One of the most significant factors that limit the lifespan and capacity of Li-ion batteries is the gradual decomposition of the binder that holds the active materials of the battery electrode together. This decomposition process is known as "binder degradation" and is caused by a combination of mechanical stress, electrochemical reactions, and thermal cycling.
Binders are a critical component of Li-ion battery electrodes, as they hold the active materials (such as graphite or lithium cobalt oxide) in place and help maintain the structural integrity of the electrode during the charge and discharge cycles. The binder is typically a polymer material, such as polyvinylidene fluoride (PVDF), that is mixed with the active material and a conductive additive (such as carbon black) to form a slurry. This slurry is then coated onto a current collector and dried to form the electrode.
During the charge and discharge cycles, the electrode experiences mechanical stress as it expands and contracts due to the insertion and removal of lithium ions. This stress can cause microcracks to form in the binder, which can lead to the formation of a passive layer on the surface of the electrode. This layer can reduce the accessibility of the lithium ions to the active material and reduce the capacity of the battery.
Additionally, electrochemical reactions can occur within the binder material, leading to its degradation. For example, if the electrolyte in the battery contains trace amounts of water, it can react with the binder to form hydrofluoric acid, which can corrode the binder material.
This degradation can also lead to the formation of a passive layer on the electrode surface, reducing the battery's capacity.
Thermal cycling can also cause the binder to degrade over time. As the battery is charged and discharged, it generates heat due to the internal resistance of the battery. This heat can cause the binder to soften and become more susceptible to mechanical stress and electrochemical reactions. Over time, the repeated cycles of heating and cooling can cause the binder to break down and lose its effectiveness.
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