Strategies to alleviate distortive phase transformations in Li-ion intercalation reactions: an example with vanadium pentoxide

Chemical and electrochemical Li-ion insertion in transition metal oxides, either via a phase transformation reaction (ions insert into specific crystallographic sites of the host lattice) or a solid solution insertion (ions distribute uniformly throughout the host lattice), enables high energy density electrochemical energy storage. Many phase transformation cathode materials, that undergo two-phase reactions, exhibit high theoretical capacities arising from multi-electron redox reactions. However, challenges in distortive phase transformations and uncontrolled phase nucleation, propagation, segregation, and co-existence continue to limit the energy density, (dis)charging rate performances, and cycling stability of available phase transformation cathode materials. Vanadium pentoxide (V2O5), a classical layered intercalation host material with high theoretical capacity, undergoes irreversible structural changes and capacity fading when intercalating more than one lithium ion per V2O5 unit in its thermodynamically stable phase. Here, we review recent synthetic strategies to alter the V-O connectivity, thereby alleviating distortive phase transformations and promoting solid solution-based Li-ion insertion in V2O5. We also summarize several widely accessible and classical molecular-based analytical tools that can provide local structural dynamics and phase transformation mechanism information on the lithiation of V2O5, including single-crystal X-ray diffraction, infrared and Raman spectroscopy, electron paramagnetic resonance, and nuclear magnetic resonance spectroscopy.

Microwave-Assisted Fabrication of High Energy Density Binary Metal Sulfides for Enhanced Performance in Battery Applications

Nanomaterials have found use in a number of relevant energy applications. In particular, nanoscale motifs of binary metal sulfides can function as conversion materials, similar to that of analogous metal oxides, nitrides, or phosphides, and are characterized by their high theoretical capacity and correspondingly low cost. This review focuses on structure-composition-property relationships of specific relevance to battery applications, emanating from systematic attempts to either (1) vary and alter the dimension of nanoscale architectures or (2) introduce conductive carbon-based entities, such as carbon nanotubes and graphene-derived species. In this study, we will primarily concern ourselves with probing metal sulfide nanostructures generated by a microwave-mediated synthetic approach, which we have explored extensively in recent years. This particular fabrication protocol represents a relatively facile, flexible, and effective means with which to simultaneously control both chemical composition and physical morphology within these systems to tailor them for energy storage applications.

Electron and Ion Transport in Lithium and Lithium-Ion Battery Negative and Positive Composite Electrodes

Electrochemical energy storage systems, specifically lithium and lithium-ion batteries, are ubiquitous in contemporary society with the widespread deployment of portable electronic devices. Emerging storage applications such as integration of renewable energy generation and expanded adoption of electric vehicles present an array of functional demands. Critical to battery function are electron and ion transport as they determine the energy output of the battery under application conditions and what portion of the total energy contained in the battery can be utilized. This review considers electron and ion transport processes for active materials as well as positive and negative composite electrodes. Length and time scales over many orders of magnitude are relevant ranging from atomic arrangements of materials and short times for electron conduction to large format batteries and many years of operation. Characterization over this diversity of scales demands multiple methods to obtain a complete view of the transport processes involved. In addition, we offer a perspective on strategies for enabling rational design of electrodes, the role of continuum modeling, and the fundamental science needed for continued advancement of electrochemical energy storage systems with improved energy density, power, and lifetime.

Gradient Design for High-Energy and High-Power Batteries

Charge transport is a key process that dominates battery performance, and the microstructures of the cathode, anode, and electrolyte play a central role in guiding ion and/or electron transport inside the battery. Rational design of key battery components with varying microstructure along the charge-transport direction to realize optimal local charge-transport dynamics can compensate for reaction polarization, which accelerates electrochemical reaction kinetics. Here, the principles of charge-transport mechanisms and their decisive role in battery performance are presented, followed by a discussion of the correlation between charge-transport regulation and battery microstructure design. The design strategies of the gradient cathodes, lithium-metal anodes, and solid-state electrolytes are summarized. Future directions and perspectives of gradient design are provided at the end to enable practically accessible high-energy and high-power-density batteries.

From Fundamental Understanding to Engineering Design of High-Performance Thick Electrodes for Scalable Energy-Storage Systems

The ever-growing needs for renewable energy demand the pursuit of batteries with higher energy/power output. A thick electrode design is considered as a promising solution for high-energy batteries due to the minimized inactive material ratio at the device level. Most of the current research focuses on pushing the electrode thickness to a maximum limit; however, very few of them thoroughly analyze the effect of electrode thickness on cell-level energy densities as well as the balance between energy and power density. Here, a realistic assessment of the combined effect of electrode thickness with other key design parameters is provided, such as active material fraction and electrode porosity, which affect the cell-level energy/power densities of lithium-LiNi_0.6 Mn_0.2 Co_0.2 O₂ (Li-NMC622) and lithium-sulfur (Li-S) cells as two model battery systems, is provided. Based on the state-of-the-art lithium batteries, key research targets are quantified to achieve 500 Wh kg^-1 /800 Wh L^-1 cell-level energy densities and strategies are elaborated to simultaneously enhance energy/power output. Furthermore, the remaining challenges are highlighted toward realizing scalable high-energy/power energy-storage systems.