Unveiling the Electrochemical Mechanism of High-Capacity Negative Electrode Model-System BiFeO₃ in Sodium-Ion Batteries: An In Operando XAS Investigation

Abstract

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.

Benefits and Development Challenges for Conversion-Alloying Anode Materials in Na-Ion Batteries

“…The conversion reaction is partially reversible because some Bi 2 S 3 is retrieved at the end of the desodiation (Figure 2C), which is beneficial for specific capacity, but not practical for cycling stability (the second cycle demonstrates diminished formation of Bi 2 S 3 ). Similar instabilities in the matrixes of some ternary CAMs, such as Bi 2 MoO 6 and BiFeO 3 , were observed (Brennhagen et al, 2022;Surendran et al, 2022). However, detailed studies on the behavior of the matrixes linked to the electrochemical performance are difficult to perform due to the complexity of the cycling mechanisms and species formed.…”

“…Compared to binary CAMs, ternary materials are significantly less studied, however, several examples based on Bi-and Sb-based oxometallates of transition metals (BiFeO 3 , Bi 2 (MoO 4 ) 3 , BiVO 4 , Sb 2 MoO 6 , SbVO 4 , Bi 2 MoO 6 ) were reported (Durai et al, 2017;Sottmann et al, 2017;Lu et al, 2019;Pan et al, 2019;Brennhagen et al, 2022;Surendran et al, 2022). The choice of the second cation (Fe, Mo, V, etc.)…”

“…Another useful operando technique, which is used to monitor specific elements in CAMs is operando XAS, where X-ray energies are scanned across the absorption edges of the target elements. This technique has been utilized to gain information on changes in oxidation states and local structures during cycling for Bi 2 O 3 , Bi 2 S 3 , BiVO 4 , Bi 2 (MoO 4 ) 3 , SnO 2, and BiFeO 3 (Kim et al, 2016;Sottmann et al, 2016b;Sottmann et al, 2017;Dixon et al, 2019;Surendran et al, 2022). In the case of Bi 2 S 3, a measurement combining operando XRD and XAS was used to reveal important aspects of the cycling mechanism (Figure 2) (Sottmann et al, 2016b).…”

Applications of all-inorganic perovskites for energy storage

“…Wang et al synthesized polycrystalline ZnSnO 3 as an anode material for SIBs, followed by a simple etching reaction to obtain hollow structures, which achieved a reversible capacity of 315 mAh g -1 at 30 mA g -1 . 95 Ammu et al, synthesized BiFeO 3 (BFO) electrode materials with conversion and alloying mechanisms for Na + storage, exhibiting intriguing specific capacity (770 mAh g -1 ) 96 The use of perovskite based insertiontype hosts is a promising method to conquer the huge volume swelling due to alloying/de-alloying in SIBs anode. Bharathi Please do not adjust margins Please do not adjust margins and splendid recyclability through alloying/de-alloying reactions.…”

“…95 Ammu et al synthesized BiFeO 3 (BFO) electrode materials with conversion and alloying mechanisms for Na + storage, exhibiting intriguing specific capacity (770 mA h g −1 ). 96 The use of perovskite based insertion-type hosts is a promising method to conquer the huge volume swelling due to alloying/de-alloying in the SIB anode. Bharathi et al synthesized perovskite Na 0.5 Bi 0.5 TiO 3 (NBTO) which exhibited high sodium storage capacity, fast sodium storage diffusion kinetics and splendid recyclability through alloying/de-alloying reactions.…”