Crystal Structure and Electrochemical Properties of A₂MPO₄F Fluorophosphates (A = Na, Li; M = Fe, Mn, Co, Ni)

Abstract

We report new solid state and hydrothermal synthetic routes to (Li,Na)2FePO4F that incorporate carbon-containing additives and result in good electrochemical properties of this Li (or Na) ion electrode material. Single crystal X-ray diffraction analysis of Na2FePO4F prepared by flux growth confirms the unusual structural features of this compound that include pairs of face-sharing metal octahedra and [6 + 1] coordination of the sodium ions. Facile Na−Li ion-exchange occurs upon reflux with lithium salts, upon electrochemical cycling in a cell (vs. Li), and also in a cell simply equilibrated at OCV. The material does not exhibit typical two-phase behavior on electrochemical cycling. A combination of a redox process which occurs with little structural strain, and ion scrambling give rise to a solid solution-like sloping voltage profile on charge−discharge, although localization of the Fe2+/3+ in the mixed valence single phase intermediate, Na1.5FePO4F drives a very small structural distortion. Temperature-dependent Mössbauer spectroscopy measurements confirm this localization, at least on short time scales (10−8 s), which persists to 370 °C. Finally, polycrystalline powders of other members of this family of compounds (Na2CoPO4F, Na2NiPO4F) were synthesized for the first time. Na2CoPO4F has a Co2+/Co3+ potential near 4.8 V. Mixed-metal phosphates of the form Na2(Fe1−x M x )PO4F, where M = Co, Mg, were also synthesized and found to be promising positive electrode materials for Li-ion or Na-ion energy storage devices.

Na2CoPO4F as a High-voltage Electrode Material for Na-ion Batteries

“…Therefore, divalent cobalt could be insignificantly reduced under the condition at high temperature with Ar flow, and in addition, the use of ascorbic acid Electrochemistry, 82(10), 909-911 (2014) as carbon source a little induces the reduction into metallic cobalt. 21 SEM observation confirmed that particle size of a carbon-modified sample becomes slightly smaller due to hindrance of crystallization (not shown here).…”

“…The large irreversible capacity of 142 mAh g ¹1 would be mainly caused by electrolyte decomposition and/or structural instability of the fluorophosphates as observed in a Li cell. 21 In contrast, the cell with the sample synthesized at 650°C exhibited 187 mAh g ¹1 and 89 mAh g ¹1 of initial charge and discharge capacity, respectively, resulting in the larger reversible capacity with suppressed irreversible capacity. Clearly, the reversible electrode performance for Na 2 CoPO 4 in the Na cells was enhanced by employing the higher-temperature synthesis even though the considerable irreversible capacity of 98 mAh g ¹1 was still remained.…”

A Comparison of Crystal Structures and Electrode Performance between Na2FePO4F and Na2Fe0.5Mn0.5PO4F Synthesized by Solid-State Method for Rechargeable Na-Ion Batteries

“…On the basis of the materials abundance in the earth's crust, iron-and manganesebased electrode materials are the promising candidates among the compounds consisting of series of 3d-transtion metals. As the ironand manganese-based systems, spinel-type lithium manganese oxides, LiMn 2 O 4 (and its derivatives), and triphylite-type lithium iron phosphate, LiFePO 4 , have been extensively studied for the practical lithium batteries. LiMn 2 O 4 and LiFePO 4 are the promising electrode materials for the large scale applications if the lithium resource is unlimited in the earth's crust.…”

“…Recently, a layered fluorinated iron phosphate, Na 2 FePO 4 F, has been reported, and electrode performance has been examined as positive electrode materials for both sodium 1,2 and lithium 1,3,4 batteries. Among the sodium and iron-based insertion materials, such as NaFeO 2 , 5 NaFeF 3 6 and NaFePO 4 , 7 Na 2 FePO 4 F shows relatively high operating voltage (ca.…”

“…1,2 As the manganese system, Na 2 MnPO 4 F was first prepared by a hydrothermal process. Na 2 MnPO 4 F 8 has a three dimensional framework structure consisting of Mn 2 F 2 O 8 chains, which is clearly different compared to that of Na 2 FePO 4 F. Na 2 [Fe x Mn 1¹x ]PO 4 F (0 < x < 1.0) samples were also synthesized by an ionothermal method, and it was found that the Na 2 MnPO 4 Ftype three dimensional structure is thermodynamically stable at x < 0.75 in Na 2 4 F with the three dimensional structure is insufficient for the battery application. 1 The layered framework structure seems to be preferable for the sodium migration process in the fluorophosphates system.…”