Enhanced Stability of a Pyrophosphate 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 Na_3.32Fe_2.34(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 Na_3.32Fe_2.11Ca_0.23(P₂O₇)₂ with exceptional stability is prepared by Ca selective doping in  this work. Operando synchrotron-based X-ray diffraction (SXRD) and in situ X-ray absorption near edge spectroscopy (XANES) results reveal that the prepared Na_3.32Fe_2.11Ca_0.23(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 undoped electrode (15.5%).