Low-Concentration Electrolyte Enables High-Voltage Positive Electrode Na4Co3(PO4)2P2O7 with Good Cycle Stability

Low energy density has been a limiting factor for further large-scale commercialization of sodium-ion batteries (SIBs). In this work, we prepare a high-voltage cathode material Na4Co3(PO4)2P2O7 with a low carbon content by a simple solid-state method. In order to counter the poor oxidative stability of the conventional electrolyte, the highly fluorinated co-solvent 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) is introduced to adjust the Na+ solvation sheath structure. Consequently, we design a low concentration SIB electrolyte (0.3 M NaPF6 in PC/FEC/TTE = 1/1/4, v/v/v). The oxidation stability of this electrolyte is greatly improved due to the special Na+ solvation sheath structure. Under high-voltage cycling, it can form a dense and thin cathode electrolyte interphase film on the cathode surface, which improves the interfacial stability between the electrolyte and the cathode material. Thus, the Na||NCPP cells in the 0.3 M-114 electrolytes maintain good cycle stability even when charged to a high voltage of 4.7 V. Meanwhile, it achieves a reversible capacity of 95.7 mA h g-1 in the initial cycle and a capacity retention of 94.25% after 150 cycles at 1 C. Moreover, it has an energy density of up to 420 W h kg-1 at 1 C. This study provides a practical way to establish a stable interface between the high-voltage positive electrode and the electrolyte in SIBs.