What are the suitable separators for sodium-ion batteries

Sodium-ion batteries have the advantages of low cost, environmental friendliness, long cycle life, and stable performance, and play a prominent role in energy storage including home energy storage, commercial energy storage, etc., low-speed electric vehicles and other fields. As a key material affecting the performance of sodium-ion batteries, sodium-ion battery separators are receiving more and more attention.

Most of the sodium ion battery separators currently studied are polymer polymer separators, glass fiber separators and other insulating materials with good sodium ion conductivity, among which polymer polyethylene or polypyrrole nanomaterials are more concentrated.

1. Performance requirements of sodium-ion battery separators

The basic performance requirements of polymer sodium ion battery separator are:

● Absolute electrical insulation. The material must ensure that the cathode and anode materials are effectively separated to avoid short circuit and cause serious safety problems;

● Good porosity. The gap of the polymer material achieves the purpose of passing sodium ions, and it should be slightly larger than the size of sodium ions to ensure the passage of ions;

● Good permeability, that is, low resistance and high ion conductivity;

● Chemical stability. To have good electrochemical corrosion resistance, no redox reaction occurs in the electrolyte;

● Better mechanical toughness. The separator needs to have a certain toughness, and the battery needs to be squeezed and stretched during the assembly process, and there should be no fracture or damage during the process;

● Thermal stability. The separator material needs to be able to be used in a wide temperature range, and it can be automatically closed or protected when the temperature exceeds the range.

2. Mainstream sodium-ion battery separator types

The mainstream types of sodium-ion battery separators in the market include ultra-high molecular weight polyethylene separators, polypropylene separators and non-woven composite separators. There are relatively few studies on separator materials for sodium-ion batteries in China top 10 lithium battery separator companies, mainly focusing on polyolefin composite separators, glass fiber filter paper separators, and organic polymer non-woven separators.

Among them, the glass fiber filter paper separator is a fiber separator made of inorganic materials. Due to its high price, thick material and low tensile strength, it is currently mostly used in laboratories and small-scale pilot tests.

Polyolefin composite separators and organic polymer non-woven separators have been mass-produced on a large scale and have been tested in terms of safety, but the thermal stability and electrolyte fusion of these two materials need to be further explored.

① Non-woven composite separators

Non-woven composite sodium ion separators have attracted much attention because of their good thermal stability, good porosity, and easy modification. Common non-woven separator materials include polyimide (PI), polyester film, cellulose film, polyamide film, aramid film, spandex film, etc. These separators can be laid up using organic fiber dry melt blowing and wet laying, the best method being electrospinning.

By controlling the pressure, the pore size of the non-woven battery separator can be effectively controlled, which contributes to the safety of the battery. The permeability and absorption rate of the non-woven battery separator are better than those of the conventional polymer polyolefin battery separator.

② Polymer polyolefin sodium ion separator

At present, the most widely used commercial membrane is polymer polyolefin composite separator. The modified material has the characteristics of high mechanical strength, good chemical stability, high wettability and low price.

A good separator should have a rich pore structure, uniform pore size distribution, suitable thickness, standard mechanical strength, suitable porosity, good thermal conductivity and chemical stability of iodine, which can help to promote sodium ion conduction. PP film and PE film are widely used in lithium batteries due to their strong corrosion resistance and high strength.

Compared lithium vs sodium battery, since the sodium battery technology is in the same line as the lithium battery, the current sodium battery basically uses the lithium battery separator. At present, commercial lithium battery separators are mainly polyethylene (PE) separators, polypropylene (PP) separators, and PE and PP composite multilayer microporous membranes.

③ Other polymer sodium ion battery separator

There is also a novel polymer ion battery separator based on dual matrix encapsulated permeation (DMEP) and a preparation method thereof. The method uses a coaxial nozzle to control the output of the solution with a certain injection rate ratio, and realizes DMEP after spinning, which solves the contradiction between the high porosity and strong tensile properties of the membrane at one time.

The polyacrylonitrile (PAN) matrix is coated on the surface of the fiber, and the DMEP membrane has better surface wettability, better electrolyte absorption rate and better porosity, and the tensile strength and breaking strain of the film are also improved accordingly.

This result is based on the irregular interpenetration of fiber-based bimatrix PAN and polyvinyl alcohol (PVA) and multipoint bonding between fibers. The DMEP membrane has good ionic conductivity (1.77 mS/cm), thermal stability (270 ℃) and electrochemical performance. Good mechanical properties, ionic conductivity and electrochemical properties indicate that DMEP composite separators have great application potential.

④ Cellulose paper sodium ion battery separator

Polyolefin microporous separator has good electrochemical and chemical stability, but it has a low melting point and poor thermal stability, and it is prone to thermal shrinkage under high temperature conditions, resulting in battery short circuit and even safety accidents.

In order not to affect the electrochemical performance of the battery and to obtain a sodium-ion battery separator with high thermal stability, cellulose paper was used to prepare a composite sodium-ion battery separator.

Cellulose paper is a kind of porous material with good adsorption, which is green, environmentally friendly, low-cost and recyclable. Cellulosic paper separators (CP separators) were used to assemble the battery and the test found that the CP separator did not adversely affect the electrochemical reaction of the positive electrode, and at the same time enabled the battery to exhibit good electrochemical performance.

⑤ Glass fiber sodium ion separator

Among the sodium-ion battery separators currently used, glass fiber separators are only used in laboratories. One of the reasons is that the mechanical strength of this separator is poor, which makes it difficult for glass fiber separators to be widely used in the industry.

However, glass fiber separators have very good pore size and porosity, which greatly affects the performance of sodium-ion batteries. In the future, glass fiber separators need to overcome the problem of poor mechanical strength to promote the development of sodium-ion batteries.

3. Conclusion

At present, the market is still dominated by lithium-ion batteries, and sodium-ion batteries are mainly concentrated in the scientific research and development stage. In scientific research and development, glass fiber separators are mostly used. The disadvantages of this kind of separator are very obvious, that is, poor mechanical strength and high price, which limit the wide-scale promotion of glass fiber separators in the market.

In the future, the cost of the separator market needs to be greatly controlled. At present, polymer separators are widely used in lithium-ion batteries, and their biggest advantage is that they are relatively cheap. If the pore size and porosity of the polymer membrane can be well controlled, it will play an important role in the marketization of the sodium ion membrane in the future.

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