The solid electrolyte interface (SEI) formed on the anode is one of the key factors that determine the life span of sodium metal batteries (SMBs). However, the continuous evolution of SEI during charging/discharging processes complicates the fundamental understanding of its chemistry and structure. In this work, we studied the underlying mechanisms of the protection effect offered by the SEI derived from sodium difluoro(oxalato)borate (NaDFOB). In situ nuclear magnetic resonance (NMR) shows that the prior reduction of DFOB anion contributes to the SEI formation, and it suppresses the decomposition of carbonate solvents. Depth-profiling x-ray photoelectron spectroscopy and high-resolution solid-state NMR reveal that the DFOB anion is gradually turned into borate and fluoride-rich SEI with cycling. The protection effect of SEI reaches the optimum at 50 cycles, which triples the life span of SMB. The detailed investigations provide valuable guidelines for the SEI engineering.

The air-sensitivity of transition metal oxide cathode materials (Na x TMO2, TM: transition metal) is a challenge for their practical application in sodium-ion batteries for large-scale energy storage. However, the deterioration mechanism of Na x TMO2 under ambient air is unclear, which hinders the precise design of air-stable Na x TMO2. Here, we revealed the origin of Na x TMO2 degradation by capturing the initial degradation status and microstructural evolution under ambient atmospheres with optimal humidity. It was found that the insertion of CO2 into Na layers along (003) planes of Na x TMO2 led to initial growth of Na2CO3 nanoseeds between TM layers, which initiated fast structure degradation with surface cracks and extrusion of Na2CO3 out of Na x TMO2. The degradation extents and pathways for Na x TMO2 could be highly associated with crystal orientation, particle morphology, and ambient humidity. Interestingly, the deteriorated Na x TMO2 could be completely healed through optimal recalcination, showing even improved air-stability and electrochemical performance. This work provides a helpful perspective on the interfacical structure design of high-performance Na x TMO2 cathodes for sodium-ion batteries.

Polyanion-type sodium superionic conductor (NASICON) Na3V2(PO4)2F (NVOPF) is a promising cathode material for sodium ion batteries (SIBs). However, NVOPF shows relatively low specific capacity and poor long-term performance at high rates. Herein, we report a remarkable improvement of NVOPF cathode material by introducing a surface coating of reduced graphene oxide (RGO). The RGO-coated Na3V2O2(PO4)2F cathode (hereafter denoted as NVOPF@RGO) delivers outstanding high-rate capability (93.6 mAh g–1 at 60 C) and ultralong cycle stability (∼87% retention after 10 000 cycles at 50 C). This surface-enhanced material also exhibits excellent full-cell performance when coupled with the Fe1–x S anode, which sustains a 94.3 mAh g–1 specific capacity after 900 cycles at 20 C. The battery performance and stability of NVOPF@RGO are among the best in the state-of-the-art NASICON-based SIBs. Electrochemical measurements have shown that the coated RGO on NVOPF not only enhances its electric conductivity but also increases the apparent sodium ion diffusivity notably. We confirmed the structural reversibility and revealed the long/short-range structural evolutions of NVOPF@RGO upon electrochemical cycling by multinuclear solid-state nuclear magnetic resonance (ssNMR) combined with X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques.

Summary Centrifugal air compressors are promising due to their zero emissions, long driving range, and wide fuel sources. The gas foil thrust bearing (GFTB) is considered as one of the key components in the centrifugal air compressor to guarantee the system is oil‐free and reliable due to there is self‐acting and high speed. However, the gas foil thrust bearings were all of lower capacity in previous research. In this study, the effects of each foil and nominal clearance on the static performance are developed experimentally. The optimal GFTB with higher capacity is obtained. The feasibility study of the proposed GFTB is carried out, which is installed on the ultra‐high‐speed centrifugal air compressor. The result shows that the proposed GFTB can run stably and efficiently in the full speed ring, and the high‐pressure ratio and large flow rate of the air compressor also can be ensured. The results play an important role in guiding the stable operation of centrifugal air compressors applied in hydrogen fuel cell vehicles (HFCVs).

In order to further explore the large deformation control technology of high ground stress soft rock tunnel, this study takes the highly weathered carbonaceous shale section of Muzhailing tunnel as the research background, the large deformation control effect of scheme 1 “three-step method” and scheme 2 “pilot tunnel expansion method” are compared and analysed by field test and numerical simulation. The results show that the numerical simulation results are consistent with the field test, and the deformation trend of the tunnel is the same. The maximum deformation of the tunnel occurs at the middle step. From the perspective of tunnel deformation, control effect is reduced by about 10% compared with scheme 1, and the deformation of both schemes does not exceed the reserved deformation (400 mm). From the perspective of construction efficiency, the construction efficiency of scheme 2 is 23.07% lower than that of scheme 1. Taking into account the deformation control effect and construction efficiency, it is recommended that the three-step method should be adopted in the construction of this section, and the research results can provide a reference for the construction of the carbonaceous slate section of the tunnel.