Layered lithium transition‐metal oxide materials, e.g., Li(Ni
1−
x
−
y
Co
x
Mn
y
)O
2
(NCM) and Li(Ni
1−
x
−
y
Co
x
Al
y
)O
2
, are the most promising candidates for lithium‐ion battery cathodes. They generally consist of ≈10 µm spherical particles densely packed with smaller particles (0.1–1 µm), called secondary and primary particles, respectively. The micrometer‐ to nanometer‐sized particles are critical to the battery performance because they affect the reaction capability of the cathode. Herein, the crystal structure of the primary particles of NCM materials is revisited. Elaborate transmission electron microscopy investigations reveal that the so‐called primary particles, often considered as single crystals, are in fact polycrystalline secondary particles. They contain low‐angle and exceptionally stable special grain boundaries (GBs) presumably created during aggregation via an oriented attachment mechanism. Therefore, this so‐called primary particle is renamed as primary‐like particle. More importantly, the low‐angle GBs between the smaller true primary particles cause the development of nanocracks within the primary‐like particles of Ni‐rich NCM cathodes after repetitive electrochemical cycles. In addition to rectifying a prevalent misconception about primary particles, this study provides a previously unknown but important origin of structural degradation in Ni‐rich layered cathodes.
The structural, electrical, and optical properties of monolayer ruthenium oxide (RuO 2 ) nanosheets (NSs) fabricated by chemical exfoliation of a layered three-dimensional form of K-intercalated RuO 2 are studied systematically via experimental and computational methods. Monolayer RuO 2 NS is identified as having a distorted h-MX 2 structure. This is the first observation of a RuO 2 NS structure that is unlike the t-MX 2 structure of the RuO 2 layers in the parent material and does not have hexagonal symmetry. The distorted h-MX 2 RuO 2 NSs are shown to have optical transparency superior to that of graphene, thereby predicting the feasibility of applying RuO 2 NSs to flexible transparent electrodes. In addition, it is demonstrated that the semiconducting band structures of RuO 2 NSs can be manipulated to be semimetallic by adjusting the crystal structure, which is related to band-gap engineering. This finding indicates that RuO 2 NSs can be used in a variety of applications, such as flexible transparent electrodes, atomic-layer devices, and optoelectronic devices.
The enhancement in electrical transport properties of exfoliated individual RuO NSs was systemically investigated for their application in flexible electronics and optoelectronics. Decoration of Ag NPs on the surface of the RuO NSs provides donor electrons and dramatically increases the electrical conductivity of the monolayer RuO NSs by up to 3700%. The n-type doping behavior was confirmed via Hall measurement analysis of the doped RuO NSs. The layer number- and temperature-dependence of the conductivity were also investigated. Moreover, carrier concentration and mobility were obtained from Hall measurements, indicating that the undoped RuO NSs had ambipolar transport and semi-metallic characteristics. Moreover, the Ag-doped RuO NS multilayer films on polycarbonate substrates were demonstrated by the Langmuir-Blodgett assembly methods, showing one-third reduction in the sheet resistance and extraordinarily high bending stability that the change in the resistance was less than 1% over 50 000 cycles.
The electrical conductivity and Seebeck coefficient of RuO nanosheets are enhanced by metal nanoparticle doping using Ag-acetate solutions. In this study, RuO monolayer and bilayer nanosheets exfoliated from layered alkali metal ruthenates are transferred to Si substrates for device fabrication, and the temperature dependence of their conductivity and Seebeck coefficients is investigated. For pristine RuO nanosheets, the sign of the Seebeck coefficient changes with temperature from 350-450 K. This indicates that the dominant type of charge carrier is dependent on the temperature, and the RuO nanosheets show ambipolar carrier transport behavior. By contrast, the sign of the Seebeck coefficient for Ag nanoparticle-doped RuO nanosheets does not change with temperature, indicating that the extra charge carriers from metal nanoparticles promote n-type semiconductor behavior.
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