Designing for minimal impact: the development of an aqueous aluminium-ion battery

Abstract

This thesis contains work related to the sustainable development of an aqueous aluminum ion battery. The battery consists of a titanium dioxide negative electrode and copper hexacynoferrate positive electrode, the electrolyte is 1 M aluminium chloride and 1 M potassium chloride. At the start of the PhD project the reported energy density was 15 Wh kg$^{-1}$ and power density of 300 W kg$^{-1}$, with a cycle life of 1750 charge/discharge cycles.

Aluminium is a viable option for secondary battery technology, not only is the theoretical energy density high (~8000 W kg$^{-1}$), but there are already well established mining, production, and recycling industries built around aluminium, making it a sustainable option too. Further, the use of an aqueous electrolyte, while limiting of the voltage range a cell can operate at - due to the electrochemical stability window of water - is an inherently safe option for an electrolyte, which also boasts an ease of manufacture.

The core concept throughout this thesis is that of minimising the environmental impacts of the battery as the design develops, and as such, a life-cycle assessment (LCA) is conducted on the current stage of the design, and compared to supercapacitors - this is due to the pseudo-capacitive, high power, nature of the battery. The design, in its current bench-based state, is found to be more environmentally friendly overall than commercial supercapacitors, and capacitors. A practical application of this battery in a dual energy storage system is studied for an EV car and bus, showing that, for a long lifetime of the EV, using dual energy storage systems has reduced environmental impacts. The LCA results were then compared to the market leader in energy storage (Li-ion) for environmental impacts.

Based on the comparison to Li-ion, development goals were set based on achieving the same or better lifetime CO$_2$ emissions, and it was found that by increasing the cycle life of the battery, and increasing the amount of utilised active material within the battery, it would become environmentally competitive with Li-ion. Following this outcome, the focus for the experimental part of the PhD was split into active material increase and lifetime extension.

To increase the active material \% within the battery, both increasing the actual amount of active material, and reducing the amount of support material were investigated. Coin-cell development of this battery to reduce support material found that a closed cell would not be appropriate - due to gas production, however 7000 stable cycles were achieved with an uncrimped cell. The loading of active material within the battery was investigated and found that a lower loading lead to increased discharge capacity (287.2 mAh g$^{-1}$ for the positive and 205 mAh g$^{-1}$ for the negative electrode). The performance of the TiO$_2$ electrodes were also investigated in terms of temperature. Given the porosity of the carbon felt, the impacts of compression on performance is being investigated in collaboration with Diamond Light Source, and initial findings are presented.

For increasing the lifetime of a battery it is prudent to understand the degradation mechanisms that occur over time as the battery loses capacity. Once this is known, design decisions and usage prescriptions can be made to mitigate or minimise these mechanisms and therefore increase the life of the battery. Based on this, a long duration experiment spanning eleven months was run at Diamond Light Source, which performed X-ray diffraction on six cycling coin cells each week. The resulting diffraction patterns, alongside X-ray computed tomography of the final coin cells have uncovered information about how the make up of the electrodes have changed over time.

Overall, this thesis shows that by working with the environmental impacts of a battery, we can produce development road-maps that both improve performance and respects the planet.

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Identifiers

ORCID for N Melzack: orcid.org/0000-0002-5578-4020

ORCID for Richard Wills: orcid.org/0000-0002-4805-7589

ORCID for Andrew Cruden: orcid.org/0000-0003-3236-2535

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