Surface Modified Membrane Via Chemical Bond And Its Application In Lithium-ion Batteries

In recent years,lithium-ion batteries(LIBs)have come to the fore due to their high energy density,excellent cycle stability and other advantages and have been widely used in various fields of social life.Especially in the large-scale application of electric vehicles and energy storage power stations,people are full of expectations for higher energy density and high safety.As an important part of the lithium-ion battery,the separator plays a key role in its safety performance.It not only avoids the direct contact of the positive and negative materials,but also has a significant impact on the battery's high and low temperature performance and cycle stability.Therefore,the research and exploration of new high-performance separators is of great significance to the development of high-performance and high-safety lithium-ion batteries.At present,commercial polyolefin separators(PP,PE)have poor thermal stability,poor wettability,and relatively single functionality,which leads to rapid capacity decay at large rates.In response to these problems,this work uses a chemical bonding strategy to modify the surface of the separator,discusses its improvement in the mechanical strength,heat resistance,wettability and other properties of the separator,and evaluates the applicability of the new separator in lithium-ion batteries.Improve the electrochemical performance and safety of lithium-ion batteries.The main work is as follows:1.Through the interface covalent grafting strategy,mesoporous silicon dioxide(mSiO₂)is covalently fixed on the surface of the PP membrane to prepare a new type of PP-g-mSiO₂ membrane.Studies have shown that the PP-g-mSiO₂membrane improves the interfacial stability,thermal stability,and liquid absorption capacity.At the same time,mSiO₂ NPs are conducive to more electrolyte storage,with a liquid absorption rate as high as 379%;Secondly,in the LiPF₆ liquid electrolyte system,the hydrogen bond formed by hydroxyl group in the PP-g-mSiO₂ separator with the PF₆^-anion F atom prevents the movement of the larger anion(PF₆^-)and improves the lithium-ion migration.rate.Its value is 0.69,which is higher than PP(0.34)and PP-SiO₂(0.45).Li-ion battery application research shows that the battery assembled with PP-g-mSiO₂ separator has a discharge capacity of 101 m A h·g^-1/5 C,and the capacity retention rate after 1000 cycles is 93%,which is higher than PP(76%)and PP-SiO₂diaphragm(88%).Owing to the covalent graft bonding,the mSiO₂ ceramic particles on the separator surface after 1000 cycles of the battery did not fall off.2.Using the intermolecular hydrogen bonding strategy,an asymmetric membrane(CP@SiO₂)composed of Polyvinylidene Fluoride(PVDF),cellulose and SiO₂nanoparticles was prepared.This unique double-layer material design gives excellent thermal stability,superior wettability(contact angle?0°)and high lithium-ion migration number(0.7).There are hydrogen bonds between fibers(F--H—O)and between fibers and SiO₂ particles so that the fibers are closely connected and the inorganic particles are uniformly loaded on the fiber surface,which can promote the uniform distribution of lithium ions and inhibit the growth of Li dendrites.With CP@SiO₂ separator,the current density is 2 m A·cm^-1,2 m Ah·cm^-1,and the overvoltage is less than 30 m V when the cycle is over 350 h.In the battery with Li Fe PO₄ as the positive electrode,the discharge capacity is 100 m Ah·g^-1 when the current density is 5C,and the discharge capacity retention rate is 89%after 1000 cycles.3.The commercial membrane was modified with dopamine to produce covalent and non-covalent bonds.As a secondary reaction platform,the modified membrane was obtained by growing ZIF-67 material and used for the study of its electrochemical performance in lithium-ion batteries.It was found that the lithium-ion battery equipped with PP/PDA@ZIF-67 membrane showed perfect performance.At a high current density of 5 C,the discharge capacity of 91 m Ah·g^-1 was higher than that of PP@ZIF-67(50 m Ah·g^-1),and at this current density for 600 cycles,the discharge capacity of78 mAh·g^-1 was still higher than that of PP@ZIF-67(20 m Ah·g^-1,400 cycles).The excellent performance of the composite membrane is attributed to(1)Metal-organic frameworks(MOFs)material has a rich pore structure and high specific surface area,which improves the wettability and thermal stability of the membrane,enhances the liquid storage capacity,and improves the ionic conductivity.(2)The introduction of dopamine could form polydopamine to adhere to its surface,and the exposed hydroxyl groups are complexed with cobalt ions,so that MOF seeds grow uniformly on the surface of the membrane,and the contact with the electrode interface is even and more.In order to be compact and reduce the interface resistance at the same time.After many cycles,the MOF material on the surface layer has not fallen off;(3)MOF has an open metal site to interact with anions in the electrolyte,releasing more Li⁺,thus improving the lithium ions mobility(0.7).In summary,this thesis focuses on the problems of poor wettability,poor thermal stability,and single function of commercial separator,and optimizes the performance of the separator.From the perspective of chemical bond,it improves the loading method of ceramic particles or metal organic frame materials in order to enhance their bonding.The firmness overcomes the problems of ceramic film falling off caused by the traditional coating process,thereby improving the electrochemical performance and safety of lithium-ion batteries,and providing some ideas and explorations for the research and development of high-performance separator.