Multi-Metallic Template Fluorination Mmtf for the Preparation of Ternary Metal Fluoride and Their First Use As Cathodes in Solid State Lithium Batteries
Authors: Eveillard Fabien , Leroux Fabrice , Batisse Nicolas , Delbègue Diane , Guerin Katia 
Published: July 2021

There is an urgent need in exploring new cathode materials for Lithium batteries in order to increase their capacities. Conversion type cathodes are relevant replacement candidates and especially metal fluorides such as FeF3 with a theoretical capacity of 712 mAh g-1 for three electrons at a voltage of 2.74 V vs. Li+/Li or CuF2 with a theoretical capacity of 528 mAh g-1 for two electrons at 3.55 V vs. Li+/Li. However, important voltage hysteresis, dissolution in organic electrolytes and poor reversibility hinder their use as reliable cathode materials in Lithium batteries. In this work, compatibility of fluorides with a PEO/LiTFSI (Poly Ethylene Oxide impregnated with Lithium bis(trifluoromethanesulfonyl)imide) solid electrolyte membrane is evaluated to avoid dissolution of active materials in organic liquid electrolytes often encountered with fluorides. Additionally, a general synthesis method is used to prepare ternary metal fluorides. Indeed, ternary metal fluorides were recently explored1–3 as a solution to buffer volume expansion occurring during conversion, hence greatly reducing voltage hysteresis and improving cyclability. These materials are usually prepared by planetary ball milling or by hydrofluoric acid HF-assisted fluorination, both methods not scalable for industrial purpose. In this work, we propose Multi-Metallic Template Fluorination MMTF as a general synthesis way to prepare ternary metal fluorides in a scalable way suitable for industrial applications.

Prussian Blue Analogs are particularly relevant fluorination templates: these compounds can be easily prepared in great quantities by coprecipitation, enabling industrial applications. Additionally, a wide variety of compositions can be synthesized while their open structure facilitates gas solid fluorination process used. Multi Metallic Template Fluorination MMTF on PBA therefore enables the preparation of fluoride compounds with desired metallic stoichiometry by tuning it in the pristine PBA. For electrochemical applications considered in this work, Copper based Prussian Blue Analog CuPBA of general formula Cu3[Fe(CN)6]2.xH2O is chosen to combine properties of copper and iron redox centers in a ternary metal fluoride cathode. By monitoring thermal transformations of CuPBA using ThermoGravimetric Analyses coupled to Mass Spectrometry (TGA MS) and XRD, different temperature domains are identified under air, assessing that the CuPBA template can be maintained up to 170 °C while oxides are formed at 350 °C. Fluorination temperatures are selected in consequence: structures of fluorinated products are characterized by combined XAS and XRD analyses. Results indicate the formation of an anti-perovskite [Cu(H2O)4]3.(FeF6)2 phase at low temperatures, showing that Multi Metallic Template Fluorination can proceed in a topotactic manner by maintaining a Multi Metallic Template in the fluorinated product. After a severe change in the local order identified by XAS at 200 °C and especially around copper cations, a biphasic CuF2/FeF3 product is formed at high temperatures.

Electrochemical signature of different CuPBA fluorination products are investigated by Cyclic Voltammetry (CV) and galvanostatic measurements with a solid electrolyte membrane. Comparisons with a mechanical milling CuF2/FeF3 reference but also with pristine CuF2 and FeF3 commercial references cycled in the same conditions are presented. Results reveal that CuPBA fluorinated at 350 °C is able to match a CuF2/FeF3 mechanical milling reference and even perform slightly better. Both materials perform much better than the mere addition of CuF2 and FeF3 signatures, confirming the superiority of ternary metal fluorides as cathode materials over binary metal fluorides stated in literature. Better performances of CuPBA Multi Metallic Template Fluorination MMTF product at high temperatures over mechanical milling reference are explained by an homogeneous dispersion of copper and iron redox centers present in the initial CuPBA template resulting in a better distribution of CuF2 and FeF3 domains in the fluorinated product at 350 °C than in the mechanical milling reference.

The first discharge capacity approaches the maximum theoretical capacity of 600 mAh g-1 and highlights the possibility to dramatically increase the capacity of Lithium batteries by potentially doubling the capacity reached with classical NMC electrodes. Multi Metallic Template Fluorination therefore enables the preparation of complex fluorinated cathodes in a scalable way for industry with interesting features over a simple mechanical milling.

The Multi Metallic Template Fluorination MMTF approach detailed in this work can be adapted to a wide range of multi metallic templates such as Metal Organic Frameworks and Layered Double Hydroxides for the preparation of mixed metal fluorides with unique features such as particular structures and/or morphologies depending on the desired application and in a scalable way.

(1) Villa et al. ACS Appl.
Mater. Interfaces
2019, 11 (1), 647–654.

(2) Wang et al. Nat Commun
2015, 6 (1), 6668.

(3) Ding et al. ACS Appl. Mater. Interfaces
2019, 11 (4), 3852–3860.

Figure 1