Thermal stability of charged LiNixMnyCozO2 (NMC, with x + y + z = 1, x:y:z = 4:3:3 (NMC433), 5:3:2 (NMC532), 6:2:2 (NMC622), and 8:1:1 (NMC811)) cathode materials is systematically studied using combined in situ time-resolved X-ray diffraction and mass spectroscopy (TR-XRD/MS) techniques upon heating up to 600 °C. The TR-XRD/MS results indicate that the content of Ni, Co, and Mn significantly affects both the structural changes and the oxygen release features during heating: the more Ni and less Co and Mn, the lower the onset temperature of the phase transition (i.e., thermal decomposition) and the larger amount of oxygen release. Interestingly, the NMC532 seems to be the optimized composition to maintain a reasonably good thermal stability, comparable to the low-nickel-content materials (e.g., NMC333 and NMC433), while having a high capacity close to the high-nickel-content materials (e.g., NMC811 and NMC622). The origin of the thermal decomposition of NMC cathode materials was elucidated by the changes in the oxidation states of each transition metal (TM) cations (i.e., Ni, Co, and Mn) and their site preferences during thermal decomposition. It is revealed that Mn ions mainly occupy the 3a octahedral sites of a layered structure (R3̅m) but Co ions prefer to migrate to the 8a tetrahedral sites of a spinel structure (Fd3̅m) during the thermal decomposition. Such element-dependent cation migration plays a very important role in the thermal stability of NMC cathode materials. The reasonably good thermal stability and high capacity characteristics of the NMC532 composition is originated from the well-balanced ratio of nickel content to manganese and cobalt contents. This systematic study provides insight into the rational design of NMC-based cathode materials with a desired balance between thermal stability and high energy density.

In this work, we present results from the application of a new in situ technique that combines time-resolved synchrotron X-ray diffraction and mass spectroscopy. We exploit this approach to provide direct correlation between structural changes and the evolution of gas that occurs during the thermal decomposition of (over)charged cathode materials used in lithium-ion batteries. Results from charged Li x Ni0.8Co0.15Al0.05O2 cathode materials indicate that the evolution of both O2 and CO2 gases are strongly related to phase transitions that occur during thermal decomposition, specifically from the layered structure (space group R3̅m) to the disordered spinel structure (Fd3̅m), and finally to the rock-salt structure (Fm3̅m). The state of charge also significantly affects both the structural changes and the evolution of oxygen as the temperature increases: the more extensive the charge, the lower the temperature of the phase transitions and the larger the oxygen release. Ex situ X-ray absorption spectroscopy (XAS) and in situ transmission electron microscopy (TEM) are also utilized to investigate the local structural and valence state changes in Ni and Co ions, and to characterize microscopic morphology changes. The combination of these advanced tools provides a unique approach to study fundamental aspects of the dynamic physical and chemical changes that occur during thermal decomposition of charged cathode materials in a systematic way.

To investigate the electronic structure of the electrochemically Li-ion deintercalated Li1 - x CoO2 system, soft X-ray absorption spectroscopy (XAS) for the oxygen K-edge and the Co LII,III-edge has been carried out intensively with compositional x value variation, compared with Co K-edge X-ray absorption near edge structure (XANES) spectroscopy. To get reasonably good XAS spectra for the electrochemically Li-ion deintercalated Li1 - x CoO2 system, we made a binder-free LiCoO2 film electrode using the electrostatic spray deposition (ESD) technique. The oxygen K-edge XAS for Li1 - x CoO2 shows more effective spectral change with respect to Li-ion content than the Co LII,III-edge XAS. The dependence of the absorption peak feature on the Li content is described in terms of the ground state of the Co and O atoms, showing the systematic variation of the hole-state site distribution between Co and oxygen atoms. From the Co LII,III-edge XAS, it is found that the broad peak shift to higher energy with the Li-ion deintercalation is due to rehybridization between Co and O atoms under the local structural distortion of CoO6 octahedra, which is also confirmed by the formation of two additional absorption peaks below the threshold energy corresponding to the oxygen 2p hole state hybridized with the 3d orbital of Co ion in the distorted CoO6 octahedral site. In the O K-edge XAS spectra for the deintercalated Li1 - x CoO2 film, the shoulder absorption peak in the energy region higher than the threshold energy could be assigned to the higher oxidation state of the oxygen site on Li deintercalation. From the Co LII,III-edge and O K-edge XAS results for the Li-ion deintercalated Li1 - x CoO2 film, it is concluded that the charge compensation for the Li-ion deintercalation process could be achieved mainly in the oxygen site and Co metal atomic site simultaneously. O K-edge and Co LII,III-edge XAS results for cycled LiCoO2 film show that the capacity fading of the LiCoO2 system is related to the decrease of Co−O bond covalency by the local structural distortion of CoO6 octahedra remaining in the cycled LiCoO2.

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NiO x thin film electrodes were prepared for use in a supercapacitor by electrochemical precipitation of Ni͑OH͒ 2 films followed by heat-treatment. The effect of electrodeposition conditions such as cathodic current density and concentration of Ni(NO 3 ) 2 solution on the surface morphology of NiO x were examined and found to have a significant effect on the surface morphology of the deposited films. The surface morphology of the NiO x films changed from dense to porous morphology with an increase in the deposition rate of Ni͑OH͒ 2 films. A maximum specific capacitance of 277 F/g was obtained for a highly porous NiO x film electrode prepared by heating the Ni͑OH͒ 2 film deposited at 4.0 mA/cm 2 in 0.1 M Ni(NO 3 ) 2 at 300°C. The charge-storage mechanism of NiO x in 1 M KOH was investigated using an electrochemical quartz crystal microbalance ͑EQCM͒ and probe beam deflection ͑PBD͒ technique. Nonmonotonic mass change was observed during redox reactions of the nickel oxide film in 1 M KOH. Analysis of the EQCM and PBD results showed that the electrochemical redox reaction of the NiO x is not a simple OH Ϫ adsorption/desorption reaction but rather composed of predominant H ϩ desorption in the initial stage of oxidation and thereafter predominant OH Ϫ adsorption in the latter stage of oxidation and vice versa during reduction.