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■ 외부초청 세미나

일시: 07.4.10 (화) 11:00
연사: Nitash P. Balsara (Department of Chemical Engineering University of California)
제목: Block Copolymer Electrolytes for Lithium Battery Applications

Polymer membranes with high ionic conductivity are critical components of solid-state batteries and fuel cells [1].  The performance of these membranes depends not only on their electrical properties but also on other properties such as shear modulus, permeability, etc.  Traditionally, high ionic conductivity is obtained in soft polymers such as rubbery poly(ethylene oxide) (PEO). However, in such systems, the rapid segmental motion responsible for fast ion transport [2] adversely affects the mechanical rigidity of the polymer, which leads to dendrite formation, reduced battery life and compromised safety.  In this study, we present a strategy where the electrical and mechanical properties of polymer electrolyte are decoupled and the requirements of high conductivity and high rigidity are simultaneously satisfied.  A series of nanostructured lamella-forming poly (styrene-b-ethylene oxide) (PS-PEO) block copolymers doped with lithium salt Li[N(SO2CF3)2], abbreviated LiTFSI were used as model polymer electrolyte. In this way, ionic transport is restricted to rubbery PEO domains while the glassy PS domains provide the structural rigidity.  A representative transmission electron microscope (TEM) image of the neat block copolymer is shown in Figure 1a.
       We focused on the lamellar samples with poly(ethylene oxide) (PEO) volume fractions, f, ranging from 0.38 to 0.55, and PEO block molecular weights, MPEO, ranging from16.3 to 98.1 kg/mol.  The low frequency storage modulus (G`) modulus at 90 ºC increased with increasing MPEO, from about 4x105 to 5x107 Pa.  Surprisingly, the conductivity of the PS-PEO/salt mixtures, s, also increased with increasing MPEO, from 6.2x10-5 to 3.6x10-4 S/cm at 90 oC.  Figure 1b shows the dependence of s  on MPEO for the temperatures of 90-120 oC.  Comparing  s  with the conductivity of pure PEO/salt mixtures, sPEO, reveals that s/fsPEO of the highest molecular weight sample is close to the theoretical upper limit for randomly oriented grains (s/fsPEO = 2/3).  In order to determine the role of structure on the ionic conductivity of these materials, we used energy-filtered electron microscopy for the direct imaging of lithium.  We subsequently performed three-dimensional reconstruction of TEM micrographs, which provides the structural information regarding the manner in which the conductive phase percolates through the copolymer electrolyte.  Current efforts focus on using these TEM experiments to determine the structure-conductivity relationships of our model polymer electrolyte and their applicability in dry lithium-ion batteries.