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Using a unique combination of advanced characterization techniques, we identify specific degradation mechanisms and quantify degradative species formed during fast charge cycling of lithium-ion battery pouch cells.

Non-aqueous redox flow batteries which can be operated under subzero temperature are significant for applications in cold regions.

Extreme fast charging (XFC, ≤15 min charging time) of Li-ion batteries (LIBs) has been proposed as an immediate target to increase the commercial appeal of electric vehicles. However, XFC of LIBs i...

Commercialization of rechargeable lithium metal batteries has long been hindered by the safety concerns associated with the undesirable lithium dendrite issues. To inhibit the uncontrolled dendrite growth, homogeneous ion transport to achieve even lithium plating is a necessity for lithium anode, but remain challenging. Inspired by traditional LixMnO2 materials with a fast Li+ transport, here we constructed 3D ionic-electronic transport channels with in-situ electrochemically lithiated MnO2. Taking advantages of the inherently fast ion transport and 3D conductive network, homogeneous ionic flux and reduced local current density can be achieved with this scaffold, contributing to even nucleation and uniform distribution of lithium plating. The dendrite-free anode shows a stable Li stripping/plating process with a low overpotential, and the full cell coupling the composite anode with LiFePO4 cathode delivers excellent cycle stability and rate capability. Our strategy offers a new avenue to advance next-generation lithium metal batteries.

Realization of extreme fast charging (XFC, ≤15 minutes) of lithium-ion batteries is imperative for the widespread adoption of electric vehicles. However, dramatic capacity fading is associated with XFC, limiting its implementation. To quantitatively elucidate the effects of irreversible lithium plating and other degradation mechanisms on the cell capacity, it is important to understand the links between lithium plating and cell degradation at both the local and global (over the full cell) scales. Here, we study the nature of local lithium plating after hundreds of XFC cycles (charging C-rates ranging from 4C to 9C) in industrially-relevant pouch cells using spatially resolved X-ray diffraction. Our results reveal a spatial correlation at the mm scale between irreversible lithium plating on the anode, inactive lithiated graphite phases, and local state-of-charge of the cathode. In regions of plated lithium, additional lithium is locally and irreversibly trapped as lithiated graphite, contributing to the loss of lithium inventory (LLI) and to a local loss of active anode material. The total LLI in the cell from irreversibly plated lithium is linearly correlated to the capacity loss in the batteries after XFC cycling, with a non-zero offset originating from other parasitic side reactions. Finally, at the global (cell) scale, LLI drives the capacity fade, rather than electrode degradation. We anticipate that the understanding of lithium plating and other degradation mechanisms during XFC gained in this work will help lead to new approaches towards designing high-rate batteries in which irreversible lithium plating is minimized.

Single-crystal cathode materials for lithium-ion batteries have attracted increasing interest in providing greater capacity retention than their polycrystalline counterparts. However, after being cycled at high voltages, these single-crystal materials exhibit severe structural instability and capacity fade. Understanding how the surface structural changes determine the performance degradation over cycling is crucial, but remains elusive. Here, we investigate the correlation of the surface structure, internal strain, and capacity deterioration by using operando X-ray spectroscopy imaging and nano-tomography. We directly observe a close correlation between surface chemistry and phase distribution from homogeneity to heterogeneity, which induces heterogeneous internal strain within the particle and the resulting structural/performance degradation during cycling. We also discover that surface chemistry can significantly enhance the cyclic performance. Our modified process effectively regulates the performance fade issue of single-crystal cathode and provides new insights for improved design of high-capacity battery materials., Performance improvement of cathode materials represents one of the most critical technological challenges for lithium-ion batteries. Here the authors show a close correlation between surface chemistry and phase distribution from homogeneity to heterogeneity, and its impact on the cyclic performance.

With the growing need for sustainable energy technologies, advanced characterization methods become more and more critical for optimizing energy materials and understanding their operation mechanisms. In this review, we focus on the synchrotron-based X-ray imaging technologies and the associated applications in gaining fundamental insights into the physical/chemical properties and reaction mechanisms of energy materials. We will discuss a few major X-ray imaging technologies, including X-ray projection imaging, transmission X-ray microscopy, scanning transmission X-ray microscopy, tender and soft X-ray imaging, and coherent diffraction imaging. Researchers can choose from various X-ray imaging techniques with different working principles based on research goals and sample specifications. With the X-ray imaging techniques, we can obtain the morphology, phase, lattice and strain information of energy materials in both 2D and 3D in an intuitive way. In addition, with the high-penetration X-rays and the high-brilliance synchrotron sources, operando/in-situ experiments can be designed to track the qualitative and quantitative changes of the samples during operation. We expect this review can broaden readers’ view on X-ray imaging techniques and inspire new ideas and possibilities in energy materials research.

Compared to the significant advances in its lithium and sodium analogue, the practical application of potassium ion batteries (KIBs) has been hindered by the sluggish K ion diffusion. Electrochemical storage via insertion/de-insertion reactions in lamellar electrode materials critically rests upon the host materials with wide interlayer spacing and the sizes of the guest ions. To uncover the underlying science behind the dimensionality in the KIBs regime, we investigated MoS2/K half-cell using uniform MoS2 rod, sheet, and sphere structures synthesized by a facile and controllable method for the first time. Impressively, when MoS2 is in the shape of a hierarchical rod, there is more intercalation pseudocapacitance (74.8% at the scan rate of 1.5 mV s−1) and the optimized electrode resulted in high-rate capacity (320 and 183 mAh g−1 at 0.1 and 1 A g−1, respectively). In contrast, MoS2 sheet and sphere contribute little pseudocapacitance (69.1% and 61.4%, respectively) and therefore exhibit low-rate capacity (232 and 110 mAh g−1 for MoS2 sheet, 185 and 96 mAh g−1 for MoS2 sphere at 0.1 and 1 A g−1, respectively). Density functional theory (DFT) results uncover that the K atom is much more liable to occupy the octahedral (Oh) site. What’s more, non-destructively 3D reconstruction of MoS2 rod electrode strongly demonstrated the integrity of the rod structure after K+ insertion. The work paves a new way for the design of two dimensional (2D) materials and offers profound insights into the charge storage mechanism of KIBs.

Extreme fast charging (XFC) of commercial lithium-ion batteries (LIBs) in ≤10-15 minutes will significantly advance the deployment of electric vehicles globally. However, XFC leads to considerable capacity fade, mainly due to graphite anode degradation. Non-destructive three-dimensional (3D) investigation of XFC-cycled anodes is crucial to connect degradation with capacity loss. Here, we demonstrate the viability of simultaneous neutron and X-ray tomography (NeXT) for ex-situ 3D visualization of graphite anode degradation. NeXT is advantageous because of the sensitivity of neutrons to Li and H and X-rays to Cu. We combine the neutron and X-ray tomography with micron resolution for material identification and segmentation on one pristine and one XFC-cycled graphite anode, thereby underscoring the benefits of the simultaneous nature of NeXT. Our ex-situ results pave the way for the design of NeXT-friendly LIB geometries that will allow operando and/or in-situ 3D visualization of graphite anode degradation during XFC.

Electrochemical alloying reactions of group IV elements, such as Si, Ge, or Sn, with lithium provide a promising route to next-generation anode materials for lithium-ion batteries (LIBs) due to their high volumetric and gravimetric capacities. However, commercialization of these anodes is still sparse owing to quick capacity fading and limited Coulombic efficiency, which arise from large volume expansion leading to particle cracking and subsequent electrochemical inactivity. As a result, the solid electrolyte interphase (SEI), originating in the decomposition of the electrolyte upon battery operation outside the electrolyte's thermodynamic stability window, grows uncontrollably. While a large number of mitigation strategies have been developed, an improved nanometer level fundamental understanding of the (de)lithiation process and SEI formation, growth, and evolution is necessary to overcome these challenges. Toward this end, many experimental and theoretical approaches have been utilized but still provide an incomplete picture. This is due to the difficulty of investigating buried interfaces and interphases of lithiation products and thin SEI layers (nanometer-scale) in situ and with the desired nanometer accuracy. In this Account, we illustrate the utilization of in situ X-ray reflectivity (XRR) to provide nanometer-scale insights on the SEI nucleation, growth, and evolution, and well as the (de)lithiation process of Si electrodes. XRR is a nondestructive and surface- and interface-sensitive technique that allows for in situ investigations during battery operation under realistic electrochemical conditions. Insight into the system is provided via the surface-normal density profile, which is interpreted in terms of thickness, density, and roughness of individual surface layers, allowing monitoring of the interfacial morphology and chemistry evolution, through which the SEI growth and Si (de)lithiation process can be resolved. We utilized a model battery anode consisting of a native oxide terminated single crystalline Si wafer in half cell configuration with standard electrolyte in a specifically designed in situ XRR electrochemical cell. We have resolved the nucleation and formation process of the inner inorganic SEI and have observed two well-defined inorganic SEI layers on Si anodes: a bottom-SEI layer (adjacent to the electrode) formed via the lithiation of the native oxide and a top-SEI layer mainly consisting of the electrolyte decomposition product, LiF. This SEI layer grows during lithiation and contracts during delithiation. Further, our results show that the lithiation of crystalline Si (c-Si) is a layer-by-layer, reaction-limited, two-phase process with a well-defined phase boundary between LixSi lithiation product and c-Si; in contrast, the delithiation of LixSi and the lithiation of amorphous Si (a-Si) are reaction-limited, single-phase processes. Moreover, we resolved the influences of current density and the Si crystallographic orientation of the reaction interface on the (de)lithiation process. The implications of our findings are discussed with regard to battery performance.

Sodium ion batteries (SIBs) are considered a potential energy storage technology due to their low cost and the abundance of sodium resources. Numerous strategies have been made to address the remaining challenges such as low electronic conductivity, huge volume change, and sluggish sodium ions kinetics. In spite of impressive advances, a comprehensive understanding of the microstructure-stability correlation remains elusive. Here, the three-dimensional (3D) Fe3S4 flower-like hierarchical materials constructed from two-dimensional (2D) nanosheet building units were synthesized via a facile and cost-effective method. Instead of materials synthesis and performance evaluation, more attentions have been paid on the underlying coupling mechanism between microstructure design, surface chemistry and 3D electronic/ionic transport pathways. It found that the thin and stable SEI layer generated by ether-based electrolytes contributes to the stable surface chemistry and fast interfacial Na+ ion transport, while 3D hierarchical structure enables short diffusion distance, volume change accomodation as well as stabilizing SEI layer. Taking advantage of the inherent high electronic conductivity in Fe3S4, we have demonstrated the paramount importance of the synergistic strategy to improve the stability of SIBs through X-ray nanotomography. This work explicitly illuminate the electrochemical stability mechanisms of 3D hierarchitectures and resolves new strategies to rational design SIBs anodes.

The field of organic electronics aspires to enable the fabrication of low-cost, solution-processed optoelectronic devices with unique mechanical, electrical, optical, and chemical properties. Criti...

Summary To shed light on the formation process and structure of the solid electrolyte interphase (SEI) layer on native oxide-terminated silicon wafer anodes from a carbonate-based electrolyte (LP30), we combined in situ synchrotron X-ray reflectivity, linear sweep voltammetry, ex situ X-ray photoelectron spectroscopy, and first principles calculations from the Materials Project. We present in situ sub-nanometer resolution structural insights and compositional information of the SEI, as well as predicted equilibrium phase stability. Combining these findings, we observe two well-defined inorganic SEI layers next to the Si anode—a bottom-SEI layer (adjacent to the electrode) formed via the lithiation of the native oxide, and a top-SEI layer mainly consisting of the electrolyte decomposition product LiF. Our study provides novel mechanistic insights into the SEI growth process on Si, and we discuss several important implications regarding ion and electron transport through the SEI layer.

Super-concentrated "water-in-salt" electrolytes recently spurred resurgent interest for high energy density aqueous lithium-ion batteries. Thermodynamic stabilization at high concentrations and kinetic barriers towards interfacial water electrolysis significantly expand the electrochemical stability window, facilitating high voltage aqueous cells. Herein we investigated LiTFSI/H2O electrolyte interfacial decomposition pathways in the "water-in-salt" and "salt-in-water" regimes using synchrotron X-rays, which produce electrons at the solid/electrolyte interface to mimic reductive environments, and simultaneously probe the structure of surface films using X-ray diffraction. We observed the surface-reduction of TFSI(-)at super-concentration, leading to lithium fluoride interphase formation, while precipitation of the lithium hydroxide was not observed. The mechanism behind this photoelectron-induced reduction was revealed to be concentration-dependent interfacial chemistry that only occurs among closely contact ion-pairs, which constitutes the rationale behind the "water-in-salt" concept., Angewandte Chemie. International Edition, 59 (51), ISSN:1433-7851, ISSN:1521-3773, ISSN:0570-0833

Batteries capable of extreme fast-charging (XFC) are a necessity for the deployment of electric vehicles. Material properties of electrodes and electrolytes along with cell parameters such as stack pressure and temperature have coupled, synergistic, and sometimes deleterious effects on fast-charging performance. We develop a new experimental testbed that allows precise and conformal application of electrode stack pressure. We focus on cell capacity degradation using single-layer pouch cells with graphite anodes, LiNi0.5Mn0.3Co0.2O2 (NMC532) cathodes, and carbonate-based electrolyte. In the tested range (10–125 psi), cells cycled at higher pressure show higher capacity and less capacity fading. Additionally, Li plating decreases with increasing pressure as observed with scanning electron microscopy (SEM) and optical imaging. While the loss of Li inventory from Li plating is the largest contributor to capacity fade, electrochemical and SEM examination of the NMC cathodes after XFC experiments show increased secondary particle damage at lower pressure. We infer that the better performance at higher pressure is due to more homogeneous reactions of active materials across the electrode and less polarization through the electrode thickness. Our study emphasizes the importance of electrode stack pressure in XFC batteries and highlights its subtle role in cell conditions.

Summary Broad use of global or spatially averaging measurements over a cell to characterize highly localized Li plating phenomena in lithium-ion batteries during fast charging has created a disconnect between measurements and the underlying causes. Consequently, the field is missing a clear path to implementing fast charging as well as to expand into extreme fast charging (XFC). Aiming to bridge these gaps, we present a detailed look into local detection of Li plating and the consequent cycle life implications for electrodes and cells under XFC by utilizing electrochemistry and high-energy X-ray diffraction. Significant heterogeneity in Li plating during XFC results in accelerated and non-uniform cycle life losses, in contrast to the prevailing acceptance that C rate is correlated to Li plating for XFC. This behavior is triggered by local electrode heterogeneity, which has yet to be identified and is not apparent in volume-averaged quantifications. A better understanding of these multiscale local electrode heterogeneities is crucial for identifying pathways to enable XFC.

To better understand failure mechanisms in lithium-ion batteries (LIBs), it is imperative to quantify the capacity fading contributions from active material loss and solid electrolyte interphase (SEI) growth. However, these two contributions are indistinguishable in traditional electrochemical measurements. In this study, we used a model system in combination with (1) precision measurements of the overall Coulombic efficiency via electrochemical experiments and (2) X-ray reflectivity (XRR) measurements of the active material losses. The model system consisted of a 515 Å thick amorphous silicon (a-Si) thin film on silicon carbide in a half-cell geometry using a carbonate electrolyte with LiPF6 salt. Schematics of the cells used in operando XRR and precision electrochemistry measurements are shown in Fig. (a) and (b) respectively. Specifically, we used surface/interface sensitive operando XRR to monitor the a-Si/LixSi thickness during (de)lithiation in order to quantify the capacity fading due to active material loss. Combined with electrochemical analysis, we quantified the capacity loss originating in SEI growth during each cycle (Fig. (c)). We have found that SEI continues to grow with cycling, and the SEI thickness is dependent on the time spent at low potentials (Fig. (d)). The continued SEI growth results in increasing overpotentials due to increased SEI resistance, which leads to a lower extent of lithiation when the cut-off voltage is reached. Finally, we extracted a proportionality constant for SEI growth following a parabolic growth law. Our results are in fair agreement with the capacity losses observed in full cell systems. We envision that the proposed methodology can be used as a qualitative measure of the ion transport properties of the SEI. The model-based approach allows for quantitative determination of lithium ion loss mechanisms in LIBs by separately tracking lithium ions within the active materials and will aid in developing ways to reduce parasitic losses. Figure 1

Predictive and quantitative understanding of ion and mass transport in liquid and polymeric electrolytes is at the heart of electrochemistry. Here, the goal is to accurately simulate the performance of an electrochemical cell, for which the temperature- and concentration-dependent transport coefficients of a given electrolyte, as well as the thermodynamic mean salt molal activity coefficient, must be known. These are the conductivity, σ, the salt diffusion coefficient, D, and the cation transference number, t+. The transport coefficients must hence be extracted with high accuracy from experimental measurements. The transference number is defined as the ratio of current carried by the cation to the total electric current. Its importance with regards to energy density and power density was recognized in the early 1990s by Doyle, Fuller and Newman (1). Nevertheless, researchers still argue about transference number values, even in baseline systems such as lithium bistrifluoromethanesulfonimidate (LiTFSI) in Poly(ethylene oxide) (PEO) (2). The transference number can be measured via the steady-state Bruce-Vincent polarization method (3, 4), the Balsara-Newman method (5), as well as pulsed field gradients NMR (pfg-NMR) (6) and pulsed field gradients electrophoretic NMR (e-NMR) (2). Despite extensive efforts towards unified results of these approaches, in particular as a function of ion concentration, a clear picture has not yet emerged. Towards this end, we developed an alternative approach towards determining the ion transport properties. Specifically, we directly and operando measured precise microscopic and macroscopic physical properties of the electrolyte upon cell polarization in a Li/electrolyte/Li cell, combined this with calculations via concentrated solution theory continuum modelling (CM), and rationalized our findings with microscopic insight from molecular dynamics (MD) simulations. We utilized a well-studied benchmark model system electrolyte consisting of PEO and LiTFSI at Li+ to EO molar ratio of r = 0.1. Under constant voltage polarization, we directly measured the velocity associated with electrolyte and ions via heterodyne synchrotron X-ray photon correlation spectroscopy (XPCS), and the TFSI- concentration gradient from electrode to electrode via X-ray absorption microscopy (XAM). The significance of our results lies in the unification of microscopic and macroscopic predictions from simulation with experimental measurements as well as the self-consistent determination of a concentration-independent transference numbers of approximately 0.2. Our study paves the way for further length- and time-scale bridging understanding of ion transport. 1. M. Doyle, T. F. Fuller, J. Newman, The Importance of the Lithium Ion Transference Number in Lithium Polymer Cells. Electrochimica Acta 39, 2073-2081 (1994). 2. M. P. Rosenwinkel, M. Schönhoff, Lithium Transference Numbers in PEO/LiTFSA Electrolytes Determined by Electrophoretic NMR. Journal of The Electrochemical Society 166, A1977-A1983 (2019). 3. J. Evans, C. A. Vincent, P. G. Bruce, Electrochemical Measurement of Transference Numbers in Polymer Electrolytes. Polymer 28, 2324-2328 (1987). 4. P. G. BRUCE, C. A. VINCENT, STEADY STATE CURRENT FLOW IN SOLID BINARY ELECTROLYTE CELLS. J. Electroanal. Chem. 225, 1-17 (1987). 5. D. M. Pesko, S. Sawhney, J. Newman, N. P. Balsara, Comparing Two Electrochemical Approaches for Measuring Transference Numbers in Concentrated Electrolytes. Journal of The Electrochemical Society 165, A3014-A3021 (2018). 6. D. M. Pesko et al., Negative transference numbers in poly(ethylene oxide)-based electrolytes. Journal of the Electrochemical Society 164, E3569-E3575 (2017).

Electrolyte ordering near solid surfaces is of vital importance in diverse fields, ranging from physical chemistry, to energy storage, and heterogeneous catalysis. However, experimental determination of the structure of the electrode–electrolyte interface and electric double layer is challenging due to limited experimental approaches. In this work we show a detailed picture of the electrode–electrolyte interface relevant to Li-ion batteries. Specifically, we probe the atomic-scale interfacial structure of a non-aqueous liquid electrolyte solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) containing lithium hexafluorophosphate (LiPF6) salt via surface sensitive Angstrom resolution X-ray reflectivity (XRR). We complement our experimental results with molecular dynamics (MD) simulations, and find good agreement between the experiment and simulation derived density profiles. The surface at open circuit voltage (OCV) induces layering of electrolyte molecules near the interface, which decays towards the bulk, and we conclude that both EC and DMC molecules in the first interfacial layer tend to adsorb parallel to the surface. With increasing salt-concentration, the layering periodicity and the degree of order increase. We discuss implications of our results to Li-ion batteries, with focus on the relation between interfacial structure and ion transport in and out of the electrode.

Correction for ‘Quantification of heterogeneous, irreversible lithium plating in extreme fast charging of lithium-ion batteries’ by Partha P. Paul et al., Energy Environ. Sci., 2021, DOI: 10.1039/d1ee01216a.

Surface sensitive X-ray reflectivity (XRR) measurements were performed to investigate the electrochemical lithiation of a native oxide terminated single crystalline silicon (100) electrode in real time during the first galvanostatic discharge cycle. This allows us to gain nanoscale, mechanistic insight into the lithiation of Si and the formation of the solid electrolyte interphase (SEI). We describe an electrochemistry cell specifically designed for in situ XRR studies and have determined the evolution of the electron density profile of the lithiated Si layer (LixSi) and the SEI layer with subnanometer resolution. We propose a three-stage lithiation mechanism with a reaction limited, layer-by-layer lithiation of the Si at the LixSi/Si interface.

Lithium-ion batteries (LIBs) capable of fast-charging are essential in the popularization of electric vehicles. Typical fast-charge requirements are charging to 80% state of charge within 10 mins. However, such high rates cause active material degradation and undesired lithium plating, leading to capacity fading and safety hazards. These issues are impacted by charging conditions, e.g. charging protocol, temperature, external pressure, etc. Here, we focus on the pressure-dependence of fast-charge batteries using an in-house designed gas bladder cell configuration. Through a systematic study, we revealed correlations between externally applied electrode stack pressure and capacity fade. Additionally, we propose possible capacity fade mechanisms during fast charge and a way to mitigate it. We utilized single-layer pouch cell batteries with a graphite anode, LiNi0.5Mn0.3Co0.2O2 (NMC532) cathode, and organic liquid electrolyte solution. Electrode stack pressure is achieved by opposing pairs of external flexible gas bladders. These gas bladders are formed by airtight sealing of a Kapton dome against each side of the flexible battery pouch. (Fig. (a)). By controlling the gas pressure within the bladders, the external pressure is precisely set and the flexible bladders locally conform to provide uniform pressure across the electrode stack. This combination bypasses issues with rigid plates where electrode thickness variations and internal electrical connections prevent uniform pressure. Additionally, the pressure is stable during battery cycling process regardless of material expansion/contraction or gassing. Furthermore, the X-ray transparent Kapton domes allow operando X-ray characterization. Using external pressures of 10, 50, and 125 psi, and on a control group with pressure applied by rigid plates, we performed a series of electrochemical experiments. All batteries were charged at 6C using CC-CV protocol and discharged at C/2 at room temperature for 140 cycles with a voltage range of 3 – 4.1 V. We found that the capacity fade decreases with increasing pressure (Fig. (b)); the capacity fade of rigid-plate cells is larger than gas-bladder cells. Additionally, the fast charge capacity increases with increasing pressure, indicating that the depth of charge/discharge in the entire cell is greater with higher external pressure (Fig. (c)). In contrast, the difference between the open circuit voltages at the end of charge and discharge is smaller at higher pressure. These two observations imply that the depth of reaction within the active material decreases with increasing pressure (Fig. (d)). Combining these two findings, we infer that there is more active material loss with lower external pressure, which is the main reason for capacity fading. The active material loss may come from the loss of electrical contact when particles crack or pulverize during fast charge, while higher external pressure can help preserve the electrical contact. Our study sheds new light on the influence of external pressure on fast-charge performance of LIBs. The use of gas-bladder cells helps to eliminate non-reproducibility in non-uniform cell thickness and material expansion/contraction. We plan to conduct operando X-ray diffraction experiments using the X-ray transparent AP cell to monitor the evolution of anode and cathode materials and Li plating during fast charge to investigate the underlying mechanism of the pressure-dependent behaviors. Figure 1

Due to the low potentials on anode materials in lithium-ion batteries, the electrolyte decomposes on the anode forming a passivation layer, termed solid electrolyte interphase (SEI). The SEI plays an essential role in battery performance since it is a major source of capacity loss and dictates cell kinetics. Nevertheless, the properties of the SEI are not yet well-understood. Here, we present the utilization of a simple and well-defined model system to understand the atomic level SEI nucleation and growth, as well as structural and chemical properties. Experiments were performed in a half-cell configuration with N-doped silicon carbide (N-SiC) wafer working electrode, Li metal counter/reference electrode, and LP30 electrolyte (1M LiPF6 in 1:1 wt% ethylene carbonate:dimethyl carbonate). N-SiC has the advantage of being electrically conductive but electrochemically inactive, allowing for a straightforward interpretation of electrochemical and structural measurements, and provides information on the intrinsic electrolyte reactivity. We used a multimodal approach combining in-situ X-ray reflectivity (XRR), precision electrochemistry at slow rates (e.g. 0.16 mV/s cyclic voltammetry), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). We obtained the evolution of the SEI’s thickness, electron density, and roughness through quantitative operando XRR measurements during the first two cyclic voltammetry (CV) cycles. Further, we found via complementary XPS experiment that the SEI was mainly composed of LiF, and we tracked the crystalline LiF growth with in-situ XRD. LiF can be formed via two reaction pathways. One is the direct reduction of PF6 -, the other is the electrocatalytic hydrogen evolution reaction of the trace amount of HF in the electrolyte, forming LiF and H2. In order to elucidate the formation mechanism of LiF, we combined the electrochemical and structural/chemical results for LP30 with precise electrochemistry using several baseline LiPF6 and/or HF containing electrolytes. The results indicate that the direct reduction of PF6 - contributes to LiF formed at ~ 1.5 V versus Li/Li+, consistent with our quantum chemistry calculations. Finally, we revealed that the ratio between organic and inorganic compounds in SEI could be tuned by changing the cycling rate. We envision that this methodology and the use of well-defined model systems can be applied to other electrolyte-electrode systems. Our results are essential in understanding the formation mechanism and properties of SEI and can aid future experimental and computational works.

The electric double layer (EDL) is the ion arrangement evolving at electrolyte interfaces. Despite its importance in many applications such as super-capacitors, lithium ion batteries (LIB), water purification, and stabilization of nanoparticles in solution, the structural motifs of the EDL are still under debate. In LIB, the molecular scale ion arrangement near the interface or EDL controls solvation and hence mass/charge transfer across the interface. Additionally, the interfacial speciation governs the reductive and oxidative processes at the interface. Thus, it has a great impact on the stability of the electrolyte. Hence, the near-interface structure plays a vital role in the solid electrolyte interphase (SEI) formation which is still poorly understood. The SEI protects the electrolyte from further decomposition and its stability, low electrical conductance, and good Li-ion transport properties are crucial for the cycling ability and long-term stability of the cell. Therefore, an atomic level elucidation of the electrolyte/electrode interface and its rearrangement dynamics due to potential changes is of utmost importance for understanding the behaviors of LIBs, and will ultimately lead to the minimization of ion transport resistance and improved long-term stability. We present an in-situ X-ray reflectivity (XR) study on the single crystalline boron-doped diamond – electrolyte interface. The metal-like electric conductivity and wide electrochemical window of the single crystalline boron-doped diamond allow us to apply a potential across the interface. We obtain structural information by modeling the potential dependent EDL to reproduce our experimental data. First proof-of-concept results on 1m CsCl(aq) show a clear trend of the XR profile on the applied potential of -1.2V – 1.2V vs AgCl. The length scale of π/qz = 4Å of the observed minima at qz = 0.75 Å−1 corresponds well to the expected length-scale of the inner-Helmholtz layer of the EDL. Our analysis suggests that the population of the inner-Helmholtz layer decreases with increasing potential. Our efforts allow to challenge the current theoretical models and simulations and will ultimately help to understand the mechanisms involved in the SEI formation and lead to the minimization of ion transport resistance as well as to an improvement of the LIB cycling ability. Figure 1

There is an ever-increasing demand for rechargeable batteries for electric vehicles, which primarily use lithium-ion batteries (LIB). One of the major proposed improvements in using LIBs for vehicles is in reducing the long recharging times (order of hours), to make it comparable to the 5-10 minute refueling times in gasoline-powered vehicles. Therefore, extreme fast charging (XFC) has been defined for LIBs, with a target to bring the charging time down to 15 minutes or less [1]. However, XFC of LIB is associated with several problems, the most prominent of which are a large loss of battery capacity over cycling and safety issues [2]. Therefore, an understanding of the how local, irreversibly plated lithium affects the local SOC of the cathode and anode, as well as tying the contributions from individual loss mechanisms quantitatively over the cell to the overall capacity fade of the cell during fast charging is necessary to design the battery for better safety and consistent performance. Towards addressing this issue, high energy X-ray diffraction (XRD) is employed, which helps build on the existing understanding of lithium plating in two ways. Firstly, XRD provides a way to quantify the amount of Li plating, as well as other loss mechanisms, and tie them to the global cell performance after XFC cycling. Second, XRD is an in-situ technique, allowing to characterize the entire battery in the fully assembled condition. Thus, local heterogeneities in the cathode and anode can be studied and correlated to heterogeneities in Li plating. In this work, sub-mm-scale XRD is used to quantify Li plating across different single layer pouch cells (3 mAh/cm2 specic capacity, with graphite anode and NMC cathode), where the charging rate and protocol are systematically varied (4C to 9C). The cells are cycled through hundreds of XFC cycles and studied in the discharged state. At the local level, the characteristics of plated lithium crystallites such as the preferred crystallographic orientations and size of plated Li on graphite are studied. Additionally, the regions with local lithium plating are correlated with the local SOC (lithium occupancy) and loss of active surface area in the cathode and anode, in the discharged state. Finally, the capacity fade of the cycled cells is correlated to the amount of dead Li, with separated contributions from irreversibly plated Li, Li trapped in graphite as LixC (which cannot be reversibly extracted from the anode) and reaction of plated Li with the electrolyte. Based on this knowledge of the properties of lithium plating and the conditions that favor it, as well as it's effect on overall battery performance, new approaches towards designing batteries can be realized, such that irreversible Li plating is minimized. This step will in turn help to guide the rational design of the next generation of XFC capable LIBs with a consistent and safe performance. References [1] T. R. Tanim, E. J. Dufek, M. Evans, C. Dickerson, A. N. Jansen, B. J. Polzin, A. R. Dunlop, S. E. Trask, R. Jackman, I. Bloom, et al. Extreme fast charge challenges for lithium-ion battery: Variability and positive electrode issues. Journal of The Electrochemical Society, 166(10):A1926{A1938, 2019. [2] A. Tomaszewska, Z. Chu, X. Feng, S. O'Kane, X. Liu, J. Chen, C. Ji, E. Endler, R. Li, L. Liu, et al. Lithium-ion battery fast charging: A review. eTransportation, 1:100011, 2019.

The cyclability of silicon anodes in lithium ion batteries (LIBs) is affected by the reduction of the electrolyte on the anode surface to produce a coating layer termed the solid electrolyte interphase (SEI). One of the key steps for a major improvement of LIBs is unraveling the SEI’s structure-related diffusion properties as charge and discharge rates of LIBs are diffusion-limited. To this end, we have combined two surface sensitive techniques, sum frequency generation (SFG) vibrational spectroscopy, and X-ray reflectivity (XRR), to explore the first monolayer and to probe the first several layers of electrolyte, respectively, for solutions consisting of 1 M lithium perchlorate (LiClO4) salt dissolved in ethylene carbonate (EC) or fluoroethylene carbonate (FEC) and their mixtures (EC/FEC 7:3 and 1:1 wt %) on silicon and sapphire surfaces. Our results suggest that the addition of FEC to EC solution causes the first monolayer to rearrange itself more perpendicular to the anode surface, while subsequent lay...

Cerium oxide, or ceria, is an important material for solid oxide fuel cells and water splitting devices. Although the ceria surface is active in catalytic and electrochemical reactions, how its catalytic properties are affected by the surface structure under operating conditions is far from understood. We investigate the structure of the coherently strained $\mathrm{Ce}{\mathrm{O}}_{2}$ ultrathin films on yttria-stabilized zirconia (001) single crystals by specular synchrotron x-ray diffraction (XRD) under oxidizing conditions as a first step to study the surface structure in situ. An excellent agreement between the experiment data and the model is achieved by using a ``stacks and islands'' model that has a two-component roughness. One component is due to the tiny clusters of nanometer scale in lateral dimensions on each terrace, while the other component is due to slightly different $\mathrm{Ce}{\mathrm{O}}_{2}$ thickness that span over hundreds of nanometers on neighboring terraces. We attribute the nonuniform thickness to step depairing during the thin film deposition that is supported by the surface morphology results on the microscopic level. Importantly, our model also shows that the polarity of the ceria surface is removed by a half monolayer surface coverage of oxygen. The successful resolution of the ceria surface structure using in situ specular synchrotron XRD paves the way to study the structural evolution of ceria as a fuel cell electrode under catalytically relevant temperatures and gas pressures.

Correction for ‘The nanoscale structure of the electrolyte–metal oxide interface’ by Hans-Georg Steinrück et al., Energy Environ. Sci., 2018, DOI: 10.1039/c7ee02724a.

While silicon (Si) has tenfold capacity of commercially used graphite, its application is still limited due to its limited cyclability. In this in situ X-ray reflectivity study, a detailed mechanistic model of the first two (de)lithiation processes of a silicon wafer is presented, which sheds light onto the fundamental difference of the reaction of Li ions with crystalline and amorphous materials. Furthermore, this study provides insight into the formation and further evolution of the inorganic solid electrolyte interphase (SEI) layer on Si anodes. The results show that the lithiation of crystalline Si is a layer-by-layer, reaction limited two-phase process, but the delithiation of LixSi (resulting in amorphous Si) and the lithiation of amorphous Si are reaction-limited single-phase processes. Furthermore, the thickness-density product of the inorganic SEI layer increases during lithiation and decreases during delithiation, resembling a “breathing” behavior; the inorganic SEI layer thickness varies between 40 and 70 A. Additionally, a low-electron-density “Li-dip” layer is found between the SEI and lithiated Si during the delithiation process, suggesting kinetically limited ion transport within the SEI, which is speculated to be one of the origins of battery's internal resistance. Several implications of the findings on battery performance in general are discussed.