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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.more » « less
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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 monoclinic
C2/m and 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 OO 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. -
Abstract Polymer–ceramic composite electrolytes are emerging as a promising solution to deliver high ionic conductivity, optimal mechanical properties, and good safety for developing high‐performance all‐solid‐state rechargeable batteries. Composite electrolytes have been prepared with cubic‐phase Li7La3Zr2O12(LLZO) garnet and polyethylene oxide (PEO) and employed in symmetric lithium battery cells. By combining selective isotope labeling and high‐resolution solid‐state Li NMR, we are able to track Li ion pathways within LLZO‐PEO composite electrolytes by monitoring the replacement of7Li in the composite electrolyte by6Li from the6Li metal electrodes during battery cycling. We have provided the first experimental evidence to show that Li ions favor the pathway through the LLZO ceramic phase instead of the PEO‐LLZO interface or PEO. This approach can be widely applied to study ion pathways in ionic conductors and to provide useful insights for developing composite materials for energy storage and harvesting.