| Sustainable development is the main theme of human development in the 21st century.The rapid development of new energy has become an irreversible trend all over the world.As one of the most important secondary batteries,lithium-ion batteries are widely applied in portable electronic devices(PED),new energy electric vehicles(EV),stationary energy storage system(ESS)and various unmanned equipment due to its small size,high energy density,quick charge and discharge process,and environmental friendliness.Lithium-ion batteries have been developing for nearly 3 decades since it came into being in 1990s,and they are developing rapidly to achieve high energy density and high safety.Continuous improvement of energy density means continuous increase of safety risks.Frequent battery fire and explosion accidents have aroused concern of publics and bring the anxiety to the whole battery industry.The failure of separator is the beginning of uncontrollable thermal runaway of lithium-ion batteries.Now,research on separator to improve battery safety mainly focuses on two directions:modification of existing polyolefin separators and development of high temperature resistant polymer separators.Polyimide(PI)is a class of high temperature resistant polymer with the unique imide ring structure in the main chain,which endow PI with excellent high and low temperature resistance,mechanical properties,insulation properties,dielectric properties,chemical corrosion resistance and radiation resistance,and it has been widely used in the fields of aviation,aerospace,microelectronics and advanced composite materials.Electrospray and electrospinning,as the fast and efficient technology for preparing nano-scale polymer materials,have been developed for about half a century and are now in the process of large-scale industrialization.The PI nanospheres and nanofiber membranes prepared by these two methods inherited the excellent comprehensive properties of pristine PI materials,and are endowed with the special surface effect,nanometer size effect and large specific surface area of nanomaterials,have attracted more and more attention.In order to overcome the deficiency of polyolefin separators and overcome the disadvantages of high temperature resistant nonwoven nanofiber membranes,such as poor mechanical properties and unstable pore structure,the following research was carried out.In the research of polyolefin modification,two types of surface inorganic functionalized PI nanospheres,PI/silica nanospheres(PI/SiO2 NS)and PI/titanium dioxide nanospheres(PI/TiO2 NS)with inorganic shell armoured structure have been successfully prepared via revers in-situ hydrolysis method and in-situ complexing hydrolysis strategy respectively.The PI/SiO2 NS and PI/TiO2 NS coated PE separators effectively improve the electrolyte affinity,thermal dimensional stability compared with untreated PE separator.The contact angle to water decreased from 122 ° to 52°,and the coated PE separators just slightly curl after treated at 140? for 30 min.NCM811/Li half-cell assembled with PI/SiO2 NS coated PE separator has a specific discharge capacity decreased from 175.2 mAh g-1 to 156.4 mAh g-1 at the rate of 1C for 100 weeks,and the capacity retention rate is 89.3%.The results of rate performance show that at 0.1 C,the specific discharge capacity of the coated PE separator remains at a high level of 201.1 mAh g-1 when the specific discharge capacity is 147.8 mAh g-1 at 5C rate,the corresponding retention rate is 73.5%.The cycling performance and rate capabilbity of coated PE exceed bare PE separator.NCM811/Li half-battery packaged with PI/TiO2 NS coated PE separator also show excellent performance.At 1C rate for 100 weeks,the specific discharge capacity decreased from 1 83.7 mAh g-1 to 166.1 mAh g-1,and the retention rate is 90.4%,which is better than that of PE separator sample(83.6%).The rate capability test results show that at 0.1C,the specific discharge capacity is 209.1 mAh g-1 and the retention rate is 74.1%at 5C,correspond to a specific discharge capacity of 154.9 mAh g-1,which is higher than that of the PE separator sample(139.8mAh g-1).The above results indicate the feasibility of PI/SiO2 and PI/TiO2 as polyolefin separators'coating materials,and the performance is significantly improved compared with pristine PE separator.Low density of nanospheres can effectively reduce the energy density loss caused by ceramic coating,which makes it a good candidate as the coating materials for polyolefin separator since its excellent performance.In the research of new type high temperature resistant polymer separator,PI nanofiber membrane(NFM)with crosslinked micromorphology is prepared by in-situ reinforcement,which is first immersing the partial imidized PI NFM into poly(amic acid)(PAA)solutions and subsequently fully imidized.The microstructure and micropore regulation are successfully achieved,the influence of different impregnating PAA concentrations on the microstructure and properties were also investigated.In the P/O@P/O system,the tensile strength is increased from 28.8 MPa to the highest 201.5 MPa by 6 times,and the initial thermal deformation temperature is increased from 300? to 360?due to the introduction of micro-crosslinking structure.NCM811/Li half-cell assembled with P/O@P/O 1%sample has a specific discharge capacity of 215.5 mAh g-1 at 0.1C,which still remains at a high level of 152.7 mAh g-1 at high rate of 5C.The capacity retention rate is 70.9%,which is higher than that of Celgard 2400 separator of 63.6%.After 100 weeks of cycle stability test at 1C,the specific discharge capacity is 176.0 mAh g-1,and the capacity retention rate was 90.1%,which is significantly higher than that of the Celgard 2400 separator.The result of the battery heating experiment also shows that it has excellent high temperature resistance and greatly improves the battery safety,the start of battery thermal runaway at 130? delayed for about 6 min.Based on the previous work,using thermoplastic 6FDA/ODA-PAA as impregnation solution,a high tensile strength PI NFM with high temperature thermal shut-down function was developed.Similarly,the mechanical properties of the NFM are increased by more than 3 times,reaching the maximum of 112.0MPa.There is no any size shrinkage after heat treatment at 300? for 30 min.Meanwhile,the 5%thermal decomposition temperature(Td5)is as high as 536?,showing excellent heat resistance,thermal dimensional stability and mechanical properties.The experiment of thermal shutdown simulation shows that it can realize the function at the high temperature of 350?,which can effectively resist the thermal shock when the high specific energy battery is thermal runaway,to avoid the separator collapse and cut off the battery reaction timely to ensure the battery safety.The NCM811/Li halfbattery assembled with this functional 6F/O@P/O 1%-PI separator has a specific capacity of 219.0 mAh g-1 at 0.1C,which still remains at a high level of 151.7 mAh g-1 with a high rate discharge of 5C,and the capacity retention rate is 69.3%,which is higher than that of Celgard 2400.After 100 weeks of cycle performance test at 1C,the capacity was 164.0 mAh g-1 and the capacity retention rate is 92.4%,which is significantly higher than that of the Celgard 2400 separator.Both of the two types PI NFM separators with crosslinking morphology overcome the defects of poor mechanical properties and unstable pore structure of nanofiber membrane,making non-woven nanofiber membrane is used in the production of the automatic winding of battery possible,meanwhile the thermal shutdown function also makes a further guarantee for the safety of batteries.At the same time,the excellent physical and electrochemical properties are verifying the feasibility of them as the high security battery separators.For scientific research,it is imperative to further improve the safety of lithium-ion batteries according to the development roadmap of energy density of lithium-ion batteries.The preparation of the separator with high porosity,high wettability,high safety for high specific energy and high power lithiumion batteries is no time to delay.The PI NFM based“flexible ceramic”coreshell PI/ZrO2 separator was fabricated by combining the coaxial inorganic functionalization and in-situ adsorption complexation hydrolysis strategy,which has high porosity and electrolyte uptake up to 85%and 420%respectively,showing a comparative wettability of ceramic.There is no size change at 300?,and the Td5 is as high as 531?.The self-extinguishing characteristics of polyimide combined with the protection of inorganic ZrO2 shell layer endues the hybrid NFM with full flame retardant characteristic,which can effectively improve the safety of the battery.The NCM811/Li half-cell assembled with this"flexible ceramic" hybrid separator showed excellent battery performance,with the retention rate of 90.9%(100 cycles at 1C).The 5C discharge capacity is still as high as 151.0 mAh g-1,and the capacity retention rate is 68.7%compared with the specific discharge capacity of 219.8 mAh g-1 at 0.1C.The comprehensive performance is far above that of Celgard 2400.The above excellent performance shows that this kind of ZrO2@PI "flexible ceramic"separator with coaxial coating structure has a favorable application prospect in high specific energy and high power lithium-ion battery. |
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