Computational Study of NaVOPO₄ Polymorphs as Cathode Materials for Na-Ion Batteries: Diffusion, Electronic Properties, and Cation-Doping Behavior

Abstract

Rechargeable sodium-ion batteries have gained considerable interest as potential alternatives to lithium-ion batteries, owing to their low cost and the wide abundance of sodium. Phosphate compounds are promising materials for sodium-ion batteries because of their high structural stability. Vanadium phosphates have shown high energy densities as cathode materials, but their Na-ion transport and cation-doping properties are not as yet fully understood. Here, we have combined density functional theory calculations and molecular dynamics techniques to study the diffusion, electronic properties, and cation doping of the α-, β-, and α I -NaVOPO 4 polymorphs. The calculated Na-ion activation energies of these compounds (0.3−0.5 eV) are typical for Na-based cathode materials and the simulations predict Na-ion diffusion coefficients of 10 −11 −10 −12 cm 2 s −1 . The cell voltage trends show an operating range of 3.1−3.3 V vs Na/Na + , with the partial substitution of vanadium by other metals (Al 3+ , Co 2+ , Fe 3+ , Mn 4+ , Ni 2+ , or Ti 4+ ) increasing the cell voltage by up to 0.2−1.0 V vs Na/Na + . Our study provides new quantitative insights into the electrochemical behavior of a potentially important class of phosphate cathode materials for sodium-ion batteries.

Implications of Oxygen–Sulfur Exchange on Structural, Electronic Properties, and Stability of Alkali‐Metal Hexatitanates

“…Na‐ and K‐ion batteries have seen a surge in interest as alternate energy sources, particularly due to their low costs, safety, and abundance . Sodium hexatitanate (Na 2 Ti 6 O 13 ) and potassium hexatitanate (K 2 Ti 6 O 13 ) have recently been considered as host structures for reversible Li‐ion insertion …”

“…Furthermore, research into solid electrolytes for all‐solid‐state batteries has seen a rapid rise in recent years. Both oxide and sulfide compounds, such as Li 10 GeP 2 S 12 , Li 10 SnP 2 S 12 , Li 7 La 3 Zr 2 O 12 , Li 3 OCl, and Li 3x La 2/3− x TiO 3 have been investigated as solid‐state electrolytes . Li 2 TiS 3 and Li 3 NbS 4 have also been tested as positive electrodes in the search for the best superionic conductors for all‐solid‐state batteries …”

“…[7][8][9][10][11][12][13][14][15] Na-and K-ion batteries have seen a surge in interest as alternate energy sources, particularly due to their low costs, safety, and abundance. [16][17][18][19][20][21] Sodium hexatitanate (Na 2 Ti 6 O 13 ) and potassium hexatitanate (K 2 Ti 6 O 13 ) have recently been considered as host structures for reversible Li-ion insertion. [8,18,19] Furthermore, research into solid electrolytes for all-solid-state batteries has seen a rapid rise in recent years.…”

“…Both oxide and sulfide compounds, such as Li 10 GeP 2 S 12 , Li 10 SnP 2 S 12 , Li 7 La 3 Zr 2 O 12 , Li 3 OCl, and Li 3x La 2/3Àx TiO 3 have been investigated as solid-state electrolytes. [20][21][22][23][24][25][26][27][28][29] Li 2 TiS 3 and Li 3 NbS 4 have also been tested as positive electrodes in the search for the best superionic conductors for all-solid-state batteries. [29] The dimensionality of the alkali-ion diffusion path has important consequences in the macroscopic conductivity of a battery material.…”

A Low‐Cost and Environmentally Friendly Mixed Polyanionic Cathode for Sodium‐Ion Storage

“…From the results,w ee stimate that the sodium diffusion barriers of NFOS are about 0.5 eV,a tt he same level as the previously reported cathodes. [13,19] To elucidate the electronic structure of the system, the partial density of states (PDOS) projected onto the Fe 3d states of the pristine and the charged sample were calculated, shown in Figure 4d.F e 2+ exhibits ahigh-spin state with the electron configuration t 2g 4 e g 2 (S = 2), where the minority spin-down states (t 2g ) locate right below the Fermi level (EF) and therefore being redox active.A ss odium ions were extracted, these spin-down states of Fe 2+ gradually shift above the EF with electrons transfer to surrounding anions, and the majority spin-up states move deeper below the EF.I nt he charged state,F e 3+ is stabilized in the highest spin state (S = 5/2) with half-filled electron configuration t 2g 3 e g 2 ,o beying the Hundsr ule.T his calculated Fe 2+ to Fe 3+ redox chemistry during the charging process agrees well with the in situ XANES measurements.…”