In-Situ TEM Studies of Fluoride Based Solid State Batteries

Rechargeable batteries with high energy density have been required for recent applications such as battery electric vehicles. Fluoride ion batteries (FIBs) can achieve high energy density by using monovalent fluoride ions as charge carriers and multi-electron reactions of electrode active materials, and have attracted much attention as a candidate to surpass the performance of the current lithium-ion battery (LIB). However, FIB using the metal/metal fluoride conversion reaction, which has a high potential for energy density, has not been fabricated with good reversibility as the current LIB. On the other hand, the FIB using a crystal capable of intercalation (insertion-extraction) reaction of fluoride ions as the active material has been developed [1]. Recently, an all-solid-state fluoride ion battery using an oxyfluorosulfide (Sr2F2Fe2OS2; SFFOS) as the intercalation-type positive active material has been reported, and the battery has operated with high capacity and good reversibility at 413 K [2]. In this study, SFFOS was evaluated as an active material for liquid-based FIB. In particular, the thermal effect on the intercalation reaction of SFFOS in ionic liquid-based electrolyte solution was measured.The SFFOS was synthesized from four kinds of powder, SrF2, SrO, Fe and S, with molar ratio of 1:1:2:2 referring to the previous papers [2, 3]. These powders were mechanically ground under argon and pressed at 6 MPa to form a pellet. The resulting pellet was vacuum sealed in a quartz tube and sintered at 1073 K for 36 hours to obtain the SFFOS. PbSnF4 as the counter electrode material was prepared from PbF2 and SnF2 by mechanical milling at 600 rpm for 6 h using a planetary ball mill, and then calcined at 673 K for 1 h under an argon atmosphere. The electrolyte solution was prepared by dissolving anhydrous tetramethylammonium fluoride (TMAF) in an ionic liquid, N,N,N-trimethyl-N-propylammonium-bis(trifluoromethanesulfonyl)amide (TMPA-TFSA), at various concentrations such as 0.075 mol/dm3 (the molar ratio of TMAF:TMPA-TFSA=1:50). The working and counter electrodes were prepared by mixing of SFFOS or PbSnF4 with acetylene black and PVdF, respectively. The galvanostatic intercalation reaction of fluoride ions into SFFOS was measured using a three-electrode cell with a Pb|PbF2 reference electrode at 298, 373, and 423 K.The galvanostatic intercalation reaction (corresponding to a charging reaction) of the SFFOS/AB/PVdF composite electrode showed the charging plateau at about 0.5 V vs. Pb|PbF2. The following deintercalation (discharging) reaction, the discharge plateau was observed around 0 V. The intercalation/deintercalation plateaus were similar to those of the previously reported all-solid-state FIB, well [2].

New generation of electrochemical energy storage devices is needed to face the continuous increase in energy stored capacity demand at lower costs from human society. Moreover, due to the constant increase in number of batteries produced, the development of more sustainable devices without the consumption of limited resources at affordable costs is an actual research challenge. The conventional lithium batteries, the most widespread kind of electrochemical energy storage devices, made by an intercalation cathode (e.g., LiCoO2, LiFePO4) and a graphite anode suffer of limited capacity for high-performance application (such as in electric vehicles) and use limited mineral resources with adverse environmental consequences.Iron fluoride is a promising conversion cathode for positive electrodes in next-gen lithium batteries due to its low cost, great abundance, and ease of extraction1,2. The main problems of metal fluoride-based cathodes are the low conductivity and the structural changes during charge/discharge cycles that limit device capacity and lifespan. To effectively increase the storage capacity, several attempts to use metal lithium anodes have been made. However, this solution is now limited to primary batteries because of dendrites growth during metal plating that causes capacity fading and safety concerns.In the last decade, batteries based on fluoride-ion shuttling are drawing attention3,4 due to the possibility to use a high-capacity conversion cathode, exploiting a multi-electron transfer, and a metal anode without dendrites formation. This kind of devices, in the early stage of development, rely on electrochemical fluorination of an electropositive metal anode and the defluorination of a more “noble” metal fluoride without a metal plating-stripping process. Nowadays, electrolyte limitation is the main challenge for the development of a fluoride-ion battery that can compete with state-of-the-art lithium-ion batteries.In this work, a surfactant-assisted solvothermal route to obtain iron fluoride hydrate (FeF3 0.33 H2O) is presented. A hierarchical grape-like structure was obtained and it was tested as cathode active material in a 2025 coin-cell working in a metal-lithium battery configuration. In these tests it showed improved performance, compared to the same synthesis without surfactant. The synthetized material was also characterized by morphological and electrochemical point of view by means of Electrochemical Impedance Spectroscopy (EIS), galvanostatic charge discharge (GCD) and cyclic voltammetry (CV).The possibility to use the same cathodic material in a room-temperature fluoride-ion battery with a metal anode was assessed by using fluoride conducting liquid electrolytes5: encouraging results about the feasibility were achieved; however, further research in the field of liquid electrolytes resulted fundamental for bringing performances of this technology to the level of actual lithium batteries.

A fluoride-ion battery (FIB) is a novel type of energy storage system that has a higher volumetric energy density and low cost. However, the high working temperature (>150 °C) and unsatisfactory cycling performance of cathode materials are not favorable for their practical application. Herein, fluoride ion-intercalated CoFe layered double hydroxide (LDH) (CoFe-F LDH) was prepared by a facile co-precipitation approach combined with ion-exchange. The CoFe-F LDH shows a reversible capacity of ∼50 mAh g-1 after 100 cycles at room temperature. Although there is still a big gap between FIBs and lithium-ion batteries, the CoFe-F LDH is superior to most cathode materials for FIBs. Another important advantage of CoFe-F LDH FIBs is that they can work at room temperature, which has been rarely achieved in previous reports. The superior performance stems from the unique topochemical transformation property and small volume change (∼0.82%) of LDH in electrochemical cycles. Such a tiny volume change makes LDH a zero-strain cathode material for FIBs. The 2D diffusion pathways and weak interaction between fluoride ions and host layers facilitate the de/intercalation of fluoride ions, accompanied by the chemical state changes of Co2+/Co3+ and Fe2+/Fe3+ couples. First-principles calculations also reveal a low F- diffusion barrier during the cyclic process. These findings expand the application field of LDH materials and propose a novel avenue for the designs of cathode materials toward room-temperature FIBs.

Propelling performance of silicon thin film lithium ion battery by appropriate dopants

The advanced copper oxide coating separator enhances the electrochemical performance of high-rate lithium-ion batteries

We synthesize a Bi0.7Fe1.3O1.5F1.7 (BFOF) phase via a non-topochemical reaction with a fluorination agent. The crystal structure is refined by Rietveld refinement on the neutron diffraction patterns as a hexagonal lattice in the R3¯ space group, along with the defect structure. The sudden decrease in magnetic susceptibility below 250 K and the linear relationship between the magnetization and the magnetic field indicate that BFOF is an antiferromagnetic material. When BFOF is used as a cathode in fluoride-ion batteries (FIBs), a discharge (charge) capacity of 360 (225) mAh/g is achieved at 140 °C. Magnetization and x-ray diffraction measurements confirm that the F ions are transferred from the cathode to the Pb counter electrode during discharge and in the opposite direction during charge, in a manner analogous to the transfer of lithium (Li) ions in Li-ion batteries. These findings contribute to the development of quaternary oxyfluorides serving as FIB cathodes.

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The capacity, energy, internal resistance and open -circuit voltage are key indicators to represent the performance of lithium ion batteries. The battery rate characteristic refers to the battery charge and discharge performance; the higher rate battery can withstand higher discharge current. This paper researched the influencing factors for consistency performance of Lithium ion batteries which were tested at 1/3C, 1.5C and 2C charge and discharge. The result shows that the charge and discharge voltage of the series-connected cell group is different slightly, and the basic electric performance (capacity, internal resistance) is not affected basically when the charge rate is less than 1C, the temperature rises slightly when charging the series-connected cell group when the charge rate is larger (higher than 1C), and proposes a method for select the appropriate battery charge and discharge rate, to eliminate the adverse batteries and ensure the good performance of batteries.