Critical overview of polyanionic frameworks as positive electrodes for Na-ion batteries

Abstract

Na-ion batteries (NIBs) are increasingly looked at as a viable alternative to Li-ion batteries due to the abundance, low cost, and thermal stability of Na-based systems. To improve the practical utilization of NIBs in applications, it is important to boost the energy and power densities of the electrodes being used, via discovery of novel candidate materials. Thus, we explore the chemical space of transition metal containing oxyfluorides (TMOFs) that adopt the perovskite structure as possible NIB electrodes. Our choice of the perovskite structure is motivated by the 'large' cationic tunnels that can accommodate Na + , while the chemistry of TMOFs is motivated by the high electronegativity and inductive effect of F − , which can possibly lead to higher voltages. We use density functional theory based calculations to estimate the ground state polymorphs, average Na (de)intercalation voltages, thermodynamic stabilities and Na + mobility on two distinct sets of compositions: the F-rich NaxMOF2, and the O-rich Na1+xMO2F where x = 0-1 and M = Ti, V, Cr, Mn, Fe, Co, or Ni. Upon identifying the ground state polymorphs in the charged compositions (i.e., MOF2 and NaMO2F), we show that F-rich perovskites exhibit higher average voltages compared to O-rich perovskites. Also, we find six stable/metastable perovskites in the F-rich space, while all O-rich perovskites (except NaTiO2F) are unstable. Finally, our Na-ion mobility calculations indicate that TiOF2-NaTiOF2, VOF2-NaVOF2, CrOF2, and NaMnOF2 can be promising compositions for experimental exploration as NIB cathodes, primarily if used in a strained electrode configuration and/or thin film batteries. Our computational approach and findings provide insights into developing practical NIBs involving fluorine-containing intercalation frameworks.

First principles investigation of anionic redox in bisulfate lithium battery cathodes

“…In the case of layered and disordered cathodes, anionic redox is almost always associated with irreversible structural changes, leading to voltage hysteresis/fading, and also oxygen evolution (resulting from oxygen dimerization). [8][9][10] Polyanionic cathodes, which are typically resistant to ''large'' structural changes, 54,55 can provide a platform for a different anionic redox mechanism. For example, the strong, covalent X-O bonds within the XO 4 polyhedral units may resist oxygen dimerization and evolution, resulting in minimal irreversible structural changes and voltage fade.…”

Pushing Forward Simulation Techniques of Ion Transport in Ion Conductors for Energy Materials

“…The Natrium Super-Ionic CONdcutor (NaSICON) with the composition Na 1+ x Zr 2 Si x P 3– x O 12 is a well-known ceramic solid-electrolyte for all-solid-state Na-ion batteries, ,− with impressive ion-conductivities of ∼4 mS cm –1 at 298 K (for composition Na 3.4 Zr 2 Si 2.4 P 0.6 O 12 ) . NaSICON provides a versatile framework, incorporating both cations (Na + and Zr 4+ ) and anion ( normalS normali O 4 4 − and normalP O 4 3 − ) moieties, which enable the number of Na + ions to vary between 1 and 4 per f.u.…”

Kinetic Monte Carlo Simulations of Sodium Ion Transport in NaSICON Electrodes

“…Having achieved widespread commercialization, rechargeable lithium (Li)-ion batteries (LIBs) are now at the risk of geopolitically constrained supply chains of key raw materials, such as cobalt, nickel, and Li. − Sodium (Na)-ion batteries (SIBs) appear to be promising alternatives to the LIB analogs, as Na-metal can be harvested directly from seawater. − Extensive research is underway to optimize electrodes and electrolytes for SIBs. − One of the material classes for NIBs is the polyanionic sodium superionic conductor (NaSICON), discovered by Hong et al, , a framework studied for its fast Na-conducting properties. Electrodes crystallizing in the NaSICON framework, with formula Na x M 2 (PO 4 ) 3 (where M = transition metal), can be highly tuned to achieve promising energy densities, ,, by changing the ratio and types of transition metals in the NaSICON, such as Na x TiV­(PO 4 ) 3 , Na x TiMn­(PO 4 ) 3 , Na x VMn­(PO 4 ) 3 , and Na x CrMn­(PO 4 ) 3 . , …”