LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on the hot topics in materials research. My name is Laura Leay. If you’ve been paying attention to the energy industry and global sustainability goals, you’ll know that there is a race to develop alternative means of powering transportation. Electric vehicles are rising in popularity but one challenge for this technology is heavy batteries with limit capacity. New research has made a vital step towards batteries with an improved energy density. Whereas batteries that are currently in widespread use are made with an excess of lithium, these batteries would have zero excess lithium and to do it, they used a nanotwinned copper foil which so far has only been looked at for other applications.
DOMINIC BRESSER: I think we have been the first to investigate this for battery applications.
LAURA LEAY: That was Dominic Bresser from the Helmholtz Institute Ulm and the Karlsruhe Institute of Technology in Germany. In existing lithium-ion battery technology the positive electrode is made from lithium and some transition metal oxide. Lithium is intercalated into the graphite structure which forms the negative electrode and sits on top of a current collector. In a zero excess battery, graphite is not required. Instead, metallic lithium is coated directly onto the current collector.
DOMINIC BRESSER: The idea here is that you theoretically you don’t have any lithium metal at the negative electrode to start with. So no reservoir; no excess. And at the same time you also don’t have graphite. So you really plate—you take the lithium from the cathode side and you plate it as metallic lithium at the negative electrode. Your battery cell really contains only the amount of lithium that you would really, ideally, need.
LAURA LEAY: At the negative electrode the current collector is made of a copper foil. Dominic’s team used a nanotwinned copper foil, which has an essentially pure, single orientation and dense twin boundaries. They found that lithium deposits more densely and much more homogenously on this nanotwinned copper foil than on commercial foils.
DOMINIC BRESSER: Seeing then how much more homogeneous the lithium deposit was on the nanotwinned copper foil compared to the conventional copper foil, this was a little surprising. The extent of the improvement was more than I was expecting. The denser structure means less surface area. And less surface area, or less surface, means less reaction with the electrolyte.
LAURA LEAY: This increases the reversibility of the lithium plating process, which means that the battery was stable after multiple charge cycles. The zero excess lithium metal plated onto the nanotwinned copper foil also suppressed dendrite formation. In older lithium metal batteries, dendritic structures of metallic lithium can grow large enough to bridge between the positive and negative electrodes, forming a short circuit, which leads to high temperatures and the possibility of a fire or an explosion. So, given that the nanotwinned copper foil avoids this potentially dangerous scenario it could solve many challenges. It’s also a novel approach.
DOMINIC BRESSER: The nanotwinned copper foil, we consider this important because usually people have only looked so far at optimized electrolyte compositions for zero excess lithium metal batteries, or they have looked at specific functional layers on top of the copper electrode. But nobody has looked at the electrode at the copper foil, the substrate, as such.
LAURA LEAY: Dominic is keen to stress that the work of his team is not a magic bullet and there is still more work to do.
DOMINIC BRESSER: Our work might contribute but it does not solve, certainly, the whole—I mean, all the challenges related to this concept.
LAURA LEAY: This work was published in a recent issue of ACS Applied Energy Materials. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on twitter, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.