Structural modification of isomorphous SO4 2--doped K2FeO4 for remediating the stability and enhancing the discharge of super-iron battery

Abstract

In the paper, the isomorphous $S{O}_4^{2-}$ doped K₂FeO₄, aimed at the remediation of the discharge and stability of the super-iron battery, was first synthesized for doping and reforming the K₂FeO₄ crystalline structure via a facile co-precipitation and mechanochemistry. Afterwards, the compared cathodes were assembled by the undoped and doped K₂FeO₄ for an evaluation of the discharge and stability in the AAA super-iron battery system. The results show that the small amounts of K₂SO₄ were doped into the K₂FeO₄ in the calculated form of K₂Fe_1-xS_xO₄ by the isomorphous substitution. The doped K₂FeO₄ cathodes/batteries exhibited an excellent discharge with a normal discharge profile. The cathodes doped by two techniques had significantly enhanced the discharge capacity of the super-iron battery with an increase of 10-30% compared to the undoped K₂FeO₄. Moreover, the stability of the K₂FeO₄ cathodes was obviously remediated by the isomorphous $S{O}_4^{2-}$ doping. The shelf time of the doped K₂FeO₄ cathodes was prolonged by an increase of about 10% in comparison of the undoped K₂FeO₄ cathode. The desirable enhancements could be attributed to doping and reforming the similar building block and isomorphous $S{O}_4^{2-}$ into the $\mathrm{Fe}{O}_4^{2-}$ tetrahedral and crystalline in the form of the isomorphous substitution and filling vacancies.

Keywords: K2FeO4; capacity; discharge; ferrates; stability; super-iron battery.

Figure 1.

The $\mathrm{Fe}{\mathrm{O}}_4^{2-}$ anion, Fe-O bond distances and angles.

Figure 2.

The orthorhombic crystalline of K₂FeO₄.

Figure 3.

The $\mathrm{S}{\mathrm{O}}_4^{2-}$ anion; Fe-O bond distances and angles.

Figure 4.

The XRD patterns of the undoped and doped K₂FeO₄ samples.

Figure 5.

The SEM images of the undoped and doped K₂FeO₄ samples ((a) the undoped sample in 1 µm scale, (b) the undoped sample in 200 nm scale, (c) the doped sample in 1 µm scale and (d) the doped sample in 200 nm scale).

Figure 6.

The discharge analysis of the $\mathrm{S}{\mathrm{O}}_4^{2-}$-doped K₂FeO₄ cathodes compared to the undoped K₂FeO₄.

Figure 7.

The stability analysis of the $\mathrm{S}{\mathrm{O}}_4^{2-}$-doped K₂FeO₄ cathodes compared to the undoped K₂FeO₄.

Figure 8.

Kinetics of the decomposition of the $\mathrm{S}{\mathrm{O}}_4^{2-}$-undoped and doped K₂FeO₄ cathodes by the double-ions co-precipitation and mechanochemistry (the initial percentage of K₂FeO₄: C₀ = 75% w/w in the cathodes).

Scheme 1.

Mechanismic chemistry of the structural modification of isomorphous $\mathrm{S}{\mathrm{O}}_4^{2-}$-doped K₂FeO₄.