Chemical Delithiation Investigation of Olivine LiFePO4
Authors: Lachal Marie , Bouchet Renaud , Boulineau Adrien , Rossignol Cécile , Alloin Fannie , Obbade Said 
Published: February 2020

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 :

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Figure 1