Application of ceramic coating in lithium battery

Lithium-ion batteries have outstanding advantages such as high voltage, high capacity, small size, light weight, environmental protection and long life. They have been widely used in various portable electronic products and electric vehicles. However, there are still certain problems in the safety of lithium-ion batteries, especially their safety problems under conditions such as high temperature, overcharge, and short circuit, which have become technical problems that must be overcome when power-type lithium-ion batteries are applied on a large scale.

At present, many lithium battery manufacturers use ceramic powder to coat negative 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-sized alumina particles. Nano alumina is one of the special functional nano materials with important application value and development prospects. It has a series of excellent characteristics such as high thermal stability, chemical stability, corrosion resistance and high hardness. It is widely used in ceramic materials and biological materials. Medical materials, semiconductor materials, catalyst carriers, surface protection layer materials and optical materials. It is precisely because of such good thermal stability that nano-alumina is considered to be a good thermal insulation material and is expected to make a significant contribution to improving the safety performance of lithium-ion batteries.

Currently, nano-alumina is mainly used to coat electrodes or diaphragms to improve the safety of the diaphragm and reduce the internal short circuit rate.

1. Anode ceramic coating

At present, ceramic powder is generally mixed with CMC and dissolved in deionized water to form a slurry. The slurry is then coated on the pole piece, and the state of the pole piece under SEM after drying is shown in Figure 1. In the pictures (a) and (b) in Figure 1, it is obvious that the ceramic coating is uniformly distributed on the surface of the negative electrode in the form of particles. The effect of ceramic coating on the performance of lithium batteries is as follows:

Figure 1. SEM of two kinds of uncirculated negative pole pieces

1. The ceramic coating has no obvious influence on the capacity of the lithium battery;

2. Adding ceramic powder will increase the internal resistance of the lithium battery. This is because the main component of the ceramic coating is Al2O3, which is non-conductive. Coating ceramic on the surface of the negative electrode material will hinder the path of electrons to the negative electrode, so the body resistance of the battery will increase;

3. The cycle performance of the ceramic coating battery is better than that of the battery without ceramic coating. In addition, by coating ceramic powder on the surface of the negative electrode, by increasing the passivation effect of the negative electrode surface and enhancing the electronic insulation, the deterioration of the electrical performance of the battery under high temperature storage conditions can be effectively suppressed. SEM analysis of the battery pole pieces after cycling is shown in the figure 2 shown.

Figure 2. SEM of negative pole piece after two cycles

It can be seen from the figure that the surface of the non-ceramic coated negative electrode is covered with a layer of fine particles, which is presumed to be a compound formed by lithium deposition during the charge and discharge process, while the surface of the ceramic coated negative electrode is relatively smooth and ceramic Evenly distributed on the surface of the pole piece. It can be speculated that the cycle performance of the battery is related to the ceramic coating. During the cycle of the battery, the negative SEI film will grow and become thicker. An overly thick SEI film will not only consume more lithium ions, but also cause lithium during charging. Ions cannot be well embedded in the negative electrode, but precipitate on the surface of the negative electrode or even on the surface of the separator, resulting in a loss of capacity during the cycle. Coating a layer of ceramic separator on the surface of the negative electrode may be able to effectively block the growth of the negative electrode SEI film, thereby reducing the loss of lithium ions during the cycle. In addition, the electrolyte will continue to decompose during the battery cycle, and the ceramic coating has a certain liquid absorption capacity, which can improve the capacity retention rate of the electrolyte during long-term charge and discharge cycles. Therefore, the ceramic coating can improve the cycle performance of the ternary lithium-ion battery.

4. The safety of ceramic coated batteries is higher than that of non-ceramic coated batteries. Two different batteries were subjected to acupuncture experiments under the same experimental conditions, and the results are shown in Figure 3.

Figure 3. Acupuncture results of two batteries

It can be seen from Figure 3 that the peak temperature of acupuncture of the ceramic-coated battery is 123.1 ℃. After the test, the battery is slightly swollen without smoke or explosion; the peak temperature of the non-ceramic-coated battery is 410 ℃. During the test, the battery exploded and smoked, the top cover was broken, and the test failed. The reason for the above phenomenon may be related to the ceramic coating on the surface of the negative electrode. Since needle punching simulates a short circuit in the battery, a large amount of heat will be generated in a short time, and the ceramic coating on the negative electrode surface can delay the heat loss during the needle punching process. Increase sharply, thereby delaying the thermal decomposition of the electrolyte, and avoiding the explosion of the battery due to a large amount of gas generated in a short time. Therefore, the ceramic coating significantly improves the safety performance of lithium-ion batteries.

2. Ceramic diaphragm

At present, researchers mainly improve battery performance in terms of positive and negative materials, separators, electrolytes, and battery design. Among them, ceramic separators are an effective way to improve battery performance. Ceramic separators can not only improve battery safety performance, but also Improve battery cycle performance and reduce self-discharge rate. There are various manufacturing methods for ceramic diaphragms, such as chemical vapor deposition and surface coating. Ceramic diaphragm can improve the cycle and safety performance of lithium ion batteries, but its preparation process is difficult to control, and the ceramic on the diaphragm is also prone to fall off during the cycle.

1. Morphological differences

Commonly used diaphragms on the market are made of PP, PE, or two kinds of composite processing. Although these microporous polyolefin separators have excellent mechanical strength and chemical stability, these separators have internal stress during the preparation process, and the stress is released in a high temperature environment, and the separator will have obvious thermal shrinkage effect, which makes the positive and negative electrodes inside the battery The direct contact of the material causes an internal short circuit and a safety failure occurs. Coating nano alumina particles 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 uniformly, 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 made by drying at 80°C for 24 hours. The micro morphology of the ceramic diaphragm is shown in Figure 4.

Figure 4. Micro morphology of PE and ceramic diaphragm

It can be seen from the figure that the coated nano-A2O3 particles completely cover the surface of the PE membrane, and there is an uneven distribution of large voids between the particles. The existence of these large voids can facilitate the insertion and extraction of Li+ and It has good liquid absorption and liquid retention performance for the electrolyte, so that it does not affect the charging and discharging performance of the lithium battery after the coating is coated.

2. Degree of heat shrinkage

The ceramic coating is helpful 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. There is a big difference in shrinkage between the two types of diaphragms. The experimental results are shown in Figure 5.

Figure 5. Two degrees of diaphragm shrinkage at different temperatures

The diaphragm shrinks at high temperature because the diaphragm has internal stress due to traction and stretching during the preparation process. In a high temperature environment, the movement of the internal molecular chain of the diaphragm causes the stress to be released and shrinks in a large area; but the ceramic coating diaphragm shrinks at 140 The morphology of the diaphragm itself has not changed except for the change in the color of the diaphragm under the baking condition of ℃. When the inorganic coatings coated on both sides of the diaphragm surface have high temperature resistance and heat insulation performance, the temperature of the base diaphragm itself is reduced, so that the diaphragm is at high temperature. The original form remains in the environment.

3. The ceramic diaphragm is beneficial to improve battery safety

Figure 6. The relationship between the internal resistance and temperature of the battery assembled with two kinds of diaphragms

The PE separator shrinks in a large area when the temperature is higher than its melting point, so that the positive and negative pole pieces inside the battery directly contact and cause an internal short circuit. Therefore, the internal resistance of the battery measured decreases rapidly; however, for the coated separator even at 150 The morphology of the separator itself will not change when baked at ℃, so there will be no short circuit inside the battery, which makes the internal resistance of the battery still increase. PE separator will lose its mechanical stability in high temperature environment, which will lead to the direct contact between the positive and negative electrodes inside the battery and cause a short circuit. The ceramic coating separator has high temperature resistance to effectively prevent short circuit inside the battery and improve the safety performance of the battery.

4. The impact of ceramic diaphragm on battery life

Lithium-ion battery separator not only isolates the positive and negative pole pieces inside the battery, but also needs to have good ion permeability. Because the inorganic coating on the separator will increase the thickness of the separator, which may affect the ion conductivity. But the experiment proves (Figure 7) its influence is weaker, but the diaphragm with ceramic coating has better cycle performance.

Figure 7. Comparison of cycle performance of two types of diaphragm batteries

PP/PE separators are non-polar, with hydrophobic surface and low surface energy. 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. Life, while the surface of inorganic ceramics is hydrophilic due to the presence of hydroxyl groups, its introduction can greatly improve the wetting and retention ability of the diaphragm 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 pieces, and is also conducive to the cycle life of the battery.

to sum up:

In summary, ceramic coatings have an important impact on the performance of lithium-ion batteries, especially the safety performance of lithium batteries. The ceramicization of the electrode and diaphragm surface can not only significantly reduce the internal short-circuit rate of the battery and improve safety, but also improve the electrolyte wettability of the electrode and diaphragm, reduce polarization, and improve the overall performance of the battery. Therefore, the application of ceramic coatings is an inevitable trend in the development of lithium-ion batteries in the future.