A two-dimensional cation-deficient Ti_0.87O₂ artificial protection layer for stable sodium metal anodes

The practical application of sodium (Na) metal batteries is still hampered by uncontrolled Na dendrite growth and unstable solid-electrolyte interphase (SEI), which lead to poor cycling stability and even serious safety concerns. Here, we present a dendrite-free and highly stable Na metal anode via directly transferring a cation-deficient Ti_0.87O₂ nanosheet constructed artificial layer onto Na surface. Profiting from the sodiophilic nature and high Young's modulus, the Ti_0.87O₂ layer can seamlessly connect to the Na surface and avoid the layer fracture caused by Na volume changes. In addition, the existence of cation defects enables Ti_0.87O₂ layer with strong Na⁺ adsorption ability that pre-uniforms the Na⁺ flux. Further, the Ti_0.87O₂ protection layer has high ionic conductivity but poor electronic conductivity; this allows Na⁺ quasi self-diffusion but restricts Na plating under the protection layer. With these advantages, the Ti_0.87O₂ layer stabilizes the electrolyte/electrode interface and guarantees dendrite-free plating/stripping of Na. As a result, significantly improved performance of the symmetric batteries and Na₃V₂(PO₄)₃ based full cells has been obtained. Importantly, the Ti_0.87O₂ layer can also protect Li anodes and further enhance their performance. Our work presents a new versatile strategy that brings metal based anodes one step closer to being a viable technology.