On the Elusive Crystallography of Lithium-Rich Layered Oxides: Novel Structural Models

Lithium-rich layered oxides (LRLOs) are one of the most attractive families among future positive electrode materials for the so-called fourth generation of lithium-ion batteries (LIBs). Their electrochemical performance is enabled by the unique ambiguous crystal structure that is still not well understood despite decades of research. In the literature, a clear structural model able to describe their crystallographic features is missing thereby hindering a clear rationalization of the interplay between synthesis, structure, and functional properties. Here, the structure of a specific LRLO, Li1.28 Mn0.54 Ni0.13 Co0.02 Al0.03 O2 , using synchrotron X-ray diffraction (XRD), neutron diffraction (ND), and High-Resolution Transmission Electron Microscopy (HR-TEM), is analyzed. A systematic approach is applied to model diffraction patterns of Li1.28 Mn0.54 Ni0.13 Co0.02 Al0.03 O2 by using the Rietveld refinement method considering the R3 ¯ $\bar{3}$ m and C2/m unit cells as the prototype structures. Here, the relative ability of a variety of structural models is compared to match the experimental diffraction pattern evaluating the impact of defects and supercells derived from the R3 ¯ $\bar{3}$ m structure. To summarize, two possible models able to reconcile the description of experimental data are proposed here for the structure of Li1.28 Mn0.54 Ni0.13 Co0.02 Al0.03 O2 : namely a monoclinic C2/m defective lattice (prototype Li2 MnO3 ) and a monoclinic defective supercell derived from the rhombohedral R3 ¯ $\bar{3}$ m unit cell (prototype LiCoO2 ).

Stacking Faults Inducing Oxygen Anion Activities in Li2 MnO3

Controllable anionic redox for a transformational increase in the energy density is the pursuit of next generation Li-ion battery cathode materials. Its activation mechanism is coupled with the local coordination environment around O, which posts experimental challenges for control. Here, the tuning capability of anionic redox is shown by varying O local environment via experimentally controlling the density of stacking faults in Li₂ MnO₃ , the parent compound of Li-rich oxides. By combining computational analysis and spectroscopic study, it is quantitatively revealed that more stacking faults can trigger smaller LiOLi bond angles and larger LiO bond distance in local Li-rich environments and subsequently activate oxygen redox reactivity, which in turn enhances the reactivity of Mn upon the following reduction process. This study highlights the critical role of local structure environment in tuning the anionic reactivity, which provides guidance in designing high-capacity layered cathodes by appropriately adjusting stacking faults.

Impact of Synthesis Chelation on the Crystallography and Capacity of Li-Rich Li1.2Ni0.13Mn0.54Fe0.13O2 Cathode Particles

The quest for removal of cobalt from battery materials has intensified in the face of intensifying demand for batteries. Cobalt-free lithium-rich Li_1.2Ni_0.13Mn_0.54Fe_0.13O₂ (LNMFO) is synthesized under variation of chelating agent ratio and pH using the sol-gel method. Systematic search of the chelation and pH space found that the extractable capacity of the synthesized LNMFO is most clearly correlated to the ratio of chelating agent to transition metal oxide; a ratio of transition metal to citric acid of 2:1 achieves greater capacity at the expense of relative capacity retention. Charge-discharge cycling, dQ/dV analysis, XRD, and Raman at different charging potentials are used to quantify the different degrees of activation of the Li₂MnO₃ phase in the LNMFO powders synthesized under different chelation ratios. SEM and HRTEM analysis are employed to understand the effect of particle size and crystallography on the activation of Li₂MnO₃ phase in the composite particles. An unprecedented use of the marching cube algorithm to evaluate atomic scale tortuosity of crystallographic planes in HRTEM revealed that subtle undulations in the planes in addition to stacking faults correlate to the extracted capacity and stability of the various LNMFO synthesized.

Quantitative analysis of diffuse electron scattering in the lithium-ion battery cathode material Li1.2Ni0.13Mn0.54Co0.13O2

In contrast to perfectly periodic crystals, materials with short-range order produce diffraction patterns that contain both Bragg reflections and diffuse scattering. To understand the influence of short-range order on material properties, current research focuses increasingly on the analysis of diffuse scattering. This article verifies the possibility to refine the short-range order parameters in submicrometre-sized crystals from diffuse scattering in single-crystal electron diffraction data. The approach was demonstrated on Li_1.2Ni_0.13Mn_0.54Co_0.13O₂, which is a state-of-the-art cathode material for lithium-ion batteries. The intensity distribution of the 1D diffuse scattering in the electron diffraction patterns of Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ depends on the number of stacking faults and twins in the crystal. A model of the disorder in Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ was developed and both the stacking fault probability and the percentage of the different twins in the crystal were refined using an evolutionary algorithm in DISCUS. The approach was applied on reciprocal space sections reconstructed from 3D electron diffraction data since they exhibit less dynamical effects compared with in-zone electron diffraction patterns. A good agreement was achieved between the calculated and the experimental intensity distribution of the diffuse scattering. The short-range order parameters in submicrometre-sized crystals can thus successfully be refined from the diffuse scattering in single-crystal electron diffraction data using an evolutionary algorithm in DISCUS.

Stacking Faults Assist Lithium-Ion Conduction in a Halide-Based Superionic Conductor

In the pursuit of urgently needed, energy dense solid-state batteries for electric vehicle and portable electronics applications, halide solid electrolytes offer a promising path forward with exceptional compatibility against high-voltage oxide electrodes, tunable ionic conductivities, and facile processing. For this family of compounds, synthesis protocols strongly affect cation site disorder and modulate Li⁺ mobility. In this work, we reveal the presence of a high concentration of stacking faults in the superionic conductor Li₃YCl₆ and demonstrate a method of controlling its Li⁺ conductivity by tuning the defect concentration with synthesis and heat treatments at select temperatures. Leveraging complementary insights from variable temperature synchrotron X-ray diffraction, neutron diffraction, cryogenic transmission electron microscopy, solid-state nuclear magnetic resonance, density functional theory, and electrochemical impedance spectroscopy, we identify the nature of planar defects and the role of nonstoichiometry in lowering Li⁺ migration barriers and increasing Li site connectivity in mechanochemically synthesized Li₃YCl₆. We harness paramagnetic relaxation enhancement to enable ⁸⁹Y solid-state NMR and directly contrast the Y cation site disorder resulting from different preparation methods, demonstrating a potent tool for other researchers studying Y-containing compositions. With heat treatments at temperatures as low as 333 K (60 °C), we decrease the concentration of planar defects, demonstrating a simple method for tuning the Li⁺ conductivity. Findings from this work are expected to be generalizable to other halide solid electrolyte candidates and provide an improved understanding of defect-enabled Li⁺ conduction in this class of Li-ion conductors.

Synthesis-structure relationships in Li- and Mn-rich layered oxides: phase evolution, superstructure ordering and stacking faults

Li- and Mn-rich layered oxides are promising positive electrode materials for future Li-ion batteries. The presence of crystallographic features such as cation-mixing and stacking faults in these compounds make them highly susceptible to synthesis-induced structural changes. Consequently, significant variations exist in the reported structure of these compounds that complicate the understanding of how the crystallographic structure influences its properties. This work investigates the synthesis-structure relations for three widely investigated Li- and Mn-rich layered oxides: Li₂MnO₃, Li_1.2Mn_0.6Ni_0.2O₂ and Li_1.2Mn_0.54Ni_0.13Co_0.13O₂. For each compound, the average structure is compared between two synthetic routes of differing degrees of precursor mixing and four annealing protocols. Furthermore, thermodynamic and synthesis-specific kinetic factors governing the equilibrium crystallography of each composition are considered. It was found that the structures of these compounds are thermodynamically metastable under the synthesis conditions employed. In addition to a driving force to reduce stacking faults in the structure, these compositions also exhibited a tendency to undergo structural transformations to more stable phases under more intense annealing conditions. Increasing the compositional complexity introduced a kinetic barrier to structural ordering, making Li_1.2Mn_0.6Ni_0.2O₂ and Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ generally more faulted relative to Li₂MnO₃. Additionally, domains with different degrees of faulting were found to co-exist in the compounds. This study offers insight into the highly synthesis-dependent subtle structural complexities present in these compounds and complements the substantial efforts that have been undertaken to understand and optimise its electrochemical properties.

Assessing the local structure and quantifying defects in Ca4Fe9O17 combining STEM and FAULTS

Defects in crystalline structures play a vital role in their properties, so their proper characterization is essential to understanding and improving the behaviour of the materials.

Stacking Versatility in Alkali-Mixed Honeycomb Layered NaKNi2TeO6

The reaction between P2-type honeycomb layered oxides Na₂Ni₂TeO₆ and K₂Ni₂TeO₆ enables the formation of NaKNi₂TeO₆. The compound is characterized by X-ray diffraction and ²³Na solid-state nuclear magnetic resonance spectroscopy, and the structure is discussed through density functional theory calculations. In addition to the honeycomb Ni/Te cationic ordering, NaKNi₂TeO₆ exhibits a unique example of alternation of sodium and potassium layers instead of a random alkali-mixed occupancy. Stacking fault simulations underline the impact of the successive position of the Ni/Te honeycomb layers and validate the presence of multiple stacking sequences within the powder material, in proportions that evolve with the synthesis conditions. In a broader context, this work contributes to a better understanding of the alkali-mixed layered compounds.

Impact of Stacking Faults and Li Substitution in LixMnO3 (0 ≤ x ≤ 2) Structural Transformations upon Delithiation

Lithium-rich layered oxides appear in most roadmaps as next generation Li-ion cathode materials owing to their superior capacity. Within this family, Li₂MnO₃ represents the archetype material and is often taken as model compound to better understand the complex structural modifications occurring in the first charging cycle. In this work, density functional theory (DFT) calculations have been used to understand the impact of stacking faults in the structural transformations occurring in Li₂MnO₃ upon delithiation, which are found to hinder the phase transformations leading to structural degradation. The formation energies of both ideal and defective Li_xMnO₃ compositions and the analysis of the encountered ground states have been used to rationalize the predicted differences in terms of structural evolution. From the understanding of the origin in the O1 phase transformation, Mg substitution is proposed as alternative strategy to improve the structural stability in this family of materials.

First-principles study of Mn antisite defect in Li2MnO3

Lithium-rich layered Li₂MnO₃is regarded as a new generation cathode material for lithium-ion batteries because of its high energy density. Due to the different preparation methods and technological parameters, there are a lot of intrinsic defects in Li₂MnO₃. One frequently observed defect in experiments is Mn antisite defect (Mn_Li). In this work, we study the energetics and electronic properties involving Mn_Liin Li₂MnO₃through first-principles calculations. We find that Mn_Lican reduce the formation energy of Li vacancies around it, but increase that of O vacancies, indicating that Mn_Licould suppress the release of O around it and facilitate capacity retention. Both O and Mn near the Mn_Lican participate in charge compensation in the delithiation process. Furthermore, the effect of Mn_Lion the migration of Li and Mn is investigated. All possible migration paths are considered and it is found that Mn_Limakes the diffusion energy barrier of Li increased, but the diffusion energy barriers of Mn from transition metal layer to Li layer are decreased, especially for the migration of the defect Mn. The insight into the defect properties of Mn_Limakes further contribution to understand the relationship between intrinsic defects and electrochemical properties of Li₂MnO₃.

Anionic redox behaviors of layered Li-rich oxide cathodes

Lithium-rich and manganese-based cathodes deliver extraordinary specific capacity with a unique anion redox, and the structural changes during the reaction from the anion keep it reversible and are accompanied by irreversible oxygen loss.

Solid state chemistry for developing better metal-ion batteries

Metal-ion batteries are key enablers in today’s transition from fossil fuels to renewable energy for a better planet with ingeniously designed materials being the technology driver. A central question remains how to wisely manipulate atoms to build attractive structural frameworks of better electrodes and electrolytes for the next generation of batteries. This review explains the underlying chemical principles and discusses progresses made in the rational design of electrodes/solid electrolytes by thoroughly exploiting the interplay between composition, crystal structure and electrochemical properties. We highlight the crucial role of advanced diffraction, imaging and spectroscopic characterization techniques coupled with solid state chemistry approaches for improving functionality of battery materials opening emergent directions for further studies.

The development of high performing metal-ion batteries require guidelines to build improved electrodes and electrolytes. Here, the authors review the current state-of-the-art in the rational design of battery materials by exploiting the interplay between composition, crystal structure and electrochemical properties.

Influence of Synthesis Routes on the Crystallography, Morphology, and Electrochemistry of Li2MnO3

With the potential of delivering reversible capacities of up to 300 mAh/g, Li-rich transition-metal oxides hold great promise as cathode materials for future Li-ion batteries. However, a cohesive synthesis-structure-electrochemistry relationship is still lacking for these materials, which impedes progress in the field. This work investigates how and why different synthesis routes, specifically solid-state and modified Pechini sol-gel methods, affect the properties of Li₂MnO₃, a compositionally simple member of this material system. Through a comprehensive investigation of the synthesis mechanism along with crystallographic, morphological, and electrochemical characterization, the effects of different synthesis routes were found to predominantly influence the degree of stacking faults and particle morphology. That is, the modified Pechini method produced isotropic spherical particles with approximately 57% faulting and the solid-state samples possessed heterogeneous morphology with approximately 43% faulting probability. Inevitably, these differences lead to variations in electrochemical performance. This study accentuates the importance of understanding how synthesis affects the electrochemistry of these materials, which is critical considering the crystallographic and electrochemical complexities of the class of materials more generally. The methodology employed here is extendable to studying synthesis-property relationships of other compositionally complex Li-rich layered oxide systems.

Improved cycling properties of a Li-rich and Mn-based Li1.38Ni0.25Mn0.75O2.38 porous microspherical cathode material via micromorphological control

The as-prepared LMNO-850 with 100–200 nm spherical-like shape primary particles exhibits superior cycling performance even at high discharge rate. The capacity fading in the first 50 cycles may be caused by interfacial side-reactions between electrode and electrolyte.

Porous and highly conducting cathode material PrBaCo2O6-δ: bulk and surface studies of synthesis anomalies

The paradigm that chemical synthesis reduces the sintering temperature as compared to solid state synthesis seems to be violated in the case of the PrBaCo2O6-δ double perovskite. The sintering temperatures for pure phase samples synthesized through the solid state route (P-SSR) and the auto-combustion route (P-ACR) were found to be 1050 and 1150 °C, respectively. The porous microstructure of P-SSR is suitable for SOFC cathode materials while that of P-ACR is pore free. High-resolution transmission electron microscopy, Raman and scanning tunneling microscopy studies reveal that there is crystal growth on a smooth surface with a preferred orientation. Our results show that this anomalous synthesis behaviour is due to anisotropic surface nucleation growth. Thermodynamically, the higher decomposition temperature in the chemical route is due to stronger electron-phonon coupling and the higher value of change in entropy. The variation in the Co-O-Co bond angle reveals Jahn-Teller vibrational anisotropy in the-b plane leading to the anisotropic synthesis behaviour. This anisotropy is the reason for the violation of the paradigm.

On the Study of Ca and Mg Deintercalation from Ternary Tantalum Nitrides

Layered CaTaN₂ and MgTa₂N₃ and cubic Mg₂Ta₂N₄ were prepared by direct solid state reaction from the binary nitrides Ta₃N₅ and A₃N₂ (A: Mg, Ca). CaTaN₂ showed a slight Ca deficiency (0.11 moles per formula), and a monoclinic distortion from previously reported R3̅m symmetry, with space group C2/m and cell parameters a = 5.4011(2), b = 3.1434(1), c = 5.9464(2) Å and β = 107.91(3)°. Ca²⁺ and Mg²⁺ deintercalation was investigated in the three compounds both chemically and electrochemically. No significant Mg²⁺ extraction could be inferred for MgTa₂N₃ and Mg₂Ta₂N₄, neither after reaction with NO₂BF₄ nor after electrochemical oxidation at 100 °C in alkyl carbonate electrolytes. Rietveld refinement of the X-ray powder diffraction pattern of chemically oxidized Ca_0.89TaN₂ indicates a decrease of the Ca content to 0.34 concomitant to the disappearance of the monoclinic distortion and expansion of the interlayer space from 5.658 to 5.762 Å, space group R3̅m and cell parameters a = 3.1103(1) and c = 17.287(1) Å. Deintercalation in this compound was also achieved electrochemically at 100 °C. Results of density functional theory calculations seem to indicate that reaction mechanisms for CaTaN₂ oxidation additional and/or alternative to deintercalation are taking place, which is likely related to the loss of crystallinity observed upon oxidation and the irreversibility of the process.

Coulombic self-ordering upon charging a large-capacity layered cathode material for rechargeable batteries

Lithium- and sodium-rich layered transition-metal oxides have recently been attracting significant interest because of their large capacity achieved by additional oxygen-redox reactions. However, layered transition-metal oxides exhibit structural degradation such as cation migration, layer exfoliation or cracks upon deep charge, which is a major obstacle to achieve higher energy-density batteries. Here we demonstrate a self-repairing phenomenon of stacking faults upon desodiation from an oxygen-redox layered oxide Na₂RuO₃, realizing much better reversibility of the electrode reaction. The phase transformations upon charging A₂MO₃ (A: alkali metal) can be dominated by three-dimensional Coulombic attractive interactions driven by the existence of ordered alkali-metal vacancies, leading to counterintuitive self-repairing of stacking faults and progressive ordering upon charging. The cooperatively ordered vacancy in lithium-/sodium-rich layered transition-metal oxides is shown to play an essential role, not only in generating the electro-active nonbonding 2p orbital of neighbouring oxygen but also in stabilizing the phase transformation for highly reversible oxygen-redox reactions.

Here the authors report the evolution of stacking faults in Na₂RuO₃ showing that there is progressive cation ordering upon charging and notably the stacking faults can disappear. This behavior is driven by cooperative Coulombic interactions and can contribute to stabilizing the phase transformations.