<i>In Situ</i> Mn K-edge X-ray Absorption Spectroscopy Studies of Electrodeposited Manganese Oxide Films for Electrochemical Capacitors

In situ Mn K-edge fluorescence X-ray absorption spectroscopy (XAS) was used to analyze the manganese oxides electrodeposited on a porous carbon paper substrate for use in electrochemical capacitors in order to determine the local and electronic structural changes in the material as a function of the applied potential in a neutral electrolyte. Within the potential range from +0.1 to +0.8 V vs SCE (reversible region), the cyclic voltammogram (CV) showed ideal capacitive characteristics. On the other hand, large current tails were observed at near both ends of the potential window in the CV when the upper and lower potential limits were expanded to +1.0 and −0.3 V vs SCE (irreversible region), which is indicative of an irreversible reaction. According to the in situ X-ray absorption near-edge structure (XANES) results, the capacitive currents of the manganese oxides in 2 M KCl in the reversible region originated from the Faradaic pseudocapacitance. The average oxidation state and local structure of the manganese oxide changed reversibly during charging/ discharging within the reversible region. On the other hand, the local and electronic structure of manganese oxide changed in an irreversible manner in the irreversible region, particularly during the redox reaction within the potential range between +0.1 to −0.3 V vs SCE. This irreversible feature of the local and electronic structure changes was attributed to the formation of the electrochemically irreversible low valence manganese oxides such as Mn2O3 and Mn3O4, and the dissolution of Mn species from the electrode

Oxygen Reduction Reaction Activity in Non-Precious Single-Atom (M–N/C) CatalystsContribution of Metal and Carbon/Nitrogen Framework-Based Sites

We examine the performance of a number of single-atom M–N/C electrocatalysts with a common structure in order to deconvolute the activity of the framework N/C support from the metal M–N4 sites in M–N/Cs. The formation of the N/C framework with coordinating nitrogen sites is performed using zinc as a templating agent. After the formation of the electrically conducting carbon–nitrogen metal-coordinating network, we (trans)metalate with different metals producing a range of different catalysts (Fe–N/C, Co–N/C, Ni–N/C, Sn–N/C, Sb–N/C, and Bi–N/C) without the formation of any metal particles. In these materials, the structure of the carbon/nitrogen framework remains unchangedonly the coordinated metal is substituted. We assess the performance of the subsequent catalysts in acid, near-neutral, and alkaline environments toward the oxygen reduction reaction (ORR) and ascribe and quantify the performance to a combination of metal site activity and activity of the carbon/nitrogen framework. The ORR activity of the carbon/nitrogen framework is about 1000-fold higher in alkaline than it is in acid, suggesting a change in mechanism. At 0.80 VRHE, only Fe and Co contribute ORR activity significantly beyond that provided by the carbon/nitrogen framework at all pH values studied. In acid and near-neutral pH values (pH 0.3 and 5.2, respectively), Fe shows a 30-fold improvement and Co shows a 5-fold improvement, whereas in alkaline pH (pH 13), both Fe and Co show a 7-fold improvement beyond the baseline framework activity. The site density of the single metal atom sites is estimated using the nitrite adsorption and stripping method. This method allows us to deconvolute the framework sites and metal-based active sites. The framework site density of catalysts is estimated as 7.8 × 1018 sites g–1. The metal M−N4 site densities in Fe−N/C and Co–N/C are 9.4 × 1018 sites–1 and 4.8 × 1018 sites g–1, respectively

Polythiophene-Wrapped Olivine NaFePO₄ as a Cathode for Na-Ion Batteries

The surface of olivine NaFePO4 was modified with polythiophene (PTh) to develop a high-performance cathode material for use in Na-ion batteries. The Rietveld refinement results of the prepared material reveal that PTh-coated NaFePO4 belongs to a space group of Pnma with lattice parameters of a = 10.40656 Å, b = 6.22821 Å, and c = 4.94971 Å. Uncoated NaFePO4 delivers a discharge capacity of 108 mAh g–1 at a current density of 10 mA g–1 within a voltage range of 2.2–4.0 V. Conversely, the PTh-coated NaFePO4 electrode exhibits significantly improved electrochemical performance, where it exhibits a discharge capacity of 142 mAh g–1 and a stable cycle life over 100 cycles, with a capacity retention of 94%. The NaFePO4/PTh electrode also exhibits satisfactory performance at high current densities, and reversible capacities of 70 mAh g–1 at 150 mA g–1 and 42 mAh g–1 at 300 mA g–1 are obtained compared with negligible capacities without coating. The related electrochemical reaction mechanism has been investigated using in situ X-ray absorption spectroscopy (XAS), which revealed a systematic change of Fe valence and reversible contraction/expansion of Fe–O octahedra upon desodiation/sodiation. The ex situ X-ray diffraction (XRD) results suggest that the deintercalation in NaFePO4/PTh electrodes proceeds through a stable intermediate phase and the lattice parameters show a reversible contraction/expansion of unit cell during cycling

Divalent Iron Nitridophosphates: A New Class of Cathode Materials for Li-Ion Batteries

Divalent Iron Nitridophosphates: A New Class of Cathode Materials for Li-Ion Batterie

Structural Origin of Overcharge-Induced Thermal Instability of Ni-Containing Layered-Cathodes for High-Energy-Density Lithium Batteries

Using a combination of time-resolved X-ray diffraction (XRD), in situ transmission electron microscopy (TEM), and first principles calculations, we explore the structural origin of the overcharge induced thermal instability of two cathode materials, LiNi0.8Co0.15Al0.05O2 and LiNi1/3Co1/3Mn1/3O2, which exhibit significant difference in thermal stabilities. Detailed TEM analysis reveals, for the first time, a complex core–shell-surface structure of the particles in both materials that was not previously detected by XRD. Structural comparison indicates that the overcharged LixNi0.8Co0.15Al0.05O2 (x xNi1/3Co1/3Mn1/3O2 consists of a similar core–shell-surface structure but a very different CdI2-type surface structure. The thermal instability of LixNi0.8Co0.15Al0.05O2 can be attributed to the release of oxygen because of the rapid growth of the rock-salt-type structure on the surface during heating. In contrast, the CdI2-type surface structure of the overcharged LixNi1/3Co1/3Mn1/3O2 particles delays the oxygen-release reaction to a much higher temperature resulting in better stability. These results gave deep insight into the relationship between the local structural changes and the thermal stability of cathode materials, which is vital to the development of new cathode materials for the next generation of lithium-ion batteries

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

Cd-Doped Li_4–<i>x</i>Cd_<i>x</i>Ti₅O₁₂ (<i>x</i> = 0.20) as a High Rate Capable and Stable Anode Material for Lithium-Ion Batteries

Li4Ti5O12 (LTO), an excellent anode for lithium-ion batteries (LIBs), suffers from low electronic conductivity, limiting its high-power rate application. An aliovalent metal ion doping strategy that tunes the electronic/ionic conductivity can mitigate this issue. In this work, we investigated a series of Cd2+ dopings on the Li4–xCdxTi5O12 (x = 0, 0.05, 0.10, and 0.20) anode material by considering its effect on structural and electrochemical performance in Li- and Na-ion batteries. Combined Rietveld refinement and X-ray absorption spectroscopy (XAS) analysis explicitly identified Cd2+ doping into the Li(8a) tetrahedral site of the cubic spinel LTO structure. According to high-resolution powder diffraction (HRPD), scanning electron microscopy (SEM), 4-point probe, and X-ray photoelectron spectroscopy (XPS), an increase in Cd2+ doping from 5 to 20% at the Li (8a) site in the LTO results in a reduction in particle size, an expansion of lattice, an increase in conductivity, and an increase in Ti3+ content to Ti4+ ratio. High-resolution scanning transmission electron microscopy (HR-STEM) confirms that cadmium ions are interstitially doped in the LTO structure. Compared to the pristine LTO electrode in the Li half cell, the Li3.80Cd0.20Ti5O12 (Cd0.20-LTO) electrode showed a significant improvement in capacity at high rates and excellent cycling performance. The improvement in performance for Cd0.20-doped LTO is a consequence of the reduction in the diffusion path and the faster Li-ion kinetics. Therefore, this Cd-doped LTO series of electrodes demonstrates advantageous features for Li-ion battery systems

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