Conventional Electrolyte System Research Articles

Push-Pull Electrolyte Design Strategy Enables High-Voltage Low-Temperature Lithium Metal Batteries.

Lithium (Li) metal batteries hold significant promise in elevating energy density, yet their performance at ultralow temperatures remains constrained by sluggish charge transport kinetics and the formation of unstable interphases. In conventional electrolyte systems, lithium ions are tightly locked in the solvation structure, thereby engendering difficulty in the desolvation process and further exacerbating solvent decomposition. Herein, we propose a new push-pull electrolyte design strategy, utilizing molecular electrostatic potential (ESP) screening to identify 2,2-difluoroethyl trifluoromethanesulfonate (DTF) as an optimal cosolvent. Importantly, DTF exhibits a moderate ESP minimum (-21.0 kcal mol-1) to strike a balance between overly strong and overly weak Li ion affinity, which allows the sulfonyl group to effectively pull Li ions without disrupting the anion-rich solvation structure. Simultaneously, the difluoromethyl group, with a high ESP maximum (37.3 kcal mol-1), pushes solvent molecules via competitive hydrogen bonding. This design reconstructs existing solvation structures and expedites Li ion desolvation. Furthermore, fluorinated DTF demonstrates excellent stability at elevated voltage and facilitates the formation of robust inorganic-rich interphases. Impressively, rapid charge transfer kinetics can be achieved employing designed electrolyte, and the LiNi0.8Mn0.1Co0.1O2 (NMC811)||Li cells demonstrate excellent charge-discharge cycling stability with a high capacity exceeding 153 mAh g-1 even at -40 °C, retaining over 93% of initial capacity after 100 cycles under a 4.8 V charging cutoff. This work provides insights into the design of low-temperature electrolytes with a wide electrochemical window, advancing the development of batteries for extreme conditions.

Inclusion of Anion Additives in the Inner Solvation Shell to Regulate the Composition of Solid Electrolyte Interphase

AbstractAn innovative approach to electrolyte engineering in carbonate electrolytes is introduced by incorporating high donor number dual anion additives into the conventional electrolyte system (1 M NaPF6 EC:PC). The active engagement of anions in the primary solvation shell effectively hinders the reduction of solvent molecules by reducing the Lowest Unoccupied Molecular Orbital (LUMO) of Na+‐solvent‐anion complex as compared to the LUMO of pure solvents or Na+‐solvent complex. The participation of anions leads to the formation of a thinner and an inorganic‐rich Solid Electrolyte Interphase on the hard carbon anode enhancing Initial Coulombic Efficiency and significantly improving its kinetics. Moreover, the system with dual anion additives exhibits oxidative stability up to 4.5 V, effectively mitigating the undesired side reactions at high voltage operation of the layered sodium nickel manganese oxide cathode. The addition of dual anion additives proves instrumental in suppressing structural degradation and transition metal dissolution during the long cycling performance of the layered oxide cathode in a sodium‐ion full cell. The synergistic effects of this dual anion additive added electrolyte on both the anode and the cathode ultimately ensured prolonged cycling of the sodium‐ion full cell. The electrolyte engineering approach outlined in this study opens the door to advancing next‐generation high‐voltage sodium batteries.

Boosting adsorption and dissociation kinetics of magnesium-chloride ion pair via bismuth/indium-based artificial interface

The formation of passivating interphases on Mg-metal anode is impeding the wide employment of rechargeable Mg-metal batteries with less safety and cost concerns. Despite engineering artificial interlayers offers an efficient solution, it still falls short of expectation because of the compromised Mg plating/stripping performances in conventional electrolyte systems. In this work, a unique bismuth/indium-based artificial interface is introduced, which enables an exceptional reversibility (lowest nucleation overpotential of 189 mV than that (583 mV) of pristine Mg anode), high rate capability (a stable charge and discharge operation at 5.0 mA cm−2 without short circuit) and good temperaturetolerance (being cyclable over 10 °C and 50 °C) for the first time. This bismuth/indium-based artificial interface is constructed by facile displacement reactions between Bi(CF3SO3)3/In(CF3SO3)3 and Mg. The dominant interphases, including Bi, In and BiOx species, can be controllably tailored via adjusting the relative ratio and concentration of Bi(CF3SO3)3/In(CF3SO3)3. Theoretical calculation results accompanied by electrochemical experiments indicate that kinetically favorable adsorption and dissociation processes of Mg-Cl ion pair on this interface, affording good magnesiophilic property and fast charge transfer kinetics, are responsible for the improved Mg-metal anode properties.

Indium Alloy as Anodes for Rechargeable Calcium Ion Batteries at Room Temperature

Rechargeable multivalent ion batteries are poised to deliver the necessary breakthroughs in energy density and stability required for extended-range electric vehicles and large-scale grid energy storage. Calcium ion batteries are of interest owing to their high theoretical energy densities, natural abundance, and safety, yet are hindered by calcium’s strong interactions with non-aqueous electrolyte solutions and a general lack of suitable intercalation hosts. In order to realize the high voltage and theoretical capacity of a calcium-ion battery, replacement of the Ca metal anode with new electrode materials that can operate reversibly in conventional electrolyte systems is necessary, beginning with fundamental studies to identify novel intercalation or alloy-type hosts which can provide high-Ca content while avoiding considerable volume expansion and capacity loss. In this work, research into a novel indium anode as a host for Ca2+ ions in conventional electrolytes at room temperature is presented.

Reversible Magnesium Metal Cycling in Additive-Free Simple Salt Electrolytes Enabled by Spontaneous Chemical Activation.

Rechargeable magnesium (Mg) batteries can offer higher volumetric energy densities and be safer than their conventional counterparts, lithium-ion batteries. However, their practical implementation is impeded due to the passivation of the Mg metal anode or the severe corrosion of the cell parts in conventional electrolyte systems. Here, we present a chemical activation strategy to facilitate the Mg deposition/stripping process in additive-free simple salt electrolytes. By exploiting the simple immersion-triggered spontaneous chemical reaction between reactive organic halides and Mg metal, the activated Mg anode exhibited an overpotential below 0.2 V and a Coulombic efficiency as high as 99.5% in a Mg(TFSI)2 electrolyte. Comprehensive analyses reveal simultaneous evolution of morphology and interphasial chemistry during the activation process, through which stable Mg cycling over 990 cycles was attained. Our activation strategy enabled the efficient cycling of Mg full-cell candidates using commercially available electrolytes, thereby offering possibilities of building practical Mg batteries.

(Digital Presentation) Free Solvent Molecules in the Electrolyte Leading to Severe Safety Concern of Ni-Rich Li-Ion Batteries

The safety of Li-ion batteries is one of the most important factors, if not the most, determining their practical applications. We have found that free carbonate-based solvent molecules in the hybrid electrolyte system can cause severe safety concerns. The 18650 battery cells using the 50% varied-alkyl chain piperidinium-based TFSI result in an immediate and aggressive explosion after applying the impact test (UN38.3) compared with the cell using the conventional electrolyte, having a mild explosion after applying the impact force with 16 s-delayed time. Furthermore, the greater concentration of gases especially C2H4 and CO in the hybrid electrolyte system compared with the conventional electrolyte system was observed by online in situ DEMS, which is evidence for the aggressive explosion of the cell using the hybrid electrolyte. In additionally, the classical MD investigation is suggested that hybrid electrolytes have a high AGG concentration of Li+ and TFSI−, producing a high concentration of the isolated EC or free solvent, which could generate more gases in the fully charged battery cell. Mixing ionic liquids with a carbonate-based solvent as the co-solvent at a fixed salt concentration of 1 M LiPF6 can lead to free carbonate-based molecules causing poor charge storage performance and safety concerns. Keywords: Ni-rich Li-ion bateriesIonic liquids; Safety; Battery explosion; Mismatch electronic properties; 1865 cylindrical cells

Concentrated LiFSI-Ethylene Carbonate Electrolytes and Their Compatibility with High-Capacity and High-Voltage Electrodes.

The unusual physical and chemical properties of electrolytes with excessive salt contents have resulted in rising interest in highly concentrated electrolytes, especially for their application in batteries. Here, we report strikingly good electrochemical performance in terms of conductivity and stability for a binary electrolyte system, consisting of lithium bis(fluorosulfonyl)imide (LiFSI) salt and ethylene carbonate (EC) solvent. The electrolyte is explored for different cell configurations spanning both high-capacity and high-voltage electrodes, which are well known for incompatibilities with conventional electrolyte systems: Li metal, Si/graphite composites, LiNi0.33Mn0.33Co0.33O2 (NMC111), and LiNi0.5Mn1.5O4 (LNMO). As compared to a LiTFSI counterpart as well as a common LP40 electrolyte, it is seen that the LiFSI:EC electrolyte system is superior in Li-metal–Si/graphite cells. Moreover, in the absence of Li metal, it is possible to use highly concentrated electrolytes (e.g., 1:2 salt:solvent molar ratio), and a considerable improvement on the electrochemical performance of NMC111–Si/graphite cells was achieved with the LiFSI:EC 1:2 electrolyte both at the room temperature and elevated temperature (55 °C). Surface characterization with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) showed the presence of thicker surface film formation with the LiFSI-based electrolyte as compared to the reference electrolyte (LP40) for both positive and negative electrodes, indicating better passivation ability of such surface films during extended cycling. Despite displaying good stability with the NMC111 positive electrode, the LiFSI-based electrolyte showed less compatibility with the high-voltage spinel LNMO electrode (∼4.7 V vs Li+/Li).

Ultra-stable flexible Zn-ion capacitor with pseudocapacitive 2D layered niobium oxyphosphides

Multivalent aqueous Zn-ion capacitors (ZICs) are promising next-generation electrochemical energy storage systems (ESSs) owing to distinctive features including good safety characteristics, low costs, and better electrochemical parameters than those of conventional supercapacitors. The key challenge of existing ZICs is their low energy density and limited cycling ability due to carbon cathode materials and conventional electrolyte systems. This paper presents a dual strategy in which the electrode and electrolyte features are engineered to improve the overall electrochemical performance of ZICs. First, the capacitance of the cathode material is improved by engineering reduced graphene oxide (rGO)-incorporating, pseudocapacitive, layered niobium oxyphosphide (NbPO) material; second, the electrochemical stability of the Zn metal anode is improved via an additive to the traditional Zn-electrolyte. The aqueous ZIC with rGO–NbPO cathode and NaClO4 additive electrolyte exhibits the highest capacitance (191.88 F g−1), maximal energy density (56.03 Wh kg−1), and excellent energy efficiency (approximately 50% to 55%). The prepared flexible solid-state rGO–NbPO ZIC has an ultra-long lifespan of over 50,000 cycles with approximately 76.81% capacitance retention (at 4 A g−1) and excellent mechanical tractability. The results provide guidance for improving the design of safe aqueous ESSs with high-level efficiency and long-term stability.

Influence of Electrolyte Additives on Lithium Titanate-Based Energy Storage Systems

The development of energy storages has been elevated to support the demands in many electronic-related applications for the present and future. Batteries and supercapacitors are the significant energy storage devices that are well-known due to their characteristic properties such as their remarkable high energy densities and power densities, respectively. According to the batteries, there are many researches on the enhancement of the electrochemical performances of the devices by using several solutions such as focusing on electrode materials. Thus, the lithium titanate (LTO) was introduced in this work. The LTO has been used as the anode material for decades due to their structures providing high Li intercalation potential and negligible volume change during lithium insertion/extraction, thermal stability and cost-effective. Nevertheless, LTO still suffers from low electronic conductivity, lithium-ion diffusion coefficient, and rate capability. One of the solutions to enhance the performance of the LTO electrode in the system is by adding electrolyte additives which can be found in recent researches. Therefore, the influences of various additives in the conventional electrolyte system such as fluoroethylene carbonate (FEC), vinyl ethylene carbonate (VEC), vinylene carbonate (VC), methylene methanedisulfonate (MMDS), glutaric anhydride (GA), and p-toluenesulfonyl isocyanate (PTSI) were investigated and studied in half-cell using LTO electrodes in this work.

Hybrid gel polymer electrolyte based on 1-methyl-1-Propylpyrrolidinium Bis(Trifluoromethanesulfonyl) imide for flexible and shape-variant lithium secondary batteries

Lithium ion conducting polymer electrolytes with broad electrochemical stability, good mechanical strength, high thermal stability, and easy processability are necessary for all-solid-state and shape-variant lithium secondary batteries. Hybrid gel polymer electrolytes incorporating an ionic liquid have been attracting attention for application in solid-state lithium secondary batteries owing to their superior thermal properties compared to conventional electrolyte systems. In this study, a variety of polymer electrolytes based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP), lithium bis(trifluoromethanesulfonyl) imide (LiTFSI), and 1-methyl-1-propylpyrrolidinium bis(trifluoromethanesulfonyl) imide (PMPyrrTFSI) are prepared, and an in-depth study of their composition dependence and electrical properties is conducted to develop the optimum composition. The composition dependent ionic conductivity of the polymer electrolyte increases with increasing LiTFSI and PMPyrrTFSI and reaches a maximum value of 6.93 × 10−4 S cm−1 at room temperature (25 °C) when the polymer electrolyte contains 30 wt% LiTFSI and 60 wt% PMPyrrTFSI. In addition, the optimized gel polymer electrolytes consisting of PVdF-HFP/LiTFSI/PMPyrrTFSI (70/30/60 by weight, i.e., 70PVdF-HFP/30LiTFSI/60PMPyrrTFSI) look transparent and exhibit high mechanical stability and excellent thermal stability up to 420 °C. Finally, the lithium iron phosphate (LiFePO4)/lithium metal solid-state cells coupled with the optimized gel polymer electrolyte are prepared, and their discharge characteristics are studied. The 70PVdF-HFP/30LiTFSI/60PMPyrrTFSI based solid-state cell delivered a maximum discharge capacity of 151 mAh g−1 at room temperature with a good rate capability and cycling performance.

Influence of lithium difluorophosphate additive on the high voltage LiNi0.8Co0.1Mn0.1O2/graphite battery

Lithium-ion batteries (LIBs) possessing high energy densities are driven by the growing demands of electric vehicles (EVs) and hybrid electric vehicles (HEVs). One of the most effective strategies to improve the energy density of LIBs is to enlarge the charge cut-off voltage via a lithium salt additive for the conventional electrolyte system. Herein, lithium difluorophosphate (LIDFP) is employed to optimize and reconstruct the composition of the structure and interface for both cathode and anode, which can effectively restrain the oxidation decomposition of electrolyte as well as refrain the dissolve out of transition metals. The LiNi0.8Co0.1Mn0.1O2 (LNCM811)/graphite pouch cell with 1 wt% LIDFP in electrolyte delivers a discharge capacity retention of 91.3% at a high voltage of 4.4 V over 100 cycles, which is higher than the 82.0% of that without LIDFP additive. Additionally, the remaining capacity of LNCM811/C battery with 1 wt% LIDFP additive which is left at 60 °C for 14 days is 85.2%, and the recovery capacity is 93.3%. The LIDFP-containing electrolyte demonstrates a great application future for the LiBs operating under the high-voltage condition and high-temperature storage performance.

Redox-Active Functional Electrolyte for High-Performance Seawater Batteries.

Rechargeable seawater batteries have gained recognition as key sustainable electrochemical systems by employing the near-infinite and eco-friendly catholyte seawater. However, their practical applications have been limited owing to the low chemical and electrochemical stability of the anode component. Herein, a stability-secured approach was developed by using sodium-biphenyl-dimethoxyethane solution as a redox-active functional anolyte for high-performance seawater batteries. This anolyte system shows high electrochemical stability, superior cycle performance, and cost-effectiveness over conventional electrolyte systems.

Non-flammable organic liquid electrolyte for high-safety and high-energy density Li-ion batteries

With increased energy density of rechargeable lithium-ion batteries for powering smart phones and electric vehicles and for their long range use, battery safety becomes more important than ever. This aspect motivated us to develop non-flammable liquid electrolyte that removes the risk of battery fire and explosion, which is urgently needed. Battery energy density and performance however should not be sacrificed to achieve just the safety. Here we report for the first time a rational design of non-flammable carbonate-based organic liquid electrolyte to satisfy safety, energy density and performance simultaneously. Our novel electrolyte, composed of 1 M lithium hexafluorophosphate salt and propylene carbonate and fluorinated linear carbonate co-solvents, at unmeasurable flash point does not fire representing non-flammable safe batteries but permits high-voltage stability to enable high-voltage charge of lithium-rich layered oxide cathode up to 5.0 V, high-energy density of 856 Wh per kg of cathode active mass and stable charge-discharge cycling performance of full-cell with graphite anode, in contrast to rapid performance fade of flammable conventional electrolyte system. The discovery of non-flammable carbonate-based organic liquid electrolyte opens up a new avenue to high-safety and high energy-density lithium-ion batteries for electric vehicles and advanced energy-storage applications.

Renewed Interest in Sulfone-Based Electrolytes for 5V Li-Ion Batteries

Harnessing the enhanced energy and power metrics offered by “5V-class” lithium-ion electrode chemistries relies on development of more robust electrolytes with expanded voltage and temperature stability windows. As a part of this effort, electrochemists have explored the promise of sulfones, highlighting their oxidative stability (generally >5.0V vs. Li/Li+ compared to ~4.3V for conventional electrolytes), safety, and compatibility with high voltage systems. Unfortunately, due to their high melting points, viscosity, and inability to independently form a stable solid electrolyte interphase (SEI), practical implementation of sulfones has generally been limited to use as minority co-solvents in more conventional electrolyte systems. In this work we explore the effects of solvation, concentration, and salt chemistry on sulfone-based electrolyte performance. In graphite anode half-cells, we characterize the formation of a stable SEI which enables full utilization of the graphite electrode for the first time in an electrolyte with a sulfone as the primary solvent. In high voltage full cells, the oxidative stability of the system is confirmed by voltammetry and galvanostatic cycling measurements. Anode and cathode surface chemistry is explored extensively with XPS, and coupled with computational calculations, a mechanism for the formation and operation of both electrode/electrolyte interphases is proposed. These results suggest sulfones warrant continued consideration as a system for enabling safe, high performance, next generation Li-ion batteries.

Correlation Between Solvation Structure and Ion-Conductive Properties of Highly-Concentrated Poly(ethylene carbonate)-Based Electrolytes

Ion-conductive solid polymer electrolytes (SPEs) are of great interest as key materials to realizing safe and flexible rechargeable batteries, particularly Li batteries. Since the discovery of ionic conductions in poly(ethylene oxide) (PEO)/metal salt complexes, numerous efforts have been made to improve their electrochemical properties. However, it is difficult to increase Li-ion conductivity that is the product of overall conductivity and Li transference number (t +). This drawback mainly arises from excessively tight complex structures between polar polymers and Li ion. To overcome the drawback and create all-solid-state batteries based on SPEs, we specifically focus on poly(alkylene carbonate)s derived from CO2/epoxide copolymerization as SPE polymer hosts [1,2]. Consequently, we have found an unusual ion-conductive behavior of poly(ethylene carbonate) (PEC)-based electrolyte, including high ionic conductivity of highly-concentrated electrolytes and extremely high t +, which in some cases is above 0.8 [3,4]. In this PEC-based electrolyte system, ionic conductivity continues to increase with increasing salt concentration, which is a behavior scarcely shown in conventional electrolyte systems. In addition, we have already confirmed operations of prototype Li/LiFePO4 cells based on PEC-based electrolytes combined with plasticizing and reinforcing additives [5,6]. In this context, we considered that elucidation of salt solvation structure is a key to better understanding the origin of the particular ion-conductive behavior, and improving the performance further. In the present study, we adopted spectroscopic studies including FT-IR, Raman spectroscopy, and solid-state 7Li magic angle spinning (MAS) NMR for PEC/lithium bis(fluorosulfonyl)imide (LiFSI) electrolytes with varying concentration. The FT-IR and Raman spectroscopy indicate that most FSI ions interact with more than one Li ion at high concentration region, which is referred to as aggregates, while C=O groups dissociates the salt. The 7Li MAS NMR suggested that a relatively weak contact between Li ions and FSI ions withdraws Li ions from a strong interaction with C=O groups. We conclude that this moderate complex structure, where C=O groups allows the highly concentrated ions to remain amorphous and mobile, leads to reasonable conductivity with extremely high t +. We believe that this finding will provide a new insight for developing SPEs having both high conductivity and high t + simultaneously.

Effect of monocationic ionic liquids as electrolyte additives on the electrochemical and thermal properties of Li-ion batteries

The conventional electrolyte system has been compared with the ionic liquid (IL) additive containing electrolyte system at room temperature as well as elevated temperature. In this work, two types of monocationic ILs such as 1-butyl-3-methylpyrrolidinium hexafluorophosphate (Pyr IL) and 1-ethyl-3-methylimidazolium hexafluorophosphate (IMI IL) are added as an additive at two different weight ratios in 1.15 M LiPF6 (EC/EMC = 3/7 v/v) electrolyte solution, the structural, electrochemical and thermal characteristics of LiNi0.80Co0.15Al0.05O2 (NCA)/carbon full-cell in different electrolyte formulations have been reconnoitered. X-ray diffraction (XRD) studies have proved that IL as an electrolyte additive does not alter the structural stability of cathode materials after cycling. Under room temperature, Pyr IL additives at 1 wt% and 3 wt% deliver better cycleability than others, with the retention ratios of 93.62% and 92.8%, respectively. At elevated temperature, only 1 wt% Pyr IL additive is giving stable capacity retention ratio of 80.74%. Ionic conductivity and self-extinguishing time (SET) values are increasing with respect to the amount of additive added to the electrolyte. Thermal studies reveal that 3 wt% Pyr IL is favorable regarding the safety of the battery as it shows shifting of peak to higher temperature of 272.10 °C. Among the IL additives evaluated in this study, addition of 1 wt% Pyr IL is the most desirable additive for achieving the best cycling performance as well as thermal behavior of Li-ion batteries.

Highly Efficient and Long-Lifetime Ozone Water Production System Realized Using a Felt Separator

The efficiency of ozone production using a conventional polymer electrolyte system with Pt electrodes decreased from 15 to 2% after operating for 100 h. The degradation of the system was caused by Pt black particles concentrated on the cathode side of the Nafion 117 membrane. The particles reacted with the membrane and produced holes on its cathode-side surface. To prevent the decomposition of the membrane, a felt separator made of polyethylene terephthalate (PET) was inserted between the Nafion 117 membrane and the cathode Pt electrode, and dilute NaCl or solutions of 0.01–1 M were circulated through the cathode to compensate for the low conductivity of the PET separator. As a result, a high current efficiency of more than 20% and a lifetime longer than 150 h have been achieved in the electrolysis system. The PET felt separator captured Pt black particles and prevented the membrane from decomposition. However, the lifetime of ozone in anode water was much shorter than that obtained using a conventional electrolyte system. A similar lifetime to that in the conventional system was obtained by substituting the PET felt separator with a quartz filter.

The Surface Film Formed on a Lithium Metal Electrode in a New Imide Electrolyte, Lithium Bis(perfluoroethylsulfonylimide) [ LiN ( C 2 F 5 SO 2 ) 2

A newly developed imide electrolyte salt, (LiBETI) was found to give very uniform, thin, and stable surface films on a lithium metal electrode in the propylene carbonate (PC) solution. LiBETI/PC was studied and compared to determine its ability to form such a stable surface film, with conventional electrolyte systems such as , , and (LiTFSI/PC). The surface film formed in LiBETI/PC system was a hemispherical, and the composition of the film consisted mainly of LiF, which is similar to that in a system. Quartz crystal microbalance (QCM) and cyclic voltammetry (after the tenth cycle) indicated that the surface film formed in LiBETI/PC (ca. 50 nm) was thinner than those in (ca. 90 nm), LiTFSI/PC (ca. 140 nm), or (ca. 255 nm). The variation of the resonance resistance (ΔR) obtained from in situ CV/QCM measurement, which has been demonstrated to be a good measure of the surface roughness, also suggested that LiBETI/PC system gave a compact and smooth surface topology during lithium deposition‐dissolution cycles. Impedance spectroscopy together with preliminary cycling tests showed that the LiBETI/PC system provides the highest cycling efficiency and improved cycleability among existing electrolyte salt systems in rechargeable battery systems employing lithium metal anodes. © 1999 The Electrochemical Society. All rights reserved.

Isotachophoretic separation of Fe(II) and Fe(III) by using 1,10-phenanthroline as a complex-forming agent

A mixture of Fe(II) and Fe(III) phenanthroline (phen) chelate complexes was isotachophoretically separated and analyzed by using a conventional electrolyte system for cation analysis (leading electrolyte: 20 m M ammonia solution, acetic acid buffer, pH 4.8; terminating electrolyte: β-alanine, pH 3.6). The migration order was Fe(III) chelate, Fe(II) chelate, phen. The recovery was 100% both for Fe(II) and Fe(III) according to PIXE analysis of the isotachophoretic (ITP) fractions. The sample mixture used (pH 2) was stable for a few days at room temperature. The sensitivity of Fe(III) chelate was 60% of that of Fe(II) chelate. This is because the ionic charge of Fe(II) chelate (+2) was larger than that of Fe(III) chelate, in which the coordination number of phen was 1.8±0.1 due to the addition of at least one pH buffer anion. The ionic charge of Fe(III) chelate was estimated as +1.4 by simulation of the concentration and effective mobility at the ITP steady state.