|M.Sc Student||Offen Inbal|
|Subject||The Development of Aluminum Anodes for Lithium-Ion|
Batteries with High Active Material Content
|Department||Department of Materials Science and Engineering||Supervisor||Professor Yair Ein-Eli|
|Full Thesis text|
Lithium-ion batteries (LIBs) are wide-spread and often used in portable electronic devices. New applications as electric vehicles raise the demand for batteries producing higher energy with lower mass and smaller dimensions. In order to achieve these demands, research of LIBs focused on developing new materials which can produce higher capacities. The negative electrode, the anode, of commercial LIBs uses graphite as anode material, as lithium can be inserted (lithiated) between its layers. Introducing materials which alloy with lithium allows the accommodation of more lithium ions within the electrode. Thus, capacities can be 2-10 times higher than produced by graphite.
Aluminum is being investigated as anode material for LIBs as it is low cost, highly abundant, highly conductive and can produce the high specific capacity of around 1000 mAh g-1 when alloyed with lithium. However, similarly to other alloying materials, the lithium accommodation is accompanied by high volume expansion. The volume alternation during lithiation and delithiation causes huge stresses, leading to aluminum pulverization. The loss of electrical contact leads to capacity loss and unstable performance of the battery. Moreover, the insulating aluminum oxide interrupts the lithiation to aluminum and aggravates the pulverization effect. Thus, aluminum research focused on these issues. Out of the many attempts, powder-based electrodes present the most promising choice for commercializing of aluminum anodes. However, it seems all studies use the common structure of active material, binder and conductive additive. Generally, the conductive additives are carbonaceous materials which decrease the active material content and the electrode density. Also, they increase capacity loss due to surface reactions, related to their high surface area. As aluminum is highly conductive, these additives might be redundant.
In this study we propose the simple binary electrode composition of Al/binder, hence removing the conductive additives and their unwanted effects. The cycling performance of these electrodes was tested, demonstrating they can produce high areal capacities, sustaining high loadings. Reversible capacity remained at about 3 mAh cm-2 over 20 cycles. However, in terms of specific capacity they underperformed electrodes containing the conductive additive carbon black (CB). The CB presented the beneficial effects of maintaining the conductivity of pulverized aluminum and buffering the aluminum expansion, thus minimizing its fracturing. Without the CB, the electrodes suffered from large capacity loss in the first cycle, as the conductivity dropped with aluminum pulverization. In further cycling, it demonstrated a more stable performance, while CB containing electrodes lost capacity continuously. It appears, the CB-binder matrix surrounding the particles fractured as cycling progresses, causing delamination of the electrode. Therefore, the electrode suffered from capacity fading over several cycles. By examining different compositions, the beneficial effects of CB were demonstrated to be effective only with high CB content of about 20-25wt%.
Herein, a different approach for Al electrode was presented, weighing the advantages and disadvantages of using conductive additives. The aluminum-binder binary composition was proven feasible and produced high capacities with higher loadings. Yet, there are drawbacks that should be addressed to make the need for carbonaceous additives eliminated.