Multi-arm Star-shaped Liquid Crystalline Block Copolymers As Polymer Electrolytes For Solid State Lithium-ion Batteries

Lithium ion batteries get wide attention because of its advantages as high energy, high voltage, charge and discharge performance, low self discharge, and long life. As a small green power source, it has been widely used in the portable electronic products such as mobile phone, notebook computer, video camera, electric motorcycle, electric bicycle, electric vehicle and aviation equipment etc.. The use of polymer electrolyte could overcome not only the leakage, short circuit, security problen caused by the traditional liquid electrolyte, but also offset the shortcoming of the inorganic solid electrolyte low conductivity, brittleness, poor mechanical. Since the solid polymer electrolyte is considered to have a good stability and reliability for lithium ion battery, it is important to design a polymer electrolyte with high conductivity. Poly (ethyleneoxide)(PEO) based polymer electrolytes have receivede extensive attentions for their potential capability to be used as alternative candidate materials in all solid state polymer lithium ion batteries. However, there are many problems such as low conductivity, bad compatibility of the interface between polymer electrolyte and electrode material, which seriously affeeting the cycle and safety. So introduction of the cyano biphenyl mesogenic to the branched polymers with PEG segments, achieving a novel liquid crystalline hyperbranched copolymer, the copolymer mixed with lithium salt to obtain the full solid state hyperbranched polymer electrolyte films. This dissertation includes four major parts.Firstly, Novel star branched amphiphilic liquid crystalline (LC) copolymers have been synthesized.4-Arm poly (ethylene oxide)-co-x-[(4-cyano-4’-biphenyl) oxy]alkyl methacrylate (TPEO-MAxLC-Φ)(x=6,Φ=20,30; x=9, Φ=10,19) containing four poly(ethylene oxide) arms (TPEO) and polymethacrylate with cyanobiphenyl mesogenic pendants (MAxLC) at the end of each arm are prepared by atom-transfer radical polymerization (ATRP). The effects of structural variations on the property, and the relationship between morphology and the ionic conductivity of the copolymer electrolytes are studied. The strong assembly of cyanobiphenyl mesogens induces the copolymers with enantiotropic mesophase, even after doped with LiC104. And lamellar structures are also achieved by cooperative assembly of hydrophobic mesogen-containing polymethacrylate groups and the amorphous hydrophilic TPEO nanoscale aggregation, especially after LC thermal annealing. The ionic conductivity has been improved greatly by incorporation of the mesogens. The cyanobiphenyl mesogens not only favor the ordered morphology to provide the efficient ion transportation pathway, but also suppress TPEO crystallization to offer the movement of TPEO chains. Among all of the electrolyte films, TPEO-MA9LC-19shows the best ion conductivity of2.24×10-5S/cm at25℃and this value reaches to5.39×10-5S/cm after annealed at LC states.Seeondly, A series of star-shaped polymers are synthesized by atom transfer radical polymerization using poly-(methoxy-poly (ethylene glycol) methacrylate)(PPEGMA) as a hydrophilic segment and poly{10-[(4-cyano-4’-biphenyl) oxy] decatyl methacrylate}(PMALC) as a hydrophobic liquid crystalline segment. The strong assembly of cyanobiphenyl mesogens induces the copolymers with enantiotropic mesophase, even after doped with LiC104. Lamellar morphology is also achieved by cooperative assembly of hydrophobic mesogen-containing polymethacrylate groups and the amorphous hydrophilic PPEGMA nanoscale aggregation, especially after liquid crystal thermal annealing. In addition, the sequential effect, that is, the position difference of the liquid crystalline segments in the copolymer electrolytes causes two quite different morphologies. The liquid crystalline segments arranged in the star polymer inner sphere (3PMALC-PPEGMA) makes it difficult for the mesogens to interact with each other efficiently, which leads to a discontinuous molecular packing. However highly ordered domains can be formed in the3PPEGMA-PMALC/LiC104electrolytes with mesogens in the star copolymer exterior, which can provide a more favorable morphology for the ions transportation. As a result, the ionic conductivity of the electrolytes can be improved by incorporation of the liquid crystalline segments into the copolymer, especially for the3PPEGMA-PMALC with the mesogen arranged in the outside of star copolymer sphere. Ionic conductivity of3PPEGMA-PMALC annealed at liquid crystalline state is1.0×10-4S/cm at25℃, which is higher than that of3PPEGMA electrolytes without mesogen groups. Thirdly, Novel star-shaped hard-soft triblock copolymers,4-arm poly (styrene)-block-poly [poly (ethylene glycol) methyl ethyl methacrylate]-block-poly{x-[(4-cyano-4’-biphenyl) oxy] alkyl methacrylate}(4PS-PPEGMA-PMAxLC)(x=3,10) with different mesogen spacer length are prepared by atom-transfer radical polymerization. The star copolymers comprised three different parts:a hard poly styrene (PS) core to ensure the good mechanical property of the solid-state polymer, and a soft, mobile poly [poly (ethylene glycol) methyl ethyl methacrylate](PPEGMA) middle sphere responsible for the high ionic conductivity of the solid poly electrolytes, and a poly{x-[(4-cyano-4’-biphenyl)oxy] alkyl methacrylate} with a birefringent mesogens at the end of each arm to tuning the electrolytes morphology. The star-shaped hard-soft block copolymers fusing hard PS core with soft PPEGMA segment can form a flexible and transparent film with dimensional stability. Thermal annealing from the liquid crystalline states allows the cyanobiphenyl mesogens to induce a good assembly of hard and soft blocks, consequently obtaining uniform nano-scale microphase separation morphology, and the longer spacer is more helpful than the shorter one. There the ionic conductivity has been improved greatly by the orderly continuous channel for efficient ion transportation, especially at the elevated temperature. The copolymer4PS-PPEGMA-PMA10LC shows ionic conductivity value of1.3×10-4S/cm (25℃) after annealed from liquid crystal state, which is higher than that of4PS-PPEGMA electrolyte without mesogen groups.Finally, Solid composite polymer electrolytes based on polyethylene oxide (PEO)/LiClO4with the star-shaped liquid-crystalline copolymer and4-cyano-4’-[(10-hydroxyalkyl) oxy](10-BPCN) biphenyl are prepared. The star-shaped liquid-crystalline copolymer,3-arm-poly{10-[(4-cyano-4’-biphenyl) oxy] decatyl methacrylate}-block-poly [methoxy-poly (ethylene glycol) methacrylate](3PMALC-PPEGMA), is composed with a conductive block (PPEGMA block) and an orientation block (PMALC block). The liquid-crystalline fillers induce high chain mobility of PEO because of the decreasing crystallinity and the cyanobiphenyl mesogen impels the blends to achieve lamellar structure. So strong assembly ability of the free mesogens ensure the composite systems to arrange with more ordered nanostructure, consequently obtaining bicontinuous nanoscale microphase separation morphology with the addition of the small liquid crystallite molecules (10-BPCN), which is favorable for ion transportation. Composite polymer electrolytes based on the ternary blend containing70/25/5(PEO/3PMALC-PPEGMA/10-BPCN) mass percent with lithium perchlorate (LiClO4) exhibits the best performance, showing an increase of more than two orders of ionic conductivity value than the pure PEO/LiClO4electrolytes and the maximum value reach to1.3×10-5S/cm (25℃) after annealed from liquid crystal state. The high lithium ion transference number and wide electrochemical stability window exhibit an acceptable performance of the system. Therefore the better miscibility and lower crystallinity, compared to pure PEO, as well as the efficient transport channel in the present system, pave a potential way to develop solid state polymer electrolytes for Li-ion batteries.