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Project Goal and Motivation:
To develop and study the behavior of a solid-state electrolytic
material utilizing microphase separation of the elements of a diblock
copolymer, comprising of poly(ethylene oxide) (PEO). It is desired that the material
satisfy both the high ionic conductivity and dimensional stability
requirements at ambient temperature and output higher efficiency than
current battery cells. Major
applications exist in the development of thin-film batteries.
Today, batteries find extensive uses in cellular phones, laptop
computers, hand-held devices and other consumer appliances and electronic
gadgets that require a portable power source.
Project Description: In the past 30 years, considerable research has been dedicated to the development of a solid electrolyte for use in batteries. The advantages of a solid polymer electrolyte battery over current liquid systems include low environmental, health, and safety hazards, low predicted material and processing costs, and greatest freedom in battery configurations1. These considerations are crucial for applications such as electric vehicles. For conduction to occur in a cell, the polymer electrolyte has to promote the dissociation and solvation of an ionic salt. While early work was conducted with potassium salts, current research on solid electrolytic systems employs lithium salts due to their high mobility and solubility in polymeric hosts such as PEO and poly(propylene oxide), PPO2,3. Early work pioneered by M. Armand on the possibility of using polymers in battery applications demonstrated that only the amorphous domains showed appreciable ionic mobility4. Despite the entirely amorphous nature of PPO, it was discarded as a viable option because it has much lower conductivities than amorphous PEO5. The stability of PEO-lithium salt complexes makes them relatively simply to prepare and handle. Ionic mobility in these systems is predicated on the mobility of the polymer host and conductivity of the system falls when chain mobility is compromised. For practical applications, it is desirable that the battery system operates at ambient temperature. However, polyethylene oxide crystallizes at temperatures below 65oC and the low mobility of PEO in the crystalline phase results in unacceptable ionic conductivities at room temperature. Apart from the electrical criteria of high ionic conductivity at room temperature, the second major challenge associated with PEO-based electrolytes is mechanical stability requirements. It is desirable that the electrolyte exists in the solid phase under operating conditions. In addition, low mechanical stability promotes the rapid deterioration of cell performance in lithium batteries containing PEO. Theory and experiments attributes this to the growth of Li dendrites in these systems and preliminary results indicate that increasing the modulus of the electrolyte will prevent the nucleation and growth of these dendrites. Since polyethylene oxide is a low strength material, the two major technical problems associated with PEO electrolytes in solid-state rechargeable lithium batteries are its inability to simultaneously satisfy both the electrical criteria of high ionic conductivity at room conditions and mechanical performance requirements of dimensional stability1. Diblock copolymers are molecules wherein two chemically dissimilar polymers are covalently bonded. Melts of diblock copolymers microphase separate into a variety of ordered phases such as spheres arranged on a body centered cubic lattice, cylinders arranged on a hexagonal lattice, gyroid, and alternating lamellae. The purpose of the second block is to organize the channels in a manner that enhances ionic conduction and inhibits dendrite growth by preventing crystallization of the PEO block and providing mechanical strength respectively. We will synthesize our block copolymers by living anionic polymerization. Characterization will be performed with standard methods such as AC impedance, gel permeation chromatography, birefringence, rheometry, X-ray scattering, and electron microscopy. We will employ cyclic voltammetry to determine the limit of electrochemical stability and impedance spectroscopy over a range of temperatures to measure conductivity. We have measured the electrical, mechanical, and morphological characteristics of a series of a poly(styrene-block-ethylene oxide) diblock copolymer PS-b-PEO systems with varying amounts of added PEO homopolymer. Characterization tools used include small angle X ray scattering (SAXS), transmission electron microscopy (TEM), optical birefringence, and rheology. Conductivity measurements were made on mixtures doped with small amounts of LiTFSI (EO:Li =50:1) using AC impedance techniques. Full results will be posted shortly. References:
1.
A.
Mayes et al., J. Electrochem. Soc. 146 32-37
(1999)
2.
R.D. Lundberg, F.E.
Bailey, R.W. Callard, J. Polym. Science 4, 1563 (1966).
3.
B.E. Fenton, J.M.
Parker, P.V. Wright, Polymer 14, 589 (1973)
4.
A. Michel,
5.
A. Michel,
6.
R.
A. Register, Y. Loo, et al., Macromolecules 34, 8968
(2001)
7.
D.S. La, E.S. Sattely,
et al., J. Am. Chem Soc. 123, 7767-7778 (2001)
8.
C.
Burguiere, S. Pascual, C. Bui, et al., Macromolecules 34,
4439-4450 (2001) 9. S.C. Schmidt, M.A. Hillmyer, Macromolecules 32, 4794-4801 (1999)
10.
T. Thurn-Albrecht,
T. Russell, Macromolecules 35, 8106-8110 (2002)
11.
H. Hyeok, H.B. Eitouni,
N.P. Balsara, J.A. Pople, Preprint (2002) 12. C. Decker, Macrol. Chem Phys. 200, 358-367 (1999)
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