Polymorphism and magnetic properties of Li2MSiO4 (M = Fe, Mn) cathode materials

Formation of oxygen vacancies in Li2FeSiO4: first-principles calculations

The formation of oxygen vacancies could affect various properties of oxides. Herein we have investigated the formation energies of an oxygen vacancy (V_O) with the relevant charge states in bulk Pnma-Li₂FeSiO₄ using first-principles calculations. The formation energies of the V_O are essentially dependent on the atomic chemical potentials that represent the experimental conditions. The calculated formation energies of an oxygen vacancy in different charge states indicate that it would be energetically favorable to fully ionize the oxygen vacancy in Li₂FeSiO₄. The presence of V_O is accompanied by a distinct redistribution of the electronic charge densities only around the Fe and Si ions next to the O-vacancy site, which shows a very local influence on the host material arising from V_O. This local characteristic is also confirmed by the calculated partial densities of states (PDOS). We also studied the influence of substitutional (Mn_Fe and Co_Fe) and cation vacancy defects (i.e., V_Fe and V_Li) in the vicinity of an O-vacancy on the formation of an O-vacancy, respectively. We find that the calculated interaction energies between these defects and the oxygen vacancy are all negative, which implies that the formation of an oxygen vacancy becomes easier when the above defects are introduced. Compared to the substitutional defects, the interaction energies between the vacancy defects and the oxygen vacancy are significantly larger. Among them, the interaction energy between V_Fe and V_O is the largest.

Fast 3D-lithium-ion diffusion and high electronic conductivity of Li2MnSiO4 surfaces for rechargeable lithium-ion batteries

High theoretical capacity, high thermal stability, the low cost of production, abundance, and environmental friendliness are among the potential attractiveness of Li2MnSiO4 as a positive electrode (cathode) material for rechargeable lithium-ion batteries. However, the experimental results indicated poor electrochemical performance in its bulk phase due to high intrinsic charge transfer resistance and capacity fading during cycling, which limit its large-scale commercial applications. Herein, we explore the surface stability and various lithium-ion diffusion pathways of Li2MnSiO4 surfaces using the density functional theory (DFT) framework. Results revealed that the stability of selected surfaces is in the following order: (210) > (001) > (010) > (100). Moreover, the Wulff-constructed equilibrium shape revealed that the Li2MnSiO4 (001) surface is the most predominant facet, and thus, preferentially exposed to electrochemical activities. The Hubbard-corrected DFT (DFT + U, with U = 3 eV) results indicated that the bulk insulator with a wide band gap (E g = 3.42 eV) changed into narrow electronic (E g = 0.6 eV) when it comes to the Li2MnSiO4 (001) surface. Moreover, the nudged elastic band analysis shows that surface diffusion along the (001) channel was found to be unlimited and fast in all three dimensions with more than 12-order-of-magnitude enhancements compared with the bulk system. These findings suggest that the capacity limitation and poor electrochemical performance that arise from limited electronic and ionic conductivity in the bulk system could be remarkably improved on the surfaces of the Li2MnSiO4 cathode material for rechargeable lithium-ion batteries.

Frequency-swept adiabatic pulses for broadband solid-state MAS NMR

We present a complete description of frequency-swept adiabatic pulses applied to isolated spin-1/2 nuclei with a shift anisotropy in solid materials under magic-angle spinning. Our theoretical framework unifies the existing descriptions of adiabatic pulses in the high-power regime, where the radiofrequency (RF) amplitude is greater than twice the spinning frequency, and the low-power regime, where the RF power is less than the spinning frequency, and so links the short high-powered adiabatic pulse (SHAP) and single-sideband-selective adiabatic pulses (S³AP) schemes used in paramagnetic solid-state NMR. We also identify a hitherto unidentified third regime intermediate between the low- and high-power regimes, and separated from them by rotary resonance conditions. We show that the prevailing benchmark of inversion performance based on (super) adiabatic factors is only applicable in the high- and intermediate-power regimes, but fails to account both for the poor performance at rotary resonance, and the impressive inversion seen in the low-power regime. For low-power pulses, which are non-adiabatic according to this definition of (super) adiabaticity, the effective Floquet Hamiltonian in the jolting frame reveals "hidden" (super) adiabaticity. The theory is demonstrated using a combination of simulation and experiment, and is used to refine the practical recommendations for the experimentalist who wishes to use these pulses.

Paramagnetic NMR in solution and the solid state

The field of paramagnetic NMR has expanded considerably in recent years. This review addresses both the theoretical description of paramagnetic NMR, and the way in which it is currently practised. We provide a review of the theory of the NMR parameters of systems in both solution and the solid state. Here we unify the different languages used by the NMR, EPR, quantum chemistry/DFT, and magnetism communities to provide a comprehensive and coherent theoretical description. We cover the theory of the paramagnetic shift and shift anisotropy in solution both in the traditional formalism in terms of the magnetic susceptibility tensor, and using a more modern formalism employing the relevant EPR parameters, such as are used in first-principles calculations. In addition we examine the theory first in the simple non-relativistic picture, and then in the presence of spin-orbit coupling. These ideas are then extended to a description of the paramagnetic shift in periodic solids, where it is necessary to include the bulk magnetic properties, such as magnetic ordering at low temperatures. The description of the paramagnetic shift is completed by describing the current understanding of such shifts due to lanthanide and actinide ions. We then examine the paramagnetic relaxation enhancement, using a simple model employing a phenomenological picture of the electronic relaxation, and again using a more complex state-of-the-art theory which incorporates electronic relaxation explicitly. An additional important consideration in the solid state is the impact of bulk magnetic susceptibility effects on the form of the spectrum, where we include some ideas from the field of classical electrodynamics. We then continue by describing in detail the solution and solid-state NMR methods that have been deployed in the study of paramagnetic systems in chemistry, biology, and the materials sciences. Finally we describe a number of case studies in paramagnetic NMR that have been specifically chosen to highlight how the theory in part one, and the methods in part two, can be used in practice. The systems chosen include small organometallic complexes in solution, solid battery electrode materials, metalloproteins in both solution and the solid state, systems containing lanthanide ions, and multi-component materials used in pharmaceutical controlled-release formulations that have been doped with paramagnetic species to measure the component domain sizes.

To Which Extent Is Paramagnetic Solid-State NMR Able To Address Polymorphism in Complex Transition-Metal Oxides?

A detailed characterization of the polymorphs constituting cathode materials, both before and after cell cycling, is mandatory to develop more stable and powerful lithium batteries. In many cases, e.g., for transition metal lithium silicates, standard diffraction techniques cannot give a clear-cut response. Here we show that broadband adiabatic fast MAS NMR can give unique information in the case of model Li₂(Mn,Fe)SiO₄ high-capacity cathode materials. By coupling ⁷Li and ²⁹Si 1D and 2D spectra, we are able to address polymorphs speciation also in the mixed Mn/Fe compositions, which is a nearly impossible task for X-rays and neutrons diffraction. We finally discuss the conditions under which this approach is useful when applied to rare nuclei such as ²⁹Si.

A2MnXO4 Family (A = Li, Na, Ag; X = Si, Ge): Structural and Magnetic Properties

Four new manganese germanates and silicates, A₂MnGeO₄ (A = Li, Na) and A₂MnSiO₄ (A = Na, Ag), were prepared, and their crystal structures were determined using the X-ray Rietveld method. All of them contain all components in tetrahedral coordination. Li₂MnGeO₄ is orthorhombic (Pmn2₁) layered, isostructural with Li₂CdGeO₄, and the three other compounds are monoclinic (Pn) cristobalite-related frameworks. As in other stuffed cristobalites of various symmetry (Pn A₂MXO₄, Pna2₁ and Pbca AMO₂), average bond angles on bridging oxygens (here, Mn-O-X) increase with increasing A/X and/or A/M radius ratios, indicating the trend to the ideal cubic (Fd3̅m) structure typified by CsAlO₂. The sublattices of the magnetic Mn²⁺ ions in both structure types under study (Pmn2₁ and Pn) are essentially the same; namely, they are pseudocubic eutaxy with 12 nearest neighbors. The magnetic properties of the four new phases plus Li₂MnSiO₄ were characterized by carrying out magnetic susceptibility, specific heat, magnetization, and electron spin resonance measurements and also by performing energy-mapping analysis to evaluate their spin exchange constants. Ag₂MnSiO₄ remains paramagnetic down to 2 K, but A₂MnXO₄ (A = Li, Na; X = Si, Ge) undergo a three-dimensional antiferromagnetic ordering. All five phases exhibit short-range AFM ordering correlations, hence showing them to be low-dimensional magnets and a magnetic field induced spin-reorientation transition at T < T_N for all AFM phases. We constructed the magnetic phase diagrams for A₂MnXO₄ (A = Li, Na; X = Si, Ge) on the basis of the thermodynamic data in magnetic fields up to 9 T. The magnetic properties of all five phases experimentally determined are well explained by their spin exchange constants evaluated by performing energy-mapping analysis.

New materials for Li-ion batteries: synthesis and spectroscopic characterization of Li2(FeMnCo)SiO4 cathode materials

Improving cathode materials is mandatory for next-generation Li-ion batteries. Exploring polyanion compounds with high theoretical capacity such as the lithium metal orthosilicates, Li2MSiO4 is of great importance. In particular, mixed silicates represent an advancement with practical applications. Here we present results on a rapid solid state synthesis of mixed Li2(FeMnCo)SiO4 samples in a wide compositional range. The solid solution in the P21/n space group was found to be stable for high iron concentration or for a cobalt content up to about 0.3 atom per formula unit. Other compositions led to a mixture of polymorphs, namely Pmn21 and Pbn21. All the samples contained a variable amount of Fe(3+) ions that was quantified by Mössbauer spectroscopy and confirmed by the TN values of the paramagnetic to antiferromagnetic transition. Preliminary characterization by cyclic voltammetry revealed the effect of Fe(3+) on the electrochemical response. Further work is required to determine the impact of these electrode materials on lithium batteries.

Broadband solid-state MAS NMR of paramagnetic systems

The combination of new magnet and probe technology with increasingly sophisticated pulse sequences has resulted in an increase in the number of applications of solid-state nuclear magnetic resonance (NMR) spectroscopy to paramagnetic materials and biomolecules. The interaction between the paramagnetic metal ions and the NMR-active nuclei often yields crucial structural or electronic information about the system. In particular the application of magic-angle spinning (MAS) has been shown to be crucial to obtaining resolution that is sufficiently high for studying complex systems. However such systems are generally extremely difficult to study as the shifts and shift anisotropies resulting from the same paramagnetic interaction broaden the spectrum beyond excitation and detection, and the paramagnetic relaxation enhancement (PRE) shortens the lifetimes of the excited signals considerably. One specific area that has therefore been receiving significant attention in recent years, and for which great improvements have been seen, is the development of broadband NMR sequences. The development of new excitation and inversion sequences for paramagnetic systems under MAS has often made the difference between the spectrum being unobtainable, and a complete NMR study being possible. However the development of the new sequences must explicitly take account of the modulation of the anisotropic shift interactions due to the sample rotation, with the resulting spin dynamics often being complicated considerably. The NMR sequences can either be helped or hindered by MAS, with the efficiency of some pulse schemes being destroyed, and others being greatly enhanced. This review describes the pulse sequences that have recently been proposed for broadband excitation, inversion, and refocussing of the signal components of paramagnetic systems. In doing so we define exactly what is meant by "broadband" under spinning conditions, and what the perfect pulse scheme should deliver. We also give a unified description of the spin dynamics under MAS which highlights the strengths and weaknesses of the various schemes, and which can be used as guidance for future research in this area. All the reviewed pulse schemes are evaluated both with simulations and experimental data obtained on the battery material LiFe(0.5)Mn(0.5)PO(4) which is typical of the complexity of the paramagnetic systems that are currently under study.

Advances in high-capacity Li2MSiO4 (M = Mn, Fe, Co, Ni, …) cathode materials for lithium-ion batteries

This review highlights the high-capacity Li₂MSiO₄ (M = Mn, Fe, Co, Ni, …) cathode materials for lithium-ion batteries.

Electrochemistry of orthosilicate-based lithium battery cathodes: a perspective

In this perspective we highlight the electrochemical features of lithium metal orthosilicates, investigated by combined in or ex situ XRD and electrochemical measurements.

Formation, structure and electrochemical performance of nano-sized Li2FeSiO4/C synthesized with the co-incorporation of citric acid and glucose followed by a two-step annealing

Nano-sized Li₂FeSiO₄/C composites are synthesized using a simple recipeviathe co-incorporation of citric acid and glucose with various molar ratios followed by a two-step annealing.