Aluminium behaviour in preparation process of lithium iron phosphate and its effects on material electrochemical performance

Lithium iron phosphate (LiFePO4) recovered from waste LiFePO4 batteries inevitably contains impurity aluminium, which may affect material electrochemical performance. Nearly all references believe that aluminium-doped LiFePO4 is a solid solution and that the material capacity increases firstly before decreasing with aluminium content. However, their reported performance of the aluminium-free LiFePO4 as a comparison is far lower than commercial LiFePO4 currently manufactured at large scales. Thus, conclusions drawn based on such a comparison are questionable. To better understand the effects of aluminium on LiFePO4 electrochemical performance, we first tracked aluminium behaviour in FePO4 center dot 2H(2)O, FePO4 precursors and LiFePO4 product. In FePO4 center dot 2H(2)O prepared from aqueous solution, aluminium exists as AlPO4 center dot 2H(2)O in two types (monoclinic and orthorhombic systems) of solid solutions (Fe1-xAlxPO4 center dot 2H(2)O). In Fe1-xAlxPO4 dehydrated from Fe1-xAlxPO4 center dot 2H(2)O, trigonal AlPO4 and FePO4 firstly form a solid solution (Fe1-xAlxPO4, berlinite). At higher aluminium content (x >= 0.026), additional AlPO 4 forms an independent phase in monoclinic system. In Li1-xFe1-xAlxPO4/C (x = 0-0.048), no evidence of solid solution formation was observed. Secondly, the effects of aluminium on Li1-xFe1-xAlxPO4 electrochemical performance were studied. It is found that, material specific discharge capacity only decreases monotonically with addition of aluminium at optimal calcination temperature (as 760 degrees C), unlike references, which increases firstly before decreasing. Finally, it is found that aluminium has no remarkable impact on material cycling stability under 100 cycles at 1C rate using coin test. (C) 2021 The Author(s). Published by Elsevier B.V.

Automated summary

The presence of aluminium impurities in lithium iron phosphate (LiFePO4) affects its electrochemical performance. Studies have shown that at optimal calcination temperature, the addition of aluminium leads to a monotonic decrease in specific discharge capacity, and aluminium has no remarkable impact on material cycling stability.