Aluminium behaviour in preparation process of lithium iron phosphate and its effects on material electrochemical performance
Lv Zhang, Dewen Zeng
Abstract
Lithium iron phosphate (LiFePO 4 ) recovered from waste LiFePO 4 batteries inevitably contains impurity aluminium , which may affect material electrochemical performance . Nearly all references believe that aluminium-doped LiFePO 4 is a solid solution and that the material capacity increases firstly before decreasing with aluminium content. However, their reported performance of the aluminium-free LiFePO 4 as a comparison is far lower than commercial LiFePO 4 currently manufactured at large scales. Thus, conclusions drawn based on such a comparison are questionable. To better understand the effects of aluminium on LiFePO 4 electrochemical performance, we first tracked aluminium behaviour in FePO 4 ·2H 2 O, FePO 4 precursors and LiFePO 4 product. In FePO 4 ·2H 2 O prepared from aqueous solution, aluminium exists as AlPO 4 ·2H 2 O in two types (monoclinic and orthorhombic systems) of solid solutions (Fe 1-x Al x PO 4 ·2H 2 O). In Fe 1-x Al x PO 4 dehydrated from Fe 1-x Al x PO 4 ·2H 2 O, trigonal AlPO 4 and FePO 4 firstly form a solid solution (Fe 1-x Al x PO 4 , berlinite). At higher aluminium content (x ≥ 0.026), additional AlPO 4 forms an independent phase in monoclinic system. In Li 1-x Fe 1-x Al x PO 4 /C (x = 0–0.048), no evidence of solid solution formation was observed. Secondly, the effects of aluminium on Li 1-x Fe 1-x Al x PO 4 /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.