A two-dimensional cation-deficient Ti0.87O2 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 Ti0.87O2 nanosheet constructed artificial layer onto Na surface. Profiting from the sodiophilic nature and high Young's modulus, the Ti0.87O2 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 Ti0.87O2 layer with strong Na+ adsorption ability that pre-uniforms the Na+ flux. Further, the Ti0.87O2 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 Ti0.87O2 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 Ti0.87O2 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.