Borrowing from Chips to Build Better Batteries
AElectric cars have become far more common over the past decade, but their batteries are still heavy, bulky and expensive. Engineers are therefore searching for designs that can store more energy in a smaller space, so that vehicles can travel further on a single charge without carrying extra weight. One of the most promising ideas is the so-called anode-free lithium battery. A team led by Professors Jinwoo Lee and Hee-Tae Jung at the Korea Advanced Institute of Science and Technology (KAIST) has now found a way to make such batteries last much longer, by borrowing techniques from the manufacture of computer chips.
BA typical lithium battery contains two electrodes, and in most designs one of them, the anode, is made from a material such as graphite. An anode-free battery leaves out this material altogether. Instead, when the battery is charged, lithium is deposited directly onto a very thin sheet of copper foil. Removing the graphite reduces both the weight and the size of the cell, so more energy can be packed into the same space. For car makers, this could eventually mean smaller, lighter battery packs that still offer a longer driving range.
CThe difficulty is that these batteries tend to wear out quickly. Each time the battery is charged, lithium builds up on the copper, and after many cycles it can grow into branching, needle-like structures known as dendrites. Dendrites reduce the battery's performance, and at the same time the protective layer that forms around the lithium can become unstable. Together, these two problems shorten the battery's working life. Earlier attempts to solve them, such as adding extra lithium or applying thicker coatings, made the battery heavier and larger, which cancelled out the main reason for choosing an anode-free design in the first place.
DThe KAIST researchers took a different approach, which works on two levels. First, they used a semiconductor technique called secondary sputtering lithography to carve a regular pattern of microscopic tubes into the surface of the copper. Each tube measures about 300 nanometres across and 150 nanometres high. These tiny features increase the usable surface area to about four times that of a flat sheet. With more space available, lithium is laid down more evenly, and the dangerous spikes are less likely to develop.
ESecond, the team covered the patterned copper with an extremely thin layer of a two-dimensional material called MXene. The coating is only about 10 nanometres thick, which is less than a thousandth of the thickness of a human hair. It acts as a foundation on which a stronger and more stable protective layer can grow while the battery is operating. In the electrolyte used in the experiments, this protective layer is rich in lithium fluoride. It limits unwanted chemical reactions between the lithium and the electrolyte, and it also restricts the growth of dendrites.
FTo confirm what was happening, the researchers examined the surfaces and cross sections of their electrodes while keeping their exposure to air to a minimum. They used several advanced instruments, including X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry and transmission electron microscopy. These tests showed that an extremely thin protective layer forms evenly across the MXene surface, and they also revealed the process by which it forms.
GAccording to Professor Lee, the study shows that fabrication methods developed for semiconductors can create both evenly spread sites for lithium to settle on and a stable protective layer. Importantly, this is achieved without changing the chemical recipe of the electrolyte or adding surplus lithium. The team believes the approach could speed up the commercial development of high-energy anode-free batteries while keeping any added weight and volume to a minimum. The work was carried out with partners at Kyungpook National University and the National NanoFab Center, and the results were published in the journal Advanced Functional Materials.
HThe project is also a good example of how ideas can travel from one industry to another. Chip makers have spent decades learning to shape materials with extraordinary precision, because smaller features allow more components to fit onto a single piece of silicon. The KAIST team applied this same kind of precision to the inside of a battery cell. The KAIST study was supported in part by national programmes that give scientists access to advanced semiconductor fabrication facilities and that aim to link semiconductor and battery research. Of course, results achieved in a laboratory do not always translate easily into commercial products, and many promising battery designs never reach the market. If the KAIST team's results can be repeated on a larger scale, the tools that made computers faster and smaller may one day help electric cars to travel further as well.