A micromechanical analysis of the unit cell of a unidirectional composite is performed using the finite element method. The circular fibers are assumed to be packed in a periodic square array. Assuming that the failure criteria for the fiber and matrix materials and also for the fiber-matrix interface are known, the failure envelope of the composite is developed using the microstresses computed in the unit cell analysis. This method is referred to as the Direct Micromechanics Method (DMM). The micromechanical methods were also used to simulate different tests to determine the strength coefficients in phenomenological failure criteria such as maximum stress, maximum strain and Tsai-Wu theories. The failure envelopes from the phenomenological failure criteria are compared with those of the DMM for the cases of biaxial and off-axis loading of a model unidirectional composite material. It is found that none of the phenomenological criteria compare well with the DMM in the entire range. A conservative failure envelope obtained using a combination of maximum stress and Tsai-Wu criteria seems to be the best choice for predicting the failure of unidirectional fiber composites.
Layered Ni-rich cathodes operating at high-voltage with superior cyclic performance are required to develop future high-energy Li-ion batteries (LIBs), yet the worse lattice oxygen escape at the high-voltage region easily causes structural instability, rapid capacity fading and safe issue upon cycling. Here, we report a dual-track strategy to fully restrain lattice oxygen escape of Ni-rich cathodes within 2.7–4.5 V by one-step Ta-doping and CeO2-coating according to their different diffusion energy barriers. The doped-Ta can alleviate charge compensation of oxygen anions as a positive charge center to reduce the lattice oxygen escape, and also induce the formation of the elongated primary particles, greatly inhibiting microcrack generation and propagation. Besides, the CeO2-coating layer effectively captures the remaining oxygen escape and then feedbacks into lattice during subsequent discharge. The resultant Ni-rich cathode enables a capacity of 231.3 mAh g−1 with a high initial Coulombic efficiency of 93.5%. A pouch-type full cell comprising graphite anode and this cathode exhibits more than 1000 times cycle life at 1C in the 2.7–4.5 V range with 90.9% capacity retention.
Ni‐rich layered oxides are at the forefront of the development of high‐energy Li‐ion batteries, yet the extensive applications are retarded by the deteriorative capacity and thermal instability. Herein, an in situ co‐precipitation strategy is implemented to achieve the novel super‐dispersed Nb‐doped Ni‐rich cathode that consists of the elongated and radially aligned primary particles with increased oxygen stable {001} planes. The unique microstructure homogenizes the intragranular and intergranular strain distribution and stabilizes the spherical secondary particles, effectively inhibiting microcrack formation and propagation and surface degradation. The super‐dispersed Nb doping prevents the Li/Ni disordering and lattice oxygen escape, thereby further strengthening the crystal structure and thermal stability. Accordingly, this cathode delivers a high reversible capacity of 229.0 mAh g−1 at 0.1 C with much better retention at 55 °C and 5 C after 100 cycles than the conventional Nb‐doped Ni‐rich cathodes. In a pouch‐type full cell, it exhibits exceptionally long life with a capacity retention of 91.9% at 1 C after 500 cycles and 80.5% at 5 C after 2000 cycles within 3.0–4.2 V, greatly prolonging the service period to cater to the lightweight and intelligence of electric vehicles.
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