Layered sodium titanate with a matched lattice: a single ion conductor in a solid-state sodium metal battery

Na metal batteries using solid-state electrolytes (SSEs) have attracted intensive attention due to their superior safety and high energy density. However, the interfacial issue is one of the biggest challenges to their working normally for the achievement of high performance. To address the high SSE/Na interfacial resistance and facilitate Na⁺ migration, an efficient approach based on a lattice-matching effect is proposed. In this work, we synthesized a sheet-like layered sodium titanate with rich oxygen vacancies formulated as Na_0.98Ti_1.3O₃ (NTO). The NTO sheet behaves like a single ion conductor with a low ion migration activation energy of ∼0.159 eV and a high ion transference number (t_Na) of 0.91, which is due to the weak interactions between the lamellar Na⁺ ions and unmoved anionic Ti-O-Ti layers in NTO. An NTO composite polymer electrolyte (CPE) was fabricated by combination with poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) and NaPF₆, and it exhibited a high ion conductivity (σ) of 1.16 × 10^-4 S cm^-1 with a t_Na of 0.73. The Na|NTO|Na symmetric cell can work normally in the initial discharge/charge cycles and the Na|NTO CPE|Na cell can endure long-term Na stripping/plating, which is associated with the matched lattice of the Na (110) and NTO (001) facets, d₍₁₁₀₎ (Na) = d₍₀₀₁₎ (NTO). Moreover, the Na|NTO CPE|Na₃V₂(PO₄)₃ (NVP) full cell presents a high discharge capacity with a good cycling performance. This is probably associated with the intrinsic oxygen vacancies in NTO, which can capture the PF₆^- anions and accelerate the dissociation of Na⁺-PF₆^- pairs in the CPE. And the decreased crystallinity of each component in NTO CPE can promote the migration of Na⁺ in NTO and along the amorphous PVDF-HFP polymer chain.