Approaching the theoretical capacity limit of Na2FeSiO4-based cathodes with fully reversible two-electron redox reaction for sodium-ion battery
Graphical abstract
Fully reversible two-electron redox reaction in orthosilicate cathodes by fluorine doping are reported. With the effect of promoting charge redistribution and accelerating the electron exchange, F-doped Na2FeSiO4 displays exceptionally high capacity of 271 mAh g−1 that has never been reported for polyanionic cathodes based on Fe2+/Fe4+ redox couple.
Introduction
The landscape of energy storage, especially in large-scale grids, has been extended beyond lithium-ion batteries (LIBs) to other electrochemical systems because of the lithium resource shortage [[1], [2], [3]]. Owing to the abundance of sodium sources and the similar energy storage principle to that of LIBs, advances in sodium-ion batteries (SIBs) will usher in a new stage of low-cost, sustainable, and large-scale energy storage systems [[4], [5], [6], [7], [8], [9]]. The large molar mass and ionic radius of sodium, however, aggravates the difficulty in the reversible sodiation/desodiation of electrodes and, correspondingly, the final electrochemical performance [[10], [11], [12]]. To improve sodium storage performance, there has been an intensive search for new cathode materials, which are generally classified as transition metal oxides, Prussian blue analogs, and polyanion compounds. Considering the severity of the safety issue for rechargeable batteries, particularly in large-scale applications, polyanion compounds have drawn widespread attention owing to their intrinsic safe characteristics during cycling, which originate from the strong inductive effects of polyanion groups [[13], [14], [15], [16]].
More than a dozen polyanionic systems have been explored as cathodes for SIBs, as shown in Fig. 1 and Table S1, although their relatively low capacity has become the main obstacle for achieving high energy density in SIBs [[17], [18], [19], [20]]. For the polyanion compounds with a single-electron redox reaction, for example, NaFePO4, there is a theoretical capacity limit of ∼140 mAh g−1 [[21], [22], [23], [24], [25], [26], [27]]. On the other hand, for the complex polyanion compounds with multipolyanion groups, such as Na3V2(PO4)3 and Na2Fe2(SO4)3, the theoretical capacities are generally lower than 130 mAh g−1, although the two-electron redox reaction might occur [[28], [29], [30], [31], [32]]. The only opportunity to achieve high capacity lies in the polyanion compounds that possess a single polyanion group in their formulas and might afford a two-electron redox reaction, for example, Na2FeSiO4 with a theoretical capacity of 276 mAh g−1. Unfortunately, preliminary work on Na2FeSiO4-based cathodes has failed to realize the full potential of their theoretical capacity [[33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43]]. The main reason for this is that the high redox potential of the polyanion group, exceeding the voltage range of the state-of-the-art electrolyte (<4.5 V vs. Na/Na+), impedes further redox reaction once one Na+ is already involved [[44], [45], [46], [47], [48]]. In addition, low electronic conductivity of Na2FeSiO4-based cathodes leads to lower capacity and rate capability compared with other cathodes [49,50]. From this point of view, it is of great significance, but very challenging, to unlock the trapped reaction in such polyanion compounds, for the sake of pursuing higher capacity cathodes for SIBs.
Herein, we have achieved the high theoretical capacity of Na2FeSiO4-based cathodes through unlocking the trapped Na+ to realize the complete two-electron redox reaction. Upon introducing fluorine, 25% F-doped Na2FeSiO4 (NFSF) exhibits an exceptionally high capacity of 271 mAh g−1 when charged up to the safe voltage limit of organic electrolyte (<4.5 V), which approaches the theoretical capacity limit and is the highest value so far among all polyanionic cathodes. The crystal structure model of Na2FeSiO4 with triclinic phase was firstly constructed to reveal the origin of the high capacity based on theoretical calculations. The Fe3+/Fe4+ redox couple was verified to be completely activated owing to the acceleration of electron exchange by fluorine and the formation of stable intermediate phases. Benefiting from its good structural stability and the protection of a thin carbon layer, F-doped Na2FeSiO4 cathodes delivered excellent cycling performance and superior rate capability.
Section snippets
Preparation of orthosilicates and fluorine-doped orthosilicates
The precursors of carbon-coated F-doped Na2FeSiO4 and Na2FeSiO4 were prepared via a facile citric acid–assisted sol-gel method. The transition metal source (FeC2O4·2H2O) and CH3COONa were mixed in ethanol with the molar ratio of Fe:Na = 1:2, and citric acid was added to ensure the molar ratio of –COOH:Fe = 1:1. For the F-doped samples, NaF was introduced into the solution as the fluorine source. After magnetic stirring at 50 C for 2 h, stoichiometric Si(OC2H5)4 (TEOS) was added as the silicon
Characterizations of fluorine-doped Na2FeSiO4
Fluorine-doped orthosilicates were prepared via a facile sol-gel method. The optimization of doping content is a key aspect of designing F−-doped cathode materials because F− tends to bond with alkali metal ions, forming an electrochemically inactive phase [[56], [57], [58], [59]]. In this work, 25% F− (versus Fe in terms of the molar ratio) can be accommodated in Na2FeSiO4 (NFS) without NaF impurity (Figs. S1–S3). The powder XRD pattern of NFSF in Fig. 2a is in good agreement with that of
Conclusion
In summary, the two-electron reaction in F-doped Na2FeSiO4 has been achieved, and the corresponding Na+ storage mechanism was clearly unraveled. A triclinic model has been constructed to acquire a better understanding of the crystal structure of F-doped Na2FeSiO4. The 25% F-doped Na2FeSiO4 cathode delivered the highest specific capacity by far of 271 mAh g−1 among all the polyanionic cathodes. In addition, excellent rate capability (150 mAh g−1 at 2C) and cycling performance (93.7% capacity
Credit author statement
Wenhao Guan: Conceptualization, Investigation, Methodology, Data curation, Writing - original draft. Qingyun Lin: Methodology, Data curation. Zhenyun Lan: Formal analysis. Wenli Pan: Validation, Data curation. Xiao Wei: Resources, Investigation. Wenping Sun: Writing—Reviewing and Editing. Runtian Zheng: Resources, Investigation. Yunhao Lu: Formal analysis. Jie Shu: Resources, Investigation. Hongge Pan: Writing—Reviewing and Editing. Mi Yan: Writing—Reviewing and Editing. Yinzhu Jiang:
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 article.
Acknowledgments
This work is supported by the National Key Research and Development Program (Grant No. 2019YFE0111200), the National Natural Science Foundation of China (Grant No. 51722105), the Zhejiang Provincial Natural Science Foundation of China (LR18B030001), the Fundamental Research Funds for the Central Universities (2018XZZX002-08), and Australian Research Council grant DP160102627 and ARENA S4 grant.
References (64)
- et al.
Designing cobalt-based coordination polymers for high-performance sodium and lithium storage: from controllable synthesis to mechanism detection
Mater. Today Energy
(2020) - et al.
Ultrahigh-rate sodium-ion battery anode enabled by vertically aligned (1T-2H MoS2)/CoS2 heteronanosheets
Mater. Today Nano.
(2020) - et al.
Progress on multiphase layered transition metal oxide cathodes of sodium ion batteries
Chin. Chem. Lett.
(2020) - et al.
Recent achievements on polyanion-type compounds for sodium-ion batteries: syntheses, crystal chemistry and electrochemical performance
J. Power Sources
(2017) - et al.
Unexpected discovery of low-cost maricite NaFePO4 as a high-performance electrode for Na-ion batteries
Energy Environ. Sci.
(2015) - et al.
Charge–discharge behavior of a Na2FeP2O7 positive electrode in an ionic liquid electrolyte between 253 and 363 K
Electrochim. Acta
(2014) - et al.
Investigation of metastable Na2FeSiO4 as a cathode material for Na-ion secondary battery
Mater. Chem. Phys.
(2016) - et al.
Na2MnSiO4 as a positive electrode material for sodium secondary batteries using an ionic liquid electrolyte
Electrochem. Commun.
(2014) - et al.
Solvothermal synthesis and electrochemical properties of Na2CoSiO4 and Na2CoSiO4/carbon nanotube cathode materials for sodium-ion batteries
Electrochim. Acta
(2018) - et al.
Interconnected mesoporous Na2FeSiO4 nanospheres supported on carbon nanotubes as a highly stable and efficient cathode material for sodium-ion battery
J. Power Sources
(2018)
Preparation and performances of novel Na2FeSiO4/C composite with more stable polymorph as cathode material of sodium-ion batteries
J. Power Sources
3D conductive CNTs anchored with Na2FeSiO4 nanocrystals as a novel cathode material for electrochemical sodium storage
Ceram. Int.
Facile solid-state synthesis of eco-friendly sodium iron silicate with exceptional sodium storage behavior
Electrochim. Acta
Fabricating 3D ordered marcoporous Na2MnSiO4/C with hierarchical pores for fast sodium storage
Electrochim. Acta
Structural and electrochemical properties of Na2FeSiO4 polymorphs for sodium-ion batteries
Electrochim. Acta
Reversible Mn2+/Mn4+ double redox in lithium-excess cathode materials
Nature
Reversible Li storage for nanosize cation/anion-disordered rocksalt-type oxyfluorides: LiMoO2–xLiF (0 ≤ x ≤ 2) binary system
J. Power Sources
Crystal structure analysis and first principle investigation of F doping in LiFePO4
J. Power Sources
An X-ray photoelectron spectroscopy study of the acidity of SiO2–ZrO2 Mixed oxides
J. Catal.
An XPS study of iron sodium silicate glass surfaces
J. Non-Cryst. Solids
Issues and challenges facing rechargeable lithium batteries
Nature
Electrodes with high power and high capacity for rechargeable lithium batteries
Science
Building better batteries
Nature
Electrical energy storage for the grid: a battery of choices
Science
Opportunities and challenges for a sustainable energy future
Nature
Room-temperature stationary sodium-ion batteries for large-scale electric energy storage
Energy Environ. Sci.
Sodium-ion battery materials and electrochemical properties reviewed
Adv. Energy Mater
Research development on sodium-ion batteries
Chem. Rev.
The scale-up and commercialization of nonaqueous Na-ion battery technologies. Adv
Energy Mater
Electrode materials for rechargeable sodium-ion batteries: potential alternatives to current lithium-ion batteries
Adv. Energy Mater
Sodium and sodium-ion batteries: 50 years of research
Adv. Energy Mater
Polyanion-type electrode materials for advanced sodium-ion batteries
Mater. Today Nano.
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Current address: School of Materials Science and Engineering, Zhejiang University, Hangzhou, Zhejiang, 310027, PR China.