Study On New Polymer Electrolytes And Anode Surface Modification For Secondary Lithium Batteries

Attempting to improve the comprehensive performance of lithiumsecondary batteries, we undertake the research work from two sides of boththe structural design of novel polymer electrolyte and polymer surfacemodification of anode. Conventional lithium secondary batteries use thecarbonate as the electrolyte solvent and the bad affinity microporous film asthe separator. However, lithium secondary batteries face serious safetyproblem, which results from the use of liquid electrolyte containing highlyvolatile and flammable organic solvents. Meanwhile, polyolefin separator iseasy caused safety issues due to its intrinsic disadvantages, such as liquidleakage, bad wettability, poor liquid electrolyte retention and poor thermaldimensional stability. On the other hand, the anode material of lithiumsecondary batteries is easy to react with liquid electrolyte under the strongreduction condition of charge process. For example, LTO is one of the mostattractive anode materials, especially suitable for an ultra-long life and high power lithium ion batteries. Unfortunately, lithium ion batteries with LTOanode are still not widely used mainly due to the unexpected interfacial sidereactions between LTO and liquid electrolyte. There will be serious swellingespecially at elevated temperatures (≥50°C). Aiming at the safety problemsof lithium secondary battery, we undertake the following research work.(1) Inorder to take place of liquid electrolyte, an all-solid-state polymer electrolyteand two kinds of gel polymer electrolytes (porous gel and pure gel) aredesigned and prepared. Ionic conductivity, mechanical performance,electrochemical stability window and the lithium anode/polymer electrolyteinterface property were investigated. Then, Li/Li symmetric, Li/LiFePO4andLi/pPAN-S cells using these polymer electrolytes were assembled and tested.(2) A uniform polyimide (PI) nanolayer is coated on surface of LTO andsystematically explores the feasibility of suppressing the unwanted interfacialside reactions at55°C. The main results were summarized as follows.1. A new composite polymer electrolyte consisting of a high content ofPSF-PEO matrix and low content butanedinitrile (SN) with LiTFSI is firstprepared by casting method. The results show that the ionic conductivities ofPSF-PEO35+LiTFSI+SN at room temperature and80°C are respectively1.6×104S cm1and1.14×103S cm1. The composite polymer electrolyte iselectrochemically stable up to4.2V (versus Li/Li+) and demonstrates good interface stability to lithium metal electrode. Moreover, it possesses excellentmechanical properties and high thermal stability in a wide temperature range.The Li/Li symmetric cell with PSF-PEO35+LiTFSI+SN displays obviouslylonger cycle life and lower polarization than the symmetric cell withPEO+LiTFSI electrolyte at65°C. Li/LiFePO4cell with the above electrolytedisplays good rechargeability at80°C.2. A novel hydrophilic polytetrafluoroethylene (PTFE)-supported gelpolymer electrolyte (GPE) membrane based on the cross-linked poly(ethyleneglycol) and poly(glycidyl methacrylate) block copolymer (PEG-b-PGMA) issuccessfully prepared by in situ cross-linking method. The physical propertiesand electrochemical properties of this GPE system are systematicallyinvestigated. The results show the cross-linked PEG-b-PGMA copolymer andpore in the optimized GPE-3membrane can absorb the largest amount ofliquid electrolyte, while the hydrophilic PTFE porous membrane offers goodmechanical support. The optimized GPE-3membrane possesses the ionicconductivity of1.30×103S cm1at25°C and is enough to be applied inlithium secondary batteries. The GPE-3is electrochemically stable up to4.5V (versus Li/Li+) and displays good compatibility to lithium metal electrode.Moreover, the optimized GPE-3membrane demonstrates superior wettability,excellent dimensional stability and non-flammability. The Li/LiFePO4cells using the GPE-3membrane show good cycling stability and rate performance,which are comparable to the cells based on conventional liquid electrolytes.The Li/GPE-3/pPAN-S cell also possesses good cycling stability and andhigher discharge specific capacity than the Li/PE-liquid electrolyte/pPAN-Scell.3. A novel Semi-IPN gel polymer electrolyte (GPE) membrane based onthe cross-linked PEGDA-co-PVC and PVDF-HFP linear polymer is firstsynthesized by UV-cured. The physical and electrochemical properties of thisoptimized Semi-IPN GPE system are systematically investigated. The resultsshow that the optimized Semi-IPN GPE can avoid leakage because of itsnon-porous character. The ionic conductivity of the Semi-IPN GPE reaches1.49×103S cm1at25°C and the electrochemical stability window up to4.2V (versus Li/Li+). It demonstrates excellent interface stability to lithium metalelectrode and superior thermal stability and mechanical properties. There is agood compromise between ionic conductivity and mechanical properties inthis Semi-IPN GPE. The resistances of Li symmetric cell with the GPE duringboth storage and cycling are more stable and lower than those with the liquidelectrolyte. Moreover, the Li/LiFePO4cells using the optimized GPE showgood cycling stability and rate performance, which are comparable to the cellsbased on conventional liquid electrolyte. 4. In view of that adjusting electrolyte composition and coating carbonlayer on LTO cannot significantly suppress the interfacial side reactions, theuniform polyimide (PI) nanolayer was coated on surface of LTO by thermalimidization, and then the effect of the PI protective layer suppressing theinterfacial side reactions between LTO and liquid electrolyte at55°C wassystematically discussed. This PI wrapping layer was characterized by FT-IR,TEM and EDS. The cycling and rate performances of PI-LTO/Li cell aresuperior to those of the LTO/Li cell at55°C. A comparison of the SEMimages of pristine LTO and PI-LTO electrodes before and after cyclingindicates that the PI protection layer can indeed lessen the side reactionsbetween LTO and liquid electrolyte at55°C. Although the initial impedanceof the PI-LTO electrode is slightly higher than that of the pristine LTO due tothe presence of PI nanolayer, its increase during cycling is not as remarkableas the latter. The PI-wrapped LTO contacted with liquid electrolyte showssmaller exothermic heat than pristine LTO and PI coating makes exothermicpeak temperature shift from263°C to276.6°C, indicating the improvedthermal stability. These results indicate PI protective layer can effectivelysuppress the side reactions between LTO and liquid electrolyte, enhancing thesafety of lithium secondary batteries using LTO anode.