Meltblown or spunbond non-woven diaphragm

Meltblown or spunbond non-woven diaphragm

There are two major types of non-woven film preparation methods, namely dry and wet. Dry methods include melt-blown method and spun-bonded method. The wet method is mainly a wet-laid method based on papermaking. The melt-blown process uses high-speed heated airflow to draw the polymer melt extruded from the slot die orifice to form ultra-fine fibers and collect them on the drum. At the same time, it relies on self-bonding to form a non-woven material. The production process is shown in Figure 1(a). The difference between spunbond process and meltblown process in equipment is mainly the spinneret. The meltblown equipment mainly uses the slit-type spinneret [Figure 1(b)], and the spunbond equipment mainly uses the spinneret [Figure 1(c)]. Non-woven fabrics prepared by meltblown, spunbond and papermaking technologies have been widely used in filter materials, medical and health materials, environmental protection materials and clothing materials. The main materials used at this stage are polyester (such as PET), polyamide (nylon) and polyolefins. The prepared diaphragm has a unique network pore structure, which can provide a good channel for ion transmission, can fully absorb, wet and effectively disperse the electrolyte, reduce the electrochemical impedance and electrochemical polarization of the battery, and have higher mechanical stretching strength. It is expected that the battery capacity and high-rate cycle performance will be greatly improved.

Meltblown or spunbond non-woven diaphragm
Figure 1 – Schematic diagram of the production process of the melt-blown method (a) and the slit-type spinneret for the melt-blown method (b) and the spinneret for the spunbond method (c)

At present, there are also two major types of PET non-woven fabric substrates that are widely used in lithium-ion battery separators. One is based on meltblown or spunbond technology, and the other is based on papermaking technology. However, when applied to lithium-ion batteries, the PET non-woven fabric substrate has a large pore size, which easily leads to a large self-discharge effect of the battery, and the non-woven fabric has a relatively high water content, which affects the safety of the battery. Therefore, in the field of lithium-ion battery separators, PET non-woven fabrics are usually selected (the manufacturing process of PET non-woven fabrics are not distinguished in the following introduction) as the separator support matrix, supplemented by other polymers or inorganic materials to prepare composite separators. Not only maintains the excellent performance of the non-woven fabric, but also greatly improves the safety of the separator.

The composite diaphragm made by Li et al. through a simple dip coating technique coated with nano-Al2O3 on the PET substrate has higher porosity, ionic conductivity and electrolyte wettability, and has good thermal stability. After assembling the battery with this diaphragm, the battery has more excellent charge-discharge cycle performance and high-rate performance. Jeong et al. prepared PVDF-HFP/PET porous composite membrane by phase transfer method. The diaphragm not only has high mechanical strength and thermal stability, but also has excellent electrochemical stability (Figure 2). Choi et al. also successfully coated SiO2/PVDF-HFP on PET non-woven fabric, which has better safety performance and battery performance.

Meltblown or spunbond non-woven diaphragm
Figure 2 – Cross-section of composite non-woven fabric (a) and schematic diagram of the microporous structure and ion transport mechanism of composite non-woven fabric (b)

The highly ordered arrangement of nanoparticles on the non-woven membrane is also an ideal method for preparing micro- or nano-sized porous materials. Cho et al. arranged PMMA colloidal particles tightly on PET non-woven fabrics to prepare a new nanocomposite membrane. Self-assembled PMMA colloidal particles provide highly ordered nanostructures, and PET non-woven fabrics provide high mechanical strength and thermal stability. The new nanocomposite diaphragm is successfully applied in high-safety, high-power lithium-ion batteries.

At present, representative commercial PET non-woven composite membranes include: Degussa (Degussa) Separion, Freudenberg (Freudenberg) non-woven ceramic diaphragm, Mitsubishi NanoBasex and Porous Power Technology’s Symmetrix® HPX, HPXF, NC2020 (non-flammability), etc.

(1) The German Degussa company fixes inorganic ceramic particles (such as aluminum oxide, silicon dioxide, etc.) on the PET non-woven fabric through the dip coating process [Figure 3 (a)]. Figure 3(b) shows the SEM image and structure diagram of the Separion diaphragm. The composite diaphragm has a high porosity and can absorb abundant electrolyte; it does not shrink (less than 1%) at 200°C, and its high temperature safety is significantly better than that of a polyolefin diaphragm. The excellent performance makes the Separion non-woven ceramic composite diaphragm have a good application prospect in power batteries.

Meltblown or spunbond non-woven diaphragm
Figure 3 – Separion diaphragm production process (a) and the SEM image and structure diagram of the Separion diaphragm (b)

(2) The German Freudenberg company uses wet-process PET ultra-thin non-woven fabric as the base material to embed inorganic ceramics in it. The obtained non-woven ceramic separator (Figure 4) is suitable for commonly used lithium-ion battery production processes (winding and Z-shaped lamination), and has excellent thermal stability and battery safety.

Meltblown or spunbond non-woven diaphragm
Figure 4 – The cross-section of the PET non-woven fabric used in the Freudenberg ceramic diaphragm (a) and the Freudenberg ceramic diaphragm (b)

(3) Mitsubishi Corporation of Japan developed a PET non-woven membrane (NanoBase0) based on the papermaking process, and coated inorganic ceramic particles on one side of NanoBase0, and developed a PET non-woven ceramic separator NanoBaseX for lithium-ion batteries. Its heat resistance is excellent (Figure 5). Compared with NanoBase0 substrate, NanoBasex has low water content and high mechanical strength. NanoBaseX and NanoBase0 have high heat stability (stored at 180°C for 30 minutes, heat shrinkage rate is less than 5%), NanoBasex has excellent electrolyte wettability, battery cycle performance and safety.

Meltblown or spunbond non-woven diaphragm
Figure 5 – Mitsubishi NanoBasex non-woven ceramic diaphragm surface morphology (a), cross-sectional morphology (b) and heat resistance test (c)

(4) Porous Power Technology of the United States uses PET non-woven fabric as the supporting substrate to launch several PVDF diaphragm products (Figure 6), including Symmetrix® HPX (PET non-woven fabric + PVDF), HPXF (PET non-woven fabric + PVDF + inorganic ceramic particles) and the newly developed NC2020 (PET non-woven fabric with flame retardant properties + PVDF + inorganic ceramic particles). Symmetrix® NC2020 is non-flammable and more resistant to thermal shrinkage than traditional diaphragms. This makes the battery more stable in the case of damage or abuse, and can prevent or delay the occurrence of thermal runaway accidents.

Meltblown or spunbond non-woven diaphragm
Figure 6 – Cross section of Symmetrix® HPX diaphragm from Porous Power Technology (a) and heat resistance of Symmetrix® series diaphragm (130℃, ASTM D1204) (b)