ranging from atomic arrangements of materials and short times for electron conduction to large format batteries and many years of operation. Characterization over this diversity of scales demands multiple methods to obtain a complete view of the transport processes involved. In addition, we offer a perspective on strategies for enabling rational design of electrodes, the role of continuum modeling, and the fundamental science needed for continued advancement of electrochemical energy storage systems with improved energy density, power, and lifetime.

Fabrication of liquid cell for in situ transmission electron microscopy of electrochemical processes

Fundamentally understanding the complex electrochemical reactions that are associated with energy devices (e.g., rechargeable batteries, fuel cells and electrolyzers) has attracted worldwide attention. In situ liquid cell transmission electron microscopy (TEM) offers opportunities to directly observe and analyze in-liquid specimens without the need for freezing or drying, which opens up a door for visualizing these complex electrochemical reactions at the nano scale in real time. The key to the success of this technique lies in the design and fabrication of electrochemical liquid cells with thin but strong imaging windows. This protocol describes the detailed procedures of our established technique for the fabrication of such electrochemical liquid cells (~110 h). In addition, the protocol for the in situ TEM observation of electrochemical reactions by using the nanofabricated electrochemical liquid cell is also presented (2 h). We also show and analyze experimental results relating to the electrochemical reactions captured. We believe that this protocol will shed light on strategies for fabricating high-quality TEM liquid cells for probing dynamic electrochemical reactions in high resolution, providing a powerful research tool. This protocol requires access to a clean room equipped with specialized nanofabrication setups as well as TEM characterization equipment.

Operando Nanoscale Imaging of Electrochemically Induced Strain in a Locally Polarized Pt Grain

Developing new methods that reveal the structure of electrode materials under polarization is key to constructing robust structure-property relationships. However, many existing methods lack the spatial resolution in structural changes and fidelity to electrochemical operating conditions that are needed to probe catalytically relevant structures. Here, we combine a nanopipette electrochemical cell with three-dimensional X-ray Bragg coherent diffractive imaging to study how strain in a single Pt grain evolves in response to applied potential. During polarization, marked changes in surface strain arise from the Coulombic attraction between the surface charge on the electrode and the electrolyte ions in the electrochemical double layers, while the strain in the bulk of the crystal remains unchanged. The concurrent surface redox reactions have a strong influence on the magnitude and nature of the strain changes under polarization. Our studies provide a powerful blueprint to understand how structural evolution influences electrochemical performance at the nanoscale.

Liquid cell electrochemical TEM: Unveiling the real-time interfacial reactions of advanced Li-metal batteries

Li metal batteries (LMBs) reveal great application prospect in next-generation energy storage, because of their high energy density and low electrochemical potential, especially when paired with elemental sulfur and oxygen cathodes. Complex interfacial reactions have long been a big concern because of the elusive formation/dissolution of Li metal at the solid-electrolyte interface (SEI) layer, which leads to battery degradation under practical operating conditions. To precisely track the reactions at the electrode/electrolyte interfaces, in the past ten years, high spatio-temporal resolution, in situ electrochemical transmission electron microscopy (EC-TEM) has been developed. A preliminary understanding of the structural and chemical variation of Li metal during nucleation/growth and SEI layer formation has been obtained. In this perspective, we give a brief introduction of liquid cell development. Then, we comparably discuss the different configurations of EC-TEM based on open-cell and liquid-cell, and focus on the recent advances of liquid-cell EC-TEM and its investigation in the electrodes, electrolytes, and SEI. Finally, we present a perspective of liquid-cell EC-TEM for future LMB research.

Multivariate hyperspectral data analytics across length scales to probe compositional, phase, and strain heterogeneities in electrode materials

The origins of performance degradation in batteries can be traced to atomistic phenomena, accumulated at mesoscale dimensions, and compounded up to the level of electrode architectures. Hyperspectral X-ray spectromicroscopy techniques allow for the mapping of compositional variations, and phase separation across length scales with high spatial and energy resolution. We demonstrate the design of workflows combining singular value decomposition, principal-component analysis, k-means clustering, and linear combination fitting, in conjunction with a curated spectral database, to develop high-accuracy quantitative compositional maps of the effective depth of discharge across individual positive electrode particles and ensembles of particles. Using curated reference spectra, accurate and quantitative mapping of inter- and intraparticle compositional heterogeneities, phase separation, and stress gradients is achieved for a canonical phase-transforming positive electrode material, α-V₂O₅. Phase maps from single-particle measurements are used to reconstruct directional stress profiles showcasing the distinctive insights accessible from a standards-informed application of high-dimensional chemical imaging.

Atomic-Scale Structure Dynamics of Nanocrystals Revealed By In Situ and Environmental Transmission Electron Microscopy

Nanocrystals are of great importance in material sciences and industry. Engineering nanocrystals with desired structures and properties is no doubt one of the most important challenges in the field, which requires deep insight into atomic-scale dynamics of nanocrystals during the process. The rapid developments of in situ transmission electron microscopy (TEM), especially environmental TEM, reveal insights into nanocrystals to digest. According to the considerable progress based on in situ electron microscopy, a comprehensive review on nanocrystal dynamics from three aspects: nucleation and growth, structure evolution, and dynamics in reaction conditions are given. In the nucleation and growth part, existing nucleation theories and growth pathways are organized based on liquid and gas-solid phases. In the structure evolution part, the focus is on in-depth mechanistic understanding of the evolution, including defects, phase, and disorder/order transitions. In the part of dynamics in reaction conditions, solid-solid and gas-solid interfaces of nanocrystals in atmosphere are discussed and the structure-property relationship is correlated. Even though impressive progress is made, additional efforts are required to develop the integrated and operando TEM methodologies for unveiling nanocrystal dynamics with high spatial, energy, and temporal resolutions.

Probing the Phase Transition during the Formation of Lithium Lanthanum Zirconium Oxide Solid Electrolyte

Lithium lanthanum zirconium oxide (LLZO) has long been considered as a promising solid electrolyte for all-solid-state lithium (Li) metal batteries because of its interfacial stability when coupled with a Li metal anode. However, the cubic phase of LLZO (c-LLZO) with a higher Li-ion conductivity has a complex atomic structure and is subject to complicated phase transition during its processing and working conditions, which remain largely elusive. Here, we reveal the phase transition process during the formation of c-LLZO nanotubes through detailed microscopic characterization by scanning and transmission electron microscopy as well as X-ray diffraction. We find four typical stages during the formation of c-LLZO along with several intermediate phases including lanthanum (La)-rich cubic lanthanum zirconium oxide (La-rich c-LZO), c-LZO, and La-rich c-LLZO. We also reveal the role of m-Li₂CO₃ and h-Li₂O₂ as the "phase mediator".

In Situ Atomic-Scale Observation of Electrochemical (De)potassiation in Te Nanowires

Potassium-ion batteries (PIBs) have great potential in energy storage due to their high abundance and low cost of potassium resources. Tellurium (Te) is a promising PIB cathode due to its high volumetric capacity and good electronic conductivity. However, the electrochemical (de)potassiation mechanism of Te remains elusive due to the lack of an effective method of directly observing the dynamic reaction at atomic resolution. Here, the phase transformations of single crystal Te on (de)potassiation are clearly revealed by in situ high-resolution transmission electron microscopy and electron diffraction. Te undergoes a consecutive phase transformation during potassiation: from Te to K₂ Te₃ in the initial potassiation, and then part of the K₂ Te₃ to K₅ Te₃ on further potassiation. The reaction has extremely high reversibility in the following depotassiation. By atomic-scale observation, an anisotropic reaction mechanism where K⁺ intercalates into Te crystalline lattice preferentially through the (001) plane (having a large d-spacing) is established during potassiation. While in the depotassiation process, K ions extract from the polycrystalline K_x Te along the same diffusion path to form single crystal Te, indicating the potassium storage is highly reversible. The strong orientation-dependent (de)potassiation mechanism revealed by this work provides implications for the future design of nanostructured cathodes for high-performance PIBs.

Direct Visualization of the Evolution of a Single-Atomic Cobalt Catalyst from Melting Nanoparticles with Carbon Dissolution

Transition metal single-atom catalysts (SACs) are of immense interest, but how exactly they are evolved upon pyrolysis of the corresponding precursors remains unclear as transition metal ions in the complex precursor undergo a series of morphological changes accompanied with changes in oxidation state as a result of the interactions with the carbon support. Herein, the authors record the complete evolution process of Co SAC during the pyrolysis a Co/Zn-containing zeolitic imidazolate framework. Aberration-corrected environmental TEM coupled with in-situ EELS is used for direct visualization of the evolution process at 200-1000 °C. Dissolution of carbon into the nanoparticles of Co is found to be key to modulating the wetting behavior of nanoparticles on the carbon support; melting of Co nanoparticles and their motion within the zeolitic architecture leads to the etching of the framework structure, yielding porous C/N support onto which Co-single atoms reside. This uniquely structured Co SAC is found to be effective for the oxidation of a series of aromatic alkanes to produce selective ketones among other possible products. The carbon dissolution and melting/sublimation-driven structural dynamics of transition metal revealed here will expand the methodology in synthesizing SACs and other high-temperature processes.

Atomic-Level Changes during Electrochemical Cycling of Oriented LiMn2O4 Cathodic Thin Films

Spinel LiMn₂O₄ is an attractive lithium-ion battery cathode material that undergoes a complex series of structural changes during electrochemical cycling that lead to rapid capacity fading, compromising its long-term performance. To gain insights into this behavior, in this report we analyze changes in epitaxial LiMn₂O₄ thin films during the first few charge-discharge cycles with atomic resolution and correlate them with changes in the electrochemical properties. Impedance spectroscopy and scanning transmission electron microscopy are used to show that defect-rich LiMn₂O₄ surfaces contribute greatly to the increased resistivity of the battery after only a single charge. Sequences of {111} stacking faults within the films were also observed upon charging, increasing in number with further cycling. The atomic structures of these stacking faults are reported for the first time, showing that Li deintercalation is accompanied by local oxygen loss and relaxation of Mn atoms onto previously unoccupied sites. The stacking faults have a more compressed structure than the spinel matrix and impede Li-ion migration, which explains the observed increase in thin-film resistivity as the number of cycles increases. These results are used to identify key factors contributing to conductivity degradation and capacity fading in LiMn₂O₄ cathodes, highlighting the need to develop techniques that minimize defect formation in spinel cathodes to improve cycle performance.

Oxygen Loss in Layered Oxide Cathodes for Li-Ion Batteries: Mechanisms, Effects, and Mitigation

Layered lithium transition metal oxides derived from LiMO₂ (M = Co, Ni, Mn, etc.) have been widely adopted as the cathodes of Li-ion batteries for portable electronics, electric vehicles, and energy storage. Oxygen loss in the layered oxides is one of the major factors leading to cycling-induced structural degradation and its associated fade in electrochemical performance. Herein, we review recent progress in understanding the phenomena of oxygen loss and the resulting structural degradation in layered oxide cathodes. We first present the major driving forces leading to the oxygen loss and then describe the associated structural degradation resulting from the oxygen loss. We follow this analysis with a discussion of the kinetic pathways that enable oxygen loss, and then we address the resulting electrochemical fade. Finally, we review the possible approaches toward mitigating oxygen loss and the associated electrochemical fade as well as detail novel analytical methods for probing the oxygen loss.

Novel Li3 VO4 Nanostructures Grown in Highly Efficient Microwave Irradiation Strategy and Their In-Situ Lithium Storage Mechanism

The investigation of novel growth mechanisms for electrodes and the understanding of their in situ energy storage mechanisms remains major challenges in rechargeable lithium-ion batteries. Herein, a novel mechanism for the growth of high-purity diversified Li3 VO4 nanostructures (including hollow nanospheres, uniform nanoflowers, dispersed hollow nanocubes, and ultrafine nanowires) has been developed via a microwave irradiation strategy. In situ synchrotron X-ray diffraction and in situ transmission electron microscope observations are applied to gain deep insight into the intermediate Li3+ x VO4 and Li3+ y VO4 phases during the lithiation/delithiation mechanism. The first-principle calculations show that lithium ions migrate into the nanosphere wall rapidly along the (100) plane. Furthermore, the Li3 VO4 hollow nanospheres deliver an outstanding reversible capacity (299.6 mAh g-1 after 100 cycles) and excellent cycling stability (a capacity retention of 99.0% after 500 cycles) at 200 mA g-1 . The unique nanostructure offers a high specific surface area and short diffusion path, leading to fast thermal/kinetic reaction behavior, and preventing undesirable volume expansion during long-term cycling.

Spatial and Temporal Analysis of Sodium-Ion Batteries

As a promising alternative to the market-leading lithium-ion batteries, low-cost sodium-ion batteries (SIBs) are attractive for applications such as large-scale electrical energy storage systems. The energy density, cycling life, and rate performance of SIBs are fundamentally dependent on dynamic physiochemical reactions, structural change, and morphological evolution. Therefore, it is essential to holistically understand SIBs reaction processes, degradation mechanisms, and thermal/mechanical behaviors in complex working environments. The recent developments of advanced in situ and operando characterization enable the establishment of the structure-processing-property-performance relationship in SIBs under operating conditions. This Review summarizes significant recent progress in SIBs exploiting in situ and operando techniques based on X-ray and electron analyses at different time and length scales. Through the combination of spectroscopy, imaging, and diffraction, local and global changes in SIBs can be elucidated for improving materials design. The fundamental principles and state-of-the-art capabilities of different techniques are presented, followed by elaborative discussions of major challenges and perspectives.

Updated Insights into 3D Architecture Electrodes for Micropower Sources

Microbatteries (MBs) and microsupercapacitors (MSCs) are primary on-chip micropower sources that drive autonomous and stand-alone microelectronic devices for implementation of the Internet of Things (IoT). However, the performance of conventional MBs and MSCs is restricted by their 2D thin-film electrode design, and these devices struggle to satisfy the increasing IoT energy demands for high energy density, high power density, and long lifespan. The energy densities of MBs and MSCs can be improved significantly through adoption of a 2D thick-film electrode design; however, their power densities and lifespans deteriorate with increased electrode thickness. In contrast, 3D architecture electrodes offer remarkable opportunities to simultaneously improve MB and MSC energy density, power density, and lifespan. To date, various 3D architecture electrodes have been designed, fabricated, and investigated for MBs and MSCs. This review provides an update on the principal superiorities of 3D architecture electrodes over 2D thick-film electrodes in the context of improved MB and MSC energy density, power density, and lifespan. In addition, the most recent and representative progress in 3D architecture electrode development for MBs and MSCs is highlighted. Finally, present challenges are discussed and key perspectives for future research in this field are outlined.

Atomistic Insights of Irreversible Li+ Intercalation in MnO2 Electrode

Tunnel-structured MnO₂ represents open-framed electrode materials for reversible energy storage. Its wide application is limited by its poor cycling stability, whose structural origin is unclear. We tracked the structure evolution of β-MnO₂ upon Li⁺ ion insertion/extraction by combining advanced in situ diagnostic tools at both electrode level (synchrotron X-ray scattering) and single-particle level (transmission electron microscopy). The instability is found to originate from a partially reversible phase transition between β-MnO₂ and orthorhombic LiMnO₂ upon lithiation, causing cycling capacity decay. Moreover, the MnO₂ /LiMnO₂ interface exhibits multiple arrow-headed disordered regions, which severely chop into the host and undermine its structural integrity. Our findings could account for the cycling instability of tunnel-structured materials, based on which future strategies should focus on tuning the charge transport kinetics toward performance enhancement.

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.

Cycling Rate-Induced Spatially-Resolved Heterogeneities in Commercial Cylindrical Li-Ion Batteries

Synchrotron high-energy X-ray diffraction computed tomography has been employed to investigate, for the first time, commercial cylindrical Li-ion batteries electrochemically cycled over the two cycling rates of C/2 and C/20. This technique yields maps of the crystalline components and chemical species as a cross-section of the cell with high spatiotemporal resolution (550 × 550 images with 20 × 20 × 3 µm³ voxel size in ca. 1 h). The recently developed Direct Least-Squares Reconstruction algorithm is used to overcome the well-known parallax problem and led to accurate lattice parameter maps for the device cathode. Chemical heterogeneities are revealed at both electrodes and are attributed to uneven Li and current distributions in the cells. It is shown that this technique has the potential to become an invaluable diagnostic tool for real-world commercial batteries and for their characterization under operating conditions, leading to unique insights into "real" battery degradation mechanisms as they occur.

A "Biconcave-Alleviated" Strategy to Construct Aspergillus niger-Derived Carbon/MoS2 for Ultrastable Sodium Ion Storage

Two-dimensional layered materials commonly face hindered electron transfer and poor structure stability, thus limiting their application in high-rate and long-term sodium ion batteries. In the current study, we adopt finite element simulation to guide the rational design of nanostructures. By calculating the von Mises stress distribution of a series of carbon materials, we find that the hollow biconcave structure could effectively alleviate the stress concentration resulting from expansion. Accordingly, we propose a biconcave-alleviated strategy based on the Aspergillus niger-derived carbon (ANDC) to construct ANDC/MoS₂ with a hollow biconcave structure. The ANDC/MoS₂ is endowed with an excellent long-term cyclability as an anode of sodium ion batteries, delivering a discharge capacity of 496 mAh g^-1 after 1000 cycles at 1 A g^-1. A capacity retention rate of 94.5% is achieved, an increase of almost seven times compared with the bare MoS₂ nanosheets. Even at a high current density of 5 A g^-1, a reversible discharge capacity around 400 mAh g^-1 is maintained after 300 cycles. ANDC/MoS₂ could also be used for efficient lithium storage. By using in situ TEM, we further reveal that the hollow biconcave structure of ANDC/MoS₂ has enabled stable and fast sodiation/desodiation.

Unveiling the abnormal capacity rising mechanism of MoS2 anode during long-term cycling for sodium-ion batteries

Transition metal sulfides are considered as one of the most potential anode materials in sodium-ion batteries due to their high capacity, low cost, and rich resources. Among plenty of options, molybdenum sulfide (MoS₂) has been the focus of research due to the graphene-like layered structure and unique electrochemical properties. Importantly, an abnormal capacity increase phenomenon was observed in the MoS₂ anode of sodium-ion batteries, but the mechanisms involved are still unclear. In this study, by analyzing the composition and structure of the material after a different number of cycles, we confirmed that the (002) plane shows a significant expansion of the interlayer spacing after the sodium ion insertion process and a phase transformation from the hexagonal phase MoS₂ (2H-MoS₂) to the trigonal phase MoS₂ (1T-MoS₂). Moreover, the ratio of 1T-MoS₂ presented an increasing trend during cycling. The dual-phase co-existence leads to enhanced electrical conductivity, higher Na affinity, and higher Na⁺ mobility, thus increasing the capacity. Our work provides a new perspective on the anomalous electrochemical behavior of sulfide anodes during long-term cycling.

The capacity rising is due to the biphasic coexistence of MoS₂ during the cycling and the progressive increase in the 1T-MoS₂ content. Simultaneously, the layer spacing expanded from 0.62 nm to 1.03 nm during the cycling process.