Plasma hydrophilic modification of battery carbon felt current collector

Carbon felt is an industrial textile material composed of randomly oriented and entangled carbon fibers. Due to its three-dimensional network structure, high conductivity, and good chemical and electrochemical stability, it has been used as a flexible current collector in redox flow batteries, fuel cells, electrochemical solar cells, lithium-ion batteries, and supercapacitors. Other carbon materials such as glassy carbon, carbon paper, and sponge carbon are also used as electrodes, but compared with carbon felt, they all have some limitations, such as high price and poor stability. In contrast, carbon felt has the advantages of low resistance, large specific surface area, high porosity, good conductivity, high stability, and low cost, and provides abundant redox reaction sites for electrochemical reactions, so carbon felt is selected as the positive electrode current collector of lead-manganese batteries.

Carbon felt is hydrophobic, has poor wettability in water, low electrolyte utilization efficiency, and insufficient electrochemical activity. It is usually modified before being used as a current collector in batteries. Currently, the methods for modifying carbon felt include plasma modification, heat treatment modification, chemical treatment modification, metal modification, polymer modification, and carbon-carbon composite modification.

plasma treatment

Plasma is considered to be the fourth state of matter besides solid, liquid and gas. It contains atoms, molecules, positive and negative ions, active free radicals, etc., in which the number of positive and negative particles is equal.

The active species and light radiation in low-temperature plasma can destroy the structure and chemical bonds on the surface of the material. The free radicals formed by the broken chemical bonds will re-bond into a new network structure, thereby changing the properties of the material surface. At the same time, the high-energy active particles in the plasma have an etching effect when bombarding the surface of the material, causing depressions or textures on the surface, resulting in changes in surface roughness.

The purpose of plasma treatment is to grow oxygen-containing functional groups or nitrogen dope on the surface of carbon felt, thereby improving the hydrophilicity of carbon felt and increasing electrochemical activity. Plasma treatment has little effect on the specific surface area of carbon felt, but mainly increases the number of functional groups.

Hydrophilicity test of carbon felt after plasma treatment

Figure 1.1 is a test diagram of the hydrophilicity of carbon felt before and after plasma modification. The contact angle between carbon felt and water is measured using a contact angle meter. As can be seen from the figure, the carbon felt before modification is hydrophobic and has poor hydrophilicity, with a contact angle of 106.2°, which leads to low electrolyte utilization efficiency. After modification, the contact angle changes from 106.2° to 0°, and the hydrophilicity is significantly improved, which increases the electrolyte utilization efficiency.

Plasma treatment can introduce hydrophilic groups on the surface of carbon felt, increase surface roughness, provide more active sites for redox reactions, and increase the reaction area, thereby improving battery performance. Low-temperature plasma treatment technology has low energy consumption and, importantly, does not produce chemical waste, is environmentally friendly, and at the same time, low-temperature plasma technology can change material properties without the use of any solvents.