Interfacial chemistry of sulfide and halide solid electrolytes in all solid-state sodium batteries: from single electrolytes to functionally partitioned architectures.

All solid-state sodium batteries (ASSSBs) are promising for safe, low cost, and large-scale energy storage, but further development is limited by the difficulty of identifying an inorganic solid electrolyte (SE) that remains stable against both high voltage cathodes and low potential sodium metal anodes. This review focuses on sulfide and halide solid electrolytes, two representative sodium based inorganic electrolyte families with complementary properties. Sulfides provide high room-temperature sodium-ion conductivity, good cold pressed contact, and efficient ion transport networks, mainly because sulfide anions are highly polarizable and form soft lattices. The same features, however, raise the valence band maximum and make sulfides prone to oxidative decomposition at high-voltage cathode interfaces. Halide electrolytes are generally more stable against oxidation and more compatible with oxide cathodes because of their deeper valence bands. However, the low lying conduction bands and reducible high valent metal cations in many halide electrolytes can lead to reduction reactions at the Na metal interface. Sulfide electrolytes are also thermodynamically unstable against Na metal. In some cases, however, their interfacial reactions can be kinetically suppressed by artificial interlayers that limit electron transfer and relieve interfacial stress. This review summarizes the structural chemistry, transport mechanisms, defect regulation, cathode oxidation behavior, anode reduction behavior, and interface engineering of sulfide and halide electrolytes. It then discusses cathode passivation, artificial anode interlayers, and sulfide-halide heterointerface compatibility. Finally, we propose functionally partitioned electrolyte architectures as an emerging design framework that assigns ion transport, oxidation resistance, reduction protection, and mechanical accommodation to distinct yet compatible electrolyte components. Although current studies in sodium systems provide preliminary evidence of feasibility, systematic investigation of the interfacial chemistry and Na ion transport across heterointerfaces between sulfide and halide electrolytes remains limited.