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On the basis of the experimental studies on viscoelastic properties of poly vinyl alcohol (PVA) films at various relative humidity (RH) and temperature conditions by dynamic mechanical analysis (DMA), the influence of both temperature and RH on the glass transition are discussed and an improved property model is developed to relate the dynamic modulus to RH and temperature. The results indicate that (1) with increasing the RH, the storage modulus of PVA decrease remarkably, while both loss modulus and tand sharply increase to reach the peak and then markedly drops. The intensity of this variation is highly dependent upon temperature. (2) Moisture increase will cause the glass transition of PVA at isothermal condition and the transition point can be detected by glass transition relative humidity (RH g ) that obtained by isothermal RH scans. (3) Similar to the relationship between T g and RH, the RH g of PVA vary linearly with temperature. The state diagram of RH g versus temperatures is nearly consistent with that of T g versus RH. (4)The present equation based on model of Mahieux and Reifsnider (Mahieux and Reifsnider, Polymer 2001, 42, 3281) can predict well the dynamic modulus of PVA at various RHs and temperatures.

Knowledge of the mechanical behaviors of polymeric separators immersed in liquid electrolytes is of great significance for predicting the long‐term performance of lithium batteries with high performance and safety. In terms of tensile tests, heating shrinkage, and dynamic mechanical analysis as well as the essential work of fracture method, the study reported here encompasses a systematic investigation of the mechanical properties of a typical commercial polypropylene separator in mixtures of ethylene carbonate and dimethyl carbonate and lithium hexafluorophosphate (LiPF6), comparing with the results in ionic liquid (IL) electrolyte composed of lithium tetrafluoroborate (LiBF4) and 1‐butyl‐3‐methylimidazolium tetrafluoroborate (BMIBF4) and dry condition. It has been found that liquid electrolytes have obvious negative effect on the dimensional stability at elevated temperature and mechanical properties, especially on crack resistance of the polymer separator. LiBF4‐BMIBF4 has much smaller damage on the strength, Young's modulus and fracture toughness of separator than the organic solution except the dynamic modulus at high temperature. Notably, the maximum tensile stress, Young's modulus and the reciprocal of relaxation time of the polymer separator are linearly dependent with strain rate under quasi‐static condition, and the relaxation time has clarified the coupling effect mechanism of liquid electrolyte and loading rate. Moreover, the non‐dimensional viscoelastic constitute equation could perfectly track the tensile behavior of wet and dry separators at different strain rate, and a property model could well characterize the temperature‐dependent storage modulus of polymer separators from rubbery to viscous state. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46441.

One of the critical challenges in advancing lithium ion battery performance is increasing mechanical stability of the solid electrolyte interphase (SEI) layers. Our work aims at developing a mathematical model to study the lithium ion concentration and stress in the SEI on the graphite anode. The main influence factors on the SEI stress have been thoroughly investigated. We find that the ion transportation of the SEI has the underlying effects on the maximum stress in the graphite active layer, especially at a high charging rate. The physical properties of the SEI should be taken into account to obtain an accurate anode stress. The tensile SEI stress along the hoop direction is dominant, and should be regarded as the leading cause of mechanical failure for the SEI. Moreover, the peak stress in the SEI is independent of the charging rate, but can be effectively reduced by rationally designing geometric and material properties of anode components by: (1) decreasing modulus of the SEI itself; (2) enhancing tensile stiffness of the current collector; and (3) making the ratio of anode radius to thickness larger than ten.