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Defect Thermodynamics in Nonstoichiometric Alluaudite-Based Polyanionic Materials for Na-Ion Batteries
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
2019 (English)In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252Article in journal, Letter (Refereed) Published
Abstract [en]

Sodium iron sulfate in the form of alluaudite Na2+2xFe2-x(SO4)3 (or NFSx) has emerged as one of the most promising cathodes for Na-ion batteries due to its highest Fe2+/3+ redox potential, low-cost, sustainability and high rate capability. Unlike most of the other cathodes, NFSx  generally crystalizes in its non-stoichiometric form with partial Na substitution for Fe sites and contains a small amount of impurities. However,  profound explanations behind this inherent behavior including others, like phase stability, configurational structure and defect formation are still  ambiguous. We have therefore performed  first principles calculations combined with a random swapping method to determine the minimum energy configurations of NFSx (with x = 0, 0.25 and 0.5) and find a correlation between the Na distribution pattern and energetics in which the site preference for Na+ ion is in a sequence of Na4 > Na1 > Na2 > Na3. Our result points out that the non-stoichiometry cannot be properly described under the framework of primitive structures. Moreover, we investigated phase stability diagrams and defect formations based on thermodynamic criteria. Our predicted phase diagrams can explain the inevitable impurity precipitation, which can be reduced as x diminishes. Defect formation analysis indicates an unlikely formation of channel blockage and identifies the dominant formation of FeNa+VNa and Nai+NaFe complexes. While the former can become spontaneous in a Na-deficient environment, the latter occurs mainly in NFS0 and accommodates the presence of non-stoichiometry.

Place, publisher, year, edition, pages
2019.
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-381171OAI: oai:DiVA.org:uu-381171DiVA, id: diva2:1302535
Available from: 2019-04-05 Created: 2019-04-05 Last updated: 2019-08-16
In thesis
1. Defect Thermodynamics and Kinetics in Polyanionic Cathodes: A Theoretical Roadmap for Na-ion based Batteries and Hybrid Supercapacitors
Open this publication in new window or tab >>Defect Thermodynamics and Kinetics in Polyanionic Cathodes: A Theoretical Roadmap for Na-ion based Batteries and Hybrid Supercapacitors
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, the framework of the density functional theory is employed to study and predict properties of polyanionic cathodes for Na-ion batteries and hybrid supercapacitors. It consists of three main parts as follows:

The first part is primarily dedicated to kröhnkite-type Na2Fe(SO4)22H2O cathode. The major goal is to probe diffusion mechanisms of Na+ ions.  The chemical potentials diagrams for the pentrary compound are determined under thermodynamic equilibrium and are used to calculate pH value for solution-based synthesis. We find that the presence of NaFe facilitates a faster migration and reduces the channel blockage issue. Moreover, the defect concentration can be tuned by controlling the pH condition. We conclude that defects and small hole polarons play a role in ionic and electronic conductivity.

The second part focuses on alluaudite-type Na2+2δFe2-δ(SO4)3 (NFSδ). We unveil the effect of the non-stoichiometry on the thermodynamics, defect nature, and voltage profiles NFSδ with δ = 0, 0.25 and 0.5. The relation between Na ion distribution and energetics is studied and reveals the necessity of using a supercell model. Chemical potential diagrams indicate an inevitable impurity precipitation in all cases, but can be reduced at low δ. Defect formation analysis shows an unlikely formation of channel blockage and can explain the impurity precipitation in experiment. Two types of phase transition are observed after half-desodiation. A higher degree of non-stoichiometry offers an improvement in specific capacity and structural reversibility for NFS0.25 and NFS0.5. The voltage profiles and formation energy reveal the Na intercalation mechanism and strategy to enhance the specific capacity.

The third part is associated with battery-type cathodes used in hybrid supercapacitors, namely the NaMPO4 and MMoO4 (where M is a transition metal). We find that triphylite NaNiPO4 shows a better electrochemical performance as compared to maricite phase due to the merit of intercalation mechanism. A mixed-NaMn1/3Co1/3Ni1/3PO4 is predicted to show faradaic behavior, mainly contributed from the Ni and Mn redox reactions, along with an improved electronic conductivity. Moreover, the effect of M substitution on phase stability, electronic properties and charge transfer is also studied in MMoO4 with M = Mn, Co and Ni. The highest capacitance is predicted for NiMoO4 amongst the others and is attributed to the higher active surface area. To compromise the capacitance and cycling stability, Mn1/3Co1/3Ni1/3MoO4 is synthesized. We predict its crystal structure by using the SQS method. Based on electronic structure, we can identify a source of the improved cycling efficiency and specific capacitance of this mixed compound.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. p. 92
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1796
Keywords
DFT, Energy Materials, Defects, Chemical potentials, Kinetics, Hybrid supercapacitors, Na-ion batteries, Polyanionic cathodes
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-381173 (URN)978-91-513-0628-5 (ISBN)
Public defence
2019-05-22, Room 80101, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2019-04-29 Created: 2019-04-05 Last updated: 2019-06-18

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