Enhancing the cut-off voltage of high-nickel layered oxide cathodes is an efficient way to obtain higher energy density of lithium-metal batteries (LMBs). However, the phase transition of the cathode materials and the uncontrolled decomposition of the electrolytes at high voltage can lead to irreversible dissolution of transition metal ions, which might cause the crossover effects on the lithium metal anodes. Nonetheless, the mechanism and electrolyte dependence of the crossover effects for Li metal anodes are still unclear. Herein, we investigate the crossover effects between LiNi 0.8 Mn 0.1 Co 0.1 O 2 and Li-metal anode in two electrolyte systems. For ether-based electrolyte, its poor oxidation stability results in massive dissolution of transition metal ions, leading to dendrite growth on anode and rapid cells failure. Conversely, ester-based electrolyte exhibits good electrochemical performances at 4.5 V with little crossover effect. This study provides an idea for electrolyte systems selection for high-voltage LMBs, and verifies that the crossover effect should not be neglected in LMBs.
Jiashu Yuan,Cuijuan Zhang,Tao Liu,Yihan Zhen,Zheng-Ze Pan,Yongdan Li
Non-aqueous redox flow batteries (NARFBs) are promising for large-scale energy storage. However, the lack of high performance membrane is still impeding their further development. Two-dimensional (2D) metal-organic framework (MOF) nanosheets-modified porous separators are prepared by the filtration method. The NARFBs constructed with the 2D MOF nanosheets-modified separator exhibit substantially higher Coulombic efficiency (91.0% vs 82.9%) without obvious compromise on voltage efficiency (93.7% vs 94.2%), and larger average discharge capacity (1.30 vs 0.86 Ah L −1 ) compared with the pristine Celgard separator at 4 mA cm −2 . Such performance is originated from the unique microstructure of 2D-MOF nanosheets . The interlayer spaces assembled by the nanosheet layers enable the fast transport of supporting electrolyte but impede the crossover of active materials and impose limited effect on the separator resistance. This work provides a facile and efficient strategy to mitigating the crossover effect in NARFB.
The oxygen reduction reaction (ORR) is traditionally performed using noble-metals catalysts, e.g. Pt. However, these metal-based catalysts have the drawbacks of high costs, low selectivity, poor stabilities, and detrimental environmental effects. Here, we describe metal-free nitrogen-doped carbon nanoblocks (NCNBs) with high nitrogen contents (4.11%), which have good electrocatalytic properties for ORRs. This material was fabricated using a scalable, one-step process involving the pyrolysis of tris(hydroxymethyl)aminomethane (Tris) at 800 °C. Rotating ring disk electrode measurements show that the NCNBs give a high electrocatalytic performance and have good stability in ORRs. The onset potential of the catalyst for the ORR is −0.05 V (vs Ag/AgCl), the ORR reduction peak potential is −0.20 V (vs Ag/AgCl), and the electron transfer number is 3.4. The NCNBs showed pronounced electrocatalytic activity, improved long-term stability, and better tolerance of the methanol crossover effect compared with a commercial 20 wt% Pt/C catalyst. The composition and structure of, and nitrogen species in, the NCNBs were investigated using Fourier-transform infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. The pyrolysis of Tris at high temperature increases the number of active nitrogen sites, especially pyridinic nitrogen, which creates a net positive charge on adjacent carbon atoms, and the high positive charge promotes oxygen adsorption and reduction. The results show that NCNBs prepared by pyrolysis of Tris as nitrogen and carbon sources are a promising ORR catalyst for fuel cells.
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