Zero-strain Co-doped Na2FeP2O7/carbon delivers superior rate capability and long-cycle stability for sodium ion batteries

Na2FeP2O7 cathode material exhibits great promise in sodium ion batteries (SIBs), due to its low cost and high structural durability. Nevertheless, the intrinsically poor electronic conductivity and sluggish Na+ diffusion kinetics of Na2FeP2O7 hinder its practical application. Herein, a synergistic cooperation of continuous amorphous carbon layer and optimized Co doping strategy is proposed to design high-performance SIB cathode (Co-doped Na2FeP2O7/C) via in-situ sol–gel method, where Co integration significantly enhances both the electronic conductivity and the Na+ diffusion kinetics, as confirmed by the combination of ex/in-situ characterizations and density functional theory calculations. Valuably, the synergy of amorphous carbon layer and appropriate Co doping restrains the structure stress to "zero-strain", Which remains beneficial for 0.10Co-NFO@C to display a high capacity of 96.12 mAh g-1 at 0.05C and a high-rate capacity of 61.75 mAh g-1 at 60C with the capacity retention of 79.9% after 5000 cycles. Moreover, optimized Co-doped Na2FeP2O7/C also shows outstanding cycling performance and rate capability when assembled into full batteries (hard carbon as the anode), demonstrating a reversible capacity of ∼110.9 mAh g-1 at 50 mA g−1, and a respectable capacity retention of 78.0% at 500 mA g−1 for 500 cycles. The strategy inspires new advances towards zero-strained cathode materials for high performance SIBs.