Structural Recognition of Some Layer-Structured Materials for Na-Ion Batteries
Authors: Wang Xuefeng , Li Yejing , Gao Yurui , Wang Zhaoxiang 
Published: February 2020

Layer-structured materials have been widely used in various applications due to their distinctive structures and physical/ chemical properties. As the electrode materials for sodium (Na)-ion batteries (NIB), their intercalation mechanisms remain unclear and their electrochemical performances are unsatisfied with low storage capacity and poor cycling stability. Therefore, it is essential to find new layered materials, explore their intercalation mechanisms and improve their electrochemical performance. In these regards, this thesis focuses on four layered materials in terms of atomic structural characterization, phase transition and intercalation mechanism, and improvement of their electrochemical performance, including anti-P2 Na0.5NbO2, Mxene Ti3C2X and transition-metal dichalcogenides MoS2.

Most of the layered AxMO2 (A=alkali ions, M= transition metals) are composed of edge-shared MO6 octahedrons, wherein A ions occupy in the interlayers with varied coordination polyhedron such as octahedrons (O), tetrahedrons (T) or trigonal prisms (P). We synthesized a new layered oxide material anti-P2 Na0.5NbO2 with negative strain effect. Its structure is composed of NbO6 trigonal prisms and NaO6 octahedrons. Analysis by X-ray absorption spectroscopy(XAS) and density functional theory (DFT) calculations indicates that the negative volume change is mainly a result of the enhanced interlayer (Na-O) interaction and the weakened Nb-Nb and Nb-O bonding in the O-Nb-O slab upon Na intercalation. As an anode material for NIB, Na0.5NbO2 exhibits high structural stability, long cycle life and prominent rate performance for NIB. Thanks to its negative stain effect, Na0.5NbO2 can be used as a “volume buffer” for the positive-strain electrode materials to construct long-term batteries with high energy density.

MXenes represent a large family of functionalized two-dimensional (2D) transition-metal carbides and carbonitrides. However, most of the understandings on their unique structures and applications are stopped at the theoretical suggestion. We clarified the surface structure and intercalation chemistry of Ti3C2X at the atomic scale by aberration-corrected scanning transmission electron microscope (STEM) and DFT calculations. The STEM studies show that the functional groups (e.g. OH-, F-, O-) and the intercalated sodium (Na) ions prefer to stay on the top sites of the centro-Ti atoms and the C atoms of the Ti3C2 monolayer, respectively. Double Na-atomic layers are found within the Ti3C2X interlayer upon extensive Na intercalation via two-phase transition and solid-solution reactions. In addition, aluminum (Al)-ion intercalation leads to horizontal sliding of the Ti3C2X monolayer. Based on these observations, the previous monolayer surface model of Ti3C2X is modified. DFT calculations using the new modeling help to understand more about their physical and chemical properties. Meanwhile, the Ti3C2X exhibits prominent rate performance and long-term cycling stability as an anode material for NIB.

The distinctive electronic and chemical properties of transition-metal dichalcogenides are closely related to their structure and intercalation chemistry. By means of STEM, we clarified the phase transition from semiconductive 2H-MoS2 to metallic 1T-MoS2 and occupancy of the intercalated Na at the atomic scale. It is shown that x = 1.5 in NaxMoS2 is a critical point for the reversibility of the structural evolution. If light of the understanding of the phase transitions and intercalation chemistry of the MoS2, the cycling stability and rate performance of MoS2 are improved by controlling its reaction depth, optimizing the binder and nano-treatment.

Figure 1