Effects of Fe and Mn co-doping on electrochemical properties of Li3V2−2xFexMnx(PO4)3/graphene cathode for lithium secondary battery

A series of Fe and Mn co-doped Li
₃
V
_2-2
xFexMnx(PO
₄
)
₃
/graphene (x = 0, 0.005, 0.010, 0.015, 0.020, and 0.025) samples have been fabricated via a sol-gel assisted microwave synthesis method. The crystal structure and morphology of the synthesized Li3V2-2xFexMnx(PO4)3/graphene was investigated using XRD and SEM systematically. Electrochemical measurement results prove that the Li
₃
V
_1.98
Fe
_0.01
Mn
_0.01
(PO
₄
)
₃
/graphene sample presents the best electrochemical performance among the as-prepared samples. The resulting Li
₃
V
_1.98
Fe
_0.01
Mn
_0.01
(PO
₄
)
₃
/graphene cathode has an excellent specific capacity of 184.4 mAh g
^-1
at 0.1C rate, and even at 10C rate, it still delivers the specific capacity of 110.5 mAh g
^-1
. The substitution of V
³⁺
with an appropriate amount of Fe
²⁺
and Mn
⁴⁺
increases the Li
⁺
diffusion coefficient assuming from the electrochemical impedance spectroscopy results, which is mainly responsible for the excellent electrochemical performance.