Constructing Safe and Durable High‐Voltage P2 Layered Cathodes for Sodium Ion Batteries Enabled by Molecular Layer Deposition of Alucone

Abstract Sodium ion batteries are a promising next‐generation energy storage device for large‐scale applications. However, the high voltage P2–O2 phase transition (>4.25 V vs Na/Na + ) and metal dissolution of P2 layered cathodes into the electrolyte result in severe capacity fading, which is a major setback to fabricate high energy devices. Hence, it is essential to design an appropriate strategy to enhance interfacial behaviors to obtain safe and stable high voltage sodium ion batteries. Herein, an ultrathin alucone layer deposited through molecular layer deposition (MLD) is employed to stabilize the structure of a P2‐type layered cathode cycled at a high cut‐off voltage (>4.45 V) for the first time. The alucone coated P2‐type Na 0.66 Mn 0.9 Mg 0.1 O 2 (NMM) cathode exhibits an 86% capacity retention after 100 cycles between 2 and 4.5 V at 1 C, demonstrating substantial improvement compared to pristine (65%) and Al 2 O 3 ‐coated (71%) NMM cathodes. Furthermore, the mechanically robust and conductive nature of the organometallic thin film enhances the rate capability relative to the pristine NMM electrode. This work reveals that the MLD of alucone on cathodes is a promising approach to improve the cycle stability of sodium ion batteries at high cut‐off voltages.