Rubidium lanthanum fluoride solid electrolyte for fluoride-ion batteries

Fluoride-ion batteries represent one of the few credible post-lithium chemistries with multivalent anion shuttling and theoretical energy densities that rival or exceed lithium-ion at the cathode level, yet the field has been starved of solid electrolytes that actually conduct fluoride ions at practical rates near room temperature. The central problem is the ion-channel geometry of most fluorite and related structures: alkali and rare-earth fluorides with small A-site cations pack too tightly, yielding migration barriers in the 0.7–1.0 eV range that render them inert at anything below several hundred degrees Celsius. The Alkali Rare-Earth Fluoride Solid Electrolyte invention — anchored to the composition RbLaF4 and its derived family — proposes a structurally motivated solution: substituting a heavy alkali such as rubidium or cesium on the A-site physically dilates the fluoride-ion conduction channels, computed via nudged-elastic-band (NEB) methods to reduce the migration barrier into the 0.45–0.55 eV range. That window is meaningfully lower than the reference barriers seen in the LaF3 and KLaF4 literature and is consistent with the conductivity targets needed to make a practical fluoride-shuttle cell. The timing matters because the fluoride-ion battery literature has just crossed an inflection point. Honda Research and other groups demonstrated room-temperature operation of fluoride-shuttle cells using liquid-fluoride electrolytes, galvanizing a secondary effort to find solid-state equivalents that eliminate the flammability and containment problems of fluoride-dissolved-in-organic-solvent approaches. That secondary effort is still largely confined to academic groups probing LaF3, CaF2, and BaSnF4 — none of which are subject to a broad composition-plus-device-use patent claiming the heavy-alkali design principle. This asset stakes an early-mover composition claim on the design principle itself: that heavy-alkali occupancy on the A-site of an AREF4 structure is the structural lever for lowering the fluoride migration barrier, covering a family spanning A = Rb, Cs, K, Na and RE = La, Ce, Pr, Nd, Sm, Gd. The asset sits within the broader integrated packaging, storage, and PFAS-treatment systems portfolio, where fluoride-ion conduction has dual relevance: as a battery electrolyte for energy storage and as a platform for electrochemical fluoride sensing and fluoride-based remediation cells. Within that portfolio this is a lead composition claim, not a defensive or backup filing, because it claims the preferred compound and the structural principle simultaneously rather than staking out exclusion territory around a competitor's already-published work.

The compound of primary interest, RbLaF4, is an alkali rare-earth tetrafluoride in which rubidium occupies the large A-site and lanthanum occupies the rare-earth site within a layered or chain-type fluoride framework. The critical design choice is the A-site. In the lighter-alkali homologues — sodium or potassium analogs — the A-site cation is too small relative to the fluoride sublattice, compressing the conduction corridors through which interstitial or vacancy-mediated fluoride migration must pass. Rubidium, with an ionic radius of approximately 1.52 Å in eight-fold coordination, is sufficiently large to prop open these corridors without collapsing the rare-earth coordination environment, and cesium (ionic radius ~1.67 Å) can be substituted in partial or full occupancy to push channel dilation further. This is the same geometric lever that governs conductivity in the perovskite oxide family — A-site size sets the bottleneck cross-section — and it has a direct computed consequence: NEB calculations anchored to the DFT-relaxed RbLaF4 structure yield a fluoride migration barrier of 0.45–0.55 eV. Published migration barriers for LaF3 (the most-studied fluoride solid electrolyte) sit in the 0.5–0.8 eV range depending on crystal direction, and KLaF4 analogs trend toward the upper half of that range, so the RbLaF4 value represents a genuine structural improvement at the computed level. The NEB simulation (internal reference SSE-RBLAF4-NEB-001) was performed with DFT as the underlying energy evaluator. This means the barrier is a single-source DFT result rather than a multi-potential consensus: one DFT source contributed. The calculation was further anchored to the existing literature on fluorite-family migration barriers to confirm the magnitude is physically plausible. Importantly, no independent machine-learning interatomic potential (MACE, CHGNet, MatterSim, or ORB) was run on this composition to provide a second opinion on structural stability or the migration energy surface. That is a meaningful gap relative to the multi-MLIP pipeline Lattice Graph applies to its most computationally mature assets. The current computational evidence is therefore best characterized as a theoretically well-motivated DFT prediction, not a multi-method validated result. The family is defined compositionally as AREF4 with A spanning Rb, Cs, K, and Na and RE spanning La, Ce, Pr, Nd, Sm, and Gd. Within that family the patent-claimed members extend to partial mixed occupancy at both the A-site (e.g., Rb1-xCsxLaF4) and the RE-site (e.g., RbLa1-yLnyF4), as well as fluoride-deficient compositions (RbLaF4-z) that would introduce charge-carrier vacancies directly. The vacancy-doped variant is especially relevant to ionic conductivity: in most fluorite-type conductors, fluoride vacancies rather than interstitials carry the dominant current, and fluorine stoichiometry control through synthesis atmosphere is a well-established route to optimizing conductivity without changing the host lattice. The bandgap and full phonon dispersion have not been reported at this stage; the space group has not been locked to a single polymorph. Multiple polymorphs of RbLaF4 are known in the literature, and polymorph identity will affect both the channel geometry and the precise migration barrier, making polymorph-resolved NEB calculations the clearest near-term simulation priority. From a materials-class perspective, heavy-alkali rare-earth fluorides sit at the intersection of two well-studied families: the tysonite-type (LaF3) conductors that dominate the fluoride solid-electrolyte literature, and the scheelite/colquiriite-type double fluorides in which alkali and rare-earth cations alternate. RbLaF4 may or may not be isostructural with the KLaF4 archetype depending on synthesis conditions, but regardless of polymorph, the structural argument that a larger A-site cation opens the anion migration corridor is general and consistent with the ionic radius trends across the series. The 0.45–0.55 eV window, if borne out experimentally, would correspond to conductivities in the 10-4 to 10-3 S/cm range at room temperature — useful for a thin-film or composite electrolyte architecture, though likely insufficient for a bulk pressed-pellet cell without further optimization. That said, bulk ionic conductivity has not been measured at this stage, which is the central open validation gate.

The addressable opportunity for solid-state fluoride-ion electrolytes spans at minimum three end markets. The first and largest is fluoride-shuttle (fluoride-ion) rechargeable batteries, where multiple-electron transfer per metal center theoretically enables gravimetric energy densities exceeding lithium-ion. Market estimates for the broader solid-state electrolyte space run into the tens of billions of dollars over the decade, but the fluoride-specific sub-segment is considerably smaller today — the technology is pre-commercial — and the relevant addressable market for a fluoride solid electrolyte licensor at this stage is best framed as the R&D pipeline of advanced battery developers who will pay licensing fees or co-development agreements to access composition rights while the underlying technology matures. An addressable market of $1–5 billion is a reasonable estimate for this segment once room-temperature fluoride cells begin reaching prototype validation stages, though that figure should be understood as an aspirational ceiling, not a present-day revenue pool. The second market is electrochemical fluoride sensing, where AREF4 materials have documented utility as sensing membranes and ion-selective layers. This is a smaller but nearer-term opportunity: fluoride sensors based on LaF3 single-crystal membranes are already commercial, and a heavy-alkali fluoride with improved conductivity could address temperature-range limitations of current sensors, particularly for high-temperature industrial and environmental monitoring applications. The third market, within the integrated packaging, storage, and PFAS-treatment systems portfolio context, is electrochemical fluoride removal or recovery from water streams — a growing priority driven by PFAS contamination regulation. Fluoride-conducting solid electrolytes could serve as selective membranes in electrodialysis cells for fluoride concentration or removal. The royalty or licensing logic in each case is a composition claim: any developer who synthesizes RbLaF4 or a closely related AREF4 compound for use in a fluoride-ion device would need to license or design around the patent family. Standard royalty rates for materials composition patents in battery electrolytes range from 1–5% of electrolyte-layer revenue or a fixed per-kilogram transfer price.

heavy-alkali fluoride-ion conductor for fluoride-shuttle batteries

The most direct acquirers or licensees are advanced battery developers specifically pursuing fluoride-ion or fluoride-shuttle chemistry — a small but growing group that includes automotive-adjacent research units (Honda Research Institute published the landmark room-temperature fluoride-ion cell), national laboratory spinouts working on post-lithium anion-shuttle cells, and specialized battery materials startups seeking composition protection for solid electrolyte layers. For these buyers, the composition claim is valuable precisely because it is early: acquiring the IP now, before experimental conductivity data either validate or invalidate the NEB prediction, costs far less than acquiring it after a competitor has confirmed the performance and the market knows what the asset is worth. Any licensee would likely negotiate milestones tied to the bulk conductivity measurement as a condition of royalty escalation. Secondary buyers exist in the ion-sensing and PFAS-treatment segments. Established electrochemical sensor manufacturers — particularly those serving water-quality monitoring, industrial-process control, or environmental compliance markets — have a demonstrated interest in fluoride-selective membrane materials, and improved conductivity in a heavy-alkali fluoride membrane translates directly to lower detection limits and faster sensor response. Within the broader portfolio's PFAS-treatment context, buyers might include water-treatment technology companies exploring electrochemical fluoride removal, where a selective fluoride-conducting membrane is a key enabling component. Licensing in these adjacent verticals could proceed in parallel with battery-focused negotiations without conflicting, since device-use claims in the patent are application-specific and can be licensed independently by field of use.

The central risk is the gap between the computed migration barrier and measured bulk ionic conductivity. The NEB result of 0.45–0.55 eV is based on a single DFT study without multi-MLIP confirmation of structural stability, and migration barriers derived from single-source DFT can shift by 0.1–0.2 eV depending on functional choice, pseudopotential, and — critically — which polymorph is used as the input geometry. RbLaF4 has multiple known polymorphs, and if the phase that crystallizes under standard synthesis conditions has a more compact channel geometry than the modeled structure, the experimental conductivity could fall well short of the predicted target. A related risk is grain-boundary resistance: even if the bulk migration barrier is favorable, polycrystalline fluoride electrolytes routinely show total conductivities one to two orders of magnitude below bulk values due to space-charge layers and grain-boundary fluoride depletion. The result would be a bulk NEB prediction that is correct but a pellet or film conductivity that is still too low for practical use without nanostructuring or sintering optimization. The de-risking roadmap is well-defined. Synthesis of phase-pure RbLaF4 by solid-state reaction or hydrothermal routes, followed by powder diffraction for polymorph identification, is achievable in a standard inorganic chemistry laboratory. Electrochemical impedance spectroscopy on pressed pellets or thin films would resolve bulk versus grain-boundary contributions. If that data is positive, multi-MLIP phonon calculations using MACE and CHGNet would provide the computational consensus currently absent and strengthen the patent's technical disclosure. If bulk conductivity falls short of the 10-4 S/cm threshold needed for practical thin-film electrolytes, the mixed-occupancy variants — particularly Rb1-xCsxLaF4 with higher cesium content — offer a structured avenue to push channel dilation further without abandoning the patent family. The IP remains defensible even during the validation phase because the composition claims do not require a conductivity performance specification, only membership in the AREF4 family as defined.