Cheap new alloy may double the anode capacity of lithium batteries
Lithium-ion batteries are at the heart of all kinds of devices, from smartphones and laptops to the ever-growing contingent of electric cars. So there is interest from many quarters in boosting their performance with advanced materials that make them lighter, more compact and able to hold more energy. A new tin-aluminum alloy developed by engineers in Texas might deliver on all three aspects, and it might even make them faster and cheaper to produce at the same time.
For years, mass-produced lithium-ion batteries have relied on graphite and copper for their anodes, the component that stores the energy as the battery charges. And for years, researchers have sought out alternative materials that could overcome the limitations of those materials, which include a high cost of production and limited storage capacity (silicon, for example, could store 10 times as much energy, although it poses another set of problems).
Creating current-day anodes is a laborious, multi-step process where graphite is powder-coated onto copper foil. But as Karl Kreder, a material scientist at the University of Texas at Austin and lead author of the new study explains, this is kind of inefficient in terms of both the manufacturing process and the battery itself.
"So the active material (graphite) is coated on top of the inactive current collector (copper)," he tells New Atlas. "This adds volume and inactive material mass to the system. By combining the current collector and active material together, a higher capacity active material can be used while simultaneously using less inactive current collecting material."
Kreder and his team achieved this with a simplified manufacturing approach that skips a complicated step, the fastidious coating process. The tin is able to be added directly into the aluminum as it is cast into blocks, creating an alloy that can then be mechanically rolled (a relatively cheap and common metallurgcal alloying process) into nanostructured metal foils. And this last step, where the particles within the material are reduced, is critically important.
"Tin is known to alloy with lithium," Kreder explains. "Unfortunately, if tin foil is used or even micrometer-sized tin particles are used, the tin will break apart when cycled due to volume expansion when it alloys with lithium. This means that if you make a battery with large tin particles it will only last for tens of charge-discharge cycles. However, if you make nanometer-sized tin particles the particles will not break apart during alloying."
The researchers call the resulting material an interdigitated eutectic alloy (IdEA) anode, which they say is one quarter the thickness and half the weight of traditional anode material. They tested it by working it into complete smaller versions of lithium-ion batteries, and then charging and discharging them to measure performance. They found that it demonstrated twice the charge storage capacity of a typical copper-graphite anode.
"The reason that this works so well is that one of the elements is active, tin, and the other one is inactive, aluminum," says Kreder. "The aluminum creates a conductive matrix in which the tin is held. The aluminum provides the structure and electrical conduction, while the tin is alloyed and de-alloyed with lithium when the battery is cycled."
A more compact anode, and in effect battery, could mean big things for manufacturers of smartphones, cars, laptops and myriad other devices. And that's to say nothing of the improved performance and cheaper manufacturing process. In their material, the researchers believe they have an early proof-of-concept for new and improved lithium-ion batteries.
"It is exciting to have developed an inexpensive, scalable process for making electrode nanomaterials," says Arumugam Manthiram, a professor and the director of the Texas Materials Institute, who led the team. "Our results show that the material succeeds very well on the performance metrics needed to make a commercially viable advance in lithium-ion batteries."
The research was published in the journal ACS Energy Letters.
Source: University of Texas