Can we design a battery that cycles forever?
Current rechargeable batteries, from the lead acid batteries introduced in 1850 to the lithium-ion batteries introduced in 1990, cycle about 300 times before noticeable changes in performance are evident. A common feature in all current battery technologies is the presence of a liquid electrolyte for conducting ions between the cathode and the anode. The products of side-reactions that are responsible for limited battery life are amplified as they diffuse rapidly through the electrolyte and contact the active centers within the electrodes. Additional safety problems arise due to the flammable nature of the organic electrolytes used in current lithium batteries. More than 50% of the energy released when a lithium-ion battery catches fire is due to combustion of the electrolyte.
A major focus of the group's activities is the development of a non-flammable solid electrolyte for lithium batteries. The electrolytes used in these studies are block copolymers with randomly oriented co-continuous microdomains. One of the microdomains provides the membrane with the required mechanical properties, while the other, which is generally soft, provides well-defined channels for transport. In contrast to both current liquid and solid electrolytes, we discovered that the lithium ion conductivity of block copolymer electrolytes increases with increasing molecular weight. Using high resolution electron microscopy to image the relatively light lithium atoms, it was found that they are localized in the middle of the conducting microdomains, where they can be more mobile, thus explaining the unique properties of our electrolytes.
Our recent work in this field is focused on developing block copolymers that simultaneously conduct ions and electrons for use in battery electrodes, and quantifying failure mechanisms in solid-state lithium batteries. We are identifying parameters and battery configurations that will enable thousands of cycles. We are motivated by large difference in cyclability of solid-state electronic devices relative to their gas-state predecessors (vacuum tubes).
Can we design materials that stay wet at elevated temperatures?
Fuel cells operating with H2 and air as inputs and electric power and H2O as the only outputs are of particular interest due to their ability to produce power without degrading the environment. Polymer electrolyte membranes with hydrophilic, proton conducting channels embedded in a structurally sound hydrophobic matrix play a central role in the operation of polymer-electrolyte fuel cells. To our knowledge, all moisture-laden polymer membranes become drier when heated in air at constant relative humidity. In contrast, the polymer developed by our lab does the opposite. It sponges up more moisture from its immediate environment as the temperature of the surrounding air rises at constant relative humidity. With this unique property, our polymer exhibits high proton conductivity at elevated temperatures. This is important for fuel cells operation because they become more efficient at higher temperatures. This unusual behavior is due to the presence of nanoscale hydrophilic channels in our membranes. Transmission electron microscopy and neutron scattering techniques were used to image and measure the size of the hydrophilic channels in the presence of water. At this scale, pores were reluctant to give up their moisture even as the temperature increased. For reasons not entirely understood, these extremely small channels continue to grab moisture from the air at temperatures as high as 90 degrees Celsius, a temperature well above the point at which other polymers dry up.
Can we design membranes that enable continuous biofuel production?
Current biofuel production processes are carried out in batch reactors. Reactions are stopped at low biofuel (ethanol and butanol) concentrations due to toxicity effects. The net effect of such fuels on the environment is controversial due to the large amount of energy required to purify the alcohols by traditional means (distillation). Our approach uses block copolymer self-assembly to create ethanol- and butanol-selective transporting pathways in membranes. Our objective is to enable the development of a continuous process for biofuel production.
In a Nutshell
Bruce Garetz (Polytechnic Institute of NYU)
Andrew Jackson (National Institute of Standards and Technology)
Hiroshi Watanabe (Kyoto University)
Andrew Minor (UC Berkeley)
John Pople (Stanford Synchrotron Radiation Lightsource)
Kenneth Downing (LBNL)
Robert Glaeser (LBNL)
Maurice Newstein (Polytechnic Institute of NYU)
Michal Banaszak (A. Mickiewicz University, Poland)
Alex Hexemer (LBNL)
Zhen-Gang Wang (Caltech)