| Lithium-ion batteries have been widely concerned by various industries since they were successfully developed.Due to their outstanding performance,they are used in various fields of life.Today,with the continuous development of new energy industry and large-scale energy storage devices,the application proportion of lithium-ion batteries has continued to increase.With the development needs,lithium-ion batteries are developing in the direction of large capacity and high specific capacity,and the increase in capacity and energy density is accompanied by the emergence of safety problems.The frequently reported safety accidents of lithium batteries have aroused the attention of the battery industry and brought certain obstacles to the development of the industry.Therefore,higher safety requirements are put forward for lithium batteries.As the main component in the battery,the separator not only to separate the positive and negative electrodes,but also to provide a channel for ion transmission.Although the separator does not undergo chemical reactions,its structure and properties have a non-negligible impact on the manufacture,life,performance,and safety of batteries.Today,polyolefin-based microporous separator is mainly used in batteries,such as PP,PE,etc.However,due to the characteristics of polyolefin itself,it has a low melting point and is not heat-resistant.It is used as a battery separator to shrink at high temperature,which is easy to cause safety problems.In addition,the low porosity and poor wettability affect ion transport and restrict its application in LIBs.In order to solve the problem of insufficient performance of polyolefin separators,it is necessary to modify them.Among many modification methods,surface coating is particularly simple and efficient.For example,ceramic coating can significantly enhance the thermal stability and wettability of the separator.Due to the poor interfacial compatibility between the inorganic ceramic layer and the organic base separator,and the inorganic ceramic layer is easy to fall off.In addition,the separator surface density is relatively large,which restricts the improvement of the specific energy of the battery.Therefore,organic polymer coatings such as PVDF and aramid appear.However,the temperature resistance of the above-mentioned polymers is not high enough.Polyimide is a type of organic compound that is extremely resistant to high temperature（long-term use temperature is generally 290°C）in current practical applications.In addition,it also has the characteristics of low temperature resistance,corrosion resistance,low dielectric,dimensional stability,and low density(1.40 g cm-3),is an ideal coating material.Therefore,this paper proposes to prepare PI nanospheres and PI nanofiber/nanospheres composite coating slurry respectively,and use mature coating technology to obtain high temperature resistant polyimide-coated lithium battery separator.In the research direction of polyimide nanosphere coating,firstly,PAA solution was synthesized,and based on the characteristics of PI molecular weight controllability,PAA solution with suitable viscosity was synthesized.The PI nanospheres were prepared by electrostatic spraying,and then configured into a slurry,and the PI nanospheres were coated with the PP separator by means of blade coating.Compared with the PP separator,the water and electrolyte contact angles of the PI nanosphere-coated composite membrane decreased from 103°and 42°to 34°and 5°,respectively,and the wettability was significantly improved.Virtually no shrinkage and exhibits excellent thermal stability.In addition,the density of PI nanospheres is low,and compared with inorganic particle coating,it is lighter,which is beneficial to the improvement of battery specific energy.The NCM811 lithium battery assembled with the PI nanosphere-coated separator has excellent rate performance,with a capacity retention rate of 69.4%at 5 C(the discharge specific capacity reaches a high level of 144.35 m Ah g-1),which is 58.9%higher than that of the PP separator.At the same time,it also has excellent cycle performance.After 200 cycles of charge and discharge at 1 C,the discharge specific capacity decreases from169.4 m Ah g-1 to 135.7 m Ah g-1,and the capacity retention rate is 80.1%（PP separator:75.4%）.The above results confirmed the feasibility of PI nanospheres as a coating material for battery separators.In the research direction of PI nanofiber/nanosphere composite coating,based on the first part of the experiment,in the second part of the study,PI nanofibers were introduced to replace part of the PI nanospheres,and the introduced PI nanofibers were used to form a network structure to increase PI.The integrity of the nanosphere coating once again improves the comprehensive performance of the modified separator.By adjusting the molecular weight and viscosity,the polyimide with nanofiber/nanosphere composite morphology was prepared by electrostatic spraying method.To explore the effect of PI nanofiber/nanosphere composite coating on the properties of polyolefin separator.At 150℃,the PI nanofiber/nanosphere composite membrane did not have any dimensional change.Compared with the pure PI nanosphere coating,the thermal stability is improved more.The affinity of the composite membrane for the electrolyte is also significantly improved.In addition,the battery assembled with the PI nanofiber/nanosphere composite membrane also showed high ionic conductivity,battery performance,high temperature safety,etc.The above results confirm the practicality of PI nanofibers/nanospheres as coating materials for polyolefin separators. |
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