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

• Charge balance is kept in single phase Li1−3xNa2xMgxMn0.8Fe0.2P1−xSixO4, x ⩽ 0.08.
• The doping strategy can expand the crystal lattice and reduce the particle size.
• The doping level as low as 0.01 results in the best electrochemical performance.

Li1−3xNa2xMgxMn0.8Fe0.2P1−xSixO4 (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 LiMn0.8Fe0.2PO4 without introducing any impurity phase when x is not higher than 0.08, but NaFePO4 appears as an impurity phase when x is as high as 0.16. Charge balance is maintained in Li1−3xNa2xMgxMn0.8Fe0.2P1−xSixO4 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, Li0.97Na0.02Mg0.01Mn0.8Fe0.2P0.99Si0.01O4 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.