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On November 10, researchers from the Materials Research Center of the Institute of Modern Physics of the Chinese Academy of Sciences worked with Lanzhou University and the Guangdong Provincial Laboratory of Advanced Energy Science and Technology to develop a polyimide high-temperature diaphragm for high-performance lithium-ion batteries using ion track technology. The results were published in the American Chemical Society's journal ACS Nano on November 5.
The role of diaphragms in lithium-ion batteries
As one of the key components of lithium-ion battery, diaphragm has the function of isolating positive and negative electrodes and conducting lithium ions, which is crucial to the safety of the battery. At present, the energy density of commercial lithium-ion batteries can reach 300 watt-hours per kilogram, and it is expected to be further improved.
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However, in the pursuit of higher energy density of lithium-ion batteries, safety issues cannot be ignored. With the increasing energy density of lithium-ion batteries, the safety of batteries has been paid more and more attention. Especially under the conditions of high power output, high temperature environment and long time use, the internal structure of the battery may change, which will affect the safety of the battery
The defect of traditional diaphragm
The traditional polyolefin separator has poor thermal stability and uneven pore structure, which is easy to shrink at high temperature and cause internal short circuit and thermal runaway.
Traditional polyolefin membranes have poor thermal stability, and the melting point is usually low, the melting point of polyethylene is about 120°C, and the melting point of polypropylene is about 160°C, which may lead to internal short circuit of the battery or thermal runaway of the battery, thereby endangering the safety of the battery( lifepo4 battery safety). In particular, when the internal temperature of the battery rises to the melting point of these materials, the polyolefin diaphragm will begin to melt, deform, or contract.
In addition, the traditional polyolefin membrane is prone to shrinkage under high temperature. Thermal shrinkage will lead to changes in the pore structure of the diaphragm, reduce the effective area of the ion channel, affect the battery performance, and in serious cases may lead to positive and negative electrode contact, short circuit or thermal runaway.
The pore structure of traditional polyolefin diaphragms is usually not uniform, and there are great differences in pore size and pore distribution. This uneven pore structure may lead to poor ion conduction inside the battery, which reduces the efficiency of the battery during charging and discharging.
The standard deviation of pore size of traditional polyolefin separator is large, so the distribution of pore size is relatively discrete, and the ion conduction channel of the battery cannot be stable and uniform. This structural problem will affect the cycle performance and charge and discharge rate of the battery, and even reduce the lithium ion battery life.
Advantages of polyimide
Polyimide is considered the ideal choice for high safety diaphragms due to its excellent thermal stability, high mechanical strength and good chemical stability.
Polyimide can maintain its structural stability at high temperatures (up to 450°C or more), and will not undergo thermal shrinkage or melting like traditional polyolefin membranes, and it has a high thermal deformation temperature and thermal degradation temperature.
This allows polyimide to work for a long time in a high temperature environment without causing an internal short circuit of the battery or triggering thermal runaway, ensuring the safety of the battery under extreme conditions.
In terms of mechanical strength, the molecular structure of polyimide is composed of rigid aromatic rings and flexible chain segments, which gives it a very high mechanical strength.
The tensile strength of the polyimide diaphragm is very high, and it can withstand large tensile and pressure, which avoids the deformation of the battery due to external force during use, and thus prevents the damage of the battery or internal short circuit. In addition, polyimide has a strong voltage resistance, which can provide sufficient structural support in high energy density batteries.
The polyimide diaphragm can resist the chemical corrosion inside the battery and extend the service life of the battery. At the same time, the pore structure of the polyimide diaphragm is relatively uniform, and the aperture distribution is narrow, which helps to improve the conduction efficiency of lithium ions, thereby improving the energy density and power density of the battery. This makes polyimide an ideal choice for high-security lithium-ion batteries.
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Therefore, in-depth research on polyimide membranes, especially the development of polyimide membranes with uniform pore structure and controlled preparation, is crucial to improve the safety of lithium-ion batteries, extend the service life and improve the overall performance.
New polyimide diaphragm technology based on ion track technology
Relying on the Lanzhou Heavy Ion Research Facility (HIRFL), researchers have developed a new process for the preparation of high-temperature resistant polyimide membranes based on ion track technology. Compared with the traditional polyolefin diaphragm, the prepared diaphragm has obvious advantages, such as mechanical strength up to 150.6 mpa, excellent high temperature resistance (the structure does not shrink at 450 degrees Celsius), narrow aperture distribution (aperture standard deviation < 6%), and vertical pore structure (tortuality is 1).
At 3 mah per square centimeter, the lithium/lithium symmetric battery using the diaphragm can be stably cycled for 1200 hours and achieve uniform, dense lithium deposition on the surface of the lithium metal electrode, indicating excellent lithium dendrite inhibition.
In addition, the lithium iron phosphate soft pack battery using the diaphragm can be stably recycled 1000 times at room temperature, the capacity retention rate is 73.25%, and shows excellent high temperature performance, can work at 150 degrees Celsius ambient temperature, which means that this makes the battery has a wider range of applications, for example,
In electric vehicles (especially in high-temperature climates), aerospace, military equipment, industrial robots, and energy storage systems in some special fields, such batteries can provide reliable energy support under extreme conditions.
Conclusion
The research work provides a new idea for the development of reliable high temperature resistant high performance lithium-ion battery diaphragm and technology, and becomes one of the effective ways and means to improve the safety of lithium-ion batteries.
This work was supported by the Guangdong Provincial Laboratory of Advanced Energy Science and Technology of China and the Natural Science Foundation of China.
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