Intercalation chemistry of layered vanadyl phosphate: a review

Hydrothermal synthesis of layered zirconium phosphates and their intercalation properties

Layered inorganic materials are widely used as fillers in nanocomposites due to their unique properties such as mechanical performance, thermal properties, flame resistance, barrier properties, corrosion resistance, catalytic activity, and multifunctionality. Among them, layered α-zirconium phosphate (α-ZrP: Zr(HPO4)2H2O) is one of the representative inorganic materials and unique properties with P-O-H groups that can exchange inorganic and organic cations and intercalate basic molecules. This paper deals with the use of this compound as an adsorbent for heavy metal cations in wastewater. Samples were prepared by hydrothermal treatment at various Zr/P ratio, water amounts, and temperatures. Furthermore, an attempt was made to widen the interlayer distance of the sample using n-butylamine. The α- zirconium phosphate prepared at 160°C for 20 h with Zr/P = 1/2 adsorbed the most amount of metal ions. Furthermore, interlayer amine modification was effective. Then, the behaviour of the adsorption process was observed by varying the time of adsorption from 1 h to 50 h, in order to clarify the change in adsorption of the samples over time.

The phosphate cathodes for aqueous zinc-ion batteries

We categorize phosphate-based cathodes in zinc-ion battery and highlight the relationship between structural properties and energy storage mechanisms. The major problems faced by each kind of materials and rational optimization strategies are summarized.

A novel approach to characterization of a relatively unstable intercalation compound under ambient conditions: revisiting a kaolinite-acetone intercalation compound

Characteristics of a kaolinite-acetone intercalation compound prepared using a kaolinite N-methylformamide intercalation compound (Kaol-NMF) as an intermediate were obtained by a set of techniques with attention to suppressing evaporation and deintercalation of acetone. X-ray diffraction (XRD) with spectroscopic analyses, Fourier-transform infrared spectroscopy (FTIR) accompanied by solid-state ¹³C and ²⁹Si nuclear magnetic resonance (NMR) spectroscopy with cross polarization (CP) and magic angle spinning (MAS) enable us to demonstrate full replacement of a pre-intercalated NMF monolayer with an acetone monolayer between the layers of kaolinite with an increase in the basal spacing from 1.08 nm (Kaol-NMF) to 1.12 nm. In addition, the appearance of an additional OH stretching band at 3630 cm^-1 and the shift of the C[double bond, length as m-dash]O stretching band to a lower wavenumber, from 1714 to 1701 cm^-1, in the FTIR spectrum, along with a downfield shift of the signal due to C[double bond, length as m-dash]O groups from 209 ppm, where a singlet was observed in the liquid-state ¹³C NMR spectrum of acetone in CDCl₃, to 219 ppm in the ¹³C CP/MAS NMR spectrum, indicate hydrogen bond formation between interlayer hydroxyl groups of kaolinite and C[double bond, length as m-dash]O groups of the intercalated acetone molecules. These careful characterization studies provide information on an interaction between kaolinite and acetone under ambient conditions.

Towards Reversible High-Voltage Multi-Electron Reactions in Alkali-Ion Batteries Using Vanadium Phosphate Positive Electrode Materials

Vanadium phosphate positive electrode materials attract great interest in the field of Alkali-ion (Li, Na and K-ion) batteries due to their ability to store several electrons per transition metal. These multi-electron reactions (from V²⁺ to V⁵⁺) combined with the high voltage of corresponding redox couples (e.g., 4.0 V vs. for V³⁺/V⁴⁺ in Na₃V₂(PO₄)₂F₃) could allow the achievement the 1 kWh/kg milestone at the positive electrode level in Alkali-ion batteries. However, a massive divergence in the voltage reported for the V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺ redox couples as a function of crystal structure is noticed. Moreover, vanadium phosphates that operate at high V³⁺/V⁴⁺ voltages are usually unable to reversibly exchange several electrons in a narrow enough voltage range. Here, through the review of redox mechanisms and structural evolutions upon electrochemical operation of selected widely studied materials, we identify the crystallographic origin of this trend: the distribution of PO₄ groups around vanadium octahedra, that allows or prevents the formation of the vanadyl distortion (O^…V⁴⁺=O or O^…V⁵⁺=O). While the vanadyl entity massively lowers the voltage of the V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺ couples, it considerably improves the reversibility of these redox reactions. Therefore, anionic substitutions, mainly O^2- by F^-, have been identified as a strategy allowing for combining the beneficial effect of the vanadyl distortion on the reversibility with the high voltage of vanadium redox couples in fluorine rich environments.

Combined NMR and X-ray diffraction study of structural aspects, dynamics and charge ordering mechanism in LixVOPO4.2H2O intercalation compounds

We carried out a detailed investigation of the local ordering and dynamics of the lithium intercalation in paramagnetic Li_xVOPO₄.2H₂O (with 0 < x ≤ 1) materials. This question was addressed using a combination of X-ray diffraction, ³¹P and ⁷Li MAS NMR experiments. We first studied the structure of the fully ordered end-member of the series, Li₁VOPO₄.2H₂O, revisiting the X-ray single crystal diffraction data on the basis of the information provided by ³¹P MAS NMR. We then carried out ⁷Li MAS and exchange NMR experiments and ³¹P MAS experiments on the polycrystalline powders obtained after partial lithium insertion in VOPO₄.2H₂O phases. These experiments evidenced an unexpected ageing of the material related with lithium dynamics between the VOPO₄ layers and a V⁴⁺/V⁵⁺ charge ordering mechanism within the layers.

Structural Engineering of 2D Nanomaterials for Energy Storage and Catalysis

Research on 2D nanomaterials is rising to an unprecedented height and will continue to remain a very important topic in materials science. In parallel with the discovery of new candidate materials and exploration of their unique characteristics, there are intensive interests to rationally control and tune the properties of 2D nanomaterials in a predictable manner. Considerable attention is focused on modifying these materials structurally or engineering them into designed architectures to meet requirements for specific applications. Recent advances in such structural engineering strategies have demonstrated their ability to overcome current material limitations, showing great promise for promoting device performance to a new level in many energy-related applications. Existing in many forms, these strategies can be categorized based on how they intrinsically or extrinsically alter the pristine structure. Achieved through various synthetic routes and practiced in a range of different material systems, they usually share common descriptors that predestine them to be effective in certain circumstances. Therefore, understanding the underlying mechanism of these strategies to provide fundamental insights into structural design and property tailoring is of critical importance. Here, the most recent development of structural engineering of 2D nanomaterials and their significant effects in energy storage and catalysis technologies are addressed.

Effective Interlayer Engineering of Two-Dimensional VOPO4 Nanosheets via Controlled Organic Intercalation for Improving Alkali Ion Storage

Two-dimensional (2D) energy materials have shown the promising electrochemical characteristics for lithium ion storage. However, the decreased active surfaces and the sluggish charge/mass transport for beyond-lithium ion storage that has potential for large-scale energy storage systems, such as sodium or potassium ion storage, caused by the irreversible restacking of 2D materials during electrode processing remain a major challenge. Here we develop a general interlayer engineering strategy to address the above-mentioned challenges by using 2D ultrathin vanadyl phosphate (VOPO₄) nanosheets as a model material for challenging sodium ion storage. Via controlled intercalation of organic molecules, such as triethylene glycol and tetrahydrofuran, the sodium ion transport in VOPO₄ nanosheets has been significantly improved. In addition to advanced characterization including X-ray diffraction, high-resolution transmission electron microscopy, and X-ray absorption fine structure to characterize the interlayer and the chemical bonding/configuration between the organic intercalants and the VOPO₄ host layers, density functional theory calculations are also performed to understand the diffusion behavior of sodium ions in the pure and TEG intercalated VOPO₄ nanosheets. Because of the expanded interlayer spacing in combination with the decreased energy barriers for sodium ion diffusion, intercalated VOPO₄ nanosheets show much improved sodium ion transport kinetics and greatly enhanced rate capability and cycling stability for sodium ion storage. Our results afford deeper understanding of the interlayer-engineering strategy to improve the sodium ion storage performance of the VOPO₄ nanosheets. Our results may also shed light on possible multivalent-ion based energy storage such as Mg²⁺ and Al³⁺.

Intercalation of 1,n-diols into strontium phenylphosphonate: how the shape of the host layers influences arrangement of the guest molecules

Strontium phenylphosphonate intercalates with 1,n diols (n=2-4, 6-8) having general formula SrC6H5PO3⋅x(HO(CH2)nOH)⋅yH2O were prepared by precipitation from strontium phenylphosphonate solution and the corresponding diols. Prepared compounds exhibit a very good stability at ambient conditions. The intercalates were characterized by X-ray diffraction, thermogravimetry and elemental analysis. Thanks to the existence of free spaces among the benzene rings the diols exhibit a peculiar intercalation behavior. This behavior is explained on the basis of molecular simulation, which facilitated to elucidate the arrangement of the diol (guest) molecules in the specifically shaped space between the layers of the host material. From the structural point of view the intercalates can be divided into two subgroups: (i) intercalates with 1,2- to 1,4-diols and (ii) intercalates with 1,6- to 1,8-diols. The alkanediols of the first group are immersed in the free spaces among the benzene groups, their molecules adopt a horseshoe shape meaning cis conformation and are bonded by both of their OH groups to one host layer. The longer alkanediol chains of the second group allow anchoring to both neighboring layers of the host forming a kind of pillared structure in the interlayer space. The diol molecules are in this case bonded to the host layers by their OH groups to the oxygen atoms of the host layers and to water molecules present in the interlayer space through hydrogen bonds. The values of the basal spacing obtained from the experimental powder X-ray patterns are in a very good agreement with the basal spacing values calculated from the models. The molecular simulation of a 1,5-pentanediol intercalate, which we were not be able to synthesize, explained why this intercalate cannot be stable.