Electron Microscopy Study Of Lithium (Sodium)Layered Oxides Cathode Materials

To understand the microstructure-property correlation upon electrochemical cycling is a fundamental issue for the investigations of cathode materials with layered structures in rechargeable lithium (sodium) ion batteries.In this work, we employed advanced analytical electron microscopy and spectroscopy, including aberration-corrected scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectra (EDS) and electron energy loss spectroscopy (EELS), to study a few representative layered materials namely P2-sodium layered material, lithium poor material and lithium rich material. Both the structure and chemisty at the bulk, surface and interface areas were carefully analysized at down to microscopic and even atomic scale. Based on these results, we could buildup the qualitively and in some cases quantitatively relationship between the microstructures and electrochemical properties.In this first part, we present the studies on P2-Na0.66Mn0,675Ni0.1625Co0.1625O2, a well-known cathode system in sodium ion battery. The pristine sample prepared via solid sintering at high temperatures was assigned to possess a P2-type layered structure (P63/mmc) with high quality by X-ray powder diffraction. Results from further HAADF-STEM and EDS analysis observed the formation of reconstructed area on its surface with a width of about 1-2 nm. And a large amount of antisite defects (with transition metal occupying the lattice sites of sodium) and obvious lattice distortions was found on these reconstructed areas. Detailed chemical analysis via EDS and EELS further confirmed an inhomogeneous elemental distribution inside these reconstructed surface areas with enriched cobalt and deficient nickel. Upon aging process, these reconstuctured surface layers further grow into wider regions ranging of 5-10 nm, which was accompanies by a spinel (Fd3m) phase to rocksalt phase (Fm3m) transition.In the second part, lithium poor material Li0.66Mn0.675Ni0.i625Co0.1625NaxO2 was prepared by an ion exchange method by using the Na0.66Mn0.675Ni0.1625Co0.162502 precursor. XRD and STEM analysis reveals that the material is a phase-separated system and most particles observed possess a 03 type layered structure with spinel or rocksalt reconstruction at the surface. Further analysis shows the surface reconstruction may due to the instability of the surface for P2 type sodium layered oxides precursor. Complete spinel-structured particles were also observed in this material, which possess a lot of twin boundary with ordered point defect at the grain boundary. However, we found that these defects can be maintained during the phase transition process during the lithium insertion reaction. For example, we found a lot of twin boundaries in the lithium rich cathode material prepared by the Li-insertion reaction with this kind of material.In the last part, lithium rich materials were discussed. A new kind of cation ordering was observed in the bulk material of Li1.2Mno.54Co0.13Ni0.13NaxO2. Surface analysis shows nickel (Ni) prefers to be enriched at the surface in perpendicular to the lithium diffusion channel, i.e., (200) surface and it tends to diffuse into the lithium (Li) layers and leads to the formation of rocksalt phase (Fm3m). However, in the case of cobalt (Co), it was found to segregate along the transition metal (TM) layers of (001) and (200) surface. Results from the aging experiments showed that cobalt-enriched layers would further result in a surface structure instability, as evidenced by the formation of a large number of antisite defects (Li-TM) and rocksalt phase structures at the (001) surface after aging. The electrochemical test shows that Co doping can result in a fast capacity decay at the first several cycles. However, it will increase the initial discharge capacity for its ability to facilitate the electrochemical activation process. Thus cobalt can be a double-edged sword for this kind of material. Furthermore, the twin boundaries in Li-rich material mentioned above can result in the segregation of cobalt and sodium, and will further affect the electrochemical behavior of the material just as the surface effect does. More complicated cases were also observed in Mn-Ni, Mn-Ni-Co-Al, Mn-Ni-Fe system. All these results will provide a theoretical guidance for the high performance lithium (sodium) layered cathode material preparation and surface modification.