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

Abstract

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 FePO4·2H2O, FePO4 precursors and LiFePO4 product. In FePO4·2H2O prepared from aqueous solution, aluminium exists as AlPO4·2H2O in two types (monoclinic and orthorhombic systems) of solid solutions (Fe1-xAlxPO4·2H2O). In Fe1-xAlxPO4 dehydrated from Fe1-xAlxPO4·2H2O, trigonal AlPO4 and FePO4 firstly form a solid solution (Fe1-xAlxPO4, berlinite). At higher aluminium content (x ≥ 0.026), additional AlPO4 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/C 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 °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.