Tracking the Chemical and Structural Evolution of the TiS2 Electrode in the Lithium-Ion Cell Using Operando X-ray Absorption Spectroscopy

Quantitative in situ synchrotron X-ray analysis of the ALD/MLD growth of transition metal dichalcogenide TiS2 ultrathin films

We present the results of a full quantitative analysis of X-ray absorption spectroscopy (XAS) performed in situ during the growth of ultrathin titanium disulfide (TiS2) films via an innovative two-step process, i.e. atomic layer deposition/molecular layer deposition (ALD/MLD) followed by annealing. This growth strategy aims at separating the growth process from the crystallization process by first creating an amorphous Ti-thiolate that is converted later to crystalline TiS2via thermal annealing. The simultaneous analysis of Ti and S K-edge XAS spectra, exploiting the insights from density functional theory calculations, allows us to shed light on the chemical and structural mechanisms underlying the main steps of growth. The nature of the bonding at the base of the interface creation with the SiO2 substrate is disclosed in this study. Evidence of a progressive incorporation of S in the amorphous Ti-thiolate is given. Finally, it is shown that the annealing step plays a critical role since the transformation of the Ti-thiolate into nanocrystalline TiS2 and the loss of S are simultaneously induced, validating the two-step synthesis approach, which entails distinct growth and crystallization steps. These observations contribute to a deeper understanding of the bonding mechanism at the interface and provide insights for future research in this field and the generation of ultra-thin layered materials.

Improving rechargeable magnesium batteries through dual cation co-intercalation strategy

The development of competitive rechargeable Mg batteries is hindered by the poor mobility of divalent Mg ions in cathode host materials. In this work, we explore the dual cation co-intercalation strategy to mitigate the sluggishness of Mg2+ in model TiS2 material. The strategy involves pairing Mg2+ with Li+ or Na+ in dual-salt electrolytes in order to exploit the faster mobility of the latter with the aim to reach better electrochemical performance. A combination of experiments and theoretical calculations details the charge storage and redox mechanism of co-intercalating cationic charge carriers. Comparative evaluation reveals that the redox activity of Mg2+ can be improved significantly with the help of the dual cation co-intercalation strategy, although the ionic radius of the accompanying monovalent ion plays a critical role on the viability of the strategy. More specifically, a significantly higher Mg2+ quantity intercalates with Li+ than with Na+ in TiS2. The reason being the absence of phase transition in the former case, which enables improved Mg2+ storage. Our results highlight dual cation co-intercalation strategy as an alternative approach to improve the electrochemical performance of rechargeable Mg batteries by opening the pathway to a rich playground of advanced cathode materials for multivalent battery applications.

Photo-Rechargeable Li-Ion Batteries using TiS2 Cathode

Photo-rechargeable (solar) battery can be considered as an energy harvesting cum storage system, where it can charge the conventional metal-ion battery using light instead of electricity, without having other parasitic reactions. Here a two-electrode lithium-ion solar battery with multifaceted TiS2 -TiO2 hybrid sheets as cathode. The choice of TiS2 -TiO2 electrode ensures the formation of a type II semiconductor heterostructure while the lateral heterostructure geometry ensures high mass/charge transfer and light interactions with the electrode. TiS2 has a higher lithium binding energy (1.6 eV) than TiO2 (1.03 eV), ensuring the possibilities of higher amount of Li-ion insertion to TiS2 and hence the maximum recovery with the photocharging, as further confirmed by the experiments. Apart from the demonstration of solar solid-state batteries, the charging of lithium-ion full cell with light indicates the formation of lithium intercalated graphite compounds, ensuring the charging of the battery without any other parasitic reactions at the electrolyte or electrode-electrolyte interfaces. Possible mechanisms proposed here for the charging and discharging processes of solar batteries, based on the experimental and theoretical results, indicate the potential of such systems in the forthcoming era of renewable energies.

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.

An Exploration of Sulfur Redox in Lithium Battery Cathodes

Secondary Li-ion batteries have enabled a world of portable electronics and electrification of personal and commercial transportation. However, the charge storage capacity of conventional intercalation cathodes is reaching the theoretical limit set by the stoichiometry of Li in the fully lithiated structure. Increasing the Li:transition metal ratio and consequently involving structural anions in the charge compensation, a mechanism termed anion redox, is a viable method to improve storage capacities. Although anion redox has recently become the front-runner as a next-generation storage mechanism, the concept has been around for quite some time. In this perspective, we explore the contribution of anions in charge compensation mechanisms ranging from intercalation to conversion and the hybrid mechanisms between. We focus our attention on the redox of S because the voltage required to reach S redox lies within the electrolyte stability window, which removes the convoluting factors caused by the side reactions that plague the oxides. We highlight examples of S redox in cathode materials exhibiting varying degrees of anion involvement with a particular focus on the structural effects. We call attention to those with intermediate anion contribution to redox and the hybrid intercalation- and conversion-type structural mechanism at play that takes advantage of the positives of both mechanistic types to increase storage capacity while maintaining good reversibility. The hybrid mechanisms often invoke the formation of persulfides, and so a survey of binary and ternary materials containing persulfide moieties is presented to provide context for materials that show thermodynamically stable persulfide moieties.

Amorphous anion-rich titanium polysulfides for aluminum-ion batteries

The strong electrostatic interaction between Al³⁺ and close-packed crystalline structures, and the single-electron transfer ability of traditional cationic redox cathodes, pose challenged for the development of high-performance rechargeable aluminum batteries. Here, to break the confinement of fixed lattice spacing on the diffusion and storage of Al-ion, we developed a previously unexplored family of amorphous anion-rich titanium polysulfides (a-TiS _x , x = 2, 3, and 4) (AATPs) with a high concentration of defects and a large number of anionic redox centers. The AATP cathodes, especially a-TiS₄, achieved a high reversible capacity of 206 mAh/g with a long duration of 1000 cycles. Further, the spectroscopy and molecular dynamics simulations revealed that sulfur anions in the AATP cathodes act as the main redox centers to reach local electroneutrality. Simultaneously, titanium cations serve as the supporting frameworks, undergoing the evolution of coordination numbers in the local structure.

Decreasing the Ion Diffusion Pathways for the Intercalation of Multivalent Cations into One-Dimensional TiS2 Nanobelt Arrays

The sparse selection of available cathode materials that allow for reversible intercalation (deintercalation) of Al³⁺ species represents a major hurdle in the development of efficient Al-ion batteries. Herein, we developed cathodes based on TiS₂ nanobelts that are capable of withstanding the high charge density of Al-ion species with minimal host lattice/ion interactions. The fabricated TiS₂ nanobelts are highly anisotropic and are directly grown on a carbon current collector yielding a spatially controlled array. The sum of evidence presented in this work indicates that one-dimensional TiS₂ nanobelt arrays can reversibly accommodate an unprecedented amount of Al ion species within their layered structure with no significant volume expansion as well as full retention of the nanobelt morphology. Thus, the one-dimensional morphology, nanoscale dimensions, short ion diffusion paths, high electrical conductivity, and absence of additives that hinder ion migration lead to Al-based TiS₂ electrochemical devices exhibiting high specific capacity, less capacity fade, and resilience under higher cycling rates at both room temperature and elevated temperatures when compared to TiS₂ platelets. We also present the effects of sulfur vacancies on the electrochemical performance of Al-based TiS_2-x nanobelt array batteries. Although Al-ion batteries are still in their infancy, we believe our TiS₂ nanobelt array cathode insertion hosts may play an important role in addressing the poor kinetics of solid-state Al-ion diffusion to enable efficient alternatives beyond lithium energy storage devices.

In Situ X-ray Absorption Spectroscopic Investigation of the Capacity Degradation Mechanism in Mg/S Batteries

The Mg/S battery is attractive because of its high theoretical energy density and the abundance of Mg and S on the earth. However, its development is hindered by the lack of understanding to the underlying electrochemical reaction mechanism of its charge-discharge processes. Here, using a unique in situ X-ray absorption spectroscopic tool, we systematically study the reaction pathways of the Mg/S cells in Mg(HMDS)₂-AlCl₃ electrolyte. We find that the capacity degradation is mainly due to the formation of irreversible discharge products, such as MgS and Mg₃S₈, through a direct electrochemical deposition or a chemical disproportionation of intermediate polysulfide. In light of the fundamental understanding, we propose to use TiS₂ as a catalyst to activate the irreversible reaction of low-order MgS _x and MgS, which results in an increased discharging capacity up to 900 mAh·g^-1 and a longer cycling life.

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

Titanium disulfide is a promising material for a range of applications, including lithium-ion battery (LIB) anodes. However, its application potential has been severely hindered by the tendency of exfoliated TiS2 to rapidly oxidize under ambient conditions. Herein, we confirm that, although layered TiS2 powder can be exfoliated by sonication in aqueous surfactant solutions, the resultant nanosheets oxidise almost completely within hours. However, we find that upon performing the exfoliation in the solvent cyclohexyl-pyrrolidone (CHP), the oxidation is almost completely suppressed. TiS2 nanosheets dispersed in CHP and stored at 4 °C in an open atmosphere for 90 days remained up to 95% intact. In addition, CHP-exfoliated nanosheets did not show any evidence of oxidation for at least 30 days after being transformed into dry films even when stored under ambient conditions. This stability, probably a result of a residual CHP coating, allows TiS2 nanosheets to be deployed in applications. To demonstrate this, we prepared lithium ion battery anodes from nano : nano composites of TiS2 nanosheets mixed with carbon nanotubes. These anodes displayed reversible capacities (920 mA h g-1) close to the theoretical value and showed good rate performance and cycling capability.

Structural characterization of an amorphous VS4 and its lithiation/delithiation behavior studied by solid-state NMR spectroscopy

Amorphous VS₄ (a-VS₄) is prepared from the crystalline one (c-VS₄). Li insertion into a-VS₄ involves the amorphous–amorphous phase transition from Peierls-distorted chain-like structure to [VS₄]³⁻ tetrahedra-based structure.