Ceramic Synthesis of Disordered Lithium Rich Oxyfluoride and the Impact of Their Defects in Electrochemical Performances
Authors: Ateba Mba Jean-Marcel , Arcon Iztok , Mali Gregor , Tchernychova Elena , Witte Ralf , Kruk Robert , Dominko Robert 
Published: February 2020

We recently focused our interest on oxyfluorides as positive electrode materials for Li-ion batteries because they were proved to exhibit higher operating voltages vs. Li than oxides due to an increased ionicity of the M–(O,F) bonds. As an example, Li2VO2F exhibits a potential of 2.7 V vs. Li+/Li which is 200 mV higher than for Li2VO3 [1-3]. Interestingly, lithium-rich oxyfluorides display high theoretical capacity (463 mA.h/g) much higher than the well-known LiCoO2 (274mA.h/g) or LiFePO4 (170 mA.h/g). Our attention was also turn to other analogous phases of Li2VO2F such as Li2FeO2F and Li2TiO2F.
Li2MIIIO2F (with M = V, Ti and Fe) were synthesized by high energetically ball milling. The obtained materials prove to be metastable (highly non-equilibrium state) with compositional inhomogeneity. Moreover, the reactants are not well mixed on the atomic scale and can produce amorphous phase. To the best of our knowledge ceramic synthesis of Li2MO2F have never been reported. Yet, the ceramic route performed at high temperature can result in more crystalline and stable phase. Recently, we have shown that the ceramic synthesis of Li2MIIIO2F is possible [4-5]. The oxyfluorides obtained in this work by ceramic synthesis crystalize in FCC (Space group N° 225). The structure can be described as a layered phase, in which lithium and the transition metal cations segregate into layers in the cubic (111) direction (Figure 1).
The process by which energy is stored in these materials constitutes a change in the design of battery materials and involves unexpected and surprisingly effective mechanisms: instead of storing lithium ions by intercalation into a stable host, lithium ions are populating and vacating lattice sites of the material itself [1,2]. This new principle leads to unprecedented higher energy and higher power density compared to other battery materials. As results, large amounts of energy and power can be stored and/or supplied from small spaces. Moreover, the presence of defect (intrinsic and/or extrinsic schottky defects) plays and important role in the electrochemical performances of lithium rich oxyfluoride. Futhermore, the study of Li2MIIIO2F materials was followed up by various techniques among which X-rays absorptions spectroscopy, NMR, Mössbauer spectroscopy and neutron diffraction.

References

[1] R. Chen, S. Ren, M. Knapp, D. Wang, R. Witter, M. Fichtner, and H. Hahn, “Disordered lithium-rich oxyfluoride as a stable host for enhanced Li+ intercalation storage,” Adv. Energy Mater., vol. 5, no. 9, 2015.
[2] R. Chen, S. Ren, M. Yavuz, A. A. Guda, V. Shapovalov, R. Witter, M. Fichtner, and H. Hahn, “Li + intercalation in isostructural Li2VO3 and Li2VO2F with O2− and mixed O2− /F − anions,” Phys. Chem. Chem. Phys., vol. 17, no. 26, pp. 17288–17295, 2015.
[3] V. Pralong, V. Gopal, V. Caignaert, V. Duffort, and B. Raveau, “Lithium-rich rock-salt-type vanadate as energy storage cathode: Li2-x
VO3,” Chem. Mater., vol. 24, no. 1, pp. 12–14, 2012.
[4] J-M. Ateba Mba, R. Dominko, “Lithium-rich transistion metal oxyfluorides with cubic dense packing as high energy density cathode material for Li-ion batteries.” The 69th Annual Meeting of the International Society of Electrochemistry, 2-7 September 2018, Bologna, Italy.
[5] J-M. Ateba Mba, I. Arčon, G. Mali, R. Dominko, “High Energy Density Li2MO2F Cathode Materials for Li-ion Batteries”, European Materials Research Society, 17-20 September 2018, Warzaw , Poland

Acknowledgement: This work has received funding from the European Union‘s Horizon 2020 research and innovation program under Grant Agreement No 711792 (LiRichFCC), from the Slovenian Research Agency research programs P2-0393 and P1-0112, from the project CALIPSOplus under the Grant Agreement 730872, from the EU Framework Program for Research and Innovation HORIZON 2020. We acknowledge access to the SR facilities of ELETTRA (beamline XAFS, pr. 20175158). We would like to thank Simone Pollastri, Mateusz Czyzycki, Giuliana Aquilanti, and Luca Olivi of ELETTRA for assistance during the experiment.

Figur1:
(left and right) Voltage profile vs. capacity at C/20 for different type of lithium rich oxyfluorides cycled in Swagelok half-cell with Li metal as anode. (Middle) a 3D representation of their corresponding crystal structure

Figure 1