Cellphone batteries often heat up and at times can burst into flames. In most cases, the cause can be traced back to lithium batteries. Despite providing long-lasting electric currents that can keep devices powered up, lithium batteries can internally short-circuit, heating up the device.
Researchers have developed a technology that can prevent lithium batteries from heating and failing. The carbon nanotube design for the battery’s conductive plate (the anode) enables the safe storage of a large quantity of lithium ions, thereby reducing the risk of fire. Further, the new anode architecture will help lithium batteries charge faster than current commercially available batteries by preventing lithium from accumulating outside the anode, which over time can cause unintended contact between the contents of the battery’s two compartments — one of the major causes of device explosions.
When lithium batteries are in use, charged particles move between the battery’s two compartments. Electrons given up by lithium atoms move from one side of the battery to the other; lithium ions travel the other direction. When charging the battery, lithium ions and electrons go back to their original compartments. The property of the anode, or the electrical conductor that houses lithium ions within the battery, plays a decisive role in the battery’s properties. A commonly used anode material is graphite. In these anodes, lithium ions are inserted between layers of graphite; however, this design limits the amount of lithium ions that can be stored within the anode and even requires more energy to pull the ions out of the graphite during charging.
Sometimes, lithium ions do not evenly deposit on the anode. Instead, they accumulate on the anode’s surface in chunks, forming tree-like structures called dendrites. Over time, the dendrites grow and eventually pierce the material that separates the battery’s two compartments. This breach causes the battery to short-circuit and can set the device ablaze. Growing dendrites also affect the battery’s performance by consuming lithium ions, rendering them unavailable for generating a current.
Another anode design involves using pure lithium metal instead of graphite. Compared to graphite anodes, those with lithium metal have a much higher energy content per unit mass or energy density. But they too can fail in the same catastrophic way due to the formation of dendrites.
To address this problem, the researchers designed anodes using highly conductive, lightweight carbon nanotubes. These carbon nanotube scaffolds contain spaces or pores for lithium ions to enter and deposit; however, these structures do not bind to lithium ions favorably. The team made two other carbon nanotube anodes with slightly different surface chemistry — one laced with an abundance of molecular groups that can bind to lithium ions and another that had the same molecular groups but in a smaller quantity. With these anodes, they built batteries to test the propensity to form dendrites.
As expected, the researchers found that scaffolds made with just carbon nano-tubes did not bind to lithium ions well. Consequently, there was almost no dendrite formation but the battery’s ability to produce large currents was also compromised. Scaffolds with an excess of binding molecules formed many dendrites, shortening the battery’s lifetime.
The carbon nanotube anodes with an optimum quantity of the binding molecules prevented the formation of dendrites. In addition, a vast quantity of lithium ions could bind and spread along the scaffold’s surface, thereby boosting the battery’s ability to produce large, sustained currents.
The anodes handle currents five times more than commercially available lithium batteries; particularly useful for large-scale batteries, such as those used in electric cars, that require quick charging.