Stabilizing P2-Type Ni–Mn Oxides as High-Voltage Cathodes by a Doping-Integrated Coating Strategy Based on Zinc for Sodium-Ion Batteries

Abstract

The key to development of high-voltage P2-type Na 0.66 Ni 0.33 Mn 0.67 O 2 is the modification methods that can effectively improve its electrochemical reversibility. Herein, a doping-integrated coating strategy based on zinc element is proposed to modify P2-type Na 0.66 Ni 0.33 Mn 0.67 O 2 , which can be achieved by a facile one-step solid-state reaction. The formation mechanism of Na 0.66 Ni 0.26 Zn 0.07 Mn 0.67 O 2 @0.06ZnO (NNZM@ 0.06ZnO) is investigated, revealing that the spinel and P3 intermediate phases appear prior to the formation of the P2 phase. Ni 2+ can be preferentially incorporated into the P2 structure in competition with Zn 2+ at high temperature, resulting in a uniform enrichment of ZnO on the surface. A small amount of Zn 2+ doping significantly suppresses the Na + /vacancy ordering effect and improves the structural reversibility. Furthermore, the electrolyte decomposition is effectively reduced because of the presence of the ZnO coating layer, leading to the formation of a thin cathode electrolyte interphase film that is favorable to fast Na + diffusion. In virtue of the Zn 2+ doping and in situ formed ZnO coating, NNZM@0.06ZnO exhibits excellent cycling stability with a capacity retention of 83.7% after 100 cycles at 100 mA g −1 and rate performance with a discharge capacity of 56.4 mAh g −1 at 2000 mA g −1 , which significantly outperforms the uncoated Na 0.66 Ni 0.26 Zn 0.07 Mn 0.67 O 2 and the Na 0.66 Ni 0.26 Zn 0.07 Mn 0.67 O 2 /0.06ZnO with the coating layer introduced by mechanical milling. This work provides a new strategy to design high-performance cathode materials for sodium-ion batteries.

Multiphase layered transition metal oxide positive electrodes for sodium ion batteries

“…The simplicity of the liquid‐based coating techniques is a strength, but still more complex than the formation of coatings spontaneously from thermodynamically favorable processes during the typical calcination process. Certain electrode compositions have been observed to spontaneously segregate certain elements to their surface, forming surface oxides with remarkably different properties than the composition in the bulk 22,91,92 . This is in contrast to the kinetic control of element distribution employed in coprecipitation or surface coating methods.…”

“…Surface coating methods such as sol‐gel and wet chemical process, 73,86,89,112 melt‐impregnation, 90 and magnetron sputtering 113 can all provide significant performance benefits, but may not achieve the high degree of conformality and thickness uniformity provided by more complex techniques such as atomic layer deposition (ALD) 70–72,94 . Surface phases may also spontaneously (partially) segregate from the bulk 22,91,92 …”

“…[70][71][72]94 Surface phases may also spontaneously (partially) segregate from the bulk. 22,91,92 Characterization of these surface phases is usually performed through a combination of surface sensitive and/or high spatial resolution techniques such as X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), and electron microscopy (SEM/TEM/STEM) paired with composition-sensitive techniques of electron energy-loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDS) or EPMA. These techniques allow for determination of surface layer composition, thickness and uniformity, oxidation state, and transport properties.…”

“…Certain electrode compositions have been observed to spontaneously segregate certain elements to their surface, forming surface oxides with remarkably different properties than the composition in the bulk. 22,91,92 This is in contrast to the kinetic control of element distribution employed in coprecipitation or surface coating methods. Thermodynamically driven surface segregation phenomenon result from differences in free energy for the segregating species between the surface and bulk to minimize the total free energy.…”

“…Utilizing similar synthesis techniques, Zhang and coworkers investigated Na 0.66 Ni 0.33 Mn 0.67 O 2 (NNM), Ni/ Zn substituted Na 0.66 Ni 0.26 Zn 0.07 Mn 0.67 O 2 (NNZM), and Na 0.66 Ni 0.26 Zn 0.07 Mn 0.67 O 2 coated with ZnO by spontaneous formation during calcination (NNZM@0.06ZnO) and contrasted with ZnO coating via mechanical milling (NNZM/0.06ZnO). 91 The experiment design rationally separates the effect of Zn doping in the bulk from the surface coating by comparing NNM and NNZM (Ni + Mn + Zn = 1), where no ZnO coating is observed. The effect of coating formation process is then investigated by adding Zn such that Ni + Mn + Zn > 1 (spontaneous NNZM@0.060ZnO and milling coated NNZM/0.06ZnO).…”