| Owing to the superior energy and power density,high voltage,long cycle life and low cost,lithium-ion batteries(LIBs)have become mainstream automotive power batteries.Recently,the event of electric vehicles fires occurred frequently,causing that the safety of power batteries became the focus of attention.With the increase of demand for energy and power density of LIBs,their safety subject to mechanical load must meet higher requirements.Therefore,it is important for preventing safety accident to study the mechanical,electrical and thermal changes of LIBs in the process of mechanical deformation and to identify the critical conditions and mechanisms for short-circuit and thermal runaway of LIBs.The working parameters of LIBs change continuously during actual use,which will inevitably affect the mechanical response of LIBs under mechanical load.In this study,the relationships between the mechanical response of cylindrical LIB cells subject to mechanical load and their working parameters,such as state of charge(SOC),charging voltage,ambient temperature and cycle number,were analyzed and summarized.Also,the mechanisms of internal short circuit and thermal runaway of LIB cells under various working conditions were deeply explored.The moment when a significant voltage drop occurred in the LIB cell was taken as the onset of the short circuit,which was also regard as the critical point of LIB cell mechanics failure.The main research results are as follows:(1)The lateral compression tests of LIB cells with various SOC were carried out.We analyzed the influence of SOC on the mechanical response of LIB cell,and revealed the reason why the mechanical behavior of LIB cells depends on their SOC.The failure modulus and structural crush stress of a LIB cell with a SOC exceeding 0.4 under compression were significantly higher than those of a LIB cell with a SOC below 0.4,because the modululs of the anode electrode for the LIB cell increased significantly with the increase of its SOC.Under compression,the probability of thermal runaway was extremely low for the cells with SOC below 0.8.(2)The lateral compression tests of LIB cells with various voltages were carried out.We analyzed the influence of charging voltages on the mechanical response of LIB cell,and revealed the mechanism of thermal runaway for the overcharged LIB cells.The failure modulus,failure stress and structural crush stress of the cells decreased with the increase of the voltages,and this trend became obvious with the cycle going,which was related to the side reactions in these cells.Thermal runaway would occur on the cells at all overcharge states.Moreover,thermal runaway would occur on the cells charged at 4.2 V to 4.4 V,when their anode tabs were located in the compression area.The thermal runaway risks of the cells overcharged to 4.5 V would reach up to 100%,which resulted from the low stability of their cathode and anode materials.The overcharge cycle had little effect on thermal runaway of LIB cells.(3)The lateral compression tests of LIB cells charged at low temperature were carried out.We analyzed the influence of charging temperatures and rates on the mechanical response of LIB cell and revealed the revalant mechanism.At-25 °C,there was a strong possibility that a premature short circuit occurred locally in the cells during charging,thus they might show complex and variable mechanical response under compression.The failure moduli and structural crush stresses of cells subject to compression tended to decrease as their ambient charging temperatures went down,because the increase of polarization and irreversible active lithium in the cells led to the decrease of their actual charging capacity.At-20 ? to 25 ?,0.5 C-charged cells exhibited higher failure moduli and crushing stresses than the 1 C-charged cells,because the higher charging rate could increase ohmic heat.The decreased actual charge capacity and the increased exothermic side reactions for the cells after low-temperature age resulted in their integral structure softening.(4)The lateral compression tests of LIB cells with different SOC after high-temperature storage were carried out.We analyzed the influence of storage temperatures and SOC on the mechanical response of LIB cell and revealed the revalant mechanism.The high SOC LIB cells after resting at the temperatures above 40 °C tended to exhibit more complex and variable mechanical response under compression than the low SOC LIB cells.The load stresses and moduli of the low SOC LIB cells were little affected by the storage temperature.The load stresses and moduli of the high SOC LIB cells decreased with the increase of the storage temperature at the same strain,which was related to the lithium content of the LIB anode and its degree of self-discharge.At 25 °C to 80 °C,the higher SOC LIB cells had greater deformation resistance.But the elevated storage temperature would reduce the SOC dependence of the LIB cell resistance to deformation.In addition,the side reactions inside the LIB cells were exacerbated during high temperature cycling,which caused the cells to exhibit higher load stress and modulus at the same strain.(5)We built a non-axisymmetric homogenization model of the battery cell,which took into account the influence of the anode tab to study the deformation mechanism of the battery cell.The internal short-circuit points,which were hot spots,were judged based on the separator failure criterion to reveal the mechanism of thermal runaway for the battery cells.The simulation results indicated that in the initial stage of the load,the short-circuit area of the cell with the tab was larger and the short circuit area was concentrated inside the cell.At the moment,the heat produced by shor-circuit points were difficult to dissipate by the external environment.In the later stage of the load,the tab would increase the compressive stress of the jellyroll in the horizontal direction near its position,causing the layered component of the jellyroll staggered and broke,the surface active particles of the electrode fall off,and the positive and negative current collectors contacted direactly.The tab also increased the compressive stress of the jellyroll in the vertical direction,so that the high hardness nickel tab would cut the jellyroll.The influence of the anode tab on the safety of the cell at the later stage of the load agreed with the experimental result that the thermal runaway of a cell occurred at the strain of 0.4 to 0.5.The simulation results also showed that the anode tabe had little effect on the deformation process and mechanical failure mode of the battery cell under radial compression,so its effect on their short-circuit position and short-circuit time was little.This conclusion was consistent with the experimental result that the short-circuit onset of the battery cells under compression were the same in the case where the position of the tab was unknown. |
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