If electric cars are to go beyond the petrol engine, the battery must be improved. The traditional lithium-ion battery has the highest weight energy density and can only be charged to about 50% of the theoretical capacity. That didn’t work when researchers tried to load more lithium batteries into the battery’s electrodes. After the first discharge/recharging cycle, the electrode began to deteriorate rapidly and no one could figure out how to prevent it.
Now there’s a clue. Researchers combined with theory of computer modeling and the advanced X-ray method, first discovered atomic way of charging by the rearrangement of atoms with electrons in the atoms and the chemical structure of battery storage between relations. This insight should provide a blueprint for battery makers to build a rich lithium electrode that can significantly improve battery performance.
Lithium ion batteries, which make full use of their potential, could increase the use of electric vehicles today by a third or more. For example, a tesla model S equipped with the company’s P100D battery pack can drive 315 miles (about 500 kilometers) at a time, up to 473 miles. Or car makers can stay within 315 miles, but lower prices and gas-powered vehicles do not have tax rebates.
“The dream is to create a affordable mass-market electric car with the same price as the petrol equivalent. Then, consumers start to save gas from day one, and everyone will switch to electricity, says doctoral student William Gent. Stanford chemistry student, the first author of the study, appeared today in Nature Communications.
Ghent works with Stanford university researcher professor William Chueh and researchers from Lawrence Berkeley national laboratory advanced light source.
Technically, the traditional lithium-ion battery is fairly simple. They have two electrodes – a positive cathode and a negatively charged anode – and there are liquid electrolytes between them. The cathode is composed of lithium and transition metals (namely nickel, manganese or cobalt).
When the battery is charged, the lithium ions move from the positive pole through the liquid electrolyte and then insert into the material that makes up the negative electrode. Transition metal ion retention. The same is true of electrons, but they pass through the circuit on the way to the negative pole. When the battery is discharged, the ions and electrons travel in opposite directions.
Lithium – rich batteries replace some of the transition metals in the electrodes with lithium. While the extra lithium is likely to increase the cathode capacity by 30 to 50 percent, there are some mysterious voltage behaviors. For example, even at low current, the average charging voltage is higher than the average discharge voltage. In a perfect battery, says Gent, the voltage is the same.
Moreover, after the charging and discharge cycle, the voltage gradually decreases. The electronic device cannot manage such erratic voltage behavior because the circuits cannot be recalibrated dynamically to handle these changes, Ghent said. That’s why lithium – rich electrodes are so impractical.
When looking for a solution, previous researchers typically study in the process of charging/discharging cycle ion how to rearrange, or how electronic stored in batteries and chemical structure of the atom. Learning at the same time is very difficult because it requires advanced analytical techniques to get the best picture, and many research teams can’t get the necessary equipment.
Ghent and his colleagues can do just that. They work in the two facilities, each device has a very bright, highly sensitive and precise adjustment of X-ray sources, so that the development of atomic rearrangement affect how electrons within the material storage. They used X-ray diffraction in the Stanford synchronous radiation light source at SLAC to detect changes in the cathode atoms and chemical structures during charge and discharge. In the advanced light source of Lawrence Berkeley national laboratory, they used resonance inelastic X-ray scattering to measure the magnetic and electronic properties of rich lithium materials.
Next, scientists used computer models to test their hypotheses. They confirmed that when a cathode loaded with lithium was charged, the normally reserved transition metal ions in conventional batteries moved. They found that the rearrangement greatly affected the voltage of the electron stored in the cathode. This will not be so bad if the ions return to their original position during discharge. But very little. And each time the battery charges and discharge, the ions move a little, causing the disorder of the atomic structure, causing strange voltage behavior.
“We hope to use this understanding to better control these materials and make them more practical,” Gent said.
He and his colleagues have begun testing different approaches to the problem. One idea is to prevent transition metal ion migration. Another approach is to design the structure to make it easier to move ions back to their original location.