A configuration entropy enabled high-performance polyanionic cathode for sodium-ion batteries

Polyanionic sodium ion cathodes have attracted lots of concern because of their excellent structural stability. However, the low specific capacity is still a pressing issue hampering their practical application. In this work, a medium-entropy NASICON-structure cathode Na${}_{3.5}$V${}_{0.5}$Mn${}_{0.5}$Fe${}_{0.5}$Ti${}_{0.5}$(PO${}_4$)${}_3$ (Me-NVMP) is proposed. The Me-NVMP achieves a highly reversible specific capacity of 165.8 mAh g${}^{-1}$ (1.8-4.4 V vs Na${}^{+}$/Na) at 0.1 C via the stepwise redox reactions of Ti${}^{3+}$/Ti${}^{4+}$-Fe${}^{2+}$/Fe${}^{3+}$, V${}^{3+}$/V${}^{4+}$-Mn${}^{2+}$/Mn${}^{3+}$, and V${}^{4+}$/V^{5+}-Mn${}^{3+}$/Mn${}^{4+}$. More impressively, the Me-NVMP yields super rate capability and cycling stability via the regulation of configuration entropy in NASICON. Specifically, the Me-NVMP cathode can preserve a capacity retention of 83.5% after 10,000 cycles at 100 C (17 A g${}^{-1}$). Furthermore, excellent cycling performance even at the temperature of 0 °C (capacity retention of 93.45% at 20 C after 1000 cycles) is also demonstrated. In situ X-ray diffraction analysis reveals that the enhanced performance can be mainly attributed to the solid-solution-type Na${}^{+}$ storage behavior in Me-NVMP. Moreover, issues such as Jahn-Teller distortion of Mn${}^{3+}$ and irreversible structural change at high voltage (>4.0 V vs Na${}^{+}$/Na) are effectively mitigated. This work inspires a new strategy to design high-performance polyanionic electrode materials. (© 2024 Wiley‐VCH GmbH)