P2-Cathode materials for sodium battery are usually active in the range of 2–4.3 V, but the decomposition of the electrolytic salt above 4 V versus Na⁺/Na is common. Arguably, the one of the concerns is the formation of HF after the reaction of the salts with water molecules, which are present as an impurity in the electrolyte. This HF ceaselessly attacks the active materials and gradually causes the failure of the electrode via electric isolation of the active materials. In this study, we report a bio-inspired b-NaCaPO₄ nanolayer on a P2-type layered Na_2/3[Ni_1/3Mn_2/3]O₂ cathode material. The coating layers successfully scavenge HF and H₂O, and excellent capacity retention was achieved with the b-NaCaPO₄-coated Na_2/3[Ni_1/3 Mn_2/3]O₂ electrode. This retention was possible because a less acidic environment was produced in the Na cells during prolonged cycling. The intrinsic stability of the coating layer also assisted in delaying the exothermic decomposition reaction of the de-sodiated electrodes. We suggest formation and reaction mechanisms for the coating layers responsible for the excellent electrode performance. The suggested technology is promising for use with cathode materials in rechargeable sodium batteries to mitigate the effects of acidic conditions in Na cells.

P2-type layered Na_2/3[Ni_1/3Mn_2/3]O₂ powders were synthesized by a conventional solid-state method. Na₂CO₃, Mn₂O₃, and NiO were thoroughly mixed. And then it was calcined at 1,000 °C for 12 h in air and then quenched, after which the pellet was immediately transferred into an Ar-filled glove box to minimize the adsorption of water and exposure to the CO₂ in the air. Calcium nitrate and phosphoric acid were first dissolved in anhydrous ethanol at room temperature, and the as-synthesized active materials were added slowly to the solution. Then, the solution containing the active material was stirred continuously at 80 °C in a dry room, accompanied by the slow evaporation of the solvent. The as-prepared and bare Na_2/3[Ni_1/3Mn_2/3]O₂ powders were heated at 700 °C for 2 h in air. Also, the coated powders were characterized by XRD, SEM, and HR-TEM. Electrochemical properties of coated powders were examined by galvanostatic cycle test and electrochemical impedance spectroscopy.

b-NaCaPO₄ coatings shows uniform coating layers, as observed by TEM. Thickness of coating layer is of about 10nm. Electrochemical test with half cells in voltage range of 2.5 - 4.3 V at 25 ^oC indicates that coated materials have better capacity retention and coulombic efficiency, rate capability, and low resistance. Thin coating layers seem to effective to improve the electrochemical properties. Also, b-NaCaPO₄ coatings give decrease of residual sodium and byproduct on the surface of particle, as confirmed by ToF-SIMS. Details will be mentioned at the conference site.