CR1620 battery

Revolutionary breakthrough in CR1620 battery: Graphene "foil" debuts

Lithium-ion batteries, as the name implies, work by moving lithium atoms between the battery's two electrodes. So increasing the battery's capacity lies primarily in finding ways to put more lithium into the electrodes. However, these researchers' tireless efforts have encountered major problems. If lithium is the majority of the electrode material, moving it out can cause the electrode to shrink. Moving it back in can cause lithium to be deposited in the wrong place, shorting the battery. A research team at Stanford University has figured out how to wrap large amounts of lithium in graphene. The resulting structure keeps the lithium in place as it leaves, allowing it to flow back to where it started. Tests of the resulting material (which they call lithium-graphene foil) show that it can give batteries nearly twice the energy density of existing CR1620 battery. An obvious solution to increasing the amount of lithium in the electrode is to simply use lithium metal itself. But this is not the simplest thing to do. Lithium metal is less reactive than other members of the periodic table (sodium and potassium), but it still reacts with air, water, and many electrolyte materials. In addition, when lithium leaves the electrode and returns, there is no way to control where it reforms into metal. After a few charge/discharge cycles, the lithium electrode begins to collapse in layers, eventually cycling enough to short-circuit the battery. To better control how the lithium behaves at the electrode, Stanford has investigated the use of some lithium-rich alloys. For example, lithium forms a complex with silicon, which typically has more than four lithium atoms for every atom of silicon. When the lithium leaves the electrode, the silicon stays behind, providing a structure to bind the lithium when it returns on the other half of the charge/discharge cycle. This solves the lithium metal problem, but it creates a new one: The silicon that stays behind when the lithium runs to the other electrode simply doesn't take up volume when the same electrode is filled with lithium-silicon mixture. It turns out that the electrode expands and contracts during the charge and discharge cycles, putting the battery under physical stress. (Note that the lithium metal electrode disappears completely, which could lead to greater mechanical stress) This seems to put us in a dilemma. Limiting the expansion and contraction of the electrode material seems to require limiting the amount of lithium that moves in and out of the electrode material. This, of course, means limiting the battery's energy density. In the new work, the researchers combined the earlier lithium-silicon work with graphene. Graphene is a one-atom-thick sheet of carbon atoms linked together, and it has many properties that are good for batteries. Powerful enough to easily transfer electricity from lithium as the battery charges and discharges. It's also very thin, which means packing lots of graphene molecules into an electrode doesn't take up a lot of space. Graphene is mechanically strong for this job. To make their electrode material, the team made nanoparticles of a lithium-silicon material. They then mixed them with graphene sheets in an eight-to-one ratio. A small amount of a plastic precursor was added, and the entire mixture was spread out on a block of plastic. Once spread, the polymer precursor creates a polymer film on top of the graphene nanoparticle mixture. This can be peeled off. The resulting material, which they call foil, contains a large number of nanoparticles surrounded by typically three to five layers of graphene. Depending on how thick you make the foil, there can be several layers of nanoparticle clusters, each separated by graphene. The graphene sheets make the material fairly strong, as you can fold and unfold it and still use it as a battery electrode. They also help keep air from reacting with the lithium. Even after two weeks of exposure to air, the foil retained about 95% of its ability to function as an electrode. Lower the fraction of graphene used in the starting mixture, and the electrode loses nearly half its capacity in the same two weeks. It works really well as an electrode. The nanoparticles do shrink when the lithium leaves, but the graphene sheets hold the structure together and keep it from shrinking. Even after 400 charge-discharge cycles, it retained 98% of its original capacity. Perhaps most importantly, when paired with a vanadium oxide cathode, the energy density was just over 500 watt-hours per kilogram. Current lithium-ion batteries are about half that. Normally, it might take a while for research results like this to leave the lab and for companies to start developing them. In this case, however, the head of the research organization already has a startup with batteries on the market. So doing this might take less time for a thorough commercial evaluation. The biggest sticking point might be the cost of graphene. Graphene is still thousands of dollars per kilogram, although it has been coming down, and a lot of people are looking for ways to make it cheaper. If they succeed, then the rest of the components of this electrode are pretty cheap. The manufacturing process seems simple.

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