Effect of Aluminum Incorporation on the Reaction Process and Reaction Products of Hydrated Magnesium Silicate

In this study, we investigated the impact of aluminium ion (Al3+) incorporation on the microstructure and the phase transformation of the magnesium silicate hydrate system. The magnesium silicate hydrate system with aluminium was prepared by mixing magnesium oxide and silica fume with different aluminium ion contents (the Al/Si molar ratios of 0.01, 0.02, 0.05, 0.1, 0.2) at room temperature. The high degree of polymerization of the magnesium silicate hydrate phases resulted in the limited incorporation of aluminium in the structure of magnesium silicate hydrate. The silicon-oxygen tetrahedra sites of magnesium silicate hydrate layers, however, were unable to substitute for silicon sites through inverted silicon-oxygen linkages. The increase in aluminium ion content raised the degree of polymerization of the magnesium silicate hydrate phases from 0.84 to 0.92. A solid solution was formed from residual aluminum-amorphous phases such as hydroxyl-aluminum and magnesium silicate hydrate phases. X-ray diffraction (XRD), field emission scanning electron microscope (F-SEM), and 29Si and 27Al MAS NMR data showed that the addition of Al3+ promotes the hydration process of MgO and has an obvious effect on the appearance of M-S-H gel. The gel with low aluminum content is fluffy, while the gel with high aluminum content has irregular flakes. The amount of Al3+ that enters the M-S-H gel increased with the increase of Al3+ content, but there was a threshold: the highest Al/Si molar ratio of M-S-H gel can be maintained at about 0.006.

Electrode Nanomaterials and Electrolytes for Aluminium-ion Batteries

<p>Energy is one of the biggest challenges of the 21st century. Factors such as the decline in availability of non-renewable power sources, the alarming levels of atmospheric CO₂, and the steady increase of the worldwide demand for energy make the worldwide transition to a fully renewable source-based production an extremely urgent necessity. Because of the intermittent nature of most renewable energy sources, battery-based energy storage systems could be a useful tool for such transition. However, current battery technologies such as lithium-ion often lack the cost-effectiveness and safety requirements necessary for large-scale grid energy storage applications; it is therefore important to search for alternative technologies which are more suitable for this purpose. Aluminium-ion batteries have recently emerged as a very promising alternative to lithium-based systems, thanks to the low cost, non-flammability, and three-electron redox chemistry of aluminium. Al-ion batteries could, in principle, offer better cost-effectiveness, higer capacity and improved safety, which would lead to a substantial advance in energy storage technology. This PhD project deals with the investigation of novel electrode nanomaterials and electrolyte systems for Al-ion batteries. Particular emphasis is put on using the special properties of nanomaterials to improve the performance of batteries and on searching for low-cost compounds to be used as alternative electrolytes. Developing these areas will enhance the cost-effectiveness of the technology, and facilitate its commercial feasibility. Vanadium oxide nanofibres and carbon nanofibres were initially tested as cathode materials. The performance of such cathodes, however, did not meet expectations: V₂O₅ nanofibres showed poor reversibility, short cycling life, and underwhelming specific capacity, while carbon nanofibres displayed a mostly capacitive, adsorption-based energy storage mechanism, with no significant ion intercalation taking place. Nevertheless, the tests performed on the latter material led to the discovery of the phenomenon of solid-electrolyte interphase formation: this process was investigated in depth and found to be mainly caused by the presence of defects on the surface of the nanofibres, favouring the decomposition of the electrolyte into insoluble species during the charging phase. Two composite materials were then tested as cathode candidates: solvothermally-prepared core-sheath C/V₂O₅ nanofibres, and a layered nanostructured electrode. The former material showed an interesting behaviour as a battery cathode, as evidence for a multiple-ion intercalation mechanism was found; this phenomenon is however short-lived, as the cathode tends to disintegrate after the first few charge-discharge cycles. In a similar fashion, the fabrication methods used to create the layered electrode were shown to be unreliable: the poor adhesion of the active material to the underlying current collector resulted in highly unstable performance of the cathode, leading to the premature failure of the battery device. Within alternative electrolytes, mixtures of inorganic and non-ionic organic compounds were studied. Eutectic mixtures of aluminium trichloride with acetamide and other small amide analogues were found to achieve good performance as battery electrolytes. Reduction of viscosity was found to be the key to improve cycling performance: this was achieved either by dilution of the electrolytes with an appropriate solvent, or by using combinations of amides to weaken the inter-molecular interactions present in the melts.</p>

Electrode Nanomaterials and Electrolytes for Aluminium-ion Batteries

<p>Energy is one of the biggest challenges of the 21st century. Factors such as the decline in availability of non-renewable power sources, the alarming levels of atmospheric CO₂, and the steady increase of the worldwide demand for energy make the worldwide transition to a fully renewable source-based production an extremely urgent necessity. Because of the intermittent nature of most renewable energy sources, battery-based energy storage systems could be a useful tool for such transition. However, current battery technologies such as lithium-ion often lack the cost-effectiveness and safety requirements necessary for large-scale grid energy storage applications; it is therefore important to search for alternative technologies which are more suitable for this purpose. Aluminium-ion batteries have recently emerged as a very promising alternative to lithium-based systems, thanks to the low cost, non-flammability, and three-electron redox chemistry of aluminium. Al-ion batteries could, in principle, offer better cost-effectiveness, higer capacity and improved safety, which would lead to a substantial advance in energy storage technology. This PhD project deals with the investigation of novel electrode nanomaterials and electrolyte systems for Al-ion batteries. Particular emphasis is put on using the special properties of nanomaterials to improve the performance of batteries and on searching for low-cost compounds to be used as alternative electrolytes. Developing these areas will enhance the cost-effectiveness of the technology, and facilitate its commercial feasibility. Vanadium oxide nanofibres and carbon nanofibres were initially tested as cathode materials. The performance of such cathodes, however, did not meet expectations: V₂O₅ nanofibres showed poor reversibility, short cycling life, and underwhelming specific capacity, while carbon nanofibres displayed a mostly capacitive, adsorption-based energy storage mechanism, with no significant ion intercalation taking place. Nevertheless, the tests performed on the latter material led to the discovery of the phenomenon of solid-electrolyte interphase formation: this process was investigated in depth and found to be mainly caused by the presence of defects on the surface of the nanofibres, favouring the decomposition of the electrolyte into insoluble species during the charging phase. Two composite materials were then tested as cathode candidates: solvothermally-prepared core-sheath C/V₂O₅ nanofibres, and a layered nanostructured electrode. The former material showed an interesting behaviour as a battery cathode, as evidence for a multiple-ion intercalation mechanism was found; this phenomenon is however short-lived, as the cathode tends to disintegrate after the first few charge-discharge cycles. In a similar fashion, the fabrication methods used to create the layered electrode were shown to be unreliable: the poor adhesion of the active material to the underlying current collector resulted in highly unstable performance of the cathode, leading to the premature failure of the battery device. Within alternative electrolytes, mixtures of inorganic and non-ionic organic compounds were studied. Eutectic mixtures of aluminium trichloride with acetamide and other small amide analogues were found to achieve good performance as battery electrolytes. Reduction of viscosity was found to be the key to improve cycling performance: this was achieved either by dilution of the electrolytes with an appropriate solvent, or by using combinations of amides to weaken the inter-molecular interactions present in the melts.</p>

Tetradiketone macrocycle for divalent aluminium ion batteries

Contrary to early motivation, the majority of aluminium ion batteries developed to date do not utilise multivalent ion storage; rather, these batteries rely on monovalent complex ions for their main redox reaction. This limitation is somewhat frustrating because the innate advantages of metallic aluminium such as its low cost and high air stability cannot be fully taken advantage of. Here, we report a tetradiketone macrocycle as an aluminium ion battery cathode material that reversibly reacts with divalent (AlCl2+) ions and consequently achieves a high specific capacity of 350 mAh g−1 along with a lifetime of 8000 cycles. The preferred storage of divalent ions over their competing monovalent counterparts can be explained by the relatively unstable discharge state when using monovalent AlCl2+ ions, which exert a moderate resonance effect to stabilise the structure. This study opens an avenue to realise truly multivalent aluminium ion batteries based on organic active materials, by tuning the relative stability of discharged states with carrier ions of different valence states.