Constructing Highly Emissive Covalent Organic Frameworks for Fe3+ Ion Detection via Wall Function

Covalent organic frameworks (COFs) represent a new type of crystalline porous polymers that possess pre-designed skeletons, uniform nanopores, and ordered π structure. These attributes make them well-suited for the design of light-emitting materials. However, the majority of COFs exhibits poor luminescence due to aggregation-caused quenching (ACQ), resulting from the strong interaction between adjacent layers. To break the limitation, the building units with three methoxy groups on the walls are used to construct TM-OMe-EBTHz-COF, which suppresses the ACQ effects to improve light-emitting activity of COF. The TM-OMe-EBTHz-COF exhibits a notable emission of yellow-green luminescence in the solid state, with a remarkably high absolute quantum yield of 21.1%. The methoxy groups and hydrazine linkage form three coordination sites, contributing to excellent performance in metal ions sensing. The TM-OMe-EBTHz-COF demonstrates high sensitivity and selectivity to Fe3+ ion. Importantly, the low detection limit is below 150 nanomolar, ranking it among the best-performing Fe3+ sensor systems.

Boosting the interfacial dynamics and thermodynamics in polyanion cathode by carbon dots for ultrafast-charging sodium ion batteries

Ultrafast-charging is the focus of next-generation rechargeable batteries for widespread economic success by reducing the time cost. However, the poor ion diffusion rate, intrinsic electronic conductivity and structural stability of cathode materials seriously hinder the development of ultrafast-charging technology. To overcome these challenges, an interfacial dynamics and thermodynamics synergistic strategy is proposed to synchronously enhance the fast-charging capability and structural stability of polyanion cathode materials. As a case study, a Na3V2(PO4)3 composite (NVP/NSC) is successfully obtained by introducing an interface layer derived from N/S co-doped carbon dots. Density functional theory calculations validate that the interfacial bonding effect of V-N/S-C significantly reduces the Na+ transport energy barrier. D-band center theory analysis confirms the downward shift of the V d-band center enhances the strength of the V-O bond and considerably inhibits irreversible phase transformation. Benefitting from this interfacial synergistic strategy, NVP/NSC achieves a high capability and excellent cycling stability with a surprisingly low carbon content (2.23%) at an extremely high rate of 100C for 10 000 cycles (87.2 mA h g-1, 0.0028% capacity decay per cycle). Furthermore, a superior performance at 5C (115.3 mA h g-1, 92.1% capacity retention after 800 cycles) is exhibited by the NVP/NSC‖HC full cell. These findings provide timely new insights for the systematic design of ultrafast-charging cathode materials.

Dual-Element-Modified Single-Crystal LiNi0.6Co0.2Mn0.2O2 as a Highly Stable Cathode for Lithium-Ion Batteries

Single-crystalline LiNi_0.6Co_0.2Mn_0.2O₂ cathodes have received great attention due to their high discharge capacity and better electrochemical performance. However, the single-crystal materials are suffering from severe lattice distortion and electrode/electrolyte interface side reactions when cycling at high voltage. Herein, a unique single-crystal LiNi_0.6Co_0.2Mn_0.2O₂ with Al and Zr doping in the bulk and a self-formed coating layer of Li₂ZrO₃ in the surface has been constructed by a facile strategy. The optimized cathode material exhibits excellent structural stability and cycling performance at room/elevated temperatures after long-term cycling. Specifically, even after 100 cycles (1C, 3.0-4.4 V) at 50 °C, the capacity retention for the Al and Zr co-doped sample reaches 92.1%, which is much higher than those of the single Al-doped (85.4%), single Zr-doped (87.1%), and bare samples (76.3%). The characterization results and first-principles calculations reveal that the excellent electrochemical properties are attributed to the stable structure and interface, in which the Al and Zr co-doping hinders cation mixing and suppresses detrimental phase transformations to reduce internal stress and mitigate microcracks, and the coating layer of Li₂ZrO₃ can protect the surface and suppress interfacial parasitic reactions. Overall, this work provides important insights into how to simultaneously build a stable bulk structure and interface for the single-crystal NCM cathode via a facile preparation process.

Heterogeneous Degradation in Thick Nickel-Rich Cathodes During High-Temperature Storage and Mitigation of Thermal Instability by Regulating Cationic Disordering

The thermal instability is a major problem in high-energy nickel-rich layered cathode materials for large-scale battery application. Due to the scarce investigation of thick electrodes at the practical full-cell level, the understanding of thermal failure mechanism is still insufficient. Herein, an intrinsic origin of thermal instability in fully charged industrial pouch cells during high-temperature storage is discovered. Through the investigation from crystals to particles, and from electrodes to cells, it is shown that serious top-down heterogeneous degradation occurs along the depth direction of the thick electrode, including phase transition, cationic disordering, intergranular/intragranular cracks, and side reactions. Such degradation originates from the abundant oxygen vacancies and reduced catalytic Ni²⁺ at cathode surface, causing microstructural defects and directly leading to the thermal instability. Nonmagnetic elements doping and surface modification are suggested to be effective in mitigating the thermal instability through modulating cationic disordering.

Wet-CO2 Pretreatment Process for Reducing Residual Lithium in High-Nickel Layered Oxides for Lithium-Ion Batteries

As the push for inexpensive vehicle electrification grows, high-energy-density cathodes for lithium-ion batteries, such as high-nickel layered oxides, have received a great deal of attention in both industry and academia. These materials, however, suffer from severe residual lithium formation, which causes slurry gelation during electrode fabrication and gas evolution during cycling. Herein, a novel cobalt hydroxide coating method on wet-CO₂ gas-treated LiNi_0.91Mn_0.03Co_0.06O₂ (Co-CO₂-NMC91) is presented. Notably, the wet-CO₂ treatment prior to a dry cobalt hydroxide coating plays a critical role in improving the coating uniformity and ultimately decreases the effective residual lithium content. Furthermore, full cells of Co-CO₂-NMC91 exhibit excellent capacity retention of 91% after 200 cycles. This study highlights how a wet-CO₂ treatment can be used to improve a typical dry coating and provides new insights toward the development of cathodes for high-energy-density LIBs without severe slurry gelation or gas evolution.

Atomic Structure of Surface-Densified Phases in Ni-Rich Layered Compounds

In this work, we report the presence of surface-densified phases (β-Ni₅O₈, γ-Ni₃O₄, and δ-Ni₇O₈) in LiNiO₂ (LNO)- and LiNi_0.8Al_0.2O₂ (LNA)-layered compounds by combined atomic level scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). These surface phases form upon electrochemical aging at high state of charge corresponding to a fully delithiated state. A unique feature of these phases is the periodic occupancy by Ni²⁺ in the Li layer. This periodic Ni occupancy gives rise to extra diffraction reflections, which are qualitatively similar to those of the LiNi₂O₄ spinel structure, but these surface phases have a lower Ni valence state and cation content than spinel. These experimental results confirm the presence of thermodynamically stable surface phases and provide new insights into the phenomena of surface phase formation in Ni-rich layered structures.