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Defect formations and pH-dependent kinetics in krohnkite Na2Fe (SO4)2·2H2O based cathode for sodium-ion batteries: Resembling synthesis conditions through chemical potential landscape
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
Kasetsart Univ, Fac Sci, Dept Phys, Bangkok 10900, Thailand.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.ORCID iD: 0000-0002-6765-2084
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Royal Inst Technol, Dept Mat Sci & Engn, SE-10044 Stockholm, Sweden.ORCID iD: 0000-0003-1231-9994
2019 (English)In: Nano Energy, ISSN 2211-2855, E-ISSN 2211-3282, Vol. 55, p. 123-134Article in journal (Refereed) Published
Abstract [en]

Thermodynamics and kinetics of intrinsic point defects in Na2Fe(SO4)(2)center dot 2H(2)O, a high-voltage cathode for Na-ion batteries, are studied by means of first-principles density functional theory. Electronic structures of charged defects are calculated to study their influences towards electronic and electrochemical properties as well as to probe hole polaron formation. As defect formation energy strongly depends on atomic chemical potentials, we initiate a systematic approach to determine their valid ranges for the pentrary Na-Fe-S-O-H compound under thermodynamic equilibria and correlate them with approximated pH parameters in solution-based synthesis. Given chemical potential landscape and formation energy, we find that Fe-Na(1+), V-Na(1-,0) and Na-Fe(1-,0) are dominant and their concentrations could be manipulated through pH condition and oxygen content in the precursor solution. It is predicted that the channel blockage due to Fe-Na would appear under strong acidic growth condition but could be diminished under weak acidic condition (4.7 <= pH <= 5.6) where Na-Fe facilitates a faster migration between each diffusion channel. Our results do not only explain the origin of intercalation mechanism and improved electronic conduction, but also demonstrates the pH influence towards conductivities in the cathode material.

Place, publisher, year, edition, pages
2019. Vol. 55, p. 123-134
Keywords [en]
Chemical potentials, Defects, DFT, Diffusions, Sodium-ion batteries
National Category
Materials Chemistry Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-374115DOI: 10.1016/j.nanoen.2018.10.038ISI: 000454636200012OAI: oai:DiVA.org:uu-374115DiVA, id: diva2:1281963
Funder
Swedish Research CouncilSwedish Research CouncilCarl Tryggers foundation Available from: 2019-01-23 Created: 2019-01-23 Last updated: 2019-04-05Bibliographically approved
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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Watcharatharapong, TeeraphatChakraborty, SudipAhuja, Rajeev

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