Using Real-Time Electron Microscopy To Explore the Effects of Transition-Metal Composition on the Local Thermal Stability in Charged Li_xNi_yMn_zCo_1–y–zO₂ Cathode Materials

Abstract

In this work, we use in-situ transmission electron microcopy (TEM) to investigate the thermal decomposition that occurs at the surface of charged Li x Ni y Mn z Co 1-y-z O 2 (NMC) cathode materials of different composition (with y, z=0.8, 0.1 and 0.6, 0.2 and 0.4, 0.3), after they have been charged to their practical upper limit voltage (4.3V). By heating these materials inside the TEM, we are able to directly characterize near surface changes in both their electronic structure (using electron energy loss spectroscopy) and crystal structure and morphology (using electron diffraction and bright-field imaging). The most Ni-rich material (y, z = 0.8, 0.1) is found to be thermally unstable at significantly lower temperatures than the other compositions-this is manifested by changes in both the electronic structure and the onset of phase transitions at temperatures as low as 100°C. Electron energy loss spectroscopy indicates that the thermally induced reduction of Ni ions drives these changes, and that this is exacerbated by the presence of an additional redox reaction that occurs at 4.2V in the y, z = 0.8, 0.1 material. Exploration of individual particles shows that there are substantial variations in the onset temperatures and overall extent of these changes. Of the compositions studied, the composition of y, z = 0.6, 0.2 has the optimal combination of high energy density and reasonable thermal stability. The observations herein demonstrate that real time electron microscopy provide direct insight into the changes that occur in cathode materials with temperature, allowing optimization of different alloy concentrations to maximize overall performance.

High‐Voltage‐Driven Surface Structuring and Electrochemical Stabilization of Ni‐Rich Layered Cathode Materials for Li Rechargeable Batteries

“…Note that a large amount (≈3.8%) of Ni in the transition metal layer (Ni oct ) migrated to octahedral sites in the Li layer (Li oct ) after electrochemical cycling at a cut‐off voltage of 4.6 V, whereas negligible migration was observed for the electrodes cycled at 4.25 and 4.9 V. The transition metal migration in layered‐type NCM materials has been considered an origin of phase transition from layered to spinel or rock‐salt phases and has been previously investigated using TEM and XAS analyses. [ 41–47 ] It has been observed that transition metals migrate from octahedral sites in the transition metal layer to octahedral or tetrahedral sites in the lithium layer at highly charged states, and the subsequent segregation of transition metals in the lithium layer triggers the phase transition from layered to spinel or rock‐salt structures. We believe that the increased Li–Ni intermixing and structural disorder at 4.6 V triggers the spinel and rock‐salt formation on the particle surface and results in deterioration of the electrochemical performance upon repeated battery cycling.…”

Anti‐Oxygen Leaking LiCoO2

“…Considering the importance of the layered oxide cathode materials in the commercialization of the lithium‐ion batteries, various experimental studies using in situ X‐ray diffraction/absorption spectroscopy, thermal analysis, in situ transmission electron microscopy (TEM) and computational efforts were carried out to characterize and understand the oxygen‐release phenomenon and the thermal degradation mechanisms in these materials. Overall, it is understood that the extraction of Li‐ions from the cathode unit cell results in the formation of under‐coordinated oxygen atoms, which destabilizes the structure .…”

“…At elevated temperatures these under‐coordinated oxygens break the bonds with the transition metals and form O 2 molecules leaving the host structure. As a result, the layered structure will rearrange to form the spinel and the rocksalt phases that contain less oxygen in their unit cell . It has been shown that the extent of oxygen release is dependent on the surface fraction of the particles …”

Rationally‐Directed Synthesis and Characterization of Nickel‐Rich Cathode Material for Lithium Ion Battery

“…Figure a illustrates the cycling performance and corresponding energy density of LNCM in half cells at 0.2 C. After 70 cycles at a current of 0.2 C, the LNCM cathode material can still deliver capacity of 169.8 mAh g −1 with a noticeable capacity retention of 90 % corresponding to an energy density of 645.6 Wh kg −1 calculated by the active material loading, which indicates good cycling stability. Differential capacity analysis has been proven an effective method to unfold the underlying electrochemical behaviour of the materials that can occur in charge‐discharge processes ,,. Here, it was also used to reveal the electrochemistry evolution of the LNCM material in half cells cycling at 0.2 C.…”