Electrochemical and Diffusional Investigation of Na2FeIIPO4F Fluorophosphate Sodium Insertion Material Obtained from FeIII Precursor

Lalit Sharma; Prasant Kumar Nayak; Ezequiel de la Llave; Haomin Chen; Stefan Adams; Doron Aurbach; Prabeer Barpanda

doi:10.1021/acsami.7b10637

Electrochemical and Diffusional Investigation of Na₂Fe^IIPO₄F Fluorophosphate Sodium Insertion Material Obtained from Fe^III Precursor

ACS Appl Mater Interfaces. 2017 Oct 11;9(40):34961-34969. doi: 10.1021/acsami.7b10637. Epub 2017 Sep 26.

Authors

Lalit Sharma¹, Prasant Kumar Nayak², Ezequiel de la Llave², Haomin Chen³, Stefan Adams³, Doron Aurbach², Prabeer Barpanda¹

Affiliations

¹ Faraday Materials Laboratory, Materials Research Center, Indian Institute of Science , C. V. Raman Avenue, Bangalore 560012, India.
² Department of Chemistry, Bar-Ilan University , Ramat-Gan 5290002, Israel.
³ Department of Materials Science and Engineering, National University of Singapore , 9 Engineering Drive 1, Singapore 117575, Singapore.

PMID: 28920668
DOI: 10.1021/acsami.7b10637

Abstract

Sodium iron fluorophosphate (Na₂Fe^IIPO₄F) was synthesized by economic solvothermal combustion technique using Fe^III precursors, developing one-step carbon-coated homogeneous product. Synchrotron diffraction and Mössbauer spectroscopy revealed the formation of single-phase product assuming an orthorhombic structure (s.g. Pbcn) with Fe^II species. This Fe^III precursor derived Na₂Fe^IIPO₄F exhibited reversible Na⁺ (de)intercalation with discharge capacity of 100 mAh/g at a rate of C/10 involving flat Fe^III/Fe^II redox plateaus located at 2.92 and 3.05 V (vs Na/Na⁺). It delivered good cycling stability and rate kinetics at room temperature. The stability of Na₂FePO₄F cathode was further verified by electrochemical impedance spectroscopy at different stages of galvanostatic analysis. Bond valence site energy (BVSE) calculations revealed the existence of 2-dimensional Na⁺ percolation pathways in the a-c plane with a moderate migration barrier of 0.6 eV. Combustion synthesized Na₂Fe^IIPO₄F forms an economically viable sodium battery material. Although the capacity of this cathode is relatively low, this study continues systematic work, which attempts to broaden the scope of reversible sodium insertion materials.

Keywords: Na-ion battery; Na2FePO4F; bond-valence site energy; combustion; fluorophosphate.