Entropy-stabilized engineering enables stable high-voltage phosphate cathode materials for sodium-ion batteries

High-voltage phosphate cathodes are a kind of promising electrode materials for constructing high-energy density sodium-ion batteries (SIBs). However, as a typical representative, Na$_4$Co$_3$(PO$_4$)$_2$P$_2$O$_7$ (NCPP), which commonly suffers from inevitably complicated structural evolution and sluggish kinetics resulting in generally unsatisfactory rate and cycling performance, which severely hinders its practical applications as ultra high-voltage cathode materials. Herein, Na$_4$Co$_2$Fe(PO$_4$)$_2$P$_2$O$_7$ (NCFPP) is designed to optimize the Co$^{2+}$/Co$^{3+}$ redox reaction, enabling a highly reversible single-phase transformation mechanism that displays enhanced rate capability and cycling stability. Furthermore, an entropy-regulation strategy is proposed to further strengthen the structural stability by suppressing undesirable topotactic phase transition. As a result, the designed medium-entropy cathode, Na$_{3.7}$Co$_{1.5}$Fe$_{0.75}$(MgAlCuZn)$_{0.2}$(PO$_4$)$_2$P$_2$O$_7$ (ME-NCFPP), delivers remarkably ultra-long cycling stability (80.3% capacity retention after 10,000 cycles at 10 C) and excellent two-year storage performance, far surpassing the NCFPP electrode. ... mehrAdditionally, the ME-NCFPP||hard carbon (HC) full cell displays excellent cycling stability. The underlying sodium-storage mechanism of the ME-NCFPP electrode is systematically unraveled through theoretical calculations combined with advanced characterization techniques, including in situ X-ray diffraction and synchrotron-based X-ray absorption spectroscopy. This work highlights the critical role of entropy engineering in suppressing multi-phase transitions, paving the way for constructing highly stable high-voltage cathodes for SIBs.