In search of new concepts to build batteries with high energy densities, electrochemical cells based on metal fluorides may be promising. Herein, we report the demonstration of secondary battery cells based on fluoride shuttle. In fluoride ion batteries, fluoride anion acts as charge transfer ion between a metal/metal fluoride pair where it will react with metal or evolve from metal fluoride depending on the flow of current.

Batteries based on a fluoride shuttle (fluoride ion battery, FIB) can theoretically provide high energy densities and can thus be considered as an interesting alternative to Li-ion batteries. Large improvements are still needed regarding their actual performance, in particular for the ionic conductivity of the solid electrolyte. At the current state of the art, two types of fluoride families can be considered for electrolyte applications: alkaline-earth fluorides having a fluorite-type structure and rare-earth fluorides having a tysonite-type structure. As regard to the latter, high ionic conductivities have been reported for doped LaF3 single crystals. However, polycrystalline materials would be easier to implement in a FIB due to practical reasons in the cell manufacturing. Hence, we have analyzed in detail the ionic conductivity of La(1-y)Ba(y)F(3-y) (0 ≤ y ≤ 0.15) solid solutions prepared by ball milling. The combination of DC and AC conductivity analyses provides a better understanding of the conduction mechanism in tysonite-type fluorides with a blocking effect of the grain boundaries. Heat treatment of the electrolyte material was performed and leads to an improvement of the ionic conductivity. This confirms the detrimental effect of grain boundaries and opens new route for the development of solid electrolytes for FIB with high ionic conductivities.

Fluoride ion batteries (FIB) provide an interesting alternative to lithium ion batteries, in particular because of their larger theoretical energy densities. These batteries are based on a F anion shuttle between a metal fluoride cathode and a metal anode. One critical component is the electrolyte that should provide fast anion conduction. So far, this is only possible in solid so-called superionic conductors, at elevated temperatures. Herein, we analyze in detail the ionic conductivity in barium fluoride salts doped with lanthanum (Ba1–x La x F2+x ). Doping by trivalent cations leads to an increase of the quantity of point defects in the BaF2 crystal. These defects participate in the ionic motion and therefore improve the ionic conductivity. We demonstrate that further improvement of the conductivity is possible by using a nanostructured material providing additional conduction paths through the grain boundaries. Using electrochemical impedance spectroscopy and AC conductivity analysis, we show that the ionic conduction in this material is controlled by the motion of vacancies through the grain boundaries. The mobility of the vacancies is influenced by the quantity of dopant but decrease for too large dopant concentrations. The optimum compositions having the highest conductivities are Ba0.6La0.4F2.4 and Ba0.7La0.3F2.3. The compound Ba0.6La0.4F2.4 was successfully used as an electrolyte in a FIB.

This article reports on the investigation of LaSrMnO 4 with K 2 NiF 4 type structure for use as an intercalation based high voltage cathode material with high capacity for fluoride ion batteries (FIBs). Charging was performed against PbF 2 based anodes and shows that fluoride intercalation proceeds stepwise to form LaSrMnO 4 F and LaSrMnO 4 F 2−x . Ex-situ X-ray diffraction experiments were recorded for different cutoff voltages for a deeper understanding of the charging process, highlighting additional potential of the method to be used to adjust fluorine contents more easily than using conventional fluorination methods. A discharging capacity of approximately 20−25 mAh/g was found, which is ∼4−5 times higher compared to what was reported previously on the discharging of BaFeO 2.5 /BaFeO 2.5 F 0.5 , approaching discharge capacities for conversion based fluoride ion batteries. Density functional theory based calculations confirm the observed potential steps of approximately 1 and 2 V for the first (LaSrMnO 4 → LaSrMnO 4 F) and second (LaSrMnO 4 F → LaSrMnO 4 F 2−x ) intercalation steps against Pb-PbF 2 , respectively. Additionally, a detailed structure analysis was performed for chemically prepared LaSrMnO 4 F 2−x (x ∼ 0.2), showing strong similarity to the product which is obtained after charging the batteries to voltages above 2 V against Pb-PbF 2 . It was observed that charging and discharging kinetics as well as coulomb efficiencies are limited for the batteries in the current state, which can be partly assigned to overpotentials arising from the use of conversion based anode composites and the stability of the charged sample toward carbon black and the current collectors. Therefore, the structural stability of LaSrMnO 4 F 2 on the deintercalation of fluoride ions was demonstrated by a galvanostatic discharging to −3 V against Pb-PbF 2 , which can be used to compensate such overpotentials, resulting in almost complete recovery of fluorine free LaSrMnO 4 with a discharge capacity of ∼100 mAh/g. This is the first report showing that selective extraction of fluoride ions from an oxyfluoride matrix is possible, and it highlights that compounds with K 2 NiF 4 type structure can be considered as interesting host lattices for the reversible intercalation/deintercalation of fluoride ions within intercalation based FIBs.

Lithium-sulphur batteries have generated tremendous research interest due to their high theoretical energy density and potential cost-effectiveness. The commercial realization of Li-S batteries is still hampered by reduced cycle life associated with the formation of electrolyte soluble higher-order polysulphide (Li2Sx, x = 4–8) intermediates, leading to capacity fading, self-discharge, and a multistep voltage profile. Herein, we have realized a practical approach towards a direct transformation of sulphur to Li2S2/Li2S in lithium-sulphur batteries by alteration of the reaction pathway. A coconut shell derived ultramicroporous carbon-sulphur composite cathode has been used as reaction directing template for the sulphur. The lithiation/delithiation and capacity fading mechanism of microporous carbon confined sulphur composite was revealed by analyzing the subsurface using X-ray photoelectron spectroscopy. No higher-order polysulphides were detected in the electrolyte, on the surface, and in the subsurface of the cathode composite. The altered reaction pathway is reflected by a single-step profile in the discharge/charge of a lithium-sulphur cell.

While the technological importance of carbon-based anodes for sodium-ion batteries is undebated, the underlying mechanism for sodium insertion and storage is still strongly disputed. Here, we present a joint experimental and theoretical study that allows us to provide detailed insights into the process of Na insertion in nongraphitizable (hard) carbon. For this purpose, we combine data from in situ Raman scattering of Na insertion in hard carbon with density functional theory-based lattice dynamics and band structure calculations for Na insertion in graphitic model structures used for a local description of graphitic domains in hard carbon. The agreement of experimental results and computational findings yields a clear picture of the Na insertion mechanism, which can be described by four different stages that are dominated by surface morphology, defect concentration, bulk structure, and nanoporosity. On the basis of the resulting model for sodium insertion, we suggest design strategies to maximize the capacity of hard carbon.

Fluoride ion batteries prepared in the discharged state using an anode composite of a mixture of Mg and MgF2 show promising cycling performances.

Fluoride ion batteries (FIBs) are among interesting electrochemical energy storage systems that are being considered as alternatives to lithium-ion batteries (LIBs). FIB offers high specific energy and energy density, thermal stability, and safety. Despite the advantages posed by the FIBs, several challenges need to be addressed to realize its full potential. We have been working on various aspects related to FIB with the aim of developing sustainable fluoride ion batteries. So far rechargeable FIBs have been demonstrated only at an elevated temperature like 150 °C and above. Here, for the first time, we demonstrate room-temperature (RT) rechargeable fluoride-ion batteries using BaSnF4 as fluoride transporting solid electrolyte. The high ionic conductivity of tetragonal BaSnF4 (3.5 × 10–4 S cm–1) enables the building of RT FIB. We built fluoride ion batteries using Sn and Zn as anodes and BiF3 as a cathode. We have investigated the electrochemical properties of two different electrochemical cells, Sn/BaSnF4/BiF3 and Zn/BiSnF4/BiF3 at various temperatures (25 °C, 60 °C, 100 °C, and 150 °C). The first discharge capacity of the Sn/BaSnF4/BiF3 and Zn/BiSnF4/BiF3 cells amounts to 120 mA h g–1 and 56 mA h g–1 at room temperature, respectively. Although Sn-based cells showed capacity fading, Zn-based cells provided relatively stable cycling behavior at low temperatures. High reversible capacities were observed at elevated operating temperature. EIS, ex-situ XRD, and SEM studies were performed on the cells to investigate the reaction mechanism.