Interfacial Chemistry Enables Highly Reversible Na Extraction/Intercalation in Layered‐Oxide Cathode Materials

Layered transition-metal oxides are promising cathode candidates for sodium-ion batteries. However, the inferior interphase formation and particulate fracture during sodiation/desodiation result in structure degradation and poor stability. Herein, the interface chemistry of P2-Na0.640Ni0.343Mn0.657O2 in an electrolyte of 1.0 mol/L NaPF6 in diglyme is unveiled to enable highly reversible Na extraction and intercalation. The uniform and robust cathode-electrolyte interphase layer is in situ formed with decomposition of diglyme molecules and anions in initial cycles. The NaF- and CO-rich CEI film exhibits high mechanical strength and ionic conductivity, which suppresses the reconstruction of its electrode interphase from P2 phase to spinel-like structure and reinforces its structure integrity without cracks. This favours facile Na+ transport and stable bulk redox reactions. It is demonstrated to show long cycling stability with capacity retention of 94.4% for 180 cycles and superior rate capability. This investigation highlights the cathode interphase chemistry in sodium-ion batteries.