Application And Research Of Chemically Modified Cellulose As Gel Polymer Electrolytes For Lithium-ion Batteries

Lithium-ion batteries（LIBs）have become ideal power sources for portable wearable electronic devices,vehicles,and smart grids in recent years due to their high energy density,long cycle life,and no memory effect.However,Li-ion batteries with commercial liquid electrolytes suffer from serious safety problems,such as volatilization,leakage,flammability,and explosion.Currently,many strategies have been explored to design batteries with higher safety,including the use of solid electrolytes instead of liquid electrolytes.Unfortunately,the low ionic conductivity and poor interfacial compatibility in solid-state electrolytes limit their further practical applications.Gel polymer electrolyte is a polymer network system swelled by a solvent.The polymer network can effectively fix the electrolyte inside,thereby improving the safety of lithium-ion batteries,and is conducive to the development of lithium-ion batteries in the direction of portability,diversification and high energy density,which is the development trend of high-safety lithium-ion batteries in the future.Therefore,the development of green,low-cost,high-strength and high-safety gel polymer electrolytes has become a recent research hotspot.As the most common natural polymer in nature,cellulose has the characteristics of abundant resources,low price,non-toxic and harmless,which lays the foundation for its application in polymer electrolytes.In this paper,cellulose is chemically modified and used as a gel polymer electrolyte in lithium-ion batteries.The main research contents are as follows:（1）Preparation of PEG-Grafted cellulose-based gel polymer electrolytesPure cellulose is difficult to be directly used in gel polymer electrolytes due to its stable structure and no affinity for organic electrolytes.In this study,a brush-like structure polymer membrane AC-PEG was prepared by grafting polyethylene glycol methacrylate（PEGMA）onto the cellulose molecular chain under UV light initiation through chemical modification.Through the test of absorption rate and contact angle,it was found that the AC-PEG membranes grafted with PEG had good wettability to the electrolyte.At the same time,the AC-PEG polymer films were characterized by tensile test and thermogravimetric analysis.The introduction of an appropriate amount of PEG helps to improve the toughness and thermal stability of the film.In addition,the electrochemical properties of the AC-PEG gel films were tested and analyzed.The AC-PEG gel films exhibited high ionic conductivity(1.82×10-3 S cm^-1)and lithium-ion transference number（0.81）,and the assembled Li Fe PO4/AC-PEG/Li batteries had stable cycling at 0.2 C and 0.5 C.（2）Preparation of crosslinked cellulose-based gel polymer electrolytesAlthough the AC-PEG polymer exhibits high affinity in the electrolyte,it does not form a cross-linked structure itself,and the AC-PEG interacts with the anions and cations in Li TFSI,resulting in infinite swelling of the AC-PEG film in the electrolytes.In this section,the crosslinked network structure is formed induced under ultraviolet light,in which Pentaerythritol Tetra（3-mercaptopropionate）（PETMP）is used as the crosslinking agent,and AC-PEG is used as the matrix material.Through the absorption rate test,tensile test and related electrochemical performance characterization,the prepared cross-linked AP film can not only achieve liquid absorption balance in the electrolyte,but also have high mechanical strength and structural stability.Based on this,the Li Fe PO₄/Li battery using the crosslinked AP gel film exhibits ultra-high cycling stability at high rates such as 0.5 C and 1 C.