Elsevier

Journal of Power Sources

Volume 469, 1 September 2020, 228417
Journal of Power Sources

Optimization of the compositions of polyanionic sodium-ion battery cathode NaFe2−xVx(PO4)(SO4)2

https://doi.org/10.1016/j.jpowsour.2020.228417Get rights and content

Highlights

  • Compositional optimization of vanadium doped iron based polyanion material.

  • Sodium-ion battery cathode.

  • Interfacial resistances and ionic diffusivity.

  • In-situ powder x-ray diffraction and Mӧssbauer spectroscopy measurements.

Abstract

Sodium (Na) super ionic conductor (NASICON) polyanionic compounds have recently attracted much attention from the battery community because of their electroactive properties and reasonably high ionic conductivities, leading to their use as a cathode in sodium-ion batteries. This article describes the compositional optimizations, crystallographic evaluations, and electrochemical behavior of a new mixed NASICON polyanionic compound, NaFe2−xVx(PO4)(SO4)2. By doping the characteristic Fe3+ sites of the FeO6 octahedrons with varying amounts of V3+, the electrochemical stability and charge transport in NaFe2(PO4)(SO4)2 were enhanced. The resulting best composition, with crystal structure NaFe1.4V0.6(PO4)(SO4)2 resolved through the Rietveld method, exhibited a stable capacity compared with the other synthesized compositions. In situ powder x-ray diffraction measurements, a single-phase intercalation/deintercalation mechanism of the NASICON structure in the measured sodium concentration window was observed with no impurity phase formation. Further electrochemical assessments revealed the interfacial charge transfer kinetics to be the rate-limiting step in the sodium concentration window. Also, the measured sodium-ion diffusivity values in the range of 6 × 10−11 to 7 × 10−11 cm2/s in the measured sodium concentration range. The results reported here highlight the potential of compositionally and morphologically optimized NaFe1.4V0.6(PO4)(SO4)2 with higher particle surface areas as a cathode material for high-performance sodium-ion batteries.

Introduction

Sodium-ion batteries are very promising candidates to replace lithium-ion batteries for large-scale energy storage systems because of their cost and safety guarantees. However, several key challenges exist in identifying suitable electrode and electrolyte materials to enable high-power and -energy density with long lifespan [[1], [2], [3], [4], [5], [6], [7], [8], [9]]. To address these challenges, several sodium-transition metals–based mixed polyanionic materials—Na4M3(PO4)2(P2O7) [10,11], Na3M(PO4)(CO3) [12], Na2MPO4F (M = Fe, Mn, Co and Ni) [13], and Na3V2(PO4)2F3−xOx [14] have been widely investigated over recent years. These materials are considered as potential cathode candidates for sodium-ion batteries because of their excellent electrochemical performance, favorable crystal structures, and good thermal and structural stability. In this setting, mixed polyanionic compounds containing (SO4)2− have recently been reported as an attractive sodium-ion cathode material because of the presence of the highly electronegative (SO4)2− anion. Lu et al. [15] reported for the first time the preliminary electrochemical results of a new series of mixed polyanionic with an alluaudite structure, NaxFey(PO4)3–z(SO4)z (0 ≤ z ≤ 3). These materials were synthesized by ball-milling a mixture of Na2Fe3(PO4)3 and Na2.56Fe1.72(SO4)3 in controlled (argon) atmospheres. Lu et al. demonstrated the presence of a highly electronegative (SO4)2− species in the crystal structure that increased the operational voltage window from 3.19 (z = 0) to 3.36 (z = 1.5) and finally, to 3.72 V (z = 3) in Na2.56Fe1.72(SO4)3. Further investigations revealed that the origin of the elevated voltage was the inductive effect caused by the highly electronegative (SO4)2− polyanion, which corroborated the overall trend of voltage enhancement in such polyanionic compounds. Additionally, Shiva et al. prepared a phosphosulfate NaFe2(PO4)(SO4)2 with a large amount of impurities, leading to an unusual electrochemical behavior resulting in a trend of capacity increase with cycling [16], which warranted further investigations. In this context, in the past several years, the authors’ research team has focused on developing new compounds and evaluating them as positive and negative electrode materials for both lithium- and sodium-ion batteries. These materials predominantly consist of crystalline α-CrPO4, alluaudite, and Na super ionic conductor (NASICON) structures [[3], [4], [5], [6],[17], [18], [19], [20], [21]].

Recently, the authors reported the synthesis and electrochemical properties of impurity-free NASICON NaFe2(PO4)(SO4)2 [22], showing good electrochemical activity with an average voltage of ~3 V. The material delivered a capacity of 89 mA h/g with 70% of the specific capacity realized (theoretical capacity = 127 mAh g−1 for a 2e reaction). In this material, insertion of 1.4 Na+ ion per formula unit NaFe2(PO4)(SO4)2 induced a 4.6% change in the cell volume. Expanding on this work, the authors here describe the synthesis protocols, structural evaluations, and electrochemical behavior of a new mixed polyanionic compound, NaFe2−xVx(PO4)(SO4)2 with a NASICON crystal structure as a potential cathode material for Na+ ion batteries. The Fe3+ (0.645 Å) sites of the FeO6 octahedrons at NaFe2−xVx(PO4)(SO4)2 were doped with V3+ (0.64 Å) to optimize the structural and electrochemical stability as well as increase the electrical conductivity of NaFe2(PO4)(SO4)2. The crystal structure was resolved by the Rietveld method from the powder x-ray diffraction (PXRD) data. The oxidation state and local environment of Fe in NaFe2(PO4)(SO4)2 was also determined using Mössbauer spectroscopy. The electrochemical performances were examined by galvanostatic cycling and cyclic voltammetry techniques. The galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) were also employed to measure the ionic diffusivity and interfacial charge transfer kinetics of the cathode material. The stability of the NASICON structure during cycling was also confirmed by in situ PXRD experiments of NaFe1.6V0.4(PO4)(SO4)2. Overall, this systematic report aims to highlight the potential of NASICON type NaFe1.6V0.4(PO4)(SO4)2 as a cathode material for sodium-ion batteries.

Section snippets

Synthesis

The NaFe2−xVx(PO4)(SO4)2 powders (0 ≤ x ≤ 2) were synthesized via sol-gel synthesis from stoichiometric mixtures of NaNO3 (Aldrich, ≥99%), Fe(NO₃)₃·9H₂O (Aldrich, ≥98%), NH4VO₃ (Aldrich, ≥99%), (NH4)2SO4 (Aldrich, ≥99%), NH4H2PO4 (Merk, ≥99%), and citric acid (CA) C6H8O7 (Riedel-deHaën). First, NH4VO3 and CA with a mole ratio of x:2 were dissolved in 40 ml of water to form a clear blue solution; then, Fe(NO₃)₃·9H₂O was dissolved in 20 ml of water and added to the blue solution (Solution A). The

Structural refinement

Following the first heat treatments at 400 °C for 12 h under argon, XRD patterns were collected; all the samples had an amorphous nature, which was determined since no XRD peak was detected. This result confirmed that 400 °C is not hot enough to crystalize this material. After the second heat treatment at 500 °C, the XRD analyses indicated that pure NASICON phases could be formed only for 0 < x ≤ 1. Full pattern-matching refinements using the space group R-3c and the cell parameters of NaFe2(PO4

Conclusions

Several new mixed polyanionic NaFe2−xVx(PO4)(SO4)2 (x = 0.2, 0.4, 0.60, 0.8, and 1.0) samples were synthesized with optimized compositions, considering structural stability and electrochemical performances. The structural stability was investigated by XRD followed by Rietveld refinement protocols to obtain accurate structural refinements of the synthesized compounds. The obtained parameters as a result of solving the crystal structure indicated NASICON phases for these materials. Of the various

CRediT authorship contribution statement

Rachid Essehli: Conceptualization, Writing - review & editing. Alaa Alkhateeb: Formal analysis. Abdelfattah Mahmoud: Data curation. Frèdéric Boschini: Data curation. Hamdi Ben Yahia: Data curation. Ruhul Amin: Conceptualization, Writing - review & editing. Ilias Belharouak: Supervision, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work is supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy Vehicle Technology Office, under contract number DE-AC05-00OR22725.

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