Unlocking Li superionic conductivity in face-centred cubic oxides via face-sharing configurations

All-solid-state batteries require solid electrolytes with high Li-ion conductivity and broad electrochemical/chemical stability. While sulfide-based superionic conductors can exceed liquid electrolyte conductivity, they suffer from narrow stability windows and moisture sensitivity. Oxide-based conductors are more stable but typically have lower conductivity, and only a few families (garnets, NASICON, perovskites) exceed 0.1 mS cm−1 at room temperature. The most common oxide framework, with fcc oxygen packing (rocksalt-type), has been largely excluded because Li migration along Oct–Tet–Oct pathways encounters large site-energy differences, leading to high activation barriers. This work tests the hypothesis that creating face-sharing Li configurations in fcc oxides via Li over-stoichiometry can raise Li site energies, flatten migration energy landscapes, enable concerted motion, and thus unlock superionic conduction in rocksalt-type oxides.

Prior work established multiple sulfide-based Li superionic conductors (for example, LGPS and argyrodites) with high room-temperature conductivities but poor air/electrochemical stability. Oxide superionic conductors with high conductivity are rarer and are mostly in structures with uncommon oxygen packing (garnets, NASICON, perovskites). Conventional fcc-oxygen rocksalt oxides have generally exhibited insufficient Li mobility due to large Oct–Tet energy offsets and high migration barriers. Face-sharing cation configurations and distorted face-sharing Li–O polyhedra have been implicated in fast ion transport in several materials (for example, Li7TiO6 and Li3V2O6 anodes), and concerted migration via strong Li–Li interactions is known in various superionic conductors. Disordered rocksalt cathodes also showed that O–TM-free tetrahedra (surrounded only by Li) lower barriers. These insights motivate engineering face-sharing Li configurations and O–TM channels in an fcc framework to enhance Li transport.

Synthesis: Li-In-Sn-O (LISO) compounds were synthesized by solid-state reaction from Li2CO3, In2O3, and SnO2 (10% excess Li2CO3 to compensate Li loss). Precursors were ball-milled (Retsch PM 200, 250 rpm, 12 h), dried at 70 °C, pelletized, calcined in air at 1,050 °C, air-quenched, ground and shaker-milled (SPEX 8000M, 30 min), then sintered at 1,050 °C and air-quenched. Heat-treatment times controlled Li content: for over-stoichiometric o-LISO, calcination 4 h and sintering 6 h; for near-stoichiometric ns-LISO, 6 h and 10 h, respectively.

Phase/structure characterization: X-ray diffraction (Rigaku MiniFlex 600; synchrotron XRD at APS 17-BM) and TOF neutron powder diffraction (SNS NOMAD) with Rietveld refinement (GSAS-II) were used to identify rocksalt (o-DRX) and a spinel-like s-phase. TEM electron diffraction and dark-field imaging mapped nanoscale s-phase domains within o-DRX; SEM imaged pellet cross-sections. ICP-OES quantified Li:In:Sn ratios.

Transport measurements: Electrochemical impedance spectroscopy (Bio-Logic VMP300, 7 MHz–100 mHz, 10 mV) with indium ion-blocking electrodes on polished, dense pellets (∼90–95% relative density) from 0 to 120 °C. Equivalent-circuit fitting yielded total ionic conductivity and Arrhenius activation energy. DC polarization with In electrodes provided electronic conductivity.

NMR: 7Li ssNMR (Bruker 16.4 T). Room-temperature MAS at 50 kHz; variable-temperature MAS at 12.5 kHz from 226–386 K. Spin-echo spectra, T1 (saturation recovery) and T2 (variable-delay Hahn echo). Chemical shifts referenced to 7Li-enriched Li2CO3.

Computations: First-principles DFT (VASP, PAW, PBE-GGA, 520 eV cutoff, 25 k-points/Å−1, convergence 1e−5 eV; forces 0.02 eV Å−1). High-throughput phase stability screening of Li1+x+yM1M2O2 ORX compositions across 16 redox-inactive cations, considering Li excess x = 0, 1/6, 1/3 and Li over-stoichiometry y = −1/12 (Li vacancies), 1/12, 1/6. Four cation orderings (γ-LiFeO2-like, spinel-like, layered-like, electrostatic ground state) with enumerated near-ground-state configurations. Stability evaluated via convex hull (energy above hull, Ehull). Interfacial reaction energies computed via convex-hull-based methodology.

AIMD: NVT ensemble, 1 fs timestep, 100 ps trajectories using Nosé–Hoover thermostat on lowest-energy o-DRX and s-phase configurations at target composition. Diffusivities obtained from Arrhenius fits; local hopping barriers from counting Oct↔Tet hops vs temperature.

• Demonstration of Li superionic conductivity in an fcc-type rocksalt-derived oxide by introducing Li over-stoichiometry to create face-sharing Li configurations.
• Over-stoichiometric Li1.17In1Sn1O4 (o-LISO) shows total Li-ion conductivity σtotal = 3.38 × 10−4 S cm−1 at room temperature with activation energy Ea = 255 meV (EIS). Electronic conductivity is 2.47 × 10−9 S cm−1, five orders of magnitude lower than ionic, confirming predominantly ionic transport.
• 7Li ssNMR reveals two Li sites (Tet and Oct). With increasing temperature, line narrowing and intensity redistribution (site1:site2 from ~1:4 at 226 K to ~1:1 at 386 K) indicate enhanced mobility and thermal population of higher-energy sites. Local hopping barriers from T1: 57 meV (Tet-like site) and 74 meV (Oct-like site), much lower than macroscopic Ea.
• Phase formation controlled by Li content: with higher Li, mixed o-DRX plus a spinel-like s-phase forms; with Li loss (longer calcination), s-phase and then Li2InO3-like impurity disappear to DRX and ultimately LiInO2-type. ICP confirms higher Li content in o-LISO than ns-LISO.
• Structural analysis: o-LISO comprises an expanded o-DRX (due to Tet Li) and a nanoscale spinel-like s-phase. Electron diffraction and dark-field imaging show s-phase nanodomains embedded in o-DRX, providing interconnected diffusion pathways.
• Mechanism: Li over-stoichiometry enforces simultaneous occupancy of face-sharing Tet–Oct Li sites, raising Li site energies and lowering migration barriers. Tet Li prefers Li-only face-sharing Oct neighbours, generating O–TM-free channels characteristic of spinel-like ordering, enabling concerted migration and 3D percolation.
• AIMD: o-DRX exhibits lower barrier (Ea,DRX ≈ 430 meV) than stoichiometric DRX (≈552 meV in ns-LISO); s-phase further reduces barriers and yields fully 3D percolating 8a–16c–8a pathways. Predicted s-phase Li conductivity is 3.17 × 10−3 S cm−1 at 300 K.
• Design guidelines from high-throughput DFT: Large redox-inactive octahedral cations (e.g., In3+) and higher Li excess reduce Ehull and stabilize Li over-stoichiometry and face-sharing configurations; Li vacancies are less favorable. Stability improves with larger octahedral cations that expand Tet sites.
• Outlook/extension: ORX compounds in In–Mg, In–Zn and In–Ti spaces also form s-phase with Li over-stoichiometry and show improved measured Li-ion conductivities reaching 10−3–10−1 S cm−1 at RT (Supplementary). o-LISO shows oxidative stability up to ~4 V and favorable chemical stability with common oxide cathodes; instability vs Li metal due to reducible In3+ is noted.

The study validates that fcc-type oxides, traditionally dismissed for superionic conduction, can achieve fast Li transport when face-sharing Li configurations are engineered via Li over-stoichiometry. Elevating Li site energies in face-sharing Tet–Oct configurations flattens the migration landscape and enables low-barrier Oct–Tet/Tet–Oct hopping. Spinel-like ordering of LiTet(LiOct)4 clusters creates O–TM-free channels and 3D percolation, further enhancing mobility. The coexistence of s-phase nanodomains within an o-DRX matrix establishes continuous diffusion pathways, explaining the observed room-temperature conductivity and low activation energy. Computational screening elucidates chemical factors stabilizing these configurations—large, redox-inactive octahedral cations and high Li excess—thus offering generalizable design rules. Collectively, the findings address the central hypothesis and broaden the scope of fcc-oxide frameworks for solid electrolytes by coupling local configurational engineering with compositional selection.

This work introduces a general strategy—Li over-stoichiometry in rocksalt-type lattices—to create face-sharing Li configurations that unlock superionic conduction in fcc oxides. Experimentally, o-LISO achieves σtotal = 3.38 × 10−4 S cm−1 at RT with Ea = 255 meV, supported by NMR-indicated low local barriers and by structural identification of an s-phase with spinel-like ordering providing 3D percolation. AIMD confirms reduced barriers and predicts higher conductivity for the s-phase. High-throughput DFT identifies large, redox-inactive cations and high Li excess as key to stabilizing over-stoichiometric rocksalt-type phases. The approach expands the chemical design space of oxide solid electrolytes and suggests future directions: replacing In3+ with more reductively stable cations for compatibility with Li metal, optimizing synthesis to maximize s-phase connectivity, and exploring broader M1–M2 chemistries predicted to stabilize face-sharing Li configurations.

• Interfacial stability: Poor stability against Li metal due to reducibility of In3+ necessitates alternative cations or protective interlayers.
• Phase heterogeneity: The s-phase forms as nanoscale domains within an o-DRX matrix; optimizing domain size/connectivity may be required to maximize bulk conductivity.
• Sensitivity to Li content: Achieving desired over-stoichiometry relies on precise control of Li loss during high-temperature processing, which may be furnace-dependent and scale-sensitive.
• Conductivity level: While markedly improved for fcc oxides, measured σtotal (∼3.4 × 10−4 S cm−1 at RT) remains below leading sulfide electrolytes; grain boundaries and porosity may contribute to higher macroscopic Ea than local barriers.
• Generality: Although high-throughput calculations provide guidelines, experimental validation across the broad predicted chemical space is ongoing and may face unforeseen synthesis and stability constraints.