Preparation And Application Of Biodegradable Poly(Vinyl Alcohol) And Chitosan Based Electrolyte Membranes

Biodegradable gel polymer electrolytes,as a stable material,can improve the environmental and safety of batteries and supercapacitors,solving environmental pollution and energy shortages.Among them,poly?vinyl alcohol??PVA?and chitosan?CS?have attracted much attention.However,they still present many problems in device applications,such as the unclear mechanism of ion migration,poor electrochemical stability and high cost.Given this,the influence of the solvent on the PVA structure was discussed.N-Methylpyrrolidone?NMP?was used as a solvent for PVA matrix to prepare Mg²⁺,Li⁺ and Na+ conducting electrolytes with low cost and high ionic conductivity.The application of electrolytes in capacitors and batteries was analyzed.Simultaneously,the Mg²⁺ conducting CS-based electrolytes were prepared by solution casting method.The migration mechanism of Mg²⁺ in the electrolyte membranes was analyzed,providing a theoretical reference for the modification of CS.In order to prepare a biodegradable PVA-based electrolyte with high ionic conductivity and low cost,the present study proposed NMP as a solvent for PVA polymer.By comparing the performances of PVA membranes prepared by H₂O and NMP solvent,it is found that NMP can destroy the intermolecular and intramolecular hydrogen bonds of PVA,leading to the free volume in PVA matrix.Consequently,the crystallinity of the PVA membrane prepared with NMP was 22% lower than that of the PVA membrane prepared with H₂O.This is beneficial to the ions migration in the polymer matrix.The PVA based electrolyte prepared by H₂O and NMP solvent was investigated.It is found that the PVA-based electrolyte prepared by H₂O can mix up to 15% of Magnesium triflate?Mg?Tf?2?with ionic conductivity of 2.36×10-6 S/cm.However,in the PVA based film prepared by NMP,Mg?Tf?2 with a mass of up to 40% of PVA can be mixed,and the ionic conductivity can reach 7.8×10-5 S/cm,indicating that there is more freedom-OH in the polymer segment,complexing more electrolyte salts.Then a appropriate mass of 1-ethyl^-3-methylimidazolium triflate?EMITf?was added in PVA/15 wt.%Mg?Tf?2 and PVA/40 wt.% Mg?Tf?2,respectively.The ionic conductivity of electrolyte prepared using NMP(?1.2×10^-3 S/cm)was nearly 10 times higher than the H₂O system??2.1×10-4 S/cm?.Therefore,NMP is more suitable for the preparation of PVA-based electrolytes.Also,the NMP system electrolytes had better applicability of EDLCs.In the potential range of 0?0.85 V,the capacitor had good cycle stability,the capacity retention rate after 1000 cycles was 100% at the current density of 0.4 A/g,and it was also 100% after 3000 cycles at the current density of 1.0 A/g.In order to avoid the safety hazard caused by electrolyte in lithium-ion batteries and lithium-ion capacitors,this paper further used NMP as solvent,lithium bis?trifluoromethanesulfonimide??Li TFSI?as electrolyte salt,1-ethyl^-3-methylimidazolium bis?trifluoromethanesulfonyl?imide salt?EMITFSI?as a plasticizer to prepare high performance Li⁺ conductive PVA-based gel electrolytes.It is found that the migration of Li⁺ can transport in PVA matrix through complexing and de-complexing with-OH.The result exhibited that the addition amount of Li TFSI was also 40% of the mass of PVA,and 10% EMITFSI was added on the basis of the mass of the mixed system,achieving an electrochemical stability window of 5.0 V and an ionic conductivity of 3.6×10^-3 S/cm.Through the application analysis and discussion of the electrolyte of the system used in lithium-ion battery and electric double-layer capacitor,it is found that the battery still had good cycle performance at 0.5 C.After 66 cycling times,the discharge specific capacity was 65.2 m A·h/g.Also,the capacitor had good cycle stability in the 0?2.0 V operating potential range.The specific capacity was 76.4 F/g after 3000 cycles at the current density of 0.6 A/g,and the capacity retention rate was 100%.The overall performance indicates that the electrolyte of this system is expected to be used in energy storage devices.This thesis further used NMP as a solvent,sodium trifluoromethanesulfonate?NaTf?as electrolyte salt,and EMITF as a plasticizer to prepare high performance Na+ conductive PVA-based electrolyte.It is found that the addition of 10% EMITf on the basis of the PVA with 30% NaTf can obtain gel electrolyte with excellent performances.The fracture stress was up to 19 MPa,and the ionic conductivity was about 3.8×10^-3 S/cm which was similar to that of Mg²⁺ and Li⁺ conducting PVA-based gel electrolyte membranes prepared with NMP.At the same time,this paper also explored the application performance of the electrolyte in the double-layer capacitor.To reduce the interface contact resistance between the electrolyte and the electrode,a spinning-casting method was proposed.The specific capacity was 103.7 F/g after 1000 cycles and the capacity retention rate was 100% in the potential range of 0?1.6 V at the current density of 0.4 A/g.At the current density of 1.0 A/g,the specific capacity was 97.8 F/g after 3000 cycles,and the capacity retention rate was 100%.Therefore,this method can improve the efficiency of assembling the capacitor and the cycle stability of the capacitor.In order to clarify the mechanism of ions migration in a CS-based electrolyte,a CS/Mg?Tf?2/EMITf gel electrolyte was prepared.Mg?Tf?2 was used as an electrolyte salt and EMITf as a plasticizer.It is found that Mg²⁺ can migrate by complexing and decomplexing with the NH2,rather than with the C=O-NHR.The study of this migration mechanism can provide a theoretical basis for the property improvment of CS-based electrolytes.