Construction Of Composite Polymer Electrolytes Based On Cellulose Derivatives And Performance In Lithium Batteries

Lithium metal anode is considered to be one of the most promising anode materials for lithium batteries due to its high energy density and extremely low potential.However,the use of liquid electrolyte in lithium batteries has caused a series of safety problems,which seriously affected the development of lithium batteries.Polymer electrolyte can ensure the good interface compatibility between electrolyte and electrode,and further ensure the safety and stability of the battery.However,most of the currently reported polymer electrolytes are composed of non-degradable synthetic polymer materials,which face serious environmental pollution problems during battery recycling.Therefore,the development of polymer electrolytes with high ionic conductivity,high chemical stability and strong mechanical properties based on green and renewable biomass resources has become one of the popular research directions in the field of solid-state lithium batteries.Cellulose has good electrolyte wettability,high mechanical properties and thermal stability,and its polar functional groups can also complex with lithium ions to achieve ion migration,which is one of the preferred materials for the next generation of polymer electrolytes.In addition,cellulose can be converted into cellulose derivatives with more types of chemical groups after proper chemical modification,which can further improve the ionic conductivity and lithium ion migration coefficient of cellulose-based polymer electrolytes.Based on this,four different types of cellulose derivative-based polymer electrolytes were prepared by selecting different types of cellulose derivatives,designing and optimizing the composition and structure of existing cellulose-based polymer electrolytes,including cellulose acetate propionate(CAP)/polyimide(PI)-based gel polymer electrolytes prepared by electrostatic spinning,CAPbased gel polymer electrolytes prepared by in situ method,CAP-reinforced polyvinylidene fluoride-hexafluoropropylene(PVDF-HFP)polymer solid electrolytes prepared by solution casting,and composite solid electrolytes prepared by simple blending of hydroxypropylmethylcellulose(HPMC)/PVDFHFP/tantalum-doped lithium-lanthanum-zirconium-oxygen(LLZTO),realizing the transition from quasi-solid state to solid state of the cellulose derivativebased polymer electrolyte.The research work includes the following aspects:(1)A novel biomass-based gel polymer electrolyte was prepared by combining the advantages of CAP and PI.The thermal stability,mechanical strength and lithium ion transport properties of the electrolyte membranes were improved by using the rigid chains of PI and the abundant polar functional groups of CAP.The microstructure and physicochemical properties of PI-CAP membranes were analyzed by modern techniques such as infrared analysis(FTIR),thermogravimetry(TG)and field emission scanning electron microscope(FE-SEM).The results show that the obtained PI-CAP membranes exhibited satisfactory electrolyte wettability,high mechanical strength and excellent thermal stability.The gel polymer electrolyte obtained by soaking the PI-CAP membrane in electrolyte and then gelling exhibited an ionic conductivity of 2.09×10-3 S/cm and a lithium ion transference number of 0.89 at room temperature.The LiFePO4 battery assembled with this electrolyte instead of a commercial separator exhibited a capacity of 158.00 mAh/g after 300 cycles at a current density of 1 C,with a capacity retention rate of 94.04%.(2)In order to achieve better interfacial contact between electrolyte and electrode,a new gel polymer electrolyte PETEA/CAP was prepared by simple in situ polymerization and gelation of CAP and pentaerythritol tetraacrylate(PETEA)in a liquid electrolyte.A stable cross-linked structure was established by using the multi-side chain structure of PETEA and the hydrogen bonding interaction between PETEA and CAP,which provided Li+migration channels,promoted the uniform deposition of Li+and avoided the growth of Li dendrites on the Li metal side.In addition,the abundant polar oxygen-containing groups in CAP promoted the dissociation of lithium salts and improved the mobility of lithium ions.The results show that the obtained PETEA/CAP gel polymer electrolyte had an ionic conductivity of 1.03×10-3 S/cm,a lithium ion transference number of 0.73,and an electrochemical window of 4.50 V at 25℃,as well as the electrolyte had good stability to lithium metal,with an overpotential of only 47.80 mV after 1000 h of cycling.Moreover,the PETEA/CAP gel polymer electrolyte produced a LiF-rich solid electrolyte interphase(SEI)film during cycling,which effectively reduced the decomposition of carbonate solvent in the electrolyte,thus ensuring interfacial stability during cycling.The assembled LiFePO4 cells exhibited a capacity retention rate of 90.60%after 500 cycles at a current density of 1 C and showed excellent rate performance.(3)Because of the existence of liquid electrolyte,the gel polymer electrolytes prepared in the above two chapters are limited to improve the safety of lithium batteries.A novel high performance solid polymer electrolyte PVDFHFP/LiTFSI/CAP(PHLC)was prepared by simply blending CAP,PVDF-HFP and LiTFSI,realizing the transition from quasi-solid to solid state.The results show that CAP played important roles in reducing the crystallinity of PVDFHFP,promoting the dissociation of LiTFSI,and providing migration paths for Li+through the C=O coordination center.The addition of CAP enhanced the motility of PVDF-HFP chain segments and improved the ionic conductivity as well as the lithium ion transference number of PHLC.The ionic conductivity of PHLC(20%CAP)polymer electrolyte was 1.25×10-4 S/cm,the lithium ion transference number was 0.49,and the electrochemical window was 4.50 V,which are much better than that of the PVDF-HFP/LiTFSI(PHL)polymer electrolyte without CAP.In addition,the ester group in CAP can complex lithium ions and reduce the concentration of lithium ions on the surface of lithium anode during battery charging,which helped to achieve uniform deposition of lithium ions on the surface of anode and played a role in inhibiting the growth of lithium dendrites.The LiFePO4|PHLC|Li batteries assembled with PHLC(20%CAP)polymer electrolyte showed excellent cycle stability and rate performance.(4)CAP is an effective polymer additive,but the poor interfacial compatibility of CAP with lithium anode severely limits its application in polymer electrolytes.A new composite solid electrolyte PVDFHFP/HPMC/LLZTO(PLHL)was prepared by a simple solution casting method.The prepared electrolyte had obvious self-supporting ability and mechanical flexibility.The high polarity of HPMC greatly facilitated the dissociation of LiTFSI.In addition,the addition of HPMC can synergistically transport lithium ions with PVDF-HFP,enhancing the ionic conductivity and lithium ion transference number of the electrolyte.During the stability test of the electrolyte-lithium anode interface,HPMC showed better compatibility than CAP,and its stability test time was extended to 400 h.Subsequently,the addition of LLZTO particles further reduced the PVDF-HFP crystallinity and expanded the amorphous region of PVDF-HFP,which accelerated the migration of Li ions.Due to the percolation effect,Li+can migrate not only in the polymer chain segments but also at the interface between PVDF-HFP and LLZTO.The electrochemical performance of the PVDF-HFP-based electrolyte was significantly improved by the synergistic effect of HPMC and LLZTO.The PLHL composite electrolyte exhibited good mechanical strength(3.00 MPa),ionic conductivity of 2.50×10-4 S/cm,lithium ion transference number of 0.70,and electrochemical window of 5.00 V.The assembled LiFePO4/PLHL/Li batteries exhibited good cycling performance and rate performance.After 300 cycles,the specific capacity of the battery was 151.00 mAh/g,and the capacity retention rate was as high as 95.60%.