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

锌离子电池(ZIB)，特别是在温和的水溶液中的体系，由于锌的阳极和电解液的丰富和相对低廉的价格、环境友好和较低的毒性水平而引起人们的极大兴趣。重要的是，ZIB可以成为所有电池公司的即用即用技术，因为它们可以使用与LIB相同的电池制造设施。理论上，由于每个金属离子有多个电子转移和相对较高的密度，锌阳极提供了超高的体积容量，与锂阳极(2061 mAh/cm3)相比，锌阳极具有5855 mAh/cm3的容量。此外，锌可以很容易地在电池中使用，而不受空气和水的严格限制。在不同的正极材料中，MnO2材料因其合适的隧道或层状结构以及丰富的环保性质和较大的工作电压窗口而受到青睐。然而，由于嵌入沟道空间有限、材料在充放电过程中的溶解和沉积导致稳定性差、机理不明(质子同时嵌入和副产物沉积)以及MnO2正极的低电子传导性等问题仍有待解决。此外，大直径的水合锌离子(~4.3？)和高极化的锌离子阻碍了结构的充分利用。本博士研究生的目标是开发用于ZIB的纳米结构MnO2，该项目分为三个阶段：阶段1)阴极设计通过引入阳离子和阴离子缺陷来解决MnO2的不稳定特性。通过引入不同的预插层金属阳离子来扩大晶格空间，稳定晶体结构。将介绍和研究阳离子、阴离子空位或其他阴离子取代物。所提出的策略可以提高电子的导电性，扩大材料的扩散通道。然后，将提供两种纳米工程策略。超薄的2D预插层MnO2将通过液体剥离工艺制备，从而促进快速扩散和有效的结构利用来存储锌离子。通过这一策略，预插层MnO2的隧道结构有可能成为锌离子存储的直接宿主，而不需要进行初始转化反应，从而提高了原始能量效率和结构稳定性。另一种方法是通过模板辅助水热合成和具有导电碳骨架的组合物来构建装饰有大量纳米孔的分级结构。这些材料将通过各种技术来表征，以了解其物理和化学性质。第二阶段电池制造在从第一阶段成功获得新材料之后，将对材料的锌离子存储性能进行评估，以了解容量、能量/功率密度、能量效率和稳定性、电化学反应和离子扩散动力学。电池正极的机理研究可以通过UOL化学中的非原位和原位设备以及钻石光源中的同步辐射资源来进行，例如用于超薄材料形态演变的原位AFM(先前的工作在图1中)。通过与One Electric Ltd.的合作，正极材料的堆积密度将得到优化。阶段3)大型设备电池性能目标将由GH、WA和One Electric Ltd.设定。学生将研究可能的正极材料大规模生产路线，并在KTP Associates的支持下开发大型储能设备的原型。

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.