Lithium-ion batteries continue to be the most effective way to store as much energy as possible in the most compact space with a minimum of weight. Scientists aim to increase energy density, power density, safety and lifetime of such batteries. Here, the electrode materials are crucial.
Anodes in lithium-ion batteries use an active material - predominantly graphite - to store energy in the form of chemical bonds. However, graphite anodes have a low charging rate and also display safety problems. As alternative active materials, lithium titanate oxide (LTO) has already been commercialized. LTO anodes offer a higher charge rate and are considered to be safer than those using graphite. However, such lithium-titanate batteries tend to have lower energy densities.
With lithium lanthanum titanate with perovskite crystal structure (LLTO), a team headed by Professor Helmut Ehrenberg, director of the Institute of Applied Materials - Energy Storage Systems (IAM-ESS) at the Karlsruhe Institute of Technology (KIT) has now investigated another promising anode material. The study, which was jointly accomplished with scientists from Jilin University in Changchun, China, and other research institutes in China and Singapore, showed that LLTO anodes have a lower electrode potential compared to commercialized LTO anodes. As a result, a higher cell voltage and a higher capacity can be achieved.
"Cell voltage and storage capacity ultimately determine the energy density of a battery," Ehrenberg explains, adding: "In the future, LLTO anodes could enable extremely safe and durable high-performance cells. This study contributes to the work of the research platform for electrochemical storage CELEST (Center for Electrochemical Energy Storage Ulm & Karlsruhe), one of the largest battery research platforms worldwide, in which the POLiS cluster of excellence is also included.
In addition to energy density, power density, safety and operating lifetime, the charging rate also determines how suitable a battery is for demanding applications. Basically, maximum discharge current and minimum charging time depend on the ion and electron transport in the solid state material and at the interfaces between electrode and electrolyte. To improve the charging rate, it is common practice to reduce the particle size of the electrode material from the micrometer scale to the nanometer scale. But as this study, published in the journal Nature Communications, shows, even perovskite structured LLTO particles of a few micrometers in size enable a higher power density and a higher charging rate than LTO nanoparticles.
The research team attributes this to the so-called pseudo-capacitive properties of LLTO: This anode material is not just able to store individual electrons, but also charge-bearing ions, which are bound by weak forces and can reversibly transfer charges to the anode. "Thanks to the larger particles, LLTO enables fundamentally simpler and more cost-effective methods for manufacturing electrodes," explains Ehrenberg.
Lu Zhang, et al.: Lithium lanthanum titanate perovskite as an anode for lithium ion batteries. Nature Communications, 2020. DOI: 10.1038/s41467-020-17233-1