Progress and Perspective of the Cathode Materials towards Bromine-Based Flow Batteries

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The Open Access journal Energy Material Advances, published in association with BIT, is an interdisciplinary platform for research in multiple fields from cutting-edge material to energy science.

Editorial board

Energy Material Advances’ editorial board is led by Feng Wu (Beijing Institute of Technology) and Jun Liu (University of Washington) and is comprised of experts who have made significant and well recognized contributions to the field.

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Research Article

Tuning Exciton Recombination Pathways in Inorganic Bismuth-Based Perovskite for Broadband Emission

Single-component emitters with broadband emission are attractive but challenging for illumination and display applications. The two-dimensional organic-inorganic hybrid perovskites have exhibited outstanding broad emission property due to low electronic dimensionality and strong exciton-phonon coupling. However, few layered all-inorganic lead-free perovskites with broadband emission have been explored, and the explicit mechanism of exciton recombination in them also needs in-depth understanding. Herein, the inorganic bismuth-based perovskite Cs3Bi2Br9 achieves the stable broadband emission under ambient temperature and pressure by tuning the exciton recombination pathways via antimony (Sb) doping, and the photoluminescence quantum yield (PLQY) realizes an enhancement from 2.9% to 15.9%. The photoluminescence excitation (PLE) spectra indicate that the doped Sb introduces newly extrinsic self-trapped states. The incorporation of Sb promotes the transfer of free excitons (FEs) to extrinsic self-trapped excitons (STEs) observed from Sb content-dependent steady-state PL spectra and, meanwhile, reduces the nonradiative recombination of the generated extrinsic STEs, which are primarily responsible for the remarkably enhanced broad emission. Furthermore, femtosecond transient absorption results elucidate a clear exciton dynamics, in which the transition from FEs to STEs might arise through the gradient energy levels, and finally extrinsic STEs at various energy states contribute to the broadband emission.

Research Article

Tailoring Defects in Hard Carbon Anode towards Enhanced Na Storage Performance

Hard carbon (HC) anodes show conspicuously commercialized potential for sodium-ion batteries (SIBs) due to their cost-effectiveness and satisfactory performance. However, the development of hard carbon anodes in SIBs is still hindered by low initial Coulombic efficiency (ICE) and insufficient cyclic stability, which are induced by inappropriate defects in the structure. Herein, we introduce a simple but effective method to tailor the defects in HC by the chemically preadsorbed K+. The soft X-ray absorption spectroscopy at the C K-edges reveals that K+ can anchor on the hard carbon via C-O-K bonds, occupying the irreversible reactive sites of Na+. Therefore, the irreversible capacity caused by some C-O bonds can be reduced. Moreover, the preadsorbed K+ can induce the rearrangement of carbon layers and lead to a high graphitization structure with fewer defects and large interlayer spacing, which not only improves the structural stability and electrical conductivity of the HC anode but also facilitates fast Na+ diffusion. Therefore, the as-obtained optimized anode demonstrates a higher ICE with better cyclic stability and superior rate capacities compared with the anode without preadsorbed K+. This work indicates that K+ preadsorbed into hard carbon is a practicable alternative to enhance the Na storage performances of HC anodes for SIBs.

Editorial

Perovskite Semiconductor Nanocrystals

Review Article

Progress and Perspective of the Cathode Materials towards Bromine-Based Flow Batteries

Bromine-based flow batteries (Br-FBs) have been one of the most promising energy storage technologies with attracting advantages of low price, wide potential window, and long cycle life, such as zinc-bromine flow battery, hydrogen-bromine flow battery, and sodium polysulfide-bromine flow battery. The research and development of aqueous Br-FBs are very fast and many achievements have been realized. However, Br-FBs suffer from the sluggish kinetics of Br2/Br- redox couple and serious self-discharge caused by the diffusion of bromine, which hinder the further commercialization and industrialization of the aqueous Br-FBs. A series of mitigation strategies have been developed to figure out these challenges, especially the modifications on electrode materials. Electrode, one of the critical components in a Br-FB, provides the reactions sites for redox couples, upon which its properties exert a significant effect on the performance of Br-FBs. Up to now, extensive research has been carried out on electrode modifications to solve the aforementioned notorious issues of Br-FBs, including surface treatment and surface modification. In this review, various electrode materials and relevant modification approaches used for Br-FBs are overviewed and summarized. Moreover, the relevant mechanisms are illustrated deeply, providing comprehensive and available instruction to pursue and develop high-performance cathodes for Br-FBs with high power density and long lifespan.

Research Article

Size-Dependent Reaction Mechanism of λ-MnO2 Particles as Cathodes in Aqueous Zinc-Ion Batteries

Manganese dioxide (MnO2) with different crystal structures has been widely investigated as the cathode material for Zn-ion batteries, among which spinel λ-MnO2 is yet rarely reported because Zn-ion intercalation in spinel lattice is speculated to be limited by the narrow three-dimensional tunnels. In this work, we demonstrate that Zn-ion insertion in spinel lattice can be enhanced by reducing particle size and elucidate an intriguing electrochemical reaction mechanism dependent on particle size. Specifically, λ-MnO2 nanoparticles (NPs, ~80 nm) deliver a high capacity of 250 mAh/g at 20 mA/g due to large surface area and solid-solution type phase transition pathway. Meanwhile, severe water-induced Mn dissolution leads to the poor cycling stability of NPs. In contrast, micron-sized λ-MnO2 particles (MPs, ~0.9 μm) unexpectedly undergo an activation process with the capacity continuously increasing over the first 50 cycles, which can be attributed to the formation of amorphous MnOx nanosheets in the open interstitial space of the MP electrode. By adding MnSO4 to the electrolyte, Mn dissolution can be suppressed, leading to significant improvement in the cycling performance of NPs, with a capacity of 115 mAh/g retained at 1 A/g for over 500 cycles. This work pinpoints the distinctive impacts of the particle size on the reaction mechanism and cathode performance in aqueous Zn-ion batteries.

Research Article

Large Scale One-Pot Synthesis of Monodispersed Na3(VOPO4)2F Cathode for Na-Ion Batteries

Na-ion batteries (NIBs) have received significant interest as potential candidates for large-scale energy storage owing to the widespread distribution of sodium and superior low-temperature performance. However, their commercial application is usually hindered by the high production cost and inadequate performance for electrode materials, particularly for cathodes. Na3(VOPO4)2F (NVOPF) has been recognized as one of the most promising cathodes for high-energy NIBs owing to the high working voltage and energy density. Here, we report a facile highly efficient room-temperature solution protocol for large-scale synthesis of NVOPF cathode for NIBs. By simply regulating pH, NVOPF can be obtained, which delivered a discharge capacity of 120.2 mAh g-1 at 0.1 C and 72% capacity retention over 8000 cycles at 25 C. Besides, the kilogram-level NVOPF products have been synthesized, and 26650 cylindrical cells were fabricated, which exhibit excellent cycling stabilities, remarkable low-temperature performance with comparable safety features. We hope our findings could provide insights on the industrial application of NVOPF in NIBs.