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Honeycomb layered oxides constitute an emerging class of materials that show interesting physicochemical and electrochemical properties. However, the development of these materials is still limited. Here, we report the combined use of alkali atoms ($\rm Na$ and $\rm K$) to produce a mixed alkali honeycomb layered oxide material, namely, $\rm NaKNi_2TeO_6$. Via transmission electron microscopy measurements, we reveal the local atomic structural disorders characterised by aperiodic stacking and incoherency in the alternating arrangement of $\rm Na$ and $\rm K$ atoms. We also investigate the possibility of mixed electrochemical transport and storage of $\rm Na$ and $\rm K$ ions in $\rm NaKNi_2TeO_6$. In particular, we report an average discharge cell voltage of about $4\, \rm V$ and a specific capacity of around $80\, \rm mAh\, g^{-1}$ at low specific currents (i.e., $< 10\, \rm mA\, g^{-1}$) when a $\rm NaKNi_2TeO_6$-based positive electrode is combined with a room-temperature $\rm NaK$ liquid alloy negative electrode using an ionic liquid-based electrolyte solution. These results represent a step towards the use of tailored cathode active materials for dendrite-free electrochemical energy storage systems exploiting room-temperature liquid alkali metal alloy materials., 39 Pages, 7 Figures, 1 Cover Art

The initial process of Li-metal electrodeposition on the negative electrode surface determines the charging performance of Li-metal secondary batteries. However, minute depositions or the early processes of nucleation and growth of Li metal are generally difficult to detect under operando conditions. In this study, we propose an optical diagnostic approach to address these challenges. Surface plasmon resonance (SPR) spectroscopy coupled with electrochemical operation is a promising technique that enables the ultrasensitive detection of the initial stage of Li-metal electrodeposition. The SPR is excited in a thin copper film deposited on a glass substrate, which also serves as a current collector enabling electrochemical Li-metal deposition. For a propylene carbonate (PC)-based Li-ion battery electrolyte, under both cyclic voltammetry and constant-current operation, Li-metal deposition is readily detected by changes in the SPR absorption dip in the reflectance spectrum. Electrochemical SPR is highly sensitive to metal deposition, with a demonstrated capability of detecting an average thickness of approximately 0.1 nm, corresponding to a few atomic layers of Li. To identify the growth mechanism, the SPR reflectance spectra of various possible Li-metal deposition processes were simulated. Comparison of the simulated spectra with the experimental data found good agreement with the well-known nucleation and growth model for Li-metal deposition from PC-based electrolytes. The demonstrated operando electrochemical SPR measurement should be a valuable tool for basic research on the initial Li-metal deposition process.

The physicochemical properties of room temperature ionic liquids (RTILs) consisting of bis(trifluoromethanesulfonyl)amide (TFSA- ) combined with 1-hexyl-1-methylpyrrolidinium (Pyr1,6 + ), 1-(butoxymethyl)-1-methylpyrrolidinium (Pyr1,1O4 + ), 1-(4-methoxybutyl)-1-methyl pyrrolidinium (Pyr1,4O1 + ), and 1-((2-methoxyethoxy)methyl)-1-methylpyrrolidinium (Pyr1,1O2O1 + ) were investigated using both experimental and computational approaches. Pyr1,1O2O1 TFSA, which contains two ether oxygen atoms, showed the lowest viscosity, and the relationship between its physicochemical properties and the position and number of the ether oxygen atoms was discussed by a careful comparison with Pyr1,1O4 TFSA and Pyr1,4O1 TFSA. Ab initio calculations revealed the conformational flexibility of the side chain containing the ether oxygen atoms. In addition, molecular dynamics (MD) calculations suggested that the ion distributions have a significant impact on the transport properties. Furthermore, the coordination environments of the Li ions in the RTILs were evaluated using Raman spectroscopy, which was supported by MD calculations using 1000 ion pairs. The presented results will be valuable for the design of functionalized RTILs for various applications.

The features of the electrode surface film during Li-metal deposition and dissolution cycles are essential for understanding the mechanism of the negative electrode reaction in Li-metal battery cells. The physical and chemical property changes of the interface during the initial stages of the reaction should be investigated under operando conditions. In this study, we focused on the changes in the optical properties of the electrode surface film of the negative electrode of a Li-metal battery. Cu-based electrochemical surface plasmon resonance spectroscopy (EC-SPR) was applied because of its high sensitivity to optical phenomena on the electrode surface and its stability against Li-metal deposition. The feature of SPR reflectance dip depends on the optical properties of the electrode surface; namely, the wavelength and depth of the reflectance dip directly connected the refractive index and extinction coefficient (color of electrode surface film), which was confirmed by reflectance simulation. In the operando EC-SPR experiment, various changes in optical properties were clearly observed during the cycles. In particular, the change in the extinction coefficient was more remarkable at the second process than the first process of Li-metal deposition. By electrochemical quartz-crystal microbalance (EQCM) measurements, surface film formation was confirmed during the first Li-metal deposition process. The remarkable change in the extinction coefficient is based on the color change of the surface film, which is caused by the chemical condition change during Li-metal deposition cycles.

Potassium-ion batteries have been envisioned to herald the age of low-cost and high-performance energy storage systems. However, the sparsity of viable components has dampened the progress of these energy devices. Thus, herein, we report coronene, a high-voltage cathode material that manifests a high-voltage of $4.1 \rm V$ enkindled by anion (de)insertion. This work not only illuminates the broad class of polycyclic aromatic hydrocarbons as prospective cathode materials but also sets a new benchmark for the performance of future organic cathode materials.

Honeycomb layered oxides with monovalent or divalent, monolayered cationic lattices generally exhibit myriad crystalline features encompassing rich electrochemistry, geometries and disorders, which particularly places them as attractive material candidates for next-generation energy storage applications. Herein, we report global honeycomb layered oxide compositions, ${\rm Ag_2}M_2{\rm TeO_6}$ ($M = \rm Ni, Mg, \textit{etc}.$) exhibiting $\rm Ag$ atom bilayers with sub-valent states within Ag-rich crystalline domains of ${\rm Ag_6}M_2{\rm TeO_6}$ and $\rm Ag$-deficient domains of ${\rm Ag}_{2 - x}\rm Ni_2TeO_6$ ($0 < x < 2$). The $\rm Ag$-rich material characterised by aberration-corrected transmission electron microscopy reveals local atomic structural disorders characterised by aperiodic stacking and incoherency in the bilayer arrangement of $\rm Ag$ atoms. Meanwhile, the global material not only displays high ionic conductivity, but also manifests oxygen-hole electrochemistry during silver-ion extraction. Within the $\rm Ag$-rich domains, the bilayered structure, argentophilic interactions therein and the expected $\rm Ag$ sub-valent states ($1/2+, 2/3+, \textit{etc}.$) are theoretically understood via spontaneous symmetry breaking of SU($2$)$\times$U($1$) gauge symmetry interactions amongst $3$ degenerate mass-less chiral fermion states, justified by electron occupancy of silver $4d_{z^2}$ and $5s$ orbitals on a bifurcated honeycomb lattice. This implies that bilayered frameworks have research applications that go beyond the confines of energy storage., 67 pages, 5 figures, 1 cover art, 1 table

A designed two-compartment cell was applied to the degradation analysis of FeF3 having high theoretical energy density. Comparing with the result of the coin cell, the two-compartment cell gave us insight that the elution of Fe was responsible for the degradation of FeF3 and LiDFOB was found as an essentially effective additive for suppressing the degradation of FeF3.

The world is at the cusp of a new era where pivotal importance is being attached to the development of sustainable and high-performance energy storage systems. Potassium-ion batteries are deemed not only as cheap battery candidates, but also as the penultimate high-voltage energy storage systems within the monovalent-cation chemistries. However, their performance and sustainability are undermined by the lack of suitable electrolytes for high-voltage operation particularly due to the limited availability of cathode materials. Here, the potential of ionic liquids based on potassium bis(trifluoromethanesulfonyl)amide (KTFSA) as high-voltage electrolytes is presented by assessing their physicochemical properties, along with the electrochemical properties upon coupling with new high-voltage layered cathode materials. These ionic liquids demonstrate a lower redox potential for potassium dissolution / deposition (with a wide voltage tolerance of around $6.0$ $\rm V$), placing them as feasible and safe electrolytes for high-voltage potassium-ion battery configuration. This is proven by matching this electrolyte with new high-voltage layered cathode compositions, demonstrating stable electrochemical performance. The present findings of electrochemically stable ionic liquids based on potassium bis(trifluoromethanesulfonyl)amide will bolster further advancement of high-performance cathode materials, whose performance at high-voltage regimes were apparently restricted by the paucity of suitable and compatible electrolytes., 29 pages, 8 figures, 1 table, 1 cover art

This study demonstrates the feasibility to investigate the electrolyte-side of the Li/electrolyte interface using an X-ray total reflection phenomenon. By means of a specially designed cell and syn...

We demonstrate the feasibility of using a combination of alkali atoms (Na and K) to develop a robust mixed-alkali honeycomb layered oxide NaKNi2TeO6. Through a series of atomic-resolution transmission electron microscopy in multiple zone axes, we reveal for the first time the local atomic structural disorders characterised by aperiodic stackings and incoherency in the alternating arrangement of Na and K atoms. Our findings indicate great structural versatility that renders NaKNi2TeO6 an ideal platform for investigating other fascinating properties such as mixed ionic transport and intriguing electromagnetic and quantum phenomena amongst honeycomb layered oxides. Finally, we unveil the possibility of inducing mixed Na- and K-ion transport electrochemistry of NaKNi2TeO6 at high voltages (~ 4V), thus epitomising it as a competent cathode candidate for the emerging dendrite-free batteries based on NaK liquid metal alloy as anodes. The results not only betoken a new avenue for developing functional materials with fascinating crystal versatility, but also prefigure a new age of ‘dendrite-free’ energy storage system designs that rely on mixed-cation electrochemistry.

Potassium-ion batteries have been envisioned to herald the age of low-cost and high-performance energy storage systems. However, the sparsity of viable components has dampened the progress of these energy devices. Thus, herein, we report coronene, a high-voltage cathode material that manifests a high-voltage of $4.1 \rm V$ enkindled by anion (de)insertion. This work not only illuminates the broad class of polycyclic aromatic hydrocarbons as prospective cathode materials but also sets a new benchmark for the performance of future organic cathode materials., Comment: 11 pages, 1 cover art

Correction for ‘The effects of the position of the ether oxygen atom in pyrrolidinium-based room temperature ionic liquids on their physicochemical properties’ by Kazuki Yoshii et al., Phys. Chem. Chem. Phys., 2020, DOI: 10.1039/d0cp02662j.

The designing of high voltage cathode materials is critical for the advancement of potassium-ion (K-ion) battery. Herein, we present a new honeycomb framework P2-type K2/3Ni1/3Co1/3Te1/3O2 (or equivalently written as K2NiCoTeO6) which exhibits the highest voltage on record (beyond 4 V versus K+/K) for layered cathode materials. This work will allow for the further development of, particularly, high voltage layered cathodes for K-ion battery.

To clarify the effects of Li pre-doping of a Si negative electrode for potential application in next-generation energy storage systems, such as Li–S and Li–O2 batteries, such electrodes were prepared by direct Li pre-doping using Li metal foil and by electrochemical pre-doping at 700 mA g−1 (Si) using a two-electrode cell. These were evaluated by comparing their charge/discharge properties, mainly by half-cell operation at a limited capacity of 2000 mAh g−1 (Si). Fluoroethylene carbonate (FEC) was added to form a stable solid electrolyte interphase film on the surface of the electrodes. The depth and homogeneity of Li pre-doping were improved by using the direct Li pre-doping method and by FEC addition, respectively. The rapid Li pre-doping of this method caused cracks and pulverization of the Si nanoparticles and promoted deep Li alloying by decreasing the Li+ diffusion distance. The tough homogeneous solid electrolyte interphase film derived from FEC suppressed electrolyte decomposition and enabled a fast Li alloying/de-alloying reaction. Addition of 10 mass% FEC to the half-cell electrolyte repaired damage to the interphase film caused by the large volume change of Si nanoparticles and improved cyclability to exceed 230 cycles.

A designed two-compartment cell was applied to the degradation analysis of FeF3 having high theoretical energy density. Comparing with the result of the coin cell, the two-compartment cell gave us insight that the elution of Fe was responsible for the degradation of FeF3 and LiDFOB was found as an essentially effective additive for suppressing the degradation of FeF3.

Rechargeable potassium-ion batteries have been gaining traction as not only promising low-cost alternatives to lithium-ion technology, but also as high-voltage energy storage systems. However, their development and sustainability are plagued by the lack of suitable electrode materials capable of allowing the reversible insertion of the large potassium ions. Here, exploration of the database for potassium-based materials has led us to discover potassium ion conducting layered honeycomb frameworks. They show the capability of reversible insertion of potassium ions at high voltages (~4 V for K2Ni2TeO6) in stable ionic liquids based on potassium bis(trifluorosulfonyl) imide, and exhibit remarkable ionic conductivities e.g. ~0.01 mS cm−1 at 298 K and ~40 mS cm–1 at 573 K for K2Mg2TeO6. In addition to enlisting fast potassium ion conductors that can be utilised as solid electrolytes, these layered honeycomb frameworks deliver the highest voltages amongst layered cathodes, becoming prime candidates for the advancement of high-energy density potassium-ion batteries. The development of potassium-ion batteries requires cathode materials that can maintain the structural stability during cycling. Here the authors have developed honeycomb-layered tellurates K2M2TeO6 that afford high ionic conductivity and reversible intercalation of large K ions at high voltages.

Novel room temperature ionic liquids (RTILs) were synthesized using phenyltrifluoroborate ([PhBF3]−), which is the simplest aryltrifluoroborate structure. The [PhBF3]−-based RTILs showed desirable transport properties, i.e., viscosity and ionic conductivity, and the anion has a rigid and bulky aromatic ring. The properties of 1-butyl-3-methylimidazolium phenyltrifluoroborate ([C4mim][PhBF3]) are comparable to those of 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]). The molecular volume of the RTILs, which was estimated using the appropriate quantum chemical calculations, highly correlated with the transport properties. The cation volume is an important factor that can control the physicochemical properties in this RTIL system. In addition, the basic skeleton structure of the cation determined the electrochemical window.

Electrochemical formation of cesium–tin alloys in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing 0.5M CsTFSA has been studied. Cathodic decomposition of BMPTFSA on a platinum electrode was suppressed by addition of CsTFSA, suggesting that Cs+ accumulated on the electrode surface and hindered the reduction of BMPTFSA. Multiple cathodic current peaks were observed on a tin electrode in the ionic liquid. Energy-dispersive X-ray analysis and X-ray diffraction results suggested formation of cesium–tin alloys after potentiostatic cathodic reduction on the tin electrode at room temperature. Keywords: Cesium, Ionic liquids, Cesium–tin alloys, Electrolytic recovery

Electrode reaction of bis(acetylacetonato)platinum(II), Pt(acac) 2 , and preparation of platinum (Pt) nanoparticles have been studied in 1-R-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (R = butyl, hexyl and decyl, which are abbreviated as BMPTFSA, HMPTFSA and DMPTFSA, respectively) ionic liquids by means of cyclic voltammetry and rotating disk electrode (RDE) method. Pt(acac) 2 was suggested to be reduced to Pt via a two-electron transfer process at a glassy carbon electrode. The diffusion coefficient of Pt(acac) 2 at 50 °C was estimated to be 1.3 × 10 −7 cm 2 s −1 in BMPTFSA, by RDE measurements. It has been demonstrated that Pt nanoparticles were able to be prepared at the glassy carbon RDE by potentiostatic electrolysis at −1.8 and −2.5 V in the ionic liquids containing Pt(acac) 2 . The prepared nanoparticles were characterized by transmission electron microscopy, energy dispersive X-ray spectroscopy and electron diffraction. No pronounced variation in the average particle sizes of Pt was observed with the rotation rates at the RDE, indicating that the average particle size was independent of the rotation rate or current density. Pt nanoparticles of average sizes of 2.1 ± 0.8, 2.8 ± 0.8 and 3.0 ± 0.8 nm were obtained after electrolysis at −1.8 V with a rotation rate of 1000 rpm in BMPTFSA, HMPTFSA and DMPTFSA, respectively, suggested that the average particle size may depend on the kind of the ionic liquid.

Palladium is well known as a useful metal for such as catalytic reaction, hydrogen absorption and improvement of corrosion resistance. Although the electrochemical reaction and deposition of palladium has been researched in some ionic liquids so far, the palladium sources have been limited to halogeno complexes such as [PdBr4]2− or [PdCl4]2−.1,2 We have already reported electrodeposition of Pd from 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing bis(acetylacetonato)palladium (Pd(acac)2).3 In the present study, the electrochemical reaction of Pd(acac)2in BMPTFSA was investigated using a rotating ring-disk electrode. BMPTFSA was prepared by metathesis reaction of LiTFSA and BMPBr, which was synthesized by quaternization reaction of 1-methylpyrrolidinium with n-butyl bromide in acetonitrile, followed by recrystallization with ethyl acetate. Pd(acac)2 was used as received. Electrochemical measurements were conducted in a glovebox of dry Ar atmosphere. A ring-disk electrode (Pt ring: 1.2 cm2; Pt disk: 0.79 cm2) was used as a working electrode. A Pt wire was used for a counter electrode. A silver wire immersed in BMPTFSA containing 0.1 M AgCF3SO3was used as a reference electrode. The counter electrode was separately set from the working and reference electrode by a glass filter. The UV-vis spectrum of BMPTFSA containing 0.25 mM Pd(acac)2 was well accorded with that of ethanol solution, suggesting that Pd(acac)2 exists as a tetra-coordinated Pd complex coordinated by the oxygen atoms of acetylacetonate in BMPTFSA. The deposition of metallic palladium was confirmed on the electrode after the potentiostatic cathodic redction at −1.9 V. An anodic current was observed at the ring electrode kept at −1.25 V when the cathodic current corresponding to reduction of Pd(acac)2 was observed on the disk electrode at the potentials more positive than –1.9 V. It has been reported that some intermediate species were detected by polarogram in N, N-dimethylformamide solution containing Pd(acac)2.4 The obtained result indicated that the intermediate species, [Pd(acac)2]−, also formed by the reduction of Pd(acac)2 in this study. Thus, it is considered that the metallic palladium may be formed by the disproportionation reaction of [Pd(acac)2]−. Acknowledgments Part of this study was financially supported by MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2012–2016. References 1. Y. Bando, Y. Katayama, and T. Miura, Electrochim. Acta, 53, 87 (2007). 2. I.-W. Sun and C. L. Hussey, J. Electroanal. Chem., 274, 325 (1989). 3. K. Yoshii, Y. Oshino, N. Tachikawa, K. Toshima, and Y. Katayama, Electrochem. Commun., 52, 21 (2015). 4. M. Hirota and S. Fujiwara, Bull. Chem. Soc. Jpn., 48, 2825 (1975).

Platinum (Pt) is widely used as a catalyst for various chemical and electrochemical reactions, such as water electrolysis, fuel cell and hydrogenation of organic species. Pt nanoparticle is often employed for many chemical reactions including methanol oxidation, conversion of exhaust gas, and so on. Recently, aprotic ionic liquids have been expected to be alternative media for electrodeposition of metals due to their wide electrochemical potential window, acceptable ionic conductivity, and negligible vapor pressure. Electrodeposition of Pt from halogeno-Pt source has been reported in the amide-type ionic liquid so far.1 Bis(acetylacetonato)metal complexes have attracted intensive interest as a precursor for the preparation of metal nanoparticles. Recently, we have reported electrodeposition of palladium from bis(acetylacetonato)palladium(II) in an amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA).2 In the present study, electrochemical behavior of bis(acetylacetonato)platinum(II) (Pt(acac)2) has been investigated in the bis(trifluoromethylsulfonyl)amide (TFSA–) ionic liquids composed of 1-butyl-1-methylpyrrolidinium (BMP+), 1-hexyl-1-methylpyrrolidinium (HMP+) and 1-decyl-1-methylpyrrolidinium (DMP+). The ionic liquids were prepared by the metathesis reactions of LiTFSA with the dialkylpyrrolidinum bromides, BMPBr, HMPBr and DMPBr in deionized water, separated by solvent extraction, finally dried under vacuum at 100 °C for 24 hours. The water content of each ionic liquid was found to be < 10 ppm by Karl Fischer titration. Pt(acac)2 was dissolved to give pale yellow transparent liquids in BMPTFSA, HMPTFSA and DMPTFSA by agitating for several hours at 60 °C. Tetra-coordinated Pt(acac)2 complex was found to exist in the ionic liquids by UV-visible spectroscopy. Cyclic voltammetry measurements of a glassy carbon electrode were conducted in BMPTFSA, HMPTFSA and DMPTFSA containing 5 mM Pt(acac)2 at various temperatures. Cyclic voltammograms showed a well-defined cathodic current peak around –1.7 V. Potentiostatic cathodic reduction of the ionic liquids containing 5 mM Pt(acac)2 at –1.7 V at 130 °C resulted in formation of metallic Pt. Thus, the cathodic current peak around –1.7 V was assignable to reduction of Pt(acac)2 to Pt(0). The reduction of Pt(acac)2 was suggested to involve formation of some intermediate species of Pt by cyclic voltammetry. Granular shaped deposit was observed on the glassy carbon substrate by scanning electron microscopy (SEM) after potentiostatic electrolysis in BMPTFSA containing 5 mM Pt(acac)2 at –1.7 V. The existence of Pt in the deposit was confirmed by energy dispersive X-ray spectroscopy (EDS). The appearance of the electrolytes turned to reddish black from pale yellow, indicating formation of Pt nanoparticles in the ionic liquids after electrolysis.1-4 Transmission electron microscopy (TEM) of the electrolytes after electrolysis confirmed formation and dispersion of Pt nanoparticles in the ionic liquids. The d-spacing values of the nanoparticles obtained in BMPTFSA, HMPTFSA and DMPTFSA were estimated to be 0.224, 0.229 and 0.225 nm, respectively, which were close to that of 0.227 nm for Pt(111). Pt nanoparticles obtained in the ionic liquids were suggested to form by disproportionation of the intermediate species of Pt. The average sizes (in diameter) of the Pt nanoparticles obtained after electrolysis in BMPTFSA, HMPTFSA and DMPTFSA were found to be 2.6 ± 0.7, 4.1 ± 1.2 and 4.3 ± 0.7 nm, respectively. Therefore, TFSA–-based ionic liquids could be promising media to prepare Pt and Pt nanoparticles with controlled size by electrolysis. Acknowledgement:Part of this work was financially supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. References [1] Y. Katayama, T. Endo, T. Miura, and K. Toshima, J. Electrochem. Soc., 160, D423 (2013). [2] K. Yoshii, Y. Oshino, N. Tachikawa, K. Toshima, and Y. Katayama, Electrochem. Commun., 52, 21 (2015). [3] Y. Katayama, Y. Oshino, N. Ichihashi, N. Tachikawa, K. Yoshii, and K. Toshima, Electrochim. Acta, 183, 37 (2015). [4] R. Fukui, Y. Katayama, and T. Miura, J. Electrochem. Soc., 158, D567 (2011).

Correction for ‘Coronene: a high-voltage anion insertion and de-insertion cathode for potassium-ion batteries’ by Minami Kato et al., New J. Chem., 2021, 45, 4921–4924.

Rechargeable potassium-ion batteries are promising candidates for low-cost largescale energy storage systems due to their cost-effectiveness, sustainability and comparable energy density. With an increasing number of publications related to electrolytes for potassium-ion batteries, (1) room temperature ionic liquids (RTILs) have also been suggested as feasible electrolytes owing to their superior properties, such as non-flammability and wide electrochemical window.(2)In our research group, we reported bis(trifluoromethanesulfonyl) amide (TFSA)-based RTILs as suitable for the operation of potassium-ion battery when coupled to high voltage cathode materials.(3) In this study, we evaluate the physicochemical properties of TFSA-based RTILs with KTFSA, in addition to assessing their application for high-voltage potassium-ion battery with honeycomb layered cathode oxides. (4) N-methyl-N-propylpyrrolidinium (Pyr13) TFSA and 1-ethyl-3-methylimidazolium (EMI) TFSA were selected as RTILs in this study. A comparison of their physicochemical properties with same concentration of KTFSA, LiTFSA, and NaTFSA reveal RTILs containing KTFSA to show relatively high ionic conductivity and wide electrochemical window. When a symmetrical cell of potassium was constructed using Pyr13TFSA with 0.5 M of KTFSA, deposition and dissolution reaction could be performed repeatedly without increase in overvoltage. Finally, we evaluated the charge/discharge performance of K2Ni2-x Co x TeO6 (where x=0.25, 0.5 and 0.75) high-voltage cathode materials using Pyr13TFSA with 0.5 M of KTFSA as an electrolyte. Results show that each positive electrode has good cycle performance, and the average voltage changed depending on the Co content in the cathode. We conclude that TFSA-based RTILs are potential candidates for high-voltage potassium-ion battery. References 1. T. Hosaka, K. Kubota, A. S. Hameed and S. Komaba, Chem. Rev. (2020). 2. T. Yamamoto, K. Matsumoto, R. Hagiwara and T. Nohira, J. Phys. Chem. C, 121, 18450 (2017). 3. T. Masese, K. Yoshii, Y. Yamaguchi, T. Okumura, Z.-D. Huang, M. Kato, K. Kubota, J. Furutani, Y. Orikasa, H. Senoh, H. Sakaebe and M. Shikano, Nat. Commun., 9, 3823 (2018). 4. K. Yoshii, T. Masese, M. Kato, K. Kubota, H. Senoh and M. Shikano, ChemElectroChem, 6, 3901 (2019). Acknowledgement This work was conducted in part under the auspices of the Japan Society for the Promotion of Science (JSPS KAKENHI Grant Numbers 19K15686).

Rechargeable potassium-ion batteries are appealing candidates for next-generation energy storage devices not only owing to their projected low-cost but also to the high voltage that can be attained.[1-3] Honeycomb layered oxide cathode materials show high voltage and facilitate reversible potassium-ion extraction and insertion (K+ (de)intercalation) at decent capacities. This makes them promising cathode candidates that can be matched to suitable anode materials to design a high-voltage (high-energy density) battery.[4-6] It is crucial to investigate the reaction mechanism of electrode materials in order to understand the structural changes accompanying reversible (de)intercalation of the large K+ that lead to unique staircase-like voltage profiles. Honeycomb layered oxides such as K2Ni2TeO6 facilitate reversible K+ (de)intercalation at high voltages with staircase-like profiles. In this presentation, we will highlight the reaction mechanism of K2Ni2TeO6, based mainly on X-ray diffraction studies. Acknowledgement This work was conducted under the support of Japan Prize Foundation, National Institute of Advanced Industrial Science and Technology (AIST) and the Japan Society for the Promotion of Science (JSPS KAKENHI Grant Numbers 19K15685). References T. Hosaka, K. Kubota, A. S. Hameed and S. Komaba, Chem. Rev. (2020). https://doi.org/10.1021/acs.chemrev.9b00463 T. Masese, K. Yoshii, Y. Yamaguchi, T. Okumura, Z.-D. Huang, M. Kato, K. Kubota, J. Furutani, Y. Orikasa, H. Senoh, H. Sakaebe and M. Shikano, Nat. Commun., 9, 3823 (2018). K. Yoshii, T. Masese, M. Kato, K. Kubota, H. Senoh and M. Shikano, ChemElectroChem, 6, 3901 (2019). T. Masese, K. Yoshii, M. Kato, K. Kubota, Z.-D. Huang, H. Senoh and M. Shikano, Chem. Commun., 55, 985 (2019). G. M. Kanyolo and T. Masese, chemRxiv,11961123 (2020). https://doi.org/10.26434/chemrxiv.11961123.v1 submitted G. M. Kanyolo, T. Masese, J. Rizell et al., Honeycomb Layered Oxides: Structure, Energy Storage, Transport, Topology and Relevant Insights. arXiv, 2003.03555 [cond-mat.mtrl-sci] (2020). https://arxiv.org/abs/2003.03555 submitted

Iron trifluoride (FeF3) has been attracted as one of the promising candidates for the positive electrode material of lithium secondary batteries due to their high theoretical capacity (712 mA g–1) and high energy density (1949 Wh kg–1). However, its poor cyclability and a large hysteresis of the voltage during charge and discharge still remain the problem to solve for the commercialization. The cycling performance of FeF3-based lithium secondary battery has been improved by various strategies, such as the addition of anion receptor, coating by other metals and tuning of electrolyte composition so far [1-3], but the suppression of degradation and the elucidation of its mechanism have not accomplished completely. In the present study, we investigated the charge and discharge behavior of the FeF3 positive electrode using a designed two-compartment type cell which has lithium ion-conducting ceramics as a separator to prevent the cross-talk of the electrolyte between the positive and negative electrode. A charge/discharge test was performed using a coin cell and two-compartment type cell using 1.0 mol dm–3 LiPF6 in 1:1 ratio of ethylene carbonate (EC) and dimethyl carbonate (DMC) as the electrolyte. Disassembling analysis of both cells including SEM observation and X-ray fluorescence analysis revealed that the elution of Fe is more responsible for the degradation of the FeF3 electrode than a thick film formation on the electrode due to the cross-talk in the coin cell. Furthermore, the cyclability was improved using the electrolyte with LiDFOB as an additive for only the positive electrode side of the two-compartment type cell, whereas the improvement of the cyclability was not found when the additive was applied to a coin cell. After the cycling tests, various analyses, such as electrochemical impedance, X-ray photoelectron spectroscopy, and transmission electron microscopy, were also conducted for the electrode with and without additives in the electrolyte to examine the degradation mechanism of FeF3 positive electrode. The obtained results were considered useful for the improvement of the cycling performance of FeF3-based lithium ion batteries. References [1] K. Kumagae, K.-I. Okazaki, K. Matsui, H. Horino, T. Hirai, J.-I. Yamaki and Z. Ogumi, J. Electrochem. Soc., 163, A1633 (2016). [2] Y. Li, X. Zhou, Y. Bai, G. Chen, Z. Wang, H. Li, F. Wu and C. Wu, ACS Appl. Mater. Interfaces, 9, 19852 (2017). [3] H. Senoh, K. Matsui, M. Shikano, T. Okumura, H. Kiuchi, K. Shimoda, K. Yamanaka, T. Ohta, T. Fukunaga, H. Sakaebe and E. Matsubara, ACS Appl. Mater. Interfaces, 11, 30959 (2019). Acknowledgement This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under the “Research and Development Initiative for Scientific Innovation of New Generation Batteries 2 (RISING2)”