Zhenduo Sun

Li-rich oxides are considered to be the most promising candidate cathode materials for high-energy Li-ion batteries due to their high specific capacity. However, the lack of a stable surface structure to inhibit interfacial side reactions is a key factor hindering the widespread adoption of Li-rich materials. In this work, a stable LiF@Spinel dual shell is constructed on the surface of Li-rich materials, in which spinel is formed by in-situ surface reconstruction and LiF is bonded to spinel through the Ni-F bond. The spinel serves as a buffer layer between the LiF coating and the Li-rich oxide, providing a three-dimensional Li-ion diffusion channel to improve the Li-ion diffusion coefficient, while the outer LiF plays a critical role in isolating the cathode from the electrolyte. Under the above dual effect, the interfacial side reactions of Li-rich materials are inhibited, thus improving their cycle stability. The obtained LiF@Spinel-coated Li-rich cathode exhibits enhanced capacity retention of 81.5% after 150 cycles at the current density of 2C, much better than 63.2% of the pristine Li-rich sample. These findings demonstrate that the construction of LiF@Spinel dual shell is a successful strategy for the modification of Li-rich materials.

The work aims to obtain the local mechanical parameters of carburized layer of CiNi steel. Tensile test and nano-indentation test were carried out for CrNi steel, stress-strain curve and load-displacement curve were then obtained. The finite element model of nano-indentation was built, and a model for obtaining the local mechanical parameters of carburized layer from load-displacement curve was established combined with dimension analysis. The mechanical parameters of pure iron and carburized layer of CrNi steel were calculated. The results show that, the dimension analysis model is accurate for predicting the mechanical properties of pure iron, the model accuracy is verified. The local mechanical parameters of carburized layer are predicted by the model, the simulated load-replacement curve based on the predicted mechanical parameters is in good agreement with the experimental result, it shows that the prediction result of the model is reasonable.

The work aims to study the influence of carburizing and nitriding on fatigue properties of 18Cr2Ni4WA high strength steel in very high cycle fatigue regime. Very high cycle fatigue tests were carried out on 18Cr2Ni4WA Steel after carburizing and nitriding respectively. The micro morphology of fatigue fracture was observed by scanning electron microscope, the failure mode and failure mechanism were discussed. The relationship between fatigue life and defect size, FGA size, fish eye size of fracture was analyzed. The characteristic size of defects is evaluated by Gumbel, Weibull and GEV distribution functions, and a modified Akiniwa fatigue life prediction model considering the relationship between FGA size and inclusion size was established. The results showed that, nitriding and carburizing treatment improve the surface fatigue limit of the steel. The fatigue life decreases with the increase of internal defect size and FGA size. After carburizing and nitriding treatment, the internal fatigue strength of the specimen decreases slightly. When the failure probability is 99%, the internal defect sizes of nitrided specimens calculated by Weibull, Gumbel and GEV distributions are 141.5 μm, 148.4 μm and 211.7 μm respectively. The calculated internal defect sizes of carburized specimens are 47 μm, 67.8 μm and 40 μm respectively. Compared with the experimental data, the fatigue strength predicted by GEV is the most appropriate. carburizing and nitriding treatment can improve the surface fatigue strength of 18Cr2Ni4WA steel, but slightly reduce the internal fatigue strength. The prediction result of the new model is conservative when the failure probability is 99%, which is suitable for engineering application.

The high cycle fatigue property of carburized 20Cr gear steel was experimentally investigated under axial loading with a stress ratio of zero. The test results show that this steel exhibits gradually decreasing S-N characteristics, and the fatigue strength corresponding to 10⁹ cycles is about 455 MPa. Based on the observation of fracture surfaces, the interior inclusion fisheye-induced fracture is the predominant fracture mode in the life regime beyond 10⁵ cycles. The fine granular area (FGA) cannot be found clearly around the inclusion. Based on the evaluation of stress intensity factor ranges (ΔK) at the front of inclusion and fisheye, the interior crack growth rate (da/dN) equation can be characterized by da/dN = 2.39 × 10⁻²¹(ΔK)^12.32. Corresponding to the tested specimen, the predicted maximum inclusion size is about 52.02 μm. The predicted fatigue strength corresponding to 10⁹ cycles by using Wang’s model is slightly higher than the experimental result, but that by using Murakami’s model is relatively conservative.

The unexpected failures of structural materials in very high cycle fatigue (VHCF) regime have been a critical issue in modern engineering design. In this study, the VHCF property of a Cr-Ni-W gear steel was experimentally investigated under axial loading with the stress ratio of R = −1, and a life prediction model associated with crack initiation and growth behaviors was proposed. Results show that the Cr-Ni-W gear steel exhibits the constantly decreasing S-N property without traditional fatigue limit, and the fatigue strength corresponding to 10⁹ cycles is around 485 MPa. The inclusion-fine granular area (FGA)-fisheye induced failure becomes the main failure mechanism in the VHCF regime, and the local stress around the inclusion play a key role. By using the finite element analysis of representative volume element, the local stress tends to increase with the increase of elastic modulus difference between inclusion and matrix. The predicted crack initiation life occupies the majority of total fatigue life, while the predicted crack growth life is only accounts for a tiny fraction. In view of the good agreement between the predicted and experimental results, the proposed VHCF life prediction model involving crack initiation and growth can be acceptable for inclusion-FGA-fisheye induced failure.