Probing Jahn-Teller Distortions and Antisite Defects in LiNiO2 with 7Li NMR Spectroscopy and Density Functional Theory.

The long- and local-range structure and electronic properties of the high-voltage lithium-ion cathode material for Li-ion batteries, LiNiO2, remain widely debated, as are the degradation phenomena at high states of delithiation, limiting the more widespread use of this material. In particular, the local structural environment and the role of Jahn-Teller distortions are unclear, as are the interplay of distortions and point defects and their influence on cycling behavior. Here, we use ex situ7Li NMR measurements in combination with density functional theory (DFT) calculations to examine Jahn-Teller distortions and antisite defects in LiNiO2. We calculate the 7Li Fermi contact shifts for the Jahn-Teller distorted and undistorted structures, the experimental 7Li room-temperature spectrum being ascribed to an appropriately weighted time average of the rapidly fluctuating structure comprising collinear, zigzag, and undistorted domains. The 7Li NMR spectra are sensitive to the nature and distribution of antisite defects, and in combination with DFT calculations of different configurations, we show that the 7Li resonance at approximately -87 ppm is characteristic of a subset of Li-Ni antisite defects, and more specifically, a Li+ ion in the Ni layer that does not have an associated Ni ion in the Li layer in its 2nd cation coordination shell. Via ex situ7Li MAS NMR, X-ray diffraction, and electrochemical experiments, we identify the 7Li spectral signatures of the different crystallographic phases on delithiation. The results imply fast Li-ion dynamics in the monoclinic phase and indicate that the hexagonal H3 phase near the end of charge is largely devoid of Li.

Jahn-Teller Distortions and Phase Transitions in LiNiO2: Insights from Ab Initio Molecular Dynamics and Variable-Temperature X-ray Diffraction.

The atomistic structure of lithium nickelate (LiNiO2), the parent compound of Ni-rich layered oxide cathodes for Li-ion batteries, continues to elude a comprehensive understanding. The common consensus is that the material exhibits local Jahn-Teller distortions that dynamically reorient, resulting in a time-averaged undistorted R3̅m structure. Through a combination of ab initio molecular dynamics (AIMD) simulations and variable-temperature X-ray diffraction (VT-XRD), we explore Jahn-Teller distortions in LiNiO2 as a function of temperature. Static Jahn-Teller distortions are observed at low temperatures (T < 250 K) via AIMD simulations, followed by a broad phase transition that occurs between 250 and 350 K, leading to a highly dynamic, displacive phase at high temperatures (T > 350 K), which does not show the four short and two long bonds characteristic of local Jahn-Teller distortions. These transitions are followed in the AIMD simulations via abrupt changes in the calculated pair distribution function and the bond-length distortion index and in X-ray diffraction via the monoclinic lattice parameter ratio, amon/bmon, and δ angle, the fit quality of an R3̅m-based structural refinement, and a peak sharpening of the diffraction peaks on heating, consistent with the loss of distorted domains. Between 250 and 350 K, a mixed-phase regime is found via the AIMD simulations where distorted and undistorted domains coexist. The repeated change between the distorted and undistorted states in this mixed-phase regime allows the Jahn-Teller long axes to change direction. These pseudorotations of the Ni-O long axes are a side effect of the onset of the displacive phase transition. Antisite defects, involving Li ions in the Ni layer and Ni ions in the Li layer, are found to pin the undistorted domains at low temperatures, impeding cooperative ordering at a longer length scale.

O3 to O1 Phase Transitions in Highly Delithiated NMC811 at Elevated Temperatures.

Nickel-rich layered oxide cathodes such as NMC811 (LixNi0.8Mn0.1Co0.1O2) currently have the highest practical capacities of cathodes used commercially, approaching 200 mAh/g. Lithium is removed from NMC811 via a solid-solution behavior when delithiated to xLi > 0.10, maintaining the same layered (O3 structure) throughout as observed via operando diffraction measurements. Although it is possible to further delithiate NMC811, it is kinetically challenging, and there are significant side reactions between the electrolyte and cathode surface. Here, small format, NMC811-graphite pouch cells were charged to high voltages at elevated temperatures and held for days to access high states of delithiation. Rietveld refinements on high-resolution diffraction data and indexing of selected area electron diffraction patterns, both acquired ex situ, show that NMC811 undergoes a partial and reversible transition from the O3 to the O1 phase under these conditions. The O1 phase fraction depends not only on the concentration of intercalated lithium but also on the hold temperature and hold time, indicating that the phase transition is kinetically controlled. 1H NMR spectroscopy shows that the proton concentration decreases with O1 phase fraction and is not, therefore, likely to be driving the O3-O1 phase transition.

Formation of new crystalline qtz-[Zn(mIm)2] polymorph from amorphous ZIF-8.

The structure of a new ZIF-8 polymorph with quartz topology (qtz) is reported. This qtz-[Zn(mIm)2] phase was obtained by mechanically amorphising crystalline ZIF-8, before heating the resultant amorphous phase to between 282 and 316 °C. The high-temperature phase structure was obtained from powder X-ray diffraction, and its thermal behaviour, CO2 gas sorption properties and dye adsorption ability were investigated.

Negative X-ray expansion in cadmium cyanide.

Cadmium cyanide, Cd(CN)2, is a flexible coordination polymer best studied for its strong and isotropic negative thermal expansion (NTE) effect. Here we show that this NTE is actually X-ray-exposure dependent: Cd(CN)2 contracts not only on heating but also on irradiation by X-rays. This behaviour contrasts that observed in other beam-sensitive materials, for which X-ray exposure drives lattice expansion. We call this effect 'negative X-ray expansion' (NXE) and suggest its origin involves an interaction between X-rays and cyanide 'flips'; in particular, we rule out local heating as a possible mechanism. Irradiation also affects the nature of a low-temperature phase transition. Our analysis resolves discrepancies in NTE coefficients reported previously on the basis of X-ray diffraction measurements, and we establish the 'true' NTE behaviour of Cd(CN)2 across the temperature range 150-750 K. The interplay between irradiation and mechanical response in Cd(CN)2 highlights the potential for exploiting X-ray exposure in the design of functional materials.

Spin-ice physics in cadmium cyanide.

Spin-ices are frustrated magnets that support a particularly rich variety of emergent physics. Typically, it is the interplay of magnetic dipole interactions, spin anisotropy, and geometric frustration on the pyrochlore lattice that drives spin-ice formation. The relevant physics occurs at temperatures commensurate with the magnetic interaction strength, which for most systems is 1-5 K. Here, we show that non-magnetic cadmium cyanide, Cd(CN)2, exhibits analogous behaviour to magnetic spin-ices, but does so on a temperature scale that is nearly two orders of magnitude greater. The electric dipole moments of cyanide ions in Cd(CN)2 assume the role of magnetic pseudospins, with the difference in energy scale reflecting the increased strength of electric vs magnetic dipolar interactions. As a result, spin-ice physics influences the structural behaviour of Cd(CN)2 even at room temperature.

High Pressure Crystal Structure and Electrical Properties of a Single Component Molecular Crystal [Ni(dddt)2] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate).

Single-component molecular conductors form an important class of materials showing exotic quantum phenomena, owing to the range of behavior they exhibit under physical stimuli. We report the effect of high pressure on the electrical properties and crystal structure of the single-component crystal [Ni(dddt)2] (where dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate). The system is isoelectronic and isostructural with [Pd(dddt)2], which is the first example of a single-component molecular crystal that exhibits nodal line semimetallic behavior under high pressure. Systematic high pressure four-probe electrical resistivity measurements were performed up to 21.6 GPa, using a Diamond Anvil Cell (DAC), and high pressure single crystal synchrotron X-ray diffraction was performed up to 11.2 GPa. We found that [Ni(dddt)2] initially exhibits a decrease of resistivity upon increasing pressure but, unlike [Pd(dddt)2], it shows pressure-independent semiconductivity above 9.5 GPa. This correlates with decreasing changes in the unit cell parameters and intermolecular interactions, most notably the π-π stacking distance within chains of [Ni(dddt)2] molecules. Using first-principles density functional theory (DFT) calculations, based on the experimentally-determined crystal structures, we confirm that the band gap decreases with increasing pressure. Thus, we have been able to rationalize the electrical behavior of [Ni(dddt)2] in the pressure-dependent regime, and suggest possible explanations for its pressure-independent behavior at higher pressures.

Synthesis, PtS-type structure, and anomalous mechanics of the Cd(CN)2 precursor Cd(NH3)2[Cd(CN)4].

We report the nonaqueous synthesis of Cd(CN)2 by oxidation of cadmium metal with Hg(CN)2 in liquid ammonia. The reaction proceeds via an intermediate of composition Cd(NH3)2[Cd(CN)4], which converts to Cd(CN)2 on prolonged heating. Powder X-ray diffraction measurements allow us to determine the crystal structure of the previously-unreported Cd(NH3)2[Cd(CN)4], which we find to adopt a twofold interpenetrating PtS topology. We discuss the effect of partial oxidation on the Cd/Hg composition of this intermediate, as well as its implications for the reconstructive nature of the deammination process. Variable-temperature X-ray diffraction measurements allow us to characterise the anisotropic negative thermal expansion (NTE) behaviour of Cd(NH3)2[Cd(CN)4] together with the effect of Cd/Hg substitution; ab initio density functional theory (DFT) calculations reveal a similarly anomalous mechanical response in the form of both negative linear compressibility (NLC) and negative Poisson's ratios.