Design and development of hybrid cathode structures for aqueous zinc ion battery systems

Abstract

Renewable energy generation is forecast to rise rapidly, increasing almost 60% from 2020 to 2026 (International Energy Agency’s 2021 report). This is the driving force for developing better and safer large-scale energy storage systems. Despite the lithium-ion batteries (LIB) dominating the global energy market due to their higher specific capacity and energy density, the poor safety feature of LIBs owing to their usage of flammable organic electrolytes is a significant concern for their application in large-scale electrical energy storage (EES) systems. Aqueous zinc ion battery (AZIB) systems are particularly attractive as a promising candidate for large-scale EES systems due to their high theoretical gravimetric capacity (820 mAh/g) and volumetric capacity (5855 mAh/cm³), low redox potential (-0.76V) of the zinc metal anodes. In addition to that, AZIB is a safer and more cost-effective alternative to the LIB systems because of the use of aqueous electrolytes with higher safety feature and higher ionic conductivity (1 S cm⁻¹). Despite the advantages, AZIB possesses complications associated with the larger ionic radii of the hydrated Zn²⁺ ions (4.3 Å) and higher electrostatic attraction of the divalent Zn²⁺ ion with the cathode structures leading to severe capacity fading and poor cycling stability. This thesis focuses on the impact of defect engineering, interlayer expansion, and the addition of conductive carbon in optimizing the electrochemical Zn²⁺ storage properties of the classical layered cathode structures such as V₂O₅, MoS₂. In the first study, we report the preparation of a set of hybrid materials consisting of Molybdenum disulfide (MoS₂) nanopatches on reduced graphene oxide (rGO) nanosheets by applying the microwave specific heating of graphene oxide (GO) and molecular molybdenum precursors followed by thermal annealing in 3% H₂ and 97% Ar. The microwave process converts GO to ordered rGO nanosheets that are sandwiched between uniform thin layers of amorphous Molybdenum trisulfide (MoS₃). The subsequent thermal annealing converts the intermediate layers into MoS₂ nanopatches with two-dimensional layered structures whose defect density is tunable by controlling the annealing temperature at 250, 325, and 600 °C, respectively. The Zn-ion storage properties strongly depend on the defects in the MoS₂ adlayer. The highly defective MoS₂/rGO hybrid prepared by annealing at 250 °C shows the highest initial Zn-ion storage capacity (~300 mAh g[subscript MoSx]⁻¹) and close to 100% coulombic efficiency, which is dominated by pseudocapacitive surface reactions at the edges or defects in the MoS₂ nanopatches. This study validates that defect engineering is critical in improving Zn-ion storage. In the second approach, the synthesis of hybrid materials consisting of Vanadium pentoxide (V₂O₅) nanoribbons (NRs) and rGO nanosheets by divalent metal cation mediated coprecipitation is adopted toward high-performance cathodes for AZIB. The divalent metal ions M²⁺ (including Zn²⁺ and Mn²⁺) effectively neutralize the negative charges on the surface of microwave exfoliated V₂O₅ NRs and GO nanosheets to form a strongly bound assembly. The hybrids are further annealed in the N₂ atmosphere to convert the GO into rGO to improve the electrical conductivity. When only Zn²⁺ ions are used during coprecipitation, the Zn-V₂O₅ NR/rGO hybrid shows a high reversible specific capacity of ~386 mAh g⁻¹ at 0.50 A g⁻¹ suffers from poor stability. This is improved by mixing some Mn²⁺ with the Zn²⁺ ions during coprecipitation. The (Mn+Zn)-V₂O₅ NR/rGO hybrid shows a slightly lower specific capacity of ~289 mAh g⁻¹ at 0.5 A g⁻¹ but with improved long-cycling stability and rate-performance due to the stronger binding of Mn²⁺ ions with the V₂O₅ host which serve as stable pillars to support the expanded V₂O₅ layers. This study ratifies the importance of morphology control in improving the ionic and electronic conductivity of the hybrid cathode structures and preventing structural collapse upon repeated intercalation/deintercalation cycles.