Operando EQCM-D with Simultaneous in Situ EIS: New Insights into Interphase Formation in Li Ion Batteries

Elucidating the Transport of Electrons and Molecules in a Solid Electrolyte Interphase Close to Battery Operation Potentials Using a Four-Electrode-Based Generator-Collector Setup

In lithium-ion batteries, the solid electrolyte interphase (SEI) passivates the anode against reductive decomposition of the electrolyte but allows for electron transfer reactions between anode and redox shuttle molecules, which are added to the electrolyte as an internal overcharge protection. In order to elucidate the origin of these poorly understood passivation properties of the SEI with regard to different molecules, we used a four-electrode-based generator-collector setup to distinguish between electrolyte reduction current and the redox molecule (ferrocenium ion Fc+) reduction current at an SEI-covered glassy carbon electrode. The experiments were carried out in situ during potentiostatic SEI formation close to battery operation potentials. The measured generator and collector currents were used to calculate passivation factors of the SEI with regard to electrolyte reduction and with regard to Fc+ reduction. These passivation factors show huge differences in their absolute values and in their temporal evolution. By making simple assumptions about molecule transport, electron transport, and charge transfer reaction rates in the SEI, distinct passivation mechanisms are identified, strong indication is found for a transition during SEI growth from redox molecule reduction at the electrode | SEI interface to reduction at the SEI | electrolyte interface, and good estimates for the transport coefficients of both electrons and redox molecules are derived. The approach presented here is applicable to any type of electrochemical interphase and should thus also be of interest for interphase characterization in the fields of electrocatalysis and corrosion.

Robust battery interphases from dilute fluorinated cations

Controlling solid electrolyte interphase (SEI) in batteries is crucial for their efficient cycling. Herein, we demonstrate an approach to enable robust battery performance that does not rely on high fractions of fluorinated species in electrolytes, thus substantially decreasing the environmental footprint and cost of high-energy batteries. In this approach, we use very low fractions of readily reducible fluorinated cations in electrolyte (∼0.1 wt%) and employ electrostatic attraction to generate a substantial population of these cations at the anode surface. As a result, we can form a robust fluorine-rich SEI that allows for dendrite-free deposition of dense Li and stable cycling of Li-metal full cells with high-voltage cathodes. Our approach represents a general strategy for delivering desired chemical species to battery anodes through electrostatic attraction while using minute amounts of additive.

Accurate Internal Monitoring of Batteries by Embedded Piezoelectric Sensors

Advancements in operando techniques have unraveled the complexities of the Electrode Electrolyte Interface (EEI) in electrochemical energy storage devices. However, each technique has inherent limitations, often necessitating adjustments to experimental conditions, which may compromise accuracy. To address this challenge, a novel battery cell design is introduced, integrating piezoelectric sensors with electrochemical analysis for surface-sensitive operando measurements. This innovative approach aims to overcome conventional limitations by accommodating commercial-grade battery electrodes within a single body, alongside a piezoelectric sensor. This enables operando electrogravimetric measurements to be realized, and the electrochemistry of a battery to be more faithfully reproduced at the sensor level. A proof of concept is carried out on both Li-ion (LiFePO4//Graphite) and Na-ion (Na3V2(PO4)2F3//Hard carbon) systems, utilizing commercially available powder electrodes. In both cases, the results reveal rational mass variations at the sensor level during the cycling of commercial electrodes with mass loadings several orders of magnitude higher, while performing Galvanostatic Charge Discharge (GCD) tests across various C-rates. This innovative design opens up possibilities for a broader application of operando electrogravimetry within the battery community, to enhance the understanding of EEI behavior and facilitate the development of more efficient energy storage solutions.

The Influence of Cathode Degradation Products on the Anode Interface in Lithium-Ion Batteries

Degradation of cathode materials in lithium-ion batteries results in the presence of transition metal ions in the electrolyte, and these ions are known to play a major role in capacity fade and cell failure. Yet, while it is known that transition metal ions migrate from the metal oxide cathode and deposit on the graphite anode, their specific influence on anode reactions and structures, such as the solid electrolyte interphase (SEI), is still quite poorly understood due to the complexity in studying this interface in operational cells. In this work we combine operando electrochemical atomic force microscopy (EC-AFM), electrochemical quartz crystal microbalance (EQCM), and electrochemical impedance spectroscopy (EIS) measurements to probe the influence of a range of transition metal ions on the morphological, mechanical, chemical, and electrical properties of the SEI. By adding representative concentrations of Ni2+, Mn2+, and Co2+ ions into a commercially relevant battery electrolyte, the impacts of each on the formation and stability of the anode interface layer is revealed; all are shown to pose a threat to battery performance and stability. Mn2+, in particular, is shown to induce a thick, soft, and unstable SEI layer, which is known to cause severe degradation of batteries, while Co2+ and Ni2+ significantly impact interfacial conductivity. When transition metal ions are mixed, SEI degradation is amplified, suggesting a synergistic effect on the cell stability. Hence, by uncovering the roles these cathode degradation products play in operational batteries, we have provided a foundation upon which strategies to mitigate or eliminate these degradation products can be developed.

Fluorine Chemistry in Rechargeable Batteries: Challenges, Progress, and Perspectives

The renewable energy industry demands rechargeable batteries that can be manufactured at low cost using abundant resources while offering high energy density, good safety, wide operating temperature windows, and long lifespans. Utilizing fluorine chemistry to redesign battery configurations/components is considered a critical strategy to fulfill these requirements due to the natural abundance, robust bond strength, and extraordinary electronegativity of fluorine and the high free energy of fluoride formation, which enables the fluorinated components with cost effectiveness, nonflammability, and intrinsic stability. In particular, fluorinated materials and electrode|electrolyte interphases have been demonstrated to significantly affect reaction reversibility/kinetics, safety, and temperature tolerance of rechargeable batteries. However, the underlining principles governing material design and the mechanistic insights of interphases at the atomic level have been largely overlooked. This review covers a wide range of topics from the exploration of fluorine-containing electrodes, fluorinated electrolyte constituents, and other fluorinated battery components for metal-ion shuttle batteries to constructing fluoride-ion batteries, dual-ion batteries, and other new chemistries. In doing so, this review aims to provide a comprehensive understanding of the structure-property interactions, the features of fluorinated interphases, and cutting-edge techniques for elucidating the role of fluorine chemistry in rechargeable batteries. Further, we present current challenges and promising strategies for employing fluorine chemistry, aiming to advance the electrochemical performance, wide temperature operation, and safety attributes of rechargeable batteries.

Operando Investigation of Solid Electrolyte Interphase Formation, Dynamic Evolution, and Degradation During Lithium Plating/Stripping

The solid electrolyte interphase (SEI) dictates the stability and cycling performance of highly reactive battery electrodes. Characterization of the thin, dynamic, and environmentally sensitive nature of the SEI presents a formidable challenge, which calls for the use of microscopic, time-resolved operando methods. Herein, we employ scanning electrochemical microscopy (SECM) to directly probe the heterogeneous surface electronic conductivity during SEI formation and degradation. Complementary operando electrochemical quartz crystal microbalance (EQCM) and ex situ X-ray photoelectron spectroscopy (XPS) provide comprehensive analysis of the dynamic size and compositional evolution of the complex interfacial microstructure. We have found that stable anode passivation occurs at potentials of 0.5 V vs Li/Li+, even in cases where anion decomposition and interphase formation occur above 1.0 V. We investigated the bidirectional relationship between the SEI and lithium plating-stripping, finding that plating-stripping ruptures the SEI. The current efficiency of this reaction is correlated to the anodic stability of the SEI, highlighting the interdependent relationship between the two. We anticipate this work will provide critical insights on the rational design of stable and effective SEI layers for safe, fast-charging, and long-lifetime lithium metal batteries.

Influence of Lithium Diffusion into Copper Current Collectors on Lithium Electrodeposition in Anode-Free Lithium-Metal Batteries

The development of "anode-free" lithium-metal batteries with high energy densities is, at present, mainly limited by the poor control of the nucleation of lithium directly on the copper current collector, especially in conventional carbonate electrolytes. It is therefore essential to improve the understanding of the lithium nucleation process and its interactions with the copper substrate. In this study, it is shown that diffusion of lithium into the copper substrate, most likely via the grain boundaries, can significantly influence the nucleation process. Such diffusion makes it more difficult to obtain a great number of homogeneously distributed lithium nuclei on the copper surface and thus leads to inhomogeneous electrodeposition. It is, however, demonstrated that the nucleation of lithium on copper is significantly improved if an initial chemical prelithiation of the copper surface is performed. This prelithiation saturates the copper surface with lithium and hence decreases the influence of lithium diffusion via the grain boundaries. In this way, the lithium nucleation can be made to take place more homogenously, especially when a short potentiostatic nucleation pulse that can generate a large number of nuclei on the surface of the copper substrate is applied.

Dynamic Molecular Investigation of the Solid-Electrolyte Interphase of an Anode-Free Lithium Metal Battery Using In Situ Liquid SIMS and Cryo-TEM

We use in situ liquid secondary ion mass spectroscopy, cryogenic transmission electron microscopy, and density functional theory calculation to delineate the molecular process in the formation of the solid-electrolyte interphase (SEI) layer under the dynamic operating conditions. We discover that the onset potential for SEI layer formation and the thickness of the SEI show dependence on the solvation shell structure. On a Cu film anode, the SEI is noticed to start to form at around 2.0 V (nominal cell voltage) with a final thickness of about 40-50 nm in the 1.0 M LiPF6/EC-DMC electrolyte, while for the case of 1.0 M LiFSI/DME, the SEI starts to form at around 1.5 V with a final thickness of about 20 nm. Our observations clearly indicate the inner and outer SEI layer formation and dissipation upon charging and discharging, implying a continued evolution of electrolyte structure with extended cycling.

Probing the Mechanically Stable Solid Electrolyte Interphase and the Implications in Design Strategies

The inevitable volume expansion of secondary battery anodes during cycling imposes forces on the solid electrolyte interphase (SEI). The battery performance is closely related to the capability of SEI to maintain intact under the cyclic loading conditions, which basically boils down to the mechanical properties of SEI. The volatile and complex nature of SEI as well as its nanoscale thickness and environmental sensitivity make the interpretation of its mechanical behavior many roadblocks. Widely varied approaches are adopted to investigate the mechanical properties of SEI, and diverse opinions are generated. The lack of consensus at both technical and theoretical levels has hindered the development of effective design strategies to maximize the mechanical stability of SEIs. Here, the essential and desirable mechanical properties of SEI, the available mechanical characterization methods, and important issues meriting attention for higher test accuracy are outlined. Previous attempts to optimize battery performance by tuning SEI mechanical properties are also scrutinized, inconsistencies in these efforts are elucidated, and the underlying causes are explored. Finally, a set of research protocols is proposed to accelerate the achievement of superior battery cycling performance by improving the mechanical stability of SEI.

Evolution of the Dynamic Solid Electrolyte Interphase in Mg Electrolytes for Rechargeable Mg-Ion Batteries

Formation and evolution of the microscopic solid electrolyte interphase (SEI) at the Mg electrolyte/electrode interface are less reported and need to be completely understood to overcome the compatibility challenges at the Mg anode-electrolyte. In this paper, SEI evolution at the Mg electrolyte/electrode interface is investigated via an in situ electrochemical quartz crystal microbalance with dissipation mode (EQCM-D), electrochemical impedance spectroscopy (EIS), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), and Fourier transform infrared spectrometry (FTIR). Results reveal remarkably different interfacial evolutions for the two Mg electrolyte systems that are studied, a non-halogen Mg(TFSI)₂ electrolyte in THF with DMA as a cosolvent (nhMg-DMA electrolyte) versus a halogen-containing all-phenyl complex (APC) electrolyte. The nhMg-DMA electrolyte reports a minuscule SEI formation along with a significant Coulomb loss at the initial electrochemical cycles owing to an electrolyte reconstruction process. Interestingly, a more complicated SEI growth is observed at the later electrochemical cycles accompanied by an improved reversible Mg deposition attributed to the newly formed coordination environment with Mg²⁺ and ultimately leads to a more homogeneous morphology for the electrochemically deposited Mg⁰, which maintains a MgF₂-rich interface. In contrast, the APC electrolyte shows an extensive SEI formation at its initial electrochemical cycles, followed by a SEI dissolution process upon electrochemical cycling accompanied by an improved coulombic efficiency with trace water and chloride species removed. Therefore, it leads to SEI stabilization progression upon further electrochemical cycling, resulting in elevated charge transport kinetics and superior purity of the electrochemically deposited Mg⁰. These outstanding findings augment the understanding of the SEI formation and evolution on the Mg interface and pave a way for a future Mg-ion battery design.

Revealing solid electrolyte interphase formation through interface-sensitive Operando X-ray absorption spectroscopy

The solid electrolyte interphase (SEI) that forms on Li-ion battery anodes is critical to their long-term performance, however observing SEI formation processes at the buried electrode-electrolyte interface is a significant challenge. Here we show that operando soft X-ray absorption spectroscopy in total electron yield mode can resolve the chemical evolution of the SEI during electrochemical formation in a Li-ion cell, with nm-scale interface sensitivity. O, F, and Si K-edge spectra, acquired as a function of potential, reveal when key reactions occur on high-capacity amorphous Si anodes cycled with and without fluoroethylene carbonate (FEC). The sequential formation of inorganic (LiF) and organic (-(C=O)O-) components is thereby revealed, and results in layering of the SEI. The addition of FEC leads to SEI formation at higher potentials which is implicated in the rapid healing of SEI defects and the improved cycling performance observed. Operando TEY-XAS offers new insights into the formation mechanisms of electrode-electrolyte interphases and their stability for a wide variety of electrode materials and electrolyte formulations.

Solid electrolyte interphase (SEI) formation on Li-ion battery anodes is critical for long-term performance. Here, the authors use operando soft X-ray absorption spectroscopy in total electron yield mode to resolve the chemical evolution of the SEI during electrochemical formation on silicon anodes.

Dissecting the Solid Polymer Electrolyte-Electrode Interface in the Vicinity of Electrochemical Stability Limits

Proper understanding of solid polymer electrolyte-electrode interfacial layer formation and its implications on cell performance is a vital step toward realizing practical solid-state lithium-ion batteries. At the same time, probing these solid-solid interfaces is extremely challenging as they are buried within the electrochemical system, thereby efficiently evading exposure to surface-sensitive spectroscopic methods. Still, the probing of interfacial degradation layers is essential to render an accurate picture of the behavior of these materials in the vicinity of their electrochemical stability limits and to complement the incomplete picture gained from electrochemical assessments. In this work, we address this issue in conjunction with presenting a thorough evaluation of the electrochemical stability window of the solid polymer electrolyte poly(ε-caprolactone):lithium bis(trifluoromethanesulfonyl)imide (PCL:LiTFSI). According to staircase voltammetry, the electrochemical stability window of the polyester-based electrolyte was found to span from 1.5 to 4 V vs Li⁺/Li. Subsequent decomposition of PCL:LiTFSI outside of the stability window led to a buildup of carbonaceous, lithium oxide and salt-derived species at the electrode-electrolyte interface, identified using postmortem spectroscopic analysis. These species formed highly resistive interphase layers, acting as major bottlenecks in the SPE system. Resistance and thickness values of these layers at different potentials were then estimated based on the impedance response between a lithium iron phosphate reference electrode and carbon-coated working electrodes. Importantly, it is only through the combination of electrochemistry and photoelectron spectroscopy that the full extent of the electrochemical performance at the limits of electrochemical stability can be reliably and accurately determined.

Aqueous Multivalent Charge Storage Mechanism in Aromatic Diamine-Based Organic Electrodes

Rechargeable batteries employing aqueous electrolytes are more reliable and cost-effective as well as possess high ionic conductivity compared to the flammable organic electrolyte solutions. Among these types of batteries, aqueous batteries with multivalent ions attract more attention in terms of providing high energy density. Herein, electrochemical behavior of an organic electrode based on a highly aromatic polymer containing 2,3-diaminophenazine repeating unit, namely poly(ortho-phenylenediamine) (PoPD), is tested in two different multivalent ions (Zn²⁺ and Al³⁺) containing aqueous electrolytes, that is, in zinc sulfate and aluminum chloride solutions. PoPD is synthesized via electropolymerization, and its ion transport and storage mechanism are comprehensively investigated by structural and electrochemical analyses. The electrochemical quartz crystal microbalance, time-dependent Fourier transform infrared, and electrochemical impedance spectroscopy analyses as well as ex situ X-ray diffraction observations established that along with the Zn²⁺ or Al³⁺ ions, reversible proton insertion/extraction also takes place. Contrary to the most of the organic electrodes that requires the use of conductive carbon additives, the electrodeposited PoPD electrode is intrinsically electrically conductive enough, resulting in a binder and additive free electrode assembly. In addition, its discharge products do not dissolve in aqueous medium. As a whole, the resulting PoPD electrode delivers excellent rate performances with prolonged cycle life in which discharge capacities of ∼110 mAh g^-1 in 0.25 M AlCl₃ and ∼93 mAh g^-1 in 1 M ZnSO₄ aqueous electrolyte after 1000 cycles at a current density of 5C have been achieved.

Scanning Electrochemical Cell Microscopy Platform with Local Electrochemical Impedance Spectroscopy

Local electrochemical impedance spectroscopy (LEIS) has been a versatile technology for characterizing local complex electrochemical processes at heterogeneous surfaces. However, further application of this technology is restricted by its poor spatial resolution. In this work, high-spatial-resolution LEIS was realized using scanning electrochemical cell microscopy (SECCM-LEIS). The spatial resolution was proven to be ∼180 nm based on experimental and simulation results. The stability and reliability of this platform were further verified by long-term tests and Kramers-Kronig transformation. With this technology, larger electric double-layer capacitance (C_dl) and smaller interfacial resistance (R_t) were observed at the edges of N-doped reduced graphene oxide, as compared to those at the planar surface, which may be due to the high electrochemical activity at the edges. The established SECCM-LEIS provides a high-spatial approach for study of the interfacial electrochemical behavior of materials, which can contribute to the elucidation of the electrochemical reaction mechanism at material surfaces.

Overview of Quartz Crystal Microbalance Behavior Analysis and Measurement

Quartz Crystal Microbalance (QCM) is one of the many acoustic transducers. It is the most popular and widely used acoustic transducer for sensor applications. It has found wide applications in chemical and biosensing fields owing to its high sensitivity, robustness, small sized-design, and ease of integration with electronic measurement systems. However, it is necessary to coat QCM with a sensing film. Without coating materials, its selectivity and sensitivity are not obtained. At present, this is not an issue, mainly due to the advancement of oscillator circuits and dedicated measurement circuits. Since a new researcher may seek to understand QCM sensors, we provide an overview of QCM from its fundamental knowledge. Then, we explain some of the recent QCM applications both in gas-phase and liquid-phase. Next, the theory of QCM is introduced by using piezoelectric stress equations and the Mason equivalent circuit, which explains how the QCM behavior is obtained. Then, the conventional equations that govern QCM behaviors in terms of resonant frequency and resistance are described. We show the behavior of QCM with a viscous film based on the acoustic wave equation and Mason equivalent circuit. Then, we present various existing QCM electronic measurement methods. Furthermore, we describe the experiment on QCM with viscous loading and its interpretation based on the Mason equivalent circuit. Lastly, we review some theoretical models to describe QCM behavior with various models.

Toward an Understanding of SEI Formation and Lithium Plating on Copper in Anode-Free Batteries

"Anode-free" batteries present a significant advantage due to their substantially higher energy density and ease of assembly in a dry air atmosphere. However, issues involving lithium dendrite growth and low cycling Coulombic efficiencies during operation remain to be solved. Solid electrolyte interphase (SEI) formation on Cu and its effect on Li plating are studied here to understand the interplay between the Cu current collector surface chemistry and plated Li morphology. A native interphase layer (N-SEI) on the Cu current collector was observed with solid-state nuclear magnetic resonance spectroscopy (ssNMR) and electrochemical impedance spectroscopy (EIS). Cyclic voltammetry (CV) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) studies showed that the nature of the N-SEI is affected by the copper interface composition. An X-ray photoelectron spectroscopy (XPS) study identified a relationship between the applied voltage and SEI composition. In addition to the typical SEI components, the SEI contains copper oxides (Cu _x O) and their reduction reaction products. Parasitic electrochemical reactions were observed via in situ NMR measurements of Li plating efficiency. Scanning electron microscopy (SEM) studies revealed a correlation between the morphology of the plated Li and the SEI homogeneity, current density, and rest time in the electrolyte before plating. Via ToF-SIMS, we found that the preferential plating of Li on Cu is governed by the distribution of ionically conducting rather than electronic conducting compounds. The results together suggest strategies for mitigating dendrite formation by current collector pretreatment and controlled SEI formation during the first battery charge.

Modeling cyclic voltammetry during solid electrolyte interphase formation: Baseline scenario of a dynamically evolving tunneling barrier resulting from a homogeneous single-phase insulating film

The solid electrolyte interphase (SEI) is an insulating film on anode surfaces in Li-ion batteries, which forms via the reaction of Li ions with reduced electrolyte species. The SEI leads to a reduction in the electrochemical current in heterogeneous electrochemical redox reactions at the electrode/electrolyte interface. Hence, the growth of the SEI is, in principle, self-limited. Toward our ultimate goal of an improved understanding of SEI formation, we develop a baseline quantitative model within Butler-Volmer electrode kinetics, which describes the cyclic voltammetry (CV) of a flat macroelectrode during SEI growth. Here, the SEI building up electrochemically during CV forms a homogeneous single-phase electronically insulating thin film due to the corresponding current. The model is based on a dynamically evolving electron tunneling barrier with increasing film thickness. Our objective is to provide a framework, which allows for both the qualitative, intuitive interpretation of characteristic features of CV measurements and the quantitative extraction of physicochemical parameters via model fitting. We also discuss the limitations of the baseline model and give a brief outlook for improvements. Finally, comparisons to exemplary CVs from the literature relevant to Li-ion battery science are presented.

Effect of Fluoroethylene Carbonate Additives on the Initial Formation of the Solid Electrolyte Interphase on an Oxygen-Functionalized Graphitic Anode in Lithium-Ion Batteries

The formation of a solid electrolyte interphase (SEI) at the electrode/electrolyte interface substantially affects the stability and lifetime of lithium-ion batteries (LIBs). One of the methods to improve the lifetime of LIBs is by the inclusion of additive molecules to stabilize the SEI. To understand the effect of additive molecules on the initial stage of SEI formation, we compare the decomposition and oligomerization reactions of a fluoroethylene carbonate (FEC) additive on a range of oxygen-functionalized graphitic anodes to those of an ethylene carbonate (EC) organic electrolyte. A series of density functional theory (DFT) calculations augmented by ab initio molecular dynamics (AIMD) simulations reveal that EC decomposition on an oxygen-functionalized graphitic (112̅0) edge facet through a nucleophilic attack on an ethylene carbon site (C_E) of an EC molecule (S2 mechanism) is spontaneous during the initial charging process of LIBs. However, decomposition of EC through a nucleophilic attack on a carbonyl carbon (C_C) site (S1 mechanism) results in alkoxide species regeneration that is responsible for continual oligomerization along the graphitic surface. In contrast, FEC prefers to decompose through an S1 pathway, which does not promote alkoxide regeneration. Including FEC as an additive is thus able to suppress alkoxide regeneration and results in a smaller and thinner SEI layer that is more flexible toward lithium intercalation during the charging/discharging process. In addition, we find that the presence of different oxygen functional groups at the surface of graphite dictates the oligomerization products and the LiF formation mechanism in the SEI.

Making Advanced Electrogravimetry as an Affordable Analytical Tool for Battery Interface Characterization

Numerous sophisticated diagnostic techniques have been designed to monitor electrode-electrolyte interfaces that mainly govern the lifetime and reliability of batteries. Among them is the electrochemical quartz crystal microbalance (EQCM) that offers valuable insights of the interfaces once the required conditions of the deposited film in terms of viscoelastic and hydrodynamic properties are fulfilled. Herein, we propose a friendly protocol that includes the elaboration of a homogeneous deposit by spray coating followed by QCM measurements at multiharmonic frequencies to ensure the film flatness and rigidity for collecting meaningful data. Moreover, for easiness of the measurements, we report the design of a versatile and airtight EQCM cell setup that can be used either with aqueous or non-aqueous electrolytes. We also present, using a model battery material, LiFePO₄, how dual frequency and motional resistance monitoring during electrochemical cycling can be used as a well-suitable indicator for achieving reliable and reproducible electrogravimetric measurements. We demonstrate through this study the essential role of the solvent assisting lithium-ion insertion at the LiFePO₄ interface with a major outcome of solvent-dependent interfacial behavior. Namely, in aqueous media, we prove a near-surface desolvation of lithium ions from their water solvation shell as compared with organic molecules. This spatial dissimilarity leads to a smoother Li-ion transport across the LFP-H₂O interface, hence accounting for the difference in rate capability of LFP in the respective electrolytes. Overall, we hope our analytical insights on interfacial mechanisms will help in gaining a wider acceptance of EQCM-based methods from the battery community.

Influence of Water Contamination on the SEI Formation in Li-Ion Cells: An Operando EQCM-D Study

The interphase formation on carbon (C) anodes in LiPF₆/EC + DEC Li-ion battery electrolyte is analyzed by combining operando electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) with in situ online electrochemical mass spectrometry (OEMS). EQCM-D enables unique insights into the anode solid electrolyte interphase (SEI) mass/thickness, its viscoelastic properties, and changes of electrolyte viscosity during the initial formation cycles. The interphase in the pure electrolyte is relatively soft (G'_SEI ≈ 0.2 MPa, η_SEI ≈ 10 mPa s) and changes its viscoelastic properties dynamically as a function of the electrode potential. With increasing electrolyte water content, the SEI becomes thicker and much more rigid. Doubly labeled D₂¹⁸O is added to the electrolyte in order to precisely track the reaction pathway of water at the anode by OEMS. In the first cycle between 2.6 and 1.7 V versus Li⁺/Li, water is reduced, and hydroxide ions initiate an autocatalytic hydrolysis of EC. With large amounts of water initially present in the electrolyte, most of the formed CO₂ gas is scavenged by reactions with hydroxide and alkoxide ions, forming a thick, rigid, and Li₂CO₃-rich early interphase on the C anode. This layer alleviates the following electrolyte decomposition processes and slows the reduction of EC < 1 V versus Li⁺/Li.

Electrochemically Induced Changes in TiO2 and Carbon Films Studied with QCM-D

Semi-solid fluid electrode-based battery (SSFB) and supercapacitor technologies are seen as very promising candidates for grid energy storage. However, unlike for traditional batteries, their performance can quickly get compromised by the formation of a poorly conducting solid-electrolyte interphase (SEI) on the particle surfaces. In this work we examine SEI film formation in relation to typical electrochemical conditions by combining cyclic voltammetry (CV) with quartz crystal microbalance dissipation monitoring (QCM-D). Sputtered layers of typical SSFB materials like titanium dioxide (TiO₂) and carbon, immersed in alkyl carbonate solvents, are cycled to potentials of relevance to both traditional and flow systems. Mass changes due to lithium intercalation and SEI formation are distinguished by measuring the electrochemical current simultaneously with the damped mechanical oscillation. Both the TiO₂ and amorphous carbon layers show a significant irreversible mass increase on continued exposure to (even mildly) reducing electrochemical conditions. Studying the small changes within individual charge-discharge cycles, TiO₂ shows mass oscillations, indicating a partial reversibility due to lithium intercalation (not found for carbon). Viscoelastic signatures in the megahertz frequency regime confirm the formation and growth of a soft layer, again with oscillations for TiO₂ but not for carbon. All these observations are consistent with irreversible SEI formation for both materials and reversible Li intercalation for TiO₂. Our results highlight the need for careful choices of the materials chemistry and a sensitive electrochemical screening for fluid electrode systems.

Elucidating the Origin of the Electrochemical Capacity in a Proton-Based Battery HxIrO4 via Advanced Electrogravimetry

Recently, because of sustainability issues dictated by societal demands, more importance has been given to aqueous systems and especially to proton-based batteries. However, the mechanisms behind the processes leading to energy storage in such systems are still not elucidated. Under this scope, our study is structured on the selection of a model electrode material, the protonic phase H_xIrO₄, and the scrutiny of the interfacial processes through suitable analytical tools. Herein, we employed operando electrochemical quartz crystal microbalance (EQCM) combined with electrochemical impedance spectroscopy (EIS) to provide new insights into the mechanism intervening at the electrode-electrolyte interface. First, we demonstrated that not only the surface or near surface but the whole particle participates in the cationic redox process. Second, we proved that the contribution of the proton on the overall potential window together with the incorporation of water at low potentials solely. This is explained by the fact that water molecules permit a further insertion of protons in the material by shielding the proton charge but at the expense of the proton kinetic properties. These findings shed a new light on the importance of water molecules in the ion-insertion mechanisms taking place at the electrode-electrolyte interface of aqueous proton-based batteries. Overall, the present results further highlight the richness of the EQCM-based methods for the battery field in offering mechanistic insights that are crucial for the understanding of interfaces and charge storage in insertion compounds.