Miniaturized Energy Storage Devices Based on Two-Dimensional Materials

Strain-controlled spin transport in a two-dimensional (2D) nanomagnet

Semiconductors with controllable electronic transport coupled with magnetic behaviour, offering programmable spin arrangements present enticing potential for next generation intelligent technologies. Integrating and linking these two properties has been a long standing challenge for material researchers. Recent discoveries in two-dimensional (2D) magnet shows an ability to tune and control the electronic and magnetic phases at ambient temperature. Here, we illustrate controlled spin transport within the magnetic phase of the 2D semiconductor CrOBr and reveal a substantial connection between its magnetic order and charge carriers. First, we systematically analyse the strain-induced electronic behaviour of 2D CrOBr using density functional theory calculations. Our study demonstrates the phase transition from a magnetic semiconductor → half metal → magnetic metal in the material under strain application, creating intriguing spin-resolved conductance with 100% spin polarisation and spin-injection efficiency. Additionally, the spin-polarised current-voltage (I-V) trend displayed conductance variations with high strain-assisted tunability and a peak-to-valley ratio as well as switching efficiency. Our study reveals that CrOBr can exhibit highly anisotropic behaviour with perfect spin filtering, offering new implications for strain engineered magneto-electronic devices.

Bending Resistance Covalent Organic Framework Superlattice: "Nano-Hourglass"-Induced Charge Accumulation for Flexible In-Plane Micro-Supercapacitors

Covalent organic framework (COF) film with highly exposed active sites is considered as the promising flexible self-supported electrode for in-plane micro-supercapacitor (MSC). Superlattice configuration assembled alternately by different nanofilms based on van der Waals force can integrate the advantages of each isolated layer to exhibit unexpected performances as MSC film electrodes, which may be a novel option to ensure energy output. Herein, a mesoporous free-standing A-COF nanofilm (pore size is 3.9 nm, averaged thickness is 4.1 nm) with imine bond linkage and a microporous B-COF nanofilm (pore size is 1.5 nm, averaged thickness is 9.3 nm) with β-keto-enamine-linkages are prepared, and for the first time, we assembly the two lattice matching films into sandwich-type superlattices via layer-by-layer transfer, in which ABA-COF superlattice stacking into a "nano-hourglass" steric configuration that can accelerate the dynamic charge transportation/accumulation and promote the sufficient redox reactions to energy storage. The fabricated flexible MSC-ABA-COF exhibits the highest intrinsic CV of 927.9 F cm-3 at 10 mV s-1 than reported two-dimensional alloy, graphite-like carbon and undoped COF-based MSC devices so far, and shows a bending-resistant energy density of 63.2 mWh cm-3 even after high-angle and repeat arbitrary bending from 0 to 180°. This work provides a feasible way to meet the demand for future miniaturization and wearable electronics.

Advanced Three-Dimensional Microelectrode Architecture Design for High-Performance On-Chip Micro-Supercapacitors

The rapid development of miniaturized electronic devices has greatly stimulated the endless pursuit of high-performance on-chip micro-supercapacitors (MSCs) delivering both high energy and power densities. To this end, an advanced three-dimensional (3D) microelectrode architecture design offers enormous opportunities due to high mass loading of active materials, large specific surface areas, fast ion diffusion kinetics, and short electron transport pathways. In this review, we summarize the recent advances in the rational design of 3D architectured microelectrodes including 3D dense microelectrodes, 3D nanoporous microelectrodes, and 3D macroporous microelectrodes. Furthermore, the emergent microfabrication strategies are discussed in detail in terms of charge storage mechanisms and structure-performance correlation for on-chip MSCs. Finally, we conclude with a perspective on future opportunities and challenges in this thriving field.

Miniaturized lithium-ion batteries for on-chip energy storage

The development of microelectronic products increases the demand for on-chip miniaturized electrochemical energy storage devices as integrated power sources. Such electrochemical energy storage devices need to be micro-scaled, integrable and designable in certain aspects, such as size, shape, mechanical properties and environmental adaptability. Lithium-ion batteries with relatively high energy and power densities, are considered to be favorable on-chip energy sources for microelectronic devices. This review describes the state-of-the-art of miniaturized lithium-ion batteries for on-chip electrochemical energy storage, with a focus on cell micro/nano-structures, fabrication techniques and corresponding material selections. The relationship between battery architecture and form-factors of the cell concerning their mechanical and electrochemical properties is discussed. A series of on-chip functional microsystems created by integrating micro-lithium-ion batteries are highlighted. Finally, the challenges and future perspectives of miniaturized lithium-ion batteries are elaborated with respect to their potential application fields.

Recent progress of heterostructures based on two dimensional materials and wide bandgap semiconductors

Recent progress in the synthesis and assembly of two-dimensional (2D) materials has laid the foundation for various applications of atomically thin layer films. These 2D materials possess rich and diverse properties such as layer-dependent band gaps, interesting spin degrees of freedom, and variable crystal structures. They exhibit broad application prospects in micro-nano devices. In the meantime, the wide bandgap semiconductors (WBS) with an elevated breakdown voltage, high mobility, and high thermal conductivity have shown important applications in high-frequency microwave devices, high-temperature and high-power electronic devices. Beyond the study on single 2D materials or WBS materials, the multi-functional 2D/WBS heterostructures can promote the carrier transport at the interface, potentially providing novel physical phenomena and applications, and improving the performance of electronic and optoelectronic devices. In this review, we overview the advantages of the heterostructures of 2D materials and WBS materials, and introduce the construction methods of 2D/WBS heterostructures. Then, we present the diversity and recent progress in the applications of 2D/WBS heterostructures, including photodetectors, photocatalysis, sensors, and energy related devices. Finally, we put forward the current challenges of 2D/WBS heterostructures and propose the promising research directions in the future.

Using X-Ray Scattering to Elucidate the Microstructural Instability of 3D Bicontinuous Nanoporous Metal Scaffolds for Use in an Aperiodic 3D Tricontinuous Conductor-Insulator-Conductor Nanocapacitor

The development of three-dimensional aperiodic energy storage devices is in part impeded by the lack of appropriate aperiodic templates that can withstand the thermal conditions required to deposit energy storage materials within their void space. Herein, the feasibility of an aperiodic three-dimensional architecture for energy storage is demonstrated for the first time by constructing a tricontinuous conductor-insulator-conductor (CIC) nanocapacitor on an aperiodic nanoporous gold scaffold. To accomplish this, the scaffold was characterized using in situ small-angle X-ray scattering (SAXS) during exposure to a thermal environment, revealing that its microstructure eventually stabilizes after undergoing a phase of rapid coarsening, indicating a departure from the 1/4 time-dependent power-law coarsening behavior usually observed at the early stage of the coarsening process. Using this stability regime, we created the CIC by intentionally precoarsening and stabilizing the scaffold before depositing two dissimilar metal oxide films in its void space by atomic layer deposition. Current-voltage characteristics and electrochemical impedance spectroscopy measurements revealed that the un-optimized 3D CIC outperformed its 2D counterpart by ∼4× in terms of capacitance. This proof-of-concept device will pave the way to the development of aperiodic three-dimensional energy storage systems with enhanced energy and power densities.

Hybrid dual-function thermal energy harvesting and storage technologies: towards self-chargeable flexible/wearable devices

The worldwide energy scarcity arising from the massive consumption of nonrenewable energy sources raised a global awareness of the need for cleaner and affordable energy solutions to mitigate climate change and ensure the world sustainable development. The rise of the Internet of Things and the fast growth of the groundbreaking market of wearable electronics boosted a major quest for self-powered technologies merging energy harvesting and energy storage functionalities to meet the demands of a myriad of market segments, such as healthcare, transportation, defense and sports. Thermoelectric devices are a green energy harvesting solution for wearable electronics since they harness the low-grade waste heat from ubiquitous thermal energy sources and convert it into electrical energy. However, these systems generate electrical energy in an intermittent manner, depend on the local heat release availability and require an additional unit to store energy. Flexible and wearable supercapacitors are a safe and eco-friendly energy storage solution to power wearables, offering advantages of security, longer cycle life, higher power density and faster charging over batteries. However, an additional unit that generates energy or that is able to charge the storage device is required. More recently, a new class of all-in-one thermally-chargeable supercapacitors blossomed to meet the requirements of the next generation of autonomous wearable electronics and ensure an endurable energy supply. This self-chargeable hybrid technology combines the functionalities of thermal energy harvesting and supercapacitive energy storage in a single multitasking device. In this Perspective, the advances in the burgeoning field of all-in-one thermally-chargeable supercapacitors for flexible/wearable applications will be critically examined, ranging from their structure and working principle to the rational design of the composing materials and of tailor-made architectures. It will start by introducing the foundations of single flexible/wearable thermoelectric generators and supercapacitors and will evolve into the pioneering venture of fully-integrated thermal energy harvesting/storage systems. It will end by highlighting the current bottlenecks and future pathways for advancing the development of this sophisticated smart technology.

Structural Engineering and Coupling of Two-Dimensional Transition Metal Compounds for Micro-Supercapacitor Electrodes

The development of portable, wearable, and miniaturized integrated electronics has significantly promoted the immense desire for planar micro-supercapacitors (MSCs) among the extremely competitive energy storage devices. However, their energy density is still insufficient owing to the low electrochemical performance of conventional electrode materials. Compared with their bulk counterparts, the large specific surface area and fast ion transport with efficient intercalation of two-dimensional (2D) transition metal compounds have spurred the research platforms for their exploitation in the creation of high-performance MSCs. This Outlook presents a systematic summary of cutting-edge research on atomically thin, layered structures of transition metal dichalcogenides, MXenes, and transition metal oxides/hydroxides. Special emphasis is given to the rapid and durable storage of ions, benefiting from the low ion diffusion barriers of host interlayer spaces. Moreover, various strategies have been described to circumvent the structural damage due to the volume change and simultaneously evincing remarkable electronic properties.