Kinetics of the Dehydration of Lithium Dihydrogenphosphate

Phosphate-based lithium materials, such as lithium iron phosphate (known as lithium ferrophosphate, LiFePO4, LFP), are among the safest materials for large-scale lithium-ion batteries due to the stability of the phosphate-bound oxygen at elevated temperatures. LFP can be melt-synthesized where the kinetics is faster, allowing for coarser and lower cost reactants. The most common lithium- and phosphate-bearing reactants can react violently upon heat-up and release a large volume of gaseous by-product. Lithium metaphosphate (LiPO3, LPO) can improve the processability and safety of the operation. In this work, we investigate the thermal decomposition of lithium dihydrogenphosphate (LiH2PO4, LHP) to LPO up to 400 degrees C. The decomposition was analyzed by isothermal and constant rate differential thermogravimetric (DTG) experiments. Activation energy profiles were estimated by an isoconversional model-free approach and kinetic model fitting. Li5H4P5O17 (L2.5) was determined to be the most stable reaction intermediate and can be isolated at temperatures between 200 and 240 degrees C. The resulting reaction is comprised of 6 reactions, where the LHP is progressively polymerized by condensation reactions leading successively to L2.5, Li3H2P3O10 (L3), Li4H2P4O13 (L4), and LPO. The first reaction step (LHP -> L2.5) was fitted with 3 reactions series/parallel describing the solid surface reaction, the viscous/liquid surface reaction, and the bulk reaction. Limiting the reaction temperature to 400 degrees C results in a solid product that can be advantageous if LPO is to be prepared in advance and dosed for LFP synthesis.