Iodide-/Polyiodide-Constrained Migration in a Layered Single-Ion Conductor for Quasi-Solid-State Sodium–Iodine Batteries: Intermediate-Assisted Charge Transfer and Multielectron Conversion

In this work, a Na⁺-preinserted layered V₂O₅ (NaVO) is developed as a solid-state electrolyte to replace liquid electrolytes (LEs). NaVO demonstrates the feature of a single-ion conductor, exhibiting a sodium ion transference number of 0.88, which can provide selective passage for Na⁺ migration with the anionic V-O-V layer unmoved. Impressively, the redox reactions and electron transfer between <i>I</i>^- and V^5+/4+ (such as V^V/IV₂O₅ + 2I^- ↔ V^IV/III₂O₅ + I₂) are beneficial for Na⁺ (de)intercalation (from)into NaVO. Meanwhile, the inner wall of the layered NaVO is occupied by the preinserted Na⁺, which can retard the penetration of the large-sized iodide/polyiodides and impede them shuttling from the iodine-cathode chamber into the anode side. Moreover, the exposed (001) crystalline plane of NaVO enables the adsorption and stabilization of the I⁺ intermediate via charge transfer of I → O → V, which can facilitate multielectron conversion of 2I^- → I₂ → 2I⁺, resulting in an increased capacity. Therefore, a NaVO/poly(vinylidene fluoride) PVDF quasi-solid-state composite polymer electrolyte (CPE) is fabricated, which presents a high σ and large <i>t</i>_Na⁺ with interfacial compatibility. And the as-assembled Na | Na and Na | I₂ cells exhibit good electrochemical performances.