Li+-ion conductivities are greater than those achieved before.
Improved composite solid polymer electrolytes (CSPEs) are being developed for use in lithium-ion power cells. The matrix components of these composites, like those of some prior CSPEs, are highmolecular- weight dielectric polymers [generally based on polyethylene oxide (PEO)]. The filler components of these composites are continuous, highly-Li+- conductive, inorganic fibers.
PEO-based polymers alone would be suitable for use as solid electrolytes, were it not for the fact that their room-temperature Li+-ion conductivities lie in the range between 10-6 and 10-8 S/cm — too low for practical applications. In a prior approach to formulating a CSPE, one utilizes nonconductive nanoscale inorganic filler particles to increase the interfacial stability of the conductive phase. The filler particles also trap some electrolyte impurities. The achievable increase in conductivity is limited by the nonconductive nature of the filler particles.
In another prior approach — the one leading to the present improved CSPEs — one utilizes a highly-Li+-conductive inorganic filler material to increase the effective Li+-conductivity of the solid electrolyte. In prior CSPE formulations following this approach, the highly-Li+-conductive fillers have been in the form of particles. It has been found that Li+-ion conductivity can be increased to about 10-4 S/cm by use of particles, but that the potential for any further increase is limited by the inherently restrictive nature of contacts between particles.
In contrast, in a CSPE of the present type, interparticle contact or the lack thereof is no longer an issue. In a typical application, a CSPE is formed as a film. The highly- Li+-conductive fibers can penetrate the entire thickness of the film and can thereby effectively constitute a relatively-long-distance Li+-transfer tunnel. The Li+-ion conductivity of the film as a whole is thus increased substantially beyond that achievable by use of particles. For example, the figure presents results of conductivity measurements on two CSPEs made from a PEOLiN( SO2CF2CF3)2 polymer electrolyte filled with 20 weight percent of the highly-Li+- conductive compound La0.55Li0.35TiO3.
An additional advantage of using filler fibers is that mechanical properties of the resulting CSPEs are superior to those attainable by use of particle fillers. One disadvantage — at the present state of development — is that relative to particles, fibers are less effective for interface stabilization and trapping of impurities. In contemplated further development, it may be possible to overcome this disadvantage by reducing fiber diameters to the order of nanometers. Other avenues of development could include selection of fiber materials having greater Li+-ion conductivities and finding ways to arrange fibers in velvetlike mats to maximize through-the-thickness conductivities.
This work was done by A. John Appleby, Chunsheng Wang, and Xiangwu Zhang of Texas A&M University for Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Materials category.Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Commercial Technology Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-17470-1.