Entropy-Enhanced Multi-Doping Strategy to Promote the Electrochemical Performance of Na4 Fe3 (PO4 )2 P2 O7.

Sodium-ion batteries (SIBs) have been regarded as promising candidates for large-scale energy storage system, and their electrochemical performance is determined by the cathode materials. Recently, the polyanion-type cathode Na4 Fe3 (PO4 )2 P2 O7 (NFPP) demonstrates decent performance, while there exists promotion space with respect to its cycle stability and rate capability. Herein, an entropy-enhanced Na4 Fe2.95 (NiCoMnMgZn)0.01 (PO4 )2 P2 O7 (HE-NFPP) cathode is proposed with improved rate performance (67.1 mAh g-1 at 50 C) and cycle performance (retention of 92.0% after 1000  cycles at 1 C). The enhancement of configuration entropy improves the structural stability of NFPP thermodynamically. In-situ XRD illustrates the sodium storage mechanism of HE-NFPP as an imperfect solid solution reaction driven by Fe2+ /Fe3+ redox with a low volume change of 4.0% (90.9% of NFPP). Through doping, the structure distortion and abrupt rearrangement are inhibited. Additionally, HE-NFPP and hard carbon (HC) are utilized to fabricate pouch cell that demonstrates an average working voltage of 3.0 V and a maximum energy density of 165 Wh kg-1 (based on the total mass of active materials). These results highlight the potential for enhancing the high-rate and long-cycle performance of NFPP as a promising cathode for SIBs through an entropy-enhanced multi-doping strategy.