Search for: All records

Na3V2(PO4)2O2xF3-2x (0≤x≤1) (NVPOF) is a promising cathode material for sodium-ion batteries, but conventional synthesis methods often require prolonged processing to obtain phase-pure material. Here, we report a 10-min microwave-assisted solution method that directly produces phase-pure, microscale NVPOF particles with well-defined tetragonal/cuboid morphologies using either NH4VO3 or V2O5 as the vanadium precursor. By systematically varying precursor chemistry, reaction temperature, and precursor concentration, we show that particle size (0.2–5 µm), crystallinity, morphology, and product yield can be tuned. The limited solubility of the V2O5 precursor leads to delayed and heterogeneous vanadium release, resulting in a bimodal particle-size distribution and possibly subtle differences in local vanadium redox environments, reflected in slightly different electrochemical behaviors. The as-synthesized micro-sized materials exhibit moderate rate capability and stable cycling. In situ XRD heating reveals a limited thermal stability window of the NVPOF sample. Guided by this result, a quick one-hour low-temperature heat treatment combined with carbon coating increases the discharge capacity to 118 mAh g-1 at C/10 and delivers 90.4% capacity retention after 300 cycles at 1C. These findings identify precursor selection and microwave reaction conditions as effective parameters for tailoring NVPOF structure and performance and provide a rapid route to NVPOF cathodes for sodium-ion batteries. 

C−H terminated nanometer scale diamonds (d = 1 to 15 nm) are synthesized from 1-fluoroadamantane at high pressure (6−8 GPa) and high temperature (500−1500 °C) in a multianvil press. High resolution transmission electron microscopy, X-ray diffraction, Raman, diffuse reflectance Fourier transform infrared, and X-ray absorption spectroscopies demonstrate the excellent crystallinity and atomically flat C−H terminated surfaces of nanodiamonds with (111) and (110) facets. The importance of hydrogen to the synthesis of nanodiamond and its faceting is discussed. Following vacancy generation, annealing and oxidation of the nanodiamonds, optically detected magnetic resonance and electron spin resonance coherence times (T2 = 0.9 and 2.1 μs) of nitrogen vacancy (NV) centers are measured. The obtained T2 values are equivalent to the shallow NV centers (depth <10 nm) in bulk diamond crystals and larger nanocrystals prepared by mechanical milling. 

Not Layered oxides with an O3 framework have attracted attention as cathode materials for sodium-ion batteries due to their high discharge capacity. Yet they are hampered for commercialization by poor cyclability due to structural instability during the sodium (de)intercalation process. The introduction of Cu in small fractions to the transition metal layers has been empirically observed to improve reversible specific capacity and cycling stability. Understanding the redox activity of Cu in O3-type NaxTMO2 materials is crucial, as it could directly influence the charge compensation mechanism, voltage profile, and structural stability. However, the precise role of Cu in O3-type sodium cathodes under high-voltage cycling conditions (>4.1 V) remains insufficiently understood. To close this gap of knowledge, we studied the effect of Cu in two representative layered oxides with the same O3 stacking but different sodium stoichiometry, Na0.9Mn1/2Fe1/3Cu1/6O2 and NaMn1/2Fe1/3Cu1/6O2. X-ray spectroscopy reveals that in Na0.9Mn1/2Fe1/3Cu1/6O2, Cu exhibits dual redox activity, Cu+/Cu2+ in the pristine state and Cu2+/Cu3+ upon charging in the sodium-deficient material, whereas only the Cu+/Cu2+ redox couple is observed in the fully stoichiometric layered oxide. The results indicate that even a slight deficiency in sodium can significantly impact the electrochemical performance and material stability and alter the elemental redox activity of Cu. 

Mn-based Li-ion battery cathodes encompass a great variety of materials structures. Decades of research effort have proven that developing a Mn-based structure featuring a high redox activity, stable cycling, and cost-effectiveness is a longstanding challenge. Motivated by such a need and inspired by the structural diversity of Mn-based cathodes, we develop a partially cation-disordered lithium niobium manganese oxide with a zigzag structure, filling the knowledge gap between zigzag-ordered and fully disordered Li–Mn-based oxides. Electrochemically, the partially disordered cathode greatly unlocks the redox activity of the zigzag lattice and maintains the cycling stability. Mechanism-wise, the partial disordering suppresses the disproportionation reaction of Mn(III) and facilitates a disordered λ-MnO2–tetragonal cation-disordered rock salt structural transformation. The work suggests the substantial opportunity of using partial disordering as the key strategy to revive locked-up redox activities and realize new energy storage mechanisms, for the pursuit of high-performance cost-effective battery materials.