On the way to a better battery: Scientists have developed a new lithium-metal battery that combines a high energy density of 560 watt-hours per kilogram with good stability. Unlike many previous forms of these batteries, a special combination of a cathode and a new type of electrolyte ensures improved durability. As a result, the battery loses little storage capacity up to more than 1,000 charge cycles, the research team reported in the specialized journal “Joule”.
Lithium-ion batteries are the “working horses” of modern technology, but they are only suitable to a limited extent for energy-hungry applications such as electric mobility. Because their energy density is insufficient to give electric cars significantly greater range and less weight. as one possible alternative So use lithium metal batteries. These batteries with an anode made of metallic lithium instead of graphite have twice the energy density.
The problem, however, is that lithium-metal batteries are barely rechargeable so far. After a few charge cycles, the metallic lithium is deposited on the anode in the form of needle-like tips. Lithium sever fracture can also be inactive the siege Form in the electrolyte, in parallel with this form sediment that ruptures holes in the separator. There are specific ways to prevent this layers of protection For the separator, which is doping for the nickel-rich cathode, but also with new electrolytes.
Energy-dense, yet fragile
Wanglin Wu of the Helmholtz Institute in Ulm (HIU) and colleagues developed a combination of these solutions. For the new lithium-metal battery, they used a low-cobalt nickel-rich film cathode (NCM88) and a thin anode made of metallic lithium. Although this mixture achieves a high energy density, it is unstable because the cathode cracks very quickly when it interacts with common electrolytes.
“The electrolyte reacts within these cracks and destroys the structure,” senior author Stefano Passerini of HIU says. “In addition, a thick, alga-like lithium-containing layer forms on the negative electrode.” So far, this has made it impossible to use this battery effectively during more than just one charge cycle.
New electrolyte protects the cathode
To avoid this, the researchers combined these ingredients with new electrolytes. This is a non-volatile, non-flammable electrolyte in which two negatively charged organic compounds, called imides, act as charge carriers. “With the help of this electrolyte, structural changes in the nickel-rich cathode can be greatly reduced,” says co-author Guk-Tae Kim of HIU.
More detailed analyzes showed that when the new electrolyte is used in the battery, it forms a protective layer on the border with the negative electrode. “This layer protects the cathode from destructive interactions with the electrolyte, thus preventing the irreversible phase shift of the hexagonal crystal lattice on its surface,” Wu and colleagues explain. At the same time, this layer ensures that microcracks do not spread further.
Still stable even after a thousand charge cycles
In comparative tests with cells of the same design but with a common organic electrolyte, the scientists examined whether this electrochemistry affects the performance and stability of a lithium-metal battery. The battery cycle delivered 0.63 mV less.
It’s different with integrating with the new electrolyte: Its voltage remained nearly stable during charging and discharging, the team reports. This lithium-metal battery’s storage capacity was initially 214 mAh per gram and dropped to just 88 percent even after 1,000 charge cycles. The Coulomb efficiency, which refers to the ratio between the drawn capacity and the supplied capacity, was also an average of 99.94 percent.
Overall, thanks to the improved cathode and new electrolyte, the lithium metal cell achieved a power density of 560 watt-hours per kilogram (Wh/kg). Combined with the increased stability, this allows future lithium-metal batteries to get a lot closer. (Joule, 2021; doi: 10.1016/j.joule.2021.06.014)
Source: Karlsruhe Institute of Technology
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