Crystal chemistry of Na insertion/deinsertion in FePO4–NaFePO4

Abstract

In this paper we examine the mechanism of Na insertion and extraction in the FePO 4 -NaFePO 4 system. Chemical preparation of the intermediate Na 1Àx FePO 4 phase has revealed the existence of a range of stable compositions with different Na + /vacancy arrangements. The mechano-chemical aspects of the charge and discharge reactions are also discussed.

Role of intermediate phase for stable cycling of Na 7 V 4 (P 2 O 7 ) 4 PO 4 in sodium ion battery

“…2B) exhibit a discrepancy between charging and discharging peaks. When measured at C/20, the charging profile shows two separate peaks at 3.88 and 3.92 V, whereas the discharging profile shows only a single peak at 3.86 V. Such distinctive behavior between the charge and discharge is similar to the case of NaFePO 4 (19,20), where the origin is speculated to be the different reaction pathways during charge and discharge (19). However, in VODP the distinctive behavior is likely to be attributed to different kinetics, rather than different reaction pathways, from the quasiopen circuit potential (QOCP) result (Fig.…”

“…However, they suffer from a substantial capacity decay with cycling, owing to crystal structure collapse and/or unstable electrode-electrolyte interfaces (10). For more stable host frameworks, various polyanion structures have also been studied as SIB cathodes as a natural extension of the success shown in the LIB materials of the same classes, including TM fluorophosphates (11)(12)(13)(14)(15), pyrophosphates (16)(17)(18) and phosphates (19)(20)(21)(22). However, most of these materials still exhibited insufficient cycle lifetimes and large voltage steps that limit their practical capacity.…”

Unexpected discovery of low-cost maricite NaFePO4as a high-performance electrode for Na-ion batteries

“…Our quantum mechanics calculations show that the underlying reason for the remarkable electrochemical activity of NaFePO 4 is the signicantly enhanced Na mobility in the transformed phase, which is $one fourth of the hopping activation barrier. Maricite NaFePO 4 [17][18][19][20][21][22]26 maricite, the thermodynamically stable phase of NaFePO 4 , has been regarded as electrochemically inactive in rechargeable batteries. 20 The NaFePO 4 composition also exhibits an olivine structure; however, the olivine structure is not stable and cannot be synthesized by conventional synthetic routes.…”

“…Recently, Na ion batteries (NIBs) have been considered as a promising alternative to LIBs since the underlying electrochemical reaction is similar to that of LIBs, but is based on the unlimited resources of Na from seawater. [13][14][15][16][17][18][19][20] The use of redox chemistry using earth abundant transition metals would provide the optimal combination with Na electrochemistry further highlighting the advantage of NIBs.In recent years, considerable research has been carried out on Fe-based electrode materials for use in NIBs. …”

In Situ Powder Diffraction Studies of Electrode Materials in Rechargeable Batteries

“…[95a] Am ajor structurald ifferencea ppeared during charginga nd discharging:o nc harging there was as equence of two two-phase reactions, whereas on discharging three phases appeared simultaneously in the ex situ XRPD data. [96] The coexistence of three phases,N aFePO 4 ,N a 2/3 FePO 4 ,a nd FePO 4 ,w as explained by the difference in the volume of the three phases:t he intermediate phase of Na 2/3 FePO 4 helped to alleviate structurals tress associated with the FePO 4 to NaFePO 4 transition. [78p] The asymmetry in phase evolution was related to the observed differencesi n the electrochemical curves.…”