Replacing the commonly used nonaqueous liquid electrolytes in rechargeable sodium batteries with polymer solid electrolytes is expected to provide new opportunities to develop safer batteries with higher energy densities. However, this poses challenges related to the interface between the Na-metal anode and polymer electrolytes. Driven by systematically investigating the interface properties, an improved interface is established between a composite Na/C metal anode and electrolyte. The observed chemical bonding between carbon matrix of anode with solid polymer electrolyte, prevents delamination, and leads to more homogeneous plating and stripping, which reduces/suppresses dendrite formation. Full solid-state polymer Na-metal batteries, using a high mass loaded Na₃V₂(PO₄)₃ cathode, exhibit ultrahigh capacity retention of more than 92 % after 2 000 cycles and over 80 % after 5 000 cycles, as well as the outstanding rate capability.