Investigation of the Structural Changes in Li[Ni_<i>y</i>Mn_<i>y</i>Co_{(1−2<i>y</i>)}]O₂ (<i>y</i> = 0.05) upon Electrochemical Lithium Deintercalation

A systematic study has been performed to investigate the structural changes of Li[Ni0.05Mn0.05Co0.90]O2, one member of the Li[NiyMnyCo(1−2y)]O2 series with low Ni/Mn content, upon electrochemical lithium deintercalation. X-ray diffraction (XRD), X-ray absorption near-edge spectroscopy (XANES), and nuclear magnetic resonance (NMR) measurements were performed, and the results from these experiments provided a detailed picture of the whole delithiation process. Oxidation of not only Ni2+, but also some Co3+, is seen in the beginning of Li extraction (less than 0.15 mol removed), the ions located closest to Mn4+ being extracted first. Further deintercalation (additional 0.2 mol of Li removal) induces an insulator to metal transition that is similar to that reported for LiCoO2. However, this reaction follows a solid solution mechanism even for this low level of substitution, rather than the two-phase reaction reported for the Ni, Mn-free oxide. When half of the Li ions are extracted, the electrochemical signature for lithium vacancy ordering in the host framework is observed. The NMR results for deintercalation of more than 50% Li were compared to those for LixCoO2 at similar stages of charge, which are reported here for the first time; they indicate that the behavior of these two phases at these potentials is very similar. When the batteries are charged to voltages higher than 4.6 V, very few lithium ions remain in the structure and the O3 to O1 phase transition occurs

Unveiling the Genesis and Effectiveness of Negative Fading in Nanostructured Iron Oxide Anode Materials for Lithium-Ion Batteries

Iron oxide anode materials for rechargeable lithium-ion batteries have garnered extensive attention because of their inexpensiveness, safety, and high theoretical capacity. Nanostructured iron oxide anodes often undergo negative fading, that is, unconventional capacity increase, which results in a capacity increasing upon cycling. However, the detailed mechanism of negative fading still remains unclear, and there is no consensus on the provenance. Herein, we comprehensively investigate the negative fading of iron oxide anodes with a highly ordered mesoporous structure by utilizing advanced synchrotron-based analysis. Electrochemical and structural analyses identified that the negative fading originates from an optimization of the electrolyte-derived surface layer, and the thus formed layer significantly contributes to the structural stability of the nanostructured electrode materials, as well as their cycle stability. This work provides an insight into understanding the origin of negative fading and its influence on nanostructured anode materials

Cation Ordering in Li[Ni_<i>x</i>Mn_<i>x</i>Co_{(1–2<i>x</i>)}]O₂-Layered Cathode Materials: A Nuclear Magnetic Resonance (NMR), Pair Distribution Function, X-ray Absorption Spectroscopy, and Electrochemical Study

Several members of the compositional series Li[NixMnxCo(1–2x)]O2 (0.01 ≤ x ≤ 1/3) were synthesized and characterized. X-ray diffraction results confirm the presence of the layered α-NaFeO2-type structure, while X-ray absorption near-edge spectroscopy experiments verify the presence of Ni2+, Mn4+, and Co3+. Their local environment and short-range ordering were investigated by using a combination of 6Li magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and neutron pair distribution function (PDF) analysis, associated with reverse Monte Carlo (RMC) calculations. The 6Li MAS NMR spectra of compounds with low Ni/Mn contents (x ≤ 0.10) show several well-resolved resonances, which start to merge when the amount of Ni and Mn increases, finally forming a broad resonance at high Ni/Mn contents. Analysis of the 6Li MAS NMR 6Li[Ni0.02Mn0.02Co0.96]O2 spectrum, is consistent with the formation of Ni2+ and Mn4+ clusters within the transition-metal layers, even at these low-doping levels. The oxidation state of Ni in this high Co content sample strongly depends upon the Li/transition metal ratio of the starting materials. Neutron PDF analysis of the highest Ni/Mn content sample Li[Ni1/3Mn1/3Co1/3]O2 shows a tendency for Ni cations to be close to Mn cations in the first coordination shell; however, the Co3+ ions are randomly distributed. Analysis of the intensity of the “LiCoO2” resonance, arising from Li surrounded by Co3+ in its first two cation coordination shells, for the whole series provides further evidence for a nonrandom distribution of the transition-metal cations. The presence of the insulator-to-metal transition seen in the electrochemical profiles of these materials upon charging correlates strongly with the concentration of the “LiCoO2” resonance

The Reaction Mechanism and Capacity Degradation Model in Lithium Insertion Organic Cathodes, Li₂C₆O₆, Using Combined Experimental and First Principle Studies

Herein, we explore the capacity degradation of dilithium rhodizonate salt (Li₂C₆O₆) in lithium rechargeable batteries based on detailed investigations of the lithium de/insertion mechanism in Li₂C₆O₆ using both electrochemical and structural ex situ analyses combined with first-principles calculations. The experimental observations indicate that the Li_<i>x</i>C₆O₆ electrode undergoes multiple two-phase reactions in the composition range of 2 ≤ <i>x</i> ≤ 6; however, the transformations in the range 2 ≤ <i>x</i> ≤ 4 involve a major morphological change that eventually leads to particle exfoliation and the isolation of active material. Through first-principles analysis of Li_<i>x</i>C₆O₆ during de/lithiation, it was revealed that particle exfoliation is closely related to the crystal structural changes with lithium deinsertion from C₆O₆ interlayers of the Li_<i>x</i>C₆O₆. Among the lithium ions found at various sites, the extraction of lithium from C₆O₆ interlayers at 2 ≤ <i>x</i> ≤ 4 decreases the binding force between the C₆O₆ layers, promoting the exfoliation of C₆O₆ layers and pulverization at the electrode, which degrades capacity retention

Decisive Factors in the Sequential Thermal Decomposition Reactions of Ni-Based Layered Cathode Materials

Understanding thermal behaviors of energy storage materials according to the charged states is essential to uncover the key factors for improving thermal stability. Herein, we trace the crystallographic changes of Ni-based layered energy storage materials during an increase in external temperature for three different states-of-charges. The most remarkable aspect is that the onset temperature of the formation of disordered spinel structure is dominantly influenced by the intermediate tetrahedron size, the space through which the cations pass. However, the completion temperature is determined by the Li contents of the Li layer. Moreover, a highly charged state triggers the rapid reduction of Ni ions, resulting in a sudden lattice expansion during the thermally induced decomposition reaction, aggravating the danger of thermal runaway. These findings give a detailed comprehension of the crystallographic behaviors of Ni-based layered materials during the thermal decomposition reaction and contribute to designing rechargeable batteries with better thermal stability

Characterization and Control of Irreversible Reaction in Li-Rich Cathode during the Initial Charge Process

Li-rich layered oxide has been known to possess high specific capacity beyond the theoretical value from both charge compensation in transition metal and oxygen in the redox reaction. Although it could achieve higher reversible capacity due to the oxygen anion participating in electrochemical reaction, however, its use in energy storage systems has been limited. The reason is the irreversible oxygen reaction that occurs during the initial charge cycle, resulting in structural instability due to oxygen evolution and phase transition. To suppress the initial irreversible oxygen reaction, we introduced the surface-modified Li­[Li_0.2Ni_0.16Mn_0.56Co_0.08]­O₂ prepared by carbon coating (carbonization process), which was verified to have reduced oxygen reaction during the initial charge cycle. The electrochemical performance is improved by the synergic effects of the oxygen-deficient layer and carbon coating layer formed on the surface of particles. The sample with suitable carbon coating exhibited the highest structural stability, resulting in reduced capacity fading and voltage decay, which are attributed to the mitigated layered-to-spinel-like phase transition during prolonged cycling. The control over the oxygen reaction of Li₂MnO₃ by surface modification affects the activation reaction above 4.4 V in the initial charge cycle and structure changes during prolonged cycling. X-ray diffraction, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy analyses as well as electrochemical performance measurement were used to identify the correlation between reduced oxygen activity and structural changes

Unraveling Reversible Redox Chemistry and Structural Stability in Sn-Doped Li-Rich Layered Oxide Cathodes

Li-rich layered oxides have received the spotlight as cathode materials to improve the energy density in recent years. However, Li-rich layered oxides accompanied by cation migration during extended cycles suffer from low-capacity retention and structural degradation through the phase transition. In this study, we synthesized a Li2IrO3 material substituting Sn for Ir, confirming that Li2Ir0.75Sn0.25O3 exhibits improved cycle performance and structural stability. This enhancement is due to the highly reversible structural changes originating from the biphasic reaction, including the O3′ phase. The intermediate O3′ phase has a distorted IrO6 octahedron by the migration of Sn, thus enlarging interslab thickness and providing a facile Li diffusion environment. More importantly, migrated Sn ions can return to the transition metal layer during the discharging process. This reversible cation migration prevents structural collapse, thus improving cycle performance. These fundamental understandings of reversible cation migration for the Li-rich materials can provide insightful factors for designing high-energy cathode materials

Unveiling the Impact of Fe Incorporation on Intrinsic Performance of Reconstructed Water Oxidation Electrocatalyst

Because of the salient impact on the performance of oxygen evolution reaction (OER), the surface dynamics of precatalysts accompanying the surface oxidation and dissolution of catalytic components demands immense research attention. Accordingly, the change in the structural integrity under high current density generally results in inconsistent OER performances. To address this challenge, here, we present the intricate design of precatalysts, strategically followed by reconstruction treatment in the presence of Fe under water oxidation condition, which significantly enhances the OER activity and long-term stability. Notably, the surface tailored heterointerface structures (Fe-doped NiOOH/CoOOH) obtained through the reconstruction of a precatalyst (Ni­(OH)2/Co9S8) with the incorporation of Fe, are abundantly enriched with electrochemically accessible high valence active sites. This results in remarkable OER activity (400 mA cm–2 at 345 mV). Density functional theory (DFT) calculations indicate that Fe-incorporated electrocatalysts give optimal binding energies of OER intermediates and show substantially reduced overpotential compared to Fe-undoped electrocatalysts