Researchers at MIT and Tsinghua University in China have found a way to more than triple the capacity of the anodes, or negative electrodes, of lithium-ion batteries while also extending their lifetime and potentially allowing for faster battery charging and discharging. The new electrode, which makes use of aluminum/titanium "yolk-and-shell" nanoparticles, is reportedly simple to manufacture and is especially promising for high-power applications.

The lithium-ion batteries in our phones, tablets and laptops store their energy-carrying ions inside negative electrodes made of graphite. Other electrode materials could in theory do a far better job by packing in more energy, but these alternatives come with their own drawbacks. Lithium could store about 10 times more energy per unit weight than graphite, but it's prone to short-circuiting and catching fire; silicon and tin could also vastly outperform graphite, but only if the battery is charged at a slow rate, which is rarely practical.

Many of the high-capacity alternatives also tend to expand and contract very noticeably as the greatly increased number of lithium ions travel to and from the electrode with each charge cycle. The repeated deformation exerts a strong mechanical stress that, over time, damages the electrode contacts and reduces the cell's capacity.

The team led by MIT professor Ju Li claim to have found a way around this problem. They have done so by creating nanoparticles with a solid titanium outer shell and an inner aluminum "yolk" that can freely expand and contract within the shell, storing and releasing ions without damaging the structure of the electrode and leading to much longer-lasting, high-capacity batteries.

Aluminium is a low-cost material that, like lithium or silicon, can store much more energy per unit weight than conventional graphite. However, it isn't usually considered a good choice for building lithium-ion batteries because the repeated expansion and shrinkage inside the electrode cause aluminium particles to shed their outer layer. Encasing the aluminum particles within a titanium dioxide shell, however, prevents the shedding, again prolonging the cell's lifetime.

To produce these nanostructures, the researchers began by placing aluminum particles about 50 nanometers in diameter in a solution of sulfuric acid and titanium oxysulfate, a process that coated the nanoparticles in a hard shell three to four nanometers thick. After a few hours in the acid, the aluminum particles shrank down to about 30 nanometers while leaving the outer shell unchanged. This gave the aluminum nanoparticles enough room to collect lithium ions and expand considerably as needed, without damaging the electric contacts of the cell.

In testing, the team found that the outer shells became slightly thicker after 500 charge cycles, but the aluminum particles were hardly damaged, even at very high charging rates. While standard graphite can store approximately 0.35 ampere-hours per gram (Ah/g), the new electrode can reportedly store over three times as much energy per unit mass (1.2 Ah/g) at a normal charging rate. Even at very fast charging rates (six minutes to full charge), the researchers reported a capacity of 0.66 Ah/g, nearly double than normal.

The low cost of aluminium, along with the reportedly simple and scalable manufacturing method, bodes well for the future. Li and colleagues say they have already successfully completed tests for full cells, which they fabricated using lithium iron phosphate for the positive electrode. Once this technology is ready for real world applications, it could lead to batteries that are longer-lasting, more energy-dense and faster-charging than today's cells.