Intercalation pathway in many-particle LiFePO4 electrode revealed by nanoscale state-of-charge mapping

5D Analysis of Capacity Degradation in Battery Electrodes Enabled by Operando CT-XANES

For devices encountering long-term stability challenges, a precise evaluation of degradation is of paramount importance. However, methods for comprehensively elucidating the degradation mechanisms in devices, particularly those undergoing dynamic chemical and mechanical changes during operation, such as batteries, are limited. Here, a method is presented using operando computed tomography combined with X-ray absorption near-edge structure spectroscopy (CT-XANES) that can directly track the evolution of the 3D distribution of the local capacity loss in battery electrodes during (dis)charge cycles, thereby enabling a five-dimensional (the 3D spatial coordinates, time, and chemical state) analysis of the degradation. This paper demonstrates that the method can quantify the spatiotemporal dynamics of the local capacity degradation within an electrode during cycling, which has been truncated by existing bulk techniques, and correlate it with the overall electrode performance degradation. Furthermore, the method demonstrates its capability to uncover the correlation among observed local capacity degradation within electrodes, reaction history during past (dis)charge cycles, and electrode microstructure. The method thus provides critical insights into the identification of degradation factors that are not available through existing methods, and therefore, will contribute to the development of batteries with long-term stability.

State-of-charge of individual active material particles in lithium ion batteries: a perspective of analytical techniques and their capabilities

The state-of-charge (SOC) is an essential parameter for battery management systems to reflect and monitor the remaining capacity of individual battery cells. In addition to its application at the cell level, the SOC also plays an important role in the investigation of redox processes of cathode active materials (CAMs) in lithium ion batteries (LIBs) during electrochemical cycling. These processes can be influenced by a large variety of factors such as active material properties, inhomogeneities of the electrode, degradation phenomena and the charge/discharge protocol during cycling. Consequently, non-uniform redox reactions can occur, resulting in charge heterogeneities of the active material. This heterogeneity can translate into accelerated aging of the CAM and a reduction in reversible capacity of the battery cell, since the active material is not fully utilized. To understand and monitor the SOC heterogeneity at the mesoscale, a wide range of techniques have been implemented in the past. In this perspective an overview of current state-of-the-art techniques to evaluate charge heterogeneities of CAMs in LIBs is presented. Therefore, techniques which utilize synchrotron radiation like X-ray absorption near-edge structure (XANES) and transmission X-ray spectroscopy (TXM) are presented as well as Raman spectroscopy and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Next to these established techniques, classification single particle inductively coupled plasma optical emission spectroscopy (CL-SP-ICP-OES) as a new approach is also discussed in this perspective. For these techniques, the areas of application, advantages as well as drawbacks are highlighted and discussed.

Precise State-of-Charge Mapping via Deep Learning on Ultrasonic Transmission Signals for Lithium-Ion Batteries

The uneven distribution of state of charge (SoC) in the lithium-ion battery is a key factor to cause fast decay of local electrochemical performance. Here, we report an acoustic method to realize SoC mapping in a pouch cell. A focused ultrasound beam is used to scan the cell, and the transmitted ultrasonic wave is analyzed with a deep learning algorithm based on the feedforward neural network. The deep learning algorithm effectively suppresses the disturbance of structural variation in different cells. As a result, the root mean squared error (RMSE) of the estimated local SoC is reduced to 3.02% when applying to different positions on different pouch cells, which is 11.07% of the RMSE by direct fitting SoC with acoustic time of flight. Combining with the progressive scanning technique, our method can realize non-destructive in situ SoC mapping with 1 mm in-plane resolution on pouch cells.

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.

Memory Effects' Mechanism in the Intercalation Batteries: The Particles' Bipolarization

To develop energy-storage devices, understanding their charge-discharge behaviors and their underlying mechanisms is mandatory. Memory effect (ME) is among the most important behaviors that should be understood, influencing the batteries' applications. In this paper, the intercalation batteries' ME and their features are justified and explained by employing the particles' bipolarization mechanism. Diffuse regions, located in both sides of the reactant/product phases, turn the particles into dipoles (bipolarized particles) during/after the processes. This bipolarization and subsequent neutralization can explain many charge-discharge behaviors, including the ME. Here, the mechanism explains and justifies all the known features and some aspects of the phenomena which have not been considered so far. According to the proposed mechanism, the aged-neutralized particles react later and in a higher voltage than the fresh-neutralized particles, causing a bump in the curve called the ME. It is the same mechanism that causes the increase in the charge voltage by increasing the open-circuit voltage rest time. Our experiments sufficiently verified the mechanism. In the paper, impacts of the average particle size, relaxation/rest time, discharge cutoff voltage of the memory-writing cycle (MWC), Li-mobility kinetics, current rate, state of charge, depth of discharge of the MWC, boundaries of the charge-discharge curve, and so forth are considered, and their influences on the ME are explained. This mechanism sheds light on the relevant characteristics of the batteries and helps design, tune, control, and engineer the behaviors.

X-Ray Spectromicroscopy Investigation of Heterogeneous Sodiation in Hard Carbon Nanosheets with Vertically Oriented (002) Planes

Hard carbon (HC) is a promising anode material for sodium-ion batteries, but the performance remains unsatisfactory and the sodiation mechanism in HC is one of the most debated topics. Here, from self-assembled cellulose nanocrystal sheets with crystallographic texture, unique HC nanosheets with vertically oriented (002) planes are fabricated and used as a model HC to investigate the sodiation mechanisms using synchrotron scanning transmission X-ray microscopy (STXM) coupled with analytical transmission electron microscopy (TEM). The model HC simplifies the 3D sodiation in a typical HC particle into a 2D sodiation, which facilitates the visualization of phase transformation at different states of charge. The results for the first time unveil that the sodiation in HC initiates heterogeneously, with multiple propagation fronts proceeding simultaneously, eventually merging into larger aggregates. The spatial correlation between the preferential adsorption and nucleation sites suggests that the heterogeneous nucleation is driven by the local Na-ion concentration, which is determined by defects or heteroatoms that have strong binding to Na ions. By identifying intercalation as the dominant sodium storage mechanism in the model HC, the findings highlight the importance of engineering the graphene layer orientation and the structural heterogeneity of edge sites to enhance the performances.

Transmission x-ray microscopy and its applications in battery material research-a short review

Transmission x-ray microscopy (TXM), which can provide morphological and chemical structural information inside of battery component materials at tens of nanometer scale, has become a powerful tool in battery research. This article presents a short review of the TXM, including its instrumentation, battery research applications, and the practical sample preparation and data analysis in the TXM applications. A brief discussion on the challenges and opportunities in the TXM applications is presented at the end.

Using In-Situ Laboratory and Synchrotron-Based X-ray Diffraction for Lithium-Ion Batteries Characterization: A Review on Recent Developments

Renewable technologies, and in particular the electric vehicle revolution, have generated tremendous pressure for the improvement of lithium ion battery performance. To meet the increasingly high market demand, challenges include improving the energy density, extending cycle life and enhancing safety. In order to address these issues, a deep understanding of both the physical and chemical changes of battery materials under working conditions is crucial for linking degradation processes to their origins in material properties and their electrochemical signatures. In situ and operando synchrotron-based X-ray techniques provide powerful tools for battery materials research, allowing a deep understanding of structural evolution, redox processes and transport properties during cycling. In this review, in situ synchrotron-based X-ray diffraction methods are discussed in detail with an emphasis on recent advancements in improving the spatial and temporal resolution. The experimental approaches reviewed here include cell designs and materials, as well as beamline experimental setup details. Finally, future challenges and opportunities for battery technologies are discussed.

Li5VF4(SO4)2: A Prototype High-Voltage Li-Ion Cathode

A Li-rich polyanionic compound based on V³⁺ with a previously unknown structure, Li₅VF₄(SO₄)₂, has been developed as a high-voltage cathode material for Li-ion batteries. The solvothermal preparation of this material, crystal structure solution, and initial electrochemical characterization are presented. An analysis based on density functional theory electronic structure calculations suggests that a high voltage close to 5 V is required to extract two Li ions and to reach the oxidation state of V⁵⁺. However, the use of conventional carbonate-based electrolytes, which exhibit increasing degradation above a potential of 4.3 V, does not permit the full capacity of this compound to be achieved at this time.

Charge distribution guided by grain crystallographic orientations in polycrystalline battery materials

Architecting grain crystallographic orientation can modulate charge distribution and chemomechanical properties for enhancing the performance of polycrystalline battery materials. However, probing the interplay between charge distribution, grain crystallographic orientation, and performance remains a daunting challenge. Herein, we elucidate the spatially resolved charge distribution in lithium layered oxides with different grain crystallographic arrangements and establish a model to quantify their charge distributions. While the holistic “surface-to-bulk” charge distribution prevails in polycrystalline particles, the crystallographic orientation-guided redox reaction governs the charge distribution in the local charged nanodomains. Compared to the randomly oriented grains, the radially aligned grains exhibit a lower cell polarization and higher capacity retention upon battery cycling. The radially aligned grains create less tortuous lithium ion pathways, thus improving the charge homogeneity as statistically quantified from over 20 million nanodomains in polycrystalline particles. This study provides an improved understanding of the charge distribution and chemomechanical properties of polycrystalline battery materials.

The authors here report on the influence of grain orientation on the charge distribution in polycrystalline materials for batteries. The quantitative characterization provides mechanistic insight into the way the grain orientation can be engineered to mitigate the charge heterogeneity.

Correlative imaging of ionic transport and electronic structure in nano Li0.5FePO4 electrodes

Bulk and surface phase separation and electronic structure variation of Li_0.5FePO₄ particles under concurrent lithiation are imaged by X-ray microscopies.

Revealing the Atomic Origin of Heterogeneous Li-Ion Diffusion by Probing Na

Tracing the dynamic process of Li-ion transport at the atomic scale has long been attempted in solid state ionics and is essential for battery material engineering. Approaches via phase change, strain, and valence states of redox species have been developed to circumvent the technical challenge of direct imaging Li; however, all are limited by poor spatial resolution and weak correlation with state-of-charge (SOC). An ion-exchange approach is adopted by sodiating the delithiated cathode and probing Na distribution to trace the Li deintercalation, which enables the visualization of heterogeneous Li-ion diffusion down to the atomic level. In a model LiNi_1/3 Mn_1/3 Co_1/3 O₂ cathode, dislocation-mediated ion diffusion is kinetically favorable at low SOC and planar diffusion along (003) layers dominates at high SOC. These processes work synergistically to determine the overall ion-diffusion dynamics. The heterogeneous nature of ion diffusion in battery materials is unveiled and the role of defect engineering in tailoring ion-transport kinetics is stressed.

Revisiting Electrochemical Techniques to Characterize the Solid-State Diffusion Mechanism in Lithium-Ion Batteries

Lithium-ion batteries (LiBs) have gained a worldwide position as energy storage devices due to their high energy density, power density and cycle life. Nevertheless, these performance parameters are yet insufficient for current and future demands diversifying their range of applications, and competitiveness against other power sources. In line with the materials science, the optimization of LiBs, first, requires an in-depth characterization and understanding of their determining steps regarding transport phenomena and electrode kinetics occurring within these devices. Experimental and theoretical studies have identified the solid-state diffusion of Li+into the composite cathode material as one of the transport mechanisms limiting the performance of LiBs, in particular at high charge and discharge rates (C-rates). Nowadays, there is however ambivalence to characterize this mass transport mechanism using the diffusion coefficient calculated either by electrochemical techniques orab initioquantum chemistry methods.  This contribution revisits conventional electrochemical methodologies employed in literature to estimate mass transport diffusivity of LiBs, in particular using LiFePO4in the cathode, and their suitability and reliability are comprehensively discussed. These experimental and theoretical methods include Galvanostatic and Potentiostatic Intermittent Titration Technique (GITT and PITT), Electrochemical Impedance Spectroscopy (EIS), Cyclic Voltammetry (CV) andab initioquantum chemistry methods. On the one hand, experimental methods seem not to isolate the diffusion mechanism in the solid phase; thus, obtaining an unreliable apparent diffusion coefficient (ranging from 10–10to 10–16 cm2 s−1), which only serves as a criterion to discard among a set of LiBs. On the other hand, atomistic approaches based onab initio, density functional theory (DFT), cannot yet capture the complexity of the local environments involved at this scale; in consequence, these approaches have predicted inadequate diffusion coefficients for LiFePO4(ranging from 10–6to 10–7 cm2 s−1) which strongly differ from experimental values. This contribution, at long last, remarks the factors influencing diffusion mechanisms and addresses the uncertainties to characterize this transport mechanism in the cathode, stressing the needs to establish methods to determine the diffusion coefficient accurately, coupling electrochemical techniques,ab initiomethods, and engineering approaches based on modeling.

Size-Dependent Memory Effect of the LiFePO4 Electrode in Li-Ion Batteries

In Li-ion batteries, the phase transition usually determines the electrochemical kinetics of some two-phase electrode materials, and it can be adopted to excellently interpret the memory effect of Li-ion batteries, therefore the size dependence of phase transition was expected to affect the memory effect significantly. In this work, we investigated the memory effect and phase transition of olivine LiFePO₄ in Li-ion batteries. Through electrochemical measurements, we found that the memory effect of LiFePO₄ was dependent on the particle size, especially after a long-time relaxation. By using the in situ X-ray diffraction, we found that the phase transition of nano-LiFePO₄ was ahead of the charging and discharging processes, while it took place concurrently or later for micro-LiFePO₄, which might be attributed to the high-specific two-phase boundary of nano-LiFePO₄. Furthermore, the phase-transition diagram was adopted to interpret the size-dependent memory effect schematically. Notably, it is the first time to report the phase transition ahead of (dis)charging for nano-LiFePO₄, which is significant to understand the phase transition of two-phase electrode materials, as well as the relevant phenomena, such as the memory effect.

In Situ/Operando X-ray Spectroscopies for Advanced Investigation of Energy Materials

Issues related to energy and the environment have now become of central and crucial importance in our societies. Low-carbon green energy will have a critical role in a necessary third industrial revolution. To reduce global greenhouse gas emissions in response to globalization and increasingly stringent carbon emission policies, large scale green energy production technologies must be established worldwide. A new age of human demand for green energy is thus coming and scientists are focused on finding new functional efficient and low-cost materials to generate clean and sustainable energy. Improving the energy conversion, generation, and storage efficiency of energy materials has always been a daunting challenge. For many important energy material systems, such as nanostructured catalysts, artificial photosynthetic systems, smart energy saving materials, and energy storage devices, monitoring the atomic and electronic structures close to the interfacial region in a real working environment is of paramount importance. Designing a better-performing material without comprehending its fundamental properties such as chemical states, atomic and electronic structures and how they are altered close to the interfacial regions during the physical and chemical reactions involved in their applications is very challenging. Understanding, controlling and tuning the interfaces in energy conversion and storage materials requires in situ/operando characterization tools, of which synchrotron X-ray spectroscopies, which have several unique features, are very suitable ones. X-ray absorption spectroscopy can be used to elucidate the local unoccupied electronic structure in the conduction band, and X-ray emission spectroscopy can be used to characterize the occupied electronic structure in the valence band. The derived resonant inelastic X-ray scattering reveals inter- and/or intra-electric transitions (i.e. d-d, f-f excitation and charge-transfer excitation) that reflect intrinsic chemical and physical properties. Scanning transmission X-ray microscopy is a chemical mapping technique with elemental sensitivity and spatial selectivity, which can therefore yield information about chemical composition in various spatial regions. This unique characteristic makes the method effective for investigating interfacial phenomena (such as electron transport, interface formation/deformation, defects, doping etc.). In situ/operando approaches have made the probing and understanding of changes in the atomic and electronic structures of energy materials in an operational environment feasible. This article presents a perspective of the pioneering developments as well as the recent achievements in in situ/operando synchrotron X-ray spectroscopies for the advanced investigation of energy materials. Four major energy material systems are identified: energy storage, energy generation, energy conversion, and energy saving material systems. Selected representative investigations of each systems are showcased and discussed demonstrating that in situ/operando synchrotron X-ray spectroscopy is truly essential for unraveling better fundamental knowledge for mechanism understanding and efficiency optimization of existing and emerging energy material systems.

Nanostructured Metal Hydrides for Hydrogen Storage

Knowledge and foundational understanding of phenomena associated with the behavior of materials at the nanoscale is one of the key scientific challenges toward a sustainable energy future. Size reduction from bulk to the nanoscale leads to a variety of exciting and anomalous phenomena due to enhanced surface-to-volume ratio, reduced transport length, and tunable nanointerfaces. Nanostructured metal hydrides are an important class of materials with significant potential for energy storage applications. Hydrogen storage in nanoscale metal hydrides has been recognized as a potentially transformative technology, and the field is now growing steadily due to the ability to tune the material properties more independently and drastically compared to those of their bulk counterparts. The numerous advantages of nanostructured metal hydrides compared to bulk include improved reversibility, altered heats of hydrogen absorption/desorption, nanointerfacial reaction pathways with faster rates, and new surface states capable of activating chemical bonds. This review aims to summarize the progress to date in the area of nanostructured metal hydrides and intends to understand and explain the underpinnings of the innovative concepts and strategies developed over the past decade to tune the thermodynamics and kinetics of hydrogen storage reactions. These recent achievements have the potential to propel further the prospects of tuning the hydride properties at nanoscale, with several promising directions and strategies that could lead to the next generation of solid-state materials for hydrogen storage applications.

Three-dimensional localization of nanoscale battery reactions using soft X-ray tomography

Battery function is determined by the efficiency and reversibility of the electrochemical phase transformations at solid electrodes. The microscopic tools available to study the chemical states of matter with the required spatial resolution and chemical specificity are intrinsically limited when studying complex architectures by their reliance on two-dimensional projections of thick material. Here, we report the development of soft X-ray ptychographic tomography, which resolves chemical states in three dimensions at 11 nm spatial resolution. We study an ensemble of nano-plates of lithium iron phosphate extracted from a battery electrode at 50% state of charge. Using a set of nanoscale tomograms, we quantify the electrochemical state and resolve phase boundaries throughout the volume of individual nanoparticles. These observations reveal multiple reaction points, intra-particle heterogeneity, and size effects that highlight the importance of multi-dimensional analytical tools in providing novel insight to the design of the next generation of high-performance devices.

Here the authors show the development of soft X-ray ptychographic tomography to quantify the electrochemical state and resolve phase boundaries throughout the volume of individual nano-particles from a composite battery electrode.

High resolution visualization of the redox activity of Li2O2 in non-aqueous media: conformal layer vs. toroid structure

A strong relationship between the surface structure and the redox activity of Li₂O₂ is visualized directly using scanning electrochemical cell microscopy, employing a dual-barrel nanopipette containing a unique gel polymer electrolyte.

The nanoscale circuitry of battery electrodes

Developing high-performance, affordable, and durable batteries is one of the decisive technological tasks of our generation. Here, we review recent progress in understanding how to optimally arrange the various necessary phases to form the nanoscale structure of a battery electrode. The discussion begins with design principles for optimizing electrode kinetics based on the transport parameters and dimensionality of the phases involved. These principles are then used to review and classify various nanostructured architectures that have been synthesized. Connections are drawn to the necessary fabrication methods, and results from in operando experiments are highlighted that give insight into how electrodes evolve during battery cycling.

Nanoscale Detection of Intermediate Solid Solutions in Equilibrated LixFePO4 Microcrystals

Redox-driven phase transformations in solids determine the performance of lithium-ion batteries, crucial in the technological transition from fossil fuels. Couplings between chemistry and strain define reversibility and fatigue of an electrode. The accurate definition of all phases in the transformation, their energetics, and nanoscale location within a particle produces fundamental understanding of these couplings needed to design materials with ultimate performance. Here we demonstrate that scanning X-ray diffraction microscopy (SXDM) extends our ability to image battery processes in single particles. In LiFePO₄ crystals equilibrated after delithiation, SXDM revealed the existence of domains of miscibility between LiFePO₄ and Li_0.6FePO₄. These solid solutions are conventionally thought to be metastable, and were previously undetected by spectromicroscopy. The observation provides experimental verification of predictions that the LiFePO₄-FePO₄ phase diagram can be altered by coherency strain under certain interfacial orientations. It enriches our understanding of the interaction between diffusion, chemistry, and mechanics in solid state transformations.

Two-dimensional lithium diffusion behavior and probable hybrid phase transformation kinetics in olivine lithium iron phosphate

Olivine lithium iron phosphate is a technologically important electrode material for lithium-ion batteries and a model system for studying electrochemically driven phase transformations. Despite extensive studies, many aspects of the phase transformation and lithium transport in this material are still not well understood. Here we combine operando hard X-ray spectroscopic imaging and phase-field modeling to elucidate the delithiation dynamics of single-crystal lithium iron phosphate microrods with long-axis along the [010] direction. Lithium diffusivity is found to be two-dimensional in microsized particles containing ~3% lithium-iron anti-site defects. Our study provides direct evidence for the previously predicted surface reaction-limited phase-boundary migration mechanism and the potential operation of a hybrid mode of phase growth, in which phase-boundary movement is controlled by surface reaction or lithium diffusion in different crystallographic directions. These findings uncover the rich phase-transformation behaviors in lithium iron phosphate and intercalation compounds in general and can help guide the design of better electrodes.

Lithium transport and phase transformation kinetics in olivine LiFePO₄ electrode remain not fully understood. Here the authors show that microsized olivine particles possess 2D lithium diffusivity and exhibit a possible hybrid mode of phase boundary migration upon cycling.

In Situ Observation of Single-Phase Lithium Intercalation in Sub-25-nm Nanoparticles

Many lithium-storage materials operate via first-order phase transformations with slow kinetics largely restricted by the nucleation and growth of a new phase. Due to the energy penalties associated with interfaces between coexisting phases, the tendency for a single-phase solid-solution pathway with exceptional reaction kinetics has been predicted to increase with decreasing particle size. Unfortunately, phase evolutions inside such small particles (tens of nanometers) are often shrouded by electrode-scale inhomogeneous reactions containing millions of particles, leading to intensive debate over the size-dependent microscopic reaction mechanisms. This study provides a generally applicable methodology capable of tracking lithiation pathways in individual nanoparticles and unambiguously reveals that lithiation of anatase TiO₂ , previously long believed to be biphasic, converts to a single-phase reaction when particle size reaches ≈25 nm. These results imply the prevalence of such a size-dependent transition in lithiation mechanism among intercalation compounds and provide important guidelines for designing high-power electrodes, especially cathodes.

Direct visualization of hydrogen absorption dynamics in individual palladium nanoparticles

Many energy storage materials undergo large volume changes during charging and discharging. The resulting stresses often lead to defect formation in the bulk, but less so in nanosized systems. Here, we capture in real time the mechanism of one such transformation—the hydrogenation of single-crystalline palladium nanocubes from 15 to 80 nm—to better understand the reason for this durability. First, using environmental scanning transmission electron microscopy, we monitor the hydrogen absorption process in real time with 3 nm resolution. Then, using dark-field imaging, we structurally examine the reaction intermediates with 1 nm resolution. The reaction proceeds through nucleation and growth of the new phase in corners of the nanocubes. As the hydrogenated phase propagates across the particles, portions of the lattice misorient by 1.5%, diminishing crystal quality. Once transformed, all the particles explored return to a pristine state. The nanoparticles' ability to remove crystallographic imperfections renders them more durable than their bulk counterparts.

It remains unclear why energy storage systems with nanoscale constituents are less susceptible to stress-induced damage than their bulk counterparts. Here, the authors probe in real time the intercalation-driven phase transitions of nanoscale palladium hydride, finding that these nanoparticles are able to fix crystallographic flaws as they form.

Thermodynamic stability of driven open systems and control of phase separation by electro-autocatalysis

Motivated by the possibility of electrochemical control of phase separation, a variational theory of thermodynamic stability is developed for driven reactive mixtures, based on a nonlinear generalization of the Cahn–Hilliard and Allen–Cahn equations. The Glansdorff–Prigogine stability criterion is extended for driving chemical work, based on variations of nonequilibrium Gibbs free energy. Linear stability is generally determined by the competition of chemical diffusion and driven autocatalysis. Novel features arise for electrochemical systems, related to controlled total current (galvanostatic operation), concentration-dependent exchange current (Butler–Volmer kinetics), and negative differential reaction resistance (Marcus kinetics). The theory shows how spinodal decomposition can be controlled by solo-autocatalytic charge transfer, with only a single faradaic reaction. Experimental evidence is presented for intercalation and electrodeposition in rechargeable batteries, and further applications are discussed in solid state ionics, electrovariable optics, electrochemical precipitation, and biological pattern formation.

Measuring spectroscopy and magnetism of extracted and intracellular magnetosomes using soft X-ray ptychography

Characterizing the chemistry and magnetism of magnetotactic bacteria (MTB) is an important aspect of understanding the biomineralization mechanism and function of the chains of magnetosomes (Fe₃O₄ nanoparticles) found in such species. Images and X-ray absorption spectra (XAS) of magnetosomes extracted from, and magnetosomes in, whole Magnetovibrio blakemorei strain MV-1 cells have been recorded using soft X-ray ptychography at the Fe 2p edge. A spatial resolution of 7 nm is demonstrated. Precursor-like and immature magnetosome phases in a whole MV-1 cell were visualized, and their Fe 2p spectra were measured. Based on these results, a model for the pathway of magnetosome biomineralization for MV-1 is proposed. Fe 2p X-ray magnetic circular dichroism (XMCD) spectra have been derived from ptychography image sequences recorded using left and right circular polarization. The shape of the XAS and XMCD signals in the ptychographic absorption spectra of both sample types is identical to the shape and signals measured with conventional bright-field scanning transmission X-ray microscope. A weaker and inverted XMCD signal was observed in the ptychographic phase spectra of the extracted magnetosomes. The XMCD ptychographic phase spectrum of the intracellular magnetosomes differed from the ptychographic phase spectrum of the extracted magnetosomes. These results demonstrate that spectro-ptychography offers a superior means of characterizing the chemical and magnetic properties of MTB at the individual magnetosome level.

Kinetic Phase Evolution of Spinel Cobalt Oxide during Lithiation

Spinel cobalt oxide has been proposed to undergo a multiple-step reaction during the electrochemical lithiation process. Understanding the kinetics of the lithiation process in this compound is crucial to optimize its performance and cyclability. In this work, we have utilized a low-angle annular dark-field scanning transmission electron microscopy method to visualize the dynamic reaction process in real time and study the reaction kinetics at different rates. We show that the particles undergo a two-step reaction at the single-particle level, which includes an initial intercalation reaction followed by a conversion reaction. At low rates, the conversion reaction starts after the intercalation reaction has fully finished, consistent with the prediction of density functional theoretical calculations. At high rates, the intercalation reaction is overwhelmed by the subsequently nucleated conversion reaction, and the reaction speeds of both the intercalation and conversion reactions are increased. Phase-field simulations show the crucial role of surface diffusion rates of lithium ions in controlling this process. This work provides microscopic insights into the reaction dynamics in non-equilibrium conditions and highlights the effect of lithium diffusion rates on the overall reaction homogeneity as well as the performance.

Visualization of anisotropic-isotropic phase transformation dynamics in battery electrode particles

Anisotropy, or alternatively, isotropy of phase transformations extensively exist in a number of solid-state materials, with performance depending on the three-dimensional transformation features. Fundamental insights into internal chemical phase evolution allow manipulating materials with desired functionalities, and can be developed via real-time multi-dimensional imaging methods. Here, we report a five-dimensional imaging method to track phase transformation as a function of charging time in individual lithium iron phosphate battery cathode particles during delithiation. The electrochemically driven phase transformation is initially anisotropic with a preferred boundary migration direction, but becomes isotropic as delithiation proceeds further. We also observe the expected two-phase coexistence throughout the entire charging process. We expect this five-dimensional imaging method to be broadly applicable to problems in energy, materials, environmental and life sciences.

Spatiotemporal phase evolution during transformations impacts materials performance in many systems. Here the authors use lithium iron phosphate particles inside a custom-designed battery to characterize in three dimensions the two-phase configuration at various states of charge via X-ray tomography.

Persistent State-of-Charge Heterogeneity in Relaxed, Partially Charged Li1- x Ni1/3 Co1/3 Mn1/3 O2 Secondary Particles

Ex situ transmission X-ray microscopy reveals micrometer-scale state-of-charge heterogeneity in solid-solution Li1- x Ni1/3 Co1/3 Mn1/3 O2 secondary particles even after extensive relaxation. The heterogeneity generates overcharged domains at the cutoff voltage, which may accelerate capacity fading and increase impedance with extended cycling. It is proposed that optimized secondary structures can minimize the state-of-charge heterogeneity by mitigating the buildup of nonuniform internal stresses associated with volume changes during charge.

Comprehensive analysis of TEM methods for LiFePO4/FePO4 phase mapping: spectroscopic techniques (EFTEM, STEM-EELS) and STEM diffraction techniques (ACOM-TEM)

Transmission electron microscopy (TEM) has been used intensively in investigating battery materials, e.g. to obtain phase maps of partially (dis)charged (lithium) iron phosphate (LFP/FP), which is one of the most promising cathode material for next generation lithium ion (Li-ion) batteries. Due to the weak interaction between Li atoms and fast electrons, mapping of the Li distribution is not straightforward. In this work, we revisited the issue of TEM measurements of Li distribution maps for LFP/FP. Different TEM techniques, including spectroscopic techniques (energy filtered (EF)TEM in the energy range from low-loss to core-loss) and a STEM diffraction technique (automated crystal orientation mapping (ACOM)), were applied to map the lithiation of the same location in the same sample. This enabled a direct comparison of the results. The maps obtained by all methods showed excellent agreement with each other. Because of the strong difference in the imaging mechanisms, it proves the reliability of both the spectroscopic and STEM diffraction phase mapping. A comprehensive comparison of all methods is given in terms of information content, dose level, acquisition time and signal quality. The latter three are crucial for the design of in-situ experiments with beam sensitive Li-ion battery materials. Furthermore, we demonstrated the power of STEM diffraction (ACOM-STEM) providing additional crystallographic information, which can be analyzed to gain a deeper understanding of the LFP/FP interface properties such as statistical information on phase boundary orientation and misorientation between domains.

Phase Boundary Propagation in Li-Alloying Battery Electrodes Revealed by Liquid-Cell Transmission Electron Microscopy

Battery cycle life is directly influenced by the microstructural changes occurring in the electrodes during charge and discharge cycles. Here, we image in situ the nanoscale phase evolution in negative electrode materials for Li-ion batteries using a fully enclosed liquid cell in a transmission electron microscope (TEM) to reveal early degradation that is not evident in the charge-discharge curves. To compare the electrochemical phase transformation behavior between three model materials, thin films of amorphous Si, crystalline Al, and crystalline Au were lithiated and delithiated at controlled rates while immersed in a commercial liquid electrolyte. This method allowed for the direct observation of lithiation mechanisms in nanoscale negative electrodes, revealing that a simplistic model of a surface-to-interior lithiation front is insufficient. For the crystalline films, a lithiation front spread laterally from a few initial nucleation points, with continued grain nucleation along the growing interface. The intermediate lithiated phases were identified using electron diffraction, and high-resolution postmortem imaging revealed the details of the final microstructure. Our results show that electrochemically induced solid-solid phase transformations can lead to highly concentrated stresses at the laterally propagating phase boundary which should be considered for future designs of nanostructured electrodes for Li-ion batteries.

Mapping polaronic states and lithiation gradients in individual V2O5 nanowires

The rapid insertion and extraction of Li ions from a cathode material is imperative for the functioning of a Li-ion battery. In many cathode materials such as LiCoO2, lithiation proceeds through solid-solution formation, whereas in other materials such as LiFePO4 lithiation/delithiation is accompanied by a phase transition between Li-rich and Li-poor phases. We demonstrate using scanning transmission X-ray microscopy (STXM) that in individual nanowires of layered V2O5, lithiation gradients observed on Li-ion intercalation arise from electron localization and local structural polarization. Electrons localized on the V2O5 framework couple to local structural distortions, giving rise to small polarons that serves as a bottleneck for further Li-ion insertion. The stabilization of this polaron impedes equilibration of charge density across the nanowire and gives rise to distinctive domains. The enhancement in charge/discharge rates for this material on nanostructuring can be attributed to circumventing challenges with charge transport from polaron formation.

Elucidation of the Conversion Reaction of CoMnFeO4 Nanoparticles in Lithium Ion Battery Anode via Operando Studies

Conversion reactions deliver much higher capacities than intercalation/deintercalation reactions of commercial Li ion batteries. However, the complex reaction pathways of conversion reactions occurring during Li uptake and release are not entirely understood, especially the irreversible capacity loss of Mn(III)-containing oxidic spinels. Here, we report for the first time on the electrochemical Li uptake and release of Co(II)Mn(III)Fe(III)O4 spinel nanoparticles and the conversion reaction mechanisms elucidated by combined operando X-ray diffraction, operando and ex-situ X-ray absorption spectroscopy, transmission electron microscopy, (7)Li NMR, and molecular dynamics simulation. The combination of these techniques enabled uncovering the pronounced electronic changes and structural alterations on different length scales in a unique way. The spinel nanoparticles undergo a successive phase transition into a mixed monoxide caused by a movement of the reduced cations from tetrahedral to octahedral positions. While the redox reactions Fe(3+) ↔ Fe(0) and Co(2+) ↔ Co(0) occur for many charge/discharge cycles, metallic Mn nanoparticles formed during the first discharge can only be oxidized to Mn(2+) during charge. This finding explains the partial capacity loss reported for Mn(III)-based spinels. Furthermore, the results of the investigations evidence that the reaction mechanisms on the nanoscale are very different from pathways of microcrystalline materials.

Engineering the Transformation Strain in LiMnyFe1-yPO4 Olivines for Ultrahigh Rate Battery Cathodes

Alkali ion intercalation compounds used as battery electrodes often exhibit first-order phase transitions during electrochemical cycling, accompanied by significant transformation strains. Despite ∼30 years of research into the behavior of such compounds, the relationship between transformation strain and electrode performance, especially the rate at which working ions (e.g., Li) can be intercalated and deintercalated, is still absent. In this work, we use the LiMnyFe1-yPO4 system for a systematic study, and measure using operando synchrotron radiation powder X-ray diffraction (SR-PXD) the dynamic strain behavior as a function of the Mn content (y) in powders of ∼50 nm average diameter. The dynamically produced strain deviates significantly from what is expected from the equilibrium phase diagrams and demonstrates metastability but nonetheless spans a wide range from 0 to 8 vol % with y. For the first time, we show that the discharge capacity at high C-rates (20-50C rate) varies in inverse proportion to the transformation strain, implying that engineering electrode materials for reduced strain can be used to maximize the power capability of batteries.

Imaging the surface morphology, chemistry and conductivity of LiNi1/3Fe1/3Mn4/3O4 crystalline facets using scanning transmission X-ray microscopy

Scanning transmission X-ray microscopy (STXM) using the X-ray fluorescence mode has been employed to elucidate the chemical structures at Ni, Fe, Mn and O sites from the (111) and (100) facets of micron-sized LiNi_1/3Fe_1/3Mn_4/3O₄ energy material particles.

Effects of Particle Size, Electronic Connectivity, and Incoherent Nanoscale Domains on the Sequence of Lithiation in LiFePO₄ Porous Electrodes

High-resolution X-ray microscopy is used to investigate the sequence of lithiation in LiFePO4 porous electrodes. For electrodes with homogeneous interparticle electronic connectivity via the carbon black network, the smaller particles lithiate first. For electrodes with heterogeneous connectivity, the better-connected particles preferentially lithiate. Correlative electron and X-ray microscopy also reveal the presence of incoherent nanodomains that lithiate as if they are separate particles.