In Situ Electron Microscopy Investigation of Sodiation of Titanium Disulfide Nanoflakes

In Situ Lattice-Resolution Revelation of the Origins of Unexplored Anisotropic Sodiation Kinetics and Phase Transition in the Niobium Sulfide Anode

Layered transition metal dichalcogenides (TMDs) have exhibited huge potential as anode materials for sodium-ion batteries. Most of them usually store sodium via an intercalation-conversion mechanism, but niobium sulfide (NbS2) may be an exception. Herein, through in situ transmission electron microscopy, we carefully investigated the insertion behaviors of Na ions in NbS2 and directly visualized anisotropic sodiation kinetics. Lattice-resolution imaging coupled with density functional theory calculations reveals the preferential diffusion of Na ions within layers of NbS2, accompanied by observable interlayer lattice expansion. Impressively, the Na-inserted layers can still withstand in situ mechanical testing. Further in situ observation vertical to the a/b plane of NbS2 tracked the illusive conversion reaction, which could result from interlayer gliding or wrinkling associated with stress accumulation. In situ electron diffraction measurements ruled out the possibility of such a conversion mechanism and identified a phase transition from pristine 3R-NbS2 to 2H-NaNbS2. Therefore, the NbS2 anode stores Na ions via only the intercalation mechanism, which conceptually differs from the well-known intercalation-conversion mechanism of typical TMDs. These findings not only decipher the whole sodiation process of the NbS2 anode but also provide valuable reference for unraveling the precise sodium storage mechanism in other TMDs.

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.

In Situ Transmission Electron Microscopy for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have attracted tremendous attentions in recent years due to the abundance and wide distribution of Na resource on the earth. However, SIBs still face the critical issues of low energy density and unsatisfactory cyclic stability at present. The enhancement of electrochemical performance of SIBs depends on comprehensive and precise understanding of the underlying sodium storage mechanism. Although extensive transmission electron microscopy (TEM) investigations have been performed to reveal the sodium storage property and mechanism of SIBs, a dedicated review on the in situ TEM investigations of SIBs has not been reported. In this review, recent progress in the in situ TEM investigations on the morphological, structural, and chemical evolutions of cathode materials, anode materials, and solid-electrolyte interface during the sodium storage of SIBs is comprehensively summarized. The detailed relationship between structure/composition of electrode materials and electrochemical performance of SIBs has been clarified. This review aims to provide insights into the effective selection and rational design of advanced electrode materials for high-performance SIBs.

In-Depth Mechanism Understanding for Potassium-Ion Batteries by Electroanalytical Methods and Advanced In Situ Characterization Techniques

The advancement of potassium ion batteries (PIBs) stimulated by the dearth of lithium resources is accelerating. Major progresses on the electrochemical properties are based on the optimization of electrode materials, electrolytes, and other components. More significantly, the prerequisites for optimizing these key compositions are in-depth and comprehensive exploration of electrochemical reaction processes, including the evolution of morphology and structure, phase transition, interface behaviors, and K⁺ movement, etc. As a result, the obtained K⁺ storage mechanism via analyzing aforementioned reaction processes sheds light on furthering practical application of PIBs. Typical electrochemical analysis methods are capable of obtaining physical and chemical characteristics. The advent of in situ electrochemical measurements enables dynamic observation and monitoring, thereby gaining extensive insights into the intricate mechanism of capacity degradation and interface kinetics. By coupling with these powerful electrochemical characterization techniques, inspiring works in PIBs will burgeon into wide realms of energy storage fields. In this review, some typical electroanalytical tests and in situ hyphenated measurements are described with the main concentration on how these techniques play a role in investigating the potassium storage mechanism for PIBs and achieving encouraging results.

Spatial and Temporal Analysis of Sodium-Ion Batteries

As a promising alternative to the market-leading lithium-ion batteries, low-cost sodium-ion batteries (SIBs) are attractive for applications such as large-scale electrical energy storage systems. The energy density, cycling life, and rate performance of SIBs are fundamentally dependent on dynamic physiochemical reactions, structural change, and morphological evolution. Therefore, it is essential to holistically understand SIBs reaction processes, degradation mechanisms, and thermal/mechanical behaviors in complex working environments. The recent developments of advanced in situ and operando characterization enable the establishment of the structure-processing-property-performance relationship in SIBs under operating conditions. This Review summarizes significant recent progress in SIBs exploiting in situ and operando techniques based on X-ray and electron analyses at different time and length scales. Through the combination of spectroscopy, imaging, and diffraction, local and global changes in SIBs can be elucidated for improving materials design. The fundamental principles and state-of-the-art capabilities of different techniques are presented, followed by elaborative discussions of major challenges and perspectives.

Intercalation Strategy in 2D Materials for Electronics and Optoelectronics

Intercalation is an effective approach to tune the physical and chemical properties of 2D materials due to their abundant van der Waals gaps that can host high-density intercalated guest matters. This approach has been widely employed to modulate the optical, electrical, and photoelectrical properties of 2D materials for their applications in electronic and optoelectronic devices. Thus it is necessary to review the recent progress of the intercalation strategy in 2D materials and their applications in devices. Herein, various intercalation strategies and the novel properties of the intercalated 2D materials as well as their applications in electronics and optoelectronics are summarized. In the end, the development tendency of this promising approach for 2D materials is also outlined.

Phase evolution and structural modulation during in situ lithiation of MoS2, WS2 and graphite in TEM

Li-ion batteries function by Li intercalating into and through the layered electrode materials. Intercalation is a solid-state interaction resulting in the formation of new phases. The new observations presented here reveal that at the nanoscale the intercalation mechanism is fundamentally different from the existing models and is actually driven by nonuniform phase distributions rather than the localized Li concentration: the lithiation process is a 'distribution-dependent' phenomena. Direct structure imaging of 2H and 1T dual-phase microstructures in lithiated MoS2 and WS2 along with the localized chemical segregation has been demonstrated in the current study. Li, a perennial challenge for the TEM, is detected and imaged using a low-dose, direct-electron detection camera on an aberration-corrected TEM and confirmed by image simulation. This study shows the presence of fully lithiated nanoscale domains of 2D host matrix in the vicinity of Li-lean regions. This confirms the nanoscale phase formation followed by Oswald ripening, where the less-stable smaller domains dissolves at the expense of the larger and more stable phases.

In Situ TEM Study on Conversion-Type Electrodes for Rechargeable Ion Batteries

Conversion-type materials have been considered as potentially high-energy-density alternatives to commercially dominant intercalation-based electrodes for rechargeable ion batteries and have attracted tremendous research effort to meet the performance for viable energy-storage technologies. In situ transmission electron microscopy (TEM) has been extensively employed to provide mechanistic insights into understanding the behavior of battery materials. Noticeably, a great portion of previous in situ TEM studies has been focused on conversion-type materials, but a dedicated review for this group of materials is missing in the literature. Herein, recent developments of in situ TEM techniques for investigation of dynamic phase transformation and associated structural, morphological, and chemical evolutions during conversion reactions with alkali ions in secondary batteries are comprehensively summarized. The materials of interest broadly cover metal oxides, chalcogenides, fluorides, phosphides, nitrides, and silicates with specific emphasis on spinel metal oxides and recently emerged 2D metal chalcogenides. Special focus is placed on the scientific findings that are uniquely obtained by in situ TEM to address fundamental questions and practical issues regarding phase transformation, structural evolution, electrochemical redox, reaction mechanism, kinetics, and degradation. Critical challenges and perspectives are discussed for advancing new knowledge that can bridge the gap between prototype materials and real-world applications.

Reaction Mechanism and Structural Evolution of Fluorographite Cathodes in Solid-State K/Na/Li Batteries

Fluorographites (CF_x ) are ultrahigh-energy-density cathode materials for alkaline-metal primary batteries. However, they are generally not rechargeable. To elucidate the reaction mechanism of CF_x cathodes, in situ transmission electron microscopy characterizations and ab initio calculations are employed. It is found that it is a two-phase mechanism upon K/Na/Li ion insertion; crystalline KF (crystalline NaF nanoparticles and amorphous LiF) is generated uniformly within the amorphous carbon matrix, retaining an unchanged volume during the discharge process. The diffusivity for K/Na/Li ion migration within the CF_x is ≈2.2-2.5 × 10^-12 , 3.4-5.3 × 10^-12 , and 1.8-2.5 × 10^-11 cm² s^-1 , respectively, which is comparable to the diffusivity of K/Na/Li ions in liquid-state cells. Encouraged by the in situ transmission electron microscopy (TEM) results, a new rechargeable all-solid-state Li/CF_x battery is further designed that shows a part of the reversible specific discharge capacity at the 2nd cycle. These findings demonstrate that a solid-state electrolyte provides a different reaction process compared with a conventional liquid electrolyte, and enables CF_x to be partly rechargeable in solid-state Li batteries.

Direct Observation of Li Migration into V5S8: Order to Antisite Disorder Intercalation Followed by the Topotactic-Based Conversion Reaction

Two-dimensional transition-metal dichalcogenides hold great potential in rechargeable lithium-ion batteries. Their electrochemical properties are closely related to the structural evolutions during lithium-ion migration. Understanding these migration/reaction mechanisms is important to help improve battery performance. Herein, we report the real-time and atomic-scale observation of phase transitions during the lithiation and delithiation for V₅S₈ via in situ electron diffraction and high-resolution transmission electron microscopy techniques. We find that the phase transformation proceeds via a sequence of order to antisite disorder intercalation and topotactic-based conversion reaction. During the intercalation reaction, the lithium ion destroys the orderings of the interstitial V with the formation of Li/V antisite. Such a reaction is found to be reversible, i.e., the extraction of lithium from Li_xV₅S₈ leads to the recovery of V orderings. The conversion reaction involves heterogeneous nucleation of Li₂S with 3-20 nm nanodomains, which maintain the crystallographic integrity with Li_xV₅S₈. These findings elucidate the complex interactions between the lithium ion and host V₅S₈ during ionic migration in solids, which should be helpful in understanding the relationship between phase transformation kinetics and battery performance.

Highly Reversible Sodiation/Desodiation from a Carbon-Sandwiched SnS2 Nanosheet Anode for Sodium Ion Batteries

The further improvement of sodium ion batteries requires the elucidation of the mechanisms pertaining to reversibility, which allows the novel design of the electrode structure. Here, through a hydrogel-embedding method, we are able to confine the growth of few-layer SnS₂ nanosheets between a nitrogen- and sulfur-doped carbon nanotube (NS-CNT) and amorphous carbon. The obtained carbon-sandwiched SnS₂ nanosheets demonstrate excellent sodium storage properties. In operando small-angle X-ray scattering combined with the ex situ X-ray absorption near edge spectra reveal that the redox reactions between SnS₂/NS-CNT and the sodium ion are highly reversible. On the contrary, the nanostructure evolution is found to be irreversible, in which the SnS₂ nanosheets collapse, followed by the regeneration of SnS₂ nanoparticles. This work provides operando insights into the chemical environment evolution and structure change of SnS₂-based anodes, elucidating its reversible reaction mechanism, and illustrates the significance of engineered carbon support in ensuring the electrode structure stability.

Cobalt-promoted fabrication of 3D carbon with a nanotube-sheet mutual support structure: scalable preparation of a high-performance anode material for Li-ion batteries

Currently, the design of carbon-based composite as a high-performance anode material for lithium-ion batteries (LIBs) presents challenges for commercial application. Herein, we developed a three-dimensional carbon-based material with a nanotube-sheet mutual support structure (MS-CNTS) engineered by the catalytic effect of Co species. The present work highlights a concise 'solvent-free' synthetic method allowing for large-scale output, which is potentially available for low cost commercial use. With the readily available acetylacetonate and cobalt (II) acetylacetonate as starting chemicals, this nanostructured carbonaceous material is fabricated with aldol condensation to construct a Co-contained carbon-link network polymer precursor followed by annealing under argon. It is composed of brim-curled graphene-like carbon nanosheets and carbon nanotubes, which support each other's structures to effectively avoid agglomeration. Therefore, it enables high performance in LIBs. In spite of the trace amount of cobalt, the carbon-based MS-CNTS anode delivers a high charge capacity of 1028 mAh g-1 at 0.1 A g-1, high rate capacity of 495 mAh g-1 at 2 A g-1, and ultra-long cycling life with a very low capacity decay of 0.008% per cycle over 1000 cycles at 0.5 A g-1, accompanied by 100% Coulombic efficiency. From full cell measurements, we further confirm the considerable promise of MS-CNTS as anodes with a long cycling life.