Deintercalated NaCoO2 and LiCoO2

Synergetic Anion-Cation Redox Ensures a Highly Stable Layered Cathode for Sodium-Ion Batteries

Sodium-ion batteries are commonly regarded as a promising candidate in large-scale energy storage. Layered iron/manganese oxide cathodes receive extensive attentions due to the element abundance and large theoretical capacity. However, these materials usually undergo obvious degradation of electrochemical performance due to the tendency of Mn dissolution and Fe migration during continuous sodium release and uptake. Herein, a strategy of anion-cation synergetic redox is proposed to suppress the structural deterioration originated from overusing the electrochemical activity of transition-metal ions, and decreased lattice strain as well as superior electrochemical performance are realized simultaneously. Results show that the Na0.8 Li0.2 Fe0.2 Mn0.6 O2 (NLFM) electrode is highly resistant to the erosion of moisture that is distinct from the traditional Mn/Fe-based electrodes. Moreover, the NLFM electrode demonstrates solid solution behavior without phase transition during cycles. The ultra-small volume change of 0.85% is ascribed to the negligible manganese dissolution and invisible transition-metal migration. The high-stable layered structure assures superior reversible capacity of ≈165 mA h g-1 , excellent rate capability, and splendid capacity retention of over 98.3% with 100 cycles. The findings deepen the understanding of the synergy between anion and cation redox and provide new insights to design the high-stable layered cathode for sodium-ion batteries.

Kinetic 2D Crystals via Topochemical Approach

The designing of novel materials is a fascinating and innovative pathway in materials science. Particularly, novel layered compounds have tremendous influence in various research fields. Advanced fundamental studies covering various aspects, including reactants and synthetic methods, are required to obtain novel layered materials with unique physical and chemical properties. Among the promising synthetic techniques, topochemical approaches have afforded the platform for widening the extent of novel 2D materials. Notably, the synthesis of binary layered materials is considered as a major scientific breakthrough after the synthesis of graphene as they exhibit a wide spectrum of material properties with varied potential applicability. In this review, a comprehensive overview of the progress in the development of metastable layered compounds is presented. The various metastable layered compounds synthesized from layered ternary bulk materials through topochemical approaches are listed, followed by the descriptions of their mechanisms, structural analyses, characterizations, and potential applications. Finally, an essential research direction concerning the synthesis of new materials is indicated, wherein the possible application of topochemical approaches in unprecedented areas is explored.

Chemistry of Quantum Spin Liquids

Quantum spin liquids are an exciting playground for exotic physical phenomena and emergent many-body quantum states. The realization and discovery of quantum spin liquid candidate materials and associated phenomena lie at the intersection of solid-state chemistry, condensed matter physics, and materials science and engineering. In this review, we provide the current status of the crystal chemistry, synthetic techniques, physical properties, and research methods in the field of quantum spin liquids. We highlight a number of specific quantum spin liquid candidate materials and their structure-property relationships, elucidating their fascinating behavior and connecting it to the intricacies of their structures. Furthermore, we share our thoughts on defects and their inevitable presence in materials, of which quantum spin liquids are no exception, which can complicate the interpretation of characterization of these materials, and urge the community to extend their attention to materials preparation and data analysis, cognizant of the impact of defects. This review was written with the intention of providing guidance on improving the materials design and growth of quantum spin liquids, and to paint a picture of the beauty of the underlying chemistry of this exciting class of materials.

State-of-the-Art Electrode Materials for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) were investigated as recently as in the seventies. However, they have been overshadowed for decades, due to the success of lithium-ion batteries that demonstrated higher energy densities and longer cycle lives. Since then, the witness a re-emergence of the SIBs and renewed interest evidenced by an exponential increase of the publications devoted to them (about 9000 publications in 2019, more than 6000 in the first six months this year). This huge effort in research has led and is leading to an important and constant progress in the performance of the SIBs, which have conquered an industrial market and are now commercialized. This progress concerns all the elements of the batteries. We have already recently reviewed the salts and electrolytes, including solid electrolytes to build all-solid-state SIBs. The present review is then devoted to the electrode materials. For anodes, they include carbons, metal chalcogenide-based materials, intercalation-based and conversion reaction compounds (transition metal oxides and sulfides), intermetallic compounds serving as functional alloying elements. For cathodes, layered oxide materials, polyionic compounds, sulfates, pyrophosphates and Prussian blue analogs are reviewed. The electrode structuring is also discussed, as it impacts, importantly, the electrochemical performance. Attention is focused on the progress made in the last five years to report the state-of-the-art in the performance of the SIBs and justify the efforts of research.

Defect Chemistry, Sodium Diffusion and Doping Behaviour in NaFeO2 Polymorphs as Cathode Materials for Na-Ion Batteries: A Computational Study

Minor metal-free sodium iron dioxide, NaFeO2, is a promising cathode material in sodium-ion batteries. Computational simulations based on the classical potentials were used to study the defects, sodium diffusion paths and cation doping behaviour in the α- and β-NaFeO2 polymorphs. The present simulations show good reproduction of both α- and β-NaFeO2. The most thermodynamically favourable defect is Na Frenkel, whereas the second most favourable defect is the cation antisite, in which Na and Fe exchange their positions. The migration energies suggest that there is a very small difference in intrinsic Na mobility between the two polymorphs but their migration paths are completely different. A variety of aliovalent and isovalent dopants were examined. Subvalent doping by Co and Zn on the Fe site is calculated to be energetically favourable in α- and β-NaFeO2, respectively, suggesting the interstitial Na concentration can be increased by using this defect engineering strategy. Conversely, doping by Ge on Fe in α-NaFeO2 and Si (or Ge) on Fe in β-NaFeO2 is energetically favourable to introduce a high concentration of Na vacancies that act as vehicles for the vacancy-assisted Na diffusion in NaFeO2. Electronic structure calculations by using density functional theory (DFT) reveal that favourable dopants lead to a reduction in the band gap.

Defect, Diffusion and Dopant Properties of NaNiO2: Atomistic Simulation Study

Sodium nickelate, NaNiO2, is a candidate cathode material for sodium ion batteries due to its high volumetric and gravimetric energy density. The use of atomistic simulation techniques allows the examination of the defect energetics, Na-ion diffusion and dopant properties within the crystal. Here, we show that the lowest energy intrinsic defect process is the Na-Ni anti-site. The Na Frenkel, which introduces Na vacancies in the lattice, is found to be the second most favourable defect process and this process is higher in energy only by 0.16 eV than the anti-site defect. Favourable Na-ion diffusion barrier of 0.67 eV in the ab plane indicates that the Na-ion diffusion in this material is relatively fast. Favourable divalent dopant on the Ni site is Co2+ that increases additional Na, leading to high capacity. The formation of Na vacancies can be facilitated by doping Ti4+ on the Ni site. The promising isovalent dopant on the Ni site is Ga3+.

Electronic spin transition in nanosize stoichiometric lithium cobalt oxide

A change in the electronic spin state of the surfaces relevant to Li (de)intercalation of nanosized stoichiometric lithium cobalt oxide LiCo(III)O(2) from low-spin to intermediate and high spin is observed for the first time. These surfaces are the ones that are relevant for Li (de)intercalation. From density functional theory calculations with a Hubbard U correction, the surface energies of the layered lithium cobalt oxide can be significantly lowered as a consequence of the spin change. The crystal field splitting of Co d orbitals is modified at the surface due to missing Co-O bonds. The electronic spin transition also has a significant impact on Co(III)-Co(IV) redox potential, as revealed by the change in the lithium (de)intercalation voltage profile in a lithium half cell.

Reinvestigation of the OP4-(Li/Na)CoO2-layered system and first evidence of the (Li/Na/Na)CoO2 phase with OPP9 oxygen stacking

The composition and synthesis conditions of the (Li/Na)CoO(2) phase with an ordered 1:1 Li/Na stacking alternating with CoO(2) slabs were determined from a careful study of the P2-Na(∼0.7)CoO(2)-O3-LiCoO(2) system. An in situ X-ray diffraction (XRD) thermal study emphasizes the metastable character of this phase that can be stabilized only by very fast quenching. Its composition, (Li(0.42)Na(0.37))CoO(2), is significantly different from the ideally expected one, (Li(0.50)Na(0.35))CoO(2), and its structure, confirmed by Rietveld refinement of the XRD pattern, presents an ideal alternate ordering of lithium, cobalt, and sodium layers within OP4-type oxygen packing. The presence of vacancies in both alkali-ion layers was confirmed by electrochemical intercalation of lithium and sodium. For the first time, a new type of layered oxide exhibiting OPP9-type oxygen packing was evidenced. Between the CoO(2) slabs, alkali ions are intercalated in the following order: Li(octa)-Na(prism)-Na(prism). This material crystallizes in the R3m space group with a(hex) = 2.828 Å and c(hex) = 46.85 Å cell parameters.

Nonmagnetic insulator state in Na1CoO2 and phase separation of na vacancies

Crystallographic, magnetic, and NMR properties of a Na1CoO2 single crystal with x approximately = 1 are presented. We identify the stoichiometric Na1CoO2 phase, which is shown to be a nonmagnetic insulator, as expected for homogeneous planes of Co3+ ions with S = 0. In addition, we present evidence that, because of slight average Na deficiency, chemical and electronic phase separation leads to a segregation of Na vacancies into the well-defined, magnetic, Na0.8CoO2 phase. The importance of phase separation is discussed in the context of magnetic order for x approximately = 0.8 and the occurrence of a metal-insulator transition for x --> 1.

59Co, 23Na NMR and electric field gradient calculations in the layered cobalt oxides NaCoO2 and HCoO2

59Co and 23Na NMR has been applied to the layered cobalt oxides NaCoO(2) and HCoO(2) at three different magnetic field strengths (4.7, 7.1 and 11.7T). The 59Co and 23Na quadrupole and anisotropic shift tensors have been determined by iterative fitting of the NMR line shapes at the three magnetic field strengths. Due to the large 59Co quadrupole interaction in NaCoO(2), a frequency-swept irradiation procedure was used to alleviate the limited bandwidth of the excitation. While the 59Co and 23Na shift and quadrupole coupling tensors in NaCoO(2) are found to be coincident and axially symmetric in agreement with the crystal symmetry requirements, the fits of the 59Co NMR spectra clearly show the presence of structural disorder in HCoO(2). The 23Na chemical shift anisotropy can be reproduced by shift tensor calculations using a point dipole model and considering that the magnetic susceptibility in NaCoO(2) is due to Van Vleck paramagnetism for Co(3+). Electric field gradient calculations using either the empirical point charge model or the ab initio full potential-linearized augmented plane wave method are compared with the experimental NMR data.