Poly(ethylene Oxide)-based Solid Polymer Electrolytes Containing Functionalized Nanosilica

Electrolyte is one of the key components of lithium-ion batteries(LIBs),which is placed between cathode and anode,and plays the role of ion conductor in the system.Though LIBs have been widely used in our daily life,the security issues still emerge due to the traditional flammable liquid electrolytes utilized in the batteries.To improve the secure performance of LIBs,replacing liquid electrolytes to solid polymer electrolytes(SPEs)is an effective way.Poly(ethylene oxide)(PEO)is regarded as a promising SPEs material for LIBs,because of its excellent compatibility with lithium salts,high safety,low cost and good electrochemical stability.Unfortunately,the implementation of the electrolytes material is hampered by the easily crystalline ethylene oxide units in PEO under ambient temperature,resulting in the constraint of ion mobility and poor ionic conductivity,and thus they can't meet the requirements of practical applications.In this dissertation,several PEO-based SPEs are fabricated by different modified methods such as blending inorganic additives to prepare composite polymer electrolytes(CPEs)and copolymerization to prepare cross-linking structure to overcome the drawback,which provides theoretical direction for preparing SPEs and may open many new opportunities for application in solid-electrolyte batteries with elevated electrochemical properties.Firstly,although various types of nanoparticles have been ubiquitously employed as additives to promote the practical performances of CPEs in LIBs,the influences of the type of the chemical bond between core and canopy of the modified nanoparticle on the properties of CPEs have rarely been investigated.Herein,two types of nanoparticle additives,namely,ionic bond modified silica nanoparticles(IBNs)and covalent bond modified silica nanoparticles(CBNs),were prepared conveniently in order to optimize the overall performance of the electrolyte.Furthermore,the CPEs were fabricated by doping IBNs or CBNs as well as lithium salt within a poly(ethylene oxide)matrix and their electrochemical properties were investigated.The dramatic enhancement in the ionic conductivity of CPEs resulted from the addition of fillers into the system,and the improvement became more significant when the fillers were IBNs,due to the increased chain mobility arised from the weaker anchoring effect of ionic bond linkage.Moreover,a broad electrochemical stability window was obtained in the presence of IBNs.Therefore,the synergistic effects of fillers structure and electrolytes composition are the key factors to improve the electrochemical performance of CPEs.Secondly,a series of solid polymer electrolytes(SPEs)were synthesized by cross-linking poly(ethylene glycol)methacrylate(PEGMA)and poly(ethylene glycol)diacrylate(PEGDA)under UV irradiation in the presence of 2-hydroxy-2-methylpropiophenone as the photoinitiator.LiClO4 was used as the lithium source,and the molar ratio between ethylene oxide segments and lithium element was fixed at 20(EO:Li+ = 20).The average molecular weights(Mn)of PEGMA and PEGDA were varied to modify the structure of SPEs.The Mn of PEGMA was 300,475 or 950 g mol-1 respectively,while the Mn of PEGDA was 200,400,600 or 1000 g mol-1 respectively.The result of ionic conductivity for each electrolyte indicated that the penetrated glycol segments of PEGMA in the cross-linked structure made more contribution to improve lithium ion mobility,and a significant increase in ionic conductivity was achieved when the glycol chains length became longer.The optimized PEGMA and PEGDA structures were obtained by the systemic experiments,which paved the way for next work.Thirdly,though hard mechanical strength of PEO based cross-linking electrolytes is achieved by UV irradiation directly,the obtained electrolyte membranes are too brittle to meet the actual application requirements.To enhance the flexibility of electrolytes,the chain transfer reagent,2-cyanoprop-2-yl-1-dithionaphthalate(CPDN),was synthesized successfully.Then,PEGMA(Mn= 475 g mol-1)and PEGDA(Mn = 400 g mol-1),which were proved to be the best structures in the former work,were polymerized via reversible addition-fragmentation chain transfer(RAFT)polymerization under thermal induced by azodiisobutyronitrile(AIBN)with the addition of CPDN.The flexibility of the prepared electrolytes was improved significantly.Furthermore,the ionic conductivity was elevated with the content of PEGMA increased,which was according with the results got from the second work.In addition,CPEs were prepared by physical blending the aforementioned IBNs in the cross-linked system fabricated by RAFT polymerization,and the ionic conductivity of the material was further improved by the addition of fillers.Finally,a series of thiol modified silica nanoparticles(SiO2-SH)were prepared successfully.Then,novel flexible organic-inorganic hybrid solid electrolytes with controlled network structures(cnHSEs)were formed via thiol-ene photopolymerization under UV irradiation,using SiO2-SH and PEGDA.The chemical connection between SiO2-SH and PEGDA promoted the homogeneous dispersion of inorganic fillers in the system,thus cnHSEs possessed the advantages of composite polymer electrolytes and cross-linked polymer electrolytes.The degree of crystallinity was depressed in cnHSEs,which enhanced the lithium ion mobility,and the flexible cnHSEs demonstrated a relatively high ionic conductivity even at low temperature of-20 ? and up to the maximum of 7.3 × 10-4 S cm-1 at 30 ?.Additionally,the lithium-ion transference number was also improved simultaneously up to 0.7 more due to the scavenger effect of silica nanoparticles embedded in the system,and cnHSEs showed a broad electrochemical stability window.Moreover,cnHSEs had excellent compatibility with various kinds of cathode material(LiFePO4 or LiCoO2),and the fabricated cell exhibited a highly reversible electrochemical reaction and stable cycling performance.