Current lithium-based batteries are based on intercalation. That is, lithium ions are forced into spaces within an electrode material such as graphite. As a result, much of the battery volume and bulk is devoted to something that does not contribute to the transport of charge between the electrodes, setting limits on the types of energy densities these technologies can reach.
As a result, much research has been done to find ways to remove one of these electrode materials. People have attempted to combine lithium metal electrodes with various materials, while other efforts have attempted to use electrodes in which lithium reacts with air to form lithium oxygen compounds. These worked in some ways, but tended to have issues that greatly reduced their useful life.
However, a recent paper describes a battery that uses lithium metal for one electrode and lithium air for the other electrode. In some measurements, the battery performs decently well over 1,000 charge/discharge cycles.
a lot of problems
The lithium metal problem is well explained. It is very difficult to deposit lithium uniformly over the surface of the electrode. Repeated charge/discharge cycles start with tiny bumps growing into spines called dendrites, where the lithium sticks to charge. Eventually the spine grows until it shorts out the system. The solution is generally thought to be a change in the electrolyte through which the lithium ions move between the electrodes. At least one company says it has developed an electrolyte that will allow lithium metal batteries to run as long as many current technologies.
The lithium-air electrode problem is very different and wide-ranging. The electrode support material must be sufficiently porous to allow air to meet the lithium and maintain it for many cycles. The reactions it hosts must avoid reactions with other substances in the atmosphere, such as water vapor, which can permanently trap lithium at the electrodes. And finally, the electrode must manage potentially complex mixtures of lithium oxides and peroxides that may form during the reaction with oxygen. In many cases, these problems were so severe that test lithium-air batteries died after dozens of cycles.
It’s not clear if there is a single solution to these problems. Also, unlike the lithium-metal counterelectrode, it is not clear whether different electrolytes contribute significantly to the resolution.
It is therefore somewhat surprising that the electrolytes in this new study appeared to help manage reactions with oxygen. But that’s not the only thing that happened with the new battery design.
conductor and catalyst
Basically two stories are needed to understand why this battery seems to work. Let’s start with the lithium air electrode. This he has two components. The first is a porous matrix made of water-repellent material. Embedded within it are catalytic nanoparticles of trimolybdenum phosphide (Mo), which the research group has a long history with.3P). Since molybdenum is relatively cheap, I started researching it in 2019, thinking it might be a good option for splitting water to produce hydrogen. A year later, they considered using it in a lithium-air battery, which also requires rearranging the bonds between the oxygen atoms.
At that point, Mo3P showed exceptional durability and was viable for over 1,200 charge/discharge cycles. However, it was not very energy efficient. For that, we clearly needed better electrolytes.
The electrolyte they used is solid at the temperatures at which batteries operate. It may be difficult to imagine a solid material through which ions can pass. However, some solids have been developed with internal channels large enough for ions to pass through. The interior of these channels contains sites through which ions can interact, allowing them to make short hops from one stable location to another as they pass through. Finally, the density of the channels allows newly arriving ions to spread relatively evenly across the surface of the electrode, avoiding problems such as dendrite formation in lithium metal.
In this case there are additional benefits. Leave the lithium air electrode exposed to air. When the researchers tried the same electrode material in a liquid electrolyte, the chemical reaction that occurred at the lithium-air electrode was only partially completed.
The solid portion of the electrolyte is carbon-based, but contains many oxygen and silicon atoms bonded to the carbon skeleton. These polar atoms help provide something that the lithium ions tend to interact with. The nanoparticles act like relay stations that move between the electrodes.They consist of LiTenGeP2S.12, a material that contains both lithium and atoms that like to interact with lithium. This ensures that the electrolyte is filled with lithium ions even when the battery is not in use, so charge flows the moment the battery is activated.
