Magnesium-doped Na2FeP2O7 cathode materials for sodium-ion battery with enhanced cycling stability and rate capability

Magnesium doping is successfully utilized to enhance the electrochemical performance of Na2FeP2O7 for sodium-ion batteries (SIBs). A series of morphological and electrochemical tests indicate that Mg doping does not affect the structures of the host phase and the abundant defects in carbon coating are beneficial for electronic conductivity. A new peak at ~2.70 V during the first few cycles in CV curves is observed, which is related to the increase of specific capacity during the lifespan test. DFT calculation results reveal the enhanced electronic conductivity in Mg doped Na2FeP2O7, which is consistent with experimental observations. The Na2FeP2O7 with an appropriate Mg doped of 5% exhibits an initial reversible capacity of 90 mAh g−1 at 0.1 C without an obvious decline after 2000 cycles at 9 C. The specific capacity of Mg doped Na2FeP2O7 does not decrease fatally even the amount of Mg doped comes to 20%. The Coulombic efficiency is very high, close to 100% in all samples. The excellent electrochemical performance of Mg doped Na2FeP2O7 presents great potential as cathodes for advanced SIBs.