Pyrophosphate Na2FeP2O7 as a low-cost and high-performance positive electrode material for sodium secondary batteries utilizing an inorganic ionic liquid

Abstract

The electrochemical performance of a Na 2 FeP 2 O 7 positive electrode has been evaluated in an inorganic ionic liquid NaFSA-KFSA (FSA = bis(fluorosulfonyl)amide) at 363 K. The electrode delivers a reversible capacity of 91 mAh g-1 with excellent rate capability (59 mAh g-1 at 2000 mA g-1) and a capacity retention of 91% over 1000 cycles, which facilitates the development low-cost and high-safety sodium secondary batteries for large-scale energy storage applications. The average oxidation state of iron increases upon sodium extraction, as evidenced by the edge shift of an X-ray absorption spectroscopy analysis. According to an extended X-ray absorption fine structure analysis, the sodium extraction is accompanied by a shortening of Fe-O bonds.

Recent Progress in Electrode Materials for Sodium‐Ion Batteries

“…[ 101,197,198 ] A stable cycle stability over 1000 times with cycle retention of 91% was achieved by utilizing ionic liquids as electrolytes. [ 199 ] The low cost, high thermal stability, and promising electrochemical properties of a series of Na 4-x Fe 2+ x /2 (P 2 O 7 ) 2 electrodes makes these materials viable competitors for NIB cathodes; [ 200 ] however, their low energy density remains a signifi cant challenge.…”

Rechargeable Na/Na0.44MnO2 cells with ionic liquid electrolytes containing various sodium solutes

“…Hagiwara et al first proposed inorganic NaFSI/KFSI (FSI ¼ bis(fluorosulfonyl)imide) intermediate-temperature IL electrolytes (with melting points of~65 C depending on the NaFSI/KFSI ratio) for NIBs [19]. They reported that NaCrO 2 and Na 2 FeP 2 O 7 cathodes can show reversible capacities of 113 and 91 mAh g À1 , respectively, in this IL at 90 C [20,21]. Recently, room-temperature pyrrolidinium-FSI and pyrrolidinium-TFSI (TFSI ¼ bis(trifluoromethylsulfonyl)imide) IL electrolytes have been proposed for NIBs [22e25].…”

Recent Advances in Sodium-Ion Battery Materials

“…The electrochemical performance of Na 2 FeP 2 O 7 electrode was greatly improved by different carbon decorations [69,70]. For example, the Na 2 FeP 2 O 7 nanoparticles embedded in carbon exhibited a high rate capability and long cycle life (84% capacity retention over 10,000 cycles) [71].…”