Chemical Delithiation Investigation of Olivine LiFePO4

LiFePO₄ is currently one of the most attractive materials for positive electrode in Li-ion batteries due to its low ecological print and for safety issues. Currently, sodium-ion batteries show a great interest and LiFePO₄ tends to be replaced by NaFePO₄ for cost advantages. As the direct synthesis of NaFePO₄ leads to the maricite, phase poorly electrochemically active, the only way to keep the olivine framework is to chemically delithiated LiFePO₄. Mechanisms of delithiation of LiFePO₄ have been previously studied to determine the model of Li ions deintercalation: Core-Shell [1-2] or Domino-Cascade [3]. Recent work [4] shows a Domino-Cascade mechanism in the case of electrochemical delithiation while the chemical delithiation will correspond to a Core-Shell process. However, the chemical delithiation have been investigated using mainly strong oxidizing agents like NO₂BF₄ or K₂S₂O₈which create some defects due to the fast and violent reaction [1]. So it is interesting to understand the chemical delithiation mechanisms versus microstructure of the powders and the delithiation conditions.

LiFePO₄was synthesized by two different routes, hydrothermal (LFP-H) and precipitation (LFP-P). The synthesis process and the phosphate sources strongly impact the purity and the microstructure of the powders.

 LFP-H presents 2 types of particles, nano-rods and platelets (Fig. 1a) while LFP-P are constituted of agglomerated nano-spheroids with grain boundaries (Fig. 1b).

The kinetic of the chemical delithiation was studied on these LiFePO₄ samples by testing different delithiation conditions: oxidizing agents (I₂, Br₂ and NO₂BF₄), solvents (acetonitrile and chloroform) and reaction time (from 15 min to 96 h). The LiFePO₄/FePO₄ ratio was determined by Rietveld refinement and by elemental nuclear analysis to quantify precisely the very low remaining lithium amount after the complete delithiation. The LiFePO₄morphology has a large impact on the chemical delithiation kinetic : LFP-H platelets are fully delithiated in less than one hour whereas LFP-P spheroids need a hundred of hours.

The HR-TEM and STEM-EELS measurements on partially delithiated samples (Fig. 2) show for the first time a shrinking core mechanism with a core of LiFePO4 surrounded by a shell of FePO4. In particular the role of the grain boundaries in between the grains is shown in the mechanism of de chemical delithiation.

Finally, FePO₄phases were tested as positive electrode material in Li-ion and Na-ion batteries. The electrochemical intercalation/deintercalation is interpreted in the perspective of the previous chemical delihiation.

References :

[1] L. Laffont, C. Delacourt, P. Gibot, M. Y. Wu, P.  Kooyman, C. Masquelier, J.M. Tarascon, Study of the LiFePO₄/FePO₄two-phase system by High Resolution Electron Energy Loss Spectroscopy, Chem. Mater. 18 (2006) 5520

[2] G. Chen, X. Song, T.J. Richardson, Electron Microscopy Study of the LiFePO₄to FePO4 Phase Transition, J. Electrochem. Solid-State Lett., 9 (2006) A295-A298

[3] C. Delmas, M. Maccario, L. Croguennec, F. Le Cras, F. Weill, Lithium deintercalation in LiFePO₄nanoparticles via a domino-cascade model, Nat. Mater. 7(2008) 665.

[4] G. Brunetti, D. Robert, P. Bayle-Guillemaud ; J. L. Rouvières, E. F. Rauch, J. F. Martin, J. F. Colin, F. Bertin, C. Cayron, Confirmation of the Domino-Cascade Model by LiFePO₄/FePO₄ Precession Electron Diffraction, Chem. Mater. 23 (2011) 4515-4524.