Analysis Of The Structure And Physicochemical Properties Of Lithium Battery Diaphragm

- Aug 02, 2019-

The diaphragm is one of the key components in the structure of lithium batteries.The diaphragm is made of plastic film, which can isolate the cathode and anode electrodes of the battery to prevent short circuit;It can also block the conduction of current in the battery through the closed-hole function when the battery overheats.The diaphragm with excellent performance plays an important role in improving the comprehensive performance of lithium battery.


Structural characteristics of lithium battery diaphragm:

1.Thickness. The thickness of lithium battery diaphragm is usually less than 25 microns. On the premise of ensuring certain mechanical strength, the thinner the diaphragm, the better. Today, most new high-powered batteries use single-layer membranes 20 or 16 microns thick.
2.Aperture and distribution. As the membrane material of lithium battery, it has a microporous structure, allowing it to absorb electrolyte. To ensure consistent electrode/electrolyte interface properties and uniform current density in the cell, the micropores should be evenly distributed throughout the diaphragm material. The size and distribution of pore size have a direct impact on the performance of lithium battery: the pore size is too large, easy to make the positive and negative electrode direct contact or easy to be lithium dendrite piercing and cause short circuit; Too small an aperture increases the resistance. The distribution of micropores is not uniform, and the local current will be too large when working,  which will affect the performance of the battery.
3.Porosity. Permeability can be characterized by the amount of gas through the membrane under certain time and pressure, which mainly reflects the patency of lithium ions through the membrane. Porosity is very important for membrane permeability and electrolyte tolerance. Most commercial lithium-ion batteries have membranes with porosity between 40% and 50%.

4.Permeability. The time it takes for a certain amount of air to pass through the diaphragm under certain conditions is called the Gurly value. The permeability of the membrane of lithium battery is the result of the comprehensive factors such as membrane porosity, pore diameter, pore shape and pore tortuosity.
5.Thermal stability: the diaphragm needs to maintain thermal stability within the temperature range of lithium battery (-20℃~60℃). Generally speaking, the PE or PP materials used in the diaphragm can meet the above requirements. Of course, there is another is because the electrolyte is sensitive to water, most manufacturers will be in the injection of liquid before the baking of about 80C, which will not have too big problems on the PP/PE diaphragm.
6.Consistency: due to the different preparation process, the consistency of lithium battery diaphragm may be different. Consistency includes self-properties such as closing temperature, and apparent consistency such as hole consistency and thickness consistency observed under

electron microscope.


Physicochemical properties of lithium battery diaphragm:

1. Wettability and wetting speed. Poor wettability of the diaphragm will increase the resistance of the diaphragm and lithium battery, and affect the cycle performance and charging and discharging efficiency of lithium battery. The wetting rate of diaphragm refers to the speed of the electrolyte entering into the membrane microhole, which is related to the surface energy, aperture, porosity, tortuosity and other characteristics of the diaphragm.
2. The diaphragm of the fluid absorption rate due to the lithium battery diaphragm material with the function of the electrolyte, so must meet the following conditions: enough to ensure that the ion channels unblocked, fluid absorption rate and in battery system, inevitably there will be a lot of side effects occur, consume a large amount of electrolyte, so there must be sufficient reserves, or it will caused by lack of electrolyte interface resistance increases, at the same time also will accelerate the consumption of the electrolyte, it will be a vicious cycle, so fluid absorption rate is an important parameter of diaphragm.
3. Chemical stability. The diaphragm in the electrolyte should maintain long-term stability, in the strong oxidation and strong reduction conditions, and do not react with electrolyte and electrode materials. The chemical stability of the diaphragm is evaluated by measuring the corrosion resistance of the electrolyte and the rate of expansion and contraction.
4. Thermal stability. Lithium batteries in the charge and discharge process will release heat, especially in the short circuit or overcharge, there will be a lot of heat. Therefore, when the temperature rises, the diaphragm should maintain the original integrity and a certain mechanical strength, continue to play the role of positive and negative electrode isolation, to prevent the occurrence of short circuit.
5. The resistance of the diaphragm of the diaphragm in resistivity is actually a microporous electrolyte resistivity, it is related to many factors, such as porosity pore the sinuosity of the electrical conductivity of electrolyte film thickness and the degree of wetting of diaphragm material electrolyte resistance testing diaphragm is more commonly used ac impedance method, because the film is very thin, often exists defects and make the error of the measured results increases, so often USES the multi-layer sample, then take the average of the measurement.
6. Autistic performance. Above a certain temperature, the components of lithium battery will occur exothermic reaction and lead to "self-heating", in addition, due to charger failure, safety current failure will lead to overcharging or battery external short circuit, these situations will generate a lot of heat. Because of thermoplastic polyolefin material is qualitative, when the temperature close to the polymer melting point, the ionic conduction of porous polymer membranes become insulation layer without holes, closed pores, resulting in the closure phenomenon, thereby blocking the ions to continue to form a circuit, protect the battery, so the polyolefin diaphragm can provide additional protection for battery.
The main function of the diaphragm is to separate the cathode and anode electrodes of lithium battery, prevent the contact between the two poles and short circuit, in addition, it also has the function of enabling electrolyte ions to pass through. It is understood that the excellent diaphragm directly affects the capacity, circulation capacity and safety performance of lithium batteries, excellent performance of the diaphragm has an important role in improving thecomprehensive performance of the battery.