Aqueous zinc-ion batteries (AZIBs) and, in particular, Zn//MnO₂ rechargeable batteries have attracted great attention due to the abundant natural resources of zinc and manganese, low cost, environmental friendliness, and high operating voltage. Among the various ways to improve the electrochemical performance of MnO₂-based cathodes, the development of MnO₂ cathodes doped with metal ions or composites of MnO₂ with conducting polymers has shown such advantages as increasing the specific capacity and cycling stability. This mini-review focuses on the strategies to improve the electrochemical performance of manganese-based cathodes of AZIBs.

In this work the nanocomposite of vanadium oxide with conducting polymer poly(3,4-ethylenedioxythiophene) (VO@PEDOT) was obtained by microwave-assisted hydrothermal synthesis. The detailed study of its structural and electrochemical properties as cathode of aqueous zinc-ion battery was performed by scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffraction analysis, X-ray photoelectron spectroscopy, thermogravimetric analysis, cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy. The initial VO@PEDOT composite has layered nanosheets structure with thickness of about 30–80 nm, which are assembled into wavy agglomerated thicker layers of up to 0.3–0.6 μm. The phase composition of the samples was determined by XRD analysis which confirmed lamellar structure of vanadium oxide V₁₀O₂₄∙12H₂O with interlayer distance of about 13.6 Å. The VO@PEDOT composite demonstrates excellent electrochemical performance, reaching specific capacities of up to 390 mA∙h∙g⁻¹ at 0.3 A∙g⁻¹. Moreover, the electrodes retain specific capacity of 100 mA∙h∙g⁻¹ at a high current density of 20 A∙g⁻¹. The phase transformations of VO@PEDOT electrodes during the cycling were studied at different degrees of charge/discharge by using ex situ XRD measurements. The results of ex situ XRD allow us to conclude that the reversible zinc ion intercalation occurs in stable zinc pyrovanadate structures formed during discharge.

ZnFe₂O₄ (ZFO) nanospheres with complex structures have been synthesized by a one-step simple solvothermal method using two different types of precursors—metal chlorides and nitrates —and were fully characterized by XRD, SEM, XPS, and EDS. The ZFO nanospheres synthesized from chloride salts (ZFO_C) were loose with a size range of 100–200 nm, while the ZFO nanospheres synthesized from nitrate salts (ZFO_N) were dense with a size range of 300–500 nm but consisted of smaller nanoplates. The different morphologies may be caused by the different hydrolysis rates and different stabilizing effects of chloride and nitrate ions interacting with the facets of forming nanoparticles. Electrochemical tests of nitrate-based ZFO nanospheres as anode materials for lithium-ion batteries demonstrated their higher cyclic stability. The ZFO_C and ZFO_N samples have initial specific discharge/charge capacities of 1354/1020 and 1357/954 mAh∙g⁻¹, respectively, with coulombic efficiencies of 75% and 71%. By the 100th cycle, ZFO_N has a capacity of 276 mAh∙g⁻¹, and for ZFO_C, only 210 mAh∙g⁻¹ remains after 100 cycles.

Vanadium oxide composites with conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) were obtained by one-step microwave-assisted hydrothermal synthesis at two different temperatures: 120 and 170 °C (denoted as V-120 and V-170, respectively). The structure and composition of the obtained samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, X-ray photoelectron spectro­scopy (XPS), and thermogravimetric (TG) analysis. The detailed study of the electro­chemical properties of the composites as cathodes of aqueous zinc-ion battery was per­formed by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) at different current densities and by electrochemical impedance spectroscopy (EIS). It was shown that V-120 demonstrated excellent electrochemical performance in the 0.3 to 1.4 V vs. Zn/Zn2+ potential range reaching specific capacities of up to 390 mA∙h∙g−1 at 0.3 A∙g−1 with excel­lent capacity stability after 1000 charge-discharge cycles. Its functional parameters were found to be much better than those of the electrodes based on the V-170 composite obtained at a higher temperature. The effect of the synthesis temperature on the electro­chemical properties is discussed in terms of the crystallographic, compositional, and thermogravimetric properties of the samples.