Sodium‐Ion Batteries: Building Effective Layered Cathode Materials with Long‐Term Cycling by Modifying the Surface via Sodium Phosphate

Abstract Surface stabilization of cathode materials is urgent for guaranteeing long‐term cyclability, and is important in Na cells where a corrosive Na‐based electrolyte is used. The surface of P2‐type layered Na 2/3 [Ni 1/3 Mn 2/3 ]O 2 is modified with ionic, conducting sodium phosphate (NaPO 3 ) nanolayers, ≈10 nm in thickness, via melt‐impregnation at 300 °C; the nanolayers are autogenously formed from the reaction of NH 4 H 2 PO 4 with surface sodium residues. Although the material suffers from a large anisotropic change in the c ‐axis due to transformation from the P2 to O2 phase above 4 V versus Na + /Na, the NaPO 3 ‐coated Na 2/3 [Ni 1/3 Mn 2/3 ]O 2 /hard carbon full cell exhibits excellent capacity retention for 300 cycles, with 73% retention. The surface NaPO 3 nanolayers positively impact the cell performance by scavenging HF and H 2 O in the electrolyte, leading to less formation of byproducts on the surface of the cathodes, which lowers the cell resistance, as evidenced by X‐ray photoelectron spectroscopy and time‐of‐flight secondary‐ion mass spectroscopy. Time‐resolved in situ high‐temperature X‐ray diffraction study reveals that the NaPO 3 coating layer is delayed for decomposition to Mn 3 O 4 , thereby suppressing oxygen release in the highly desodiated state, enabling delay of exothermic decomposition. The findings presented herein are applicable to the development of high‐voltage cathode materials for sodium batteries.