The structural origin of enhanced stability of Na3.32Fe2.11Ca0.23(P2O7)2 cathode for Na-ion batteries

The storage of renewable energy depends largely on sustainable technologies such as sodium-ion batteries with high safety, long lifespan, low cost, and non-toxicity. Pyrophosphate Na3.32Fe2.34(P2O7)2 cathode could meet this requirement, however, its structural stability needs to be further enhanced for practical purposes. To overcome this problem, Na-deficient Na3.32Fe2.11Ca0.23(P2O7)2 with exceptional stability is prepared by Ca selective doping in this work. In operando synchrotron-based X-ray diffraction (SXRD) and in situ X-ray absorption near edge spectroscopy (XANES) results reveal that the prepared Na3.32Fe2.11Ca0.23(P2O7)2 is a single-phase solid-solution reaction with high reversibility. A strong correlation between the voltage curve and lattice parameters is deciphered for the first time. Additionally, the atomic-doping-engineering strategy could significantly enhance the thermal and electrochemical stability of the electrode materials, contributing to their good structural reversibility and enhanced operational safety. Specifically, after 1000 cycles at 1 C, the Ca doped electrode achieves a high capacity retention of 81.7%, which is much better than that of the un-doped electrode (15.5%). Our work may pave a new avenue for designing safe and low-cost cathode materials for battery applications with long cycle life.