Why is lithium battery life diminished by repeated charge-discharge cycles? Lawrence Berkeley National Laboratory materials chemist Guoying Chen thinks the answer has to do with a phenomenon called particle cracking.

Guoying ChenShe led a team of researchers who studied a lithium manganese nickel oxide cathode, which is viewed as one of the next generation battery materials because of its high energy. The problem is that it has a unique high charge and discharge voltage. This also provokes enhanced reactivity from the electrolyte and leads to a less stable battery.

In order to carry out the study, researchers made single crystals a few microns in size rather than using commercial grade particles, which often have built-in variations. "With our samples we could carry out analysis based on observation of individual particles without worrying about contributions from other random factors not in our control, such as grain boundaries and porosity," says Chen.

The key process is phase transformation, which occurs when lithium comes out of the particles as the battery is being charged and goes back when it is discharged.

The researchers used transmission microscopy imaging combined with x-ray absorption analysis to map out the chemical and phase distribution on their particles at a very high spatial resolution. They discovered that the charge/discharge cycling involved multiple phases simultaneously on the same particle. The impact of volume differences between the phases, a reduction of more than 6% in total, caused the particles to crack, a phenomenon that becomes more significant as the particle approaches the fully delithiated state.

"If you have cracking, it means fresh surface keeps getting exposed, thus causing more reactions with the electrolyte, which consumes the electrolyte and reduces the lifetime of the battery," Chan says. "If we can minimize or eliminate the cracking issue, we will probably see much improved stability."

The researchers are exploring two ideas to minimize the cracking: smaller but optimally sized particles and avoiding fully charging them.