Ars Technica reports on a new sort of cathode that’s made of materials that are abundant (therefore cheap), that store electrons and let them move rapidly (so it’s a good conductor), and that repairs the usual degradation that makes other batteries wear out over time:
The researchers ultimately targeted a material with the formula Li1.3Fe1.2Cl4. Simulations showed that it would form a material that places the iron and chloride at the center of structures that looks like two four-sided pyramids placed with their bases facing each other (gamers would recognize this as a d8). Each of these had a variable number of lithium atoms at each of the corners of these structures, and molecular simulations showed that lithium ions could readily move between these locations, allowing the material to shuffle ions around the material rapidly. These locations also give the ions a place to sit when stored.
Making it involved creating a mix of lithium chloride and two different formulations of iron chloride. They were pulverized and mixed by rapid rotation with a bunch of solid balls, and the pulverized mixture was then heated overnight at 200°C. The result was a material that could be incorporated into batteries.
When put to use in a test setup, the material had a similar energy density to iron phosphate cathodes, which are noted more for their durability than energy density. Somewhat unusually, it actually maintained more of its capacity when charging at higher rates (most materials do better at a slower rate of charge). And it was quite durable, retaining over 90 percent of its capacity after 3,000 cycles when charged and discharged at a rate that would fill the battery in under 15 minutes. (Again, capacity decayed more rapidly at lower charging rates.)
The material’s conductivity wasn’t great, but the researchers found they could improve it by mixing in some conductive carbon (about two percent by weight). In addition, they showed that it could be layered on top of a high-capacity cathode material, acting as a solid-state electrolyte that both allows ions to flow through and stores them if the capacity of the cathode material is saturated.
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The melting point also changes; combined with the heat associated with charge/discharge, this may contribute to a key change in the material’s properties: The material undergoes a transition from brittle to ductile, meaning it’s easier to deform. This ductility made the material self-healing. “Cracks and voids present in the pristine Li1.3Fe1.2Cl4 electrode are completely healed upon charging,” the researchers wrote. That self-healing is almost certainly the primary factor behind the ability of this cathode material to hold on to 90 percent of its capacity after the equivalent of 10 years of daily charging.
Beyond the self-healing, there’s a lot to like about this material. It’s fast charging, has reasonable capacity, and uses cheap and abundant raw materials. The biggest drawback is probably the manufacturing process described in the paper, as it’s hard to scale the pulverization process that was used in the lab. The researchers have an idea about how to do better, but it’s still not clear how readily this material can be incorporated into battery manufacturing.