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KFM: Fachverband Kristalline Festkörper und deren Mikrostruktur

KFM 7: Materials for the Storage and Conversion of Energy (joint session MM/KFM)

KFM 7.2: Talk

Monday, March 18, 2024, 12:00–12:15, C 264

Design criteria for zero-strain cathode materials of the tungsten bronze type compounds A_xFeF₃ (A=Li, Na, K) by first-principles — •Aljoscha F. Baumann¹, Daniel Mutter², Daniel F. Urban^1,2, and Christian Elsässer^1,2 — ¹Freiburg Materials Research Center, 79104 Freiburg, Germany — ²Fraunhofer IWM, 79108 Freiburg

Mechanical stresses in the microstructure of cathode materials during charge/discharge cycles can reduce the long-term stability of intercalation-type alkali-metal-ion batteries. In this context, crystalline compounds exhibiting zero-strain (ZS) behavior are of particular interest. For instance, near zero-strain sodiation was experimentally measured in the tetragonal tungsten-bronze (TTB) type compound Na_xFeF₃. [Han, J. Mater. Chem. A, 4, 7382] By atomistic simulations, using a first-principles method based on density functional theory, we investigated the potential of iron-based fluoride compounds with tungsten-bronze structures as ZS cathode materials. The simulations were conducted systematically to study the intercalation of the alkali metal ions Li⁺, Na⁺, and K⁺ into the TTB and two related tungsten-bronze structures of the perovskite and hexagonal types. As the alkali-metal ions intercalate, the oxidation state of the Fe ions decreases, which leads to an enlargement of their surrounding Fluorine octahedra. We found that this effect can be partially compensated by the volume reduction of the F- polyhedra surrounding the alkali ions. We discuss the structural and chemical prerequisites of the host lattice for enabling a ZS insertion mechanism for ions in crystals.

Keywords: Alkali-metal-ion intercalation battery; Electrode materials; Zero-strain; Tungsten bronze; Atomistic simulation