An Advanced High-Entropy Cathode Achieves a Multi-Electron Reaction via the Activation of Multicationic Redox in Polyanionic Phosphates for Sodium-Ion Batteries

Abstract

Sodium superionic conductor (NASICON) type Na 3 V 2 (PO 4 ) 3 has received much attention as one of the promising cathodes for sodium-ion batteries due to its stable three-dimensional framework structure; however, it suffers from poor electronic conductivity and unsatisfactory cycling stability during Na + de/intercalation. Herein, we developed a high-entropy cathode of Na 3 VFe 0.5 (TiMnZrCuAl) 0.5 (PO 4 ) 3 (HE-NVP), utilizing the high-entropy effect to enhance the structural stability and introducing electrochemically active elements of Ti, Fe, and Mn to facilitate multioxidation reduction and mitigate the structural degradation. After high-entropy doping, the V 4+ /V 5+ of the HE-NVP at the high potential is successfully activated in a wide voltage range, and six redox couples (V 2+/3+ (1.6 V), Ti 3+/4+ (2.2 V), Fe 2+/3+ (2.5 V), V 3+/4+ /Mn 2+/3+ (3.6 V), and V 4+/5+ (4.0 V)) are reversibly converted to realize the reversible participation of multiple electrons in electrochemical processes. Consequently, the as-synthesized HE-NVP achieves a high specific capacity of 158.8 mAh g –1 at 0.5C, corresponding to an energy density of 524 Wh kg –1, and can be operated stably at 2C for 170 cycles with a capacity retention of up to 95%. The density functional theory calculations show a decrease in the bandgap of the HE-NVP, leading to an enhancement of the electronic conductivity, while the ex situ X-ray diffraction reveals a single-phase mechanism to store Na +, small volume variation, and good reversibility during charging and discharging for the HE-NVP. This work provides a flexible and broad strategy to achieve high energy density with long-term cycle stability in polyanionic materials by combining multielectron reaction with high entropy effects.

Topics & Concepts