Internal and External Co‐Engineering of Stable Cathode Interface Improves Cycle Performance of Polymer Sodium Batteries

Abstract The development of polymer sodium batteries requires cathode materials with stable interfaces to avoid poor interfacial contact and interfacial side reactions during cycling. Here, a co‐engineering strategy is deployed to tailor the cathode internal structure and improve the cathode interface stability through bonding interactions. Internally, the effect of low‐cost Fe substitution in the obtained Na 0.67 Mn 2/3 Fe 1/3 O 2 cathode material renders favorable effects in several aspects. First, the increased lattice constant facilitates Na + intercalation and thereby lowers the diffusion barrier of Na + ions. Second, it increases the electronic conductivity, thereby improving the reaction reversibility. Third, the MnO bond length is shortened, which alleviates the Jahn‐Taylor effect and improves structural stability. In addition to these internal effects, the FeOB bond interactions due to Fe substitution promote the decomposition of the tris(trimethylsilane)borate additive and the formation of a dense and uniform cathode electrolyte interface film, leading to improved cycling stability. Owing to the co‐engineering of both internal structure and surface modification, the polymer solid‐state sodium battery with a stable interface exhibits a specific capacity of 85.2 mAh g ‐1 after 800 cycles at 1 C.