Electrochemical Quartz Crystal Microbalance Research Articles

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.

Thermo-Electro-Responsive Redox-Copolymers for Amplified Solvation, Morphological Control, and Tunable Ion Interactions.

Electro-responsive metallopolymers can possess highly specific and tunable ion interactions, and have been explored extensively as electrode materials for ion-selective separations. However, there remains a limited understanding of the role of solvation and polymer-solvent interactions in ion binding and selectivity. The elucidation of ion-solvent-polymer interactions, in combination with the rational design of tailored copolymers, can lead to new pathways for modulating ion selectivity and morphology. Here, we present thermo-electrochemical-responsive copolymer electrodes of N-isopropylacrylamide (NIPAM) and ferrocenylpropyl methacrylamide (FPMAm) with tunable polymer-solvent interactions through copolymer ratio, temperature, and electrochemical potential. As compared to the homopolymer PFPMAm, the P(NIPAM0.9-co-FPMAm0.1) copolymer ingressed 2 orders of magnitude more water molecules per doping ion when electrochemically oxidized, as measured by electrochemical quartz crystal microbalance. P(NIPAM0.9-co-FPMAm0.1) exhibited a unique thermo-electrochemically reversible response and swelled up to 83% after electrochemical oxidation, then deswelled below its original size upon raising the temperature from 20 to 40 °C, as measured through spectroscopic ellipsometry. Reduced P(NIPAM0.9-co-FPMAm0.1) had an inhomogeneous depth profile, with layers of low solvation. In contrast, oxidized P(NIPAM0.9-co-FPMAm0.1) displayed a more uniform and highly solvated depth profile, as measured through neutron reflectometry. P(NIPAM0.9-co-FPMAm0.1) and PFPMAm showed almost a fivefold difference in selectivity for target ions, evidence that polymer hydrophilicity plays a key role in determining ion partitioning between solvent and the polymer interface. Our work points to new macromolecular engineering strategies for tuning ion selectivity in stimuli-responsive materials.

Epitope imprinting of Sip D protein of Salmonella Typhi bacteria through multiple monomers approach

Typhoid fever is an endemic disease in India. An early stage detection of this disease will reduce the burden on healthcare industry and prevent many fatalities. Here a MIP sensor is fabricated to detect Salmonella typhi bacteria through its antigenic epitope sequence identified through immunoinformatic tools. Epitope imprinting through multiple monomers was utilized to deposit the sensing matrix on gold coated electrochemical quartz crystal microbalance (EQCM) electrode. Methacrylic acid, 2-methacryloyloxyethyl phosphorylcholine and benzyl methacrylate were used as monomers while ethylene glycol dimethacrylate as crosslinker and azobisisobutyronitrile was used as initiator. The analytical performance of MIP-EQCM sensor was studied by electrochemical as well as piezoelectric measurements. Selectivity of sensor was examined thorugh mismatched peptide sequences and certain plasma proteins also. The detection limit was found to be 0.27 ng mL−1 by DPV and 0.87 ng mL−1 by piezoelectric measurements. The sensor exhibited high selectivity towards imprinted epitope. Additionally, the sensor demonstrated the specific and selective detection in real sample of typhoid infected patients. Thus, the proposed MIP-EQCM sensor could be proposed as diagnostic tool for typhoid fever.

An electrochemical technique for sensing uranium adsorption and desorption

Uranium is a toxic, heavy metal that can pose elevated health risks if leached into the environment. Uranium mobility is dependent on many factors, including speciation, solution pH, ligands in solution, and presence of surfaces. Surface adsorption is one phenomenon that inhibits uranium mobility in the environment and is studied as a naturally occurring phenomenon as well an intentional tool for environmental remediation. This work presents and validates a potentiometric, electrochemical technique for sensing uranium adsorption on and desorption from an electrochemically active surface solely through changes of the electrode potential. This novel electrochemical technique presents a new lens to study adsorption that complements external techniques (e.g., spectroscopy). Indication of adsorption and desorption via the electrochemical technique are gravimetrically validated using an electrochemical quartz crystal microbalance. This work contributes a unique, complementary technique that corroborates the adsorption of uranium on an electrode surface.

2D Germanane‐MXene Heterostructures for Cations Intercalation in Energy Storage Applications

AbstractHeterostructures offer an exceptional possibility of combining individual 2D materials into a new material having altered properties compared to the parent materials. Germanane (GeH) is a 2D material with many favorable properties for energy storage and catalysis, however, its performance is hindered by its low electrical conductivity. To address the low electrochemical performance of GeH, a heterostructure of GeH and Ti3C2Tx is fabricated. The Ti3C2TX is a layered material belonging to the family of MXenes. The resulting heterostructure (GeMXene) at a defined mass ratio of GeH and Ti3C2Tx shows superior capacitive performance that surpasses that of both pristine materials. The effect of the size of cations and anions for intercalation into GeMXene in different aqueous salt solutions is studied. GeMXene allows only cation intercalation, which is evidenced by the gravimetric electrochemical quartz crystal microbalance (EQCM) technique. The capacitive performance of the GeMXene is compared in neutral, acidic, and alkaline electrolytes to determine the best electrochemical performance. This unleashes the potential use of GeMXene heterostructure in different electrolytes for supercapacitors and batteries. This work will pave the way to explore the heterostructures of other 2D materials such as novel MXenes and functionalized germanane for highly energy‐storage efficient systems, and beyond.

Zinc‐Ion and Proton as Joint Charge Carriers of S‐MoO2 for High‐Capacity Aqueous Zinc‐Ion Batteries

AbstractMolybdenum‐based materials are regarded as promising candidates for aqueous zinc‐ion batteries (AZIBs) because of their multi‐valences and high specific capacity. However, the structural instability of MoS2 and sluggish reaction kinetics of MoO2 restrict their further development in AZIBs. Herein, the MoO2 with in situ inherited sulfur atoms (S‐MoO2) is successfully prepared by heat treatment of MoS2 in static air. Benefiting from the synergistic effects of inherited S atoms and introduced O vacancies, the S‐MoO2 exhibits higher specific/rate capacities (236 mAh g−1 at 0.1 A g−1 and 105 mAh g−1 at 5.0 A g−1) and better cycling stability (81% capacity retention after 2000 cycles at 2.0 A g−1) than the perfect MoO2. More significantly, the in situ electrochemical quartz crystal microbalance (EQCM) and ex situ spectroscopic techniques comprehensively elucidate that zinc‐ion and proton as joint charge carriers insert/extract into/from S‐MoO2 through the (011) and (020) planes with high reversibility. This work provides a guideline for understanding the multi‐ion storage mechanism of cathode materials for high‐capacity AZIBs.

Electrodeposited Poly(5-Amino-2-Naphthalenesulfonic Acid-co-o-Aminophenol) as the Electrode Material for Flexible Supercapacitor.

Copolymers of 5-amino-2-naphthalenesulfonic acid (ANS) and o-aminophenol (oAP) are electropolymerized on carbon cloth substrate from aqueous solutions, and the electropolymerization process is investigated using electrochemical quartz-crystal microbalance. The surface of the copolymer (PANS-co-oAP) appears rough and is capable to store charge as the battery-type electrode in 1m H2 SO4 (102.9mAhg-1 at 1Ag-1 ) or in 1m ZnSO4 (79.75mAhg-1 at 1Ag-1 ) aqueous solutions. Compared with PANS and PoAP, the high specific capacity of the PANS-co-oAP is originated from the increased number of electrochemically active sites and increased diffusion rates of ions. Evidence of amino/imino and hydroxyl/carbonyl groups redox processes and cation insertion and extraction are given by ex situ X-ray photoelectron spectroscopy. When used as the electrode material in the flexible solid-state supercapacitors, the specific capacitance is at 37.9Fg-1 which does not significantly alter with the bending angle. The flexible solid-state supercapacitor shows a specific energy of 5.4Whkg-1 and a power density of 250.3Wkg-1 at 0.5Ag-1 , and a high capacitance retention (88.2%) after 3000 cycles at 5Ag-1 is achieved.

A catalase enzyme biosensor for hydrogen peroxide at a poly(safranine T)-ternary deep eutectic solvent and carbon nanotube modified electrode

A new electrochemical enzyme biosensor has been developed to monitor hydrogen peroxide. The sensor uses a multiwalled carbon nanotube modified glassy carbon electrode covered by a poly(safranine T) polymer film prepared by potential cycling electropolymerization in ternary deep eutectic solvent (DES), prepared from choline chloride (ChCl), ethylene glycol (EG) plus malonic acid (MA) or lactic acid (LA) with composition optimization. It is shown that this leads to greater polymer growth and improved film properties compared to binary DES; the process was monitored by an electrochemical quartz crystal microbalance. The best ternary DES composition was found to be 16 % ChCl:MA / 84 % ChCl:EG. Electrochemical characterisation was done by cyclic voltammetry and electrochemical impedance spectroscopy, and the morphology of the synthesized polymer films was examined by scanning electron microscopy. Catalase enzyme was immobilized on the modified electrode and the enzyme biosensor used for measurement of hydrogen peroxide, with a very low detection limit of 34 nM. The selectivity was found to be excellent in relation to common interferents and the biosensor showed very good recovery in peroxide measurements in commercial samples.

Comprehensive Study on the Ion-Selective Behavior of MnOx for Electrochemical Deionization.

Manganese oxide is an effective active material in several electrochemical systems, including batteries, supercapacitors, and electrochemical deionization (ECDI). This work conducts a comprehensive study on the ion-selective behavior of MnOx to fulfill the emptiness in the energy and environmental science field. Furthermore, it broadens the promising application of MnOx in the ion-selective ECDI system. We propose a time-dependent multimechanism ion-selective behavior with the following guidelines by utilizing a microfluidic cell and the electrochemical quartz crystal microbalance (EQCM) analysis. (1) Hydrated radius is the most critical factor for ions with the same valence, and MnOx tends to capture cations with a small hydrated radius. (2) The importance of charge density rises when comparing cations with different valences, and MnOx prefers to capture divalent cations with a strong electrostatic attraction at prolonged times. Under this circumstance, ion swapping may occur where divalent cations replace monovalent cations. (3) NH4+ triggers MnOx dissolution, leading to performance and stability decay. The EQCM evidence has directly verified the proposed mechanisms, and these data provide a novel but simple method to judge ion selectivity preference. The overall ion selectivity sequence is Ca2+ > Mg2+ > K+ > NH4+> Na+ > Li+ with the highest selectivity values of βCa//Li and βCa//Na around 3 at the deionization time = 10 min.

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.

Greening Industrially Applied Toxic Cadmium Plating with γ‐Ni2Zn11 Alloy in Deep Eutectic Solvents: Promising Electroplating Efficiency and Chemical Corrosion Resistance

Cadmium is an industrially applied plating metal used in aerospace applications, but electrodeposition of cadmium is toxic, and current production method will soon be banned. Therefore, electrodeposition of alternative alloys in environmentally friendly solutions is an attractive research area. An additive‐free, nonaqueous, deep eutectic solvent (DES) containing Ni and Zn halide salts is developed in a greener environmental method. Gamma‐phase nanocrystalline Ni2Zn11 is coated onto hot‐rolled mild steel via pulse reverse current (PRC) in the developed nonaqueous solvent. The employed DES’ relatively low viscosity and sufficient conductivity enable the simultaneous reduction of Ni2+ and Zn2+ cations on microelectrode. Electrochemical quartz crystal microbalance (eQCM) analysis proves the absence of the commonly reported problem of electrode blocking during electrodeposition, revealing an efficiency of 86.07%. Appropriate PRC parameters are applied for scaled macroelectrode deposition. The plating profile exhibits 20 μm‐thick alloy deposition without cracks around 3 h. Scanning electron microscopy–electron‐dispersive spectroscopy and X‐ray diffraction analyses reveal that 15.5 wt% Ni is in the nanograin phase, and all crystal planes belong to the γ‐Ni2Zn11 phase, which is needed as an alternative to cadmium plating. Hardness and corrosion tests performed on the γ‐Ni2Zn11 coating reveal better hardness and corrosion resistance with supporting morphological evidence.

Efficient formation of poly(ethylene glycol) polymer brushes on gold electrodes via surface-initiated electrochemically mediated ATRP (SI-eATRP)

Electrochemically mediated surface-initiated atom transfer radical polymerization (SI-eATRP) is an interesting grafting from technique used to grow, in a controlled way, polymeric films from a wide range of surfaces, including metal substrates. Aqueous SI-eATRP offers an appealing strategy for the preparation of polymers by using water-soluble monomers, employing an oxidatively stable catalyst complex. The goal of this work is to tailor compact poly-(oligo(ethylene glycol)methacrylate) (p-OEGMA) brushes on gold electrodes by combining the formation of self-assembled monolayers (SAMs) containing a suitable ATRP initiator molecule, and the SI-eATRP polymerization approach. We have investigated the effects of the SAM-initiator nature, the applied potential, and the duration of the polymerization process on the properties of the grafted p-OEGMA-brushes. In addition, we employed electrochemical quartz crystal microbalance measurements to evidence the living character of the polymerization. The final properties of the tailored p-OEGMA-brush were studied by cyclic voltammetry and electrochemical impedance spectroscopy, obtaining information on the electronic and ionic blocking behaviour of these films, under different experimental conditions. Further information on structural conformation, composition, and organization was obtained from different characterization techniques: IRRAS and XPS spectroscopies, contact angle measurements, and SEM and AFM microscopies. These techniques confirmed that an efficient formation of p-OEGMA-brushes via SI-eATRP was achieved. Thus, the designed SI-eATRP system offers an interesting option to functionalize a wide variety of substrates with water-soluble and biocompatible monomers. Such coatings present potential applications in various fields, including biomedical applications.

An Aqueous Electrolyte Regulator for Highly Stable Zinc Anode Under −35 to 65 °C

AbstractThe reversibility and long‐term cycling stability of aqueous zinc‐ion batteries (AZIBs) in a wide temperature range have rarely been explored. Herein, diethylene glycol monoethyl ether (DG) is introduced as an electrolyte additive to enhance Zn performance within a wide temperature range of −35 to 65 °C. Operando synchrotron Fourier transform infrared spectroscopy analysis combined with molecular dynamics simulations reveal that the introduction of DG disrupts the initial hydrogen bonding network of the aqueous electrolyte, restructuring the solvation structure surrounding Zn2+ ions and mitigating water‐induced parasitic reactions. Adding DG reduces the freezing point of the aqueous electrolyte without compromising its incombustibility. Moreover, operando electrochemical quartz crystal microbalance with dissipation monitoring (EQCM‐D) and X‐ray photoelectron spectroscopy demonstrated that the coordinated DG and OTF− undergo reductive decomposition, forming a self‐healing solid electrolyte interphase comprising an inorganic/organic ZnF2‐ZnS, which can effectively suppress the notorious side reactions and guide the uniform Zn deposition. Consequently, the symmetric Zn/Zn cells demonstrate excellent cycling stability for 3500 h under 1 mA cm−2 at 25 °C, and for 1000 h under 1 mA cm−2 at both −35 and 65 °C. Full batteries with a DG‐containing electrolyte exhibit a long lifespan of 5000 cycles at 2 A g−1.

Is Ethanol Essential for the Lithium-Mediated Nitrogen Reduction Reaction?

The lithium-mediated nitrogen reduction reaction (Li-NRR) is a promising method for decentralized ammonia synthesis using renewable energy. An organic electrolyte is utilized to combat the competing hydrogen evolution reaction, and lithium is plated to activate the inert N2 molecule. Ethanol is commonly used as a proton shuttle to provide hydrogen to the activated nitrogen. In this study, we investigate the role of ethanol as a proton shuttle in an electrolyte containing tetrahydrofuran and 0.2 M lithium perchlorate. Particularly designed electrochemical experiments show that ethanol is necessary for a good solid-electrolyte interphase but not for the synthesis of ammonia. In addition, electrochemical quartz crystal microbalance (EQCM) demonstrates that the SEI formation at the onset of lithium plating is of specific importance. Chemical batch synthesis of ammonia combined with real-time mass spectrometry confirms that protons can be shuttled from the anode to the cathode by other species even without ethanol. Moreover, it raises questions regarding the electrochemical nature of Li-NRR. Finally, we discuss electrolyte stability and electrochemical electrode potentials, highlighting the role of ethanol on electrolyte degradation.

Label-free genosensing of dengue serotypes with an electrodeposited reduced graphene oxide-tris(bipyridine)ruthenium(II)

Constructing a label-free electrochemical transducer platform without compromising inherent biocompatibility against specific bioreceptor remains challenging, particularly probing nucleic acid hybridization at electrode interface without external redox-mediator. Here, we show that electrochemically reduced graphene oxide-tris(bipyridine)ruthenium(II) (ErGO-TBR) nanosheets electrodeposited on carbon screen printed electrode can quantify hybridization of clinically important target sequences specific to serotypes of dengue virus (DENV) non-structural 1 (NS1) protein. Different variables including deposition potential, time, and electrolytic composition were optimized for fabrication of label-free transducer platform. Structural and electrochemical properties of ErGO-TBR/SPE were comprehensively elucidated using microscopic and spectroscopic techniques. Electrochemical quartz crystal microbalance (EQCM) analysis reveals the growth of electrodeposited redox-active species on the electrode interface. Surface functional group investigations suggested that TBR deposited on the basal and edges of ErGO substrate via electrostatic and π-π interactions. Functionalization of bio-affinity layer (B) on ErGO-TBR/SPE enables better loading of probe DNA (PDNA) toward specific detection of DENV target DNA (TDNA) with an ultralow detection limit promising for clinical diagnosis. Scalable chronoamperometry-based redox-active surface growth, customizable bioactivation strategy and external mediator-less probing of nucleic acid hybridization make the present system suitable for other translational application in healthcare diagnosis.

Proton is Essential or Not: A Fresh Look on Pseudocapacitive Energy Storage of PANI Composites.

Protonation has been considered essential for the pseudocapacitive energy storage of polyaniline (PANI) for years, as proton doping in PANI chains not only activates electron transport pathways, but also promotes the proceeding of redox reactions. Rarely has the ability for PANI of storing energy without protonation been investigated, and it remains uncertain whether PANI has pseudocapacitive charge storage properties in an alkaline electrolyte. Here, this work first demonstrates the pseudocapacitive energy storage for PANI without protonation using a PANI/graphene composite as a model material in an alkaline electrolyte. Using in situ Raman spectroscopy coupled with electrochemical quartz crystal microbalance (EQCM) measurements, this work determines the formation of -N= group over potential on a PANI chain and demonstrates the direct contribution of OH- in the nonprotonation type of oxidation reactions. This work finds that the PANI/graphene composite in an alkaline electrolyte has excellent cycling stability with a wider operation voltage of 1V as well as a slightly higher specific capacitance than that in an acidic electrolyte. The findings provide a new perspective on pseudocapacitive energy storage of PANI-based composites, which will influence the selection of electrolytes for PANI materials and expand their application in energy storage fields.

Electrodeposition of Silver in Cyanide-Free Solution Containing Pyrimidine Derivative as a Complexing Agent

Pyrimidine derivatives, for example, 2,4-pyrimidinedione (uracil), 1-methyluracil and 5-methyluracil (thymine) can bind with metal ions in alkaline solution. 5-methyluracil (thymine) as complexing agent in electroplating solution has not been investigated due to low solubility in water. The aims of this study were to determine the potential of 5-methyluracil (thymine) as complexing agent for cyanide-free silver electroplating solution. The influence of complexing agent and polyethyleneimine on the silver electrodeposition reaction were studied. Several methods which are electrochemical quartz crystal microbalance (EQCM) and electrochemical impedance spectroscopy (EIS) have been used to study the electrochemical behavior of cyanide-free silver electroplating solution. The surface morphology and brightness of silver deposits obtained from each cyanide-free silver electroplating solution were evaluated. A semi-bright deposit was obtained from thymine and uracil-complexing silver solution while matte deposit from 5,5-dimethylhydantoin-complexing silver solution. A bright silver deposit could be obtained in the presence of polyethyleneimine. The reduction current value decreases as the amount of polyethyleneimine added increases due to inhibition effect on the silver electrodeposition reaction. The results of electrochemical quartz crystal microbalance measurement showed that the addition of polyethyleneimine in thymine-complexing silver solution does not significantly affect the resonance frequency change.

Electrically Controlled Adsorptive Membranes with Tunable Affinity for Selective Chromium (VI) Separation from Water.

Ionic contaminants such as Cr(VI) pose a challenge for water purification using membrane-based processes. However, existing membranes have low permeability and selectivity for Cr(VI). Therefore, in this study, we prepared an electrically controlled adsorptive membrane (ECAM-L) by coating a loose Cl--doped polypyrrole layer on a carbon nanotube substrate, and we evaluated the performance of ECAM-L for Cr(VI) separation from water. We also used electrochemical quartz crystal microbalance measurements and molecular dynamics and density functional theory calculations to investigate the separation mechanisms. The adsorption and desorption of Cr(VI) could be modulated by varying the electrostatic interactions between ECAM-L and Cr(VI) via potential control, enabling the cyclic use of the ECAM-L without additional additives. Consequently, the oxidized ECAM-L showed high Cr(VI) removal performance (<50 μg/L) and treatment capacity (>3500 L/m2) at a high water flux (283 L/m2/h), as well as reusability after the application of a potential. Our study demonstrates an efficient membrane design for water decontamination that can selectively separate Cr(VI) through a short electric stimulus.

Developing Tin Anodes for Aqueous Batteries

Aqueous batteries based on metal anodes present an attractive alternative to lithium-ion for long-duration energy storage due to their low-cost, high-safety, and theoretically high energy density. Still, despite many efforts towards development and commercialization, metal anode systems remain hampered by irreversible hydrogen evolution (HER), detrimental shape change, and passivation during cycling. Recognizing the immediacy required to enable the renewable energy transition, our research is focused on developing alternative anode chemistry based on Tin (Sn) for aqueous batteries.Sn metal has been used in aqueous battery systems as additives or current collectors to mitigate side reactions due to its high overpotential for HER. However, little attention was paid to its potential use as a redox active material in aqueous electrolytes. In this talk, we will discuss our efforts to develop Sn as an aqueous anode with high capacity and cyclability. We will first present our development of pouch cells to enable aqueous cells while minimizing the side reactions from cell components other than the active materials. Preliminary data demonstrate the feasibility of using Sn as anode active materials via a 4-electron process in aqueous systems. To investigate the mechanism and improve the Coulombic efficiency, we perform operando X-ray diffraction (XRD), rotating ring disk electrochemistry (RRDE), electrochemical quartz crystal microbalance (EQCM), and 117Sn nuclear magnetic resonance (Sn-NMR) on the Sn system. Lastly, we will show our improvement based on the mechanistic understanding that enables Sn-based full cells with high Coulombic efficiency, high areal capacity, high utilization rate and long capacity retention.

New Mechanism Insights on Ca-Ion Storage in Small Molecular Crystal

Rechargeable calcium-ion batteries have become intriguing alternatives for grid-scale energy storage devices. However, the strong electrostatic interaction caused by the intrinsic divalent nature of Ca2+ leads to sluggish ion diffusion kinetics, thus resulting in low capacity. Herein, a small molecular crystal organic material is employed as the electrode material for the aqueous calcium-ion batteries. This kind of organic material could not only bypass the sluggish ion diffusion through carbonyl enolization but also avoid electrode inevitable dissolution due to the intermolecular hydrogen bonding, and flexible structure. In addition, the robust crystal structure allows fast Ca-ion diffusion and storage. Electrochemical characterization including electrochemical quartz crystal microbalance combined with first-principles density calculations and in(ex)-situ spectroscopy methods reveals the whole Ca-ion storage mechanism at the atomistic and macroscopic levels. The reversible enolization redox process, diffusion path, and structure change of molecular crystals have been clearly clarified. Consequently, the materials yield a premier rate capability (70.2 mAh g−1 at 5 A g−1), and excellent cycling stability (retention of 80.3% over 1000 cycles at 2 A g−1). This study extends the boundary of molecular crystal systems for battery chemistry to construct high-performance multivalent ion batteries.