Application of Ceramic Coatings in Lithium Batteries

At present, many lithium battery manufacturers use ceramic powder-coated pole pieces or ceramic separators and other materials related to "ceramic powder" to improve the safety of lithium batteries. In fact, ceramic powder is not "ceramic", but nano alumina and boehmite materials.

Alumina and boehmite have a series of excellent properties such as high thermal stability, chemical stability, corrosion resistance and high hardness, and are widely used in ceramic materials, biomedical materials, semiconductor materials, catalyst carriers, and surface protective layer materials. and optical materials.

At present, special alumina and boehmite products are mainly used to coat electrodes or separators to improve the safety of the separators and reduce the internal short circuit rate.

1. The edge coating of the pole piece is of great significance to the safety and yield of the battery.

Materials such as boehmite can also be used to coat the pole pieces of lithium battery cells to improve the safety performance and yield of lithium batteries.

(1) Coating of the edge of the positive pole piece: Since the positive pole piece is generally smaller than the negative pole piece, the edge of the wide side of the pole piece is prone to burrs during cutting. Once the diaphragm is pierced and contacts the negative pole, the battery will be short-circuited.

The surface of boehmite is smooth, and the edge of the positive electrode can be filled after coating, so that the surface after cutting is smooth and free of burrs.

(2) Coating the edge of the negative electrode sheet: the surface of the negative electrode is rough, coated with ultra-small particle size of Bomite, the hole of the negative electrode becomes uniform, can improve the hydrophilicity of the electrolyte, so that the lithium ion is more smooth in the process of charging and discharging.  The anode edge coating has not become the mainstream, and has been applied to consumer batteries in ATL. 

2. Ceramic diaphragm.

Ceramic separator is an effective way to improve battery performance. Ceramic separator can not only improve the safety performance of the battery, but also improve the cycle performance of the battery and reduce the self-discharge rate. Ceramic separators can improve the cycle and safety performance of lithium-ion batteries, but the preparation process is difficult to control, and the ceramics on the separators are also prone to fall off during the cycle.

(1) Morphological differences.

Common separators on the market are made of PP, PE, or a combination of the two.

Coating nano-alumina or boehmite powder on the surface of the separator can effectively improve the safety of lithium batteries. After dissolving and mixing the ceramic powder with PVDF and NMP, and dispersing evenly, the coating machine is turned on to coat the ceramic powder on the PE diaphragm. The thickness of the ceramic coating can be controlled, and then the ceramic diaphragm is obtained by drying at 80°C for 24 hours.

The coated nano-Al2O3 completely cover the surface of the PE separator, and there is a heterogeneous distribution of larger voids between the particles. The existence of these larger voids can facilitate the insertion and extraction of Li+ and has a good effect on the electrolyte. Liquid absorption and liquid retention performance, so as not to affect the charge-discharge performance of the lithium battery after coating the separator.

(2) Degree of heat shrinkage.

The ceramic coating is beneficial to improve the high temperature resistance of the diaphragm. Put the ceramic diaphragm and the ordinary diaphragm in a box at different temperatures for 2 hours, and the shrinkage rate of the two diaphragms is very different. The diaphragm will shrink at high temperature because the diaphragm has internal stress due to traction and stretching during the preparation process.

When the inorganic coating applied on both sides of the surface of the diaphragm has high temperature resistance and heat insulation performance, the temperature of the base diaphragm itself is lowered, so that the diaphragm still maintains its original shape in a high temperature environment.

(3) Ceramic diaphragms help improve battery safety.

When the temperature is higher than its melting point, the PE separator will shrink in a large area, so that the positive and negative pole pieces inside the battery are directly contacted, resulting in an internal short circuit, so the internal resistance of the measured battery decreases rapidly; however, for the coated separator, even at 150 The shape of the separator itself will not change after baking at ℃, so there will be no short circuit inside the battery, so that the internal resistance of the battery is still increasing.

The PE separator will lose its mechanical stability in a high temperature environment, resulting in a short circuit due to direct contact between the positive and negative electrodes inside the battery, while the ceramic coated separator has high temperature resistance, which can effectively prevent short circuit inside the battery and improve the safety performance of the battery.

(4) The effect of ceramic separator on battery life.

The lithium-ion battery separator not only isolates the positive and negative pole pieces inside the battery, but also needs to have good ion permeability. Since the thickness of the separator will be increased after coating the separator with an inorganic coating, it may affect the ion conductivity.

PP/PE separators are all non-polar, with hydrophobic surface and low surface energy, and it is difficult to wet and maintain polar organic electrolytes such as ethylene carbonate and propylene carbonate, which directly affects the cycle performance and use of the battery. Due to the presence of hydroxyl groups, the surface of inorganic ceramics is hydrophilic, and its introduction can greatly improve the wetting and holding ability of the separator or electrode to the electrolyte, and greatly improve the cycle performance of the battery.

At the same time, the nano-alumina particles have a large specific surface area, which can improve the wettability and liquid retention of the electrolyte to the pole piece, and is also beneficial to the cycle life of the battery.

Ceramic coatings have an important impact on the performance of lithium-ion batteries, especially the safety performance of lithium batteries. The ceramicization of the surface of the electrode and the separator can not only significantly reduce the internal short-circuit rate of the battery and improve the safety, but also improve the electrolyte wettability of the electrode and the separator, reduce the polarization, and improve the comprehensive performance of the battery such as cycling.