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Nickel phosphide (Ni 5 P 4 ) nanosheets are synthesized using in situ chemical vapor deposition of P on Ni foam. The thickness of the as-synthesized Ni 5 P 4 film is determined to be ∼5 nm, using atomic force microscopy (AFM). The small thickness shortens the diffusion path of Li ions and results in fast ion transport. In addition, the 2D Ni 5 P 4 nanosheets seamlessly connect to the Ni foam, which facilitates electron transfer between Ni 5 P 4 and the Ni current collector. Therefore, the binder/carbon free-nickel supported Ni 5 P 4 shows fast rate performance as an anode for lithium-ion batteries (LIBs). The specific capacity of 2D Ni 5 P 4 is obtained as 600 mA h g −1 at a cycling rate of 0.1C, approaching the theoretical capacity of 768 mA h g −1 . Even at a rate of 0.5C, the capacity remains as 450 mA h g −1 over 100 cycles. A capacity >100 mA h g −1 is retained at a very high rate of 20C. Ni 5 P 4 also exhibits a low voltage of ∼0.5 V with respect to Li metal, which makes it a suitable negative electrode for LIBs. In operando 31 P NMR and 7 Li NMR are employed to probe the lithiation and de-lithiation mechanisms upon electrochemical cycling. 

Abstract Lattice oxygen redox yields anomalous capacity and can significantly increase the energy density of layered Li‐rich transition metal oxide cathodes, garnering tremendous interest. However, the mechanism behind O redox in these cathode materials is still under debate, in part due to the challenges in directly observing O and following associated changes upon electrochemical cycling. Here, with¹⁷O NMR as a direct probe of O activities, it is demonstrated that stacking faults enhance O redox participation compared with Li₂MnO₃domains without stacking faults. This work is concluded by combining both ex situ and in situ¹⁷O NMR to investigate the evolution of O at 4i, 8j sites from monoclinicC2/mand 6c(1), 6c(2), 6c(3) sites from the stacking faults (P3₁12). These measurements are further corroborated and explained by first‐principles calculations finding a stabilization effect of stacking faults in delithiated Li₂MnO₃. In situ¹⁷O NMR tracks O activities with temporal resolution and provides a quantitative determination of reversible O redox versus irreversible processes that form short covalent OO bonds. This work provides valuable insights into the O redox reactions in Li‐excess layered cathodes, which may inspire new material design for cathodes with high specific capacity.