Improving the cyclability of sodium-ion cathodes by selection of electrolyte solvent

doi:10.1016/j.jpowsour.2011.09.008

Journal of Power Sources

Volume 197, 1 January 2012, Pages 314-318

https://doi.org/10.1016/j.jpowsour.2011.09.008 Get rights and content

Abstract

A composite material containing orthorhombic Na_1.8FePO₄F and carbon is prepared by mechanical activation and ceramic procedures. The material is studied in sodium test cells as a potential candidate for sodium-ion battery cathodes. The effect of the solvents in the electrolyte on the electrochemical performance is analysed by X-ray absorption spectroscopy. The structural changes on cycling are small, while the changes in the oxidation state of iron agree with the sodium insertion–extraction processes. The oxidation state is especially affected by the upper limit of the voltage window, and the discharge capacity is strongly affected when using propylene carbonate solvent. Capacity and capacity retention are higher for sodium cells using mixtures of ethylene carbonate and diethyl carbonate as the solvent of NaPF₆ electrolytes.

Highlights

► Na_1.8FePO₄F/C composite material is prepared by a new mechanical-ceramic method. ► The local structure of Na_xFePO₄F electrodes were examined for the first time by XAS. ► The local changes from Na_1.8FePO₄F to NaFePO₄F, and then to Na₂FePO₄F are small. ► Both Fe²⁺ and Fe³⁺ are in high spin configurations in Na_xFePO₄F electrodes. ► Capacity and capacity retention are improved by using NaPF₆ electrolyte in EC:DEC.

Introduction

Sodium-ion batteries have been envisaged as a promising alternative to lithium-ion cells as the current cost and accessibility of lithium may prevent the extensive use of lithium-based batteries. However, sodium electrodes have shown poor cycling properties and this would limit their applications. Almost two decades ago the concept of sodium-ion batteries as an alternative to lithium-ion batteries was discussed by Doeff et al. [1]. In this respect NaCoO₂ – the analogue of LiCoO₂ – was thoroughly studied as the positive electrode in sodium-ion cells using carbon [2], [3] and conversion oxide materials [4] as anodes. Recently, Ellis et al. [5] reported on a sodium/lithium iron phosphate, A₂FePO₄F (A = Na, Li), that could serve as a cathode in either lithium-ion or sodium-ion cells. The use of ionic liquids by Tarascon group has given interesting results on the ionothermal preparation of the sodium fluorophosphates, Na₂FePO₄F, cathode [6].

The optimization of Na₂FePO₄F material for use in sodium test cells is the aim of this work. The study is focused in the changes affecting the cathode material with a common anode on changing the electrolyte. It is well known that safety problems led to discard the use of LiClO₄ in lithium-ion batteries. In this study, NaPF₆ was chosen instead of NaClO₄ in order to avoid safety problems associated to the use of perchlorates. The different organic solvents used in the study are common in lithium and sodium batteries. Firstly, we present a new preparation route based on mechanical activation and ceramic procedures. Then the effect of the electrolyte was analysed by X-ray absorption spectroscopy (XAS). The XAS study of the electrodes was done with different organic solvents. Finally, the choice of electrolyte and potential window were assessed by the analysis of electrochemical results.

Section snippets

Experimental

The synthesis of Na₂FePO₄F was carried out in two steps. Firstly, a nano-FePO₄ precursor was synthesized by spontaneous precipitation from aqueous solutions as described by Huang et al. [7]. In a second step, the resultant FePO₄ was mixed with sodium carboxymethylcellulose (NaCMC, average Mw ∼250,000) and NaF (99% purity) in equimolar concentrations and with 10 ml of hot ethanol near the boiling point. The mixture was ground in a planetary ball-mill for 30 min at 500 rpm in air to get a

Results and discussion

The X-ray diffraction (XRD) pattern of the resulting Na₂FePO₄F/C composite and the corresponding Rietveld refinement (Fig. 1) confirmed that the phosphate material can be indexed in the orthorhombic Pbcn space group in good agreement with previously reported data on Na₂FePO₄F [5]. However, by allowing the refinement of the Na site occupancy, a final stoichiometry of Na_1.8FePO₄F with a = 5.2287₃ Å, b = 13.864₁ Å, and c = 11.7615₈ Å. (R_wp = 14.9%, R_exp = 14.9%, R_Bragg = 8.6%) was obtained. The origin of the

Conclusions

A new preparation route is found to obtain Na₂FePO₄F/C composites. The resulting material contains the fluorophosphate in the known orthorhombic form, being ca. 10% substoichimetric in sodium due to a partial reduction of iron. The composite electrode provides good cycling performance vs sodium in test cells. The local structures of pristine Na₂FePO₄F, and charged and discharged electrodes have been examined by XANES. The results show that Fe²⁺ and Fe³⁺ are found in high spin configurations in

Acknowledgements

This work was carried out in the MICINN MAT2008-05880 and Junta de Andalucía FQM-6017 contracts and ALISTORE-ERI.

References (15)

R. Alcántara et al.
Electrochem. Commun.
(2001)
Y. Huang et al.
Electrochim. Acta
(2009)
A. Deb et al.
Electrochim. Acta
(2005)
M.M. Doeff et al.
J. Electrochem. Soc.
(1993)
R. Alcántara et al.
J. Electrochem. Soc.
(2002)
R. Alcántara et al.
Chem. Mater.
(2002)
B.L. Ellis et al.
Nat. Mater.
(2007)

There are more references available in the full text version of this article.

Cited by (64)

State-of-the-art review on electrolytes for sodium-ion batteries: Potential recent progress and technical challenges
2023, Journal of Energy Storage
Lithium batteries play a prominent role as a critical technology for advancing electric vehicles. However, establishing lithium-based technologies for mass storage encounters critical challenges such as materials availability and cost-efficiency. Hence, strategic approaches should be developed to address the existent challenges. Using sodium as new sustainable chemistry to replace lithium-based technologies tends to exhibit promising solution as the most appealing alternative. While exploring new electrode materials which has attracted significant interest from eminent researchers for sodium-ion batteries, research activities related to electrolyte are less attention paid. This paper reviews the most recent articles on developing and improving the electrolytes for sodium-ion batteries, particularly liquid electrolytes. This is the latest comprehensive discussion related to sodium-ion batteries with different type of electrolytes and a particular focus on the advantages/disadvantages in order to improve efficiency of these novel technologies as well as comprehensive discussion on the application of advanced nanomaterials towards these devices.
Assembly and electrochemical testing of renewable carbon-based anodes in SIBs: A practical guide
2022, Journal of Energy Chemistry
Citation Excerpt :
Some examples are NaCrO2 [36,37], NaxCoO2 [38–40], NaxMnyO2 [41–43], NaxFeyMnyO2 [44,45], NaxNiyMnzCOtO2 [46,47], NaxNiyMnyO2 [48,49], and NaxNiyFezMnyO2 [50–52]. Polyanionic compounds are also potential candidates, including olivine-type (NaFePO4) [53,54], NASICON-type (Na3V2(PO4)3) [55,56], sulfates (Na2M(SO4)2·nH2O) [57], fluorophosphates (NaxMPO4F) [58–62], fluorosulfates (NaMSO4F) [63], silicates (Na2MSiO4) [64], and pyrophosphates as Na7V3(P2O7)4 [65]. Moreover, Prussian blue analogues (PBAs) have gained interest during the last years because of their low cost, open framework structure, and long cycle stability [66,67].
Sodium-ion batteries (SIBs) are considered as a promising candidate to replace lithium-ion batteries (LIBs) in large-scale energy storage applications. Abundant sodium resources and similar working principles make this technology attractive to be implemented in the near future. However, the development of high-performance carbon anodes is a focal point to the upcoming success of SIBs in terms of power density, cycling stability, and lifespan. Fundamental knowledge in electrochemical and physicochemical techniques is required to properly evaluate the anode performance and move it in the right direction. This review aims at providing a comprehensive guideline to help researchers from different backgrounds (e.g., nanomaterials and thermochemistry) to delve into this topic. The main components, lab configurations, procedures, and working principles of SIBs are summarized. Moreover, a detailed description of the most used electrochemical and physicochemical techniques to characterize electrochemically active materials is provided.
Thermal risk evaluation of concentrated electrolytes for Li-ion batteries
2021, Journal of Power Sources Advances
Concentrated electrolytes have been attracting increasing attention due to their unique properties. However, despite the concern about their thermal stability, few research has been done on their exothermic behaviors, especially with the coexistence of electrodes. Herein, we report the results of detailed investigation into the thermal properties of LiBF₄, LiPF₆, LiTFSI, and LiFSI/carbonate concentrated solutions and their thermal behaviors with the coexistence of fully lithiated graphite. Concentrated LiBF₄ solutions showed no practical application possibilities because they were unstable on C₆Li. Increasing the salt concentration decreased the thermal stability of LiPF₆/PC solutions with the coexistence of C₆Li. The organic salt dominated the thermal behavior of the solution when mixed with C₆Li. A drastic exothermic reaction happened at 210–220 °C when C₆Li was mixed with LiFSI solutions, indicating a very high thermal risk of LiFSI carbonate solutions as LIB electrolytes. In contrast, LiTFSI solutions showed much milder reactions with C₆Li. On the other hand, because of the different LiF content in SEI, the exothermic onset temperature of the C₆Li mixture with the concentrated solution increased in the order of LiFSI > LiTFSI > LiPF₆. Comprehensively, concentrated LiTFSI electrolytes should be a good choice for LIB from the standpoint of battery safety.
Transport studies of NaPF<inf>6</inf> carbonate solvents-based sodium ion electrolytes
2021, Electrochimica Acta
Characterizing fundamental ion transport is necessary for the modification and development of electrolytes. In this study, we investigated the local and bulk transport properties of 1 M NaPF₆ in the binary EC:PC (1:1), and ternary 5EC:3PC:2DEC (5:3:2) solvents, as well as their 2 wt.% FEC modulated versions. We determined ¹H, ¹⁹F and ²³Na Nuclear Magnetic Resonance (NMR) spin-lattice relaxation time (T₁), self-diffusion coefficient (D), linewidth and chemical shift. These were complimented with density, viscosity and ionic conductivity measurements, all as a function of temperature. Both the viscosity and ionic conductivity displayed VFT behavior and their Walden products showed a dependence on temperature. This coupled with their having the lower Walden products indicated less salt dissociation in the 5EC:3PC:2DEC and 5EC:3PC:2DEC + 2 wt.% electrolytes. NMR D ¹⁹F and ²³Na values for these electrolytes were also similar at 298K indicating cation-anion association, but diverged with increasing temperatures, with that for the anion being greater than the cation. The addition of the FEC provides greater local dynamics for the individual solvents in the less polar electrolyte but appeared to impede that of the ions.
Engineering optimization approach of nonaqueous electrolyte for sodium ion battery with long cycle life and safety
2021, Green Energy and Environment
Electrolyte design strategies are closely related to the capacities, cycle life and safety of sodium–ion batteries. In this study, we aimed to optimize electrolyte with the focus on engineering aspects. The basic physicochemical properties including ionic conductivity, viscosity, wettability and thermochemical stability of the electrolytes using NaPF₆ as the solute and the mixed solvent with different components of EMC, DMC or DEC in PC or EC were systematically measured. Ah pouch cell with NaNi_1/3Fe_1/3Mn_1/3O₂/hard carbon electrodes was used to evaluate the performance of the prepared electrolytes. By using the Inductive Coupled Plasma Emission Spectrometer (ICP), X-ray photoelectron spectroscopy (XPS), Thermogravimetric-differential scanning calorimetry (TG-DSC) and Accelerating Rate Calorimeter (ARC), we show that an optimized electrolyte can effectively promote the formation of a protective interfacial layer on two electrodes, which not only retards parasitic reactions between the electrodes and electrolyte but also suppresses dissolution of metal ions from the cathode. With an optimized electrolyte, a NaNi_1/3Fe_1/3Mn_1/3O₂/hard carbon cell can attain 56.16% capacity retention under the low temperature of −40 °C, and can be able to retain 80% capacity retention after more than 2500 cycles while presenting excellent thermal safety.
Impact of carbonate-based electrolytes on the electrochemical activity of carbon-coated Na<inf>3</inf>V<inf>2</inf>(PO<inf>4</inf>)<inf>2</inf>F<inf>3</inf> cathode in full-cell assembly with hard carbon anode
2021, Journal of Colloid and Interface Science
An immense effort has been put into developing high-performance electrodes to commercialize sodium-ion batteries, but research on developing an efficient electrolyte is lacking. This study aims to find the best carbonate-based electrolyte systems by incorporating the existing ideas reported in this field. The sodium superionic conductor (NASICON) type Na₃V₂(PO₄)₂F₃-C (NVPF-C) was chosen as a cathode, and its compatibility with four different carbonate-based electrolyte solutions was studied in the half-cell assembly. Additionally, full-cell assembly with hard carbon as an anode is also explored. Binary and ternary combinations of the solvents ethylene carbonate, propylene carbonate, and dimethyl carbonate were employed with and without fluoroethylene carbonate as an additive. A systematic study was performed, including the in-situ impedance technique, and to determine the compatibility. Detailed galvanostatic studies for NVPF-C based half-cells, as well as hard carbon/NVPF-C full-cells, are performed, which shows that 1 M NaClO₄ in propylene carbonate:dimethyl carbonate + fluoroethylene carbonate is a better electrolyte composition for this assembly. Subsequently, a temperature study was carried out on this electrolyte to test its performance.

View all citing articles on Scopus

View full text

Short communicationImproving the cyclability of sodium-ion cathodes by selection of electrolyte solvent

Abstract

Highlights

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Electrochem. Commun.

Electrochim. Acta

Electrochim. Acta

J. Electrochem. Soc.

J. Electrochem. Soc.

Chem. Mater.

Nat. Mater.

Short communication
Improving the cyclability of sodium-ion cathodes by selection of electrolyte solvent