The materials currently attracting most interest as positive electrodes for Lithium-ion batteries are Li and Mn-rich layered oxides that exhibit outstanding energy densities at an affordable cost.¹ A common feature for all these layered oxides is a high capacity “plateau”, observed only at the end of the first charge, once all the transition metal ions are already at the tetravalent state. That behavior has been explained by the reversible participation of oxygen anions in the redox processes, thanks to hybridization between their p levels and the d levels of the transition metals. This reaction is reversible within the bulk, occurring without any major structural modification, while oxidized oxygen ions are lost at the surface causing irreversible structural reorganizations at the outer part of the particles, those being at the origin of a continuous voltage decay upon cycling.^2,3 We will show how we tried to stabilize concentration gradients and core-shell composites with Li and Mn-rich layered oxides in the bulk and stoichiometric layered oxides at the outer part of the spherical aggregates,⁴ with the goal to combine high energy density and chemical stability respectively.

We will also highlight that Tavorite-type compositions offer a very rich crystal chemistry, among which LiVPO₄F has the highest theoretical energy density (i.e. 655 Wh/kg).⁵ New Tavorite-type compositions were recently obtained: LiVPO₄OH and LiVPO₄F_1-yO_y, for these latter by direct syntheses or by aging of LiVPO₄F upon oxidation in air.^6-8 We will show how we can tailor the structure, the potential and the reaction mechanism involved, playing with the composition of the Tavorite-type phases. We will discuss how detrimental/positive the defects can be on the electrochemical properties of the mixed oxy-fluorophosphates LiVPO₄F_1‑yO_y.

Acknowledgements:

These researches are funded by Région Nouvelle Aquitaine for layered oxides and by the French National Research Agency ANR (Labex STORE EX and project HIPOLITE) for polyanionic materials. The authors thank also the French network RS2E (http://www.energie-rs2e.com), the European network ALISTORE-ERI (http://www.alistore.eu), FEDER and Région Haut-de-France.

References:

[1] Croguennec, L.; Palacin, M. R., Journal of the American Chemical Society 2015, 137, 3140-3156

[2] Koga, H.; Croguennec, L.; Ménétrier, M.; Douhil, K.; Belin, S.; Bourgeois, L.; Suard, E.; Weill, F.; Delmas, C.,^, Journal of the Electrochemical Society 2013, 160, A786-A792

[3] Genevois, C. ; Koga, H. ; Croguennec, L. ; Ménétrier, M. ; Delmas, C. ; Weill, F., Journal of Physical Chemistry C 2015, 119, 75-83

[4] Pajot et al., in preparation

[5] C. Masquelier and L. Croguennec, Chemical Reviews 2013, 113, 6552−6591

[6] Boivin, E.; Chotard, J.-N.; Ménétrier, M.; Bourgeois, L.; Bamine, T.; Carlier, D.; Fauth, F.; Suard, E.; Masquelier, C.; Croguennec, L., Journal of Material Chemistry A 2016, 4, 11030–11045.

[7] Boivin, E.; Chotard, J.-N., Ménétrier, M.; Bourgeois, L.; Bamine, T.; Carlier, D.; Fauth, F.; Masquelier, C.; Croguennec, L., Journal of Physical Chemistry C 2016, 120(46), 26187-26198

[8] Boivin et al., in preparation