In an article recently published in the journal ACS Materials Letters, researchers presented the construction of medium- and high-entropy polyanionic lithium superionic conductors having argyrodite structure, which crystallizes in the F−43m space group. They also demonstrated the effect of entropy (compositional disorder) on the room-temperature ionic conductivity and activation energy of such polyanionic ceramic materials.
Study: High-Entropy Polyanionic Lithium Superionic Conductors. Image Credit: Immersion Imagery/Shutterstock.com
High-entropy materials (HEMs) have garnered significant attention over the years owing to their tunable and unexpected properties. The introduction of different elements on the same crystallographic site leads to an increase in the configurational entropy, ΔSconfig, in a particular kind of structure, which is the basis of the high-entropy concept.
High-entropy alloys, as well as ceramics, are known to adhere to the high-entropy concept and therefore can be considered to be high-entropy materials. However, polyanionic materials, which are widely utilized as ion conductors and electrode materials, are not yet subjected to the high-energy concept.
Polyanionic-based materials have an interesting characteristic where they can provide a rigid three-dimensional framework for fast diffusion of ions via interstitial space. Ideally, all the sulfide- and ceramic oxide-based Li-ion conductors containing covalently bonded polyanions are used as solid electrolytes in solid-state batteries. Among them, lithium thiophosphates with high ionic conductivity and suitable mechanical properties have significant potential to be placed in contact with various battery components.
Argyrodites, having the general formula Li6PS5X (X=Cl, Br, I), and comprising of [PS4]3-tetrahedra and uncoordinated X- and S2- anions, offer a structural framework to facilitate the diffusion of lithium. Extensive investigations on argyrodite-type materials have been conducted in the past, wherein high ionic conductivity at room temperature was attained as a consequence of the substitution at phosphorus, chalcogenide, and halide sites.
This makes them a suitable candidate to achieve high ΔSconfig and can also aid in understanding the impact of configurational entropy on the transport properties of Li-ion.
Although there are numerous reports present in the literature on enhanced lithium and proton conduction observed in high-entropy rock salt oxides, critical analysis of the effect of ΔSconfig on the charge transport still needs to be performed. Also, in a majority of the works, entropy was introduced through cation mixing only. Hence, anion mixing in the covalent or ionic host lattice sites of the argyrodite structure opens up a new avenue to achieve high entropy.
About the Study
In the present study, the researchers prepared polyanionic materials, namely Li6PS5[Cl0.33Br0.33I0.33] (HEAR1), Li6P[S2.5Se2.5][Cl0.33Br0.33I0.33] (HEAR2), and Li6.5[Ge0.5P0.5][S2.5Se2.5]-[Cl0.33Br0.33I0.33] (HEAR3) through a classical solid-state chemistry route.
Consecutive substitutions on their halide, chalcogenide, and phosphorus sites were performed to vary or increase the configurational entropy and ΔSconfig values were calculated as 1.24R, 2.28R, and 2.98R for HEAR1, HEAR2, and HEAR3, respectively. HEAR2 and HEAR3 are depicted as HEMs based on initial studies of HEAs.
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The structural and elemental properties of the as-grown materials were determined by complementary synchrotron and neutron scattering techniques. Their phosphorus local environments were probed through 31P magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. Electrochemical impedance spectroscopy (EIS) and 7Li pulsed field gradient (PFG) NMR spectroscopy were used to evaluate the Li-ion conductivity of as-prepared materials.
Throughout the structural analysis of the prepared polyanionic materials, an unequal distribution of elements was observed over the respective crystallographic sites. The results show that the fully polyanionic high-entropy argyrodite materials, HEAR1 and HEAR2, have nearly comparable room-temperature Li-ion conductivity and activation energy (from 7 Li PFG NMR spectroscopy).
The ionic conductivity and activation energy both dropped with increasing cation substitution (HEAR3), resulting in the lowest value known for argyrodite-type Li-ion conductors, 0.22 eV. Applying the Meyer-Neldel rule, the researchers found that the activation energy is linearly related to the pre-exponential factor of diffusivity, D0. This shows that as configurational entropy increases, the lattice softens, affecting bulk ion transport in the case of HEAR3.
Furthermore, XRD and NPD demonstrated that all three materials have similar structural properties, such as similar halide/chalcogenide site inversion spanning from 26% to 33% and slight modifications in the Li substructure.
In conclusion, this study presents an effective method for the synthesis of high ΔSconfig multielement substituted lithium argyrodites via anion mixing. It was found that increasing entropy had minimal effect on their room-temperature Li-ion conductivity (~10-3 S cm-1) but significantly lowered the conduction activation energy to 0.22 eV.
The former is most likely due to the analyzed samples' structural similarity. Increasing the number of elements, however, changes or increases both the configurational and vibrational entropies (lattice softening) in polyanionic materials, which can have either synergistic or antagonistic impacts on their charge transport properties.
Overall, the authors hope that the findings will encourage additional research into high-entropy ion conductors, potentially allowing for the finetuning of their electrical and (electro)chemical properties. Given the wide compositional design space accessible for investigation, these enhanced properties present a significant advantage in the quest of Li-ion conductors.
Strauss, F., Lin, J., Duffy, M. et al. High-Entropy Polyanionic Lithium Superionic Conductors. ACS Materials Lett. 4, XXX, 418–423 (2022). https://pubs.acs.org/doi/10.1021/acsmaterialslett.1c00817