Investigating Local Degradation and Thermal Stability of Charged Nickel-Based Cathode Materials through Real-Time Electron Microscopy

In this work, we take advantage of in situ transmission electron microscopy (TEM) to investigate thermally induced decomposition of the surface of Li_<i>x</i>Ni_0.8Co_0.15Al_0.05O₂ (NCA) cathode materials that have been subjected to different states of charge (SOC). While uncharged NCA is stable up to 400 °C, significant changes occur in charged NCA with increasing temperature. These include the development of surface porosity and changes in the oxygen K-edge electron energy loss spectra, with pre-edge peaks shifting to higher energy losses. These changes are closely related to O₂ gas released from the structure, as well as to phase changes of NCA from the layered structure to the disordered spinel structure, and finally to the rock-salt structure. Although the temperatures where these changes initiate depend strongly on the state of charge, there also exist significant variations among particles with the same state of charge. Notably, when NCA is charged to <i>x</i> = 0.33 (the charge state that is the practical upper limit voltage in most applications), the surfaces of some particles undergo morphological and oxygen K-edge changes even at temperatures below 100 °C, a temperature that electronic devices containing lithium ion batteries (LIB) can possibly see during normal operation. Those particles that experience these changes are likely to be extremely unstable and may trigger thermal runaway at much lower temperatures than would be usually expected. These results demonstrate that in situ heating experiments are a unique tool not only to study the general thermal behavior of cathode materials but also to explore particle-to-particle variations, which are sometimes of critical importance in understanding the performance of the overall system

Oxygen-Release-Related Thermal Stability and Decomposition Pathways of Li_<i>x</i>Ni_0.5Mn_1.5O₄ Cathode Materials

The thermal stability of charged cathode materials is one of the critical properties affecting the safety characteristics of lithium-ion batteries. New findings on the thermal-stability and thermal-decomposition pathways related to the oxygen release are discovered for the high-voltage spinel Li_<i>x</i>Ni_0.5Mn_1.5O₄ (LNMO) with ordered (<i>o</i>-) and disordered (<i>d</i>-) structures at the fully delithiated (charged) state using a combination of in situ time-resolved X-ray diffraction (TR-XRD) coupled with mass spectroscopy (MS) and X-ray absorption spectroscopy (XAS) during heating. Both <i>o</i>- and <i>d</i>- Li_<i>x</i>Ni_0.5Mn_1.5O₄, at their fully charged states, start oxygen-releasing structural changes at temperatures below 300 °C, which is in sharp contrast to the good thermal stability of the 4V-spinel Li_<i>x</i>Mn₂O₄ with no oxygen being released up to 375 °C. This is mainly caused by the presence of Ni⁴⁺ in LNMO, which undergoes dramatic reduction during the thermal decomposition. In addition, charged <i>o</i>-LNMO shows better thermal stability than the <i>d</i>-LNMO counterpart, due to the Ni/Mn ordering and smaller amount of the rock-salt impurity phase in <i>o</i>-LNMO. Two newly identified thermal-decomposition pathways from the initial Li_<i>x</i>Ni_0.5Mn_1.5O₄ spinel to the final NiMn₂O₄-type spinel structure with and without the intermediate phases (NiMnO₃ and α-Mn₂O₃) are found to play key roles in thermal stability and oxygen release of LNMO during thermal decomposition

Quantification of Honeycomb Number-Type Stacking Faults: Application to Na₃Ni₂BiO₆ Cathodes for Na-Ion Batteries

Ordered and disordered samples of honeycomb-lattice Na₃Ni₂BiO₆ were investigated as cathodes for Na-ion batteries, and it was determined that the ordered sample exhibits better electrochemical performance, with a specific capacity of 104 mA h/g delivered at plateaus of 3.5 and 3.2 V (vs Na⁺/Na) with minimal capacity fade during extended cycling. Advanced imaging and diffraction investigations showed that the primary difference between the ordered and disordered samples is the amount of number-type stacking faults associated with the three possible centering choices for each honeycomb layer. A labeling scheme for assigning the number position of honeycomb layers is described, and it is shown that the translational shift vectors between layers provide the simplest method for classifying different repeat patterns. It is demonstrated that the number position of honeycomb layers can be directly determined in high-angle annular dark-field scanning transmission electron microscopy (STEM-HAADF) imaging studies. By the use of fault models derived from STEM studies, it is shown that both the sharp, symmetric subcell peaks and the broad, asymmetric superstructure peaks in powder diffraction patterns can be quantitatively modeled. About 20% of the layers in the ordered monoclinic sample are faulted in a nonrandom manner, while the disordered sample stacking is not fully random but instead contains about 4% monoclinic order. Furthermore, it is shown that the ordered sample has a series of higher-order superstructure peaks associated with 6-, 9-, 12-, and 15-layer periods whose existence is transiently driven by the presence of long-range strain that is an inherent consequence of the synthesis mechanism revealed through the present diffraction and imaging studies. This strain is closely associated with a monoclinic shear that can be directly calculated from cell lattice parameters and is strongly correlated with the degree of ordering in the samples. The present results are broadly applicable to other honeycomb-lattice systems, including Li₂MnO₃ and related Li-excess cathode compositions

Quantification of Honeycomb Number-Type Stacking Faults: Application to Na₃Ni₂BiO₆ Cathodes for Na-Ion Batteries

Ordered and disordered samples of honeycomb-lattice Na₃Ni₂BiO₆ were investigated as cathodes for Na-ion batteries, and it was determined that the ordered sample exhibits better electrochemical performance, with a specific capacity of 104 mA h/g delivered at plateaus of 3.5 and 3.2 V (vs Na⁺/Na) with minimal capacity fade during extended cycling. Advanced imaging and diffraction investigations showed that the primary difference between the ordered and disordered samples is the amount of number-type stacking faults associated with the three possible centering choices for each honeycomb layer. A labeling scheme for assigning the number position of honeycomb layers is described, and it is shown that the translational shift vectors between layers provide the simplest method for classifying different repeat patterns. It is demonstrated that the number position of honeycomb layers can be directly determined in high-angle annular dark-field scanning transmission electron microscopy (STEM-HAADF) imaging studies. By the use of fault models derived from STEM studies, it is shown that both the sharp, symmetric subcell peaks and the broad, asymmetric superstructure peaks in powder diffraction patterns can be quantitatively modeled. About 20% of the layers in the ordered monoclinic sample are faulted in a nonrandom manner, while the disordered sample stacking is not fully random but instead contains about 4% monoclinic order. Furthermore, it is shown that the ordered sample has a series of higher-order superstructure peaks associated with 6-, 9-, 12-, and 15-layer periods whose existence is transiently driven by the presence of long-range strain that is an inherent consequence of the synthesis mechanism revealed through the present diffraction and imaging studies. This strain is closely associated with a monoclinic shear that can be directly calculated from cell lattice parameters and is strongly correlated with the degree of ordering in the samples. The present results are broadly applicable to other honeycomb-lattice systems, including Li₂MnO₃ and related Li-excess cathode compositions

Using Real-Time Electron Microscopy To Explore the Effects of Transition-Metal Composition on the Local Thermal Stability in Charged Li_<i>x</i>Ni_<i>y</i>Mn_<i>z</i>Co_{1–<i>y</i>–<i>z</i>}O₂ Cathode Materials

In this work, we use <i>in situ</i> transmission electron microscopy (TEM) to investigate the thermal decomposition that occurs at the surface of charged Li_<i>x</i>Ni_<i>y</i>Mn_<i>z</i>Co_{1–<i>y</i>–<i>z</i>}O₂ (NMC) cathode materials of different composition (with <i>y</i>, <i>z</i> = 0.8, 0.1, and 0.6, 0.2, and 0.4,and 0.3), after they have been charged to their practical upper limit voltage (4.3 V). 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 (<i>y</i>, <i>z</i> = 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 (i) the thermally induced reduction of Ni ions drives these changes, and (ii) this is exacerbated by the presence of an additional redox reaction that occurs at 4.2 V in the <i>y</i>, <i>z</i> = 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 <i>y</i>, <i>z</i> = 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

Quantification of Honeycomb Number-Type Stacking Faults: Application to Na₃Ni₂BiO₆ Cathodes for Na-Ion Batteries

Ordered and disordered samples of honeycomb-lattice Na₃Ni₂BiO₆ were investigated as cathodes for Na-ion batteries, and it was determined that the ordered sample exhibits better electrochemical performance, with a specific capacity of 104 mA h/g delivered at plateaus of 3.5 and 3.2 V (vs Na⁺/Na) with minimal capacity fade during extended cycling. Advanced imaging and diffraction investigations showed that the primary difference between the ordered and disordered samples is the amount of number-type stacking faults associated with the three possible centering choices for each honeycomb layer. A labeling scheme for assigning the number position of honeycomb layers is described, and it is shown that the translational shift vectors between layers provide the simplest method for classifying different repeat patterns. It is demonstrated that the number position of honeycomb layers can be directly determined in high-angle annular dark-field scanning transmission electron microscopy (STEM-HAADF) imaging studies. By the use of fault models derived from STEM studies, it is shown that both the sharp, symmetric subcell peaks and the broad, asymmetric superstructure peaks in powder diffraction patterns can be quantitatively modeled. About 20% of the layers in the ordered monoclinic sample are faulted in a nonrandom manner, while the disordered sample stacking is not fully random but instead contains about 4% monoclinic order. Furthermore, it is shown that the ordered sample has a series of higher-order superstructure peaks associated with 6-, 9-, 12-, and 15-layer periods whose existence is transiently driven by the presence of long-range strain that is an inherent consequence of the synthesis mechanism revealed through the present diffraction and imaging studies. This strain is closely associated with a monoclinic shear that can be directly calculated from cell lattice parameters and is strongly correlated with the degree of ordering in the samples. The present results are broadly applicable to other honeycomb-lattice systems, including Li₂MnO₃ and related Li-excess cathode compositions

Explore the Effects of Microstructural Defects on Voltage Fade of Li- and Mn-Rich Cathodes

Li- and Mn-rich (LMR) cathode materials have been considered as promising candidates for energy storage applications due to high energy density. However, these materials suffer from a serious problem of voltage fade. Oxygen loss and the layered-to-spinel phase transition are two major contributors of such voltage fade. In this paper, using a combination of X-ray diffraction (XRD), pair distribution function (PDF), X-ray absorption (XAS) techniques, and aberration-corrected scanning transmission electron microscopy (STEM), we studied the effects of micro structural defects, especially the grain boundaries, on the oxygen loss and layered-to-spinel phase transition through prelithiation of a model compound Li₂Ru_0.5Mn_0.5O₃. It is found that the nanosized micro structural defects, especially the large amount of grain boundaries created by the prelithiation can greatly accelerate the oxygen loss and voltage fade. Defects (such as nanosized grain boundaries) and oxygen release form a positive feedback loop, promote each other during cycling, and accelerate the two major voltage fade contributors: the transition metal reduction and layered-to-spinel phase transition. These results clearly demonstrate the important relationships among the oxygen loss, microstructural defects and voltage fade. The importance of maintaining good crystallinity and protecting the surface of LMR material are also suggested

Structural Changes in Reduced Graphene Oxide upon MnO₂ Deposition by the Redox Reaction between Carbon and Permanganate Ions

We explore structural changes of the carbon in MnO₂/reduced graphene oxide (RGO) hybrid materials prepared by the direct redox reaction between carbon and permanganate ions (MnO₄^–) to reach better understanding for the effects of carbon corrosion on carbon loss and its bonding nature during the hybrid material synthesis. In particular, we carried out near-edge X-ray absorption fine structure spectroscopy at the C K-edge (284.2 eV) to show the changes in the electronic structure of RGO. Significantly, the redox reaction between carbon and MnO₄^– causes both quantitative carbon loss and electronic structural changes upon MnO₂ deposition. Such disruptions of carbon bonding have a detrimental effect on the initial electrical properties of the RGO and thus lead to a significant decrease in electrical conductivity. Electrochemical measurements of the MnO₂/reduced graphene oxide hybrid materials using a cavity microelectrode revealed unfavorable electrochemical properties that were mainly due to the poor electrical conductivity of the hybrid materials. The results of this study should serve as a useful guide to rationally approaching the syntheses of metal/RGO and metal oxide/RGO hybrid materials

Correlating Structural Changes and Gas Evolution during the Thermal Decomposition of Charged Li_<i>x</i>Ni_0.8Co_0.15Al_0.05O₂ Cathode Materials

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_<i>x</i>Ni_0.8Co_0.15Al_0.05O₂ 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 <i>R</i>3̅<i>m</i>) to the disordered spinel structure (<i>Fd</i>3̅<i>m</i>), and finally to the rock-salt structure (<i>Fm</i>3̅<i>m</i>). 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

Structural Changes and Thermal Stability of Charged LiNi_<i>x</i>Mn_<i>y</i>Co_<i>z</i>O₂ Cathode Materials Studied by Combined <i>In Situ</i> Time-Resolved XRD and Mass Spectroscopy

Thermal stability of charged LiNi_<i>x</i>Mn_<i>y</i>Co_<i>z</i>O₂ (NMC, with <i>x</i> + <i>y</i> + <i>z</i> = 1, <i>x</i>:<i>y</i>:<i>z</i> = 4:3:3 (NMC433), 5:3:2 (NMC532), 6:2:2 (NMC622), and 8:1:1 (NMC811)) cathode materials is systematically studied using combined <i>in situ</i> 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 3<i>a</i> octahedral sites of a layered structure (<i>R</i>3̅<i>m</i>) but Co ions prefer to migrate to the 8<i>a</i> tetrahedral sites of a spinel structure (<i>Fd</i>3̅<i>m</i>) 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