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[en] We utilize transmission electron microscopy in conjunction with electron energy loss spectroscopy to investigate local degradation that occurs in LixNi0.8Co0.15Al0.05O2 cathode materials (NCA) after 30 cycles with cutoff voltages of 4.3 V and 4.8 V at 55 °C. NCA has a homogeneous crystallographic structure before electrochemical reactions; however, we observed that 30 cycles of charge/discharge reactions induced inhomogeneity in the crystallographic and electronic structures and also introduced porosity particularly at surface area. These changes were more noticeable in samples cycled with higher cutoff voltage of 4.8 V. Effect of operating temperature was further examined by comparing electronic structures of oxygen of the NCA particles cycled at both room temperature and 55 °C. The working temperature has a greater impact on the NCA cathode materials at a cutoff voltage of 4.3 V that is the practical the upper limit voltage in most applications, while a cutoff voltage of 4.8 V is high enough to cause surface degradation even at room temperature.
[en] Fluorine-doped tin oxide (SnOx:F) films on SUS 316 were prepared as a function of substrate temperature using electron cyclotron resonance-metal organic chemical vapor deposition (ECR-MOCVD) in order to achieve corrosion-resistant and low contact resistance bipolar plates for polymer electrolyte membrane fuel cells (PEMFCs). The SnOx:F films coated on SUS 316 substrate in the heating range from 200 to 500 0C were characterized by x-ray diffraction (XRD), Auger electron microscopy (AES) and field emission-scanning electron microscopy (FE-SEM). To simulate the aggressive PEMFC environment, all electrochemical experiments were conducted in 1 M H2SO4+2 ppm HF solution at 70 0C. With increases in the heat treatment temperature from 300 to 500 0C, it was shown that both corrosion resistance and interfacial contact resistance (ICR) substantially increase. The AES data revealed that the amount of fluorine decreases with increasing temperature in our experimental range. The deposition temperature appears to be one of the critical process parameters on the formation of the corrosion-protective layer for PEMFC bipolar plates. It is probably caused by microstructural evolution before/after potentiodynamic corrosion tests under the PEMFC environment.
[en] The structural stability of cathode materials during electrochemical reactions, in particular, under high‐rate discharge, is pertinent to the design and development of new electrode materials. This study investigates the structural inhomogeneity that develops within a single LiNiCoAlO (NCA83) particle during a fast discharging process under different cutoff voltages. Some of the NCA83 particles discharged from a high cutoff voltage (4.8 V) developed surface areas in which the layered structure was recovered, although the interiors retained the degraded spinel structure. These micro‐ and nano‐scale structural inversions from high cutoff voltage seem highly correlated with structural evolutions in the initial charged state, and may ultimately degrade the cycling stability. This study advances understanding of the structural inhomogeneity within primary particles during various electrochemical processes and may facilitate the development of new Ni‐rich cathode materials. (© 2019 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)
[en] Highlights: • Uniform and ultrathin carbon layer coating on LFP particles using liquid CO2. • Uniform carbon distribution on hierarchical porous nano/micron LFP. • High energy densities (109 Wh kg−1, 458 Wh L−1) at 30 C achieved. • High power densities (3.3 kW kg−1 at 30 C) and long-term cyclability achieved. • Excellent cycling tolerance under challenging conditions. A liquid carbon dioxide (l-CO2) based coating approach is developed for ultrathin, uniform, and conformal carbon coating of hierarchically mesoporous LiFePO4 (LFP) nano/microspheres for fabricating high-energy-density and high-power-density carbon coated LFP (C-LFP) with long-term cyclability. The unique properties of l-CO2 result in an ultrathin carbon layer (1.9 nm) distributed all over the primary nano-sized LFP particles (20–140 nm in diameter), forming a core (LFP)-shell (carbon) structure. This unique structure provides facile penetration of liquid electrolytes and rapid electron and Li-ion transport. C-LFP exhibits high reversible capacity, high energy and power density (168 mAh g−1 at 0.1 C, 109 Wh kg−1 and 3.3 kW kg−1 at 30 C, respectively) with excellent long-term cyclability (84% cycle retention at 10 C after 1000 cycles). In addition, the ultrathin and uniform carbon layer of the mesoporous microspheres allows a high tap density (1.4 g cm−3) resulting in a high volumetric energy density (458 Wh L−1 at a 30 C rate). Furthermore, C-LFP presents a high capacity and stable cycling performance under low-temperature and high-temperature environment. Well-developed carbon coating approach in this study is simple, scalable, and environmentally benign, making it very promising for commercial-scale production of electrode materials for large-scale Li-ion battery applications.
[en] A hybrid nanocomposite of FeF3·0.5H2O and MWCNTs is synthesized as a high–performance cathode material for room–temperature sodium–ion batteries. The composite exhibits remarkably high capacity (197 mAh g−1) and stable cycle performance (148 mAh g−1 at the 100th cycle with a 0.05 C rate) accompanying with the unique nanostructure of the material and the incorporation of MWCNTs. The addition of MWCNTs not only increases the conductivity of the active material but also plays a role to design a unique morphology where nano-sized FeF3·0.5H2O particles are grown both inside and outside of the MWCNTs. The sodium diffusion coefficient of the composite material is determined by galvanostatic intermittent titration technique during dis/charging and the values are in the range of 10−12–10−14 cm2 s−1 which are lesser than lithium diffusion coefficient of FeF3 (10−10–10−12 cm2 s−1). In situ X–ray diffraction coupled with ex situ high resolution transmission electron microscopy is employed to investigate the phase transition behavior. The results reveal both crystalline and amorphous phases upon Na insertion. Furthermore, ex situ NEXAFS spectroscopy (at the F K–edge and Fe L3–edge) is conducted at different potential steps to determine the change in oxidation state and local structure. NEXAFS spectra reveal the conversion reaction mechanism and reversibility of the material as evidenced by the de/formation of NaF during de/sodiation process. The combined study of in situ XRD and NEXAFS will give valuable information on de/sodiation reactions in FeF3·0.5H2O.