Substantially reduced energy and power capabilities of lithium-ion cell operating at low temperatures pose a technical barrier for market penetration of hybrid electric vehicles and pure electric vehicles. The present work delineates Li-ion cell behaviors at low temperatures by a combined experimental and modeling approach. An electrochemical-thermal coupled model, incorporating concentration-and temperature-dependent transport and kinetic properties, is applied and validated against 2.2Ah 18650 cylindrical cells over a wide range of temperatures (−20 • C to 45 • C) and discharge rates. Simulation and experimental results demonstrate the dramatic effects of cell self-heating upon electrochemical performance. A nonisothermal Ragone plot accounting for these important thermal effects is proposed for the first time for Li-ion cells and more generally for thermally coupled batteries. Detailed resistance analysis indicates that performance limits at −20 • C depend on not only discharge rates but also thermal conditions. Optimization of cell design parameters and material properties is performed for 1 C rate discharge starting from −20 • C, where the principal performance limitations are found to be Li + diffusion in the electrolyte and solid-state Li diffusion in graphite particles, instead of charge-transfer kinetic or ohmic resistance.

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A set of lithium-ion cells containing a LiNi 0.8 Co 0.15 Al 0.05 O 2-based positive electrode and a graphite negative electrode were cycled nonintrusively at high power ͑5C rate͒ and elevated temperature ͑40°C͒. The aged cells were characterized at prescribed cycle numbers ͑up to 5250 cycles͒ by a three-electrode cell, capacity measurement, and electrochemical impedance spectroscopy ͑EIS͒. Excellent cyclability of these cells under typical hybrid-electric vehicle conditions is demonstrated by 18% capacity fade after 5250 cycles, and the discharge capacity shows a mainly parabolic behavior with the cycle number ͑N͒ ͑dependent on N 1/2 ͒ in the initial stage and a linear behavior ͑dependent on N͒ for subsequent cycles. Using a lithium reference electrode further reveals that the capacity fade during cycling is primarily caused by the positive electrode, where discharge capacity may be limited by a decrease in active lithium intercalation sites in the oxide particles. The increase in full-cell impedance with cycling is evident from the increase in midfrequency arc width ͑R w ͒, composed of charge-transfer kinetic resistance ͑R ct ͒ and Li + transport resistance through the solid electrolyte interphase ͑SEI͒, R SEI. More specifically, the cell-impedance rise comes mainly from the rising R ct and R SEI of the positive electrode. Based on individual electrode EIS spectra and equivalent-circuit analysis, it is found that the R SEI rise in the positive electrode is far more influential than the change in R ct. Therefore, property modification and thickening of the SEI layer of the positive electrode during cycling appear to be dominant factors in cell-impedance rise and power fade.

Johnson-Cook constitutive model is still the most used model in metal cutting simulation, although several drawbacks reported in the literature. A high number of Johnson-Cook model parameters can be found in the literature for the same work material. One question that may arise is "What is the most suitable set of Johnson-Cook model parameters for a given material?". The present paper puts in evidence some issues related with the selection of these parameters from the literature. In this contribution, two sets of Johnson-Cook model parameters for Ti-6A-4V are evaluated, using three types of metal cutting models. These models are based on three different formulations: Lagrangian, Arbitrary Eulerian-Lagrangian (ALE) and Couple Lagrangian-Eulerian (CEL). This evaluation is based on the comparison between measured and predicted chip geometry, chip compression ratio, forces, plastic deformation and temperature distributions.