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Mapping Sodium Intercalation Mechanism, Electrochemical Properties and Structural Evolution in Non-stoichiometric Alluaudite Na2+2δFe2-δ(SO4)3 Cathode Materials
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. (Condensed Matter Theory Group)
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory. Indian Inst Technol Indore, Discipline Phys, Indore 453552, Madhya Pradesh, India. (Condensed Matter Theory Group)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. (Condensed Matter Theory Group)ORCID iD: 0000-0003-1231-9994
2019 (English)In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 7, p. 17446-17455Article in journal (Refereed) Published
Abstract [en]

In the scientific advancement of future cathode materials, alluaudite sodium iron sulfate Na2+2δFe2−δ(SO4)3 (NxFyS) has emerged as one of the most promising candidates for sustainable sodium-ion batteries due to its high Fe2+/3+ redox potential (3.8 V vs.Na/Na+), low cost, and high rate capability. Usually, this material occurs in a non-stoichiometric form with partial Na+ substitutions on Fe sites, where δ is close to 0.25 (N2.5F1.75S) depending on the synthesis conditions. While many contemporary works have primarily been directed to study this non-stoichiometric compound, our previous theoretical prediction unveiled the possibility to synthesize stoichiometric alluaudite (N2F2S), which is expected to deliver higher specific capacity (∼120 mA h g−1) as compared to the non-stoichiometric derivatives. This provokes curiosity toward the non-stoichiometric effect on the electrochemical activities and sodium intercalation mechanism in alluaudite materials. In this work, we therefore perform rigorous first-principles calculations to study the structural evolution, electrochemical behavior, and voltage profile of NxFyS with y = 2, 1.75, and 1.5. We reveal the likelihood of two phase transitions after half desodiation process, whereas the probability is reduced with a higher degree of non-stoichiometry, suggesting improvement in the structural reversibility for N2.5F1.75S and N3F1.5S. The prediction of the voltage profiles shows the benefit of non-stoichiometry in enhancing the specific capacity and identifies the structural rearrangement of Fe2O10 dimers as the hidden reason behind the irreversible sharp peak experimentally observed in differential galvanostatic profiles.

Place, publisher, year, edition, pages
2019. Vol. 7, p. 17446-17455
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-381172DOI: 10.1039/c9ta03930aISI: 000476913600026OAI: oai:DiVA.org:uu-381172DiVA, id: diva2:1302536
Funder
Swedish Research CouncilStandUpCarl Tryggers foundation Available from: 2019-04-05 Created: 2019-04-05 Last updated: 2019-08-22Bibliographically 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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