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Sodium-ion storage technologies are promising candidates for large-scale grid systems due to the abundance and low cost of sodium. However, compared to well-understood lithium-ion storage mechanisms, sodium-ion storage remains relatively unexplored. Herein, we systematically determine the sodium-ion storage properties of anatase titanium dioxide (TiO2(A)). During the initial sodiation process, a thin surface layer (~3 to 5 nm) of crystalline TiO2(A) becomes amorphous but still undergoes Ti4+/Ti3+ redox reactions. A model explaining the role of the amorphous layer and the dependence of the specific capacity on the size of TiO2(A) nanoparticles is proposed. Amorphous nanoparticles of ~10 nm seem to be optimum in terms of achieving high specific capacity, on the order of 200 mAh g-1, at high charge/discharge rates. Kinetic studies of TiO2(A) nanoparticles indicate that sodium-ion storage is due to a surface-redox mechanism that is not dependent on nanoparticle size in contrast to the lithiation of TiO2(A) which is a diffusion-limited intercalation process. The surface-redox properties of TiO2(A) result in excellent rate capability, cycling stability and low overpotentials. Moreover, tailoring the surface-redox mechanism enables thick electrodes of TiO2(A) to retain high rate properties, and represents a promising direction for high-power sodium-ion storage.

Aqueous ammonium ion batteries (AAIBs) have attracted considerable attention due to their high safety and rapid diffusion kinetics. Unlike spherical metal ions, NH4+ forms hydrogen bonds with host materials, leading to a unique storage mechanism. A variety of electrode materials have been proposed for AAIBs, but their performance often falls short in terms of future energy storage needs. Hence, there is a critical need to design and develop advanced electrode materials for AAIBs. 2D materials, with their tunable interlayer spacing, remarkable interfacial chemistry and abundant surface functional groups, are an ideal choice for electrode materials for NH4+ storage. This review highlights the latest research on 2D electrode materials for AAIBs, providing insights into their working principles, NH4+ storage mechanisms and control strategies for designing high-performance AAIBs. Furthermore, a summary and future perspectives on 2D electrode materials in the development of AAIBs are provided, aiming to promote the advancement of high-performance AAIBs. [ABSTRACT FROM AUTHOR]

Despite the extensive research on MnO2 as a pseudocapacitor electrode material, there has been no report on heterostructures of multiple phase MnO2. Here we report the combination of two high-capacitance phases of MnO2, namely, α-MnO2 nanowires and δ-MnO2 ultrathin nanoflakes, to form a core-branch heterostructure nanoarray. This material and structure design not only increases the mass loading of active materials (from 1.86 to 3.37 mg cm2), but also results in evident pseudocapacitance enhancement (from 28 F g-1 for pure nanowires to 178 F g-1 for heterostructures at 5 mV s-1). The areal capacitance is up to 783 mF cm-2 at 1 mV s-1. Upon 20 000 cycles, the heterostructure array electrode still delivers a reversible capacitance above 100 F g-1 at 4.5 A g-1. Kinetic analysis reveals that capacitances due to both capacitive and diffusion controlled processes have been enlarged for the self-branched heterostructure array. This work presents a new route to improve the electrochemical performance of MnO2 as a binder-free supercapacitor electrode.

The virtues of electrolytic MnO2 aqueous batteries are high theoretical energy density, affordability and safety. However, the continuous dead MnO2 and unstable Mn2+/MnO2 electrolysis pose challenges to the practical output energy and lifespan. Herein, we demonstrate bifunctional cationic redox mediation and catalysis kinetics metrics to rescue dead MnO2 and construct a stable and fast electrolytic Zn–Mn redox-flow battery (eZMRFB). Spectroscopic characterizations and electrochemical evaluation reveal the superior mediation kinetics of a cationic Fe2+ redox mediator compared with the anionic ones (e.g. I– and Br–), thus eliminating dead MnO2 effectively. With intensified oxygen vacancies, density functional theory simulations of the reaction pathways further verify the concomitant Fe-catalysed Mn2+/MnO2 electrolysis kinetics via charge delocalization and activated O 2p electron states, boosting its rate capability. As a result, the elaborated eZMRFB achieves a coulombic efficiency of nearly 100%, ultra-high areal capacity of 80 mAh cm–2, rate capability of 20 C and a long lifespan of 2500 cycles. This work may advance high-energy aqueous batteries to next-generation scalable energy storage. [ABSTRACT FROM AUTHOR]

Electrochromic (EC) devices represent an emerging energy-saving technology, exhibiting the capability to dynamically modulate light and heat transmittance. Despite their promising potential, the commercialization of EC devices faces substantial impediments such as high cost, intricate fabrication process, and low optical contrast inherent in conventional EC materials relying on the ion insertion/extraction mechanism. In this study, we introduce an innovative "electrode-free" electrochromic (EC) device, termed the EECD, which lacks an EC-layer on the electrodes during device assembling and in the bleached state. This device features a simplified fabrication process and delivers superior optical modulation. It achieves a high optical contrast ranging from 68-85% across the visible spectrum and boasts a rapid response time, reaching 90% coloring in just 17 seconds. In addition, EECD exhibits stable cycling for over 10,000 cycles without noticeable degradation and maintains functionality across a broad temperature range (0 °C to 50 °C). Furthermore, the fabricated large-area devices (40 cm × 40 cm) demonstrate excellent tinting uniformity, suggesting excellent scalability of this approach. Our study establishes a paradigmatic breakthrough for EC smart windows. Cheng Yang and co-workers develop an "electrode-free" electrochromic window, featuring a simplified fabrication process and delivering superior optical modulation capability. [ABSTRACT FROM AUTHOR]

Voltage oscillation at subzero in sodium-ion batteries (SIBs) has been a common but overlooked scenario, almost yet to be understood. For example, the phenomenon seriously deteriorates the performance of Na 3 V 2 (PO 4 ) 3 (NVP) cathode in PC (propylene carbonate)/EC (ethylene carbonate)-based electrolyte at -20 °C. Here, the correlation between voltage oscillation, structural evolution, and electrolytes has been revealed based on theoretical calculations, in-/ex-situ techniques, and cross-experiments. It is found that the local phase transition of the Na 3 V 2 (PO 4 ) 3 (NVP) cathode in PC/EC-based electrolyte at -20 °C should be responsible for the oscillatory phenomenon. Furthermore, the low exchange current density originating from the high desolvation energy barrier in NVP-PC/EC system also aggravates the local phase transformation, resulting in severe voltage oscillation. By introducing the diglyme solvent with lower Na-solvent binding energy, the voltage oscillation of the NVP can be eliminated effectively at subzero. As a result, the high capacity retentions of 98.3% at -20 °C and 75.3% at -40 °C are achieved. The finding provides insight into the abnormal SIBs degradation and brings the voltage oscillation behavior of rechargeable batteries into the limelight., Competing Interests: Competing interests statement:The authors declare no competing interest.

Sulfur-based aqueous batteries (SABs) are deemed promising candidates for safe, low-cost, and high-capacity energy storage. However, despite their high theoretical capacity, achieving high reversible value remains a great challenge due to the thermodynamic and kinetics problems of elemental sulfur. Here, the reversible six-electron redox electrochemistry is constructed by activating the sulfur oxidation reaction (SOR) process of the elaborate mesocrystal NiS 2 (M-NiS 2 ). Through the unique 6e - solid-to-solid conversion mechanism, SOR efficiency can reach an unprecedented degree of ca. 96.0%. The SOR efficiency is further revealed to be closely associated with the kinetics feasibility and thermodynamic stability of the M-NiS 2 intermedium in the formation of elemental sulfur. Benefiting from the boosted SOR, compared with the bulk electrode, the M-NiS 2 electrode exhibits a high reversible capacity (1258 mAh g -1 ), ultrafast reaction kinetics (932 mAh g -1 at 12 A g -1 ), and long-term cyclability (2000 cycles at 20 A g -1 ). As a proof of concept, a new M-NiS 2 ‖Zn hybrid aqueous battery exhibits an output voltage of 1.60 V and an energy density of 722.4 Wh kg cath -1 , which opens a new opportunity for the development of high-energy aqueous batteries., (© The Author(s) 2022. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd.)

The relatively sluggish lithium ion diffusion of LiNi 1/3 Co 1/3 Mn 1/3 O 2 (NCM) is one of the fatal factors which can significantly prevent its widespread usage in high-power applications. In this work, the monodispersed hierarchical porous yolk-shell-like LiNi 1/3 Co 1/3 Mn 1/3 O 2 (YS-NCM) with exposure to {010} electrochemical active facets was successfully synthesized, aiming to elevate the lithium ion diffusion ability and thus to enhance the electrochemical performance. The hierarchical porous nano-/microsphere morphology as well as the voids between the yolk and the shell allow for shortened Li + diffusion pathways, leading to improved Li + diffusion capability. These voids are also beneficial for providing more buffers for the volume changes during repeated charge and discharge. Additionally, the exposure of {010} electrochemical active facets provides more open structure for unimpeded Li + migration. Therefore, by this design strategy, the lithium ion transport kinetics is greatly improved, yielding superior electrochemical performances. When examined as the cathode material for lithium-ion batteries (LIBs), the YS-NCM-based cells have achieved superior rate capability and stable cycling performance, rendering it as a promising cathode candidate for practical lithium-ion battery applications., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The control of structure and morphology in an electrode design for the development of large-power lithium ion batteries is crucial to create efficient transport pathways for ions and electrons. Herein, we report a powerful combinational strategy to build omnibearing conductive networks composed of titanium niobium oxide nanorods and carbon nanofibers (TNO/CNFs) via an electrostatic spinning method and a hydrothermal method into free-standing arrays with a three-dimensional heterostructure core/shell structure. TNO/CNF electrode exhibits significantly superior electrochemical performance and high-rate capability (241 mA h g -1 at 10C, and 208 mA h g -1 at 20C). The capacity of the TNO/CNF electrode is 257 mA h g -1 after 2000 cycles at 20C, which is much higher than that of the TNO electrode. In particular, the TNO/CNF electrode delivers a reversible capacity of 153.6 mA h g -1 with a capacity retention of 95% after 5000 cycles at ultrahigh current density. Superior electrochemical performances of the TNO/CNF electrode are attributed to the unique composite structure., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)