Solvent exfoliation stabilizes TiS2 nanosheets against oxidation, facilitating lithium storage applications

Soft-in-Rigid Strategy Promoting Rapid and High-Capacity Lithium Storage by Chemical Scissoring

Large and rapid lithium storage is hugely demanded for high-energy/power lithium-ion batteries; however, it is difficult to achieve these two indicators simultaneously. Sn-based materials with a (de)alloying mechanism show low working potential and high theoretical capacity, but the huge volume expansion and particle agglomeration of Sn restrict cyclic stability and rate capability. Herein, a soft-in-rigid concept was proposed and achieved by chemical scissoring where a soft Sn-S bond was chosen as chemical tailor to break the Ti-S bond to obtain a loose stacking structure of 1D chain-like Sn1.2Ti0.8S3. The in situ and ex situ (micro)structural characterizations demonstrate that the Sn-S bonds are reduced into Sn domains and such Sn disperses in the rigid Ti-S framework, thus relieving the volume expansion and particle agglomeration by chemical and physical shielding. Benefiting from the merits of large-capacity Sn with an alloying mechanism and high-rate TiS2 with an intercalation mechanism, the Sn1.2Ti0.8S3 anode offers a high specific capacity of 963.2 mA h g-1 at 0.1 A g-1 after 100 cycles and a reversible capacity of 250 mA h g-1 at 10 A g-1 after 3900 cycles. Such a strategy realized by chemical tailoring at the structural unit level would broaden the prospects for constructing joint high-capacity and high-rate LIB anodes.

Pressure-modulated lattice structural evolution in TiS2

Titanium disulfide (TiS2) has drawn considerable attention in materials, physics, and chemistry thanks to its potential applications in batteries, supercapatteries and thermoelectric devices. However, the simplified and controlled synthesis of high-quality TiS2 remains a great challenge. In this study, a straightforward widely accessible approach to the one-step chemical vapor transport (CVT) process is presented. Meanwhile, combining high-pressure (HP) Raman spectroscopy measurements and first-principles calculations, the pressure-induced phase transition of TiS2 from P3̄m1 phase (phase I) to C2/m phase (phase II) at 16.0 GPa and then to P6̄2m phase (phase III) at 32.4 GPa was disclosed. The discovery of HP being within the Weyl semi-metallic phase represents a significant advancement towards understanding the electronic topological states, discovering new physical phenomena, developing new electronic devices, and gaining insight into the properties of elementary particles.

Sonication-assisted liquid phase exfoliation of two-dimensional CrTe3 under inert conditions

Liquid phase exfoliation (LPE) has been used for the successful fabrication of nanosheets from a large number of van der Waals materials. While this allows to study fundamental changes of material properties' associated with reduced dimensions, it also changes the chemistry of many materials due to a significant increase of the effective surface area, often accompanied with enhanced reactivity and accelerated oxidation. To prevent material decomposition, LPE and processing in inert atmosphere have been developed, which enables the preparation of pristine nanomaterials, and to systematically study compositional changes over time for different storage conditions. Here, we demonstrate the inert exfoliation of the oxidation-sensitive van der Waals crystal, CrTe3. The pristine nanomaterial was purified and size-selected by centrifugation, nanosheet dimensions in the fractions quantified by atomic force microscopy and studied by Raman, X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX) and photo spectroscopic measurements. We find a dependence of the relative intensities of the CrTe3 Raman modes on the propagation direction of the incident light, which prevents a correlation of the Raman spectral profile to the nanosheet dimensions. XPS and EDX reveal that the contribution of surface oxides to the spectra is reduced after exfoliation compared to the bulk material. Further, the decomposition mechanism of the nanosheets was studied by time-dependent extinction measurements after water titration experiments to initially dry solvents, which suggest that water plays a significant role in the material decomposition.

Black Phosphorus Stabilized by Titanium Disulfide and Graphite via Chemical Bonds for High-performance Lithium Storage

Black phosphorus (BP) anode has received extensive attentions for lithium-ion batteries (LIBs) due to its ultrahigh theoretical specific capacity (2596 mAh g^-1) and superior electronic conductivity (≈10² S m^-1). However, the enormous volume variations during lithiation/delitiation processes greatly limit its applications. Herein, a new BP-titanium disulfide-graphite (BP-TiS₂-G) nanocomposite composed of BP, titanium disulfide and graphite has been prepared by a facile and scalable high-energy ball milling method. The experimental data proves that PC and PS bonds have been successfully introduced at the interface, which can effectively maintain the structural integrity of the BP-TiS₂-G electrode when evaluated as an anode material for LIBs. In addition, lithium-ion diffusion kinetics have been demonstrated to be enhanced from the synergistic effect of PC and PS bonds. As a result, the BP-TiS₂-G anode shows outstanding cycling stability (906.2 mAh g^-1 after 1300 cycles at 1.0 A g^-1) and superior rate performance (313.8 mAh g^-1 at 10.0 A g^-1). Our work shows the synergistic effects of different chemical bonds to stabilize BP can be a potential strategy for the development of high-performance alloy-type anodes for rechargeable batteries.

Liquid Processing of Interfacially Grown Iron-Oxide Flowers into 2D-Platelets Yields Lithium-Ion Battery Anodes with Capacities of Twice the Theoretical Value

Iron oxide (Fe₂ O₃ ) is an abundant and potentially low-cost material for fabricating lithium-ion battery anodes. Here, the growth of α-Fe₂ O₃ nano-flowers at an electrified liquid-liquid interface is demonstrated. Sonication is used to convert these flowers into quasi-2D platelets with lateral sizes in the range of hundreds of nanometers and thicknesses in the range of tens of nanometers. These nanoplatelets can be combined with carbon nanotubes to form porous, conductive composites which can be used as electrodes in lithium-ion batteries. Using a standard activation process, these anodes display good cycling stability, reasonable rate performance and low-rate capacities approaching 1500 mAh g^-1 , consistent with the current state-of-the-art for Fe₂ O₃ . However, by using an extended activation process, it is found that the morphology of these composites can be significantly changed, rendering the iron oxide amorphous and significantly increasing the porosity and internal surface area. These morphological changes yield anodes with very good cycling stability and low-rate capacity exceeding 2000 mAh g^-1 , which is competitive with the best anode materials in the literature. However, the data implies that, after activation, the iron oxide displays a reduced solid-state lithium-ion diffusion coefficient resulting in somewhat degraded rate performance.

The Role of Additives in Suppressing the Degradation of Liquid-Exfoliated WS2 Monolayers

Group VI transition metal dichalcogenides (TMDs) are considered to be chemically widely inert, but recent reports point toward an oxidation of monolayered sheets in ambient conditions, due to defects. To date, the degradation of monolayered TMDs is only studied on individual, substrate-supported nanosheets with varying defect type and concentration, strain, and in an inhomogeneous environment. Here, degradation kinetics of WS₂ nanosheet ensembles in the liquid phase are investigated through photoluminescence measurements, which selectively probe the monolayers. Monolayer-enriched WS₂ dispersions are produced with varying lateral sizes in the two common surfactant stabilizers sodium cholate (SC) and sodium dodecyl sulfate (SDS). Well-defined degradation kinetics are observed, which enable the determination of activation energies of the degradation and decouple photoinduced and thermal degradation. The thermal degradation is slower than the photoinduced degradation and requires higher activation energy. Using SC as surfactant, it is sufficiently suppressed. The photoinduced degradation can be widely prevented through chemical passivation achieved through the addition of cysteine which, on the one hand, coordinates to defects on the nanosheets and, on the other hand, stabilizes oxides on the surface, which shield the nanosheets from further degradation.

Dispersant-assisted liquid-phase exfoliation of 2D materials beyond graphene

The extensive research on liquid-phase exfoliation (LPE) performed in the last 10 years has enabled a low cost and mass scalable approach to the successful production of a range of solution-processed 2-dimensional (2D) materials suitable for many applications, from composites to energy storage and printed electronics. However, direct LPE requires the use of specific solvents, which are typically toxic and expensive. Dispersant-assisted LPE allows us to overcome this problem by enabling production of solution processed 2D materials in a wider range of solvents, including water. This approach is based on the inclusion of an additive, typically an amphiphilic molecule, designed to interact with both the nanosheet and the solvent, enabling exfoliation and stabilization at the same time. This method has been extensively used for the LPE of graphene and has been discussed in many reviews, whilst little attention has been given to dispersant-assisted LPE of 2D materials beyond graphene. Considering the increasing number of 2D materials and their potential in many applications, from nanomedicine to energy storage and catalysis, this review focuses on the dispersant-assisted LPE of transition metal dichalcogenides (TMDs), hexagonal boron nitride (h-BN) and less studied 2D materials. We first provide an introduction to the fundamentals of LPE and the type of dispersants that have been used for the production of graphene, we then discuss each class of 2D material, providing an overview on the concentration and properties of the nanosheets obtained. Finally, a perspective is given on some of the challenges that need to be addressed in this field of research.

Liquid-Phase Exfoliation of Rhenium Disulfide by Solubility Parameter Matching

In this work, we provide a detailed account of the liquid-phase exfoliation (LPE) of rhenium disulfide (ReS₂), a promising new-generation two-dimensional material. By screening LPE in a wide range of solvents, we show that the most optimal solvents are characterized by similar Hildebrand or dispersive Hansen solubility parameters of 25 and 18 MPa^1/2, respectively. Such values are attained by solvents such as N-methyl-2-pyrrolidone, N,N-dimethylformamide, and 1-butanol. In line with solution thermodynamics, we interpret the conditions for high-yield exfoliation as a matching of the solvent and ReS₂ solubility parameters. Using N-methyl-2-pyrrolidone as an exemplary exfoliation solvent, we undertook a detailed analysis of the exfoliated ReS₂. In-depth morphological, structural, and elemental characterization outlined that the LPE procedure presented here produces few-layer, anisotropically stacked, and chemically pure ReS₂ platelets with long-term stability against oxidation. These results underscore the suitability of LPE to batch-produce few-layer and pristine ReS₂ in solvents that have a solubility parameter close to 25 MPa^1/2.

Production of Quasi-2D Platelets of Nonlayered Iron Pyrite (FeS2) by Liquid-Phase Exfoliation for High Performance Battery Electrodes

Over the past 15 years, two-dimensional (2D) materials have been studied and exploited for many applications. In many cases, 2D materials are formed by the exfoliation of layered crystals such as transition-metal disulfides. However, it has recently become clear that it is possible to exfoliate nonlayered materials so long as they have a nonisotropic bonding arrangement. Here, we report the synthesis of 2D-platelets from the earth-abundant, nonlayered metal sulfide, iron pyrite (FeS₂), using liquid-phase exfoliation. The resultant 2D platelets exhibit the same crystal structure as bulk pyrite but are surface passivated with a density of 14 × 10¹⁸ groups/m². They form stable suspensions in common solvents and can be size-selected and liquid processed. Although the platelets have relatively low aspect ratios (∼5), this is in line with the anisotropic cleavage energy of bulk FeS₂. We observe size-dependent changes to optical properties leading to spectroscopic metrics that can be used to estimate the dimensions of platelets. These platelets can be used to produce lithium ion battery anodes with capacities approaching 1000 mAh/g.

The Rate Performance of Two-Dimensional Material-Based Battery Electrodes May Not Be as Good as Commonly Believed

Two-dimensional (2D) materials show great potential for use in battery electrodes and are believed to be particularly promising for high-rate applications. However, there does not seem to be much hard evidence for the superior rate performance of 2D materials compared to non-2D materials. To examine this point, we have analyzed published rate-performance data for a wide range of 2D materials as well as non-2D materials for comparison. For each capacity-rate curve, we extract parameters that quantify performance which can then be analyzed using a simple mechanistic model. Contrary to expectations, by comparing a previously proposed figure of merit, we find 2D-based electrodes to be on average ∼40 times poorer in terms of rate performance than non-2D materials. This is not due to differences in solid-state diffusion times which were similarly distributed for 2D and non-2D materials. In fact, we found the main difference between 2D and non-2D materials is that ion mobility within the electrolyte-filled pores of the electrodes is significantly lower for 2D materials, a situation which we attribute to their high aspect ratios.