Solid-State Lithium Electrolytes: Characteristic of Floating Li Inside of Anion Framework

Abstract

The transition from liquid to solid electrolytes is driven by the need for enhanced safety and higher energy density in advanced batteries. Solid-state electrolytes (SSEs) eliminate flammability and leakage risks but suffer from low ionic conductivity at ambient conditions due to lattice constraints and high migration barriers. Breakthroughs in SSEs materials such as Li₁₀GeP₂S₁₂ (LGPS), Li₇La₃Zr₂O₁₂ (LLZO), and Argyrodite-type Li₆PS₅Cl reveal a unique phenomenon: lithium ions exhibit “floating” behavior within a stable anionic framework, enabling quasi-fluid migration through interconnected channels. This work explores the physicochemical nature of “floating Li,” emphasizing weak interactions, multi-path coupling, and framework flexibility as key factors reducing migration barriers. We further propose an electronic-density-based approach using the interaction region indicator (IRI) to extract characteristic descriptors for high-conductivity SSEs. Comparative analysis of IRI maps across different electrolytes demonstrates distinct patterns associated with low-electron-density migration channels. These insights establish a paradigm shift from single-path models to networked migration behavior and suggest that integrating chemical bonding theory, lattice dynamics, and data-driven screening can accelerate the rational design of next-generation solid electrolytes.

Keywords

argyrodite / electron-density character extraction / floating lithium-ions / network-migrations / solid-state electrolytes

References

[1] Q. Zhao, S. Stalin, C. Z. Zhao, and L. A. Archer, “Designing Solid-State Electrolytes for Safe, Energy-Dense Batteries,” Nature Reviews Materials 5, no. 3 (2020): 229-252.

[2] T. K. Schwietert, A. Vasileiadis, and M. Wagemaker, “First-Principles Prediction of the Electrochemical Stability and Reaction Mechanisms of Solid-State Electrolytes,” JACS Au 1, no. 9 (2021): 1488-1496.

[3] K. Nie, Y. Hong, J. Qiu, et al., “Interfaces Between Cathode and Electrolyte in Solid State Lithium Batteries: Challenges and Perspectives,” Frontiers in Chemistry 6 (2018): 616.

[4] H. Huo and J. Janek, “Solid-State Batteries: From ‘All-Solid’ to ‘Almost-Solid,” National Science Review 10, no. 6 (2023): nwad098.

[5] J. Sung, J. Heo, D. H. Kim, et al., “Recent Advances in All-Solid-State Batteries for Commercialization,” Materials Chemistry Frontiers 8, no. 8 (2024): 1861-1887.

[6] F. Zheng, M. Kotobuki, S. Song, M. O. Lai, and L. Lu, “Review on Solid Electrolytes for All-Solid-State Lithium-Ion Batteries,” Journal of Power Sources 389 (2018): 198-213.

[7] I. Hanghofer, M. Brinek, S. L. Eisbacher, et al., “Substitutional Disorder: Structure and Ion Dynamics of the Argyrodites Li6PS5Cl, Li6PS5Br and Li6PS5i,” Physical Chemistry Chemical Physics 21, no. 16 (2019): 8489-8507.

[8] N. Kamaya, K. Homma, Y. Yamakawa, et al., “A Lithium Superionic Conductor,” Nature Materials 10, no. 9 (2011): 682-686.

[9] R. Murugan, V. Thangadurai, and W. Weppner, “Fast Lithium Ion Conduction in Garnet-Type Li₇La₃Zr₂o₁₂,” Angewandte Chemie International Edition 46, no. 41 (2007): 7778-7781.

[10] D. Tapler, B. Gadermaier, J. Spychala, et al., “Unraveling Ultrafast Li-Ion Dynamics in the Solid Electrolyte LiTi₂(PS₄)₃ by NMR Down to Cryogenic Temperatures,” Journal of the American Chemical Society 147, no. 23 (2025): 20023-20032.

[11] Y. Mo, S. P. Ong, and G. Ceder, “First Principles Study of the Li₁₀GeP₂S₁₂ Lithium Super Ionic Conductor Material,” Chemistry of Materials 24, no. 1 (2012): 15-17.

[12] Z. Xu, X. Chen, H. Zhu, and X. Li, “Anharmonic Cation–Anion Coupling Dynamics Assisted Lithium-Ion Diffusion in Sulfide Solid Electrolytes,” Advanced Materials 34, no. 49 (2022): 2207411.

[13] K. Jun, Y. Chen, G. Wei, X. Yang, and G. Ceder, “Diffusion Mechanisms of Fast Lithium-Ion Conductors,” Nature Reviews Materials 9, no. 12 (2024): 887-905.

[14] Y. Kato, S. Hori, and R. Kanno, “Li10GeP2S12-Type Superionic Conductors: Synthesis, Structure, and Ionic Transportation,” Advanced Energy Materials 10, no. 42 (2020): 2002153.

[15] R. Guo, Y. Zhong, P. Yu, et al., “Activating Forbidden Intercage-Ionic-Diffusivity by Anion-Gradient-Disordered Interphase for Ultrastable Argyrodite-Based All-Solid-State Lithium Metal Batteries,” Small 21, no. 23 (2025): 2500764.

[16] E. Zhao, L. He, Z. Zhang, et al., “New Insights Into Li Distribution in the Superionic Argyrodite Li₆PS₅Cl,” Chemical Communications 57, no. 82 (2021): 10787-10790, https://doi.org/10.1039/D1CC03083C.

[17] D. H. S. Tan, E. A. Wu, H. Nguyen, et al., “Elucidating Reversible Electrochemical Redox of Li6PS5Cl Solid Electrolyte,” ACS Energy Letters 4, no. 10 (2019): 2418-2427.

[18] T. Lu and Q. Chen, “Interaction Region Indicator: A Simple Real Space Function Clearly Revealing Both Chemical Bonds and Weak Interactions,” Chemistry–Methods 1, no. 5 (2021): 231-239.

[19] M. K. Tufail, P. Zhai, M. Jia, N. Zhao, and X. Guo, “Design of Solid Electrolytes With Fast Ion Transport: Computation-Driven and Practical Approaches,” Energy Material Advances 4 (2023): 0015.

[20] M. Wu, Z. Wei, Y. Zhao, and Q. He, “Recent Applications of Theoretical Calculations and Artificial Intelligence in Solid-State Electrolyte Research: A Review,” Nanomaterials 15, no. 3 (2025): 225.

[21] S. Park and J. Kang, “Atomistic Insights Into Room-Temperature Ion Conduction Mechanisms in Li₁₀GeP₂S₁₂ Via Machine Learning Interatomic Potentials,” Journal of Applied Physics 138, no. 13 (2025): 135001.

[22] S. Zhang, J. Ma, S. Dong, and G. Cui, “Designing All-Solid-State Batteries by Theoretical Computation: A Review,” Electrochemical Energy Reviews 6, no. 1 (2023): 4.

[23] I. Hanghofer, M. L. Brinek, S. Eisbacher, et al. Substitutional Disorder: Structure and Ion Dynamics of the Argyrodites Li₆PS₅Cl, Li₆PS₅Br and Li₆PS₅I. Published Online April 17, 2019.

[24] L. Zhou, N. Minafra, W. G. Zeier, and L. F. Nazar, “Innovative Approaches to Li-Argyrodite Solid Electrolytes for All-Solid-State Lithium Batteries,” Accounts of Chemical Research 54, no. 12 (2021): 2717-2728.

[25] T. D. Kühne, M. Iannuzzi, M. Del Ben, et al., “CP2K: An Electronic Structure and Molecular Dynamics Software Package—Quickstep: Efficient and Accurate Electronic Structure Calculations,” Journal of Chemical Physics 152, no. 19 (2020): 194103.

[26] T. Lu, “A Comprehensive Electron Wavefunction Analysis Toolbox for Chemists, Multiwfn,” Journal of Chemical Physics 161, no. 8 (2024): 082503.

RIGHTS & PERMISSIONS

2026 The Author(s). Battery Energy published by Xijing University and John Wiley & Sons Australia, Ltd.