A Medium‐Entropy Phosphate Cathode with Multielectron Redox Reaction for Advanced Sodium‐Ion Batteries

Abstract Achieving multi‐sodium storage and high operating voltage is key to boosting energy density of NASICON‐type materials. However, the activation of more redox couples is usually accompanied by asymmetric and irreversible electrochemical reactions, thus causing fast capacity fading. To address this issue, a medium‐entropy concept is proposed, and a novel medium‐entropy Na 3 Mn 2/3 V 2/3 Ti 2/3 (PO 4 ) 3 /C@CNTs (ME‐NMVTP) cathode is designed. The as‐prepared ME‐NMVTP achieves a successive redox reaction, delivering a highly reversible specific capacity of 147.9 mA h g −1 at 50 mA g −1 together with a long‐term lifespan of 1000 cycles at 500 mA g −1 (capacity retention of 88.3%), which is superior to low‐entropy cathodes such as Na 4 MnV(PO 4 ) 3 /C@CNTs (LE‐NMVP) and Na 3 MnTi(PO 4 ) 3 /C@CNTs (LE‐NMTP). Moreover, benefiting from the entropy effect, solid‐solution and biphasic reactions with reversible structure evolution and small volume change are achieved during the multi‐sodium storage process. First‐principles calculation and kinetic analysis results affirm the enhanced electronic conductivity and facilitated Na + migration of the ME‐NMVTP cathode derived from the synergistic effect of the three transition‐metal elementals in a medium‐entropy crystalline state. The strategy of engineering medium‐entropy to construct cathodes with superior performance is expected to be widely applicable to other electrode materials.