Elucidation of Operating Mechanism of BiFeO3 As a Negative Electrode for Na-Ion Batteries Using in Operando XAS Investigation

Abstract

Sodium-ion batteries (SIBs) are gaining much attention in the global market of rechargeable batteries especially in the field of large-scale electrical energy storage systems. Considering the increasing demand for the efficient energy storage systems, it is necessary to design new electrode materials, which are capable to deliver higher reversible capacity. A wide spectra of positive electrode materials were employed in sodium ion batteries, whereas more development is required in the field of negative electrodes. Anode materials of sodium-ion batteries were mainly classified under insertion, conversion and alloying type materials1. Among the conversion type materials, Perovskite materials have gained significant attention as conversion type anode materials in sodium-ion batteries, owing to the high theoretical capacity possessed by these materials2,3.BiFeO3 is one such perovskite which possess the potential to deliver higher reversible capacity through conversion and alloying reactions4.In the present work, BiFeO3 was synthesized using a sol-gel method. Structural and morphological characterizations were done with X-ray diffraction and Field emission scanning electron microscopy techniques. Further, electrochemical studies were carried out in a sodium half-cell in the voltage range 0.01-3.00 V vs. Na+/Na. In the cyclic voltammogram, pronounced difference is observed between the first and subsequent cycles and to understand the reason behind this, elucidation of operating mechanism is required. In situ/ex situ XRD techniques are usually not sufficient to get information regarding the operating mechanism of conversion type electrode materials as these converts into metallic nanoclusters (without a long-range order) distributed in Na2O matrix during the insertion of Na+ ions. Hence, in this work, element specific X-ray absorption spectroscopic (XAS) technique was applied to elucidate the oxidation state and coordination environment changes of BiFeO3 during cycling in a sodium half-cell. Evidences for the electrochemical activity of Bi through conversion reaction (Bi3+ - Bi0) and alloying reaction (with metallic Na) during first discharge was obtained from XAS results. Further, involvement of Fe in the charge storage mechanism through conversion of its oxidized (Fe3+) form to metallic Fe was also observed, which is absent in previous literature reports. In the subsequent charging, reversible dealloying and subsequent oxidation of Bi and oxidation of Fe were noted. The Bi LIII edge and Fe K edge XAS results during second discharge reveals that similar conversion and alloying reactions occur as in the case of first discharge by reduction of Bi and Fe oxides. However, Fe and Bi oxides exists in different oxidation states at the end of first charge than in pristine BiFeO3.In addition to the elucidation of operating mechanism, the electrochemical performance of BiFeO3 was optimized by varying binder and electrolyte compositions. The best electrochemical performance was obtained when Fluoroethylene carbonate was used as an electrolyte additive. Acknowledgement A. Bhaskar gratefully acknowledge financial support from “DST-IISc Energy Storage Platform on Supercapacitors and Power Dense Devices” through the MECSP-2K17 program under grant no. DST/TMD/MECSP/2K17/20”. D. Dixon acknowledges the financial support from funding agency, Science and Engineering Board (SERB), New Delhi, India through Ramanujan Fellowship, under the grant number SB/S2/ RJN-162/2017. A. Surendran is grateful to UGC New Delhi (Ref. no.:63/CSIR-UGC NET DEC. 2017) for the UGC SRF grant and Academy of Scientific and Innovative Research (AcSIR), Ghaziabad- 201002, India. A.T acknowledges the Council of Scientific and Industrial Research (CSIR) for a senior research fellowship and Innovative Research (AcSIR), Ghaziabad- 201002. DESY, Hamburg, Germany is acknowledged for the beamtime allocation at the P65 beamline at PETRA III and we acknowledge the beamline scientist Dr. Edmund Welter for the support. The CIF, CSIR-CECRI is acknowledged for technical support with material characterization. Reference [1] Wu, C., Dou, S. X. & Yu, Y. The State and Challenges of Anode Materials Based on Conversion Reactions for Sodium Storage. Small 14, 1–20 (2018).[2] M. Nakayama, H. Ikuta, Y. Uchimoto, M. Wakihara, Changes in local structure during electrochemical Li insertion into A-site deficient perovskite oxides, La1/3NbO3, J. Phys. Chem. B,107(2003)10715-10721[3] M. Kong, K. Liu, J. Ning, J. Zhou, H. Songa, Perovskite framework NH4FeF3/carbon composite nanosheets as a potential anode material for Li and Na ions storage, J. Mater. Chem. A, 5(2017)19280-19288[4] X. Dinga, Y.Liub, Hollow bismuth ferrite combined graphene as advanced anode material for sodium-ion batteries, 30(2020)153-159