Active material and interphase structures governing performance in sodium and potassium ion batteries

Flexible electrochemical energy storage devices and related applications: recent progress and challenges

Given the escalating demand for wearable electronics, there is an urgent need to explore cost-effective and environmentally friendly flexible energy storage devices with exceptional electrochemical properties. However, the existing types of flexible energy storage devices encounter challenges in effectively integrating mechanical and electrochemical performances. This review is intended to provide strategies for the design of components in flexible energy storage devices (electrode materials, gel electrolytes, and separators) with the aim of developing energy storage systems with excellent performance and deformability. Firstly, a concise overview is provided on the structural characteristics and properties of carbon-based materials and conductive polymer materials utilized in flexible energy storage devices. Secondly, the fabrication process and strategies for optimizing their structures are summarized. Subsequently, a comprehensive review is presented regarding the applications of carbon-based materials and conductive polymer materials in various fields of flexible energy storage, such as supercapacitors, lithium-ion batteries, and zinc-ion batteries. Finally, the challenges and future directions for next-generation flexible energy storage systems are proposed.

Quantifying Interfacial Ion Transfer at Operating Potassium-Insertion Battery Electrodes within Highly Concentrated Aqueous Solutions

Electrode/electrolyte interfacial ion transfer is a fundamental process occurring during insertion-type redox reactions at battery electrodes. The rate at which ions move into and out of the electrode, as well as at interphase structures, directly impacts the power performance of the battery. However, measuring and quantifying these ion transfer phenomena can be difficult, especially at high electrolyte concentrations as found in batteries. Herein, we report a scanning electrochemical microscope method using a common ferri/ferrocyanide (FeCN) redox mediator dissolved in an aqueous electrolyte to track changes in alkali ions at high electrolyte concentrations (up to 3 mol dm-3). Using voltammetry at a platinum microelectrode, we observed a reversible E1/2 shift of ∼60 mV per decade change in K+ concentrations. The response showed high stability in sequential measurements and similar behavior in other aqueous electrolytes. From there, we used the same FeCN mediator to position the microelectrode at the surface of a potassium-insertion electrode. We demonstrate tracking of local changes in the K+ concentration during insertion and deinsertion processes. Using a 2D axisymmetric, finite element model, we further estimate the effective insertion rates. These developments enable characterization of a key parameter for improving batteries, the interfacial ion transfer kinetics, and future work may show mediators appropriate for molar concentrations in nonaqueous electrolytes and beyond.

The Enormous Potential of Sodium/Potassium-Ion Batteries as the Mainstream Energy Storage Technology for Large-Scale Commercial Applications

Cost-effectiveness plays a decisive role in sustainable operating of rechargeable batteries. As such, the low cost-consumption of sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) provides a promising direction for "how do SIBs/PIBs replace Li-ion batteries (LIBs) counterparts" based on their resource abundance and advanced electrochemical performance. To rationalize the SIBs/PIBs technologies as alternatives to LIBs from the unit energy cost perspective, this review gives the specific criteria for their energy density at possible electrode-price grades and various battery-longevity levels. The cost ($ kWh-1 cycle-1) advantage of SIBs/PIBs is ascertained by the cheap raw-material compensation for the cycle performance deficiency and the energy density gap with LIBs. Furthermore, the cost comparison between SIBs and PIBs, especially on cost per kWh and per cycle, is also involved. This review explicitly manifests the practicability and cost-effectiveness toward SIBs are superior to PIBs whose commercialization has so far been hindered by low energy density. Even so, the huge potential on sustainability of PIBs, to outperform SIBs, as the mainstream energy storage technology is revealed as long as PIBs achieve long cycle life or enhanced energy density, the related outlook of which is proceeded as the next development directions for commercial applications.

Emerging high voltage V4+/V5+ redox reactions in Na3V2(PO4)3-based cathodes for sodium-ion batteries

Na3V2(PO4)3 (NVP) cathode materials with the advantages of long cycle life and superior thermal stability have been considered promising cathode candidates for SIBs. However, the unsatisfactory energy density derived from low theoretical capacity and operating voltage (3.35 V vs. Na+/Na, based on the V3+/V4+ redox couple) inevitably limits their practical application. Therefore, the activation of the V4+/V5+ redox couple (∼4.0 V vs. Na+/Na) in NVP-based cathode materials to boost the energy density of SIBs has attracted extensive attention. Herein, we first analyze the challenges of activation of the V4+/V5+ redox couple in NVP-based cathode materials. Subsequently, the recent achievement of NVP-based cathode materials with activated V4+/V5+ redox reactions for SIBs is overviewed. Finally, further research directions of high voltage V4+/V5+ redox reactions in NVP-based cathodes are proposed. This review provides valuable guidance for developing high energy density NVP-based cathode materials for SIBs.

Synergistic interface and structural engineering for high initial coulombic efficiency and stable sodium storage in metal sulfides

Transition metal sulfides (TMS) have gained significant attention as potential anode materials for sodium ion batteries (SIBs) due to their high theoretical capacity and abundance in nature. Nevertheless, their practical use has been impeded by challenges such as large volume changes, unstable solid electrolyte interphase (SEI), and low initial coulombic efficiency (ICE). To address these issues and achieve both long-term cycling stability and high ICE simultaneously, we present a novel approach involving surface engineering, termed as the "dual-polar confinement" strategy, combined with interface engineering to enhance the electrochemical performance of TMS. In this approach, CoS crystals are meticulously coated with polar TiO2 and embedded within a polar S-doped carbon matrix, forming a composite electrode denoted as CoS/TiO2-SC. Significantly, an ether-based electrolyte with chemical stability and optimized solvation properties synergistically interacts with the Co-S-C bonds to create a stable, ultra-thin SEI. This concerted effect results in a notably high ICE, reaching approximately 96%. Advanced characterization and theoretical simulations confirm that the uniform surface modification effectively facilitates sodium ion transport kinetics, restrains electrode pulverization, and concurrently enhances interaction with the ether-based electrolyte to establish a robust SEI. Consequently, the CoS/TiO2-SC electrode exhibits high reversible capacity, superior rate capability, and outstanding cycling stability.

MnFe Prussian Blue Analogue Open Cages for Sodium-Ion Batteries: Simultaneous Evolution of Structure, Morphology, and Energy Storage Properties

Prussian blue analogues (PBAs) exhibiting hollow morphologies have garnered considerable attention owing to their remarkable electrochemical properties. In this study, a one-pot strategy is proposed for the synthesis of MnFe PBA open cages. The materials are subsequently employed as cathode electrode in sodium-ion batteries (SIBs). The simultaneous evolution of structure, morphology, and performance during the synthesis process is investigated. The findings reveal substantial structural modifications as the reaction time is prolonged. The manganese content in the samples diminishes considerably, while the potassium content experiences an increase. This compositional variation is accompanied by a significant change in the spin state of the transition metal ions. These structural transformations trigger the occurrence of the Kirkendall effect and Oswald ripening, culminating in a profound alteration of the morphology of MnFe PBA. Moreover, the shifts in spin states give rise to distinct changes in their charge-discharge profiles and redox potentials. Furthermore, an exploration of the formation conditions of the samples and their variations before and after cycling is conducted. This study offers valuable insights into the intricate relationship between the structure, morphology, and electrochemical performance of MnFe PBA, paving the way for further optimizations in this promising class of materials for energy storage applications.

Progress and perspectives on iron-based electrode materials for alkali metal-ion batteries: a critical review

Iron-based materials with significant physicochemical properties, including high theoretical capacity, low cost and mechanical and thermal stability, have attracted research attention as electrode materials for alkali metal-ion batteries (AMIBs). However, practical implementation of some iron-based materials is impeded by their poor conductivity, large volume change, and irreversible phase transition during electrochemical reactions. In this review we critically assess advances in the chemical synthesis and structural design, together with modification strategies, of iron-based compounds for AMIBs, to obviate these issues. We assess and categorize structural and compositional regulation and its effects on the working mechanisms and electrochemical performances of AMIBs. We establish insight into their applications and determine practical challenges in their development. We provide perspectives on future directions and likely outcomes. We conclude that for boosted electrochemical performance there is a need for better design of structures and compositions to increase ionic/electronic conductivity and the contact area between active materials and electrolytes and to obviate the large volume change and low conductivity. Findings will be of interest and benefit to researchers and manufacturers for sustainable development of advanced rechargeable ion batteries using iron-based electrode materials.

2D Materials Nanoarchitectonics for 3D Structures/Functions

It has become clear that superior material functions are derived from precisely controlled nanostructures. This has been greatly accelerated by the development of nanotechnology. The next step is to assemble materials with knowledge of their nano-level structures. This task is assigned to the post-nanotechnology concept of nanoarchitectonics. However, nanoarchitectonics, which creates intricate three-dimensional functional structures, is not always easy. Two-dimensional nanoarchitectonics based on reactions and arrangements at the surface may be an easier target to tackle. A better methodology would be to define a two-dimensional structure and then develop it into a three-dimensional structure and function. According to these backgrounds, this review paper is organized as follows. The introduction is followed by a summary of the three issues; (i) 2D to 3D dynamic structure control: liquid crystal commanded by the surface, (ii) 2D to 3D rational construction: a metal-organic framework (MOF) and a covalent organic framework (COF); (iii) 2D to 3D functional amplification: cells regulated by the surface. In addition, this review summarizes the important aspects of the ultimate three-dimensional nanoarchitectonics as a perspective. The goal of this paper is to establish an integrated concept of functional material creation by reconsidering various reported cases from the viewpoint of nanoarchitectonics, where nanoarchitectonics can be regarded as a method for everything in materials science.

Dirac t-Boron Nitride Monolayer as an Appealing Binder-Free Anode for Alkali Metal Ion Batteries

The development of universal anode materials with superlative electrochemical performance poses a great challenge for rechargeable alkali metal (AM) ion battery technologies. In the present work, the viability of the gapless Dirac t-BN (tetragonal boron nitride) monolayer as a lightweight binder-free anode has been systematically evaluated via comprehensive first-principles calculations. Aside from the desirable electronic conductivity, the t-BN monolayer exhibits an excellent ionic conductivity as well due to its moderate affinity for Li, Na, and K atoms with favorable in-plane barriers of 0.36, 0.18, and 0.19 eV, respectively. Meanwhile, the presence of B4N4 octagons allows the AM atoms to penetrate through the t-BN monolayer. Excitingly, the host material delivers an ultrahigh specific capacity up to 1080 mA h g-1 for Li, 5400 mA h g-1 for Na, and 2160 mA h g-1 for K in the wake of low mean open-circuit voltages of 0.033, 0.203, and 0.300 V at the half-cell level. According to the standard hydrogen electrode methodology, the energy densities are forecasted to be as large as 3240, 13500, and 5680 mW h g-1 for Li, Na, and K ion batteries, respectively, with robust thermal stability up to at least 400 K. The safety and cycling durability of the t-BN monolayer are jointly corroborated via the moderate mechanical strengths and ab initio molecular dynamics simulations at the maximum intercalated states, as well as via the small lattice changes and its ultrahigh tolerable ultimate tensile strain. These findings unambiguously promise that the t-BN monolayer can serve as an appealing candidate for anode applications in AM ion batteries.

Materials Nanoarchitectonics at Dynamic Interfaces: Structure Formation and Functional Manipulation

The next step in nanotechnology is to establish a methodology to assemble new functional materials based on the knowledge of nanotechnology. This task is undertaken by nanoarchitectonics. In nanoarchitectonics, we architect functional material systems from nanounits such as atoms, molecules, and nanomaterials. In terms of the hierarchy of the structure and the harmonization of the function, the material created by nanoarchitectonics has similar characteristics to the organization of the functional structure in biosystems. Looking at actual biofunctional systems, dynamic properties and interfacial environments are key. In other words, nanoarchitectonics at dynamic interfaces is important for the production of bio-like highly functional materials systems. In this review paper, nanoarchitectonics at dynamic interfaces will be discussed, looking at recent typical examples. In particular, the basic topics of "molecular manipulation, arrangement, and assembly" and "material production" will be discussed in the first two sections. Then, in the following section, "fullerene assembly: from zero-dimensional unit to advanced materials", we will discuss how various functional structures can be created from the very basic nanounit, the fullerene. The above examples demonstrate the versatile possibilities of architectonics at dynamic interfaces. In the last section, these tendencies will be summarized, and future directions will be discussed.

Sodium Layered/Tunnel Intergrowth Oxide Cathodes: Formation Process, Interlocking Chemistry, and Electrochemical Performance

Manganese-based layered oxides are prospective cathode materials for sodium-ion batteries (SIBs) due to their low cost and high theoretical capacities. The biphasic intergrowth structure of layered cathode materials is essential for improving the sodium storage performance, which is attributed to the synergistic effect between the two phases. However, the in-depth formation mechanism of biphasic intergrowth materials remains unclear. Herein, the layered/tunnel intergrowth Na0.6MnO2 (LT-NaMO) as a model material was successfully prepared, and their formation processes and electrochemical performance were systematically investigated. In situ high-temperature X-ray diffraction displays the detailed evolution process and excellent thermal stability of the layered/tunnel intergrowth structure. Furthermore, severe structural strain and large lattice volume changes are significantly mitigated by the interlocking effect between the phase interfaces, which further enhances the structural stability of the cathode materials during the charging/discharging process. Consequently, the LT-NaMO cathode displays fast Na+ transport kinetics with a remarkable capacity retention of ∼70.5% over 300 cycles at 5C, and its assembled full cell with hard carbon also exhibits high energy density. These findings highlight the superior electrochemical performance of intergrowth materials due to interlocking effects between layered and tunnel structures and also provide unique insights into the construction of intergrowth cathode materials for SIBs.

Impact of electrolyte decomposition products on the electrochemical performance of 4 V class K-ion batteries

In the pursuit of long-life K-ion batteries (KIBs), half-cell measurements using highly reactive K metal counter electrodes are a standard practice. However, there is increasing evidence of electrolyte decomposition by K metal impacting electrode performance. Herein, we systematically explored the K metal-treated electrolytes KPF6, KN(SO2F)2 (KFSA), and their combination in ethylene carbonate/diethyl carbonate (EC/DEC), referred to as K-KPF6, K-KFSA, and K-KPF6:KFSA, respectively, after storage in contact with K metal. Through mass spectrometry analysis, we identified significant formation of carbonate ester-derived decomposition products such as oligocarbonates for K-KPF6, while K-KFSA predominantly generates anions combining FSA- with the solvent structures. Using three-electrode cells, we delineated the positive effects of the K-KFSA and K-KPF6:KFSA electrolytes on graphite negative electrode performance and the negative impact of oligocarbonates in K-KPF6 on K2Mn[Fe(CN)6] positive electrodes. The interactions between the decomposition products and the electrodes were further evaluated using density functional theory calculations. Full cell measurements using K-KPF6:KFSA showed an improved energy density and capacity retention of 78% after 500 cycles compared with an untreated electrolyte (72%). Hard X-ray photoelectron spectroscopy indicated the incorporation of the FSA-derived structures into the solid electrolyte interphase at graphite, which was not observed in K metal-free cells. Overall, this work indicates further complexities to consider in KIB measurements and suggests the potential application of decomposition products as electrolyte additives.

Zn and N co-doped three-dimensional honeycomb-like carbon featured with interconnected nano-pools for dendrite-free zinc anode

The zinc-ion battery (ZIB) has been extensively researched as one of the promising electrochemical power sources. However, the problem of Zn-dendrite formation during repeated plating and stripping process seriously hinders the development of ZIBs. Herein, three-dimensional (3D) honeycomb-like porous carbon (HPC) with co-doping of zinc and nitrogen is prepared through confining growth of nanoscale zeolite imidazole framework-8 (ZIF-8) on the well-designed nano-pools walls of HPC followed by pyrolysis at 600 ℃ to obtain the final product ZnN/HPC-600, which exhibits large surface area and abundant zincophilic interfaces, ensuring homogeneous distribution of electronic field and low polarization during cycling process. Importantly, ZnN/HPC-600 facilitates the uniform distribution and migration of Zn²⁺ in this nano-pools structure, avoiding the growth of dendritic Zn crystal during charging stage. The symmetric and asymmetric cells with Zn/ZnN/HPC-600 anodes are assembled, demonstrating excellent cycling reversibility, good rate performance and long-term stability. Besides, a Zn||MnO₂ full cell with Zn/ZnN/HPC-600 anode also exhibits robust cycling stability, fast reaction kinetics and almost 100 % coulombic efficiency. This work offers a novel and efficient carbonaceous nano-pools strategy to realize dendrite-free zinc anode in ZIBs.

Surface-driven fast sodium storage enabled by Se-doped honeycomb-like macroporous carbon

Selenium (Se) is an ideal doping agent to modulate the structure of carbon materials to improve their sodium storage performance but has been rarely investigated. In the present study, a novel Se-doped honeycomb-like macroporous carbon (Se-HMC) is prepared by a surface crosslinking method using diphenyl diselenide as the carbon source and SiO₂ nanospheres as the template. Se-HMC has a high Se weight percentage above 10%, with a large surface area of 557 m² g^-1. Owing to the well-developed porous structure in combination with Se-assisted capacitive redox reactions, Se-HMC exhibits surface-dominated Na storage behaviors, thus presenting large capacity and fast Na storage capability. To be specific, Se-HMC delivers a high reversible capacity of 335 mA h g^-1 at 0.1 A g^-1, and after an 800-cycle repeated charge/discharge test at 1 A g^-1, the capacity is stable with no dramatic loss. Remarkably, the capacity remains 251 mA h g^-1 under a very large current density of 5 A g^-1 (≈20 C), demonstrating an ultrafast Na storage process. As far as we know, such a good rate performance has been rarely achieved for carbon anodes before.

Supercritical carbon dioxide assisted formation of crystalline materials for various energetic applications

This highlight gives an overview of recent advances in production of crystalline materials for high energy density applications for rechargeable batteries and solar cells or energetic compounds in supercritical carbon dioxide medium.