In the quest to build batteries that will store electricity at prices we can afford, neither polymer nor ceramic electrolytes are proving satisfactory. A new solution combining features of both might bridge the gap to the clean energy future.
Batteries require two electrodes – an anode and a cathode – and an electrolyte to carry charge between them. Producing a trio that can operate for thousands of cycles, under required conditions, at an affordable price is among the great technological challenges of our age.
Professor Nitash Balsara of the Lawrence Berkeley National Laboratory is working on the electrolyte part of the equation. Currently, most rechargeable batteries use liquid electrolytes, but these run the risk of bursting into flames and have side reactions that reduce their capacity with time.
In theory, solid electrolytes could be better, being both less flammable and less vulnerable to the products of side reactions migrating to electrodes. However, polymer versions work best at problematically high temperatures. Electrodes move as they are charged and discharged and ceramic or glass electrolytes lose contact in the process unless pressure is applied. "It needs something like 1 ton over every square centimeter [0.16 square inches], so you need a big truck sitting on the battery as it cycles," Balsara said in a statement.
In the Proceedings of the National Academy of Sciences Balsara and his coauthors reveal a hybrid that appears to resolve these problems. Its capacity to conform to the electrodes, its conductivity at room temperature, and its efficiency and stability makes it, they argue, a strong contender to be the electrolyte of the future. “Our work opens a previously unidentified route for developing compliant solid electrolytes that will address the challenges of lithium batteries,” the authors write.
The hybrid is made by attaching perfluoropolyether chains to the surface of small glass particles and adding salt. A film made from the product is stable and highly conductive at room temperature. By adjusting the ratio of polymer to glass, the authors were also able to make it bind to the electrodes without pressure.
One of the problems battery engineers face is the potential to improve one of the components, only to find what they have made is incompatible with better versions of the parts it needs to work with. However, when tested in combination with next-generation cathode candidates, the hybrid works, even at voltages where most other electrolytes fail.
Although sulfur cathodes cannot operate at the high voltages of some competitors, their low cost and high capacity inspire hope that they may play a major role in future batteries. Unlike many alternatives, the glass/polymer hybrid minimizes lithium polysulfide dissolution, making it, the authors argue, “ideally suited for lithium-sulfur cells.”
Balsara acknowledges that, so far at least, the perfect electrolyte has not been achieved, since his product is still substantially less conductive than liquid versions. Nevertheless, heclaimed: “It's probably good enough for some applications.” Moreover, he hopes that more tweaking of the sizes and concentrations of components could improve conductivity further.