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Understanding Li+ion diffusion pathways in Li-rich layered transition metal (TM) oxides is crucial for understanding the sluggish kinetics in anionic O2–redox. Although Li diffusion within the alkali layers undergoes a low-barrier octahedral–tetrahedral–octahedral pathway, it is less clear how Li diffuses in and out of the TM layers, particularly given the complex structural rearrangements that take place during the oxidation of O2–. Here, we develop simultaneous electron ptychography and annular dark field imaging methods to unlock the Li migration pathways in Li1.2Ni0.13Mn0.54Co0.13O2associated with structural changes in the charge–discharge cycle. At the end of TM oxidation and before the high-voltage O oxidation plateau, we show that the Li migrating out of the TM layers occupies the alkali-layer tetrahedral sites on opposite sides of the TM layers, forming Li–Li dumbbell configurations, consistent with the density functional theory calculations. Also occurring are the TM migration and phase transition from O3 to O1 stacking, leading to unstable tetrahedral Li and the absence of Li contrast in imaging. Upon further Li deintercalation to 4.8 V, most of the tetrahedral Li are removed. After discharging to 2 V, we did not identify the reformation of tetrahedral Li but observed permanently migrated TMs at the alkali-layer sites, disfavoring the Li occupying the tetrahedral sites for diffusion. Our findings suggest a landscape of Li diffusion pathways in Li-rich layered oxides and strategies for minimizing the disruption of Li diffusion.

Li-rich metal oxides, such as Li1.2Ni0.13Mn0.54Co0.13O2, can deliver high specific capacities because of the redox of lattice O2−in addition to the cations. Observing oxygen distortions is key to understand the redox process. Electron ptychography is a phase-reconstruction method in 4D scanning transmission electron microscopy, providing atomic-resolution phase images with high signal-to-noise ratio and dose efficiency. Herein, we use electron ptychography to image the oxygen shift in Li1.2Ni0.13Mn0.54Co0.13O2during the first cycle. The picometer-scale precision measurement shows distinct oxygen shifts in the bulk and surface after charging and compares with various theoretical anionic redox models. The shift after discharging is not seen to recover in the bulk accounting for voltage hysteresis; however, it recovers close to the surface, although with a phase change. We suggest that Li1.2Ni0.13Mn0.54Co0.13O2proceeds distinct oxygen redox in the bulk and surface. The altered oxygen sublattice after first cycle potentially explains the changed voltage profiles of following cycles.

O-redox in compounds with Li on the transition-metal layers (TML) has recently been attributed to the formation of molecular O2on charge, trapped in the lattice. Here, we show that a similar process occurs for P2-Na0.67[Mn0.72Mg0.28]O2, which contains Mg2+on the TML. The molecular O2is identified by high-resolution RIXS and quantified by magnetometry, showing that it equates to the charge passed. This O2is trapped in voids that are formed by Mg2+out-of-plane displacement and Mn4+in-plane disordering and is then reduced on discharge associated with a large voltage hysteresis. In contrast to compounds containing Li+in the TML, in which the honeycomb ordering and the high-voltage plateau are irreversibly lost after the first cycle, in P2-Na0.67[Mn0.72Mg0.28]O2, the plateau reappears partially on the second charge due to the partial reversibility of Mn in-plane and Mg out-of-plane migration and the local reformation of the honeycomb ordering.

In conventional intercalation cathodes, alkali metal ions can move in and out of a layered material with the charge being compensated for by reversible reduction and oxidation of the transition metal ions. If the cathode material used in a lithium-ion or sodium-ion battery is alkali-rich, this can increase the battery’s energy density by storing charge on the oxide and the transition metal ions, rather than on the transition metal alone1–10. There is a high voltage associated with oxidation of O2−during the first charge, but this is not recovered on discharge, resulting in reduced energy density11. Displacement of transition metal ions into the alkali metal layers has been proposed to explain the first-cycle voltage loss (hysteresis)9,12–16. By comparing two closely related intercalation cathodes, Na0.75[Li0.25Mn0.75]O2and Na0.6[Li0.2Mn0.8]O2, here we show that the first-cycle voltage hysteresis is determined by the superstructure in the cathode, specifically the local ordering of lithium and transition metal ions in the transition metal layers. The honeycomb superstructure of Na0.75[Li0.25Mn0.75]O2, present in almost all oxygen-redox compounds, is lost on charging, driven in part by formation of molecular O2inside the solid. The O2molecules are cleaved on discharge, reforming O2−, but the manganese ions have migrated within the plane, changing the coordination around O2−and lowering the voltage on discharge. The ribbon superstructure in Na0.6[Li0.2Mn0.8]O2inhibits manganese disorder and hence O2formation, suppressing hysteresis and promoting stable electron holes on O2−that are revealed by X-ray absorption spectroscopy. The results show that voltage hysteresis can be avoided in oxygen-redox cathodes by forming materials with a ribbon superstructure in the transition metal layers that suppresses migration of the transition metal.

Atomic level understanding of water on titanium dioxide (TiO2), in particular the mineral form anatase, is exceptionally important, considering its practical applications including photocatalytic water-splitting, photoinduced hydrophilicity , and water purification.1−3 Although much effort has been devoted to the study of water adsorption and dissociation on anatase, investigating the fundamental steps of single water splitting remains challenging. These challenges arise from the difficulty of chemical identification of single molecular water and its dissociation products. Here, we have unequivocally identified single water molecules and hydroxyls on the TiO2anatase (101) surface using scanning tunneling microscopy (STM) in combination with inelastic tunneling spectroscopy (IETS). Labeling of single molecules on the semiconductor surface was confirmed by demonstrating the isotope shift of the vibrational signatures and by first-principles density functional theory (DFT). The chemical identification of individual water molecules on anatase opens a direct path to follow the water splitting process on the single molecule level.

The adsorption of molecular oxygen has important implications for the catalytic activity of oxide surfaces, especially for oxidative TiO2catalysts. In the case of anatase TiO2(101), several studies have noted the importance of surface defects which stabilize the adsorption of oxygen molecules because of charge transfer. Further oxygen adsorption via this mechanism is precluded once a stoichiometric TiO2surface is attained. Here, the adsorption of oxygen molecules was re-enabled through a simple modification of sample cleaning procedures, resulting in the formation of a well-ordered trellis arrangement of oxygen molecules. The trellis is strongly passivating and does not affect the band gap of the pristine anatase substrate. Furthermore, they induce the extension of anatase terraces up to 100 Å wide. The atomic and electronic structures of the trellis were investigated using scanning tunneling microscopy experimentation and density functional theory calculations.

We investigate the Raman spectrum of the low-temperature orthorhombic phase of the organic–inorganic halide perovskite CH3NH3PbI3, by combining first-principles calculations with high-resolution low-temperature Raman measurements. We find good agreement between theory and experiment and successfully assign each of the Raman peaks to the underlying vibrational modes. In the low-frequency spectral range (below 60 cm–1), we assign the prominent Raman signals at 26, 32, 42, and 49 cm–1to the Pb–I–Pb bending modes with either Agor B2gsymmetry and the signal at 58 cm–1to the librational mode of the organic cation. Owing to their significant intensity, we propose that these peaks can serve as clear markers of the vibrations of the [PbI3]−network and of the CH3NH3+cations in this perovskite, respectively. In particular, the ratios of the intensities of these peaks might be used to monitor possible deviations from the ideal stoichiometry of CH3NH3PbI3.

The molecular structure and interactions of interfacial water, i.e., the first monolayers of water adsorbed on surfaces, are critical to determine the microscopic properties of solid–liquid interfaces. In particular, interfacial water on TiO2anatase (101) is of specific relevance to industrial, environmental, and medical applications. However, owing to the complexity and challenges in sample preparation, direct observation has been limited to individual or short-range ordered water molecules adsorbed on anatase. In this article, we report the direct observation of an unprecedented long-range ordered water monolayer on TiO2anatase (101). The adsorption structure and inter/intramolecular interactions of the water molecules have been investigated using scanning tunneling microscopy (STM) at 5 K in conjunction with density functional theory calculations. Additionally, the effect of STM tip-induced electric field was calculated to elucidate the ground-state structure of the water monolayer. The water molecules commensurately adsorb on the anatase (101) substrate; each oxygen atom of water forms a dative bond with one of its lone pair electrons to a surface Ti5catom. The remaining lone pair electron and OH bonds are oriented in the surface plane resulting in hydrophobic properties. The herein presented results are important to understand the complex wetting properties of the TiO2anatase surface.

The search for improved energy-storage materials has revealed Li- and Na-rich intercalation compounds as promising high-capacity cathodes. They exhibit capacities in excess of what would be expected from alkali-ion removal/reinsertion and charge compensation by transition-metal (TM) ions. The additional capacity is provided through charge compensation by oxygen redox chemistry and some oxygen loss. It has been reported previously that oxygen redox occurs in O 2p orbitals that interact with alkali ions in the TM and alkali-ion layers (that is, oxygen redox occurs in compounds containing Li+–O(2p)–Li+interactions). Na2/3[Mg0.28Mn0.72]O2exhibits an excess capacity and here we show that this is caused by oxygen redox, even though Mg2+resides in the TM layers rather than alkali-metal (AM) ions, which demonstrates that excess AM ions are not required to activate oxygen redox. We also show that, unlike the alkali-rich compounds, Na2/3[Mg0.28Mn0.72]O2does not lose oxygen. The extraction of alkali ions from the alkali and TM layers in the alkali-rich compounds results in severely underbonded oxygen, which promotes oxygen loss, whereas Mg2+remains in Na2/3[Mg0.28Mn0.72]O2, which stabilizes oxygen.

Using first-principles calculations, we perform a comprehensive and systematic analysis to establish the role of van der Waals (vdW) interactions and anharmonicity in the vibrational properties of the low-temperature orthorhombic phase of the hybrid perovskite CH3NH3PbI3. To this end, we consider the most common approaches for including vdW effects in our phonon calculations: the semiempirical Grimme approximations, the Tkatchenko-Scheffler dispersion corrections, and the vdW density-functional method. The vibrational normal modes are first calculated within the harmonic approximation. We consider the LDA and GGA approximations to the exchange-correlation functional and include spin–orbit coupling (SOC) effects. On top of the harmonic calculations, we also evaluate the anharmonicity of the normal modes and the phonon–phonon coupling by solving one-dimensional and two-dimensional nuclear Schrödinger equations, respectively, via the finite-displacement method. We observe that both the LDA and GGA approximations work remarkably well in describing the vibrational properties of CH3NH3PbI3. We find that vdW effects and relativistic effects do not have any significant impact on the vibrational properties of CH3NH3PbI3. Our study also reveals that the spinning modes of the organic CH3NH3+cations carry considerable anharmonicity but that the anharmonic coupling between different modes is relatively small.

In this work, we investigate the vibrational properties of the hybrid organic/inorganic halide perovskite MAPbI3(MA = CH3NH3) in the range 6–3500 cm–1by combining first-principles density-functional perturbation theory calculations and low-temperature infrared (IR) absorption measurements on evaporated perovskite films. By using a group factor analysis, we establish the symmetry of the normal modes of vibration and predict their IR and Raman activity. We validate our analysis via explicit calculation of the IR intensities. Our calculated spectrum is in good agreement with our measurements. By comparing theory and experiment, we are able to assign most of the features in the IR spectrum. Our analysis shows that the IR spectrum of MAPbI3can be partitioned into three distinct regions: the internal vibrations of the MA cations (800–3100 cm–1), the cation librations (140–180 cm–1), and the internal vibrations of the PbI3network (<100 cm–1). The low-frequency region of the IR spectrum is dominated by Pb–I stretching modes of the PbI3network with Busymmetry and librational modes of the MA cations. In addition, we find that the largest contributions to the static dielectric constant arise from Pb–I stretching and Pb–I–Pb rocking modes, and that one low-frequency B2uPb–I stretching mode exhibits a large LO–TO splitting of 50 cm–1.