| Currently,the common separator used in lithium ion battery mainly consists of polyolefin,which has problems of poor electrolyte wettability and thermal stability.Besides,the common separator comes from non-renewable fossil raw materials,which is difficult to meet the needs of high-performance and eco-friendly lithium ion battery.As a kind of natural polymer material,cellulose possesses strong polarity,good structural stability,good wettability and thermal stability,as well as extensive sources,abundant reserves and renewable degradation.Therefore,cellulose endows the potential to prepare the high-performance separator for lithium ion battery.However,the separator made directly from cellulose fiber for papermaking has some problems,such as large aperture and low strength,while the separator from low length-diameter ratio nanocellulose endows the compact structure,and they are difficult to meet the requirements of the lithium-ion battery.In this thesis,some problems of the current separator for lithium ion battery are explored to be solved in the following two aspects.(1)High aspect ratio cellulose nanofibers(CNF)with a length of 5 ? m and an aspect ratio of 1000 were prepared from fast-growing poplar as the raw material of plant fibers.CNF separators with different thicknesses were prepared by vacuum filtration.The test results show that when working at a temperature above 100°C,the dimensional shrinkage rate of the commercial diaphragm reaches a maximum of 42%.As a comparison,the volume of the CNF separator hardly changes,reflecting better thermal dimensional stability than the commercial separator.The electrolyte absorption rate,surface wetting angle and wetting area of the CNF separator were improved by 2 times,4 times and 8 times,respectively,compared with the commercial separator,showing that the electrolyte wettability of the CNF separator was significantly improved compared with the commercial separator.In particular,the specific capacity of the cell corresponding to the CNF separator with a thickness of 20 ? m is 155.1m Ah/g after 100 cycles of charge and discharge at 0.5 C,and 138.2 m Ah/g after 100 cycles at2 C.When the loading amount is 7.64 mg/cm~2,the specific capacity is stable at 152.3 m Ah/g after 50 cycles at 0.5 C,which is better than that of commercial separator cell,reflecting good electrochemical performance.(2)High aspect ratio bacterial cellulose nanofibers(BC)with a length of tens of microns and an aspect ratio of over 1000 were prepared from bacterial cellulose,and then BC membranes with different thicknesses were prepared by vacuum filtration.The test results show that the volume of the BC separator does not change significantly when working at a temperature of 160°C,showing good thermal dimensional stability.The electrolyte absorption rate,surface contact angle,and wetting area of the BC separator were improved by4 times,3 times,and 2 times,respectively,compared with the commercial separator,reflecting better electrolyte wettability.When the thickness of the BC separator is 20 ? m,the coulombic efficiency of the corresponding cell achieves 99%after 100 cycles of charge and discharge at 0.5 C,the discharge specific capacity remains at 155.2 m Ah/g,and the specific capacity remains at 137.0m Ah/g after 100 cycles at 2 C.When the loading amount is 7.64mg/cm~2,the specific capacity is stable at 152.6 m Ah/g after 50 cycles at 0.5 C,which is better than that of commercial separator cell,showing good electrochemical performance.The above results show that high aspect ratio cellulose nanofibers with an aspect ratio greater than 1000 can be prepared using poplar cellulose and bacterial cellulose as raw materials.Their corresponding battery separators all have better electrolyte wettability,thermal stability and electrochemical performance than commercial polyolefin separators.This research provides a new technical approach for the construction of next-generation lithium-ion battery separators based on green sustainable resources. |
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