Preparation Of Polymeric Separator For High Performance Lithium-based Rechargeable Batteries

Lithium-based rechargeable batteries,such as lithium-ion(Li-ion)battery and lithium-sulfur(Li-S)battery,possess wide application promise in the field of powering various portable electronic devices and electric vehicles for their favorable advantages of long cycling life,no memory effect,and high-energy efficiency.In a Li-ion or Li-S battery,the separator plays a key role in guaranteeing the battery safety by physically isolating the cathode and anode while providing ions transport between the electrodes.Currently,polyolefin-based microporous separators such as polypropylene(PP)and polyethylene(PE)are the main products because of their good chemical and electrochemical stability,reliable mechanical strength,and low cost.However,these polyolefin separators exhibit severe thermal shrinkage at elevated temperatures,which may consequently lead to internal short circuits and thermal runaway,and eventually cause an outbreak of fire or even an explosion.Moreover,the inert surface of polyolefin separators shows poor compatibility with the polar organic electrolyte solvents,which result in a reduced battery performance.Furthermore,polyolefin-based substrates exhibit insufficient ability to localize polysulfide species within the cathode side when assembled into Li-S batteries.To solve these problems,this thesis has mainly done the following research,(1)Fast assemble of polyphenol derived coatings on PP separator for high performance Li-ion batteries.Nature polyphenol tannic acid(TA)is facilely assembled on the PP separator triggered by sodium periodate.The TA derived coatings can effectively endow the substrate with enhanced wettability(water contact angles dropped from 121.7 to 53.2°)and accordingly improved electrochemical properties(ionic conductivity increased from 0.72 to 0.91 mS cm-1)without damaging their pore structures.Importantly,the modified separator can definitely promote the cycle stability(coulombic efficiency exceeding 99.9%)and rate performance for the LiCoO2(LCO)/graphite(C)cells assembled with it.(2)Novel water-based ceramic coating for PE separator with improved wettability and thermal resistance.PE separator is transferred from hydrophobic to hydrophilic,and then the aqueous ZrO2 slurry is coated on the substrate evenly.Wettability(electrolyte contact angled fropped from 43.7° to only 5.7°)and thermal stability(thermal shrinkage only 3.2%after being treated at 140 ℃ for 1h)of the modified separator improved significantly.NCM-523/C cells assembled with the ZrO2 coated separators exhibited much enhanced cycling performance when cycled at 45 ℃.(3)Poly(imide-amide)(PAI)coated PE separator with enhanced heat resistance and wettability for high performance Li-ion batteries.PAI is coated on both side of PE separator through non-solvent induced phase separation with appropriate porous structure.The PAI-PE separator displays enhanced wettability(electrolyte up take increased from 82.1 to 157.3 wt%)and heat resistance with pretty low shrinkage after being treated at 140 ℃ for 1h.Significantly,NCM-523/cells exhibit excellent cycling stability and rate performance at 45 ℃ in the case of PAI-PE substrate.(4)Insight into the effect of separator wettability on the ionic conduction and interface properties for lithium-metal anode batteries.Polyphenol and polyamine are assembled on the surface of PE separator through layer-by-layer method.Wettability and electrolyte uptake of the modified separator(MPE)improved significantly without damage its morphology and pore structure.Ionic conductivity and lithium-ion transference number of MPE increased to 0.42 mS cm-1 and 0.49,respectively.Galvanostatic measurements are performed on Li symmetric cells to investigate the effect of separator wettability on interface properties.It is found that MPE separator favors the electrochemical process by providing better interface compatibility and more uniform deposition of Li+,which correspondingly mitigate the formation of Li dendrites.Lithium-metal anode cells LCO/Li shows superior battery performance in case of the MPE substrate with improved wettability.These results are expected to be instructive for the design of more durable Li electrodes.(5)In-situ phase inversion towards integrated electrode/separator with multifunction for long-life Li-S batteries.The integration of sulfur-carbon complex and poly(m-phenylene isophthalamide)separator(S@PMIA)is achieved though in-situ phase inversion method.The PMIA separator containing polar groups(-NH,C=O)is designed with finger-like and asymmetric porous structure,which is confirmed to be effective to confine the shuttle of polysulfides by both physically "trapping" and chemically "repellence",and inhibit the growth of dendrites at Li ande.Significantly,the S@PMIA/Li cell exhibit excellent cycling performance with an initial reversible capacity of 802.7 mAh g-1 and retains 548.7 mAh g-1 after 1000 cycles at 1C,degradation rate of only 0.032%per cycle.