Li⁺/Na⁺ Hybrid Ion Conduction Mechanism in the Superionic Conductor Li_3-xNa_xZr₂Si₂PO₁₂

Abstract

Hybrid ion conductors that transport multiple ionic conductive species provide a useful platform for understanding how mixed-ion transport governs ionic conductivity within a single phase. However, the controlled introduction of multiple mobile ions into solid-state electrolytes and a mechanistic understanding of their migration within the framework remain challenging. Herein, a skeleton-retained Li⁺↔Na⁺ cationic exchange was used to simultaneously induce Li⁺ and Na⁺ cations into the NASICON-type framework of Li_3-xNa_xZr₂Si₂PO₁₂ (0 < x < 3). We show that the interpenetration of NaO₆ and NaO₈ coordination polyhedra significantly influences the ionic conductivity of hybrid ion conductors. Computational analysis indicates that Na⁺ transfer from octahedral NaO₆ sites to octa-coordinated NaO₈ sites is thermodynamically favorable, accompanied by Li⁺ relocation from NaO₈ to tetrahedral LiO₄ environments at former NaO₆ sites, thereby promoting Li⁺/Na⁺ site segregation. The increased occupation of Na⁺ at NaO₈ sites not only suppresses Na⁺ mobility due to bottleneck limitations but also hinders the formation of a continuous Li⁺ migration network, thereby reducing the room-temperature ionic conductivity from 1.78 to 0.50 mS·cm^-1. Upon re-exchange, Na⁺ in the NaO₈ sites is replaced by Li⁺ in penta-coordinated LiO₅, which re-establish percolating ion-transport pathways for Li⁺ and enable reversible recovery of the overall conductivity. These results reveal a fast dual-ion conduction mechanism enabled by the interpenetrating occupation of Li⁺ and Na⁺ across the available sites. This work opens a new avenue for the development of hybrid ion conductors.

Keywords

Hybrid ion conductor / Li⁺/Na⁺ cationic exchange / Dual-ion transport mechanism / Interpenetrating occupation

References

[1] Yagi S, Ichitsubo T, Shirai Y, Yanai S, Doi T, Murase K, Matsubara E. A concept of dual-salt polyvalent-metal storage battery[J]. J. Mater. Chem. A, 2014, 2(4): 1144-1149. http://doi.org/10.1039/c3ta13668j.

[2] Stoyanova R, Koleva V, Stoyanova A. Lithium versus mono/polyvalent ion intercalation: Hybrid metal ion systems for energy storage[J]. Chem. Rec., 2019, 19(2-3): 474-501. http://doi.org/10.1002/tcr.201800081.

[3] Karlsson C, Zanghellini E, Swenson J, Roling B, Bowron D T, Börjesson L. Structure of mixed alkali/alkaline-earth silicate glasses from neutron diffraction and vibrational spectroscopy[J]. Phys. Rev. B, 2005, 72(6): 064206. http://doi.org/10.1103/PhysRevB.72.064206.

[4] Belostotsky V. Defect model for the mixed mobile ion effect[J]. J. Non-Cryst. Solids, 2007, 353(11-12): 1078-1090. http://doi.org/10.1016/j.jnoncrysol.2006.12.027.

[5] Gombotz M, Hiebl C, Stainer F, Wilkening H M R. Solids with two mobile ions: Proton H⁺ self-diffusion in Li-H exchanged garnet-type Li₆La₃ZrTaO₇ as seen by solid-state ¹H NMR relaxation[J]. J. Phys. Chem. C, 2023, 127(23): 10960-10967. http://doi.org/10.1021/acs.jpcc.3c02330.

[6] Wang Y R, Wang C X, Yi X, Hu Y, Wang L, Ma L B, Zhu G Y, Chen T, Jin Z. Hybrid Mg/Li-ion batteries enabled by Mg²⁺/Li⁺ co-intercalation in VS₄ nanodendrites[J]. Energy Storage Mater., 2019, 23: 741-748. http://doi.org/10.1016/j.ensm.2019.06.001.

[7] Jamnik J, Maier J, Pejovnik S. A powerful electrical network model for the impedance of mixed conductors[J]. Electrochim. Acta, 1999, 44(24): 4139-4145. http://doi.org/10.1016/s0013-4686(99)00128-0.

[8] Stolz L, Hochstaedt S, Roeser S, Hansen M R, Winter M, Kasnatscheew J. Single-ion versus dual-ion conducting electrolytes: The relevance of concentration polarization in solid-state batteries[J]. ACS Appl. Mater. Interfaces, 2022, 14(9): 11559-11566. http://doi.org/10.1021/acsami.2c00084.

[9] Wu B B, Wang S Y, Evans W J, Deng D Z, Yang J H, Xiao J. Interfacial behaviours between lithium ion conductors and electrode materials in various battery systems[J]. J. Mater. Chem. A, 2016, 4(40): 15266-15280. http://doi.org/10.1039/c6ta05439k.

[10] Yu T, Ning W J, Li H Y, Guo S H, Zhou H S. Dual-ion conductors: from liquid to solid[J]. Nanoscale Horiz., 2024, 9(5): 667-674. http://doi.org/10.1039/d4nh00011k.

[11] Lou C J, Liu J, Sun X, Zhang W D, Xu L G, Luo H J, Chen Y J, Gao X, Kuang X J, Fu J P, Xu J, Su L, Ma J W, Tang M X. Correlating local structure and migration dynamics in Na/Li dual ion conductor Na₅YSi₄O₁₂[J]. Proc. Natl. Acad. Sci. U.S.A., 2024, 121(33): e2401109121. http://doi.org/10.1073/pnas.2401109121.

[12] Yan G, Yu S, Nonemacher J F, Tempel H, Kungl H, Malzbender J, Eichel R A, Krüger M. Influence of sintering temperature on conductivity and mechanical behavior of the solid electrolyte LATP[J]. Ceram. Int., 2019, 45(12): 14697-14703. http://doi.org/10.1016/j.ceramint.2019.04.191.

[13] Xu Q, Tsai C L, Song D, Basak S, Kungl H, Tempel H, Hausen F, Yu S, Eichel R A. Insights into the reactive sintering and separated specific grain/grain boundary conductivities of Li_1.3Al_0.3Ti_1.7(PO₄)₃[J]. J. Power Sources, 2021, 492: 229631. http://doi.org/10.1016/j.jpowsour.2021.229631.

[14] Kim J, Kim Y, Kim Y, Kim Y. Novel electrochemical ion-exchange of Li_xNa_3-xZr₂Si₂PO₁₂(LNZSP): a Li-ion conducting solid electrolyte for battery applications[J]. J. Mater. Chem. A, 2025, 13(47): 40776-40787. http://doi.org/10.1039/d5ta06485f.

[15] Kwon G, Gwon H, Bae Y, Jung C, Ko D S, Kim M G, Yoon K, Yoon G, Kim S, Jung I S, Lee S, Kim T, Kim J S, Kim T Y, Kim Y S. Disorder-driven sintering-free garnet-type solid electrolytes[J]. Nat. Commun., 2025, 16: 3256. http://doi.org/10.1038/s41467-025-58108-7.

[16] Zimmermann B, Fuchs T, Westphal J, Janek J, Lepple M. Structural analysis of tin-substituted high-entropy Li-garnet electrolytes for solid-state batteries[J]. ACS Org. Inorg. Au, 2025, 5(3): 211-220. http://doi.org/10.1021/acsorginorgau.5c00021.

[17] Park H, Jung K, Nezafati M, Kim C S, Kang B. Sodium ion diffusion in nasicon (Na₃Zr₂Si₂PO₁₂) solid electrolytes: Effects of excess sodium[J]. ACS Appl. Mater. Interfaces, 2016, 8(41): 27814-27824. http://doi.org/10.1021/acsami.6b09992.

[18] Zhu L, Wang Y W, Chen J C, Li W L, Wang T T, Wu J, Han S Y, Xia Y H, Wu Y M, Wu M Q, Wang F W, Zheng Y, Peng L M, Liu J J, Chen L Q, Tang W P. Enhancing ionic conductivity in solid electrolyte by relocating diffusion ions to under-coordination sites[J]. Sci. Adv., 2022, 8(11): eabj7698. http://doi.org/10.1126/sciadv.abj7698.

[19] Okhotnikov K, Charpentier T, Cadars S. Supercell program: a combinatorial structure-generation approach for the local-level modeling of atomic substitutions and partial occupancies in crystals[J]. J. Cheminform., 2016, 8: 17. http://doi.org/10.1186/s13321-016-0129-3.

[20] Kresse G, Furthmüller J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set[J]. Comput. Mater. Sci., 1996, 6(1): 15-50. http://doi.org/10.1016/0927-0256(96)00008-0.

[21] Kresse G, Furthmüller J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set[J]. Phys. Rev. B, 1996, 54(16): 11169-11186. http://doi.org/10.1103/PhysRevB.54.11169.

[22] Blöchl P E. Projector augmented-wave method[J]. Phys. Rev. B, 1994, 50(24): 17953-17979. http://doi.org/10.1103/PhysRevB.50.17953.

[23] Kresse G, Joubert D. From ultrasoft pseudopotentials to the projector augmented-wave method[J]. Phys. Rev. B, 1999, 59(3): 1758-1775. http://doi.org/10.1103/PhysRevB.59.1758.

[24] Perdew J P, Burke K, Ernzerhof M. Generalized gradient approximation made simple[J]. Phys. Rev. Lett., 1996, 77(18): 3865-3868. http://doi.org/10.1103/PhysRevLett.77.3865.

[25] Chadi D J. Special points for Brillouin-zone integrations[J]. Phys. Rev. B, 1977, 16(4): 1746-1747. http://doi.org/10.1103/PhysRevB.16.1746.

[26] He X F, Zhu Y Z, Epstein A, Mo Y F. Statistical variances of diffusional properties from ab initio molecular dynamics simulations[J]. npj Comput. Mater., 2018, 4(1): 18. http://doi.org/10.1038/s41524-018-0074-y.

[27] Hong H Y P. Crystal structures and crystal chemistry in the system Na_1+xZr₂Si_xP_3-xO₁₂[J]. Mater. Res. Bull., 1976, 11(2): 173-182. http://doi.org/10.1016/0025-5408(76)90073-8.

[28] Goodenough J B, Hong H Y P, Kafalas J A. Fast Na⁺-ion transport in skeleton structures[J]. Mater. Res. Bull., 1976, 11(2): 203-220. http://doi.org/10.1016/0025-5408(76)90077-5.

[29] Adetona A J, Wang G, Walkley B, Sinclair D C, Reaney I M. Effect of ZrO₂ stoichiometry on the structural and electrical properties of Na₃Zr_2-xSi₂PO_12-2x solid electrolyte[J]. J. Eur. Ceram. Soc., 2024, 44(5): 2989-2997. http://doi.org/10.1016/j.jeurceramsoc.2023.12.071.