Application of alumina in lithium battery materials

Aluminum oxide (Al2O3) is a white crystalline powder that is odorless, tasteless, non-toxic, high hardness, and heat-resistant compound. Its melting point is 2054 ℃ and boiling point is 2980 ℃. The ultrafine alumina powder material with uniform particle size has characteristics such as porosity, high dispersibility, insulation, and heat resistance.

How are alumina and batteries related?

Lithium ion batteries are mainly composed of positive electrode, negative electrode, separator, electrolyte solution, etc. High purity alumina is classified according to purity, mainly into 4N (purity 99.99%), 4N5 (purity 99.995%), and 5N (purity 99.999%) and above levels. 5N grade high-purity alumina plays a crucial role in lithium battery separators and positive electrode materials. In addition, high-performance Al2O3 is widely used as an inorganic filler in solid-state batteries.

Coating membrane

The separator is the technical barrier in lithium-ion battery materials, with a cost proportion second only to positive electrode materials, accounting for about 10% to 14%. In some high-end batteries, the cost proportion of separators can even reach 20%. There are three main types of coating materials on the current market: inorganic coating, organic coating, and organic+inorganic coating.

Inorganic coating refers to inorganic ceramic powders represented by alumina and boehmite, which can improve the high temperature resistance of membranes. High purity alumina, as an inorganic coating material, has high thermal stability and chemical inertness, making it a good choice for ceramic coatings on battery separators. Its advantages include:

1) Aluminum oxide coating has high temperature resistance and can maintain the intact shape of the diaphragm at 180 ℃;

2) Aluminum oxide coating can neutralize free HF in the electrolyte, improving the acid resistance and safety performance of the battery;

3) Nano alumina can form a solid solution in lithium batteries, improving rate and cycling performance;

4) Nano alumina powder has good wetting properties and certain liquid absorption and retention abilities;

5) Aluminum oxide coating can increase the tortuosity of micropores and has lower self discharge than ordinary separators.

With the continuous improvement of safety requirements for power batteries, consumer batteries, and energy storage batteries, the market application scenarios of inorganic coated separators such as high-purity alumina continue to be rich, and the market prospects are broad. According to GGII statistics, the amount of inorganic coating film used in 2021 was 1.57 billion square meters, with a compound annual growth rate of 41.17% from 2016 to 2021. Considering that there is still significant room for improvement in the coating ratio of lithium iron phosphate batteries in the power battery and energy storage fields, and emerging coating technologies are gradually being adopted in the consumer battery field, it is expected that the coating separator market will continue to maintain a rapid development trend. According to GGII's forecast, the expected usage of inorganic coating film in 2025 is 3.9 billion square meters, and the compound annual growth rate from 2021 to 2025 will be as high as 25.54%.

Positive electrode additive material

At present, although lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium iron phosphate (LiFePO4), and ternary material (Li-Ni-Co-Mn-O) are four commercially available positive electrode materials for lithium-ion batteries, they have certain shortcomings in terms of safety, cycling performance, and capacity retention. In order to improve the stability of positive electrode materials, researchers have adopted different modification methods, such as doping (Mo, Mg), surface coating [metal oxides (Al2O3, ZnO), fluorides (AlF3)], and the combination of two methods (Cr doping and Li3PO4 coating, Al doping and LiAlO2 coating). Surface coating is considered an effective method for modifying positive electrode materials. Among numerous coating materials, Al2O3 is widely used due to its wide source and low price, as well as its ability to effectively enhance the electrochemical performance of positive electrode materials.

The role of Al2O3 surface coating

In lithium-ion battery cathode materials, surface coating with Al2O3 can effectively improve the capacity retention, long cycling performance, and thermal stability of the cathode material. The positive effects of Al2O3 surface coating on the performance of positive electrode materials may include: as a hydrogen fluoride scavenger, it removes HF from the electrolyte solution and inhibits the dissolution of transition metals in the positive electrode material; Form a physical protective barrier on the surface of the positive electrode material to suppress unnecessary side reactions between the positive electrode material and the non-aqueous electrolyte; Lithium oxide is formed on the surface of the positive electrode material to increase the diffusion rate of lithium ions and reduce the charge transfer resistance; Reduce exothermic reactions and improve the thermal stability of positive electrode materials; Al2O3 reacts with LiPF6 to generate electrolyte additive LiPO2F2, which enhances the cycling performance and lifespan of the battery; Inhibit Jahn Teller effect and enhance the cycling stability of the electrode.

Polymer solid electrolyte filler

The safety issue of liquid batteries is currently an urgent problem that needs to be solved. Compared to liquid electrolytes, solid electrolytes have many advantages that liquid electrolytes do not possess: high mechanical strength, non flammability, controllable shape, ability to suppress the growth of lithium dendrites, wide electrochemical window, small self discharge, compatibility with high-voltage materials, and ability to improve battery energy density. At present, the solid-state electrolytes with high research popularity mainly exist in three forms: polymers, sulfides, and oxides, each with its own advantages and disadvantages.

Polymer electrolytes use polymer materials as the matrix material for electrolytes, which have high mechanical strength and relatively high stability for lithium electrodes. However, the high crystallinity of polymers at room temperature is not conducive to the formation of conductive networks in polymer solid electrolytes, resulting in low room temperature ionic conductivity of polymer electrolytes. To address the issue of high crystallinity of polymers at room temperature, it is common to prepare polymer composite solid electrolytes by doping inorganic fillers into pure polymer electrolytes, in order to reduce the crystallinity of the polymer and improve the room temperature conductivity of the polymer electrolyte. The commonly used inorganic fillers can be roughly divided into two categories: one is inorganic active fillers, such as Li1.5Al0.5Ge1.5 (PO4) 3 (LAGP), Li7La3Zr2O12 (LLZO), Li1.3Al0.3Ti1.7 (PO4) 3 (LATP) and other fast ion conductors. Another type of filler is inert filler, such as nano Al2O3, which cannot conduct lithium ions on its own, but can promote the formation of a conductive network by reducing the room temperature crystallinity of the polymer matrix, thereby improving the ion conductivity of the solid electrolyte. As an inorganic inert filler, the advantage of Al2O3 over micrometer sized LAGP and LLZO lies in its ability to form continuous phases in the polymer composite electrolyte prepared by doping, resulting in relatively high electrolyte uniformity and mechanical strength. However, it is necessary to treat the surface of Al2O3 by grafting some substances to consume the hydroxyl groups on the surface of Al2O3, reduce its agglomeration, improve its dispersibility in PEO, and thereby enhance the various properties of polymer composite electrolytes.