Evolution of fluoride shuttle battery reactions and three-dimensional morphology changes of BiF3 microparticles in an ethylene carbonate-based liquid electrolyte

Fluoride shuttle batteries (FSBs) use defluorination of metal fluorides and fluorination of metals, and they are considered as candidates of next-generation batteries with high energy densities. During FSB reactions of orthorhombic and cubic BiF₃ (o-BiF₃ and c-BiF₃, respectively) in an ethylene carbonate-based liquid electrolyte, in situ Raman mapping and in situ laser scanning confocal microscopy (LSCM) for three-dimensional analysis were conducted almost simultaneously. As the potential of o-BiF₃vs. Pb (E[WE]) was decreased to 0.4 V, desorption of F− started at the protrusions of o-BiF₃ particles. After defluorination, E[WE] was increased to 0.6 V, and c-BiF₃ appeared at protrusions of the surfaces of Bi. However, at the surfaces where o-BiF₃ partially remained, o-BiF₃ grew rather than c-BiF₃. The apparent volumes of particles increased during defluorination and fluorination. The results are important for determining reaction mechanisms, and the results indicate the possibility of the use of ethylene carbonate-based liquid electrolytes

Electrochemical performance of a lead fluoride electrode mixed with carbon in an electrolyte containing triphenylboroxine as an anion acceptor for fluoride shuttle batteries

In fluoride shuttle batteries (FSBs), the addition of an anion acceptor (AA) is required to dissolve the supporting electrolyte salt in an organic solvent. Based on theoretical calculations and practical experiments, the effectiveness of triphenylboroxine (TPhBX) as an AA for FSB was verified. Using an electrolyte with TPhBX as an AA, the specific capacities of the following two types of lead fluoride (PbF₂) electrodes were evaluated: (i) PbF₂ pulverized using a ball mill, (ii) pulverized PbF₂ mixed with carbon using a ball mill. The experimental results indicate that mixing PbF₂ with carbon using a ball mill increases the specific capacity of PbF₂ electrode

Electrochemical properties of lead fluoride electrode in fluoride shuttle battery

Recently, the reversible discharge and charge reaction of BiF₃ electrode for fluoride shuttle battery (FSB) that can be used as a promising candidate for next-generation battery are observed using a liquid-based electrolyte. In this study, we investigate the electrochemical performance of PbF₂ as an active material for the FSB. To increase the electronic conductivity, the PbF₂ was mixed with carbon, and the composite material between PbF₂ and carbon, PbF₂/C, is formed. High discharge and charge capacities are obtained for PbF₂/C during the first cycle. Although the discharge and charge capacities gradually decreased, the discharge and charge reactions occurred in the second and third cycles. To confirm the progress of the discharge and charge reactions, the crystal structure change of the active material during discharging and charging in the first and second cycles is evaluated using X-ray diffraction (XRD). From the XRD results, the formation of Pb and PbF₂ during discharging and charging can be confirmed, indicating that the discharge (PbF₂ + 2e− → Pb + 2F−) and charge (Pb + 2F− → PbF₂ + 2e−) reactions progress in both the first and second cycles

Improved electrochemical performances in a bismuth fluoride electrode prepared using a high energy ball mill with carbon for fluoride shuttle batteries

Bismuth fluoride (BiF₃) is a promising positive electrode material for fluoride shuttle batteries (FSBs) owing to its high theoretical specific capacity (302 mA h g⁻¹). However, it exhibits low practical capacity. The methods for preparing the electrode are known to have significant effects on battery performance. The mixture between BiF₃ and carbon, BiF₃/C, prepared by high energy ball milling method has been already approved in lithium ion batteries. With this method, a significant improvement over the discharge and charge capacities of the BiF₃/C electrode has been achieved. In this work, for the first time, BiF₃/C electrode has been used for FSB. Using BiF₃/C electrode significantly increased the discharge and charge capacities. To confirm the progress of the discharge and charge reactions of BiF3/C electrode, the crystal structure of active materials and oxidation state of Bi for the BiF₃/C electrode during discharging and charging has been investigated by X-ray diffraction and X-ray absorption fine structure. The results reveal that, with higher capacity values, discharge and charge reactions related to BiF₃/C have been realized

Complexation of F⁻ by Li⁺ and Mg²⁺ Ions as Inorganic Anion Acceptors in Lactone-Based Li⁺/F⁻ and Mg²⁺/F⁻ Hybrid Electrolytes for Fluoride Shuttle Batteries

The development of high-quality fluoride-ion transporting electrolytes is a crucial demand for fluoride shuttle batteries (FSBs). However, the uncontrolled chemical and electrochemical activities of fluoride ions narrow the available potential window, hindering the development of high-voltage FSB cells. We present a method for upgrading recently developed lactone-based liquid fluoride electrolytes by complexation of F⁻ with Li⁺ and Mg²⁺ ions. In the resultant Li⁺/F⁻ and Mg²⁺/F⁻ hybrid electrolytes, Li2F+ and MgF+ were the most probable soluble complexes, and the effective fluoride concentrations could reach ∼0.15 M along with excess Li⁺(Mg²⁺) ions. Unique interactions between F⁻ and Li⁺(Mg²⁺) were observed using ¹⁹F nuclear magnetic resonance spectroscopy. Li⁺(Mg²⁺) ions thus served as inorganic anion acceptors with ultimate redox stabilities to expand the negative potential window of the electrolytes to near −3 V vs SHE. The proposed complex formation was also supported by a conductometric titration method. We demonstrated the superior and versatile electrochemical performances of the Li⁺/F⁻ hybrid electrolyte, which enabled reversible charge/discharge reactions of various metal electrodes and composite electrodes in a wide range of redox series. Further, the Li⁺/F⁻ hybrid electrolyte opened valid new reaction paths for aluminum, making it a promising negative electrode in high-voltage FSB cells

Oxidation behaviour of lattice oxygen in Li-rich manganese-based layered oxide studied by hard X-ray photoelectron spectroscopy

The oxidation/reduction behaviours of lattice oxygen and transition metals in a Li-rich manganese-based layered oxide Li[Li0.25Ni0.20Mn0.55]O1.93 are investigated by using hard X-ray photoelectron spectroscopy (HAX-PES). By making use of its deeper probing depth rather than in-house XPS analyses, we clearly confirm the formation of O- ions as bulk oxygen species in the active material. They are formed on the 1st charging process as a charge compensation mechanism for delithiation and decrease on discharging. In particular, the cation-anion dual charge compensation involving Ni and O ions is suggested during the voltage slope region of the charging process. The Ni ions in the material are considered to increase the capacity delivered by a reversible anion redox reaction with the suppression of O2 gas release. On the other hand, we found structural deterioration in the cycled material. The O- species are still observed but are electrochemically inactive during the 5th charge-discharge cycle. Also, the oxidation state of Ni ions is divalent and inactive, although that of Mn ions changes reversibly. We believe that this is associated with the structural rearrangement occurring after the activation process during the 1st charging, leading to the formation of spinel- or rocksalt-like domains over the sub-surface region of the particles

Direct observation of reversible oxygen anion redox reaction in Li-rich manganese oxide, Li2MnO3, studied by soft X-ray absorption spectroscopy

Li-rich layered oxides have attracted attention as promising positive electrode materials for next-generation lithium-ion secondary batteries because of their high energy storage capacity. The participation of the oxygen anion has been hypothesized to contribute to these oxides' high capacity. In the present study, we used O K-edge and Mn L-edge X-ray absorption spectroscopy (XAS) to study the reversible redox reactions that occur in single-phase Li-rich layered manganese oxide, Li2MnO3. We semiquantitatively analyzed the oxygen and manganese reactions by dividing the charge/discharge voltage region into two parts. The O K-edge XAS indicated that the electrons at the oxygen site reversibly contributed to the charge compensation throughout the charge/discharge processes at operating voltages between 2.0 and 4.8 V vs. Li+/Li0. The Mn L-edge XAS spectra indicated that the Mn redox reaction occurred only in the lower-voltage region. Thus, at higher potentials, the electrons, mainly at the oxygen site, contributed to the charge compensation. Peaks whose energies were similar to peroxide appeared in and then disappeared from the O K-edge spectra obtained during the reversible redox cycles. These results indicate that the reorganization of the oxygen network in the crystal structure affects the redox components. By using two kinds of detection modes with different probing depths in XAS measurements, it was found that these redox reactions are bulk phenomena in the electrode

Delithiation/lithiation behavior of LiNi<inf>0.5</inf>Mn<inf>1.5</inf>O<inf>4</inf> studied by in situ and ex situ ^6,7Li NMR spectroscopy

Delithiation and lithiation behaviors of ordered spinel LiNi0.5Mn1.5O4 and disordered spinel LiNi0.4Mn1.6O4 were investigated by using in situ (in operando) 7Li NMR and ex situ 6Li MAS NMR spectroscopy. The in situ 7Li monitoring of the ordered spinel revealed a clear appearance and subsequent disappearance of a new signal from the well-defined phase Li0.5Ni0.5Mn1.5O4, suggesting the two-phase reaction processes among Li1.0Ni0.5Mn1.5O4, Li0.5Ni0.5Mn1.5O4, and Li0.0Ni0.5Mn1.5O4. Also, for the disordered spinel, Li0.5Ni0.4Mn1.6O4 was identified with a broad distribution in Li environment. High-resolution 6Li MAS NMR spectra were also acquired for the delithiated and lithiated samples to understand the detailed local structure around Li ions. We suggested that the nominal Li-free phase Li0.0Ni0.5Mn1.5O4 can accommodate a small amount of Li ions in its structure. The tetragonal phases Li2.0Ni0.5Mn1.5O4 and Li2.0Ni0.4Mn1.6O4, which occurred when the cell was discharged down to 2.0 V, were very different in the Li environment from each other. It is found that 6, 7Li NMR is highly sensitive not only to the Ni/Mn ordering in LiNi0.5Mn1.5O4 but also to the valence changes of Ni and Mn on charge-discharge process

Difference of rate performance between discharge and charge reactions for bismuth fluoride electrode in lithium-ion battery

The conversion-based BiF₃ is a promising cathode material for lithium-ion batteries due to its high theoretical capacity (302 mAh g⁻¹). Nanocomposites of BiF₃ and carbon (BiF₃/C) are known to improve the electrochemical performance by increasing the electronic conductivity of the electrode. Here we investigate the electrochemical performance of BiF3/C at high C-rates. In particular, we newly investigate the difference of high C-rate performance between discharge and charge reactions. The discharge and charge capacities in the first cycle were almost the same at 0.1 C. In contrast, the discharge capacity was higher than charge capacity at 10 C. Further, during cycling at 10 C, the charge capacity drastically decreased, but the discharge capacity remained high. The rate performance of the discharge reaction was higher than that of the charge reaction, especially after cycling