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.
The structures of molecules form the cornerstone of our chemical knowledge. Lowering of symmetry in closed-shell molecules is often attributed to the Pseudo Jahn-Teller (PJT) distortions wherein non-adiabatic coupling (NAC) between the ground state and excited states creates vibrational instability along specific normal modes. Nevertheless, other factors like steric interactions are also well known in the literature to induce structural distortions. In this article, we consider two specific cases of molecular distortions - the first one being spontaneous for contorted polyaromatic hydrocarbons (c-PAH) where non-bonded repulsions between the two pairs of syn H-atoms in tribenzopyrene, TBP (1), can enforce either a C → C or C → C distortion. PJT-effects account for the correct preference of the C structure over C (by 4.6 kcal mol). The second case (non-spontaneous symmetry breaking) is that of benzene (2) and coronene (3) which upon application of sufficiently strong static external electric field develop vibrational instability along q(a) to cause D → C and D → C distortions for 2 and 3 respectively. An external electric field (F) was applied parallel to the aromatic ring of 2-3 for investigation of non-spontaneous symmetry breaking. Such electric field induced structural distortion is understood on the basis of excess charge accumulation of the planar rings which is circumvented by symmetry lowering. PJT effects seem to have significant consequences for identification of global minima amongst several local minimal molecular structures.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.