LBL Scientists Invent New Class of Polymer Cathodes for Batteries

Spring 1990

By Jeffery Kahn, [email protected]

A battery can be made as large as the imagination, but the energy it stores is limited by the laws of chemical thermodynamics. No matter the genius of the inventor, these laws define limits which cannot be exceeded.

The energy theoretically available in a battery is defined by the bonding forces that can be liberated when two electrode compounds react. To build a better battery, inventors either can discover more energetic electrode compound combinations, or can engineer a means to reduce the electrical losses customary in a battery. Even the best cells realize only about 25 percent of the specific energy that, in theory, is available. Specific energy is a measure of a battery's energy output in relation to its weight.

Recently, LBL researchers invented a novel new class of polymer cathodes that makes possible a new family of rechargeable batteries. On paper, these cells have from double to triple the specific energy values of state-of-the-art commercial batteries. In laboratory tests, prototypes deliver considerably more energy in relation to their size and weight than any commercial rechargeable electrochemical cell.

The test cells also retain their staying power. They perform like new with virtually no loss of energy after 100 cycles or recharges, and more than 350 cycles have been demonstrated. Though still in the research stage with many engineering pitfalls ahead, commercial development now appears likely.

The Materials and Chemical Sciences Division's Lutgard C. De Jonghe and Steven Visco invented the new cells along with graduate student Meilin Liu and visiting French scientist Michel B. Armand.

"These cells can be made for all types of uses, from the sustained low power demands of a watch, to the high power demands of electric vehicles. By choosing from among our new class of positive electrode materials, we can vary the cell's performance," says De Jonghe, who also is a materials science professor at UC Berkeley.

The new cells combine cutting edge features such as solid state thin films and a lithium anode with the unique new cathode materials.

Lithium, a silvery-white metallic element, is the lightest metal. Highly reactive, it is used as an anode in nonrechargeable commercial coin cell-size cells. Despite enormous potential, lithium's promise has been limited by the lack of a complimentary cathode material that can endure through the hundreds of cycles necessary in a first class rechargeable cell.

De Jonghe, Visco, and company have discovered a class of cathode compounds which react with lithium, and which are readily rechargeable. In the inventors' words, they're solid redox polymer electrodes. Journalists who have written about the discovery call them plastic. Indeed, says Visco, the compounds are plastic, and are used commercially as fuel lines and window seals, but are new in a battery.

The key ingredient in the cathodes is a class of organo-sulfur compounds. These particular organo-sulfur compounds are disulfide polymers, very large molecules that consist of identical building blocks which are connected together in long chains through sulfur- sulfur bonds.

Electrical energy is produced when electrons released from lithium oxidation cleave these sulfur-sulfur bonds in the polymer, depolymerizing the cathode. To recharge the cell, the process is reversed and the molecules are rejoined. This unique depolymerization-polymerization reaction has never before been used in a battery.

Sulfur-sulfur chemical bonds, which are broken and rejoined, are at the root of this depolymerization-polymerization process. While unique in a battery, a related reaction is commonplace in many life forms. Sulfur-sulfur bonds often are fundamental to the folding together of chains of amino acids into proteins.

Like many scientific discoveries, the new cells were developed in a roundabout fashion. Researchers were searching for a way to lower the impracticably high operating temperature of another cell.

Recounts Visco, "The starting point was this very promising sodium-sulfur cell for electric vehicles that operates at 350 C. The operating temperature is so high because of the sulfur cathode. We were experimenting with replacement cathode materials, something that would operate at a lower temperatures in a liquid state with the sodium anode. That's how we got into disulfides."

Many disulfides were tried, but the number of times these disulfude cathodes could be recharged was poor because they reacted with solvents used in the electrode. Then it dawned on scientists to try a new approach. Instead of liquids, they focused on solids. An all solid-state thin film cell has inherent stability advantages including long cycle life and negligible self-discharge.

"We were looking at disulfide polymers, and found them to be very stable in combination with polyether electrolytes. Very rapidly, things started to go right," says Visco.

One polyether electrolyte, polyethylene oxide, similar to the material found in kitchen plastic wrap, was found to perform particularly well. The compound is multipurpose. It serves both as the electrolyte and the separator between the electrodes, conducting an internal flow of ions within the battery. And, it also allows the transport of ions within the cathode.

The cells created by De Jonghe, Visco, Liu, and Armand are continuing to evolve. To date, more than 15 disulfide polymers have been evaluated, with hundreds of useful compounds yet to be examined. De Jonghe notes that different compounds have different electrochemical characteristics, allowing various cell types to be made for a range of applications. Some compounds are more suitable for ambient temperature operation. Others operate at 80-100 C and could provide substantially more energy and power than the room- temperature versions.

As researchers explore commercial opportunities and continue research and development efforts, their quest for a workable sodium battery has not been forgotten.

"Ultimately," says De Jonghe, "these batteries with the new cathodes might use sodium rather than lithium. Sodium is even cheaper and more plentiful than lithium. In a battery to power a car, sodium might be preferable. The new cathodes will work with sodium if we can find a polymer electrolyte separator which won't react with sodium. That, however, is a whole new problem."