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Fluoride ions are used in battery electrolytes in fluoride shuttle batteries. Since organic solvents are used in battery electrolytes, there is a growing demand to develop appropriate methods for quantifying fluoride ion concentration in organic solvents. In this study, a fluoride ion-selective electrode (ISE) for organic solutions is proposed as an electrode of the second kind. A Ag|AgF electrode was made via the anodization of a silver wire in propylene carbonate (PC) containing dissolved fluoride ions. The resultant electrode exhibits a stable linear response of the open circuit potential to the logarithm of the fluoride ion concentration in PC solutions over a range of 10-4-10-2 mol dm-3. The lower and upper limits of the linear response were interpreted in terms of the solubility and the formation of a silver fluoride complex. The use of this electrode of the second kind is suitable for the analysis of fluoride ions in organic solutions and is a promising concept for the development of ISEs for the detection of ions in organic solutions under highly restrictive conditions.

This study attempted to stabilize the nanosurface of LiNiO2 (LNO) electrodes by varying the electrolyte concentration, significantly influencing its initial electrochemical behaviors for use in aqueous lithium-ion batteries. The charge/discharge capacities, reversibility, and cyclability of LNO were improved during initial cycles with an increase in the concentration of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). As determined by the galvanostatic intermittent titration technique, the superior diffusivity of Li+ ions in the LNO electrode is also obtained in the concentrated electrolyte. Nanoscale observation of the LNO surface revealed that its morphology is maintained relatively well in the concentrated electrolyte while it is destroyed in dilute electrolytes after the initial electrochemical cycles. These results are considered to be attributable to the variation of the interface condition in the electrical double layer with an increase in the electrolyte concentration, thus stabilizing the nanosurface of LNO by suppressing the dissolution of Ni ions from the surface. Additionally, in situ X-ray diffraction analysis demonstrated that LNO shows more stable phase transitions and volume changes as the electrolyte concentration increases, indicating that its structural changes in bulk can be directly related to the state of the nanosurface, which has a positive impact on the initial electrochemical behaviors in this system.

Carbon nanosphere (CNS) electrodes are the candidate of sodium-ion battery (SIB) negative electrodes with small internal resistances due to their small particle sizes. Electrochemical properties of low-crystallized CNS electrodes in dilute and concentrated sodium bis(trifluoromethanesulfonyl) amide/ethylene carbonate + dimethyl carbonate (NaTFSA/EC + DMC) were first investigated. From the cyclic voltammograms, both lithium ion and sodium ion can reversibly insert into/from CNSs in all of the electrolytes used here. The cycling stability of CNSs in concentrated electrolytes was better than that in dilute electrolytes for the SIB system. The interfacial charge-transfer resistances at the interface between CNSs and organic electrolytes were evaluated using electrochemical impedance spectroscopy. In the Nyquist plots, the semicircles at the middle-frequency region were assigned to the parallel circuits of charge-transfer resistances and capacitances. The interfacial sodium-ion transfer resistances in concentrated organic electrolytes were much smaller than those in dilute electrolytes, and the rate capability of CNS electrodes in sodium salt-concentrated electrolytes might be better than in dilute electrolytes, suggesting that CNSs with concentrated electrolytes are the candidate of SIB negative electrode materials with high rate capability. The calculated activation energies of interfacial sodium-ion transfer were dependent on electrolyte compositions and similar to those of interfacial lithium-ion transfer.

To improve ionic conductivity, solid-state electrolytes with polarizable anions that weakly interact with mobile ions have received much attention, a recent example being lithium/sodium-rich antiperovskite M3HCh (M = Li, Na; Ch = S, Se, Te). Herein, in order to clarify the role of anions in antiperovskites, the M3FCh family, in which the polarizable H- anion at the octahedral center is replaced by the ionic F- anion, is investigated theoretically and experimentally. We unexpectedly found that the stronger attractive interaction between F- and M+ ions does not slow down the M+ ion diffusion, with the calculated energy barrier being as low as that of M3HCh. This fact suggests that the low-frequency rotational phonon modes of the octahedron of cubic M3FCh (and M3HCh) are intrinsic to facilitate the fast ionic diffusion. A systematic analysis further reveals a correlation between the tolerance factor t and the ionic transport: as t decreases within the cubic phase, the rotational mode becomes softer, resulting in the reduction of the migration energy. The cubic iodine-doped Li3FSe has a room-temperature ionic conductivity of 5 × 10-5 S/cm with a bulk activation energy of 0.18 eV.

This study investigated the effects of the concentrations of LiNO3 aqueous electrolytes on the initial electrochemical properties of LiNiO2 (LNO) electrodes. Electrochemical measurements revealed t...

Graphitic materials cannot be applied for the negative electrode of sodium-ion battery because the reversible capacities of graphite are anomalously small. To promote electrochemical sodium-ion intercalation into graphitic materials, the interfacial sodium-ion transfer reaction at the interface between graphitized carbon nanosphere (GCNS) electrode and organic electrolyte solutions was investigated. The interfacial lithium-ion transfer reaction was also evaluated for the comparison to the sodium-ion transfer. From the cyclic voltammograms, both lithium-ion and sodium-ion can reversibly intercalate into/from GCNS in all of the electrolytes used here. In the Nyquist plots, the semi-circles at the high frequency region derived from the Solid Electrolyte Interphase (SEI) resistance and the semi-circles at the middle frequency region owing to the charge-transfer resistance appeared. The activation energies of both lithium-ion and sodium-ion transfer resistances were measured. The values of activation energies of the interfacial lithium-ion transfer suggested that the interfacial lithium-ion transfer was influenced by the interaction between lithium-ion and solvents, anions or SEI. The activation energies of the interfacial sodium-ion transfer were larger than the expected values of interfacial sodium-ion transfer based on the week Lewis acidity of sodium-ion. In addition, the activation energies of interfacial sodium-ion transfer in dilute FEC-based electrolytes were smaller than those in concentrated electrolytes. The activation energies of the interfacial lithium/sodium-ion transfer of CNS-1100 in FEC-based electrolyte solutions were almost the same as those of CNS-2900, indicating that the mechanism of interfacial charge-transfer reaction seemed to be the same for highly graphitized materials and low-graphitized materials each other. Graphic abstract

The formation of graphite intercalation compounds (GICs) in aqueous solutions has attracted much attention, but reversibility in the formation/deformation of GICs is a challenging issue to construct highly safe rechargeable batteries. In this study, we used an operando analysis (X-ray diffraction and Raman spectroscopy) to discuss the feasibility of using fluoride-containing polyatomic anions in the formation of GICs in aqueous highly concentrated solutions. We found that the intercalation of anions containing a C₂F₅ moiety (such as [N(SO₂CF₃)(SO₂CF₂CF₃)]⁻ or [N(SO₂CF₂CF₃)₂]⁻) does not occur in the bulk of graphite, but only in the surface region. In addition, anions containing a CF₃ moiety show different behaviors: SO₃CF₃⁻ shows greater reversibility and larger stage-number than N(SO₂CF₃)₂⁻ in the formation of GICs. These results provide design guidelines for the reversible intercalation and de-intercalation of anions and their application as a cathode material in aqueous rechargeable batteries.

This study investigated the fundamental mechanisms of the loss of capacity of LiNiO2 (LNO) electrodes for Li+ insertion/deinsertion with a special focus on the origin of this deterioration in an aqueous system. In situ Raman spectra revealed that the intercalation of H+ ions formed a NiOOHx film at the surface of LNO during the initial electrochemical cycles; this NiOOHx film was also confirmed by X-ray photoelectron spectroscopy and transmission electron microscopy analysis. The formation of an electrochemically inactive spinel-like phase (Ni3O4) at the subsurface was triggered by the absence of Li in the NiOOHx film at the surface. These structural changes of LNO, accelerated by the intercalation of H+ ions, were considered to be the fundamental cause of the greater loss of capacity in the aqueous system.

The performance of the graphite anode of lithium-ion batteries is greatly affected by the solid electrolyte interphase (SEI) generated at the first charge. However, there are few studies on the kinetics of the lithium-ion intercalation/de-intercalation reaction in graphite to investigate the effect of SEI. In this study, the correlation between the interfacial lithium-ion transfer resistance (Rct) and the double layer capacitance (Cdl) of graphite composite electrodes coated with various SEIs was investigated. It was found that the value of 1/RctCdl was different for each SEI, that is, the frequency (or rate) of intercalation and de-intercalation of lithium ions into graphite was different for each SEI. The activation energy of Rct was almost the same for all the electrolyte solutions. These results indicate that the pre-exponential factor of the Arrhenius equation governing the rate of interfacial ion transfer in a practical graphite anode is dependent on the nature of SEI.

Electrocatalysts with high activity for the oxygen evolution reaction (OER) play a pivotal role in various electrochemical devices such as water electrolyzers and metal–air batteries. Electronic in...

In electrochemical reactions involving protons or hydroxide ions, local pH in the vicinity of a working electrode dynamically changes. In this study, we introduce the measurements of the local pH i...

This study attempted to stabilize the nanosurface of LiNiO

To improve ionic conductivity, solid-state electrolytes with polarizable anions that weakly interact with mobile ions have received much attention, a recent example being lithium/sodium-rich antiperovskite M

Here, highly concentrated sodium bis(fluorosulfonyl)amide (NaFSA) aqueous electrolytes were examined for the aqueous electrolytes of electric double-layer capacitors with wide potential window. Firstly, fundamental properties of electrolytes were investigated using conductivity measurements and Raman spectroscopy. During cyclic voltammetry of activated carbon electrodes, aluminum corrosion was observed for positive electrode side. After current collectors of positive electrodes were changed to titanium, capacitor reactions were confirmed for both of positive electrode and negative electrode sides in NaFSA aqueous electrolytes. In addition, resistances of activated carbon electrodes in NaFSA aqueous electrolytes were smaller than those in LiTFSA aqueous electrolytes.

Graphite intercalation compounds of bis(fluorosulfonyl)amide (FSA-GICs) are electrochemically synthesized in a highly concentrated aqueous solution. While only water decomposition occurs at the graphite electrode in a dilute aqueous solution (1 mol kg−1 NaFSA), redox peaks clearly appear in a highly concentrated aqueous solution (19 mol kg−1 NaFSA). Under the application of a constant current, the electrode potential reaches 1.7 V (vs. Ag/AgCl), which is far beyond the upper limit of the potential window, in 19 mol kg−1 NaFSA aq., and the formation of FSA-GIC is confirmed by X-ray diffraction patterns. Acceptor-type GICs using organic anions are observed for the first time in highly concentrated aqueous solutions of NaFSA. Keywords: Concentrated aqueous solutions, Graphite intercalation compounds, Water decomposition, Potential window

Sodium-ion batteries cannot employ graphite which is a typical negative electrode material for lithium-ion batteries. This is principally because sodium-ion cannot intercalate deeply into graphite, which has been a mystery for many years. Here, the mechanism of electrochemical sodium-ion intercalation into graphitic materials was investigated by using Raman spectroscopy and X-ray diffraction measurement to solve the question. Low stage sodium graphite intercalation compound (Na-GIC) was formed electrochemically only near the surface of graphite by potential holding above the sodium metal deposition potential. On the other hand, the high stage Na-GIC was formed electrochemically in the bulk at the sodium metal deposition potential. In addition, the apparent diffusion distance and the apparent diffusion coefficient of sodium-ion inside graphite were calculated using chronopotentiograms and potentiostatic intermittent titration technique. As a result, the sodium-ion diffusion inside spherical graphite was not slow enough to explain the limited reactivity. Hence, the limitation of sodium-ion intercalation into graphite might be originated from not the kinetic limitation inside graphite but the thermodynamic limitation.

This study investigated the fundamental mechanisms of the loss of capacity of LiNiO

Invited for this month's cover are the groups of Shih-kang Lin at the National Cheng Kung University and Takeshi Abe at Kyoto University. The image shows how interfacial chemistry design can play a role in unlocking higher-energy-density and fast-charging Li

Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) is a promising electrocatalyst for the oxygen evolution reaction (OER) in alkaline solution. The OER activities of BSCF are gradually enhanced by prolonging the duration of electrochemical operation at OER potentials, but the underlying cause is not fully understood. In this study, we investigated the role of chemical operation, equivalent to immersion in alkaline solution, in the time-course of OER enhancement of BSCF. Interestingly, the time-course OER enhancement of BSCF was promoted not only by electrochemical operation, which corresponds to potential cycling in the OER region, but also by chemical operation. In situ Raman measurements clarified that chemical operation had a lower rate of surface amorphization than electrochemical operation. On the other hand, the leaching behavior of A-site cations was comparable between chemical and electrochemical operations. Since the OER activity of BSCF was stabilized by saturating the electrolyte with Ba2+, “chemical” A-site leaching was key to inducing the time-course OER enhancement on perovskite electrocatalysts. Based on these results, we provide a fundamental understanding of the role of chemical operation in the OER properties of perovskites.

Sodium-ion intercalation into the bulk of graphite hardly occurs. In this study, a nano-sized graphitizable carbon nanospheres (CNS) covered with basal planes were used to shorten the diffusion len...

Current methods for improving the electrochemical properties of lithium-ion battery electrode materials demand an understanding of its surface property and chemistry. We investigate the electrochemical property of a thin-film Li4Ti5O12 (LTO) layer on a hard carbon (e.g., glass-like carbon) ideal model electrode and propose that its unique properties make it an effective protective coating layer to improve the performance and stability of commercially obtained hard carbon powder. The LTO layer displayed a varying degree of coverage with the number of coatings, which minimized the initial reversible capacity loss because of the continuous electrolyte reduction due to the surface film formation on the GC electrode surface and improved reversibility. With the successful addition of the protective coating layers, the total resistances for interfacial charge transfer was significantly decreased. Using an in situ technique to probe the surface film's electrochemical characteristics, we systematically reveal that the LTO layer functioned as an inner, compact layer with its in situ surface film formation on the surface and resulted in a stable interface and displayed exemplary coverage and shielded most of the direct contact between the GC electrode and electrolyte solution. Furthermore, the LTO layer displayed a notable increase in current density, indicating the increased lithium-ion activity (aLi+) at the interface between the GC electrode and the LTO layer resulting in outstanding cyclability and rate performance.

The pH is one of the most important factors for electrochemical reactions. The solution pH is often measured prior to the electrochemical measurements, but it is not sufficient to consider only bulk pH, especially for the measurements in neutral pH solutions. When electrochemical reactions occur that produce/consume protons or hydroxide ions, the local pH in the vicinity of an electrode becomes considerably different from the bulk pH. This local pH is important in considering electrodeposition and corrosion. Furthermore, it also has an critical effect on the operating voltage of aqueous batteries and capacitors, as local pH affects the electrochemical stability of water.1 However it is difficult to measure the local pH, because the measurements have to be made in a small region adjacent to the electrode. Herein, rotating ring-disk electrode (RRDE) is a promising tool for the local pH measurements, though it has not been applicable to neutral pH solutions.2 In this study, the theory of local pH measurements using RRDE was extended to apply to solutions over a wide pH range.3 This extended theory was corroborated by measurements of the local pH change during hydrogen oxidation/evolution reactions on platinum disk electrode in solutions with near-neutral pH. This reversible reaction makes it possible to control the local pH on the disk electrode by the applied potential. The platinum ring electrode successfully detected the local pH changes by potentiometry, and the measured local pH was in good agreement with the theoretical predictions (Fig. 1a). In addition, using the RRDE modified with iridium oxide (IrOx) ring, we were able to evaluate the dynamic changes in the local pH of the working electrodes during oxygen evolution reaction in Ar-saturated neutral-pH solution (Fig. 1b).4 Thus, RRDE with IrOx ring has the potential to be a widely applicable tool for measuring the local pH of the electrode during any electrochemical reactions. Reference Y. Yokoyama, T. Fukutsuka, K. Miyazaki, and T. Abe, J. Electrochem. Soc., 165, A3299–A3303 (2018). W. J. Albery and A. R. Mount, J. Chem. Soc. Faraday Trans. 1, 85, 1181–1188 (1989). Y. Yokoyama, K. Miyazaki, Y. Miyahara, T. Fukutsuka, and T. Abe, ChemElectroChem, 6, 4750–4756 (2019). Y. Yokoyama, K. Miyazaki, Y. Kondo, Y. Miyahara, T. Fukutsuka, and T. Abe, Chem. Lett., 49, 195–198 (2020). Figure 1

Introduction Sodium-ion batteries (SIBs) are widely focused on as post lithium-ion batteries from the view of element strategy[1]. Hence, SIBs are attractive for large scale energy storage like electric vehicles. For the large scale energy storage, batteries with high rate capability are desired. Carbon nanosphere (CNS) covered with basal planes is expected as the candidate for carbon negative electrode material with high rate capability because CNS electrodes showed the high rate capability in LIBs owing to small particle sizes and short diffusion distances[2]. To enhance the rate capability of electrode materials, the internal resistances of electrodes should be investigated. In LIBs, the interfacial lithium-ion transfer at the interface between electrodes and electrolytes is one of the rate determining step[3]. However, the kinetic properties of interfacial sodium-ion transfer at the interface between carbon negative electrodes and electrolytes is unclear. In this study, the activation energies of interfacial sodium-ion transfer reaction at the interface between CNS electrodes and organic electrolyte solutions were evaluated. Experimental Electrochemical measurements were carried out using a three-electrode cell. Working electrode was CNS composite electrode. CNS treated at 1100°C and 2900°C were used and denoted as CNS-1100 and CNS-2900, respectively. Reference electrode was Ag/Ag+ electrode and counter electrode was natural graphite composite electrode. The electrolyte solutions were 0.9, 4 mol kg- 1 sodium bis(trifluoromethanesulfonyl)amide (NaTFSA)/ethylene carbonate (EC) + dimethyl carbonate (DMC) (1:1 by vol.) and 0.7 mol kg- 1 NaTFSA/fluoroethylene carbonate (FEC). Hereafter, all potentials are referred to as Fc/Fc+. Cyclic voltammetry (CV) was conducted between open circuit potential (OCV) and various potentials, and the scan rate was set at 0.1 mV s−1. Electrochemical impedance spectroscopy (EIS) was carried out with ac amplitude of 10 mV in a frequency range of 100 kHz − 10 mHz at temperatures ranging from 30°C to 10°C. Results Figure 1 shows the cyclic voltammograms of CNS-1100 composite electrodes in 0.9 mol kg- 1 NaTFSA/EC+DMC(1:1 by vol.). The irreversible currents were observed below −2 V and more than half of the irreversible currents disappeared from the 2nd cycle, indicating the formation of solid electrolyte interphase on CNS electrodes at the same manner of lithium ion-system. The redox peaks also appeared around −3 V, and were assigned to the peaks of reversible sodium-ion insertion into/from CNS-1100. Figure 2 shows the Nyquist plots of CNS-1100 composite electrode in 0.9 mol kg−1 NaTFSA/EC+DMC(1:1 by vol.). In the Nyquist plots, based on the potential dependence of the semi-circle, the semi-circle at the middle frequency region was assigned to the charge-transfer resistance. The same kinds of electrochemical behaviors were observed in the cases of other electrolytes and CNS-2900 electrodes. In the meeting, the activation energies of interfacial sodium-ion transfer will be reported. Acknowledgement This work was partially supported by ESICB, Kyoto University. References [1] Komaba et al., Adv. Funct. Mater., 21 (2011) 3859. [2] Yoshizawa et al., Carbon, 44 (2006) 2558. [3] Ogihara et al., J. Electrochem. Soc., 159 (2012) A1034. Figure 1

Enhancement of oxygen reduction and evolution reaction (ORR and OER) performance has been considered to play a key role for the development of next-generation energy storage devices such as metal-air batteries and water electrolyzers. Recently, mixed-anion compounds have been found to be highly efficient bifunctional electrocatalysts for ORR and OER[1,2]. Our group has been also clarified that layered perovskite oxychloride Sr2Co1 − x Fe x O3Cl serves as a good bifunctional catalyst, whose OER activity outperformed that of Ba0.5Sr0.5Co0.8Fe0.2O3 −δ [3]. While detailed mechanism of its enhanced OER activity has been discussed previously[4], that of ORR has not been evaluated in detail, even though ORR pathway is generally considered to be much more complicated. Moreover, further exploration of highly active bifunctional mixed-anion perovskites has to be continued for better understanding of oxygen electrocatalysis on mixed-anion perovskite compounds. In the present research, therefore, we focused on the evaluation of ORR pathway for Sr2Co1 − x Fe x O3Cl, and oxygen electrocatalysis on other mixed-anion perovskite compounds in alkaline media. Solid-state method was applied to the synthesis of Sr2Co1 − x Fe x O3Cl and other mixed-anion perovskites. Electrochemical measurements were conducted by a three-electrode cell with a rotating disk electrode (RDE). Glassy carbon RDEs coated by a catalyst layer consisting of catalysts, Vulcan XC-72 (Cabot), and AS-4 (Tokuyama) were used for the working electrode. Pt wire and reversible hydrogen electrode were used for the counter and reference electrodes, respectively. Oxygen-saturated 1 mol dm− 3 KOH solution was used for ORR activity tests, and Argon-saturated 1 mol dm− 3 KOH solution containing 5 mmol dm− 3 H2O2 was used for the activity tests of peroxide reduction reaction (PRR). Peroxide decomposition rates of Sr2Co1 − x Fe x O3Cl were performed by measuring evolved oxygen gas in the same manner as previously described[5]. While the highest OER activity was observed at x = 0.2, ORR activity of Sr2Co1 − x Fe x O3Cl was monotonously decreased with increase of Fe doping. Figure 1 shows decomposition rates of Sr2Co1 − x Fe x O3Cl in 1 mol dm− 3 KOH. Surprisingly, their HO2 − decomposition rate was higher than those of cobalt-based perovskite oxides such as LaCoO3, whose ORR activity was comparable to those of oxychlorides. Further discussion about ORR pathway of Sr2Co1 − x Fe x O3Cl, and oxygen electrocatalysis of other mixed-anion perovskites will be provided at the conference. Figure 1. H2O2 decomposition rates of Sr2Co1 − x Fe x O3Cl powders. References [1] A. Miura, C. Rosero-Navarro, Y. Masubuchi, M. Higuchi, S. Kikkawa, and K. Tadanaga, Angew. Chem. Int. Ed., 55 (2016) 7963. [2] M. A. Ghanem, P. Arunachalam, A. Almayouf, and M. T. Weller, J. Electrochem. Soc., 163 (2016) H450. [3] Y. Miyahara, K. Miyazaki, T. Fukutsuka, and T. Abe, Chem. Commun., 53 (2017) 2713. [4] Y. Miyahara, K. Miyazaki, T. Fukutsuka, and T. Abe, 232nd ECS Meeting, (2017) A1-30. [5] H. M. Zhang, Y. Shimizu, Y. Teraoka, N. Miura, and N. Yamazoe, J. Catal., 121 (1990) 432. Figure 1