High-entropy Li-garnets as new solid-state electrolytes in Li-ion batteries

Research Associate: Benjamin Zimmermann

High entropy oxides (HEOs) are a new class of materials that exhibit outstanding unique and tunable properties, making them promise for future efficient applications for energy conversion and storage. HEOs are complex single-phase oxides consisting of five or more cations in approximately equal amounts. Thereby, the metal cations arrange randomly on the cation lattice of the crystal structure, resulting in high configurational entropy. Even compositions that have a positive enthalpy of formation can be stabilized at a certain temperature when the entropy term of the Gibbs energy becomes larger than the enthalpy term, so-called entropy stabilization, extending the materials engineering space. Since the first publication in 2015 by Rost et al.,^[1] many compositions and crystal structures have been synthesized and their properties have been studied.

A possible application for high entropy oxides is in lithium-ion batteries as solid-state electrolyte. In this context, garnet-like structures are in the focus of recent research due to their excellent stability with lithium metal and good Li-ion conductivity. Another advantage is the improved safety. Most commercial lithium-ion batteries contain organic solvents in which Li salts are dissolved. In case of a short-circuit or high thermal load, these can burn or explode. First reports about the Li-ion conductivity of garnet structures with the nominal composition Li₅La₃M₂O₁₂ (M = Nb, Ta) were made by Weppner et al. in 2003.^[2] Since then, many compounds with the general formula Li_xA₃B₂O₁₂ (X = 5‑7; A = La, Bi, Y, Al; B = Sc, Zr, Ti, Hf, Ta, Nb)^[3,4] have been studied. The focus lies on the cubic modification because it has about a 100 times higher ion conductivity than the tetragonal modification. If the concept of high entropy is applied to these garnet structures, the properties can be further boosted by using different valence ratios on the sublattices and combining ion properties. Another goal is to stabilize the high-temperature cubic structure at room temperature.^[5]

The key aspect of this work is the solid-state and wet-chemical synthesis of multicomponent-oxides in said Li-Garnet system and their characterization. The samples will undergo a variety of measurements including XRD and thermodynamic analysis. The goal is to deepen the understanding of component-property relationships and the influence of synthesis parameters on conductivity and phase properties.

Figure 1: Wet-chemical synthesis of HEOs (top) and solid-state synthesis (bottom).

References:

[1] Rost, C.; Sachet, E.; Borman, T. et al. Entropy-stabilized oxides. Nat. Commun. 2015, 6, 8485.

[2] Thangadurai, V.; Kaack, H.; Weppner, W. J. F. Novel Fast Lithium Ion Conduction in Garnet-
      Type Li₅La₃M₂O₁₂ (M = Nb, Ta). JACS 2003, 86, 437-440.

[3] Fu, Z.; Ferguson, J. Processing and characterization of an Li₇La₃Zr_0.5Nb_0.5Ta_0.5Hf_0.5O₁₂ high‐
        entropy Li–garnet electrolyte. JACS 2022, 10, 6175-6183.

[4] Miara, L. J.; Richards, W. D.; Wang, Y. E. et al., First-Principles Studies on Cation Dopants and
     Electrolyte|Cathode Interphases for Lithium Garnets. Chem. Mater. 2015, 27, 4040-4047.

[5] Zeng, Y.; Ouyang, B.; Liu, J. et al. High-entropy mechanism to boost ionic conductivity. Science
     2022, 378, 1320-1324.