As the world builds larger wind and solar power systems, the demand for large backup systems for the economy is growing rapidly to provide electricity when the sun sets and the air is calm. Today's lithium-ion batteries are still too expensive for most such applications, and other options such as pumped storage require specific terrain that is not always available.
Now, researchers at MIT and elsewhere have developed a new type of battery, made entirely of rich and inexpensive materials, that can help fill the gap.
MIT professor Donald Sadoway and 15 others wrote in Nature describing the new battery architecture, which uses aluminum and sulfur as two electrode materials, with a molten salt electrolyte in the middle.
"I want to invent something better and better than lithium-ion batteries, for small fixed storage, and ultimately for cars [for a purpose]," explains Sadoway.
In addition to being expensive, lithium-ion batteries also contain flammable electrolytes, making them less suitable for transportation. Therefore, Sadoway studied the periodic table, looking for cheap, earth-rich metals that could replace lithium. He then discovered that the earth's richest metal aluminum had the electrochemical properties suitable for efficient batteries. After many experiments, they eventually had three ingredients combined with cheap and easy access to —— aluminum, no different from supermarket foil; sulfur, usually waste from processes such as petroleum refining; and widely available salt."The ingredients are cheap and safe to —— and it doesn't burn," Sadoway said.
In their experiments, the team showed that batteries can withstand hundreds of cycles at extremely high charging rates, and that each battery is expected to cost about a sixth of that of similar lithium-ion batteries. They show that the charging rate depends largely on the operating temperature, with 110 degrees Celsius (230 degrees F) charging 25 times as fast as 25 degrees Celsius (77 degrees F).
Surprisingly, the team chose the molten salt as the electrolyte just because of its low melting point, proving to have an accidental advantage. One of the biggest problems with battery reliability is the formation of dendrites, narrow metal spikes formed on one electrode that will eventually grow to the other electrode, causing a short circuit and affecting efficiency. But this particular salt, which occurs, is very good at preventing this failure.
Their choice of chloroaluminates "basically eliminates these runaway dendrites, while also allowing for very fast charging," Sadoway said."We experimented with a very high charging rate, for less than a minute, and we never lost the battery by a short circuit to the dendrites.”
More importantly, the battery does not require an external heat source to maintain its operating temperature. Heat is naturally generated electrochemically through the charging and discharge of the battery. He said the new battery formula is ideal for the size of a single home or small and medium-sized business power substation, generating tens of kilowatt-hours of storage capacity.
The research team includes members from Peking University, Yunnan University, and Wuhan University of Technology in China; the University of Louisville, Kentucky; the University of Waterloo, Canada; the Oak Ridge National Laboratory, Tennessee; and the Massachusetts Institute of Technology. This work was supported by the MIT Energy Initiative, the MIT Despanander Center for Technological Innovation, and the ENN Group.