Solid State and Structural Chemistry Unit


BANGALORE – 560 012

Speaker: Dr. M. Anji Reddy

Helmholtz Institute Ulm Electrochemical Energy Storage

Helmholtzstraße, Germany


Unlocking the Potential of Metal Fluorides for Electrochemical Energy Storage

Date & Time : Friday 15th February 2019 at 11:00 AM
Venue : SSCU Auditorium
Fluorine has outstanding potential as an electrode due to its high electronegativity and comparably lightweight. Employing fluorine in elemental form as a component of electrode material (in contrast to Li) is difficult, however, due to its high chemical reactivity and gaseous state. Instead, the fluoride anion (F-) may be utilized as an electrochemically stable transport ion between two electrodes. By choosing an appropriate metal/metal fluoride system combined with suitable fluoride transporting electrolyte, novel electrochemical cells can be built. Indeed, primary electrochemical cells based on fluoride transfer were realized four decades back [1], but largely overlooked until the demonstration of rechargeable fluoride ion batteries (FIBs) [2]. Since the demonstration of rechargeable FIBs, we are working on various aspects related to FIBs with the aim of developing sustainable fluoride ion batteries [3-6]. So far rechargeable FIBs have been demonstrated only at an elevated temperature like 150 °C and above. Recently, for the first time, we have demonstrated room-temperature (RT) rechargeable fluoride-ion batteries using BaSnF4 as fluoride transporting solid electrolyte [7]. BaSnF4 exhibits high ionic conductivity of 3.5×10-4 S cm-1 at RT but limited by low electrochemical stability window. In contrast tysonite-type, La0.9B0.1F2.9 electrolyte shows a large electrochemical stability window, but it has the drawback of lower ionic conductivity at RT (0.4×10−6 S cm−1). To overcome these limitations of the low electrolyte stability of BaSnF4 and low ionic conductivity of La0.9B0.1F2.9, we developed an interlayer electrolyte. Pressing a thin layer of La0.9Ba0.1F2.9 together with BaSnF4 enhanced the total conductivity of the pellet (compared to pure La0.9Ba0.1F2.9) while it physically isolated the less stable and highly conductive electrolyte (BaSnF4) from the anode (Ce). This approach allowed the demonstration of relatively high voltage FIBs at RT which otherwise inoperable either with BaSnF4 or La0.9Ba0.1F2.9 electrolyte alone [8]. Apart from applications in FIBs, transition metal fluorides could be used as high capacity cathode materials for lithium-ion batteries (LIBs), due to their ability to reversibly react with Li at relatively high potentials. However, metal fluorides are electrical insulators, exhibiting slow reaction kinetics. Consequently, efficient synthesis of metal fluoride-carbon nanocomposites is crucial to get adequate electrochemical performance. We have developed a new and facile one-step method for the chemical synthesis of novel carbon-metal fluoride nanocomposites and established their feasibility as cathode materials for LIBs [9-10]. For sodiumion batteries (SIBs) we propose weberite-type sodium metal fluorides (SMF), a new class of high voltage and high energy density materials which are so far unexplored as cathode materials for SIBs [11]. Weberite-type SMF is the only class of compounds that can offer high energy density than the state-of-the-art LIB cathodes. More than 70 compounds with Na2MM’F7 composition adopts the weberite-type structure, which demonstrates the high stability of the structure. We have modeled 22 known and 10 new compounds. Apart from the high energy density, the weberite-type structure shows low Na diffusion barriers with pseudo-3D diffusion paths. The high energy density combined with low diffusion barriers for Na makes this type of compounds promising as cathode materials for SIBs. Also, we found few lithium metal fluorides serve as excellent solid Li+ conducting electrolytes for solid-state lithium batteries, overcoming the short comes of ceramic and sulphide based solid electrolytes [12]. Overall, the presentation will highlight the potential of metal fluorides in electrochemical energy storage systems.


1. J. A. Schoonman, J. Electrochem. Soc. 1976, 123, 1772.

2. M. Anji Reddy and M. Fichtner. J. Mater. Chem. 2011, 21, 17059.

3. C. Rongeat, M. Anji Reddy, R. Witter, and M. Fichtner, J. Phys. Chem. C, 2013, 117, 4943.

4. C. Rongeat, M. Anji Reddy, T. Diemant, J. Behm, M. Fichtner, J. Mater. Chem. A, 2014, 2, 20861.

5. C. Rongeat, M. Anji Reddy, R. Witter, M. Fichtner, ACS Appl. Mater. Interfaces, 2014, 6, 2103.

6. M. Irshad, J. Chable, R. Witter, M. Fichtner, M. Anji Reddy, ACS Appl. Mater. Interfaces, 2018, 10, 17249.

7. M. Irshad, R. Witter, M. Fichtner, and M. Anji Reddy, ACS Appl. Energy Mater. 2018, 1, 4766.

8. M. Irshad, R. Witter, M. Fichtner, and M. Anji Reddy, submitted.

9. M. Anji Reddy et al, Adv. Energy Mater. 2013, 3, 308.

10. M. Anji Reddy et al., Energy Technol. 2016, 4, 201.

11. H. Euchner, O. Clemens, M. Anji Reddy, submitted.

12. M. Feinauer, H. Euchner, M. Fichtner, M. Anji Reddy under preparation.