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- and V5+/4+ (such as VV/IV2O5 + 2I- ↔ VIV/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 tNa+ with interfacial compatibility. And the as-assembled Na | Na and Na | I2 cells exhibit good electrochemical performances.