Polymer and glass or ceramic solid electrolytes each come with their own set of issues. Polymer electrolytes don’t conduct well at room temperature and need to be heated up. Ceramic electrolytes, on the other hand, do conduct well at room temperature but require a great deal of pressure to maintain contact with the electrodes.

Berkeley Lab battery scientist Nitash Balsara, working with collaborator Joseph DeSimone of the University of North Carolina at Chapel Hill led the effort on the hybrid electrolyte. Balsara was one of the co-founders of battery startup Seeo, founded in 2007 to develop a solid block copolymer electrolyte. Balsara and DeSimone have also co-founded a startup company called Blue Current, which aims to commercialize a perfluoropolyether-based nonflammable electrolyte they developed together (described in a 2014 PNAS paper, Wong et al., earlier post).

The new glass-polymer hybrid was made by taking particles of glass, attaching perfluoropolyether chains to the surface of the particles, adding salt, and then making a film out of these components. By tuning the polymer-to-glass ratio, the researchers were able to come up with a compliant electrolyte with high conductivity at room temperature and excellent electrochemical stability.

The electrolyte is compliant, which means it can readily deform to maintain contact with the electrode as the battery is cycled, and also has unprecedented room temperature conductivity for a solid electrolyte.

—Nitash Balsara

Although the conductivity is not as good as that of a liquid electrolyte, being about 10 to 15 times lower, it’s probably good enough for some applications, Balsara said.

We don’t necessarily need to match a liquid electrolyte because nearly all of the current in the hybrid electrolyte is carried by the lithium ion. In conventional lithium electrolytes only 20 to 30 percent of the current is carried by the lithium ion. Nevertheless, it is likely that playing around with different glass compounds, particle size, and length and concentration of the polymer chains will result in improved conductivity.

—Nitash Balsara

The researchers also demonstrated that their hybrid electrolyte should be stable with two of the most promising next-generation cathode candidates that are being developed, sulfur and high-voltage cathodes such as lithium nickel manganese cobalt oxide.

People would like to use 5-volt cathodes, but electrolytes that are stable against those 5-volt cathodes are not readily available. We have demonstrated this electrolyte is stable at 5 volts, though we have not incorporated the hybrid electrolyte in the cathode yet.

—Nitash Balsara

Further experiments demonstrated that the hybrid electrolyte can be well suited to work with a sulfur cathode, which operates at a relatively low voltage but has the advantages of being high capacity and very inexpensive. A major failure mode in lithium-sulfur cells with conventional liquid electrolytes is the dissolution of intermediate compounds formed as sulfur in the cathode is converted to lithium sulfide into the electrolyte. However, the intermediates were found to be insoluble in the glass-polymer electrolyte.

“Although much work remains to be done, we believe that our work opens a previously unidentified route for developing hybrid solid electrolytes that will address the current challenges of lithium batteries,” the researchers wrote in the PNAS paper.

Funding for the research at Berkeley Lab was provided by DOE’s Office of Science through the Joint Center for Energy Storage Research, a DOE Energy Innovation Hub. Part of the work was done at the Stanford Synchrotron Radiation Lightsource at SLAC National Accelerator Laboratory and at the Advanced Light Source at Berkeley Lab, both DOE Office of Science User Facilities.

Resources

• Dominica H. C. Wong, Jacob L. Thelen, Yanbao Fu, Didier Devaux, Ashish A. Pandya, Vincent S. Battaglia, Nitash P. Balsara, and Joseph M. DeSimone (2014) “Nonflammable perfluoropolyether-based electrolytes for lithium batteries” PNAS 111 (9) 3327-3331 doi: 10.1073/pnas.1314615111