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 Na3.5V0.5Mn0.5Fe0.5Ti0.5(PO4)(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 Ti3+/Ti4+-Fe2+/Fe3+, V3+/V4+-Mn2+/Mn3+, and V4+/V5+-Mn3+/Mn4+. 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 degrees 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 Mn3+ 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.