Mechanism Study Of The Ionic Conduction In Cyclodextrin-polymer/li~+ Solid Polymer Electrolytes By Solidstate Nuclear Magnetic Resonance

Compared with traditional electrolytes,solid polymer electrolytes(SPEs)have many superior properties,such as high energy density,inhibition of lithium metal dendrite growth and easy processing et al.Thus the research of SPEs becomes hot in the field of solid state electrolyte.In this thesis,several highly conductive CD-polymer/Li~+SPEs have been successfully prepared via the supramolecular self-assembly method with Poly(ethyl oxide)(PEO),cyclodextrin(CD)and alkali metal salts.By using various solid-state NMR technology,the dynamics of lithium ions and polymers in the materials are investigated in great details.Based on these results,the conductive mechanism in the materials is discussed.Finally,the materials are succesfully assembled into an all-solid-state lithium metal battery.The main contents of this thesis are listed in the following:1.In chapter 2,a series of solid state NMR methodology are developed.This methodology can be used to investigate the chemical environment and distribution of lithium ions in solid electrolytes.By using this NMR methodology,the chemical environment and distribution of lithium ions in?-CD-PEO_n/Li~+are investigated.Based on this,the dynamics of polymer chains and lithium ions and the conductive mechanism are also studied and discussed.The results of solid state NMR clearly show that there are two different states of lithium ions in the system,that is,the ones close to polymers and the others close to CDs respectively.The lithium ions close to the polymer chains have a high density and fast mobility while the lithium ions close to CDs have a low spatial density and slow mobility.We then conclude that the lithium ions close to the polymer chains are the main source of the charge carriers in this material.This is welll in line with the observation in the electrochemistry measurement in Chapter 5.The work in this chapter has developed a NMR methodology for the following research and provides the strategy to improve the ionic conductivity of the solid polymer electrolytes having tunnel structures.2.In Chapter 3,the effect of the chemical structure regularity of polymer chains on the conductivity of CD-polymer/Li~+materials is studied.For this purpose,we use polypropylene glycol(PPG)and block copolymers(PG-co-EG-co-PG)in stead of PEO,to synthesize two new kinds of solid polymer electrolyte materials,?-CD-PPG/Li~+and?-CD-copolymer/Li~+.The solid-state NMR results show that the polymer chains in?-CD-PPG/Li~+sample have poor mobility because of the steric effects,and in turn the low ionic conductivity.Whereas,in?-CD-copolymer/Li~+system,the polymer chains demonstrate a high mobility and a high ionic conductivity.We thus conclude that introducing the chemical structure regularity of polymer chains can improve the conductivity of the CD-polymer/Li~+materials,but the steric effects between the polymer chains and the assembled CDs need to be considered.3.In Chapter 4,the effects of the conformation sequence of polymer chains on the conductivity of materials are studied.In this chapter,serveal?-CD-PEO_n/Li~+materials are chosen for the study.By controlling the Li ion concentration,the conformation sequence of PEO chains in the samples can be changed from the ttg conformational sequence to the all-trans conformational sequence.The solid-state NMR results show that PEO chains with all-trans conformational sequence have a very high segmental mobility.This significantly enhances the mobility of the lithium ions.In several samples having low lithium ion concentrations,the Li ions show a very high mobility which is similar to the observations in LISICON ceramic materials.The work in this chapter provides a direction to create fast lithium ion transportation in solid polymer electrolytes.4.In Chapter 5,the application of the SPEs developed in the previous chapter is studied.Several all-solid lithium-metal batteries are successfully assembled using the?-CD-PEO_n/Li~+SPEs developed in the previous chapters.The electrochemical cycle is achieved in the all-solid lithium-metal batteries.The electrochemical parameters such as electrochemical window,ion transference number,DC resistance are measured.The excellent electrochemical properties show the feasibility to assemble all-solid high-power batteries by using such solid polymer electrolytes.The work in this chapter provide some basic knowledge to the practical application of solid polymer electrolytes developed in this thesis.The work of this thesis shows that the tunnel-structure polymer solid electrolyte based on the self-assembly of CD and polymer has a promissing application prospect.It also shows that NMR technology has great advantages of studying the microstructure,molecular motions in such types of complicated material system.The work in this thesis is helpful for one's understanding of the conduction mechanism of SPE and may provide the useful knowledge for the design of new and highly conductive SPE materials.