Nanostructural design of MnO2 cathodes for rechargeable aqueous Zn-ion batteries

Zn-ion batteries (ZIBs), especially systems in mild aqueous electrolytes, are the subject of intense interest due to the abundance and relatively low cost of Zn anodes and electrolytes, environmental benignity and lower-level of toxicity. Importantly, ZIBs can be a ready-to-use technique for all battery companies as they can use the same battery fabrication facilities as LIBs.

Theoretically, zinc anodes offer an ultrahigh volumetric capacity of 5855 mAh/cm3, compared to that of Li anodes (2061 mAh/cm3), due to multiple electron transfer per metal ion and relatively higher density. Moreover, Zn can be easily employed in cells without strict restrictions of air and water. Among different cathode materials, MnO2 materials are favourable due to their suitable tunnel or layered structures and the abundance, environmentally friendly nature and large working voltage window. However, problems such as limited intercalated channel space, poor stability due to dissolution and deposition of such materials during charge/discharge, unclarified mechanism (simultaneous proton intercalation and side product deposition) and low electron conductivity of MnO2 cathodes are yet to be solved. In addition, the large diameter of hydrated Zn2+ (~4.3 Å) and the high polarisation of Zn2+ inhibit the full utilisation of the structures. This PhD studentship aims at developing nanostructured MnO2 for ZIBs, and the project is divided into three stages:

Stage 1) Cathode design
The unstable features of MnO2 will be resolved by inducing cation and anion defects. Different pre-intercalated metal cations will be induced to expand the lattice space and stabilise the crystal structures. The cation, anion vacancy or other anion replacements will be introduced and investigated. The proposed strategies can improve electron conductivity, expand the diffusion channels of the materials. Then, two nano-engineering strategies will be provided. Ultra-thin 2D pre-intercalated MnO2 will be fabricated via a liquid exfoliation process, thus facilitating rapid diffusion and efficient structure utilisation for Zn-ion storage. Through this strategy, the tunnel structure of pre-intercalated MnO2 have the possibility to be direct hosts for Zn-ion storage without an initial conversion reaction, thus improving the pristine energy efficiency and structural stability. Another approach is to build hierarchical structures decorated with considerable nanopores via template-assisted hydrothermal synthesis and the composition with conductive carbon frameworks. The materials will be characterised by various techniques to understand the physical and chemical properties.

Stage 2) Battery fabrication
After successfully obtaining new materials from Stage 1, Zn-ion storage properties of materials will be evaluated to understand capacity, energy/power density, energy efficiency and stability, electrochemical reactions and ion-diffusion kinetics. The mechanism study of the battery cathode can be carried out by ex-situ and in-situ equipment in UoL Chemistry and synchrotron-based resources in Diamond Light Source, such as in-situ AFM for morphological evolution of ultra-thin materials (previous work in Figure 1). The packing density of the cathode materials will be optimised via the collaboration with One Electrical Ltd.

Stage 3) Large-scale device
Battery performance target will be set by GH, WA and One Electrical Ltd. The student will investigate possible mass production routes for cathode materials, and develop prototypes for large-scale energy storage devices with the support from KTP associates.