Quantification of the Mass and Viscoelasticity of Interfacial Films on Tin Anodes Using EQCM-D

Revealing the Structural Evolution of Electrode/Electrolyte Interphase Formation during Magnesium Plating and Stripping with operando EQCM-D

Rechargeable magnesium batteries could provide future energy storage systems with high energy density. One remaining challenge is the development of electrolytes compatible with the negative Mg electrode, enabling uniform plating and stripping with high Coulombic efficiencies. Often improvements are hindered by a lack of fundamental understanding of processes occurring during cycling, as well as the existence and structure of a formed interphase layer at the electrode/electrolyte interface. Here, a magnesium model electrolyte based on magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2 ) and MgCl2 with a borohydride as additive, dissolved in dimethoxyethane (DME), was used to investigate the initial galvanostatic plating and stripping cycles operando using electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D). We show that side reactions lead to the formation of an interphase of irreversibly deposited Mg during the initial cycles. EQCM-D based hydrodynamic spectroscopy reveals the growth of a porous layer during Mg stripping. After the first cycles, the interphase layer is in a dynamic equilibrium between the formation of the layer and its dissolution, resulting in a stable thickness upon further cycling. This study provides operando information of the interphase formation, its changes during cycling and the dynamic behavior, helping to rationally develop future electrolytes and electrode/electrolyte interfaces and interphases.

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.

Exploring the Promise of Multifunctional "Zintl-Phase-Forming" Electrolytes for Si-Based Full Cells

Li-M-Si ternary Zintl phases have gained attention recently due to their high structural stability, which can improve the cycling stability compared to a bulk Si electrode. Adding multivalent cation salts (such as Mg²⁺ and Ca²⁺) in the electrolyte was proven to be a simple way to form Li-M-Si ternary phases in situ in Si-based Li-ion cells. To explore the promise of Zintl-phase-forming electrolytes, we systematically investigated their application in pouch cells via electrochemical and multiscale postmortem analysis. The introduction of multivalent cations, such as Mg²⁺, during charging can form Li_xM_ySi ternary phases. They can stabilize Si anions and reduce side reactions with electrolyte, improving the bulk stability. More importantly, Mg²⁺ and Ca²⁺ incorporate into interfacial side reactions and generate inorganic-rich solid-electrolyte interphase, thus enhancing the interfacial stability. Therefore, the full cells with Zintl-phase-forming electrolytes achieve higher capacity retentions at the C/3 rate after 100 cycles, compared to a baseline electrolyte. Additionally, strategies for mitigating the electrode-level fractures of Si were evaluated to make the best use of Zintl-phase-forming electrolytes. This work highlights the significance of synergistic impact of multifunctional additives to stabilize both bulk and interface chemistry in high-energy Si anode materials for Li-ion batteries.

Electrocatalytic synthesis of adipic acid coupled with H2 production enhanced by a ligand modification strategy

Adipic acid is an important building block of polymers, and is commercially produced by thermo-catalytic oxidation of ketone-alcohol oil (a mixture of cyclohexanol and cyclohexanone). However, this process heavily relies on the use of corrosive nitric acid while releases nitrous oxide as a potent greenhouse gas. Herein, we report an electrocatalytic strategy for the oxidation of cyclohexanone to adipic acid coupled with H₂ production over a nickel hydroxide (Ni(OH)₂) catalyst modified with sodium dodecyl sulfonate (SDS). The intercalated SDS facilitates the enrichment of immiscible cyclohexanone in aqueous medium, thus achieving 3.6-fold greater productivity of adipic acid and higher faradaic efficiency (FE) compared with pure Ni(OH)₂ (93% versus 56%). This strategy is demonstrated effective for a variety of immiscible aldehydes and ketones in aqueous solution. Furthermore, we design a realistic two-electrode flow electrolyzer for electrooxidation of cyclohexanone coupling with H₂ production, attaining adipic acid productivity of 4.7 mmol coupled with H₂ productivity of 8.0 L at 0.8 A (corresponding to 30 mA cm⁻²) in 24 h.

Adipic acid is an important building block of polymers, although its production relies on harmful reagents. Here, authors examined surfactant-modified nickel hydroxide for adipic acid electrosynthesis coupled with hydrogen gas evolution.

Effect of Electrode Thickness on Quality Factor of Ring Electrode QCM Sensor

As a key type of sensor, the quartz crystal microbalance (QCM) has been widely used in many research areas. Recently, the ring electrode QCM sensor (R-QCM) with more uniform mass sensitivity has been reported. However, the quality factor (Q-factor) of the R-QCM has still not been studied, especially regarding the effect of electrode thickness on the Q-factor. Considering that the Q-factor is one of crucial parameter to the QCM and it is closely related to the output frequency stability of the QCM, we study the effect of different electrode thicknesses on the Q-factor of the R-QCM in this paper. On the other hand, we clarify the relationship between the electrode thickness and the Q-factor of the R-QCM. The measurement results show that the average Q-factor increases with increases in the thickness of ring electrodes generally; however, the resonance frequency of the QCM resonator decreases with increases in the thickness. The low half-bandwidth (2Γ < 1630 Hz) of the R-QCM shows that the frequency performance is good. Additionally, the R-QCM has a higher Q-factor (Q > 6000), which indicates that it has a higher frequency stability and can be applied in many research areas.

From bulk to interface: electrochemical phenomena and mechanism studies in batteries via electrochemical quartz crystal microbalance

Understanding the bulk and interfacial behaviors during the operation of batteries (e.g., Li-ion, Na-ion, Li-O₂ batteries, etc.) is of great significance for the continuing improvement of the performance. Electrochemical quartz crystal microbalance (EQCM) is a powerful tool to this end, as it enables in situ investigation into various phenomena, including ion insertion/deinsertion within electrodes, solid nucleation from the electrolyte, interphasial formation/evolution and solid-liquid coordination. As such, EQCM analysis helps to decipher the underlying mechanisms both in the bulk and at the interface. This tutorial review will present the recent progress in mechanistic studies of batteries achieved by the EQCM technology. The fundamentals and unique capability of EQCM are first discussed and compared with other techniques, and then the combination of EQCM with other in situ techniques is also covered. In addition, the recent studies utilizing EQCM technologies in revealing phenomena and mechanisms of various batteries are reviewed. Perspectives regarding the future application of EQCM in battery studies are given at the end.

In Situ Probing of Mass Exchange at the Solid Electrolyte Interphase in Aqueous and Nonaqueous Zn Electrolytes with EQCM-D

Multivalent chemistry provides intriguing benefits of developing beyond lithium ion energy storage technologies and has drawn extensive research interests. Among the multivalent candidates, metallic zinc anodes offer an attractive high volumetric capacity at a low cost for designing the secondary ion batteries. However, the interfacial mass exchange at the Zn electrolyte/anode boundary is complicated. The least understood solid electrolyte interphase (SEI) occurs simultaneously with the reversible metal deposition, and its dynamic progression is unclear and difficult to capture. One major challenge to investigate such a dynamic interface is the lack of in situ analytical methods that offer direct mass transport information to reproduce the realistic battery operating conditions in an air-sensitive, nonaqueous electrolyte environment with a high iR drop. Work reported here reveals an in-depth analysis of the complex and dynamic SEI at the Zn electrolyte/electrode interface utilizing a multiharmonic quartz crystal microbalance with a dissipation method combined with the spectroscopic analysis. Key differences are observed for the SEI formation in the nonaqueous Zn(TFSI)₂ electrolyte in contrast to the aqueous ZnCl₂ electrolyte for reversible Zn deposition. A large disproportional loss of coulombs relative to the gravimetric mass change is prominently observed at the initial electrochemical cycles in the nonaqueous Zn electrolyte, and results suggest an in situ formation of an ionically permeable SEI layer that is compositionally featured with a rich content of organic S and N components. Further overtone-dependent dissipation analysis implies the changes in viscoelasticity at the electrode interface during the early SEI formation in the nonaqueous Zn(TFSI)₂ electrolyte.

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.

The Study of the Binder Poly(acrylic acid) and Its Role in Concomitant Solid-Electrolyte Interphase Formation on Si Anodes

We use neutron reflectometry to study how the polymeric binder, poly(acrylic acid) (PAA), affects the in situ formation and chemical composition of the solid-electrolyte interphase (SEI) formation on a silicon anode at various states of charge. The reflectivity is correlated with electrochemical quartz crystal microbalance to better understand the viscoelastic effects of the polymer during cycling. The use of model thin films allows for a well-controlled interface between the amorphous Si surface and the PAA layer. If the PAA perfectly coats the Si surface and standard processing conditions are used, the binder will prevent the lithiation of the anode. The PAA suppresses the growth of a new layer formed at early states of discharge (open circuit voltage to 0.8 V vs Li/Li⁺), protecting the surface of the anode. At 0.15 V, the SEI layer underneath the PAA changes in chemical composition as indicated by an increase in the scattering length density and thickness as the layer incorporates components from the electrolyte, most likely the salt. At lithiated and delithiated states, the SEI layer changes in chemical composition and grows in thickness with delithiation and shrinks during lithiation.

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.

Nanoporous carbon for electrochemical capacitive energy storage

This review summarizes the recent advances of nanoporous carbon materials in the application of EDLCs, including a better understanding of the charge storage mechanisms by combining the advanced techniques and simulations methods.

Microgravimetric and Spectroscopic Analysis of Solid-Electrolyte Interphase Formation in Presence of Additives

Electrochemical quartz crystal microbalance (EQCM) with damping monitoring is applied for real-time analysis of solid-electrolyte interphase (SEI) formation in diphenyl octyl phosphate (DPOP) and vinylene carbonate (VC) modified electrolytes. Fast SEI formation is observed for the DPOP containing electrolyte, whereas slow growth is detected in VC-modified and reference electrolytes. QCM measurements in a dry state show considerable reduction of the mass quantity for DPOP and reference samples and minor mass decrease for the SEI layer formed in the presence of VC. The results indicate that VC enhances SEI stability, whereas the addition of DPOP or no additive results in incorporation of loosely attached species, leadubg to SEI instability. Resonance frequency damping, Δw, and dissipation factor, D, are used for analyzing mechanical properties of the SEI layers. The apparent increase of Δw and D during SEI formation in presence of DPOP suggests a pronounced viscoelasticity of the layer. QCM results are compared with surface morphology and chemical composition, revealing excellent agreement of the applied characterization approaches.

Real-time insight into the doping mechanism of redox-active organic radical polymers

Organic radical polymers for batteries represent some of the fastest-charging redox active materials available. Electron transport and charge storage must be accompanied by ion transport and doping for charge neutrality, but the nature of this process in organic radical polymers is not well understood. This is difficult to intuitively predict because the pendant radical group distinguishes organic radical polymers from conjugated, charged or polar polymers. Here we show for the first time a quantitative view of in situ ion transport and doping in organic radical polymers during the redox process. Two modes dominate: doping by lithium ion expulsion and doping by anion uptake. The dominance of one mode over the other is controlled by anion type, electrolyte concentration and timescale. These results apply in any scenario in which electrolyte is in contact with a non-conjugated redox active polymer and present a means of quantifying doping effects.

In Situ Real-Time Mechanical and Morphological Characterization of Electrodes for Electrochemical Energy Storage and Conversion by Electrochemical Quartz Crystal Microbalance with Dissipation Monitoring

Quartz crystal microbalance with dissipation monitoring (QCM-D) generates surface-acoustic waves in quartz crystal plates that can effectively probe the structure of films, particulate composite electrodes of complex geometry rigidly attached to quartz crystal surface on one side and contacting a gas or liquid phase on the other side. The output QCM-D characteristics consist of the resonance frequency (MHz frequency range) and resonance bandwidth measured with extra-ordinary precision of a few tenths of Hz. Depending on the electrodes stiffness/softness, QCM-D operates either as a gravimetric or complex mechanical probe of their intrinsic structure. For at least 20 years, QCM-D has been successfully used in biochemical and environmental science and technology for its ability to probe the structure of soft solvated interfaces. Practical battery and supercapacitor electrodes appear frequently as porous solids with their stiffness changing due to interactions with electrolyte solutions or as a result of ion intercalation/adsorption and long-term electrode cycling. Unfortunately, most QCM measurements with electrochemical systems are carried out based on a single (fundamental) frequency and, as such, provided that the resonance bandwidth remains constant, are suitable for only gravimetric sensing. The multiharmonic measurements have been carried out mainly on conducting/redox polymer films rather than on typical composite battery/supercapacitor electrodes. Here, we summarize the most recent publications devoted to the development of electrochemical QCM-D (EQCM-D)-based methodology for systematic characterization of mechanical properties of operating battery/supercapacitor electrodes. By varying the electrodes' composition and structure (thin/thick layers, small/large particles, binders with different mechanical properties, etc.), nature of the electrolyte solutions and charging/cycling conditions, the method is shown to be operated in different application modes. A variety of useful electrode-material properties are assessed noninvasively, in situ, and in real time frames of ion intercalation into the electrodes of interest. A detailed algorithm for the mechanical characterization of battery electrodes kept in the gas phase and immersed into the electrolyte solutions has been developed for fast recognition of stiff and viscoelastic materials in terms of EQCM-D signatures treated by the hydrodynamic and viscoelastic models. Working examples of the use of in situ hydrodynamic spectroscopy to characterize stiff rough/porous solids of complex geometry and viscoelastic characterization of soft electrodes are presented. The most demonstrative example relates to the formation of solid electrolyte interphase on Li₄Ti₅O₁₂ electrodes in the presence of different electrolyte solutions and additives: only a few cycles (an experiment during ∼30 min) were required for screening the electrolyte systems for their ability to form high-quality surface films in experimental EQCM-D cells as compared to 100 cycles (200 h cycling) in conventional coin cells. Thin/small-mass electrodes required for the EQCM-D analysis enable accelerated cycling tests for ultrafast mechanical characterization of these electrodes in different electrolyte solutions. Hence, this methodology can be easily implemented as a highly effective in situ analytical tool in the field of energy storage and conversion.

In situ analytical techniques for battery interface analysis

Interface is a key to high performance and safe lithium-ion batteries or lithium batteries.

In situ real-time gravimetric and viscoelastic probing of surface films formation on lithium batteries electrodes

It is generally accepted that solid–electrolyte interphase formed on the surface of lithium-battery electrodes play a key role in controlling their cycling performance. Although a large variety of surface-sensitive spectroscopies and microscopies were used for their characterization, the focus was on surface species nature rather than on the mechanical properties of the surface films. Here we report a highly sensitive method of gravimetric and viscoelastic probing of the formation of surface films on composite Li₄Ti₅O₁₂ electrode coupled with lithium ions intercalation into this electrode. Electrochemical quartz-crystal microbalance with dissipation monitoring measurements were performed with LiTFSI, LiPF₆, and LiPF₆ + 2% vinylene carbonate solutions from which structural parameters of the surface films were returned by fitting to a multilayer viscoelastic model. Only a few fast cycles are required to qualify surface films on Li₄Ti₅O₁₂ anode improving in the sequence LiPF₆ < LiPF₆ + 2% vinylene carbonate << LiTFSI.

The solid-electrolyte interphase formed on Li-battery electrodes strongly affects their cycling performance, however the mechanical properties of the surface films are not well-known. Here the authors report a sensitive gravimetric/viscoelastic method to probe surface film formation on composite electrodes, coupled with Li-ion intercalation.