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_3.5V_0.5Mn_0.5Fe_0.5Ti_0.5(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 °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.