Imagine a thirsty plant's roots eagerly drawing water from the soil. This natural process is not just about the availability of water but also how the soil's texture allows the water to seep in and reach the roots. In lithium-ion batteries, a similar concept applies to the electrodes & electrolytes. Here, the electrodes are like the plant's roots, and the electrolyte is akin to the soil's moisture/water. For a battery to operate at its best, it is important that the electrolyte intimately wets the electrode materials, ensuring that every spot & area is saturated, much like the roots need to access every drop of water to thrive. The wettability property is influenced by the following parameters, each providing insights into the battery's potential performance:
Surface energy: It is the inherent tension present on the electrode's surface that affects how the electrolyte spreads upon contact. A higher surface energy usually correlates with better wetting.
Contact angle: It is the angle at which the electrolyte droplet meets the electrode surface. A smaller contact angle indicates better wetting as the electrolyte 'hugs' the electrode more closely.
Porosity of the electrode material: The more porous the electrode, the more pathways there are for the electrolyte to infiltrate and interact with the electrode material.
Roughness of the electrode surface: It can enhance or hinder wettability. A certain level of roughness can be beneficial, creating more surface area for the electrolyte to wet.
Thoroughly evaluating these parameters allows us to predict how the battery will perform. Poor wettability can lead to inadequate ion transport, resulting in lower battery capacity and power. It can also cause uneven electrode degradation, which compromises the battery's life cycle.
The image illustrates various methods to measure the wetting properties. The 'Wetting Balance Test Scale' gauges the force of wetting when a sample contacts a liquid, measuring the attraction between the liquid's surface tension and the sample's surface. The 'Wetting Balance Test Tensiometer', on the other hand, focuses on the interaction between a liquid droplet and a solid surface, measuring the force needed to pull the droplet from the surface, which indicates the wetting strength. The 'Droplet Spread Test' visually assesses how a liquid droplet spreads on a surface, and the 'Capillary Rise Test' measures the height a liquid climbs against gravity in a small tube or porous material attached to the solid surface. These methods offer critical insights into the wetting behavior of materials that constitute the electrodes and separators in lithium-ion cells.
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