Li1−3xNa2xMgxMn0.8Fe0.2P1−xSixO4 as cathode materials for lithium ion batteries

Abstract Li 1−3 x Na 2 x Mg x Mn 0.8 Fe 0.2 P 1− x Si x O 4 ( x  = 0, 0.01, 0.02, 0.04, 0.08, 0.16) are studied as cathode materials for lithium ion batteries. Na, Mg and Si can be incorporated into the structure of LiMn 0.8 Fe 0.2 PO 4 without introducing any impurity phase when x is not higher than 0.08, but NaFePO 4 appears as an impurity phase when x is as high as 0.16. Charge balance is maintained in Li 1−3 x Na 2 x Mg x Mn 0.8 Fe 0.2 P 1− x Si x O 4 because the oxidation states of Na, Mg, Mn, Fe, P and Si can be considered as +1, +2, +2, +2, +5 and +4, respectively. The first charge/discharge capacity, the discharge capacity at each current density and the capacity retention upon cycling first increase and then decrease with increasing x , and the maximum values are always obtained when x is equal to 0.01. At the current density of 20  C , Li 0.97 Na 0.02 Mg 0.01 Mn 0.8 Fe 0.2 P 0.99 Si 0.01 O 4 has a discharge capacity as high as 84 mA h g −1 and 98.3% of the initial discharge capacity is retained after 100 cycles.