Careful development and optimization of negative electrode (anode) materials for Na-ion batteries (SIBs) are essential, for their widespread applications requiring a long-term cycling stability. BiFeO3 (BFO) with a LiNbO3-type structure (space group R3c) is an ideal negative electrode model system as it delivers a high specific capacity (770 mAh g–1), which is proposed through a conversion and alloying mechanism. In this work, BFO is synthesized via a sol–gel method and investigated as a conversion-type anode model-system for sodium-ion half-cells. As there is a difference in the first and second cycle profiles in the cyclic voltammogram, the operating mechanism of charge–discharge is elucidated using in operando X-ray absorption spectroscopy. In the first discharge, Bi is found to contribute toward the electrochemical activity through a conversion mechanism (Bi3+ → Bi0), followed by the formation of Na–Bi intermetallic compounds. Evidence for involvement of Fe in the charge storage mechanism through conversion of the oxide (Fe3+) form to metallic Fe and back during discharging/charging is also obtained, which is absent in previous literature reports. Reversible dealloying and subsequent oxidation of Bi and oxidation of Fe are observed in the following charge cycle. In the second discharge cycle, a reduction of Bi and Fe oxides is observed. Changes in the oxidation states of Bi and Fe, and the local coordination changes during electrochemical cycling are discussed in detail. Furthermore, the optimization of cycling stability of BFO is carried out by varying binders and electrolyte compositions. Based on that, electrodes prepared with the Na-carboxymethyl cellulose (CMC) binder are chosen for optimization of the electrolyte composition. BFO–CMC electrodes exhibit the best electrochemical performance in electrolytes containing fluoroethylene carbonate (FEC) as the additive. BFO–CMC electrodes deliver initial capacity values of 635 and 453 mAh g–1 in the Na-insertion (discharge) and deinsertion (charge) processes, respectively, in the electrolyte composition of 1 M NaPF6 in EC/DEC (1:1, v/v) with a 2% FEC additive. The capacity values stabilize around 10th cycle and capacity retention of 73% is observed after 60 cycles with respect to the 10th cycle charge capacity.

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Sodium-ion battery (NIB) system is an emerging technology and can be considered as a suitable alternative for lithium-ion batteries (LIBs) due to the large abundance and distribution of sodium on earth and similar working principles to LIB. Among those cathodes for NIBs, layered transition metal oxides (NaxMO2) receive more attention because of their higher capacity, appropriate operating potentials, higher ionic conductivity, and ease of synthesis [1]. According to the stacking sequence of oxygen layers and Na occupation sites, layered transition metal oxides are mainly classified as P2, O3, P3, and O2 structures. The letters P and O imply that the sodium occupies trigonal prismatic sites and octahedral sites, respectively. The numbers indicate the no. of oxygen stacking layers [2]. Among these, the P2 type layered transition metal oxides gained more recognition as cathode materials for NIBs due to their superior rate capability from the migration of sodium ions through the face-sharing trigonal prismatic sites [3]. However, the intercalation/de-intercalation of large sodium ions creates some structural deterioration and irreversibility. Designing multiphase materials is an effective strategy to improve the electrochemical performance of the material to avail the synergistic effects from each phase [3,4]. In this work, a cobalt-free layered-spinel composite was synthesized by sol-gel method as positive electrode material for NIBs. It is highly attractive, as it is cobalt-free and hence, cost-effective and environmentally benign. The layered phase provides a smoother diffusion pathway and the spinel phase could enhance the electronic conductivity [3,4]. The presence of layered and spinel phases was confirmed by the X-ray diffraction technique. Scanning electron microscopic investigations reveal particles of layered morphology with well-defined edges. The electrochemical investigations were done in Na-half cells in the voltage range of 1.5- 4.0 V vs. Na+/Na. The cyclic voltammogram of the layered-spinel composite in Na half-cell shows two sets of peaks corresponding to the redox activity of Mn and Ni. When the upper cut-off voltage was increased above 4 V, contributions from the Fe electrochemical activity were also observed. To investigate the sodium storage performance, galvanostatic charge-discharge studies were done. The material displayed an initial discharge capacity of 171 mAh g-1 and promising high-rate behavior. To investigate the electrochemical mechanism, in operando X-ray absorption spectroscopic studies were done and the results will be discussed in detail. Acknowledgement A.Bhaskar gratefully acknowledges 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 SERB, New Delhi, India through Ramanujan Fellowship, under the grant number SB/S2/ RJN-162/2017. A. Thottungal is grateful to CSIR New Delhi for the CSIR SRF grant and AcSIR, Ghaziabad- 201002, India. DESY, Hamburg, Germany is acknowledged for the beamtime allocation at the P65 beamline at PETRA III and the beamline scientist Dr. Edmund Welter is acknowledged for his support. This work contributes to the research performed at CELEST, and was partially funded by the DFG under Project ID 390874152 (POLiS Cluster of Excellence). Reference D. Slater , D. Kim , E. Lee and C. S. Johnson , Adv. Funct. Mater., 2013, 23 , 947 —958 Delmas, C. Fouassier and P. Hagenmuller, Phys. B, 1980, 99, 81 Zheng, P. Yan, W. H. Kan, C. Wang and A. Manthiram, J. Electrochem. Soc., 2016, 163(3), A584 Hou , J. Yin , X. Lu , J. Li , Y. Zhao and X. Xu , Nanoscale, 2018, 10 , 6671 —6677