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Electrode regulation for high-performance and sustainable aqueous zinc-ion batteries

Jiao, Yiding; (2022) Electrode regulation for high-performance and sustainable aqueous zinc-ion batteries. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

Thanks to water-based mild electrolytes, aqueous zinc-ion batteries are endowed with high power density, low cost, and good safety. They are expected to be suitable for next-generation energy storage systems in the post-lithium era. Unfortunately, the development of aqueous zinc-ion batteries is hindered by their unsustainable electrodes. For the cathode, manganese materials, including a series of manganese oxides and manganates, are favoured due to their high discharge plateau and low cost while suffering from severe capacity degradation due to irreversible structural transformation and dissolution of Mn2+. For the anode, zinc metal is used in aqueous zinc-ion batteries due to the low redox potential (-0.76 V vs standard hydrogen electrode) and high theoretical capacity (820 mAh·g-1 and 5,854 mAh·L-1 ) and environmental friendliness. However, side reactions such as corrosion reaction with mild acid electrolyte, formation of inert by-product on the electrode surface, and dendrite growth obstruct zinc anode's practical application. Therefore, this thesis develops and investigates high-performance and sustainable MnO2 cathodes and zinc anodes for aqueous zinc-ion batteries. Strategies for stabilising cathode and anode are systematically investigated, evaluating their physical/electrochemical properties, and revealing their underlying connections. The mechanism is hence carefully evaluated. Therefore, the aqueous zinc-ion batteries exhibit high electrochemical performance and sustainability, superior to recently reported research. The details of the included study are summarised in the following three aspects: (1) MnO2 cathode is widely applied as a cathode material for aqueous zinc-ion batteries due to their high discharge voltage plateau (1.30–1.35 V vs Zn2+/Zn), abundant reserves, simple fabrication, low cost, and environmental friendliness. However, MnO2 cathodes are challenged by structural transformation upon cycling, leading to inevitable capacity degradation. Pre-intercalating metal ions in manganese polymorphs has been demonstrated to be a practical approach to stabilise manganese oxide-based cathodes via electrostatic force between preintercalated ions and MnO6 octahedra. However, no previous reports systematically revealed the amount of pre-intercalation and its stabilisation effect on electrochemical performance. In chapter 2, two δ-MnO2 materials are synthesised with different K pre-intercalation amounts (i.e., K0.21MnO2 and K0.28MnO2). The physical and electrochemical characterisations reveal their differences in composition and cycling stability, which resulted in the “phase selection” effect from the K: Mn ratio. With a high K pre-intercalation and K: Mn ratio, the phase transformation between δ-MnO2 and α-MnO2 can be suppressed. Therefore, aqueous zinc-ion batteries from a K0.28MnO2 cathode exhibit > 95% capacity retention for over 1,000 cycles with a sufficient specific capacity of 300 mAh·g-1 , which significantly prolongs the cycling life of manganese oxide-based cathodes. This section provides a general principle for designing pre-intercalated manganese oxide-based cathodes for aqueous zinc-ion batteries with sustainability. (2) Despite the numerous efforts made to improve the cycling stability of manganese oxide-based cathodes, it remains a challenge to enhance the specific capacity of manganese oxide-based cathodes. The low specific capacity of manganese materials (normally < 100 mAh·g-1 at current densities > 2 A·g-1 ) generally results from the restriction of one-electron transfer reaction (308 mAh·g-1 for Mn (III)/Mn (IV)) and a low pseudocapacitive contribution. In chapter 3, a highly porous CaMnO3 material is designed with open frameworks and a manganese state of +4. The dissolution of the CaMnO3 is tackled by plasma treatment and carbon coating. The modified CaMnO3 cathode could deliver a superior capacity output for > 150 mAh·g-1 at 2 A·g-1 , exceeding current reports on manganese oxidebased cathodes. The charge storage mechanism was systematically investigated via ex-situ X-ray diffraction and X-ray photoelectron spectroscopy. This work presents a general strategy to improve the electrochemical performance of manganese oxide-based cathodes, enlarging the specific contact area with electrolyte while enabling a cathode electrolyte interphase for cycling stability, which brings manganese oxide-based cathodes closer to the practical application. (3) To extend the cycle life of manganese oxide-based cathodes, numerous efforts have been devoted to material design, including strategies such as preintercalation, carbonaceous coating, building heterostructures and doping. However, these strategies are typically complicated and only ensure a limited extension effect. In chapter 4, an effective electro-activation strategy is demonstrated to revive the degraded Zn-MnO2 batteries. By designing an appropriate electro-activation protocol, it was revealed that the inert β-MnOOH by-product could be removed and a pristine β-MnO2 cathode reformed. By this means, the degraded aqueous zinc-ion batteries can be electro-activated for more than 20 times, allowing a cycle life of > 20,000 cycles, far exceeding current reports. Furthermore, the universality of this strategy in other manganese materials was revealed. (4) To further improve the sustainability of aqueous Zn-MnO2 batteries, it is urgent to tackle the unstable Zn anodes. The side reactions on the zinc anode during cycling can be concluded as corrosion, passivation, and dendrite growth. The corrosion side reaction refers to the spontaneous reaction of zinc metals with the mild acid electrolytes (ZnSO4 and Zn(CF3SO3)2), which leads to evident zinc loss during cycling. The passivation reaction occurs with pH shifting on the anode surface during cycling, while the irreversible zinc hydroxides are formed on the anode surface, thereby reducing the reactive surface area. Dendrites are typical branchlike crystals easily formed on the anode when the charge is accumulated. The arbitrary dendrites usually penetrate the separator and cause a short circuit to the full cells, reducing cycle life and posing safety hazards. In chapter 5, a "polymer glue" coating on the Zn anode is developed to solve these issues systematically, consisting of polymer frameworks of polyethene oxide and dissolved LiN(CF3SO2)2 ionic conductive agent. The polymer glue enables more than 1,000 hours of cycling life at an ultra-high depth of discharge of 90%, signifying its potential in the application.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Electrode regulation for high-performance and sustainable aqueous zinc-ion batteries
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Copyright © The Author 2021. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request.
UCL classification: UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Chemistry
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL
URI: https://discovery.ucl.ac.uk/id/eprint/10156144
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